AN ABSTRACT OF THE DISSERTATION OF
Sorasaree Tonsiengsom for the degree of Doctor of Philosophy in Chemistry presented
on November 28, 2006.
Title: STUDIES TOWARD THE TOTAL SYNTHESIS OF ALKALOIDS:
NAGELAMIDE A AND D, AGELASTATIN D, DRAGMACIDIN A-C, SALACIN
AND ALMAZOLES
Abstract approved:
Kevin P. Gable
Studies toward the total syntheses of highly potent cytotoxic alkaloids
including the bromopyrrole alkaloids and indole alkaloids were conducted and are
described. Studies carried out in the course of this dissertation consist of five total
syntheses of natural products that include bromopyrrole alkaloids nagelamide A, D
and agelastatin D as well as indole-based alkaloids dragmacidin A-C, salacin and
almazoles.
The total synthesis of dimeric bromopyrroles, nagelamide A, was achieved in
8 steps from ornithine by using NCS oxidative dimerization of 2-aminoimidazole as a
key step. The total synthesis of nagelamide D was accomplished in 6 steps using acidpromoted dimerization as a key feature. These methods provide a short and rapid entry
into the syntheses of nagelamides without the use of protecting groups on nitrogen.
In studies toward the synthesis of agelastatin D, the ABD-ring system was
derived from a β–functionalization of linear imidazolone. The studies carried out in
the course of this thesis have set in place a major ABD-ring core for the agelastatin D.
Only the construction of the C-ring through a one-carbon bridge remains to be done.
In the synthesis of bisindole alkaloids, a short synthetic strategy for
dragmacidin A, B and C was accomplished by involving the dimerization of
oxotryptamines to give bis(indolyl)pyrazines, which upon reduction and selective
methylation with sodium cyanoborohydride in acetic acid or formic acid afforded the
target piperazine natural products as the key steps.
The application of the interrupted Pictet-Spengler cyclization involving
halotryptamine spirocyclization with aldehydes having various functionalities has
been investigated. The methodology appears to work well with aldehydes containing
alcohol or ester groups but not with ketones or protected aldehydes. Furthermore, we
have demonstrated the synthesis of salacin via halotryptamine spirocyclization.
A short synthesis of almazole C and D are described. The key steps involve the
peptide coupling and Gabriel-Robinson oxazole synthesis with chiral, nonracemic keto
amides. An integral aspect of the research involves the preparation of the key β–
oxotryptophan synthon and demonstration of its utility. These investigations have lead
to a revision of the structure of almazole D as 5-(3-indolyl)oxazole.
©
Copyright by Sorasaree Tonsiengsom
November 28, 2006
All Rights Reserved
STUDIES TOWARD THE TOTAL SYNTHESIS OF ALKALOIDS:
NAGELAMIDE A AND D, AGELASTATIN D, DRAGMACIDIN A-C,
SALACIN AND ALMAZOLES
by
Sorasaree Tonsiengsom
A DISSERTATION
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Presented November 28, 2006
Commencement June 2007
Doctor of Philosophy dissertation of Sorasaree Tonsiengsom presented on
November 28, 2006
APPROVED:
Major Professor, representing Chemistry
Chair of the Department of Chemistry
Dean of the Graduate School
I understand that my dissertation will become part of the permanent collection of
Oregon State University libraries. My signature below authorizes the release of my
dissertation to any reader upon request.
Sorasaree Tonsiengsom, Author
ACKNOWLEDGMENTS
First of all, I wish to express my appreciation and gratitude to my advisor, Dr.
David Horne, for his encouragement as well as many hours of attention and guidance
he has devoted to this research study. I would also like to thank Drs. Max Deinzer,
Kevin Gable, Paul Blakemore, and Goran Jovanovic for serving on my doctoral
dissertation committee and for their helpful suggestions and comments.
The financial support of the chemistry department of Oregon State University
is gratefully acknowledged. Special gratitude is extended to Drs. Kenichi Yakushijin
and Fumiko Y. Miyake for their continued assistance with technical insights into this
work. In addition, several individuals including Eddie Lee, Yulong Ma, and those
from the White research group, the Carter research group, and the Blakemore research
group have provided useful comments and assistance in different ways when I worked
on this dissertation and their help is acknowledged.
Last but not least, I would like to thank my parents, sister, and my husband.
Without their love and support I would never have finished this work.
TABLE OF CONTENTS
Page
CHAPTER 1 GENERAL INTRODUCTION…………………………….. 1
Bromopyrrole Alkaloids………………………………….... 1
Indole Alkaloids………………………………………….... 3
References…………………………………………………. 7
CHAPTER 2 SYNTHESIS OF NAGELAMIDE A AND D…………….. 10
Isolation, Structure Determination and
Biological Activities……………………………………….. 11
The Oxidation of 2-Aminoimidazole by Elemental
Bromine or Acid ……...……………………………........... 12
Synthesis of Nagelamide A and D………………………… 14
References…………………………………………………. 21
CHAPTER 3 STUDIES TOWARD THE SYNTHESIS OF
AGELASTATIN D…………………................................... 22
Isolation, Structure Determination and
Biological Activities………………………………………. 22
Previous Synthetic Work………………………………….. 24
Synthesis of Agelastatin D………………………………… 26
References…………………………………………………. 34
TABLE OF CONTENTS (Continued)
Page
CHAPTER 4 SYNTHESIS OF BISINDOLYLPIPERAZINE MARINE
ALKALOIDS DRAGMACIDIN A, B AND C:
REDUCTION OF 2,5-BIS(3’-INDOLYL)PYRAZINES
TO 2,5-BIS(3’-INDOLYL)PIPERAZINES………………. 36
Isolation and Biological Activities………………………… 36
Previous Synthetic Work………………………………….. 37
Synthesis of Dragmacidin A, B and C…………………….. 39
References…………………………………………………. 47
CHAPTER 5 THE STEREOSELECTIVE INTRAMOLECULAR
IMINIUM ION SPIROCYCLIZATION:
SYNTHESIS OF SALACIN……………………………… 49
Isolation, Structure Determination and
Biological Activities………………………………………. 49
Previous Synthetic Work: The Construction of the
Spiroalkaloids Using (L)-Tryptophan or Other Derivatives.. 49
The Methodology for Stereoselective
Spiro[3,3-pyrrolidine]oxindole Construction……………… 52
Synthesis of Salacin……………………………………….. 54
References…………………………………………………. 64
TABLE OF CONTENTS (Continued)
Page
CHAPTER 6 SYNTHESIS OF 5-(3-INDOLYL)OXAZOLE
NATURAL PRODUCTS AND STRUCTURE REVISION
OF ALMAZOLE D……………………………………….. 65
Isolation, Structure Determination and
Biological Activities………………………………………. 65
Previous Synthetic Work………………………………….. 67
Synthesis of Almazoles……………………………………. 68
References…………………………………………………. 73
CHAPTER 7 EXPERIMENTAL SECTION…………………………….. 75
GENERAL CONCLUSION………………………………………………. 124
BIBLIOGRAPHY…………………………………………………………. 126
APPENDIX………………………………………………………………... 131
LIST OF FIGURES
Figure
Page
1.1
Linear, tetracyclic and dimerized oroidin alkaloids……………….. 2
1.2
Structures of spirooxindole alkaloids……………………………… 4
1.3
Structures of bisindole alkaloids…………………………………... 5
1.4
Structures of naturally occurring 2,4-disubstituted oxazoles……… 6
1.5
Structures of naturally occurring 5-(3-indolyl)oxazoles…………... 7
2.1
Nagelamide A-H and dimeric bromopyrrole alkaloids……………. 10
2.2
The structures of nagelamide A and D…………………………….. 12
2.3
HMBC Analysis of 2.30-2.32……………………………………... 18
3.1
The structures of agelastatins……………………………………… 22
4.1
The structures of dragmacidins…………………………………… 36
4.2
NOE Experiment of 5-Br-oxotryptamine and 6-Br-oxotryptamine.. 41
4.3
The coupling constants for trans and cis piperazines……………… 44
5.1
The structure and NOE analysis of salacin 1.17…………………... 49
5.2
The NOE analysis of 5.36…………………………………………. 58
5.3
The 1H NMR of 5.45 and related spirooxindoles…………………. 61
5.4
The 1H and 13C NMR of major and minor rotamers
of N-formamide 5.46………………………………………………. 62
5.5
The NOE analysis of the rotational isomer of
the N-formamide bond in 1.17…………………………………….. 63
6.1
The structures of almazoles……………………………………….. 65
LIST OF FIGURES (Continued)
Figure
Page
6.2
The 13C NMR data of almazole C, D, prealmazole C and
martefragin A……………………………………………………… 66
6.3
Propose the revised structure of almazole D by the Horne group… 67
6.4
The comparison of proposed almazole d and revised structure of
almazole D and their derivatives………………………………….. 73
LIST OF TABLES
Table
4.1
Page
The comparison of 1H NMR spectral data of dragmacidin C …….. 47
STUDIES TOWARD THE TOTAL SYNTHESIS OF ALKALOIDS:
NAGELAMIDE A AND D, AGELASTATIN D, DRAGMACIDIN A-C,
SALACIN AND ALMAZOLES
CHAPTER I
GENERAL INTRODUCTION
1.1 Bromopyrrole Alkaloids
Introduction
Pyrrole and 2-amimino imidazole derivatives are common structural units
found in metabolites possessing various states of bromination. 4,5-Dibromopyrrole-2carboxylic acid, the amide, and nitrile have been isolated from Agelas oroids.1 These
brominated pyrrole units are the most common class of secondary metabolites from
Agelas species. Oroidin alkaloids are a family of C11N5 natural products isolated as
secondary metabolites from marine sponges. Oroidin 1.1 is sometimes called the first
member of the family and is generally regarded as the parent member of the simplest
of these compounds (Fig 1.1). Oroidin was initially isolated from the Mediterranean
axinellid sponge Agelas oroides in 19711 and the structure confirmed in 1973.2 This
compound is historically the central example in a series of similar alkaloids as follows:
the linear oroidin members; hymenidin,3 which is the monobromo compound and
clathrodin4 which is the debromo species. Oroidin was hypothesized as a biosynthetic
precursor of dimers: sceptrin,5 nagelamides6 or cyclized alkaloids isolated from marine
sponges; agelastatins (Fig 1.1).
2
O
N
N
H
H2N
N
H
H
N
Br
R1
N
H
H
N
R2
1.1 oroidin : R1 = R2 = Br
1.2 hymenidin : R1 = H, R2 = Br
1.3 clathrodin : R1 = R2 = H
NH2
O
NH
HN
NH
H
N
O
Br
HN
HN
NH2
1.4 sceptrin
Br
NH
Br
N
R
HN
O
H
N
Br
O
N
H
N
H
NH2
N
HN
NH2
Br
1.5 nagelamide A : R = H
1.6 nagelamide B : R = OH
Me
N
O
Br
D
H
C
NH
N
HH
R
A B
NH
HO
O
1.7 agelastatin A : R = H
1.8 agelastatin B : R = Br
HO
Br
Me
N
H
NH
H OH
NH
N
O
agelastatin C
1.9
HO
O
Br
H
N
H
O
NH
HH
NH
N
O
agelastatin D
1.10
Figure 1.1. Linear, tetracyclic and dimerized oroidin alkaloids.
The oroidin alkaloids possess antibacterial, 5 antifungal, 5 antiserotonergic,3 αadrenoceptor blocking7 and mild cytotoxic activities.8 Recently, the first example of a
cytotoxic agent toward tumor cells in the oroidin alkaloid family, agelastatin A 1.7,
was reported by Pietra.9 Agelastatin A was isolated from Agelas dendromorpha and
represents a novel skeleton for an oroidin alkaloid. This highly fused tetracyclic
bromopyrrole skeleton could be derived biogenetically from a hymenidin-like
precursor. The studies described in this section focus on the synthesis of the oroidin
family of the marine alkaloid; dimerized oroidin: nagelamide A and D and tetracyclic
oroidin: agelastatin D (Fig 1.1).
3
1.2 Indole Alkaloids
Introduction
Indole alkaloids from marine origin and from plants have been widely
documented in the last 50 years for their chemistry and biology. Many of these
compounds have received attention due to their structural novelty and biological
significance. There are numerous indole alkaloids known, including compounds with
the true indole nucleus and those derived from it such as dihydroindole,
pseudoindoxyl, and oxoindole. A natural representative with an indole nucleus is
biogenic tryptamine. In this section, we focus on a variety of functionalized indole
derivatives such as spirooxindole alkaloids and halogenated bisindole alkaloids. The
spirooxindole alkaloid family is one of the tryptamine-based metabolites. Halogenated
bisindole alkaloids include dimeric indoles tethered by a heterocyclic chromophore
such as dragmacidins or indolyloxazole alkaloids were substituted indoles such as
almazoles.
Spirooxindole alkaloids
A number of oxindole alkaloids derived from tryptamine or tryptophan
displaying significant biological activity are shown in Fig 1.2. The spiropyrrolidine
oxindoles such as spirotryprostatin A 1.11 and B 1.12 are known to inhibit the
mammalian cell cycle in the G2/M phase and were isolated from the fermentation
broth
of
Aspergillus
fumigatus
BM939.10,11
Rhynchophylline
1.13
and
isorhynchophylline 1.14, spirooxindole alkaloids, were isolated from the Uncaria
plants which have been widely used to treat ailments such as hypertension and
cardiovascular conditions.12 Elacomine 1.15 and isoelacomine 1.16 were isolated from
the roots of the shrub Elaeagnus commutate.13 Salacin 1.17, a spirooxindole alkaloid,
was isolated from a Thai medicinal plant.14
4
O
H
HO
O
HN
N
HN
N
N
O
H3CO
O
H
H
N CO2Me
HN
Et
O
CO2Me
OCH3
spirotryprostatin A
1.11
O
O
O
HN
N
O
HN
HN
N
N
O
elacomine
1.15
rhynchophylline
1.13
H
Et
CO2Me
spirotryprostatin B
1.12
N CO2Me
H
OCH3
isorhynchophylline
1.14
HO
isoelacomine
1.16
O
HN
NCHO
O
salacin
1.17
Figure 1.2. Structures of spirooxindole alkaloids.
Bisindole alkaloids
Bisindole alkaloids display significant biological activities. Bisindole alkaloids
and their analogs are one of the rapidly growing groups of sponge metabolites. These
alkaloids exhibit potent bioactivities including antiviral, antitumor, antibacterial and
anti-inflammatory activities (Fig 1.3).
Hyrtiosin B 1.18, the first simple bisindole alkaloid, was found in the
Okinawan marine sponge Hyrtios erecta and showed in vitro cytotoxic activity against
human epidermoid carcinoma KB cells.15 There are a variety of bisindole derivatives
linked by a heterocyclic system, including the topsentins, nortopsentins, rhopaladins,
hamacanthins, and dragmacidin family of natural products. Bartik and co-workers
5
reported three bisindole metabolites; topsentin A, bromotopsentin and deoxytopsentin
from Topsentia genitrix in 1987.16 The topsentin family is the first example of the
brominated bisindole alkaloids which have a 2-acylimidazole inserted between two
indole units that have shown cytotoxic and antitumor activities. Nortopsentin A-C
1.22-1.23, bisindole alkaloids possessing an imidazole moiety, were isolated from the
halichondride sponge Spongosorites ruetzleri and exhibited antifungal and antitumor
activity.17,18
N
H
N
O
HO
OH
O
N
H
N
H
R1
R2
hyrtiosin B
1.18
N
H
N
H
NH O
N
R2
R1
1.22 nortopsentin A : R1 = R2 = Br
1.23 nortopsentin B : R1 = Br, R2 = H
1.24 nortopsentin C : R1 = H, R2 = Br
H
N
N
H
N
H
1.25 rhopaladin A : R1 = OH, R2 = Br
1.26 rhopaladin B : R1 = OH, R2 = H
1.27 rhopaladin C : R1 = H, R2 = Br
1.28 rhopaladin D : R1 = R2 = H
Br
H
N
R3
N
R1
N
R1
R2
N
N
H
N
H
O
R3
1.19 topsentin : R1 = R2 = R3 = OH
1.20 bromotopsentin : R1 = Br, R2 = H, R3 = OH
1.21 deoxytopsentin : R1 = R2 = R3 = H
HN
R1
O
Br
N
H
R2
hamacanthin A
1.29
N
H
H
N
Br
N
R4
1.30 dragmacidin : R1 = OH, R2 = Br, R3 = H, R4 = Me
1.31 dragmacidin A : R1 = R2 = R3 = R4 = Me
1.32 dragmacidin B : R1 = R2 = H, R3 = R4 = Me
1.33 dragmacidin C : R1 = R2 = R3 = R4 = H
Figure 1.3. Structures of bisindole alkaloids.
6
Kobayashi and co-workers reported the isolation of new bisindole alkaloids
containing an imidazolinone moiety named rhopaladins A-D, from Okinawan tunicate
Rhopalaea sp.19 Hamacanthin A 1.29,20 is a representative of a group of bisindole
metabolites containing a 5,6-dihydro-1(2H)-pyrazinone moiety between bromoindole
units. This compound was isolated from marine sponge Hamacantha sp. and showed
antimicrobial activity. Dragmacidin 1.30, isolated marine sponge Dragmacidon sp., is
the first example of the dragmacidin family which has a piperazine linkage between
indole units.21
Oxazole alkaloids
Oxazole alkaloids are found in both marine and terrestrial sources. 2,4Disubstituted oxazoles are powerful bioactive marine metabolites. An example is
hennoxazole, a bioactive bisoxazole, isolated from a sponge, Polyfibrospongia sp (Fig
1.4).22
R1
OMe
O
OR2 H
O
N
O
N
1.34 hennoxazole A : R1 = OH, R2 = Me
1.35 hennoxazole B : R1 = OH, R2 = Et
1.36 hennoxazole C : R1 = OH, R2 = Bu
1.37 hennoxazole D : R1 = H, R2 = Me
Figure 1.4. Structures of naturally occurring 2,4-disubstituted oxazoles.
However, indole alkaloids bearing an unusual 2,5-disubstituted oxazole moiety
occur in a small number of natural products, and a lot of them are found in a red alga
off the coast of Senegal (Fig 1.5). The 5-(3-indolyl)oxazole ring system occurs in a
small
number
of
natural
products.
The
pimprinine
family,
pimprinine,23
pimprinethine,24 WS-30581A and B,25 and pimprinaphine26 is the first of the simple of
7
2,5-disubstituted oxazole moiety on the indole. Martefragin A, an indolyl-peptide
substituted on an oxazole moiety, was isolated from a red alga, Martensia fragillis and
showed inhibitory activity against lipid peroxidation.27 Almazole A-D are the oxazole
ring
having
2,5-inserted
between
indole
and
peptide
N,N-dimethyl-L-
phenylalaninamide moieties.28-30 The almazole family is found mostly in red seaweed
and only almazole D 1.47 showed antibacterial activity against Gram-negative
bacteria. These indolyloxazole alkaloids can be regarded as masked tryptamine
derivative.
O2C
N
O
R
N
O
NH
N
H
N
H
martefragin A
1.43
1.38 pimprinine : R = Me
1.39 pimprinethine : R = Et
1.40 WS-30581A : R = Pr
1.41 WS-30581B : R = Bu
1.42 pimprinaphine : R = Bn
N
N
O
O
NHR O
O
NMe2
1.44 almazole A : R = CHO
1.45 almazole B : R = H
N
H
O
NMe2
almazole C
1.46
N OH
H
N
N
almazole D
1.47
Figure 1.5. Structures of naturally occurring 5-(3-indolyl)oxazoles.
1.3 References
1. Forenza, S.; Minale, L.; Riccio, R.; Fattorusso, E.; J. Chem. Soc., Chem.
Commun., 1971, 1129.
2. Garcia, E. E.; Benjamin, L. E.; Fryer, R. I. J. Chem. Soc., Chem. Commun.,
1973, 78.
8
3. Kobayashi, Y.; Ohizumi, Y.; Nakamura, H.; Hirata, Y.; Wakamatsu, K.;
Miyazawa Experientia 1986, 42, 1176.
4. Morales, J. J.; Rodriguez, A. D. J. Nat. Prod. 1991, 54, 629.
5. a) Faulkner, D. J.; Walker, R. P.; Engen, D. V.; Clardy, J. J. Am. Chem. Soc.
1981, 103, 6772. b) Keifer, P. A.; Schwartz, R. E.; Koker, M. E. S.; Hughes, R.
G.; Rittschof, D.; Rinehart, K. L. J. Org. Chem. 1991, 56, 2965. c) Kobayashi,
J.; Tsuda, M.; Ohizumi, Y. Experientia 1991, 47, 301. d) Kobayashi, J.; Tsuda,
M.; Murayama, T.; Nakamura, H.; Ohizumi, Y.; Ishibashi, M.; Iwamura, M.;
Ohta, T. Tetrahedron 1990, 46, 5579.
6. Endo, T.; Tsuda, M.; Okada, T.; Mitsuhashi, S.; Shima, H.; Kikuchi, K.;
Mikami, Y.; Fromont, J.; Kobayashi, J. J. Nat. Prod. 2004, 67, 1262-1267.
7. Kobayashi, Y.; Ohizumi, Y.; Nakamura, H.; Hirata, Y.; Wakamatsu, K.;
Miyazawa, T. Experientia 1986, 42, 1064.
8. Cimino, G.; De Rosa, S.; De Stefano, S.; Mazzarella, L.; Puliti, R.; Sodano, G.
Tetrahedron Lett. 1982, 23, 767.
9. D’Ambrosio, M.; Guerriero, A.; Debitus, C.; Ribes, O.; Pusset, J.; Leroy, S.;
Pietra, F. J. Chem. Soc., Chem. Commun. 1993, 1305.
10. Cui, C. -B.; Kakeya, H.; Osada, H. J. Antibiot. 1996, 49, 832-835.
11. Cui, C. -B.; Kakeya, H.; Osada, H. Tetrahedron 1996, 52, 12651.
12. Shi, J.; Yu, J. –X.; Chen, X. –P.; Xu, R. –X. Acta Pharm. Sinica 2003, 24, 97101.
13. Pellegrini, C.; Weber, M.; Borschberg, H. –J. Helv. Chim. Acta 1996, 79, 151168.
14. Ponglux, D.; Wongseripipatana, S.; Aimi, N.; Nishimura, M.; Ishikawa, M.;
Sada, H.; Haginiwa, J.; Sakai, S. Chem. Pharm. Bull. 1990, 38, 573-575.
15. Kveder, S.; Iskric, S. Biochem. J. 1965, 94, 509-512.
16. Bartik, K.; Braekman, J. –C.; Daloze, D.; Stoller, C.; Huysecom, J.;
Vandevyver, G.; Ottinger, R. Can. J. Chem. 1987, 65, 2118.
17. Sakemi, S.; Sun, H. H. J. Org. Chem. 1991, 56, 4304.
9
18. Sun, H. H.; Sakemi, S.; Gunasekera, S.; Kashman, Y.; Lui, M.; Burres, N.;
McCarthy, P. U. S. Patent 4970226; Chem. Abstr. 1991, 115, 35701z.
19. Sato, H.; Tsuda, M.; Watanabe, K.; Kobayashi, J. Tetrahedron 1998, 54, 8687.
20. Gunasekera, S. P.; McCarthy, P. J.; Kelly, -B. M. J. Nat. Prod. 1994, 57, 1437.
21. Kohmoto, S.; Kashman, Y.; McConnell, O. J.; Rinehart, K. L.; Wright, A.;
Koehn, F. J. Org. Chem. 1988, 53, 3116.
22. Ichiba, T.; Yoshida, W. Y.; Scheuer, P. J.; Higa, T.; Gravalos, D. G. J. Am.
Chem. Soc. 1991, 113, 3173-3174.
23. Bhate, D. S.; Hulyalker, R. K.; Menon, S. K. Experientia 1960, 16, 504.
24. Noltenmeyer, M.; Sheldrick, G. M.; Hoppe, H. –U.; Zeeck, A. J. Antibiot.
1982, 35, 549-555.
25. Umehara, K.; Yoshida, K.; Okamoto, M.; Iwami, M.; Tanaka, H.; Kohsaka,
M.; Imanaka, H. J. Antiobiot. 1984, 37, 1153-1160.
26. Koyama, Y.; Yokose, K.; Dolby, L. J. Agric. Biol. Chem. 1981, 19, 1437.
27. Takahashi, S.; Matsunaga, T.; Hasegawa, C.; Saito, H.; Fujita, D.; Kiuchi, F.;
Tsuda, Y. Chem. Pharm. Bull. 1998, 46, 1527-1529.
28. N’Diaye, I.; Guella, G.; Chiasera, G.; Mancini, I.; Pietra, F. Tetrahedron Lett.
1994, 35, 4827-4830.
29. Guella, G.; Mancini, I.; N’Diaye, I.; Pietra, F. Helv. Chim. Acta 1994, 77,
1999-2006.
30. N’Diaye, I.; Guella, G.; Mancini, I.; Pietra, F. Tetrahedron Lett. 1996, 37,
3049-3050.
10
CHAPTER II
SYNTHESIS OF NAGELAMIDE A AND D
Kobayashi recently reported eight new dimeric bromopyrrole alkaloids,
nagelamide A-H, which have been isolated from the Okinawan marine sponge Agelas
sp (Fig 2.1).1 These compounds exhibited antibacterial activity against Gram-positive
bacteria. The other members of the family of dimeric bromopyrrole alkaloids or
dimeric
oroidin
alkaloids
(Fig
2.1)
include
ageliferin,
bromoageliferin,
dibromoageliferin which are [2+4] dimers of oroidin. Oxysceptrin is formally a [2+2]
dimer of oroidin. Mauritiamine is an oxidative dimerization product of oroidin.2
Br
Br
NH
Br
H
N
Br
NH
O
N
R
HN
O
N
H
Br
N
H
NH2
N
NH2
Br
N
H
HN
Br
HN
N
H
N
HN
Br
NH
NH
N
N
NH2
R2
10'
NH2
2.1 nagelamide C : ∆9(10), 9'(10')
2.2 nagelamide D : 9, 9', 10, 10'-tetrahydro
Br
R1
N
H
NH2
H2N
O
9'
10
Br
1.5 nagelamide A : R = H
1.6 nagelamide B : R = OH
Br
N
H
HN
N
9
HN
O
H
N
Br
O
N
H
H
N
Br
O
2.3 nagelamide E : R1 = R2 = H
2.4 nagelamide F : R1 = Br, R2 = H
2.5 nagelamide G : R1 = R2 = Br
Br
O
R
O HN
N
H
N
N
H
N
NH2
HN
NH2
2.6 nagelamide H : R = N
2.7 mauritiamine : R = O
SO3
Figure 2.1. Nagelamide A-H and dimeric bromopyrrole alkaloids.
11
H2N
Br
R1
O
N
H
HN
Br
NH
N
N
H
H
N
N
NH2
R2
Br
HN
N
H
N
H
Br
NH2
O
HN
NH
HN
NH
HN
H
N
O
O
2.8 ageliferin : R1 = R2 = H
2.9 bromoageliferin : R1 = Br, R2 = H
2.10 dibromogeliferin : R1 = R2 = Br
NH2
oxysceptrin
2.11
Figure 2.1. Nagelamide A-H and dimeric bromopyrrole alkaloids (continued).
2.1 Isolation, Structure Determination and Biological Activities
Nagelamides A (1.5) and D (2.2), isolated from the Okinawan marine sponge
Agelas (SS-1003), are members of dimeric bromopyrrole alkaloids.1 The structures
were elucidated from spectral studies and shown to be dimeric bromopyrrole
alkaloids. Detailed UV analyses revealed λmax = 279 nm (ε 27 800) and a FABMS
spectrum of nagelamide A showed the pseudomolecular ion peak at m/z 775, 777, 779,
781 and 783 (1:4:6:4:1) which is indicative of the presence of the pyrrole
chromophore with four bromine atoms in the molecule. The 2D NMR studies
including COSY, TOCSY, HMQC, HMBC and ROESY identified the structures of
nagelamide A and D (Fig 2.2). The HMBC spectrum implied that two aminoimidazole
rings were attached to C10 because there are correlations for H10/H15 and
H10/C15′. The ROESY spectrum disclosed two dibromopyrrole carbonyl moieties
connected through amide bonds through the cross-peaks for H4/NH7 and H4′/NH7′.
The coupling constant (15.9 Hz) and the ROESY correlation for H10/H20′ indicated
that nagelamide A 1.5 has the E-geometry within the s-trans diene system.
Nagelamide A and D exhibited antibacterial activity against Gram-positive
Micrococcus luteus (MIC, 2.08 and 4.17 µg/mL, respectively) and Bacillus subtilis
(MIC, 16.7 and 33.3 µg/mL, respectively). There are no reports of the total syntheses
of these compounds.
12
Br
Br
NH
Br
H
N
Br
NH
4
O
15
9
HN7
O
10'
9'
N
N
O
4
NH2
N
H
15'
7'
N
H
4'
10
Br
H
N
Br
HN
HN
O
7'
N
H
N
10
N
H
15'
9
'
10'
NH2
1
H-1H COSY
HMBC
ROSEY
N
HN
4'
NH2
Br
15
9
7
NH2
Br
nagelamide D : 9, 9', 10, 10'-tetrahydro
2.2
nagelamide A
1.5
Figure 2.2. The structures of nagelamide A and D.
2.2 The Oxidation of 2-Aminoimidazole by Elemental Bromine or Acid
The proposed chemistry is based on the oxidation of 2-aminoimidazoles by
elemental bromine or acid. Foley and Büchi first investigated the oxidation of 2aminoimidazoles
dibromophakellin
by
elemental
2.14.3
The
bromine
key
in
step
the
biomimetic
involved
synthesis
of
oxidative-cyclization
of
dihydrooroidin hydrochloride with bromine as shown in Scheme 2.1.
Scheme 2.1. The biomimetic synthesis of dibromophakellin
O
N
H2N
H
N
N
H
N
H
Br
Br2
CH3COOH
O
NH
Br
2.12·HCl
Br
O
N
N
N
N
H H
NH2
tBuOK
quant
Br
Br
dibromophakellin
2.14
N
Br
NH
N
Br
NH2
2.13
13
From subsequent research by the Horne group, the oxidative dimerization of 2aminoimidazoles by molecular bromine in the synthesis of parazoanthoxanthin A
2.20a involved the initial ipso oxidation of 2.15 to produce diazafulvene 2.17 and 2.18
followed by dimerization to give the homodimer 2.16 which underwent oxidative
cyclization to produce the 10 electrons azulene chromophore as the key step (Scheme
2.2).4 A key finding was the choice of solvent MeSO3H which allowed for the desired
product formation. Under MeSO3H condition, there was no observed of any C-ring
dimerization product resulting from diazafulvene 1.19, which was found in the
synthesis of dibromophakellin 2.14 by Foley and Büchi3 and in the synthesis of
mauritiamine using NCS in TFA by the Horne group.5
Scheme 2.2. The synthesis of parazoanthoxanthin A
N
0.5 eq Br2, MeSO3H, rt
H2N
N
2.15·HCl
N
N
H2N
NH2
N
H
N
H
2.16·2HCl
N
H2N
N
H
3 eq Br2, MeSO3H
rt, 73%
N
H2N
N
H2N
N
H
N
H
2.17
2.18
N
N
H
N
NH2
parazoanthoxanthin A
2.20a
H2N
N
2.19
Besides oxidation with bromine, Büchi and co-workers also reported acidcatalyzed oxidative dimerization of imidazole 2.21 in the synthesis of zoanthoxanthins
(Scheme 2.3).6 These methodologies would lead to the rapid synthesis of dimeric
bromopyrrole alkaloids such as nagelamide A and D.
14
Scheme 2.3. The synthesis of zoanthoxanthins
N
H2N
N
N
N
H
NH2
parazoanthoxanthin A
2.20a, 15%
N
H
N
OH
N
10% HCl
H2SO4
H2N
H2N
2.21
N
H2N
N
H
N
H
2.17
2.18
N
N
H2N
N
N
H
NH2
pseudozoanthoxanthin A
2.20b, 8%
2.3 Synthesis of Nagelamide A and D
Retrosynthetic analysis
The retrosynthetic analysis of nagelamide (Scheme 2.4) indicates it could be
derived from amidation of dibromopyrrole 2.23 and the dimer of 2-aminoimidazole
2.22. The dimer intermediate of nagelamide A could be derived from the acidpromoted dimerization of monomer 2.26. On the other hand, the construction of the
dimer intermediate of nagelamide D could be synthesized via the oxidative
homodimerization of monomer 2.25 according to Horne’s procedure.4 The 2aminoimidazole derivative, i.e. the hypothetic forerunner, could be prepared from
ornithine 2.27.
15
Scheme 2.4. Retrosynthetic analysis of nagelamide A and D
O
HN
H
N
6
Br
1
NH2
3
H2N
N
H
Br
Br
10
N
13
15'
N
N
13'
15 10' 11' N
NH2
H2N
NH
1'
N
H
NH2
N
H
H
9'
Br
O
N
1.5 or 2.2
·2HCl
N
H
NH2
+
H
N
Cl3C
2.23
2.22
Br
Br
O
N
NH2
NH2
H2N
N
H
2.25·2HCl
H2N
NH2
CO2H
N
H2N
N
N
H
2.24
H2N
N
H
2.27·2HCl
ornithine
NH2
2.26·2HCl
Synthesis of key 2-aminoimidazoles
Our synthesis began with the preparation of 2-aminoimidazole 2.25, by
Akabori reduction of ornithine methyl ester, followed by condensation with
cyanamide at pH 4.5 and cyclization with 15% HCl (Scheme 2.5).7
Scheme 2.5. Preparation of 2-aminoimidazole
H2N
NH2
CO2H·2HCl
2.28
N
1. MeOH(HCl)
2. 5% Na(Hg); H2NCN
95°C, 2.5h, 15% HCl
62%
NH2
H2N
N
H
2.26·2HCl
N
NH2
H2N
N
H
2.25·2HCl
1. NCS, MeOH, rt
2. MeOH/xylene 135°C
40% in 2 steps
16
The installation of the double bond in 2-aminoimidazole 2.25 was carried out
in 2 steps. Oxidation of 2.25•2HCl with NCS in methanol at room temperature
followed by heating the resulting product in MeOH/xylene at 135 °C gave vinyl
imidazole 2.26•2HCl in 40% yield.
NCS Oxidative dimerization
With 2-aminoimidazole 2.25 in hand, we focused on the oxidative
heterodimerization by NCS. Oxidation of 2.25•2HCl with NCS in methanesulfonic
acid for 16 h, followed by addition of 2-aminoimidazole 2.27, gave heterodimer 2.28
in 40% yield (Scheme 2.6). The formation of dimer 2.28 results from initial ipso
oxidation of 2.25 to produce intermediate 2.24 followed by exocyclic addition of
monomer 2.27. Construction of the dimeric bromopyrroles was then immediately
accomplished because of the dimer instability. Condensation of 2.28 with
dibromo(trichloroacetyl)pyrrole 2.238 in DMF in the presence of sodium carbonate
produced dimeric bromopyrrole 2.29•2HCl in 68% yield.
Scheme 2.6. The oxidative heterodimerization by NCS
NH2
N
NH2
H2N
N
H
2.25·2HCl
1. 1.1 eq NCS, MeSO3H, rt
2. 1.1 eq
N
NH2 ·0.5H2SO4
N
2.27
H
16h, 40%
O
Br
Br
N
1.2 eq 2.23 H
DMF, rt, 1h
68%
HN
CCl3
H
N
Br
Br
O
N
N
N
H
N
H
H2N
NH2
2.29·2HCl
N
N
N
H
N
H
H2N
NH2
2.28
17
As a result of the success of oxidative heterodimerization, we turned our
attention to the oxidative homodimerization by NCS (Scheme 2.7). Treatment of 2aminoimidazole 2.25•2HCl with 0.5 eq NCS in methanesulfonic acid at room
temperature for 1 day homodimer 2.30 was obtained in 30-35% yield after purification
by chromatography with MeOH(NH3). The structure of dimer 2.30 was confirmed by
HMBC analysis, which showed the correlations for H-6/C-5 and H-6/C-4′ (Fig. 2.3).
On the other hand, treatment of 2.25•2HCl with 0.5 eq NCS in methanesulfonic acid
stirred for 1 day, followed by addition of 0.5 eq NCS and heating to 80-90 °C for 12 h
gave homodimer 2.31 in 30% yield after purification by chromatography with
MeOH(NH3). HMBC Analysis of 2.31 showed the correlations for H-5/C-6, H-7/C-4
and H-6′/C-5′ (Fig. 2.3). Further treatment of 2.25•2HCl with a total of 1.5 equiv of
NCS caused oxidative cyclization to 14-electrons azulene ring chromophore 2.32 in
25% yield and 2.32 was confirmed by HMBC analysis. The proposed mechanism of
these dimers showed in Scheme 2.8.
Scheme 2.7. The oxidative homodimerization by NCS
NH2
7
N
NH2
H2N
N
H
0.5 eq NCS, MeSO3H
rt, 1d
30-35%
6
N
H2N
N
NH2
1
N
H
5
2.25·2HCl
6'
5'
N
H
2.30
NH2
NH2
0.5 eq NCS, MeSO3H, rt, 1d;
0.5 eq NCS, 80-90°C, 12h
30%
N
N
N
H
N
H
H2N
NH2
2.31
NH2
NH2
0.5 eq NCS, MeSO3H, rt;
0.5 eq NCS, 1d;
0.5 eq NCS, 1d
25%
H2N
N
N
H2N
N
NH
2.32
NH2
18
NH2
NH2
7
H2N
HMBC
HMBC
6
N
H2N
NH2
N
H
5
6'
5'
N
N
N
1
NH2
N
H
N
H
N
H
2.30
NH2
NH
N
H2N
NH2
NH2
N
N
H2N
NH2
HMBC
2.32
2.31
Figure 2.3. HMBC Analysis of 2.30-2.32.
Scheme 2.8. Proposed mechanism for production of 2.30-2.33
O
N
O3SCH3
Cl N
N
H
O
0.5 eq NCS
NH3
MeSO3H
N
H2N
N
H
NH3
H2N
Cl
N
N
-HCl
NH3
H2N
NH3
H2N
N
H
N
H
O
2.25
H
Cl N
NH3
NH3
O
NH3
H
N
Cl
N
NH2 0.5 eq
NCS
H2N
N
H
N
H
N
N
H2N
NH2
N
H
-HCl
N
N
N
H
N
H
H2N
N
H
NH2
2.30
NH3
NH3
NH3
N
N
H2N
N
H
N
H
H+
NH3
NH3
N
N
NH2 0.5 eq H2N
NCS
NH3
-HCl
NH2
N
Cl H
N
H
O
N
N
N
H
N
H
H2N
NH2
2.31
Cl N
NH3
NH3
NH3
O
NH3
NH3
N
N
H2N
NH2
N
H
N
H
H2N
N
HN
H3N
NH3
N
NH
NH3
NH2
H2N
N
N
H3N
N
NH
2.32
NH2
19
Synthesis of nagelamide D
To complete the synthesis of nagelamide D, we had to install the
dibromopyrrole group into the dimer 2.30, a core structure of nagelamide D. Due to
the instability of the dimer, we immediately coupled dimer 2.30 with dibromopyrrole
2.23 to furnish dimeric bromopyrrole 2.2 in 80% yield (Scheme 2.9). The structure of
2.2 was confirmed by 1H,
13
C NMR and 2D NMR including, COSY and HMBC
analyses (Scheme 2.9). The molecular formula of synthetic 2.2 was C22H25O2N10Br4
by HRFABMS [m/z 777.4641 [M+H]+] (lit.1 m/z 776.8739 [M+H]+). The UV
absorption of 2.2 in methanol showed λmax = 295 nm (lit.1 UV (MeOH) λmax = 295
nm). These spectral data revealed that this compound is in full agreement with
nagelamide D.1
Scheme 2.9. The synthesis of nagelamide D
O
Br
NH2
Br
N
N
N
H
N
H
H2N
HN
N
H
NH2
O
CCl3
2.23
N
N
N
H
N
H
H2N
Br
NH2
2.30
NH2
Br
N
H
NH
·2HCl
O
nagelamide D
2.2
HN 7
8
H2N
13
11 15'
11'
15
8'
2'
Br
1'
N
H
5' 7'
NH
O
2
Br
10
N
H
Br
1
Br
4
9
N
H
N
6
N
13'
N
H
9'
·2HCl
Br
Br
DMF, rt, 1d
80%
O
H
N
NH2
HMBC
COSY
20
Synthesis of nagelamide A via acid-catalyzed dimerization
With the unsaturated 2-aminoimidazole 2.26•2HCl salt in hand, we could
synthesize nagelamide A via the acid-promoted dimerization (Scheme 2.10). The free
base of 2.26 was obtained after salt 2.26 was purified by chromatography on silica gel
with MeOH:MeOH(NH3) (8:2) as eluent. The acid-promoted dimerization of free base
2.26 in methanesulfonic acid at room temperature for 1 day gave homodimer
2.33•4HCl. Purification of salt 2.33 on silica gel with MeOH(NH)3 gave free base in
40% yield, which subsequently underwent acylation with dibromopyrrole 2.23 to
produce dimeric bromopyrrole 1.5 in 80% yield. Comparison of UV, IR, and NMR
spectral data of our synthetic nagelamide A with literature values for the natural
nagelamide firmly established identity.1
Scheme 2.10. The synthesis of nagelamide A
NH2
N
NH2
H2N
N
H
MeSO3H
rt, 1d
40%
N
N
N
H
N
H
H2N
NH2
2.26·HCl
2.33
NH2
O
Br
Br
HN
N
H
CCl3
O
H
N
Br
Br
2.23
N
N
N
H
N
H
H2N
DMF, rt, 1d
80%
Br
Br
N
H
NH2
NH
·2HCl
O
nagelamide A
1.5
21
In summary, we have achieved the synthesis of nagelamide A and D via the
oxidative dimerization of 2-aminoimidazoles 2.25 and 2.26. The method provides a
rapid entry into the synthesis of nagelamides without the use of protecting groups on
the nitrogen. The first total synthesis of nagelamide A and D were completed in 8 and
6 steps, respectively, starting from ornithine.
2.4 References
1. Endo, T.; Tsuda, M.; Okada, T.; Mitsuhashi, S.; Shima, H.; Kikuchi, K.;
Mikami, Y.; Fromont, J.; Kobayashi, J. J. Nat. Prod. 2004, 67, 1262-1267.
2. a) Faulkner, D. J.; Walker, R. P.; Engen, D. V.; Clardy, J. J. Am. Chem. Soc.
1981, 103, 6772. b) Keifer, P. A.; Schwartz, R. E.; Koker, M. E. S.; Hughes, R.
G.; Rittschof, D.; Rinehart, K. L. J. Org. Chem. 1991, 56, 2965. c) Kobayashi,
J.; Tsuda, M.; Ohizumi, Y. Experientia 1991, 47, 301. d) Kobayashi, J.; Tsuda,
M.; Murayama, T.; Nakamura, H.; Ohizumi, Y.; Ishibashi, M.; Iwamura, M.;
Ohta, T. Tetrahedron 1990, 46, 5579.
3. Foley, L. H.; Büchi, G. J. Am. Chem. Soc. 1982, 104, 1776-1777.
4. a) Xu, Y. –z.; Yakushijin, K.; Horne, D. A. J. Org. Chem. 1996, 61, 95699571. b) Xu, Y. –z.; Yakushijin, K.; Horne, D. A. Tetrahedron Lett. 1992, 33,
4385-4388.
5. Olofson, A.; Yakashijin, K.; Horne, D. A. J. Org. Chem. 1997, 62, 7918-7919.
6. a) Braun, M.; Büchi, G. J. Am. Chem. Soc. 1976, 98, 3049-3050. b) Braun, M.;
Büchi, G.; Bushey, D. F. J. Am. Chem. Soc. 1978, 100, 4208-4213.
7. Originally performed with thiocyanate, see a) Akabori, S. Chem. Ber. 1933,
66, 151-158. b) Lawson, A.; Morley, H. V. J. Chem. Soc. 1955, 1695-1698.
8. Bailey, D. M.; Johnson, R. E. J. Med. Chem. 1973, 16, 1300-1302.
22
CHAPTER III
STUDIES TOWARD THE SYNTHESIS OF AGELASTATIN D
3.1 Isolation, Structure Determination and Biological Activities
Agelastatins are members of the cytotoxic tetracyclic oxindole alkaloid family.
The pyrroloaminopropylimidazole alkaloids A-D 1.7-1.10 are characteristically found
in sponges from the family Axinellidae. Agelastatins A 1.7 and B 1.8 were isolated
from the Agelas dedromorpha in 1993 by Pietra and co-workers.1 One year later, the
absolute configuration of agelastatin A was proposed to be (5aS,5bS,8aS,9aS) using
molecular-mechanics calculations. Importantly, these alkaloids displayed significant
cytotoxic and antileukimic properties. Agelastatin A exhibited potent cytotoxicity
against L1210 in mice and human KB nasopharyngeal tumor cell lines at low drug
concentrations (IC50 = 0.075 µg/mL),1c inhibited GSK-3β3 with IC50 of 12 µM and
could play a role in preventing Alzheimer’s disease. Agelastatin C 1.9 and D 1.10
were isolated later from the West Australian sponge Cymnastela sp in 1998.2
Me
N8 O
9
Br
8a D 7
H
C 5b NH
1
N 9a5a H H 6
R
A B
NH
HO
3a
HO
Br
H
HO
O
NH
H OH
NH
N
5
O
Me
N
O
1.7 agelastatin A : R = H
1.8 agelastatin B : R = Br
agelastatin C
1.9
Br
H
N
H
O
NH
HH
NH
N
O
agelastatin D
1.10
Figure 3.1. The structures of agelastatins.
With only small quantities available, the structure of agelastatin D was
determined by
1
H NMR spectroscopy and chemical correlation of 1.10 with
agelastatin A (Scheme 3.1). The result indicated that agelastatin D 1.10 is a lower
homologue of 1.7 and lacks an N-methyl group. There is no report of any biological
23
activity for this compound. Agelastatins have received much attention due to their
unique structural and biological significance.
Scheme 3.1. The chemical correlation of agelastatin A and D
Me O
Br
KOH, MeI
DMSO
1.7 and 1.10
Me
N
H
O
N
H H Me
N
N
Me
O
3.1
The biogenesis of agelastatin A was proposed by Pietra (Scheme 3.2), from
enzyme-driven C8 attack at C4 in a hymenidin-like precursor 3.2, which based on the
similarity in the skeletal connectivity between the agelastatins and the axillenid
congener oroidin, and pyrrole nitrogen attack at the developing positive C7, followed
by re-functionalization at C4 and C5. This, however, is only speculative and probably
does not accurately represent the actual biosynthetic pathway.
Scheme 3.2. Proposed biogenesis of agelastatin A
H3C
N
5
4
Br
N
7
O
3.2
8 O
NH
O
NH
agelastatins
24
3.2 Previous Synthetic Work
The total synthesis of agelastatin A
This unusual pyrroloketopiperazine has made the agelastatin an attractive
target for total synthesis. Thus far, the total synthesis of agelastatin A 1.7 has been
achieved by several groups.4-7 The first total synthesis of 1.7 was accomplished by
Weinreb’s group using a hetero Diels-Alder cycloaddition of cyclopentadiene with Nsulfinyl methyl carbamate 3.7 to construct the cyclopentadiene 3.5, which is the
precursor of the carbocyclic C-ring (Scheme 3.3).4 The key features to introducing the
CBD tricycles are a Sharpless/Kresze allylic amination, internal Michael addition and
a D-ring annulation by addition of methyl isocyanate to an α-amino ketone.4 The
synthesis was achieved with a longest linear sequence (9.7% overall yield) from Nsulfinyl methyl carbamate 3.7 and commercially available 3.6 in 12 steps.
Scheme 3.3. Weinreb’s synthesis of agelastatin A
H
agelastatin A
1.7
C
NH
N
O
O
Ts
NP
HH
SES
NS+ +
N
Ts
H
C
O
NP
H
O
3.3
3.4
3.5
3.6
O
+
S
N
O3.7
CO2Me
Feldman’s group reported the enantioselective synthesis of (-)-1.7 by using a
vinylcarbene C-H insertion of an alkylidenecarbene intermediate to construct the Cring core as a key step (Scheme 3.4).5 This synthesis has been accomplished in 14
steps (3.85% overall yield) from epichlorohydrin 3.11a.
25
Scheme 3.4. Feldman’s synthesis of agelastatin A
H
TMS
agelastatin A
1.7
O
O
C
N
H
H
N
O
SES
Ts
NH
O
H
Me
N
oNB
C
Ts
O
O
IPhOTf
epichlorohydrin
3.11a
Me
N
oNB
O
N
Ts
O
Cl
O
Me
N
oNB
3.9
Me
N
oNB
N
O
N
H
H
3.8
O
O
O
O
H
3.11
3.10
Hale first described the formal asymmetric synthesis of the Weinreb’s C-ring
intermediate 3.12 from a Hough-Richardson aziridine (Scheme 3.5).6a
Scheme 3.5. Hale’s synthesis of agelastatin A
O
route II
NH
agelastatin A
1.7
O
route I
H
TMS
NH
N
O
NH Bn
N
H
N
Me
H O
3.13
O
H
NBoc
HH
C
SES
O
Weinreb's Advanced
intermediate
3.12
HN
O
H
O
NH
HH
HN
SES
3.14
O
NH
HH
SES
3.15
O
Ph
O
I
O
O
OMe
O
N
H
OMe
SETO
O
NH
3.17
MeO
NHSES
3.16
26
Later, this group performed the total synthesis of (-)-1.7 by using a basecatalyzed intramolecular Michael addition of the chiral cyclopentenone 3.13.6b The
carbocyclic ring system would be derived from a ring-closing metathesis reaction of
3.15 which would be prepared from a Vasella reductive ring-opening of iodide 3.16.
The latter compound derived from known aziridine 3.17. This synthesis was achieved
in 26 steps (0.59% overall yield).
In a recent synthesis of agelastatin A by Davis and Deng (Scheme 3.6),7
construction of the C-ring core intermediate was achieved through sulfiniminemediated enantioselective synthesis of diaminoketodiene 3.19 and ring-closing
metathesis. This synthesis has been accomplished in 11 steps (9.3% overall yield)
from ethyl(dibenzylamino)acetate 3.20b and acrolein-derived sulfinimine (R)-(-)3.20c.
Scheme 3.6. Davis and Deng’s synthesis of agelastatin A
O
O
C
NH
agelastatin A
1.7
NBn2
3.18
p-Tolyl
H
N
H
O
O
O
S
NBn2
NH
H
N
H
3.19
NH
CO2Et
Bn2N
CO2Et
+
p-Tolyl
O
S
N
NBn2
3.20a
3.20b
(R)-(-)-3.20c
3.3 Synthesis of Agelastatin D
Recently, Taglialatela-Scafati and co-workers reported the isolation of
cyclooroidin 3.21 from the sponge Agelas oroides.8 This discovery suggested that the
pyrroloketopiperazine 3.22 may serve as a precursor in the biosynthesis of the
agelastatins.
27
O
NH2
HN
N
D
NH
NH
Br
Br
N
Br
Br
NH
A
N
B
agelastatin D
1.10
NH
O
O
cyclooroidin
3.21
3.22
Proposed synthesis of agelastatin D
Previous studies by our group in the synthesis of slagenins 3.28-3.30
demonstrated that the tetrahydrofuro[2,3-d]imidazolidin-2-one core could be derived
from an intramolecular cyclization by β-functionalization of linear imidazolone 3.25
in the presence of methanesulfonic acid (Scheme 3.7).9
Scheme 3.7. The synthesis of slagenin A, B and C
O
Cl3C
H
N
NH2
O
N
H
3.24
N
O
O
N
H
Br
5% HCl, reflux
2h; NaOH, 85%
NH
O
H
N
N
H
O
N
H
OH
N
H
MeSO3H
23°C, 3h
90%
Br
3.25
H
N
O
H
N
N
H
O
DMF, rt, 1h
90%
3.34
H
N
H
N
H
N
NCS
MeOH, 90%
Br
3.27
3.26
Br
H OMe
N
O
N
O
H H
O
N
H
H
N
Br
3.28 slagenin B
H OCH3
N
O
N
O
H H
O
N
H
3.29 slagenin C
H2O, H+
H OH
N
O
O
N
H H
O
N
H
H
N
3.30 slagenin A
Br
H
N
Br
28
Retrosynthetic analysis
Our synthetic strategy relies on the synthesis of bromoagelastatin D 3.31 as a
precursor of agelastatin D 1.10, with selective removal of the beta bromine substituent
of 3.31 taking place via a protobromination reaction. In contrast to Weinreb’s
approach, we envisioned that functionalized pyrroloketopiperazine 3.31 to arise by a
controlled cyclization of β-activated linear imidazolone 3.32. The intermediate 3.32
could be a key intermediate in the construction of the ABD-rings of agelastatins in one
step via intramolecular cyclization of β-activated imidazolones. This hypothetical
intermediate could be derived from an aminopropyl imidazolone, a useful intermediate
in the putative biomimetic syntheses of related imidazolone sponge metabolites,
slagenins9 and axinohydantoins.10
Scheme 3.8. Retrosynthetic analysis of agelastatin D
HO
Br
H
A
N
C
B
O
H
N
HN
O
D
D
NH
Br
NH
HH
NH
Br
A
N
B
NH
O
O
3.31
agelastatin D 1.10
H
N
H
N
O
D
N
H
O
N
H
3.32
NH2
O
H
N
N
H
Br
A
3.23
+
Br
O
Cl3C
H
N
Br
2.23
Br
29
Synthesis of agelastatin D
Our synthesis began with the preparation of the aminopropyl imidazolone 3.23,
by Akabori reduction of ornithine methyl ester 2.28a followed by condensation with
potassium cyanate in 60% yield (Scheme 3.9).11 Subsequent oxidation of 3.23 with
NBS in the presence of methanol at room temperature for 30 m gave α-methoxy 3.33
in 70% yield. The demethylation of 3.33 in the presence of TFA for 5 h produced salt
3.34. Purification of salt 3.34 gave free base 3.34 in 35% yield. Acylation of free base
3.34 with 2,3-dibromo(trichloroacetyl)pyrrole 2.2312 furnished carboxamide 3.32 in
excellent yield.
Scheme 3.9. Synthesis of key intermediate 3.32
H2N
H
N
5% Na(Hg); KOCN
15% HCl
60%
NH2
CO2Me ·2HCl
N
H
2.28a
3.23
OMe
H
N
1.2 eq NBS, MeOH
-78°C, 70%
NH2
O
TFA, rt, 5h, 35%
NH2
O
N
H
3.33
Br
H
N
NH2
O
N
H
1.2 eq
2.23
3.34
Br
N
H
DMF, rt, 16h
78%
O
CCl3
H
N
O
N
H
O
N
H
3.32
H
N
Br
Br
Synthesis of key intermediate
The direct installation of a double bond in 3.34 from 3.23 proved problematic;
thus we decided to do the acylation before oxidative elimination (Scheme 3.10).
Acylation of the free base 3.23 with 2.23 produced carboxamide 3.35 in 78% yield.
Subsequent oxidation of amide 3.35 with 1.6 eq NBS in presence of methanol at -78
30
°C to room temperature for 30 min gave a mixture of dibromo-α-methoxy 3.36 and
tribromo-α-methoxy 3.37 in 1:3 ratio. It is difficult to control the production of
dibromopyrrole 3.36 as a major product, so we decided to make tribromo 3.37 instead.
Oxidation of 3.35 with 1.8 eq NBS gave tribromo 3.37 in 80% yield, however
treatment of 3.35 with 2 eq NBS gave tribromo-α-carbonyl 3.38 as a major product
due to over oxidation.
Scheme 3.10. Synthesis of key intermediate
Br
H
N
NH2
O
N
H
3.23
NBS
MeOH, -78°C to rt
1.2 eq Br
N
2.23
H
DMF, rt, 16h
90%
H
N
O
CCl3
N
H
N
H
O
OMe
O
N
H
O
O
H
N
H
N
N
H
3.35
Br
H
N
+
Br
Br
OMe
O
H
N
N
H
O
N
H
Br
3.36
H
N
3.37
Br
Br
Br
NBS 3.36 : 3.37: 3.38
1.6 eq
1:3:0
62%
1.8 eq
0:9:1
80%
3.35
2 eq. NBS
MeOH, -78°C to rt
H
N
O
N
H
O
N
H
O
3.38
80%
H
N
H
N
Br
+
Br
O
N
H
OMe
O
N
H
3.37
20%
Br
H
N
Br
Br
Intramolecular cyclization of β-activated imidazolone: Synthesis of ABD-ring
system
With tribromo-α-methoxy 3.37 in hand, we next focused our efforts on the
formation of the ABD-ring system. The formation of the piperazinone ring system was
accomplished by elimination of the methoxy group in 3.37 via reflux in anhydrous
pyridine for 2 days to produce an allylic amide intermediate, followed by an
31
intramolecular Michael addition, resulting in our key ABD-ring intermediate 3.39 in
modest yield (35%).
Scheme 3.11. Synthesis of ABD-ring of agelastatin D
O
H
N
O
OMe
O
N
H
N
H
HN
H
N
Br
NH
Br
anh pyr, reflux
2d, 35%
Br
Br
N
Br
NH
3.37
Br
O
3.39
Synthesis of C-ring system
After successful construction of the A, B and D ring system of agelastatin, the
next objective was the construction of the C-ring by installation of a one-carbon
bridge. Our approach to the one-carbon bridge was based on studies performed by
Speckamp13c and Overman13b who showed that the bridged azatricyclic 3.41, 3.42 and
3.44 were formed in high yield via N-acyliminium ions (eq 1 and eq 2).
OCHO
H
OEt
HCO2H
rt
N
O
H
eq 1
+
N
N
O
3.40
OCHO
(~1:1)
3.41
O
3.42
I
H
TfOH, nBu4NI
DCM, rt
73-97%
N
O
3.43
N
O
3.44
eq 2
32
We started with the second oxidation of 3.39 leading to the trans-dimethoxy
adduct 3.45 in 45% yield (Scheme 3.12). Next, refluxing trans-dimethyl 3.45 in
anhydrous pyridine for 1 day resulted in endo-piprazinone 3.46 in low yield along
with decomposed starting material. The trans stereochemistry of the adduct 3.45 was
confirmed by Nuclear Overhauser Effect spectroscopy (NOESY) (Scheme 3.12). NOE
is a phenomenon involving polarization transfer between nuclei which do not have
scalar (through bond) couplings, but are close together in space. Ideally the nuclei
being observed should be 3-5 Å apart. Irradiation of the C4-H singlet at 4.49 ppm
showed NOEs to both methoxy groups at δ 3.14 and 3.22 ppm. When the C4 methoxy
group (geminal to the ring proton) was irradiated, an NOE was seen only to the ring
proton and the geminal imidazole N3 proton, which resonated at 7.92 ppm. No
enhancement was observed for the adjacent C4 methoxy group (Scheme 3.12). Also
there was no observed NOE signal to either the C4 ring proton or the geminal
imidazole N-1 proton. These data established the stereochemical relationship of the
methoxy groups as trans.
Scheme 3.12. Synthesis of intermediate 3.46
O
O
HN
NH
Br
Br
NH
Br
O
O
0.75 eq NBS
MeOH, -78°C
45%
N
H MeO
N
H
N
MeO
Br
H
N
NH
O
N
H
Br
Br
3.39
HN
NOE
O
3.2
4.5
7.9
MeO H H
N
4.6 H
3.1 4 3
5 1
MeO
N
H
N
Br
7.6
Br
Br
N
Br
NH
Br
3.45
H
N
anh pyr, reflux
1d, 35%
Br
3.45
NOE Analysis of 3.45
O
O
3.46
33
In the course of this reaction, the mechanistic pathway leading to adduct 3.45
probably involves initial incorporation of halogen to aminoimidazolone followed by
substitution with nucleophile solvent (Scheme 3.13). The trans dimethoxy adduct 3.45
predominates thus implicating the formation of intermediate with the stepwise
addition of another MeOH form the least hindered face.
Scheme 3.13. Mechanistic pathway of 3.45
H
N
R
O
H
N
N
H
R
O
N
H
3.39
MeOH
H
N
R
N
H
OMe
O
Br
NBS
H
N
R
N
H
OMe
O
MeOH
H
N
O
R
Br
N
H
MeOH
H
N
O
R
OMe
OMe
3.45
N
H
The future synthetic plan
The final task remaining in the synthesis of agelastatin D 1.10 involves
installation of the C-ring as shown in Scheme 3.14. A one carbon bridge could be
derived in theory from the acid-mediated ring closure of 3.46 which would lead to the
C-ring via N-acyliminium ion. Subsequent hydration of the resulting product 3.47
would give the B/D transoid/cisoid mixtures according to Pietra’s procedure.1c
Hydrogenation of the resulting product followed by bromination would provide
agelastatin D.
34
Scheme 3.14. Synthetic plan to finish synthesis of agelastatin D
O
H
N
HN
D
NH
Br
Br
A
N
Br
B
H
A
Br
Br
NH
3.46
N
H
N
D
C
B
NH
HH
NH
O
3.48
Br
Br
H
A
N
Br
O
HO
Br
TfOH
CHCl3, rt
B
NH
1. H2, Pd/C
2. NBS
acidic H2O/acetone
H
NH
3.47
O
HO
O
O
D
C
Br
H
A
N
C
B
H
N
O
D
NH
HH
NH
O
agelastatin D
1.10
In summary, we were able to construct the ABD-ring core of agelastatin from
the key intermediate 3.35 by oxidative elimination and cyclization in 5 steps. Next we
will have to construct the C-ring from the N-acyliminium ion of endo piprazinone
3.46, followed by completion of to the synthesis of agelastatin D.
3.4 References
1. a) D’Ambrosio, M.; Guerriero, A.; Debitus, C.; Ribes, O.; Pusset, J.; Leroy, S.;
Pietra, F. J. Chem. Soc., Chem. Commun. 1993, 1305-1306. b) D’Ambrosio,
M.; Guerriero, A.; Chiasera, G.; Pietra, F. Helv. Chim. Acta 1994, 77, 18951902. c) D’Ambrosio, M.; Guerriero, A.; Ripamonti, M.; Debitus, C.;
Waikedre, J.; Pietra, F. Helv. Chim. Acta 1996, 79, 727-735.
2. Hong, T. W.; Jimenez, D. R.; Molinski, T. F. J. Nat. Prod. 1998, 61, 158-161.
3. Meijer, L.; Thunnissen, A. -M. W. H.; White, A. W.; Garnier, M.; Nikolic, M.;
Tsi, L. -H.; Walter, J.; Cleverley, K. E.; Salinas, P. C.; Wu, Y. -Z.; Biernat, J.;
Mandelkow, D. -M.; Kim, S. -H.; Pettit, G. R. Chem. Biol. 2000, 7, 51-63.
4. Stien, D.; Anderson, G. T.; Chase, C. E.; Koh, Y. -h.; Weinreb, S. M. J. Am.
Chem. Soc. 1999, 121, 9574-9579.
35
5. a) Feldman, K.; Saunders, J. C. J. Am. Chem. Soc. 2002, 124, 9060-9061. b)
Feldman, K. S.; Saunders, J. C.; Wrobleski, M. L. J. Org. Chem. 2002, 67,
7096-7109.
6. a) Hale, K. J.; Domostoj, M. M.; Tocher, D. A.; Irving, E.; Scheinmann, F.
Org. Lett. 2003, 5, 2927-2930. b) Domostoj, M. M.; Irving, E.; Scheinmann,
F.; Hale, K. J. Org. Lett. 2004, 6, 2615-1618.
7. Davis, F. A.; Deng, J. Org. Lett. 2005, 7, 621-623.
8. Fattorusso, E.; Taglialatela-S., O. Tetrahedron Lett. 2000, 41, 9917.
9. Barrios Sosa, A. C.; Yakushijin, K.; Horne, D. A. Org. Lett. 2000, 2, 34433444.
10. Barrios Sosa, A. C.; Yakushijin, K.; Horne, D. A. J. Org. Chem. 2002, 67,
4498-4500.
11. Originally performed with thiocyanate, see a) Akabori, S. Chem. Ber. 1933,
66, 151-158. b) Lawson, A.; Morley, H. V. J. Chem. Soc. 1955, 1695-1698.
12. Bailey, D. M.; Johnson, R. E. J. Med. Chem. 1973, 16, 1300-1302.
13. N-acyliminium ion, see a) Brodney, M. A.; Padwa, A. J. Org. Chem. 1999, 64,
556-565. b) Brosius, A. D.; Overman, L. E. J. Org. Chem. 1997, 62, 440-441.
c) Schoemaker, H. E.; Dijkink, J.; Speckamp, W. N. Tetrahedron 1978, 34,
163-172.
36
CHAPTER IV
SYNTHESIS OF BISINDOLYLPIPERAZINE MARINE ALKALOIDS
DRAGMACIDIN A, B AND C:
REDUCTION OF 2,5-BIS(3′-INDOLYL)PYRAZINES TO
2,5-BIS(3′-INDOLYL)PIPERAZINES
4.1 Isolation and Biological Activities
Dragmacidins, isolated from the sponge genera Dragmacidon, Hexadella, and
Spongorites, as well as from the tunicate Didemnum candidum, are members of a
small group of 2,5-bis(3′-indolyl)piperazine marine alkaloids.1 Structures for
dragmacidin A (1) and dragmacidin B (2) were elucidated from spectral studies, which
indicated that they contain piperazine heterocycle linkages two indole units in a headto-tail fashion.1a The indole and alkyl moieties are positioned in a chair-like all transdiequatorial orientation about the piperazine ring. The relative stereochemistry of
dragmacidin C (3), however, has remained obscure.1b In 2002, Kawasaki showed that
the relative stereochemistry of dragmacidin C was determined to be cis by the
comparison of synthetic cis and trans dragmacidin C with the natural product.
R2
N
2
Br
N
H
H
N
H
N
Br
5
5
2
N
R1
1.31 dragmacidin A : R1 = H, R2 = Me
1.32 dragmacidin B : R1 = R2 = Me
Br
H
N
N
H
Br
N
H
1.33 dragmacidin C
2,5-bis(6′-bromo-3′-indolyl)piperazine
Figure 4.1. The structures of dragmacidins.
37
4.2 Previous Synthetic Work
The total synthesis of dragmacidins
There are several groups who have reported the total synthesis of dragmacidins
A, B and C. The first total synthesis of dragmacidin B was accomplished by Cava’s
group.2
The
bisindole
feature
was
prepared
from
bromination
of
1,4-
dimethylpiperazine-2,5-dione, followed by addition of 6-bromoindole that gave
dragmacidin B as shown in Scheme 4.1.
Scheme 4.1. Cava’s synthesis of dragmacidin B
O
CH3
N
NBS, AIBN
CCl4
N
O
CH3
4.1
O
Br
N
H
H
N
Me
N
N
Me
O
O
Br
CH3
N
Br
N
O
CH3
4.2
Br
N
H
4.3
DMF
43% in 2 steps
Br
BH3·THF
25%
dragmacidin B
1.32
4.4
Kawasaki and co-workers reported the first total synthesis of dragmacidin A in
2000.3a The key feature for the construction of the 2,5-bis(3′-indolyl)piperazines were
the condensation of indolylglycines followed by cyclization and reduction (Scheme
4.2). Two years later, Kawasaki succeeded in the synthesis of dragmacidins A, B and
C as well as confirmed the configuration of stereochemistry of dragmacidin C as cis.
38
Scheme 4.2. Kawasaki’s synthesis of dragmacidin A, B and C
N3
O
1.
Ph3P
4.6
O
N
OMe
benzene, reflux
Ac
2. TMSN3, MeSO3H
MS-4Å, CH2Cl2
4.5
0°C to rt
Br
O
O
1. Ph3P, H2O, THF, rt;
Ac2O, DMAP, CH2Cl2
2. Boc2O, NaHCO3, rt
99% from 4.7
O
Br
N
Ac
4.7
O
Boc
HN
Boc
HN
O
RhCl(PPh3) 4.9
EtOH, H2O, 70°C
O
Br
N
Ac
NH2
OH
Br
O
Br
4.10
Boc
Boc
HN
MeO2C
NH
+
N
Ac
O
N
HN
MeO2C
NH
Br
Br
O
N
Ac
Ac
O
1. HCO2H, rt
2. NH3, MeOH
0°C
70%
Br
N
H
N
H
4.13
Br
Br
BH3, THF
0°C to rt
45%
O
dragmacidin A
1.31
NaBH4, HCO2H
70°C, 22%
4.14
O
1. HCO2H, rt
2. NH3, MeOH
0°C
70%
H
N
H
N
N
H
H
N
H
N
N
H
N
Ac
4.13, 40%
4.12, 67%
4.12
N
Ac
BOP, DIEA
4.11, THF, 0°C
N
Ac
4.8
OMe
dragmacidin B
1.32
Br
O
BH3, THF
0°C to rt
45%
cis dragmacidin C
1.33
4.15
Recently, Horne’s group succeeded in the facile synthesis of dragmacidin B
via the dimerization of oxotryptamines to give 2,5-bis(3′-indolyl)pyrazine 4.17
followed
by
selective
reduction
and
reductive
methylation
with
sodium
39
cyanoborohydride reduction as key steps (Scheme 4.3).4 This is the most efficient
synthesis to date.
Scheme 4.3. Horne’s synthesis of dragmacidin B
H
N
N
O
NH2
Br
N
H
4.16
Br
N
xylene/EtOH
Argon, 130°C, 3d
Br
N
H
4.17
NaBH3CN
HCO2H, 70%
dragmacidin B
1.32
4.3 Synthesis of Dragmacidin A, B and C
Proposed synthesis of dragmacidins
Lyle and Thomas studied the reduction of pyrazinium salts using NaBH4 which
yield a 9:1 mixture of trans and cis piperazines, respectively (eq 1).5 Gribble and coworkers reported the reduction-alkylation of an indole with NaBH4 in acidic media (eq
2).6
N
NaBH4
H
N
H
N
+
N
N
Ph
4.19
Ph
4.18
eq 1
N
9:1
Ph
4.20
NaBH4
eq 2
+
N
H
4.21
RCO2H
N
H
4.22
N
R
4.23
R = Me, Et
40
These methods inspired us to believe that the construction of the 2,5-bis(3′indolyl)piperazines could be prepared from the dimerization of oxotryptamine, leading
to the pyrazine (Scheme 4.4). The resulting pyrazine could undergo selective reduction
and reductive methylation with sodium cyanoborohydride. The proposed chemistry is
based on our recent success in the synthesis of dragmacidin B using sodium
cyanoborohydride
reduction
in
an
acidic
media
to
transform
2,5-bis(3′-
indolyl)pyrazines into 2,5-bis(3′-indolyl)piperazines. This methodology could be
applied in the synthesis of dragmacidin A and C as well.
Scheme 4.4. Retrosynthetic analysis
H
N
Br
N
N
dragmacidin
A, B and C
Br
N
H
4.17
O
NH2
Br
N
H
4.16
indole
4.21
Synthesis of oxotryptamine: A critical synthon
Our approach to the piperazine ring system begins with indole-3-carbonyl
nitrile 4.24,7 which is readily prepared from indole.
41
Scheme 4.5. Preparation of oxotryptamines
O
1. (COCl)2, ether
0°C to rt, 1h
N
H
4.21
O
CN
2. CuCN, CH3CN, toluene
ether, 110°C, 7h
53%
N
H
4.24
NH2
H2, Pd/C
AcOH, 16h
90%
N
H
4.25
O
R1
Br2
AcOH, HCO2H
NH2
R2
N
H
4.26 R1 = Br, R2 = H; 59%
4.16 R1 = H, R2 = Br; 21%
The addition of oxalyl chloride to indole followed by refluxing the resulting
product in CH3CN with CuCN for 7 h resulted in indole-3-carbonyl nitrile in 53%
yield. Hydrogenation of 4.24 over Pd/C yielded oxotryptamine 4.25 in excellent yield.
Bromination of oxotryptamine gave an isomeric mixture of 5- and 6bromooxotryptamines 4.26 and 4.16 in approximately 2:1 ratio, respectively. 5- and 6Bromoindole was separated by flash chromatography, and the bromine position in
these compounds was determined by NOE measurements (Fig 4.2). The irradiation of
the CH2 proton (3.90 ppm) showed NOE signals for singlet protons (8.33 and 8.40
ppm). These data are consistent with 5-bromooxotryptamine 4.26. On the other hand,
irradiation of the CH2 proton (3.97 ppm) in 6-bromooxotryptamine 4.16 showed NOE
signals for singlet (8.35 ppm) and the doublet (8.12 ppm).
NOE
NOE
O
8.12, d
O
8.33, s
NH2
7.31, d
Br
NH2
3.90, s
3.97, s
Br
7.67, s
N
H
8.35, s
6-bromooxotryptamine
4.16
7.47, d
7.35, d
N
H
8.40, s
5-bromooxotryptamine
4.26
Figure 4.2. NOE Experiment of 5-Br-oxotryptamine and 6-Br-oxotryptamine.
42
Synthesis of 2,5-bis(3′-indolyl)pyrazines
Upon heating oxotryptamine 4.25 in a xylene/EtOH (5:1) solution under a
sealed atmosphere of argon for 3 days, followed by exposure to air for 1 day and
filtration, gave yellow solid pyrazine 4.27 in 67% yield (Scheme 4.5). This successful
result prompted us to pursue the dimerization of bromooxotryptamine 4.26 and 4.16,
which would lead directly to the desired piperazine ring system. In a manner similar to
that used above, a thermal tandem cyclocondensation-autoxidation of 5- and 6bromotryptamine afforded piperazine 4.28 and 4.17 in good yield.
Scheme 4.5. Synthesis of 2,5-bis(3′-indolyl)pyrazines
H
N
N
O
R1
R2
NH2
N
H
4.25 R1 = R2 = H
4.26 R1 = Br, R2 = H
4.16 R1 = H, R2 = Br
130°C, Ar
EtOH/xylene
sealed tube
R1
R2
R2
R1
N
N
H
4.27 R1 = R2 = H; 67%
4.28 R1 = Br, R2 = H; 60%
4.17 R1 = H, R2 = Br; 60%
NaBH3CN reduction of 2,5-bis(3′-indolyl)pyrazine 4.27 and 4.28
The next important operation in the synthesis was to transform pyrazines 4.27,
4.28 and 4.17 into piperazines. It has been reported by Gribble and co-workers that
indoles can be readily reduced to indolines using borohydrides and carboxylic acids.6
Recently our group reported the conversion of pyrazines to piperazines by using
NaBH3CN.4 Treatment of pyrazine 4.27 with 30 eq NaBH3CN in acetic acid gave
trans-piperazine 4.29 in 67% yield as the major product along with five additional
minor products (Scheme 4.6). According to Lyle and Thomas studied the reduction of
pyrazinium salts using NaBH4, the stereochemistry is established by the attack of
hydride from a borohydride ion on an imine as A (Scheme 4.6). The hydride prefers to
43
attack along route a, which proceed via an energetically favored chair-like transition
state over the route b via a boat-like transition state. Using excess NaBH3CN in formic
acid as solvent, piperazine 4.27 underwent Eschweiler-Clark like reductive
methylation to afford the thermodynamically more stable trans diequatorial isomer
4.35 as a major product.
Scheme 4.6. NaBH3CN reduction of 2,5-bis-(3′-indolyl)pyrazine
R2
N
H
N
R2
N
5
5
2
30 eq NaBH3CN
AcOH
2
N
R1
N
H
H
N
N
R1
+
N
H
4.32 R1 = R2 = H; <5%
4.33 R1 = R2 = Et; <5%
4.34 R1 = H, R2 = Et; <5%
4.29 R1 = R2 = H; 67%
4.30 R1 = R2 = Et; <5%
4.31 R1 = H, R2 = Et; <5%
4.27
Me
N
H
N
Me
N
5
2
2
60 eq NaBH3CN
HCO2H
N
Me
N
H
+
N
H
4.35
54%
H
N
5
N
Me
4.36
<5%
a
HN
cis
a
b
N
boat-like TS
H3C
CH3
H
trans
chair-like TS
an energetically favored
b A
The trans and cis substituted piperazines can be characterized by the 1H NMR
coupling constant between the methine H2 and H5 hydrogens in the piperazine ring.
For the trans products 4.29-4.31 and 4.35, the methine H2 and H5 coupling constants
are a doublet of doublets, (J = 10.3, 2.7 Hz), which suggests that the piperazine ring
44
exists in the chair conformation with both indole substituents occupying equatorial
positions (Fig 4.3). On the other hand, cis product 4.32-4.34 and 4.36 showed smaller
coupling constants for H2 and H5 (dd, J = 5.9, 3.5 Hz). These coupling constants
suggest that the piperazine ring exists as a boat-like ring conformation.
R2
N
2
H
N
R2
N
5
5
2
N
R1
N
H
H
N
N
R1
N
H
trans-product
a chair conformation
cis-product
a boat-like ring conformation
4.29 R1 = R2 = H; H2,H5: δ 4.07, dd, J = 10.1, 2.3 Hz
4.32 R1 = R2 = H; H2,H5: δ 4.30, dd, J = 5.9, 3.5 Hz
4.30 R1 = R2 = Et; H2,H5: δ 3.87, dd, J = 10.5, 3.0 Hz
4.34 R1 = H, R2 = Et; H2: δ 4.52, dd, J = 5.4, 3.7 Hz
H5: δ 3.98, dd, J = 6.5, 3.2 Hz
4.31 R1 = H, R2 = Et; H2: δ 4.39, dd, J = 10.3, 2.4 Hz
H5: δ 3.64, dd, J = 10.3, 3.1 Hz
3.5 dd
J = 11.0, 2.2 Hz
Me
H
N
H
N
H
H
N
3.95 brs
Me
H
N
5
H
2
N
H 2.50 dd, J = 11.0, 10.8 Hz
H
Me 2.87 dd, J = 11.0, 2.2 Hz
trans-4.35
a chair conformation
N
H
2
H
N
5
H 3.1 dd, J = 11.1, 6.4 Hz
N
H
Me 2.87 dd, J = 11.1, 3.3 Hz
cis-4.36
a boat-like ring conformation
Figure 4.3. The coupling constants for trans and cis piperazines.
45
Repetition of the above sequence starting with 2,5-bis(5′-bromo-3′indolyl)pyrazine 4.28 afforded trans piperazines 4.37 and cis 4.38 in 8:1 ratio (Scheme
4.6).
Scheme 4.6. NaBH3CN reduction of 2,5-bis-(5′-bromo-3′-indolyl)pyrazine
4.03 dd
J = 11.0, 2.6 Hz
4.28
25 eq NaBH3CN
AcOH
H
N
5
2
Br
N
H
N
H
4.25 dd
J = 5.9, 3.5 Hz
H
N
trans-4.37
60%
H
N
H
N
Br +
5
Br
2
Br
N
H
N
H
cis-4.38
<5%
Synthesis of bisindole piperazine natural products dragmacidin A, B and C
We next attempted the synthesis of bisindole piperazine natural products
dragmacidins A, B and C in a similar manner as shown in Scheme 4.7. Reduction of
2,5-bis(6′-bromo-3′-indolyl)pyrazine 4.17 with 50 eq NaBH3CN in formic acid
produced dragmacidin A 1.31 and dragmacidin B 1.32 in 14% and 56% yield,
respectively (Scheme 4.7). The spectral data of these synthetic products matched those
of the natural products. On the other hand, treatment of 4.17 with 25 eq NaBH3CN in
acetic acid gave trans-dragmacidin C 4.39 and cis-dragmacidin C 1.33 in 61% and
<5% yield, respectively.
46
Scheme 4.7. Synthesis of dragmacidins A-C
Me
N
50 eq NaBH3CN
HCO2H
H
N
N
H
Br
Br
Me
N
Br
1.31 dragmacidin A
14%
Br
N
Me
+
N
H
H
N
N
H
1.32 dragmacidin B
56%
4.17
H
N
H
N
Br
H
N
5
2
25 eq NaBH3CN
AcOH
5
2
N
H
Br
Br
H
N
N
H
4.39 trans-dragmacidin C
61%
N
H
+
Br
N
H
1.33 cis-dragmacidin C
<5%
Due to the obscurity of the relative stereochemistry of dragmacidin C,
Faulkner and co-workers isolated dragmacidin C and assigned the relative
stereochemistry of both substituents at C-2 and C-5 to be equatorial. Later Kawasaki
synthesized the cis- and trans-isomer of dragmacidin C and confirmed the relative
stereochemistry of dragmacidin C to be the cis-isomer with boat-like conformation
based on 1H-NMR data. Herein, we have demonstrated our synthetic dragmacidin C
with natural product and with Kawasaki’s synthesis (Table 4.1). The spectral data of
these synthetic products were consistent with those of natural products.
47
Table. 4.1. The comparison of 1H NMR spectral data of dragmacidin C
H-2 (H-5)
H-3 (H-6)
Natural (DMSO-d6)
4.30 (6.0,3.0)
3.03 m
Natural (acetone-d6)*
4.30 (6.0, 3.0)
3.16 (12.0, 3.0), 3.26 (12.0, 6.0)
Synthetic cis 1.33*
4.29 (6.0, 3.0)
3.14 (11.7, 3.0), 3.25 (11.7, 6.0)
4.03 (10.8, 2.6)
2.84 (11.6, 10.8), 3.13 (11.6, 2.6)
Synthetic cis 1.33
4.32 (5.4, 3.4)
3.18 (11.8, 3.2), 3.28 (11.8, 57)
Synthetic trans 4.39
4.05 (10.1, 2.4)
2.86 (11.5, 10.1), 3.13 (11.5, 2.4)
Synthetic trans 4.39
*
(DMSO-d6)
*
Kawasaki’s 1H NMR data
In summary, a convenient method for the synthesis of 2,5-bis(3′-
indolyl)piperazines from 2,5-bis(3′-indolyl)pyrazines using sodium cyanoborohydride
in acid media and the short synthesis of bis(indolyl) piperazine natural products has
been achieved.
4.4 References
1. a) Morris, S. A.; Andersen, R. J. Tetrahedron 1990, 46, 715-720. b) Fahy, E.;
Potts, B. C. M.; Faulkner, D. J.; Smith, K. J. Nat. Prod. 1991, 54, 564- 569.
2. Whitlock, C. R.; Cava, M. P. Tetradedron Lett. 1994, 35, 371-374.
3. a) Kawasaki, T.; Enoki, H.; Matsumura, K.; Ohyama, M.; Inagawa, M.;
Sakamoto, M. Org. Lett. 2000, 2, 3027-3029. b) Kawasaki, T.; Ohno, K.;
Enoki, H.; Umemoto, Y.; Sakamoto, M. Tetrahedron Lett. 2002, 43, 42454248.
4. Miyake, F.; Yakushijin, K.; Horne, D. Org. Lett. 2000, 2, 3185-3187.
5. Lyle, R. E.; Thomas, J. J. Org. Chem. 1965, 30, 1907-1909.
48
6. a) Gribble, G. W.; Lord, P. D.; Skotnicki, J.; Dietz, S. E.; Eaton, J. T.; Johnson,
J. J. Am. Chem. Soc. 1974, 96, 7812-7814. b) Gribble, G. W.; Hoffman, J. H.
Synthesis 1977, 859-860.
7. Hogan, I. T.; Sainsbury, M. Tetrahedron 1984, 40, 681-682.
49
CHAPTER V
THE STEREOSELECTIVE INTRAMOLECULAR IMMINIUM ION
SPIROCYCLIZATION: SYNTHESIS OF SALACIN
5.1 Isolation, Structure Determination and Biological Activities
Salacin is a spirooxindole alkaloid isolated from the leaves of Uncaria
salaccensis, a Thai medicinal plant in 1990.1 The relative stereochemistry of
tryptamine-based alkaloid 1.17 was elucidated by 1H NMR and NOE analysis. The
irradiation of H9 showed NOE to H6 and H14 whereas no NOE was observed
between H9 and H3. This indicated the side chain is trans to the carbonyl group of
spirooxindole. Only the 1H NMR of the major rotamer has been reported and there is
no report of 13C NMR data or any biological activity information for this compound.
O
HN
12
6
5
9
2
3
9
14
NCHO
O
6
HN
14
16
salacin
1.17
O
H
O
NCHO
3
NOE analysis of 1.17
Figure 5.1. The structure and NOE analysis of salacin 1.17.
5.2 Previous Synthetic Work: The Construction of the Spiroalkaloids Using (L)Tryptophan or Other Derivatives
A growing number of oxindole alkaloids derived from tryptamine or
tryptophan exhibit a wide rang of important biological activity.2 The successful
construction of the spirooxindole alkaloid using (L)-tryptophan or other derivatives
has been achieved by several groups. However stereochemical control and high yields
are lacking. Danishefsky approached the synthesis of spirotryprostatin B 1.12 using
classical Mannich conditions between oxindole 5.1 and prenyl aldehyde; however the
resulting products showed low selectivity (Scheme 5.1).3 The loss of stereochemical
50
control results from the retro-Mannich process as shown in Scheme 5.2 which affords
thermodynamic product mixtures.
Scheme 5.1. Danishefsky’s synthesis of spirotryprostatin B
CO2Me
O
HN
NH
CO2Me
NH
HN
O
CHO
5.2
CO2Me
N
H
5.1
NH2
O
·HCl
(desired)
5.3a
2:3:3:3
mixture
Et3N, pyridine
5.3b
CO2Me
HN
NH
HN
CO2Me
O
NH
O
5.3c
O
5 steps
5.3d
O
HN
N
N
O
H
spirotryprostatin B
1.12
Scheme 5.2. Classical Mannich reaction and retro-Mannich equilibrium
NH
NH2
RCHO
N
H
O
5.4
R
OH
"mannich
condition"
O
N
O
H
5.5
OH
HN
NH
HN
N
R
5.7
R
5.6
NH
HN
O R
5.8
51
Ganesan achieved the total synthesis of spirotryprostatin B by using an Nacyliminium Pictet-Spengler methodology to introduce the prenyl and proline
moieties; however, these conditions gave low yield (Scheme 5.3).4
Scheme 5.3. Ganesan’s synthesis of spirotryprostatin B
CO2Me
N
N
H
N-acyliminium
CO2Me
Pictet-Spengler
46%
N
H
H O
5.9
1. NBS
2. base
68%
N
Fmoc
5.10 1.4:1 (Hα:Hβ)
O
O
H
HN
N
1. LDA
2. PhSeBr
2%
N
O
H
spirotryprostatin B
1.12
5.11
Horne recently reported a novel approach to spiro[oxindole-3,3′-pyrrolidines]
by halotryptamine spirocyclization in the synthesis of indole alkaloids, isoelacomine
1.16 and elacomine 1.15 with high stereochemical control (Scheme 5.4).5 This
contrasts with previous classical Mannich approaches in that high stereochemical
control is achieved via what is believed to be a kinetic process.
Scheme 5.4. Horne’s synthesis of elacomine and isoelacomine
Br
O
NH2
Br
N
H
Br
HN
CHO
NH
MeOH
then TFA
80%
HN
Br
5.12
O
5.13b
5.13a
Br
O
HN
ClCO2Me, DCM
Et3N, 82%
N CO2Me
+
N CO2Me
HN
O
Br
5.14a
5.14b
NH
52
Scheme 5.4. Horne’s synthesis of elacomine and isoelacomine (Continued)
O
O
HN
HN
N CO2Me 1. NaOMe, CuI, DMF, 76%
2. BBr3, 23°C, 88%
Br
N CO2Me
HO
5.14a
isoelacomine
1.16
Br
HO
N CO2Me 1. NaOMe, CuI, DMF, 73%
2. BBr3, 23°C, 53%
HN
O
O
5.14b
5.3 The Methodology
Construction
N CO2Me
HN
elacomine
1.15
for
Stereoselective
Spiro[3,3-pyrrolidine]oxindole
2-Halo-tryptamines are useful synthons for the indole-based natural products
because a halogen atom blocks Pictet-Spengler type conditions at the α–position and
would force reaction at the β-position leading to spirooxindole (i.e an interrupted
Pictet-Spengler reaction). The TFA protonation of the resulting pyrrolidone nitrogen
helps prevent an undesired retro-Mannich process. Subsequently hydrolysis of the
haloindolenine intermediate would furnish the desired spirooxindole in a one pot
reaction (Scheme 5.5).
Scheme 5.5. Spirocyclization of 2-halotryptamines
O
Cl
NH2
N
H
X
5.15 X = Cl or Br
1. RCHO
2. activation
with "L+"
[one pot]
N
N L
R
5.16
hydrolysis
[one pot]
HN
N L
R
5.17
53
Preparation of 2-halotryptamines
Tryptamine 5.18·HCl could undergo regioselective chlorination at the 2position using NCS in a 20% formic acid in acetic acid solution. 2-Chlorotryptamine
salt 5.15a was formed in 70% yield. The 2-bromotryptamine salt 5.15b could be
prepared by following an analogous protocol for 2-chlorotryptamine salt by using
NBS (Scheme 5.6).
Scheme 5.6. Preparation of 2-halotryptamines
NH2
N ·HCl or ·HBr
H
5.18
1.1 eq NCS
20% HCO2H/AcOH
0°C, 70%
NH2
·HCl
N
Cl
H
5.15a
NH2
1.2 eq NBS
20% HCO2H/AcOH
0°C, 80%
·HBr
Br
N
H
5.15b
The interrupted halo Pictet-Spengler reaction: Stereocontrolled spirocyclization
of 2-chlorotryptamine and aldehyde
Previous work on the interrupted halo Pictet-Spengler reaction showed that
condensation of free base chlorotryptamine 1.15a with isovaleraldehyde in DCM for 2
h at room temperature followed by TFA protonation of the resulting Schiff base lead
to 5.20 and 5.21 in good yield and high diasteroselectivity (97:3) (Scheme 5.7).5 The
relative stereochemistry was confirmed by 1H NMR and NOE studies. The major
product of this reaction has the R group trans to the oxindole carbonyl due to steric
interactions between the R group and chlorine moieties; therefore the reaction prefers
to go via 5.23 over 5.22 (Scheme 5.8). The less sterically hindered aldehyde such as
propanal also gave high diastereoselectivity as well.
54
Scheme 5.7. Stereocontrolled spirocyclization of 2-chlorotryptamine and aldehydes
O
NH2
RCHO
DCM, 0°C
Cl
N
H
N
R
HN
TFA, DCM
80-90%
+
NH
HN
R
N
Cl
H
5.19
5.15a
NH
5.20
O R
5.21
a R = CH2CH(CH3)2
b R = Et
97:3
95:5
These results suggest that the initial iminium ion cyclization leading to the
formation of the spirochloroindolenine 5.19 proceeds through an irreversible
kinetically controlled pathway. The observed stereoselectivities could be explained by
minimization of the steric interactions in the transition state between the R group and
chloride moiety as shown (Scheme 5.8). The steric interaction in structure 5.23 is less
than it is in structure 5.22 therefore favoring 5.23 over 5.22. Furthermore, the
protonation of the resulting pyrrolidone nitrogen by TFA helps prevent an undesirable
retro-Mannich process which would lead to the loss of stereochemical control.
Scheme 5.8. Stereochemical rationale
H
N
R
O
HN
Cl
O R
minor
R
NH
NH
HN
H
N
Cl
disfavored
5.22
NH
N
H
favored
5.23
R
major
5.4 Synthesis of Salacin
The proposed chemistry is based on our recent success in a novel approach to
spiro[oxindole-3,3′-pyrrolidines] by halotryptamine spirocyclization with excellent
stereochemical control.5 This methodology could impact the general applicability of
55
the method to provide access to a wide range of related alkaloids such as salacin with
high stereoselectivity. The construction of the spirooxindole unit of salacin could be
prepared from tryptamine and an aldehyde (Scheme 5.9).
Scheme 5.9. Retrosynthetic analysis
O
HN
O
NCHO
O
route I
HN
O
NH
O
H
O
1.17
5.24
5.25
+
NH2
route II
O
HN
O
N
H
O
5.26
N
H
X
5.15 X= Br, Cl
O
5.27
There are two possible synthetic routes to salacin, each using different
aldehydes (Scheme 5.9). In route I, the spirooxindole ring could be derived from
condensation of halotryptamines and aldehyde 5.25 bearing a C-O bond at the C4
carbon atom. In route II, salacin could be derived from an oxidative cleavage at the
enamine double bond of a precursor molecule 5.26. Tetracyclic 5.26 could result from
the condensation of halotryptamine 5.15 and known aldehyde 5.27.7
Preparation of aldehydes and synthetic application of 2-halotryptamines
Our studies began with the preparation of aldehyde 5.25 from a known alcohol
5.30 which can be prepared from Grignard reaction between brominated 5.28 and
propanal (Scheme 5.10). Dess-Martin oxidation of alcohol 5.30 gave ketone 5.31 in
90% yield and subsequent deprotection of the dioxolane was effected in 75% yield to
afford aldehyde 5.25 (Scheme 5.10).
56
Scheme 5.10. Preparation of aldehyde 5.25
Br
O
O
5.28
O
1. Mg, THF
2. EtCHO 5.29
52%
O
O
DMP, DCM
90%
H
OH
5.30
O
H
5.31
O
O
TsOH, acetone-H2O
reflux, 75%
H
O
5.25
With aldehyde 5.25 in hand, our attention next turned to the spirocyclization
reaction (Scheme 5.11). Condensation of 2-chlorotryptamine with ketone aldehyde
5.25 (DCM, MgSO4, 2 h, -78 °C) and activation with TFA (6 eq, 2 h) afforded
pentacyclic compound 5.32 in 30% yield.
Scheme 5.11. Synthetic application of 2-halotryptamines with aldehyde 5.25
NH2
N
H
Cl
1. 2.1 eq 5.25, DCM, -78°C
2. 6 eq TFA, 0°C, 2h
30%
5.15a
N
N
H
5.32
3.20
NOE
4.32
N
N
H
1.46
3.04
1.45
2.83
7.51
7.08
5.32
The structure of 5.32 was assigned based on the 1H,
13
C NMR and NOE
analyses. Irradiation of the methylene proton (2.83 ppm) of 5.32 showed NOEs to
methyl proton (1.46 ppm) and methylene protons (3.20 and 4.32 ppm). Irradiation of
the methylene proton (3.04 ppm) showed NOEs to methyl proton (1.45 ppm) and
methine (7.08 ppm).
57
Attempts to utilize standard procedures were unsuccessful, likely due to the
Schiff base undergoing cyclization with the ketone before spirocyclization can occur,
leading to the pyrrole ring. This suggests that the ketone is too reactive compared to
the Schiff base, so we became interested in the use of different carbonyl groups to
explore whether carbonyl derivatives would serve as useful functional groups for
spirocyclization reactions. Interestingly, this represents the first example of C-C bond
formation substituted at the 2-halo position of indole.
Scheme 5.12. Proposed mechanism for production of 5.32
N
NH2
N
H
5.25
DCM, 0°C
Cl
N
H
NH
O
Cl
N
H
Cl
O
5.15a
H+
N
N
H
Cl
N
N
H
N
TFA
Cl
N
H
Cl
O
H
O
N
N
H Cl
N
H
N
N
H
N
N
N
H
N
H
OH
5.32
O
O
N
58
Our investigation started with known aldehyde 5.35 which was accessible in
two steps in 60% yield from γ-butylrolactone 5.33 (Scheme 5.13).6 In similar manner,
ester aldehyde 5.35 underwent spirocyclization reaction and gave spirooxindole 5.36;
however attempts to purify 5.36 by flash chromatography resulted in the amidation of
5.36 to 5.37 in 70% yield.
Scheme 5.13. Preparation of aldehyde 5.35 and synthetic application with 2halotryptamines
O
O
6 eq TEA
O
MeOH, 2h
OMe
HO
PCC, NaOAc
DCM, rt, 3h
85% in 2 steps
O
5.33
OMe
H
O
5.34
5.35
O
NH2
Cl
N
H
5.15a
HN
1. 1.5 eq 5.35, DCM, 0°C
2. 6 eq TFA, 0°C
O
HN
NH
O
N
O
OMe
5.37
5.36
column
H:EtOAc:MeOH(NH3)
6:3.5:0.5
70%
The relative stereochemistry of 5.36 was confirmed by 1H NMR and NOE
studies. The R group trans to the oxindole carbonyl was confirmed by 1-D selective
NOESY experiments (Fig 5.2).
NOE
2.20, 2.35
7.18 1.45
7.05
H H
7.22
9
6.95
6
14
5
OMe
O
3.38
NCHO
HN
O
H 3.4
2.11, 2.5
5.36
Figure 5.2. The NOE analysis of 5.36.
59
Irradiation of the H-9 doublet (7.18 ppm) of 5.36 showed NOEs to methylene
protons H-5, H-6 and H-14 at 3.38, 2.11, and 1.45 ppm, respectively. No NOE was
observed between H-9 and H-3. These data established the stereochemical relationship
of the alkyl chain group and oxindole carbonyl as trans.
With these preliminary results, we wondered whether aldehyde 5.27, via route
II, would undergo spirocyclization. Known aldehyde 5.27 could be prepared according
to Kuehne’s procedure (Scheme 5.14).7 Enamine formation of butyraldehyde followed
by Michael addition to methyl acrylate produced aldehyde methyl ester 5.40.
Subsequent acetylation, LiAlH4 reduction and PCC oxidation of 5.42 gave known
aldehyde 5.27 in good yield. However condensation of bromotryptamine with 5.27
followed by treatment with TFA furnished tetracyclic 5.43 in 20% yield without any
spiro product. This suggests that TFA might deprotect the acetyl group to generate
aldehyde which undergoes imine formation instead of spirocyclization.
Scheme 5.14. Preparation of aldehyde 5.27 and synthetic application with 2halotryptamines
O
pyrrolidine, K2CO3
overnight, rt
40%
H
5.38
O
0°C to rt, 6h;
reflux, 40h;
AcOH, reflux, 8h
54%
5.39
CO2CH3
OH
, p-TsOH
HO
benzene, reflux, 1d
Dean-Stark trap
83%
CO2CH3
H
CO2CH3 , CH3CN
N
5.40
O
O
5.41
O
1M LiAlH4, THF
0°C to reflux, overnight
85%
Br-tryptamine
5.15b
OH
O
O
PCC, NaOAc
3h, quant
H
O
5.42
1. 1.5 eq 5.27, DCM, 0°C
2. 6 eq TFA, 0°C, 2h
20%
N
H
5.43
N
O
5.27
60
We turned our attention back to revised route I and decided to change the C3carbonyl of aldehyde 5.25 to a protected alcohol in order to prevent pyrrole formation
as shown in Scheme 5.15. Alcohol 5.29 could be converted to aldehyde 5.44 via
benzyl protection and deacetylation of the acetyl group in excellent yield.
Spirocyclization of chlorotryptamine with aldehyde 5.44 gave spiooxindole 5.45 in
50% yield and high diastereoselectivity (9:1).
Scheme 5.15. Preparation of aldehyde 5.44 and synthetic application with 2halotryptamines
1. BnBr, NaH
THF, Bu4NI, 95%
2. TsOH, acetone-H2O
reflux, 85%
5.29
O
H
5.44 OBn
O
NH2
N
H
Cl
1. 1.5 eq 5.44, DCM, 0°C
2. 6 eq TFA, 0°C, 2h
50%
HN
5.15a
NH
OBn
5.45
The relative stereochemistry of 5.45, however, could not be unequivocally
confirmed at this stage because of the overlap of the proton signals between H3
methine and the methylene proton of the benzyl group and the oxindole ring and
benzene ring. Although the relative stereochemistry could not be confirmed by NOE
analysis, we were be able to compare 1H NMR data of the resulting spirooxindole
product with other closely related spirooxindoles synthesized previously by our group
as in the synthesis of isoelacomine and elacomine (Fig 5.3).5 According to 1H NMR
data, in spirooxindole 5.20, the R group trans to the oxindole carbonyl, showed the
CH2 protons at 2.65 and 2.28 ppm while the CH2 of 5.21, the R group cis to the
oxindole carbonyl, had resonances at 2.65 and 2.45 ppm. The CH2 protons of 5.45
showed at 2.49 and 2.09 ppm which is correlated to the 1H NMR of the oxindole
alkaloid in which the R group is trans to the oxindole carbonyl.
61
2.65
2.45
OHH H
H
HN
OHH H
H
3.9
HN
7.11
H
7.08
1.11
7.42
NH
4.10
H
7.32
3.7
NH
6.96
2.65
2.28
7.35
1.7, 0.9
1.5
deoxyisoelacomine·HCl
5.21
7.4
2.09
OHH H
H
3.75
HN
4.0
0.76
3.42
NH
OBn
H
0.75
1.32
7.12
2.49
3.45
0.83
1.6, 1.2
3.13
5.45
deoxyelacomine·HCl
5.20
Figure 5.3. The 1H NMR of 5.45 and related spirooxindoles.
End Game
With spirooxindole 5.45 in hand, we turned our attention to completion of the
salacin synthesis. We concentrated on the rotational isomer of the N-formamide bond.
There are a number of examples of natural products bearing secondary N-formyl
substituents
that
show
rotational
isomer
characteristics
such
as
N,N-
dimethylformamide, halichondramide and halishigamide D.8 N-formylation of
spirooxindole 5.20 with acetic formic anhydride gave the formamide 5.46 in 85%
yield (Scheme 5.16). 1H and 13C NMR analysis showed the rotamer effect (Fig 5.4).
Scheme 5.16. N-formylation of spirooxindole alkaloid
O
NH2
N
H
Br
H
1. 1.5 eq , DCM, 0°C
2. 6 eq TFA, 0°C, 2h
50%
HN
H
NH
O
O
O
HN
3
THF, 0°C,
10 m, 85%
9
5.20
5.15b
In addition, the comparison of
O
O
NCHO
14
5.46
13
C NMR data between major and minor
rotational isomers of N-formamide 5.46 showed the larger difference in 13C chemical
shift when the carbons are close to the nitrogen atom. The R group trans to the
oxindole carbonyl was confirmed by 1-D selective NOESY experiments. Irradiation of
62
the H-14 doublet (1.53 ppm) of 5.46 showed NOEs to H-9 at 7.07 ppm. No NOE was
observed between H-9 and H-3. These data established the stereochemical relationship
of the alkyl chain and the oxindole carbonyl as trans.
1.27, 1.53
2.11, 2.48
7.13
7.07
7.24
14
9
7.0
NCHO
3
HN
1H
O
1H
NMR of major isomer 5.46
123.1
129.7
122.6
141.1
43.5
HN
O H
181.3
110.9
NCHO
60.2
25.3
129.58
23.2
34.7
57.3
22.8
40.1
125.3
25.3
129.1
110.9
8.38
H
4.49
NMR of minor isomer 5.46
22.8
40.8
124.8
13C
NCHO
HN
8.32
H4.16
O
1.30, 1.85
0.9
7.13 2.11, 2.48
1.37
7.24
0.6
7.07
3.83, 3.98
7.0
0.9
1.34
0.7
3.83, 3.98
129.7
161.4
141.1
13
45.7
56.7
HN
O H
179.7
NMR of major isomer 5.46
23.2
35.4
59.9
NCHO
162.8
C NMR of minor isomer 5.46
Figure 5.4. The 1H and 13C NMR of major and minor rotamers of N-formamide 5.46.
Scheme 5.17. The end game
O
O
HN
NH
OBn
H
O
O
O
HN
NCHO
OBn
THF, 0°C, 10m
quant
5.47
5.45
O
1. H2, Pd/C, MeOH, quant
2. PCC, NaOAc, DCM, quant
HN
3
9
14
salacin
1.17
NCHO
O
63
With this result, N-formylation of spirooxindole 5.45 with acetic formic
anhydride followed by deprotection of the resulting N-formamide 5.47 gave 2°
alcohol, which underwent PCC oxidation leading to 1.17 in excellent yield (Scheme
5.17).
The 1H and
13
C NMR resonances of 1.17 were assigned based on HSQC,
COSY, and NOE analysis. The alkyl group cis to the benzene ring (trans to the
oxindole carbonyl) was confirmed by 1D selective NOESY experiments. Irradiation of
the major isomer 1.17 at H-14 (2.16 ppm) showed NOEs to H-9 at 7.25 ppm (Fig 5.5).
When the H-9 doublet is irradiated, an NOE was seen to the methylene proton of the
alkyl chain and the pyrrolidine protons, which resonated at 1.66 and 3.67, 2.22 ppm,
respectively. No NOE was observed between H-9 (7.25 ppm) and H-3 (4.05 ppm). In
addition, irradiation of the minor isomer at H-9 (7.40 ppm) showed NOEs to H-9 at
1.78 and 2.14 ppm but no NOE with H-3 (4.25 ppm). These data established the
stereochemical relationship of the alkyl chain group and oxindole carbonyl as trans.
NOE
7.10
7.25
7.32
2.16, 1.66
2.32
H
14
9
7.01
2.3
0.9
O
3.67, 4.03
3
HN
O
NCHO
H 4.05
2.39, 2.22
NOE analysis of major 1.17
Figure 5.5. The NOE analysis of the rotational isomer of
the N-formamide bond in 1.17.
In summary, the application of halotryptamine spirocyclization with aldehydes
having various functionalities has been described. The methodology appears to work
well with aldehydes containing alcohol or ester groups but not with ketones or
protected aldehydes. Furthermore, we have shown the synthesis of salacin can be
carried out via halotryptamine spirocyclization.
64
5.5 References
1. Ponglux, D.; Wongseripipatana, S.; Aimi, N.; Nishimura, M.; Ishikawa, M.;
Sada, H.; Haginiwa, J.; Sakai, S. Chem. Pharm. Bull. 1990, 38, 573-575.
2. a) Cui, C. –B.; Kakeya, H.; Osada, H. J. Antibiot. 1996, 49, 832-835. b) Cui,
H.; Kaeya, H.; Osada, H. Tetrahedron 1996, 52, 12651-12666.
3. von Nussbaum, F.; Danishefsky, S. J. Angew. Chem. Int. Ed. 2000, 39, 21752178.
4. Wang, H.; Ganesan, A. J. Org. Chem. 2000, 65, 4685-4693.
5. Miyake, F. Y.; Yakushijin, K.; Horne, D. A. Org. Lett. 2004, 6, 711-713.
6. a) Gannett, P. M.; Nagel, D. L.; Reilly, P. J.; Lawson, T.; Sharpe, J.; Toth, B.
J. Org. Chem. 1988, 53, 1064-1071. b) Corey, E. J.; Albright, J.; Barton, A. E.;
Hashimoto, S. J. Am. Chem. Soc. 1980, 102, 1435-1436.
7. Bornmann, W. G.; Kuehne, M. E. J. Org. Chem. 1992, 57, 1752-1760.
8. Shin, J.; Lee, H. -S.; Kim, J. -Y.; Shin, H. J.; Ahn, J. -W.; Paul, V. J. J. Nat.
Prod. 2004, 67, 1889-1892.
65
CHAPTER VI
SYNTHESIS OF 5-(3-INDOLYL)OXAZOLE NATURAL PRODUCTS
AND STRUCTURE REVISION OF ALMAZOLE D
6.1 Isolation, Structure Determination and Biological Activities
Pietra and co-workers reported the isolation of almazole C 1.461 and almazole
D 1.472, an indole alkaloid bearing an unusual 2,5-disubstituted oxazole moiety from
red alga off the coast of Senegal (Fig. 6.1). The structure of almazole C was
determined from spectral data and comparison with almazole A and B.3 The FAB-MS
showed C21H21N3O and the fragments m/z 287 [M-Me2N] and 240 [M-C7H7] which
belong to the N,N-dimethyl-L-phenylalanine moiety. The NMR signals; 1H NMR: 7.3
ppm; 13C NMR: 160.68, 148.84, and 119.86 ppm indicated 2,5-substiution of oxazole.
The NOE correlation of H4/H4′ suggests that the oxazole moiety is connected to the
indole. There is no report on the biological activity of this compound. Almazole D was
discovered to be the first and only bioactive member of the almazole family, being
antibacterial against Gram-negative Serratia marcescens and Salmonella typhi XLD.2
The putative structure of almazole D was determined by detailed spectroscopic
analysis, chemical modification studies involving the treatment of almazole D with
diazomethane that yielded the methylated analog and comparison with almazole C.2
N
N
3
5'
NHR
O
2'
5'
O
7
1.44 almazole A : R = CHO
1.45 almazole B : R = H
5'
3
2'
O
8
N
N
O
4
4
N
H
7
almazole C
1.46
N
H
HO
N
1"
2'
N
almazole D
1.472
Figure 6.1. The structures of almazoles.
6"
66
Although, there was no detection of a C8 carbonyl group, the author assumed
this extra carbonyl group was located between the indole and the oxazole moieties
according to FAB-MS which showed the fragments m/z 375 [M+H]+, m/z 331 [MMe2N], m/z 239, and m/z 144. The structure of almazole C and the proposed for
almazole D are shown in Fig. 6.1.
There are spectroscopic reasons to question this structural assignment. The
C-NMR chemical shift of the C3 carbon in almazole D and its derivative are δ 103-
13
105 ppm, which are different from the chemical shift of the C3 carbon in prealmazole
C 6.21 (115.6 ppm) in which exists the carbonyl group substituent at C3 of the indole
system. On the other hand, the chemical shifts of the C3 carbon in almazole D and its
derivative have similar values to the C3 carbon of 5-(3-indolyl)oxazole such as
almazole C (105.7 ppm) and martefragin A 1.434 (105.4 ppm) (Fig.6.2).
not detected
O
105.1
103.9
O
H
N
115.6
O
158.5
N RO
N
170.38
H
N
O
N
H
previously proposed structure of
1.47 almazole D : R=H δC3 = 105.1
6.1 derivative : R=CH3 δC3 = 103.9
by Pietra
prealmazole C
6.2
168.9
O2C
128.5
N
105.7
105.4
O
N
N
H
almazole C
1.46
N
N
O
NH
N
H
martefragin A
1.43
Figure 6.2. The 13C NMR data of almazole C, D, prealmazole C and martefragin A.
According to
13
C NMR analysis of the C3 indole moiety, the proposed
structure of almazole D does not match the results for the extensive series of
oxotryptamine derivatives prepared by the Horne group and those reported in the
67
literature values. This analysis suggests that the proposed structure of almazole D
might be incorrect. It is possible that almazole D could be one of the 5-(3indolyl)oxazoles such as almazole C or martefragin A. To further examine this point,
we attempted the synthesis of both almazole C and a proposed structural revision of
almazole D (Fig 6.3).
NaO2C
N
O
N
N
H
revised structure of
almazole D 6.3
Figure 6.3. Proposed structure of almazole D by the Horne group.
6.2 Previous Synthetic Work
The biomimetic synthesis of almazole C was accomplished by Pietra.1 The key
steps involved peptide formation and Robinson-Gabriel cyclization (Scheme 6.1).
Scheme 6.1. The synthesis of almazole C by Pietra
O
O
O
Br
N
H
Br2, MeOH
reflux, 2h
6.4
1. HMTA/CHCl3
rt, 2h
2. 37% aq. HCl/EtOH
1:9, rt, 1d
N
H
6.5
O
N
HO
O
6.6
1. CDI, DMF, rt, 45m
2. 4.25, rt, overnight
prealmazole C
6.7
N
H
4.25·HCl
N
H
N
N
H
NH2
O
POCl3, 60°C
2d, 50%
almazole C
1.46
68
6.3 Synthesis of Almazoles
In the retrosynthetic route to almazole C and D (Scheme 6.2), the oxazole
moiety of the natural products could be derived from a Robinson-Gabriel cyclization
of the prealmazole intermediate 6.9. Prealmazole 6.9 could be assembled from a
peptide coupling between 6.6 and the key synthons: oxotryptamine 4.25 and
oxotryptophan methyl ester 6.10, respectively. Both of these synthons, in principle,
could be prepared from indole 4.21.
Scheme 6.2. Retrosynthetic analysis of almazole C and D
R
O
N
H
N
N
O
N
N
H
R
N
H
6.8
6.9
N
O
HO
NH2
N
H
4.21
O
N
H
R
+
4.25 R= H
6.10 R= CO2CH3
O
6.6
Synthesis of oxotryptamine and oxotryptophan methyl ester
The synthesis of almazole required developing a new preparation of
oxotryptamine 4.25 and oxotryptophan methyl ester 6.10 as key synthons. Starting
from indole, indole-3-carbonyl nitrile 4.24 was prepared according to the procedure of
Hogan and Sainsbury (Scheme 6.3).5 The addition of oxalyl chloride to indole
followed by refluxing the resulting product in CH3CN with CuCN for 7 h gave indole3-carbonyl nitrile 4.24 in 53% yield. Subsequent hydrogenation of 4.24 in acetic acid
gave oxotryptamine in 50% overall yield. Treatment of indole-3-carbonyl nitrile 4.24
69
with methyl isocyanoacetate 4.26 gave oxazole ester 6.11 in 70% isolated yield. Acid
hydrolysis of the resulting product 6.11 afforded oxotryptophan methyl ester 6.10 in
76% yield and minor amounts oxotryptamine (5%).
Scheme 6.3. The preparation of oxotryptamine and oxotryptophan methyl ester
O
1. (COCl)2, ether
0°C to rt, 1h
N
H
4.21
O
2. CuCN, CH3CN, toluene
ether, 110°C, 7h
53%
N
H
4.24
MeO2C
1.2 eq
4.24
MeO2C
H2, Pd/C
AcOH, 16h
90%
CN
1.2 eq DBU, 10h
70%
O
N
H
6.11
N
H
4.25
O
N
NC
4.26
NH2
MeOH/HCl
60°C, 3d
seal-tube, 76%
NH2
N
H
CO2Me
6.10
Synthesis of 5-(3-Indolyl)oxazole
With oxotryptamine in hand, an indole alkaloid bearing a 2,5-disubstituted
oxazole moiety could be prepared from N-acylation between oxotryptamine and acid
chloride followed by dehydration with oxazole cyclization in the presence of POCl3
leading to 2,5-disubstituted oxazole in 85-90% yield (Scheme 6.4). The typical
13
C
NMR chemical shift of the C3 carbon of oxotryptamine derivative 6.12 appeared at
114.1 ppm, while the C3 carbon of 5-(3-indolyl)oxazole 6.13 appeared at higher field
than 103.9 ppm. With this preliminary result in hand, we moved forward to the
synthesis of almazole C and D.
70
Scheme 6.4. Synthesis of 5-(3-Indolyl)oxazole
O
O
NH2 1.2 eq R
114.1
O
Cl
1.5 eq TEA, THF
30m, 80-90%
N
H
4.25
N
H
N
114.1
R
O
N
H
O
POCl3
rt, 24h
85-90%
6.12 R = CH2CH2CH3
R
N
H
6.13 R = CH2CH2CH3
Synthesis of almazole C
Condensation
of
optically
pure
N,N-dimethyl-L-phenylalanine
6.6,
diethylphosphorylcyanide (DEPC)6 with 4.25 produced prealmazole C 6.7, [α]23D =
+99 (c = 0.27 MeOH) (lit.1 [α]23D = +38, c = 0.25 MeOH) (Scheme 6.5).
Scheme 6.5. Synthesis of almazole C
N
HO
O
O
NH2 1.2 eq
O
115.6
H
N
6.6
N
H
1.2 eq DEPC, TEA
rt, 12h, 85%
4.25
N
H
N
O
POCl3, 60°C
2d, 50%
prealmazole C
6.7
N
105.6
O
N
N
H
almazole C
1.46
It should be noted that a large discrepancy in specific rotation exists between
our work and that reported by Pietra and coworkers. Treatment of 6.7 with POCl3 at
23 °C for 1 day did not produce any appreciable amount of almazole C 1.46; however,
heating the reaction to 60 °C for 2 days produced good yields of 1.46, [α]23D = +156 (c
= 0.27, MeOH). At 90 °C, almazole C 1.46 can be obtained in shorter reaction time
71
but the higher temperature afforded 1.46 in lower optical purity ([α]23D = +91, c =
0.27, MeOH). The optical purity of 1.46 resulting from both sets of reaction conditions
was also evaluated by NMR analysis in the presence of (R)-Mosher’s acid.7 Under the
60 °C cyclization conditions, >97% enantiomeric product purity was obtained as
indicated by the presence of a single methyl signal seen at 2.85 ppm in CDCl3. Under
the 90 °C conditions, however, epimerization was evident in the product by the
presence of two methyl signals 2.85 and 2.84 ppm. Comparison of 1H and 13C spectra
of almazole C and the natural product revealed a perfect match.
Synthesis of almazole D
Scheme 6.6. Synthesis of almazole D
N
HO
O
NH2
N
H
6.10
O
O
N
H
CO2Me 1.2 eq DEPC, TEA
rt, 12h, 60%
H
N
N
POCl3
60°C, 5d
53%
O
CO2Me
6.14
MeO2C
104.7
1.2 eq
6.6
NaO2C
N
O
N
N
H
1. 1N NaOH, MeOH
2d, rt, 90%
2. 1.5 eq NaOH, D2O
6.15
According to preliminary
104.1
N
O
N
N
H
6.3
13
C NMR assignments of 5-(3-indolyl)oxazole,
almazole D appeared to be similar to almazole C except for the carboxylate group at
the C4′ carbon of the oxazole ring. Our attention next turned to the synthesis of
oxazole, and the revised structure proposed for almazole D. The carboxylate group
could be introduced by utilizing oxotryptophan methyl ester 6.10 and processed in a
fashion similar to that described in the earlier synthesis of almazole C to give the final
product 6.3 (Scheme 6.6).
72
Treatment of oxotryptophan methyl ester 6.10 with N,N-dimethyl-Lphenylalanine 6.6 in the presence of triethylamine for 12 h at room temperature gave
the peptide product 6.14 in 60% yield and a 1:1 mixture of diastereomers. The
Robinson-Gabriel cyclization of 6.14 at 60 °C for 5 days lead to 2,5-disubstituted
oxazole 6.15 in 53% yield ([α]23D = +115, c = 0.13, MeOH) and recovered starting
material in ~1:1 ratio. The cyclization of prealmazole D was slower than prealmazole
C due to larger steric interactions. To complete the reaction, heating at 90 °C was
required; however at this temperature significant epimerization occurred as shown by
the presence of two-NMe2 product signals at 2.97 and 2.95 ppm in the presence of (R)Mosher’s acid. The 1H and
13
C NMR spectra of methyl ester 6.15 and methylated
almazole D are identical thereby indicating a structural revision for 1.47 to 6.15 is
needed. In addition, subsequent sponification of the ester gave carboxylic acid 6.3 in
90% yield. The 1H and
13
C NMR spectra of 6.3 (MeOH-d4) as well as the UV
spectrum in the presence of NaOH (1.5 eq) were identical to data of authentic
almazole D, thereby unambiguously establishing the structure of almazole D is 6.3
(Fig 6.4). Furthermore the optical rotation of 6.3, [α]23D = +15 (c = 0.15, MeOH), was
similar to that of the proposed almazole D, [α]23D = +20 (c = 0.07, MeOH). These
optical rotations indicated that the absolute configuration is similar to the natural
product.
73
not detected
not detected
O
O
152.2
105.1
O
N HO
H
N MeO
H
N
previously proposed structure
of
almazole D 1.47 by Pietra
169.5
NaO2C
104.1
163.9
MeO2C
N
104.7
N
123.3
N
159.5
O
155.3
N
H
revised structure of
almazole D
6.3
N
178.0
157.4
O
155.3
159.5
methoxy derivative
6.1
129.2
N
O
158.5
N
170.38
153.0
103.9
N
N
H
revised structure of
almazole D methyl ester, Horne group
6.15
Figure 6.4. The comparison of proposed almazole D and revised structure of
almazole D and their derivatives.
In summary, we have revised the structure of almazole D and its total synthesis
as a 5-(3-indolyl)oxazole. We synthesized almazole C in 5 steps and almazole D in 7
steps starting from indole using a peptide coupling and Gabriel-Robinson cyclization
of the key synthon oxotryptophan as the key steps.
6.4 References
1. Guella, G.; Mancini, I.; N’Diaye, I.; Pietra, F. Helv. Chim. Actra 1994, 77,
1999-2006.
2. N’Diaye, I.; Guella, G.; Mancini, I.; Pietra, F. Tetrahedron Lett. 1996, 37,
3049-3050.
3. N’Diaye, I.; Guella, G.; Chiasera, G.; Mancini, I.; Pietra, F. Tetrahedron Lett.
1994, 35, 4827-4830.
4. Takahashi, S.; Matsunaga, T.; Hasegawa, C.; Saito, H.; Fujita, D.; Kiuchi, D.;
Tsuda, Y. Chem. Pharm. Bull. 1998, 46, 1527-1529.
5. Hogan, I. T.; Sainsbury, M. Tetrahedron 1984, 40, 681-682.
74
6. Yamada, S.; Kasai, Y.; Shioiri, T.; Yokoyama, Y. Tetrahedron 1976, 32, 2211.
7. a) Baxter, C. A. R.; Richards, H. C. Tetrahedron Lett. 1972, 3357-3358. b)
Belvisi, L.; Gennari, C.; Poli, G.; Scolastico, C.; Salom, B. Tetrahedron:
Asymm. 1993, 4, 273-280. c) Pellegrini, C.; Strassler, C.; Weber, M.;
Borschberg, H. -J. Tetrahedron: Asymm. 1994, 5, 1979-1992.
75
CHAPTER VII
EXPERIMENTAL SECTION
General techniques: Starting materials were obtained from common commercial
suppliers and used without further purification. Unless otherwise stated, concentration
under reduced pressure refers to a rotatory evaporator at water aspirator pressure. OneD and 2D nuclear magnetic resonance (NMR) spectra were acquired using a Bruker
AM-400 and Bruker AC-300 spectrometers. Spectra were obtained in CDCl3, DMSOd6, MeOH-d4 and acetone-d6 solution in 5 nm diameter tubes, and the chemical shift
in ppm is quoted relative to the residual signal of chloroform (δΗ 7.25 ppm, or δC 77.0
ppm), dimethyl sulfoxide (δΗ 2.50 ppm, or δC 39.5 ppm), and methanol (δΗ 3.31 ppm,
or δC 49.0 ppm). Multiplicities in the 1H NMR spectra are described as: s = singlet, d =
doublet, t = triplet, q = quartet, m = multiplet, br = broad; coupling constants are
reported in Hz. Exchangeable protons were identified by acquisition of a 1H spectrum
after addition of a drop of D2O to the NMR tube containing a solution of the sample.
Infrared (IR) spectra were obtained using a Nicolet 5DXB FT-IR spectrometer using a
thin film supported between NaCl plates or KBr discs. Melting points were recorded
using open capillary tubes on a Buchi melting point apparatus and are uncorrected.
Optical rotations were measured at ambient temperature (22 °C) in MeOH solution
with a polarimeter using a 1 mL capacity cell with 1 dm path length. Low-resolution
FAB mass spectra (LRMS) and high-resolution FBA mass spectra (HRMS) were
obtained on a Kratos MS 50 and JEOL MSRoute spectrometer. Ion mass/charge (m/z)
ratios are reported as values in atomic mass units.
76
7.1 Nagelamide A and D
Br
N
H
O
CCl3
4 eq Br2
AcOH, rt, 1d
90%
4,5-Dibromo-2-(trichloroacetyl)pyrrole
2.23.
Br
N
H
2.23
To
a
O
CCl3
stirred
solution
of
2-
(trichloroacetyl)pyrrole (50.0 g, 0.24 mol) in acetic acid (500 mL) was added bromine
25.5 mL (0.98 mol, 4 equiv) in acetic acid (200 mL), 45-60 m at room temperature.
The reaction mixture was stirred for 1 d. The solvent was removed in vacuo to afford a
residue which was dissolved in ether and neutralized with 10% K2CO3. The ether layer
was dried with MgSO4 and filtered and concentrated to yield 78.4 g (0.21 mol, 90%)
of 2.23 as colorless solid: IR (KBr) : 3289, 3131, 1658, 1440, 1404, 1368, 1160 cm-1;
1
H NMR (CDCl3, 300 MHz) δ 9.85 (s, 1H), 7.37 (d, J = 2.9 Hz, 1H);
13
C NMR
(CDCl3, 100 MHz) δ 172.2 (s), 124.1 (s), 123.2 (d), 113.3 (s), 102.6 (s), 94.0 (s).
H2N
NH2
CO2H
2.28·2HCl
1. MeOH(HCl)
2. 5% Na(Hg); H2NCN,
95°C, 2.5h, 15% HCl
62%
N
NH2
H2N
N
H
2.25·2HCl
L-ornithine methyl ester dihydrochloride 2.28a. Commercially available Lornithine•HCl (100 g, 0.59 mol) was stirred in 1000 mL MeOH saturated with HCl for
12 h at room temperature. The solvent was evaporated and ornithine methyl ester
crystallized with addition of ethyl acetate to yield a white solid 120 g (0.72 mol, 92%).
1
H NMR (DMSO-d6, 300 MHz) δ 8.81 (bs, 3H), 8.29, (bs, 3H), 3.97 (m, 1H), 3.75 (s,
3H), 2.78 (t, 2H), 1.88 (m, 2H), 1.68 (m, 2H).
2-Amino-4-(3-aminopropyl)-1H-imidazole 2.25. A solution of L-ornithine methyl
ester dihydrochloride (25.0 g, 0.12 mol) in 250 mL of water was cooled between 0-5
°C. The pH was adjusted to 1.5-2.0 by addition of 15 % HCl. Over the course of 1-2 h,
77
5% Na/Hg (556 g, 1.12 mol, 10 equiv) was added, while maintaining the temperature
and pH in the giving range. The pH was then adjusted to 4.3 by the addition of 1 N
NaOH. Cyanamide 48.0 mL (0.57 mol, 5 equiv) was added and the solution heated at
95 °C for 2.5 h. Removal of the solvent in vacuo afforded a light yellow residue which
was washed with ether (3 × 200 mL). Methanol was then added to the residue and
NaCl removed by filtration. The filtrate, after evaporation, gave a pale yellow solid.
Recrystalization from ethanol gave 2-amino-imidazole salt as colorless needles (18.0
g, 0.85 mol, 62%): 1H NMR (DMSO-d6, 400 MHz) δ 12.27 (bs, 1H), 11.75 (bs, 1H),
8.21 (bs, 3H), 7.40 (s, 2H), 6.64 (s, 1H), 2.75 (m, 2H), 2.52 (m, 2H), 1.84 (m, 2H); 13C
NMR (DMSO-d6, 100 MHz) δ 147.3 (s), 125.8 (s), 109.4 (d), 38.2 (t), 25.9 (t), 21.5
(t).
N
H2N
N
H
2.25·2HCl
NH2 1. 1.2 eq NCS, MeOH, rt
2. xylene/MeOH, 135°C
40% in 2 steps
N
H2N
NH2
N
H
2.26·2HCl
3-Amino-1-(2-aminoimidazole-4-yl)-prop-1-ene 2.26. To a stirred solution of 2amino-4-(3-aminopropyl)-1H-imidazole salt (0.60 g, 2.81 mmol) in 10 mL of
methanol at room temperature was added NCS (0.41 g, 3.38 mmol, 1.2 equiv). After 1
h, methanol was removed in vacuo without heat and the resulting residue was washed
with ether (3 × 100 mL) and acetone (3 × 100 mL) to give crude dimethoxy imidazole
salt as unstable colorless oil. Resulting crude product was dissolved in 10 mL
methanol and 10 mL m-xylene and heat at 135 °C for 3 h. Xylene was decanted, after
cooling to room temperature, and washed with ether (3 × 100 mL) and acetone (2
× 100 mL). Addition of 5 mL of methanol to the residue and filtration yield 0.21 g
(0.01 mol, 40%) of pure 2.26•HCl as colorless solid: mp 220-222 °C; 1H NMR
(DMSO-d6, 400 MHz) δ 6.40 (s, 1H), 6.17 (d, J = 15.7 Hz, 1H), 5.86 (dt, J = 15.7, 5.9
Hz, 1H), 5.42 (bs, 2H), 5.03 (bs, 3H), 3.25 (d, J = 5.9 Hz, 2H); 13C NMR (DMSO-d6,
100 MHz) δ 151.2 (s), 130.5 (s), 125.1 (d), 120.6 (d), 117.0 (s), 43.9 (t).
78
NH2
N
NH2
H2N
N
H
2.25·2HCl
1. 1.1 eq NCS, MeSO3H, rt
2. 1.1 eq
N
NH2 ·0.5H2SO4
N
2.27
H
16h, 40%
N
N
N
H
N
H
H2N
NH2
2.28·3HCl
Heterodimer 2.28. To a stirred solution of 2.25•HCl in (1.00 g, 4.69 mmol) in 10 mL
of methanesulfonic acid was added NCS (0.69 g, 5.16 mmol, 1.1 equiv) at room
temperature. After 10 m, 2.27 (0.68 g, 5.16 mmol, 1.1 equiv) was added and resulting
solution was stirred for 16 h, diluted with acetone and decanted (2 × 30 mL). Flash
chromatography 100% MeOH(NH3) of the residue afforded dimer 2.28. Addition of
conc. HCl to a methanol solution of the free base and concentration in vacuo gave 0.61
g (1.85 mmol, 40%) 2.28•3HCl: 1H NMR (DMSO-d6, 300 MHz) δ 12.42 (bs, 2H),
11.98 (bs, 2H), 8.36 (bs, 3H), 7.45 (s, 4H), 6.77 (s, 2H), 4.14 (t, J = 7.2 Hz, 1H), 2.74
(m, 2H), 2.17 (m, 2H);
C NMR (DMSO-d6, 75 MHz) δ 147.2 (s), 125.4 (s), 110.3
13
(d), 36.5 (t), 29.6 (t), 29.5 (d).
O
Br
NH2
Br
N
N
N
H
N
H
H2N
NH2
H
N
HN
N
H
O
CCl3
4
Br
N
N
N
H
N
H
H2N
DMF, rt, 1h
68%
Br
NH2
2.29 ·2HCl
2.28
4,5-Dibromo-1H-pyrrole-2-carboxylic acid [3,3-bis-(2-amino-1H-imidazol-4-yl)propyl]-amide 2.29. To a stirred solution of dimer 2.28 (0.03 g, 0.14 mmol) in 10 mL
of DMF under nitrogen was added 4,5-dibromopyrrol-2-yl trichloromethylketone
(0.06 g, 0.15 mmol, 1 equiv) at room temperature. After 1 h, the reaction mixture was
diluted
with
ether
and
decanted
(2
× 100
mL).
Flash
chromatography
(CH2Cl2:MeOH(NH3), 9:1) of the residue afforded 2.29 as light brown solid. Addition
of conc. HCl to a methanol solution of the free base and concentration in vacuo gave
79
0.05 g (0.09 mmol, 68%) of 2.29•2HCl: mp 160-162 °C; IR (KBr) : 3298, 3152, 1679,
1629, 1565, 1523, 1324 cm-1; 1H NMR (DMSO-d6, 300 MHz) δ 12.81 (bs, 1H), 12.34
(bs, 2H), 11.90 (bs, 2H), 8.59 (bs, 1H), 7.39 (s, 4H), 7.03 (d, J = 1.9 Hz, 1H), 6.72 (s,
2H), 3.99 (t, J = 7.1 Hz, 1H), 3.17 (d, J = 5.2 Hz, 2H), 2.06 (d, J = 6.5 Hz, 2H); 13C
NMR (DMSO-d6, 75 MHz) δ 159.8 (s), 147.9 (s×2), 129.0 (s), 127.0 (s×2), 114.1 (d),
111.0 (d×2), 105.2 (s), 98.7 (s), 37.1 (t), 32.5 (t), 30.5 (d).
NH2
N
NH2
H2N
N
H
2.25·2HCl
0.5 eq NCS, MeSO3H
rt, 1d
30-35%
N
N
N
H
N
H
H2N
NH2
2.30
NH2
Homodimer 2.30. To a stirred solution of 2.25•2HCl in (1.00 g, 4.69 mmol) in 10 mL
of methanesulfonic acid was added NCS (0.31 g, 2.35 mmol, 0.5 equiv) at room
temperature. The reaction mixture was stirred for 1 d, diluted with acetone and
decanted (2 × 200 mL) and CH2Cl2 and decanted (2 × 200 mL). Flash chromatography
100% MeOH(NH3) of the residue afforded dimer 2.30 as colorless sticky oil. Addition
of conc. HCl to a methanol solution of the free base and concentration in vacuo gave
0.41-0.45 g (1.47 mmol, 30-35%) of 2.30•4HCl: 1H NMR (DMSO-d6, 300 MHz)
δ 12.32 (bs, 1H), 12.26 (bs, 2H), 12.00 (bs, 1H), 8.34 (bs, 3H), 8.24 (bs, 3H), 7.46 (s,
2H), 7.37 (s, 2H), 6.89 (s, 1H), 4.46 (t, J = 7.1 Hz, 1H), 2.79 (m, 4H), 2.65 (m, 2H),
2.33 (m, 1H), 2.10 (m, 1H), 1.84 (m, 2H); 13C NMR (DMSO-d6, 75 MHz) δ 148.0 (s),
147.6 (s), 126.3 (s), 123.0 (s), 121.0 (s), 111.1 (d), 38.9 (t), 37.5 (t), 30.6 (t), 29.1 (d),
27.6 (t), 20.9 (t).
80
NH2
N
H2N
N
H
NH2
2.25·2HCl
0.5 eq NCS, MeSO3H, rt, 1d;
0.5 eq NCS, 80-90°C, 12h
30%
N
N
N
H
N
H
H2N
NH2
2.31
NH2
Homodimer 2.31. To a stirred solution of 2.25 in (1.00 g, 4.69 mmol) in 10 mL of
methanesulfonic acid was added NCS (0.31 g, 2.35 mmol, 0.5 equiv) at room
temperature. The reaction mixture was stirred for 1 d and added NCS (0.31 g, 2.35
mmol, 0.5 equiv) at room temperature and heat at 80-90 °C for 12 h; then diluted with
acetone and decanted (2 × 200 mL) and CH2Cl2 and decanted (2 × 200 mL). Flash
chromatography 100% MeOH(NH3) of the residue afforded dimer 2.31: as colorless
sticky oil. Addition of conc. HCl to a methanol solution of the free base and
concentration in vacuo gave 0.37 g (1.34 mmol, 30%) of 2.31•4HCl: mp 160-162 °C;
1
H NMR (DMSO-d6, 300 MHz) δ 13.00 (bs, 1H), 12.58 (bs, 1H), 12.38 (bs, 2H), 8.56
(bs, 3H), 8.17 (bs, 3H), 7.61 (s, 2H), 7.50 (s, 2H), 6.82 (s, 1H), 6.26 (t, J = 6.6 Hz,
1H), 3.49 (m, 2H), 2.72 (m, 2H), 2.49 (t, J = 6.6 Hz, 2H), 1.80 (m, 2H).
NH2
N
H2N
N
H
NH2
2.25·2HCl
0.5 eq NCS, MeSO3H, rt;
0.5 eq NCS, 1d;
0.5 eq NCS, 1d
25%
H2N
N
N
NH
N
H2N
NH2
2.32
Azulene 2.32. To a stirred solution of 2.25 in (1.00 g, 4.69 mmol) in 10 mL of
methanesulfonic acid was added NCS (0.31 g, 2.35 mmol, 0.5 equiv) at room
temperature. The reaction mixture was stirred for 1 d and added NCS (0.31 g, 2.35
mmol, 0.5 equiv) at room temperature. After stirred 1 d, the reaction mixture was
added NCS (0.31 g, 2.35 mmol, 0.5 equiv) at room temperature and stirred for 1 d.
The reaction mixture was diluted with acetone and decanted (2 × 200 mL) and CH2Cl2
and decanted (2 × 200 mL). Flash chromatography 100% MeOH(NH3) of the residue
81
afforded azulene 2.32. Addition of conc. HCl to a methanol solution of the free base
and concentration in vacuo gave 0.31 g (1.14 mmol, 25%) of 2.32•4HCl: mp 266-268
°C; UV (MeOH) λmax 278, 398 nm, UV (MeOH:HCl, 1:1) λmax 297, 420 nm, UV
(conc. HCl) λmax 297, 390 nm; IR (KBr) : 3329, 3087, 1700, 1655, 1551, 1470, 1310
cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 8.83 (bs, 3H), 8.44 (bs, 3H), 8.39 (s, 1H), 7.92
(bs, 2H), 7.80 (bs, 2H), 4.51 (s, 2H), 3.74 (t, J = 7.4 Hz, 2H), 3.23 (t, J = 7.4 Hz, 2H);
C NMR (DMSO-d6, 75 MHz) δ 159.6 (s), 159.0 (s), 151.5 (s), 150.4 (s), 144.5 (s),
13
141.6 (s), 129.0 (s), 125.8 (s), 123.3 (d), 43.0 (t), 38.4 (t), 29.5 (t).
O
Br
NH2
N
N
N
H
N
H
H2N
Br
N
2.1 eq 2.23 H
NH2
NH2 2.30
HN
CCl3
H
N
Br
Br
O
N
N
N
H
N
H
H2N
DMF, rt, 1d
80%
Br
Br
N
H
NH2
NH
·2HCl
O
nagelamide D
2.2
Nagelamide D 2.2. To a stirred solution of dimer 2.30 (0.03 g, 0.11 mmol) in 10 mL
of DMF under nitrogen was added 4,5-dibromopyrrol-2-yl trichloromethylketone
(0.08 g, 0.23 mmol, 2.1 equiv) at room temperature. After 1 d, the reaction mixture
was diluted with ether and decanted (2 × 100 mL). Flash chromatography
(CH2Cl2:MeOH(NH3), 3:1) of the residue afforded 2.2 as light brown solid. Addition
of conc. HCl to a methanol solution of the free base and concentration in vacuo gave
0.06 g (0.08 mmol, 80%) of 2.2•2HCl: mp 235-237 °C; UV (MeOH) λmax 295 nm; IR
(KBr) : 3281, 3160, 1681, 1624, 1566, 1524, 1419, 1324 cm-1; 1H NMR (DMSO-d6,
400 MHz) δ 12.78 (d, J = 2.5 Hz, 1H), 12.77 (d, J = 2.5 Hz, 1H), 12.22 (s, 1H), 12.14
(s, 1H), 12.10 (s, 1H), 11.88 (s, 1H), 8.47 (t, J = 5.3 Hz, 1H), 8.43 (t, J = 5.4 Hz, 1H),
7.41 (s, 2H), 7.28 (s, 2H), 7.04 (d, J = 2.5 Hz, 1H), 7.02 (d, J = 2.6 Hz, 1H), 6.81 (s,
1H), 4.14 (t, J = 7.5 Hz, 1H), 3.17 (m, 4H), 2.48 (m, 2H), 2.22 (m, 1H), 2.03 (m, 1H),
82
1.71 (m, 2H); 13C NMR (DMSO-d6, 75 MHz) δ 159.8 (s×2), 147.0 (s), 146.6 (s), 128.2
(s), 128.1 (s), 126.6 (s), 122.8 (s), 120.0 (s), 113.0 (d×2), 109.8 (s), 104.4 (s), 104.3
(s), 97.8 (s×2), 38.1 (t), 36.5 (t), 31.7 (t), 28.8 (t), 28.6 (d), 20.6 (t); HRFABMS m/z
calcd for C22H26O2N10Br4 [M+H]+: 776.8301; found: 777.4641.
O
Br
NH2
Br
N
N
N
H
N
H
H2N
2.1 eq 2.23
NH2
NH2
2.31
HN
N
H
CCl3
H
N
Br
Br
O
N
N
N
H
N
H
H2N
DMF, rt, 1d
80%
Br
Br
N
H
NH2
NH
·2HCl
O
2.31a
Amide 2.31a. To a stirred solution of dimer 2.31 (0.02 g, 0.07 mmol) in 5 mL of DMF
under nitrogen was added 4,5-dibromopyrrol-2-yl trichloromethylketone (0.05 g, 0.15
mmol, 2.1 equiv) at room temperature. After 1 d, the reaction mixture was diluted with
ether and decanted (2 × 100 mL). Flash chromatography (CH2Cl2:MeOH(NH3), 3:1)
of the residue afforded 2.31a as brown solid. Addition of conc. HCl to a methanol
solution of the free base and concentration in vacuo gave 0.045 g (0.06 mmol, 80%) of
2.31a•2HCl: mp 218-220 °C; UV (MeOH) λmax 299 nm; IR (KBr) : 3273, 3155, 1679,
1618, 1562, 1522, 1419, 1390, 1323, 1235 cm-1; 1H NMR (DMSO-d6, 300 MHz)
δ 12.82 (d, J = 2.4 Hz, 1H), 12.75 (d, J = 2.4 Hz, 1H), 12.73 (s, 1H), 12.43 (s, 1H),
12.29 (s, 1H), 12.20 (s, 1H), 8.76 (t, J = 5.3 Hz, 1H), 8.44 (t, J = 5.4 Hz, 1H), 7.51 (s,
2H), 7.41 (s, 2H), 7.03 (d, J = 2.7 Hz, 1H), 6.97 (d, J = 2.5 Hz, 1H), 6.70 (s, 1H), 6.24
(t, J = 6.6 Hz, 1H), 3.86 (bt, J = 5.4 Hz, 2H), 3.13 (m, 2H), 2.36 (bt, J = 7.0 Hz, 2H),
1.72 (m, 2H); 13C NMR (DMSO-d6, 75 MHz) δ 159.7 (s×2), 148.8 (s), 147.6 (s), 130.4
(d), 129.0 (s), 128.7 (s), 126.6 (s), 126.1 (s), 118.0 (s), 115.2 (s), 114.1 (d), 113.8 (d),
112.9 (d), 105.6 (s), 105.1 (s), 98.8 (s), 98.7 (s), 38.9 (t), 38.4 (t), 28.9 (t), 22.2 (t);
HRFABMS m/z calcd for C22H24O2N10Br4 [M+H]+: 778.8769; found: 778.1000.
83
NH2
N
NH2
H2N
N
H
MeSO3H, rt, 1d
40%
N
N
N
H
N
H
H2N
NH2
2.26·2HCl
NH2
2.33
Dimer 2.33. A free base 2.26 (1.00 g, 4.76 mmol) was dissolved in 10 mL of
methanesulfonic acid and stirred for 1 d at room temperature. The reaction mixture
was diluted with acetone and decanted (2 × 30 mL), followed by addition of CH2Cl2
and decantation (2 × 50 mL). Flash chromatography (CH2Cl2:MeOH(NH3), 6:4) of the
residue afforded 2.33•4HCl in 0.52 g (1.88 mmol, 40%) as pale yellow oil: 1H NMR
(DMSO-d6, 300 MHz) δ 12.95 (bs, 1H), 12.71 (bs, 1H), 12.35 (bs, 1H), 12.04 (s, 1H),
8.43 (bs, 3H), 8.42 (bs, 3H), 7.49 (s, 4H), 6.99 (d, J = 15.8 Hz, 1H), 6.89 (s, 1H), 6.12
(dt, J = 6.1 Hz, 1H), 4.54 (t, J = 7.2 Hz, 1H), 3.56 (m, 2H), 2.74 (m, 2H), 2.35 (m,
1H), 2.17 (m, 1H);
13
C NMR (DMSO-d6, 75 MHz) δ 148.0 (s), 147.2 (s), 125.0 (s),
123.4 (s), 121.1 (d), 120.6 (s), 120.2 (d), 110.5 (d), 40.4 (t), 36.6 (t), 29.5 (t), 28.5 (d).
O
Br
NH2
Br
N
N
N
H
N
H
H2N
2.1 eq 2.23
NH2
NH2
2.33
HN
N
H
CCl3
H
N
Br
Br
O
N
N
N
H
N
H
H2N
DMF, rt, 1d
80%
Br
Br
N
H
NH2
NH
O
1.5 ·2HCl
Nagelamide A 1.5. To a stirred solution of dimer 2.33 (0.02 g, 0.07 mmol) in 5 mL of
DMF under nitrogen was added 4,5-dibromopyrrol-2-yl trichloromethylketone (0.06g,
0.15 mmol, 2.1 equiv) at room temperature. After 1 d, the reaction mixture was diluted
with ether and decanted (2 × 100 mL). Flash chromatography (CH2Cl2:MeOH(NH3),
3:1) of the residue afforded 1.5 as brown solid. Addition of conc. HCl to a methanol
solution of the free base and concentration in vacuo gave 0.046 g (0.06 mmol, 80%) of
84
1.5•2HCl: mp 228-230 °C; UV (MeOH) λmax 278.8, 290 nm; IR (KBr) : 3277, 3159,
1685, 1629, 1562, 1522, 1419, 1390, 1323 cm-1; 1H NMR (DMSO-d6, 300 MHz)
δ 12.79 (s, 2H), 12.56 (s, 1H), 12.51 (s, 1H), 12.25 (s, 1H), 11.93 (s, 1H), 8.57 (t, J =
5.5 Hz, 1H), 8.52 (t, J = 5.2 Hz, 1H), 7.39 (bs, 4H), 7.02 (d, J = 3.1 Hz, 1H), 7.01 (d, J
= 3.0 Hz, 1H), 6.82 (s, 1H), 6.47 (d, J = 15.9 Hz, 1H), 6.07 (dt, J = 15.9, 6.2 Hz, 1H),
4.28 (t, J = 7.4 Hz, 1H), 3.89 (m, 2H), 3.13 (m, 2H), 2.24 (m, 1H), 2.03 (m, 1H); 13C
NMR (DMSO-d6, 75 MHz) δ 158.9 (s), 158.7 (s), 147.6 (s), 147.1 (s), 128.2 (s), 128.1
(s), 126.7 (d), 126.1 (s), 122.3 (s), 121.1 (s), 116.1 (d), 113.1 (d×2), 110.1 (d), 104.6
(s), 104.4 (s), 97.9 (s×2), 40.4 (t), 36.4 (t), 31.5 (t), 28.7 (d); HRFABMS m/z calcd for
C22H24O2N10Br4 [M+H]+: 778.1000; found: 778.1000.
7.2 Agelastatin D
H2N
H
N
NH2 1. MeOH(HCl)
CO2H
2. 5% Na(Hg); KOCN
2.28·2HCl
15% HCl, 60%
NH2
O
N
H
3.23
(3-Aminopropyl)-1H-imidazolidin-2-one 3.23. A solution of L-ornithine methyl
ester dihydrochloride (22.0 g, 0.11 mol) in 300 mL H2O was prepared and cooled
between 0-5 °C. The pH was adjusted between 1.2 and 1.5 by addition of a 15%
solution of HCl. Over the course of 1 h, 5% Na(Hg) (545 g, 1.11 mol, 10 equiv) was
added in 1 g pieces maintaining the pH and temperature constant. After bubbling
ceased the solution was decanted, potassium cyanate was added (9.41 g, 0.11 mol, 1
equiv), adjusting the pH to 3.4, and the solution was refluxed for 3 h. The solvent was
removed in vacuo and residual water was azeotroped with absolute ethanol to give a
light yellow residue. Ethanol was added to the residue and NaCl was filtered off.
Addition of a solution of EtOH:CH2Cl2 (2:1) resulted in precipitation of inorganic
impurities which were filtered off. Evaporation of the solvent gave a precipitate which
was washed with a minimum amount of ethanol to provide 12.87 g (0.07 mol, 60%) of
the desired product as a colorless solid: IR (KBr) : 3102, 3065, 1666, 1631, 1492,
85
1148 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 9.85 (bs, 1H), 9.51 (s, 1H), 8.11 (bs,
3H), 6.01 (s, 1H), 2.74 (t, J = 7.3 Hz, 2H), 7.26 (t, J = 7.2 Hz, 2H), 3.50 (q, J = 7.2
Hz, 2H);
C NMR (DMSO-d6, 100 MHz) δ 155.4 (s), 121.0 (s), 104.7 (d), 38.4 (t),
13
25.9 (t), 22.5 (t).
H
N
NH2
O
N
H
3.23·HCl
1.2 eq NBS
MeOH, -78°C
70%
H
N
OMe
NH2
O
N
H
3.35
4-(3-Amino-1-methoxy-propyl)-1,3-dihydro-imidazol-2-one 3.35. To a solution of
imidazolone 3.23 (5.00 g, 28.2 mmol) in 125 mL CH3OH was added NBS (6.02 g,
33.8 mmol, 1.2 equiv) was added at -78 °C. The reaction was stirred at room
temperature for 3 h and removed the solvent under reduced pressure. Flash
chromatography (CH2Cl2:MeOH(NH3), 6:4) of the residue afforded 3.4 g (0.02 mol,
70%) of 3.35 as light yellow oil: 1H NMR (DMSO-d6, 400 MHz) δ 6.30 (s, 1H), 4.00
(t, J = 6.9 Hz, 1H), 3.09 (s, 3H), 2.67 (t, J = 6.9 Hz, 2H), 1.96 (m, 1H), 1.78 (m, 1H);
C NMR (DMSO-d6, 100 MHz) δ 155.1 (s), 120.4 (s), 107.6 (d), 72.4 (q), 55.0 (d),
13
36.8 (t), 33.6 (t).
H
N
OMe
NH2
O
N
H
3.33
TFA, rt, 5h, 35%
H
N
NH2
O
N
H
3.34
4-(3-Amino-propenyl)-1,3-dihydro-imidazol-2-one 3.34. The methoxy-imidazolone
(0.20 g, 1.21 mmol) was dissolved in TFA and stirred at room temperature for 5 h.
The solvent was evaporated without heat under reduced pressure. Flash
chromatography (CH2Cl2:MeOH(NH3), 7:3) of the residue afforded 16 mg (0.11
mmol, 35%) of 3.34 as light yellow oil: IR (KBr) : 3183, 3022, 1671, 1684 cm-1; 1H
NMR (DMSO-d6, 300 MHz) δ 10.43 (s, 1H), 10.08 (s, 1H), 8.01 (bs, 3H), 6.49 (s,
1H), 6.25 (d, J = 15.8 Hz, 1H), 5.71 (dt, J = 15.8, 6.8 Hz, 1H), 3.50 (d, J = 6.8 Hz,
86
2H);
13
C NMR (DMSO-d6, 100 MHz) δ 155.3 (s), 123.9 (s), 120.7 (d), 116.3 (d),
110.9 (d), 41.1 (t).
H
N
NH2
O
N
H
1.2 eq 2.23
N
H
H
N
N
H
O
DMF, rt, 16h
78%
3.34
O
H
N
Br
Br
3.32
4-Bromo-1H-pyrrole-2-carboxylic acid [3-(2-oso-2,3-dihydro-1H-imidazol-4-yl)allyl]-amide 2.32. To a solution of 3-amino-1-(imidazolidin-2-one-4-yl)prop-1-ene
3.34 free base (0.05 g, 0.33 mmol) in 2 mL of DMF was added dibromopyrrol-2-yl
trichoromethylketone 2.23 (0.15 g, 0.40 mmol, 1.2 equiv). The reaction was stirred at
room temperature under N2 for 16 h. Ether was added to the reaction mixture (3 × 50
mL) and decanted. Flash chromatography (CH2Cl2:MeOH, 9:1) of the residue afforded
0.10 g (0.25 mmol, 78%) of dibromoamide 3.32 as white solid: mp 240-242 °C; IR
(KBr) : 3286, 3145, 1684, 1615 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 12.70 (s, 1H),
10.24 (s, 1H), 9.88 (s, 1H), 8.35 (t, J = 5.6 Hz, 1H), 6.98 (d, J = 2.5 Hz, 1H), 6.37 (s,
1H), 6.03 (d, J = 15.9 Hz, 1H), 5.78 (dt, J = 18.9, 5.6 Hz, 1H), 3.91 (t, J = 5.3 Hz,
2H); 13C NMR (DMSO-d6, 100 MHz) δ 159.1 (s), 155.3 (s), 128.6 (s), 122.1 (s), 121.5
(d), 119.1 (s), 113.1 (s), 108.9 (d), 105.1 (s), 98.3 (s), 40.6 (t); HRMS m/z calcd for
C11H12N4O2Br [M++1]: 311.0136; found: 311.0142.
H
N
NH2
O
N
H
3.23
H
N
1.2 eq 2.23, DMF, rt
16h, 90%
O
O
N
H
N
H
H
N
Br
Br
3.35
4,5-Dibromo-1H-pyrrole-2-carboxylic acid [3-(2-oxo-2,3-dihydro-1H-imidazol-4yl)-propyl]-amide 3.35. To a solution of (3-aminopropyl)-1H-imidazolidin-2-one free
base 3.23 (0.35 g, 2.43 mmol) in 2 mL of DMF was added 4,5-dibromopyrrol-2-yl
trichoromethylketone (1.08 g, 2.92 mmol, 1.2 equiv). The reaction was stirred at room
temperature under N2 for 16 h. Ether was added to the reaction mixture (3 × 100 mL)
87
and decanted and the resulting residue was triturated with methanol. The product was
filtered and purified by recrystalization from a mixture of methanol and water (1:1) to
afford 0.87 g (2.21 mmol, 90%) of the desired couple product 3.35 as a colorless solid:
mp 180-182 °C; IR (KBr) : 3245, 3141, 1687, 1622 cm-1; 1H NMR (DMSO-d6, 300
MHz) δ 12.65 (bs, 1H), 9.72 (s, 1H), 9.40 (s, 1H), 8.10 (s, 1H), 6.90 (s, 1H), 5.95 (s,
1H), 3.16 (m, 2H), 2.23 (m, 2H), 1.65 (m, 2H);
13
C NMR (DMSO-d6, 100 MHz)
δ 159.4 (s), 155.4 (s), 128.7 (s), 121.8 (s), 112.9 (d), 104.9 (s), 104.2 (d), 98.2 (s),
38.45 (t), 28.2 (t), 13.1 (t); HRMS m/z calcd for C11H13N4O2Br2 [M++1]: 390.9405;
found: 390.9405.
H
N
O
N
H
O
N
H
3.35
H
N
H
N
Br
Br
1.6 eq NBS, MeOH
-78°C to rt, 62%
O
N
H
O
OMe
N
H
3.36
H
N
Br
Br
4,5-Dibromo-1H-pyrrole-2-carboxylic acid [3-methoxy-3-(2-oxo-2,3-dihydro-1Himidazol-4-yl)-propyl]-amide 3.36. To a stirred solution of imidazolone 3.35 (0.10 g,
0.26 mmol) in 10 mL CH3OH under nitrogen was added NBS (0.73 g, 0.41 mmol, 1.6
equiv) at -78 °C. The reaction was stirred at room temperature for 1 d and
concentrated under reduced pressure. Flash chromatography (CH2Cl2:MeOH(NH3),
8:2) of the residue afforded 0.66 g (1.56 mmol, 62%) of 3.36 as white solid: mp 214216 °C; IR (KBr) : 3147, 2937, 1694, 1634, 1562, 1234, 1112 cm-1; 1H NMR (DMSOd6, 300 MHz) δ 12.65 (bs, 1H), 9.92 (s, 1H), 9.65 (s, 1H), 8.08 (t, J = 5.52 Hz, 1H),
6.90 (s, 1H), 6.27 (s, 1H), 3.86 (t, J = 5.9 Hz, 1H), 3.19 (m, 2H), 3.11 (s, 3H) 1.95 (m,
1H), 1.77 (m, 1H); 13C NMR (DMSO-d6, 100 MHz) δ 159.3 (s), 155.6 (s), 128.7 (s),
121.1 (s), 112.9 (d), 108.0 (d), 104.9 (s), 98.2 (s), 73.0 (d), 55.4 (q), 36.0 (t), 33.9 (t).
88
H
N
H
N
O
N
H
O
N
H
3.35
N
H
O
H
N
N
H
Br
Br
O
OMe
H
N
Br
Br
Br
3.37 80%
1.8 eq NBS, MeOH
-78°C to rt
H
N
O
O
O
N
H
N
H
H
N
Br
Br
Br
3.38 20%
NBS oxidation of 3.35. To a stirred solution of imidazolone 3.35 (3.00 g, 7.65 mmol)
in 300 mL CH3OH under nitrogen was added NBS (2.45 g, 13.7 mmol, 1.8 equiv) at 78 °C. The reaction was stirred at room temperature for 1 d and concentrated under
reduced pressure. Flash chromatography (CH2Cl2:MeOH(NH3), 8:2) of the residue
afforded 3.37 and 3.38 in 3.06 g, 6.12 mmol, 80 % and 0.74 g, 1.53 mmol, 20% yield,
respectively:
3,4,5-Tribromo-1H-pyrrole-2-carboxylic acid [3-methoxy-3-(2-oxo-2,3-dihydro1H-imidazol-4-yl)-propyl]-amide 3.37. mp 234-236 °C; IR (KBr) : 3101, 2921,
1696, 1631, 1547, 1503, 1425, 1234 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 13.06
(bs, 1H), 9.97 (s, 1H), 9.71 (s, 1H), 7.63 (t, J = 5.2 Hz, 1H), 6.31 (s, 1H), 3.93 (t, J =
5.8 Hz, 1H), 3.16 (m, 2H), 3.11 (s, 3H), 1.95 (m, 1H), 1.81 (m, 1H);
13
C NMR
(DMSO-d6, 100 MHz) δ 158.6 (s), 155.6 (s), 127.0 (s), 121.0 (s), 108.4 (d), 105.1 (s),
102.4 (s), 99.75 (s), 73.8 (d), 55.9 (q), 35.6 (t), 33.6 (t);
3,4,5-Tribromo-1H-pyrrole-2-carboxylic
acid [3-oxo-3-(2-oxo-2,3-dihydro-1H-
imidazol-4-yl)-propyl]-amide 3.38. mp 261-263 °C; 1H NMR (DMSO-d6, 300 MHz)
δ 13.12 (bs, 1H), 10.80 (s, 1H), 10.52 (s, 1H), 7.63 (s, 1H), 3.54 (m, 2H), 2.85 (m,
2H); 13C NMR (DMSO-d6, 100 MHz) δ 186.8 (s), 158.3 (s), 154.5 (s), 126.1 (s), 123.8
(s), 122.0 (d), 105.1 (s), 102.6 (s), 99.9 (s), 36.5 (t), 35.6 (t).
89
O
H
N
O
OMe
N
H
O
N
H
Br
3.38
H
N
HN
NH
Br
Br
Br
anh pyr, reflux
2d, 35%
N
Br
NH
Br
3.39
O
6,7,8-Tribromo-4-(2-oxo-2,3-dihydro-1H-imidazol-4-ylmethyl)-3,4-dihydro-2Hpyrrolo[1,2-a]pyrazin-1-one 3.39. A solution of α-methoxy-imidazolone 3.38 (0.2 g,
0.4 mmol) in 30 mL anhydrous pyridine was refluxed for 2 d. The reaction mixture
was concentrated under reduced pressure to give dark brown residue. The crude
product was dissolved in MeOH, filtered and concentrated under reduced pressure.
Flash chromatography (CH2Cl2:MeOH(NH3), 17:3 to 9:1) of the residue afforded 65
mg (0.14 mmol, 35%) of pyrazinone 3.39 as white solid: mp 258-260 °C; IR (KBr) :
3209, 2914, 1678, 1643, 1529, 1432, 1330 cm-1; 1H NMR (MeOH-d4, 400 Hz) δ 6.05
(s, 1H), 4.70 (m, 1H), 3.86 (dd, J = 13.5, 4.1 Hz, 1H), 3.56 (d, J = 13.5 Hz, 1H), 2.83
(d, J = 7.09 Hz, 2H); 13C NMR (MeOH-d4, 100 MHz) δ 158.4 (s), 155.3 (s), 121.2 (s),
117.7 (s), 107.9 (s), 107.6 (s×2), 103.9 (s), 54.0 (d), 42.3 (t), 28.2 (t).
O
HN
O
NH
0.75 eq NBS
MeOH, -78°C
45%
Br
N
Br
NH
Br
O
Br
Br
3.39
H MeO
N
H
N
MeO
H
N
O
N
H
Br
3.45
6,7,8-Tribromo-4-(4,5-dimethoxy-2-oxo-imidazolidin-4-ylmethyl)-3,4-dihydro2H-pyrrolo[1,2-a]pyrazin-1-one 3.45. To a stir solution of cycloimidazolone (0.10 g,
0.22 mmol) in MeOH 10 mL was added NBS (0.03 g, 0.16 mmol, 0.75 equiv) at -78
°C. The reaction was stirred at room temperature for 1 d and concentrated under
reduced pressure. Flash chromatography (CH2Cl2:MeOH(NH3), 17:3) of the residue
afforded 0.05 g (0.09 mmol, 45%) of dimethoxy adduct 3.45 as white solid: mp 256258 °C; IR (KBr) : 3216, 3062, 1726, 1664, 1533, 1428, 1358, 1330, 1110, 1065 cm-1;
90
1
H NMR (DMSO-d6, 300 Hz) δ 8.02 (s, 1H), 7.90 (s, 1H), 7.60 (s, 1H), 4.65 (bs, 1H),
4.46 (s, 1H), 3.62 (s, 1H), 3.19 (s, 3H) 3.11 (s, 3H), 2.18 (dd, J = 14.3, 9.4 Hz, 1H),
1.78 (dd, J = 14.3, 4.1 Hz, 1H); 13C NMR (DMSO-d6, 75 MHz) δ 160.6 (s), 157.8 (s),
123.1 (s), 107.3 (s), 104.5 (s), 102.8 (s), 91.1 (s), 88.9 (d), 55.1 (q), 52.7 (d), 49.3 (q),
43.3 (t), 34.5 (t); HRMS m/z calcd for C13H15N4O4Br3 [M++1]: 532.0000; found
532.7000.
O
O
Br
Br
H MeO
N
H
N
MeO
HN
H
N
N
H
NH
O anh pyr, reflux Br
1d, 35%
Br
N
NH
Br
Br
3.45
O
3.46
6,7,8-Tribromo-4-(2-oxo-2,3-dihydro-1H-imidazol-4-ylmethyl)-2H-pyrrolo[1,2a]pyrazin-1-one 3.46. A solution of dimethoxy adduct 3.45 (0.03 g, 0.06 mmol) in 30
mL anhydrous pyridine was refluxed for 1 d. The reaction mixture was concentrated
under reduced pressure. Flash chromatography (CH2Cl2:MeOH(NH3), 9:1 to 17:3) of
the residue afforded 82 mg (0.02 mmol, 35%) of endo pirazinone 3.46 as white solid:
mp 258-260 °C; IR (KBr) : 3416, 3923, 1657, 1630, 1565, 1462, 1393, 1369 cm-1; 1H
NMR (DMSO-d6, 400 Hz) δ 10.86 (d, J = 5.8 Hz, 1H), 9.80 (s, 1H), 9.52 (s, 1H), 6.48
(d, J = 5.8 Hz, 1H), 5.93 (s, 1H), 3.99 (s, 1H);
13
C NMR (DMSO-d6, 100 MHz)
δ 155.3 (s), 154.0 (s), 124.1 (s), 119.1 (s), 117.3 (d), 115.6 (s), 110.3 (s), 106.2 (d),
101.7 (s), 100.9 (s), 26.9 (t).
91
7.3 Dragmacidin A, B and C
O
1. (COCl)2, ether
0°C to rt, 1h
2. CuCN, CH3CN, toluene
ether, 110°C, 7h
53%
N
H
4.21
CN
N
H
4.24
Indolyl-3-carbonyl nitrile 4.24. To anhydrous ether (150 mL) was added indole (10.0
g, 85.3 mmol) at 0 °C. To the solution was dropwise added oxalyl chloride (8.50 mL,
93.8 mmol, 1.2 equiv), and the mixture was stirred for 1 h. To the mixture was then
added copper cyanide (14.2 g, 158 mmol, 1.7 equiv), acetonitrile (10 mL), and toluene
(150 mL) at room temperature, and the mixture was stirred for 7 h at 110 °C. The
reaction mixture was filtered and washed with dry THF, and the liquid part treated
with activated carbon and boiling a few min, filtration and concentration to get crude
mixture, and purified by flash chromatography using CH2Cl2 to give a light brown
solid 4.24 in 36.5 g (6.21 mol, 53%): 1H NMR (DMSO-d6, 300 MHz) δ 12.90 (bs,
1H), 8.63 (d, J = 3.0 Hz, 1H), 8.64 (d, J = 6.5, 2.2 Hz, 1H), 8.05 (dd, J = 6.2, 2.3 Hz,
1H), 7.58 (td×2, J = 6.5, 2.1 Hz, 2H);
13
C NMR (DMSO-d6, 75 MHz) δ 159.0 (s),
141.8 (d), 138.0 (s), 125.4 (d), 124.7 (s), 124.3 (d), 122.8 (d), 116.7 (s), 114.8 (s),
113.8 (d); HRMS m/z calcd for C10H10N2O [M+]: 174.0794; found: 175.0000.
O
O
CN
N
H
4.24
H2, Pd/C
AcOH, 16h
90%
NH2
N
H
4.25
β-oxotryptamine 4.25. A mixture of indolyl-3-carbonyl nitrile (5.00 g, 29.4 mmol)
and 10 % palladium carbon (1.5 g) in 150 mL of acetic acid was placed under a
balloon of hydrogen. After 16 h, the reaction mixture was filtered over the pad of
celite. After evaporation of the filtrate under reduced pressure, the resulting residue
was treated with 20 mL of conc. HCl/EtOH 20% v/v and concentrated. EtOH was
92
added to the resulting residue and concentrated in vacuo. This EtOH
addition/evaporation sequence was repeated three times. The resulting residue was
rinsed with Et2O and decanted. Trituration with EtOH (10 mL) afforded 4.25•HCl
(4.63 g, 0.03 mol, 90%) as a light tan solid. Flash chromatography of the filtrate over
SiO2 using a 9:1 solution of CH2Cl2:MeOH(NH3) as the eluent yielded (10%) of 4.25
as the free base and the resulting filtrate was concentrated: 1H NMR (DMSO-d6, 300
MHz) δ 12.45 (bs, 1H), 8.50 (d, J = 2.9 Hz, 1H), 8.36 (bs, 3H), 8.15 (d, J = 7.1 Hz,
1H), 7.52 (d, J = 7.1 Hz, 1H), 7.23 (td×2, J = 7.0, 1.2 Hz, 2H), 4.34 (d, J = 5.1 Hz,
2H). 13C NMR (DMSO-d6, 75 MHz) δ 195.3 (s), 136.5 (s), 133.0 (d), 125.4 (s), 122.7
(d), 121.6 (d), 121.2 (d), 114.3 (s), 112.1 (d), 48.2 (t); HRMS m/z calcd for C10H11N2O
[M+H]+: 174.0794; found: 175.0871.
O
O
NH2
N
H
4.25
R1
Br2
AcOH, HCO2H
NH2
R2
N
H
4.26 R1 = Br, R2 = H; 59%
4.16 R1 = H, R2 = Br; 21%
5-Bromoxotryptamine 4.26 and 6-Bromooxotryptamine 4.16. To a stirred solution
of oxotryptamine 4.25 (2.00 g, 11.5 mmol) in AcOH (60 mL)-HCO2H (30 mL) was
added bromine (0.6 mL, 11.5 mmol, 1.0 equiv) at 23 °C. After 20 m, the reaction
mixture was concentrated under reduced pressure. Flash chromatography of the
resulting residue over SiO2 using a 19:1-9:1 gradient of CHCl3:MeOH(NH3) as the
eluent gave 4.26 (1.7g, 6.71 mmol, 59%) and 4.16 (0.6 g, 2.37 mmol, 21%)
5-Bromoxotryptamine 4.26. (free base) IR (KBr) : 3354, 1650, 1521, 1490, 1442 cm; H NMR (DMSO-d6, 300 MHz) δ 8.38 (s, 1H), 8.31 (d, J = 1.8 Hz, 1H), 7.45 (d, J =
1 1
8.6 Hz, 1H), 7.33 (dd, J = 8.6, 1.8 Hz, 1H), 3.88 (s, 2H);
13
C NMR (DMSO-d6, 100
MHz) δ 196.3 (s), 136.1 (s), 135.2 (d), 128.0 (s), 126.1 (d), 124.2 (d), 115.3 (s), 115.1
93
(d), 114.6 (s), 49.1 (t); HRFABMS m/z calcd for C10H10N2O79Br [M+H]+: 252.9977;
found: 252.9979.
6-Bromoxotryptamine 4.16. (free base) mp 260-262 °C; IR (KBr) : 3340, 1637,
1597, 1521, 1453 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 8.35 (s, 1H), 8.10 (d, J = 8.4
Hz, 1H), 7.65 (d, J = 1.4 Hz, 1H), 7.31 (dd, J = 8.4, 1.4 Hz, 1H), 3.86 (s, 2H);
13
C
NMR (DMSO-d6, 100 MHz) δ 196.3 (s), 138.3 (s), 134.8 (d), 125.4 (d), 125.3 (s),
123.7 (d), 116.1 (s), 115.7 (d), 115.1 (s), 49.2 (t); HRFABMS m/z calcd for
C10H10N2O79Br [M+H]+: 252.9977; found: 252.9978.
H
N
N
O
R1
R2
NH2
N
H
130°C, Ar
EtOH/xylene
sealed tube
R1
R2
4.25 R1 = R2 = H
4.26 R1 = Br, R2 = H
4.16 R1 = H, R2 = Br
R2
R1
N
N
H
4.27 R1 = R2 = H; 67%
4.28 R1 = Br, R2 = H; 60%
4.17 R1 = H, R2 = Br; 60%
2,5-Bis(3′-indolyl)pyrazine 4.27. A mixture of oxotryptamine (0.50 g, 2.87 mmol) in
p-xylene (60 mL)-EtOH (12 mL) was heated in a sealed tube at 130 °C for 3 d under
nitrogen with the exclusion of air. After cooling to room temperature, the reaction
mixture was exposed to air for 1 d, concentrated and filtered to afford 4.27 (0.3 g, 0.96
mmol, 67%) as a yellow solid. mp 280-282 °C; R (KBr) : 3387, 1558, 1457, 1422,
1341 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 11.63 (bs, 2H), 9.12 (s, 2H), 8.44 (d, J =
7.6 Hz, 2H), 8.23 (d, J = 2.2 Hz, 2H), 7.47 (d, J = 7.9 Hz, 2H), 7.17 (t×2, J = 7.3 Hz,
4H);
13
C NMR (DMSO-d6, 100 MHz) δ 146.7 (s), 140.1 (d), 137.0 (s), 125.6 (d),
125.2 (s), 122.0 (d), 121.5 (d), 120.1 (d), 112.7 (s), 111.9 (d); HRFABMS m/z calcd
for C20H15N4 [M+H]+: 311.1297; found: 311.1300.
94
2,5-Bis(5′-bromo-3′-indolyl)pyrazine 4.28. Following as analogous protocol for
4.27, pyrazine 4.28 as obtained from 4.26 (0.30g, 1.19 mmol) in 32.6 g, 0.07 mmol, 60
% yield. mp 242-244 °C; IR (KBr) : 3415, 1550, 1444, 1428, 1316 cm-1; 1H NMR
(DMSO-d6, 400 MHz) δ 11.87 (d, J = 1.9 Hz, 2H), 9.12 (s, 2H), 8.69 (d, J = 1.9 Hz,
2H), 8.34 (d, J = 2.8 Hz, 2H), 7.48 (d, J = 8.6 Hz, 2H), 7.34 (dd, J = 8.6, 1.9 Hz, 2H);
C NMR (DMSO-d6, 100 MHz) δ 147.2 (s), 141.0 (d), 136.5 (s), 127.9 (s), 127.8 (d),
13
125.4 (d), 124.6 (d), 114.8 (d), 113.7 (s), 113.1 (s); HREIMS m/z calcd for C20H12N4
79
Br2 [M]+: 465.9429; found: 465.9427.
2,5-Bis(6′-bromo-3′-indolyl)pyrazine 4.17. Following as analogous protocol for
4.27, pyrazine 4.17 as obtained from 4.16 (0.30 g, 1.19 mmol) in 32.6 g, 0.07 mmol,
60 % yield. mp 314-316 °C; IR (KBr) : 3393,1549, 1447, 1417, 1172 cm-1; 1H NMR
(DMSO-d6, 400 MHz) δ 11.77 (d, J = 2.8 Hz, 2H), 9.13 (s, 2H), 8.39 (d, J = 8.6 Hz,
2H), 8.27 (d, J = 2.8 Hz, 2H), 7.66 (d, J = 1.7 Hz, 2H), 7.28 (dd, J = 8.6, 1.7 Hz, 2H);
C NMR (DMSO-d6, 100 MHz) δ 147.2 (s), 141.0 (d), 138.7 (s), 127.4 (d), 125.1 (s),
13
124.1 (d), 123.9 (d), 115.6 (s), 115.3 (d), 113.7 (s); HRFABMS m/z calcd for
C20H12N4 79Br2 [M]+: 465.9429; found: 465.9427.
R2
N
H
N
H
N
N
R1
N
H
4.29 R1 = R2 = H; 67%
4.30 R1 = R2 = Et; <5%
4.31 R1 = H, R2 =Et; <5%
N
N
N
H
30 eq NaBH3CN
AcOH
+
R2
N
4.27
N
H
N
R1
H
N
4.32 R1 = R2 = H; <5%
4.33 R1 = R2 = Et; <5%
4.34 R1 = H, R2 = Et; <5%
95
NaBH3CN reduction of 4.27 in CH3CO2H. To a stirred solution of 4.27 (0.10 g, 0.32
mmol) in AcOH at 0 °C (50 mL) under nitrogen was added NaBH3CN (0.60 g, 9.70
mmol, 30 equiv). After 2 h, the reaction mixture was concentrated in vacuo and the
resulting residue was washed with ether and triturated with a small amount of EtOH to
yield trans-2,5- bis(3′-indolyl)piperazine 4.29 (67 mg, 0.21 mmol, 67%) as a colorless
solid. Flash chromatography of the combined ether washing and filtrate using a 19:1
solution of CH2Cl2:MeOH(NH3) as the eluent gave a residue that was subjected to
further purification by PTLC using a 19:1-9:1 CH2Cl2:MeOH(NH3) gradient to yield
five additional minor products 4.30-4.34 graining in yield from 2-4% each.
trans-2,5-Bis(3′-indolyl)piperazine 4.29. mp 160-162 °C; IR (KBr) : 3146, 1629,
1501, 1456, 1836 cm-1; 1H NMR (DMSO-d6, 300 MHz) δ 10.85 (bs, 2H), 7.70 (d, J =
7.7 Hz, 2H), 7.34 (d, J = 8.0 Hz, 2H), 7.25 (d, J = 2.1 Hz, 2H), 7.05 (t, J = 7.0 Hz,
2H), 6.97 (t, J = 7.0 Hz, 2H), 4.07 (dd, J = 10.1, 2.3 Hz, 2H), 3.18 (dd, J = 11.5, 2.3
Hz, 2H), 2.87 (dd, J = 11.5, 10.1 Hz, 2H);
C NMR (DMSO-d6, 100 MHz) δ 137.1
13
(s), 127.0 (s), 122.6 (d), 121.7 (d), 120.1 (d), 119.0 (d), 117.8 (s), 112.3 (d), 55.0 (d),
54.2 (t); HRFABMS m/z calcd for C20H21N4 [M+H]+: 317.1766; found: 317.1761.
cis-2,5-Bis(3′-indolyl)piperazine 4.32. 1H NMR (acetone-d6, 300 MHz) δ 10.05 (bs,
2H), 7.73 (d, J = 7.9 Hz, 2H), 7.58 (d, J = 1.8 Hz, 2H), 7.37 (d, J = 8.0 Hz, 2H), 7.07
(t, J = 8.1 Hz, 2H), 6.98 (t, J = 8.0 Hz, 2H), 4.30 (dd, J = 5.9, 3.5 Hz, 2H), 3.30 (dd, J
= 11.6, 5.9 Hz, 2H), 3.17 (dd, J = 11.6, 3.5 Hz, 2H); 13C NMR (acetone-d6, 100 MHz)
δ 137.1 (s), 127.6 (s), 123.3 (d), 121.4 (d), 119.6 (d), 118.8 (d), 117.9 (s), 111.5 (d),
52.1 (d), 50.2 (t); HRFABMS m/z calcd for C20H21N4 [M+H]+: 317.1766; found:
317.1756.
trans-1,4-Diethyl-2,5-bis(3′-indolyl)piperazine 4.30.
1
H NMR (acetone-d6, 400
MHz) δ 10.16 (bs, 2H), 8.03 (d, J = 7.8 Hz, 2H), 7.43 (d, J = 8.0 Hz, 2H), 7.37 (d, J =
2.3 Hz, 2H), 7.14 (td, J = 8.1, 1.2 Hz, 2H), 7.06 (td, J = 8.0, 1.1 Hz, 2H), 3.87 (dd, J =
96
10.5, 3.0 Hz, 2H), 3.12 (dd, J = 11.1, 3.0 Hz, 2H), 2.74 (q×2, J = 7.4 Hz, 2H), 2.62
(dd, J = 11.1, 10.5 Hz, 2H), 2.02, (q×2, J = 6.9 Hz, 2H), 0.88 (t, J = 7.2 Hz, 6H); 13C
NMR (acetone-d6, 100 MHz) δ 137.5 (s), 127.3 (s), 123.4 (d), 121.7 (d), 120.7 (d),
118.9 (d), 116.6 (s), 111.7 (d), 60.9 (d), 59.7 (t), 48.2 (t), 11.5 (q); HRFABMS m/z
calcd for C24H29N4 [M+H]+: 373.2392; found: 373.2357.
cis-1,4-Diethyl-2,5-bis(3′-indolyl)piperazine 4.33. 1H NMR (acetone-d6, 300 MHz)
δ 10.45 (bs, 2H), 7.97 (d, J = 7.9 Hz, 2H), 7.57 (d, J = 1.7 Hz, 2H), 7.49 (dm, J = 8.1
Hz, 2H), 7.19 (td, J = 8.1, 1.2 Hz, 2H), 7.11 (td, J = 8.0, 1.2 Hz, 2H), 4.46 (bd, J = 8.4
Hz, 2H), 3.51 (dd, J = 12.0, 2.3 Hz, 2H), 3.22 (m, 2H), 2.97 (m, 2H), 2.52 (bs, 2H),
1.03 (t, J = 7.2 Hz, 6H); 13C NMR (acetone-d6, 100 MHz) δ 137.3 (s), 126.9 (s), 125.0
(d), 122.5 (d), 119.9 (d), 119.7 (d), 112.3 (d), 111.3 (s), 59.8 (d), 56.8 (t), 48.2 (t), 10.3
(q); HRFABMS m/z calcd for C24H29N4 [M+H]+: 373.2392; found: 373.2354.
trans-1-Ethyl-2,5-bis(3′-indolyl)piperazine 4.31. 1H NMR (acetone-d6, 300 MHz)
δ 10.11 (bs, 2H), 7.99 (d, J = 7.7 Hz, 1H), 7.84 (d, J = 8.1 Hz, 1H), 7.40 (dm, J = 7.9
Hz, 2H), 7.34 (d, J = 1.8 Hz, 1H), 7.31 (d, J = 2.4 Hz, 1H), 7.13-7.00 (m, 4H), 4.39
(dd, J = 10.3, 2.4 Hz, 1H), 3.64 (dd, J = 10.3, 3.1 Hz, 1H), 3.35 (dd, J = 10.8, 2.4 Hz,
1H), 3.31 (dd, J = 11.7, 10.3 Hz, 1H), 3.08 (dd, J = 11.7, 3.2 Hz, 1H), 2.74 (q×2, J =
7.4 Hz, 1H), 2.33 (dd, J = 10.8, 10.3 Hz, 1H), 2.07 (q×2, J = 6.9 Hz, 1H), 0.90 (t, J =
7.2 Hz, 3H); 13C NMR (acetone-d6, 100 MHz) δ 137.5 (s), 137.3(s), 127.4 (s), 127.1
(s), 123.3 (d), 122.0 (d), 121.7 (d), 121.6 (d), 120.6 (d), 119.9 (d), 118.9 (d), 118.8 (d),
118.3 (s), 116.7 (s), 111.7 (d×2), 61.0 (d), 59.8 (t), 54.9 (t), 54.3 (d), 48.5 (t), 11.3 (q);
HRFABMS m/z calcd for C22H25N4 [M+H]+: 345.2079; found: 345.2072.
cis-1-Ethyl-2,5-bis(3′-indolyl)piperazine 4.34. 1H NMR (acetone-d6, 300 MHz)
δ 10.14 (bs, 2H), 7.81 (d, J = 8.0 Hz, 1H), 7.75 (bs, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.64
(bs, 1H), 7.43-7.37 (m, 2H), 7.12-6.95 (m, 4H), 4.52 (dd, J = 5.4, 3.7 Hz, 1H), 3.98
(dd, J = 6.5, 3.3 Hz, 1H), 3.23 (dd, J = 11.4, 6.5 Hz, 1H), 3.21 (dd, J = 11.2, 5.4 Hz,
97
1H), 3.04 (dd, J = 11.4, 3.3 Hz, 1H), 2.79 (dd, J = 11.2, 3.7 Hz, 1H), 2.48 (m, 1H),
2.27 (m, 1H), 1.03 (t, J = 7.1 Hz, 3H); 13C NMR (acetone-d6, 100 MHz) δ 137.1 (s),
136.8 (s), 128.2 (s), 127.6 (s), 124.4 (d), 124.0 (d), 121.5 (d), 121.4 (d), 119.8 (d),
119.3 (d), 118.9 (d×2), 117.3 (s), 114.8 (s), 111.6 (d×2), 58.3 (d), 55.2 (t), 51.4 (d),
50.1 (t), 49.0 (t), 12.1 (q); HRFABMS m/z calcd for C22H25N4 [M+H]+: 345.2079;
found: 345.2099.
Me
N
N
H
H
N
N
Me
4.35
54%
N
60 eq NaBH3CN
HCO2H
N
N
H
H
N
+
Me
N
4.27
N
H
H
N
N
Me
4.36
<5%
NaBH3CN reduction of 4.27 (50 mg, 0.16 mmol) in formic acid at 0 °C (25 mL) under
nitrogen was added NaBH3CN (0.60 g, 9.70 mmol). After 1 d, the reaction mixture
was concentrated in vacuo. Flash chromatography of the resulting residue using a 19:1
solution of CH2Cl2:MeOH(NH3) as the eluent gave 30 mg (0.09 mmol, 54%) of trans1,4-dimethyl-2,5-bis(3′-indolyl)piperazine 4.35 and minor amount of cis-1,4-dimethyl2,5-bis(3′-indolyl)piperazine 4.36 (<5%).
trans-1,4-Dimethyl-2,5-bis(3′-indolyl)piperazine 4.35. IR (KBr) : 3135, 1621, 1538,
1453, 1325 cm-1; 1H NMR (DMSO-d6, 300 MHz) δ 10.92 (bs, 2H), 7.84 (d, J = 7.7
Hz, 2H), 7.35 (d, J = 8.0 Hz, 2H), 7.29 (d, J = 2.0 Hz, 2H), 7.07 (t, J = 7.4 Hz, 2H),
6.98 (t, J = 7.3 Hz, 2H), 3.53 (dd, J = 10.9, 2.2 Hz, 2H), 2.87 (dd, J = 11.2, 2.2 Hz,
98
2H), 2.57 (dd, J = 11.2, 10.9 Hz, 2H), 1.97 (s, 6H);
13
C NMR (DMSO-d6, 75 MHz)
δ 137.3 (s), 127.1 (s), 124.2 (d), 121.8 (d), 120.6 (s), 119.2 (d), 115.6 (d), 112.3 (d),
63.8 (t), 62.3 (d), 43.8 (q); HRFABMS m/z calcd for C22H25N4 [M+H]+: 345.2079;
found: 345.2079.
cis-1,4-Dimethyl-2,5-bis(3′-indolyl)piperazine 4.36.
1
H NMR (acetone-d6, 300
MHz) δ 10.21 (bs, 2H), 7.81 (d, J = 7.6 Hz, 2H), 7.80 (bs, 2H), 7.42 (d, J = 7.8 Hz,
2H), 7.10 (t, J = 7.7 Hz, 2H), 7.03 (t, J = 7.8 Hz, 2H), 3.95 (bs, 2H), 3.01 (dd, J =
11.1, 6.4 Hz, 2H), 2.63 (dd, J = 11.1, 3.3 Hz, 2H), 2.13 (s, 6H); 13C NMR (acetone-d6,
100 MHz) δ 136.7 (s), 128.5 (s), 125.1 (d), 121.5 (d), 119.4 (d), 119.2 (d), 113.4 (s),
111.8 (d), 58.5 (d and t), 43.1 (q); HRFABMS m/z calcd for C22H25N4 [M+H]+:
345.2079; found: 345.2077.
H
N
H
N
Br
H
N
N
Br
N
N
H
Br
N
H
N
H
4.37
60%
Br
25 eq NaBH3CN
AcOH
+
H
N
4.28
Br
H
N
Br
N
H
N
H
4.38
<5%
NaBH3CN Reduction of 4.28 in CH3CO2H. To a stirred solution of 5bromooxotryptamine (66.0 mg, 0.14 mmol) in acetic acid at 0 °C (25 mL) under
nitrogen was added NaBH3CN (0.22 g, 3.50 mmol, 25 equiv). After 2 h, the reaction
mixture was concentrated in vacuo. Flash chromatography of the resulting residue
using a 19:1-9:1 gradient of CH2Cl2:MeOH(NH3) as the eluent gave 40 mg (0.08
99
mmol, 60%) of trans-2,5-bis(5′-bromo-3′-indolyl)piperazine 4.37 and minor amount
of cis-2,5-bis(5′-bromo-3′-indolyl)piperazine 4.38 (<5%).
trans-2,5-Bis(5′-bromo-3′-indolyl)piperazine 4.37. mp 250-252 °C; IR (KBr) : 3127,
1569, 1490, 1455, 1310 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 11.10 (bs, 2H), 7.93
(d, J = 1.5 Hz, 2H), 7.32 (d, J = 8.6 Hz, 2H), 7.32 (d, J = 2.0 Hz, 2H), 7.17 (dd, J =
8.6, 1.4 Hz, 2H), 4.03 (dd, J = 11.0, 2.6 Hz, 2H), 3.13 (dd, J = 11.3, 2.6 Hz, 2H), 1.89
(dd, J = 11.3, 11.0 Hz, 2H);
C NMR (DMSO-d6, 100 MHz) δ 135.9 (s), 128.9 (s),
13
124.5 (d), 124.2 (d), 122.7 (d), 117.5 (s), 114.3 (d), 111.7 (s), 54.6 (d), 54.1 (t);
HRFABMS m/z calcd for C20H19N479Br2 [M+H]+: 472.9976; found: 472.9974.
cis-2,5-Bis(5′-bromo-3′-indolyl)piperazine 4.38. 1H NMR (acetone-d6, 300 MHz)
δ 10.28 (bs, 2H), 7.94 (d, J = 1.9 Hz, 2H), 7.59 (bs, 2H), 7.35 (d, J = 8.6 Hz, 2H), 7.17
(dd, J = 8.6, 1.9 Hz, 2H), 4.25 (dd, J = 5.9, 3.5 Hz, 2H), 3.26 (dd, J = 11.9, 5.9 Hz,
2H), 3.13 (dd, J = 11.9, 3.5 Hz, 2H);
C NMR (acetone-d6, 100 MHz) δ 135.8 (s),
13
129.4 (s), 125.0 (d), 124.0 (d), 122.6 (d), 117.6 (s), 113.4 (d), 111.7 (s), 52.0 (d), 49.9
(t); HRFABMS m/z calcd for C20H19N479Br2 [M+H]+: 472.9976; found: 472.9969.
100
Me
N
H
N
Br
N
H
Br
H
N
N
H
Br
1.31 dragmacidin A
14%
N
50 eq NaBH3CN
HCO2H
N
Br
N
H
+
4.17
Me
N
Br
N
H
H
N
Br
N
Me
1.32 dragmacidin B
56%
Dragmacidin A and dragmacidin B. To a stirred solution of 4.17 (66.0 mg, 0.14
mmol) in formic acid at 0 °C (25 mL) under nitrogen was added NaBH3CN (0.45 g,
7.10 mmol, 50 equiv). After 4 h, the reaction mixture was concentrated in vacuo. Flash
chromatography
of
the
resulting
residue
using
a
19:1-9:1
gradient
of
CH2Cl2:MeOH(NH3) as the eluent gave a residue that was subjected to further
purification by PTLC using 19:1-9:1 CH2Cl2:MeOH(NH3) gradient to yield 10 mg
(0.02 mmol, 14%) of dragmacidin A 1.31 and 40 mg of dragmacidin B 1.32 (0.08
mmol, 56%).
Dragmacidin A 1.31. 1H NMR (acetone-d6, 400 MHz) δ 10.33 (bs, 2H), 7.94 (d, J =
8.5 Hz, 1H), 7.84 (d, J = 8.4 Hz, 1H), 7.64 (bs, 2H), 7.41 (bs, 1H), 7.38 (bs, 1H), 7.20
(dm, J = 8.0 Hz, 2H), 4.44 (dd, J = 10.4, 2.6 Hz, 1H), 3.41 (dd, J = 10.5, 3.0 Hz, 1H),
3.31 (dd, J = 11.0, 10.5 Hz, 1H), 3.20 (dd, J = 11.0, 2.6 Hz, 1H), 3.09 (dd, J = 11.0,
3.0 Hz, 1H), 2.39 (dd, J = 11.0, 10.4 Hz, 1H), 2.11 (s, 3H); 13C NMR (acetone-d6, 100
MHz) δ 138.7 (s), 138.5 (s), 126.5 (s×2), 124.9 (d), 123.7 (d), 122.6 (d), 122.4 (d×2),
122.1 (d), 118.6 (s), 117.1 (s), 115.3 (s), 115.2 (s), 115.04 (d), 114.9 (d), 64.4 (t), 63.2
101
(d), 54.6 (t), 54.3 (d), 44.2 (q); HRFABMS m/z calcd for C21H21N479Br2 [M+H]+:
487.0133; found: 487.0131.
Dragmacidin B 1.32. 1H NMR (acetone-d6, 300 MHz) δ 10.29 (bs, 2H), 7.92 (d, J =
8.5 Hz, 2H), 7.61 (d, J = 1.8 Hz, 2H), 7.37 (d, J = 1.4 Hz, 2H), 7.17 (dd, J = 8.5, 1.8
Hz, 2H), 3.58 (dd, J = 10.5, 2.9 Hz, 2H), 2.92 (dd, J = 11.3, 2.9 Hz, 2H), 2.62 (dd, J =
11.3, 10.5 Hz, 2H), 2.04 (s, 6H);
C NMR (acetone-d6, 75 MHz) δ 138.3 (s), 126.1
13
(s), 124.5 (d), 122.2 (d), 122.0 (d), 116.5 (s), 114.8 (s), 114.6 (d), 63.6 (t), 62.3 (d),
43.0 (q); HREIMS m/z calcd for C22H23N479Br2 [M+H]+: 501.0289; found: 501.0297.
H
N
Br
H
N
5
2
N
H
Br
H
N
Br
4.39 trans-dragmacidin C
61%
N
N
Br
N
H
N
H
25 eq NaBH3CN
AcOH
+
H
N
4.17
Br
H
N
5
2
N
H
Br
N
H
1.33 cis-dragmacidin C
<5%
trans-Dragmacidin C 4.39 and cis-dragmacidin C 1.33. To a stirred solution of 6bromooxotryptamine (65.0 mg, 0.14 mmol) in acetic acid at 0 °C (30 mL) under
nitrogen was added NaBH3CN (0.22 g, 3.50 mmol, 25 equiv). After 2 h, the reaction
mixture was concentrated in vacuo. Flash chromatography of the resulting residue
using a 19:1-9:1 gradient of CH2Cl2:MeOH(NH3) as the eluent gave 40 mg (0.08
102
mmol, 61%) of trans-dragmacidin C 4.39 and minor amounts of cis-dragmacidin B
1.33 (<5%).
trans-Dragmacidin C 4.39. mp 268-270 °C; IR (KBr): 3133, 1617, 1539, 1455, 1328
cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 11.03 (bs, 2H), 7.69 (d, J = 8.5 Hz, 2H), 7.52
(d, J = 1.7 Hz, 2H), 7.29 (d, J = 2.3 Hz, 2H), 7.10 (dd, J = 8.5, 1.7 Hz, 2H), 4.05 (dd,
J = 10.1, 2.4 Hz, 2H), 3.13 (dd, J = 11.5, 2.4 Hz, 2H), 2.86 (dd, J = 11.5, 10.1 Hz,
2H);
13
C NMR (DMSO-d6, 75 MHz) δ 138.0 (s), 126.0 (s), 123.9 (d), 121.98 (d),
121.95 (d), 117.9 (s), 114.8 (d), 114.5 (s), 54.6 (d), 53.9 (t); HRFABMS m/z calcd for
C20H19N479Br2 [M+H]+: 472.9976; found: 472.9976.
cis-Dragmacidin B 1.33. 1H NMR (acetone-d6, 300 MHz) δ 10.26 (bs, 2H), 7.72 (d, J
= 8.5 Hz, 2H), 7.62 (bs, 2H), 7.60 (d, J = 1.8 Hz, 2H), 7.13 (dd, J = 8.5, 1.8 Hz, 2H),
4.32 (dd, J = 5.4, 3.4 Hz, 2H), 3.28 (dd, J = 11.8, 5.4 Hz, 2H), 3.18 (dd, J = 11.8, 3.4
Hz, 2H); 13C NMR (DMSO-d6, 75 MHz) δ 137.8 (s), 126.5 (s), 125.1 (d), 121.8 (d×2),
117.1 (s), 114.7 (d), 114.3 (s), 51.5 (d), 49.9 (t); HREIMS m/z calcd for C20H19N479Br2
[M+H]+: 472.9976; found: 472.9968.
7.5 Salacin
NH2
N
H
5.18·HCl
1.1 eq NCS
20% HCO2H/
AcOH, 0°C
70%
NH2
N
H
Cl
5.15a·HCl
Chlorotryptamine 5.15a. To a suspension of tryptamine (10.0 g, 62.4 mmol) in 200
mL ethanol, was added conc. HCl at 0 °C. The reaction mixture was stirred about 20
m at room temperature. The reaction mixture was filtered, washed with CH2Cl2 and
EtOH and dried with vacuum pump. Tryptamine salt (1.00 g, 56.0 mmol) was
dissolved in 15 mL formic acid and 50 mL acetic acid and continued stirred until
completely dissolved. N-chlorosuccinimide (0.74 g, 55.4 mmol, 1.1 equiv) was added
103
and stirred about 20 m. The reaction was concentrated under reduced pressure. Flash
chromatography (CH2Cl2:MeOH(NH3), 19:1) of the residue afforded 0.69 g (3.55
mmol, 70%) of 5.15a: 1H NMR (CDCl3, 300 MHz) δ 7.50 (d, J = 7.1 Hz, 1H), 7.24 (d,
J = 7.89 Hz, 1H), 7.16 (dt, J = 7.1, 1.20 Hz, 1H), 7.11 (dt, J = 7.1, 1.2 Hz, 1H), 3.04 (t,
J = 6.6 Hz, 2H), 2.90 (t, J = 6.3 Hz, 2H).
NH2
NH2
1.2 eq NBS
20% HCO2H/AcOH
0°C, 80%
N
H
N
H
5.18·HBr
Br
5.15b·HBr
Bromotryptamine 5.15b. To a suspension of tryptamine (1.00 g, 6.24 mmol) in 100
mL ether, was added 1.06 mL HBr (1.5 equiv) at room temperature. The reaction
mixture was stirred for 20 m at room temperature. The reaction mixture was filtered
and washed with CH2Cl2 and EtOH and dried with vacuum pump. Tryptamine salt
(1.00 g, 4.14 mmol) was dissolved in 15 mL formic acid and 50 mL acetic acid and
continued to stir until completely dissolved. N-bromosuccinimide (0.81 g, 4.56 mmol,
1.2 equiv) was added and stirred for 20 m. The reaction was concentrated under
reduced pressure. Flash chromatography (CH2Cl2:MeOH(NH3), 19:1) of the residue
afforded 1.07 g (4.37 mmol, 80%) of 5.15b: 1H NMR (CDCl3, 300 MHz) δ 11.80 (s,
1H), 7.85 (s, 3H), 7.55 (dd, J = 7.1, 1.3 Hz, 1H), 7.30 (dd, J = 7.1, 1.2 Hz, 1H), 7.11
(t, J = 7.0 Hz, 1H), 7.04 (t, J = 7.0 Hz, 1H), 2.96 (s, 4H); 13C NMR (CDCl3, 100 MHz)
δ 137.1 (s), 127.7 (s), 122.6 (d), 120.3 (d), 118.5 (d), 111.8 (s), 110.2 (d), 109.9 (s),
39.6 (t), 23.6 (t).
Br
O
1. Mg, THF
2. EtCHO
52%
O
5.28
O
O
H
OH
5.30
1-[1,3]-Dioxolan-2-yl-pentan-3-ol 5.33. The Grignard reagent was prepared by
addition of 4.0 g (22.1 mmol, 1.4 equiv) of 2-(2-bromoethyl)-1,3-dioxolane 5.28 in 30
104
mL of dry THF to 0.57 g (23.6 mmol, 1.4 equiv) of Mg over a period of 1.5 h at 30-35
°C. Stirring was continued for 1 h at 30 °C; then cool down to 0 °C, a solution of
propyl aldehyde (0.98 g, 16.9 mmol, 1 equiv) in 15 mL of THF was added dropwise.
The mixture was stirred about 2 h and was poured into ice-cold aqueous NH4Cl
solution, extracted twice with ether, washed with water, dried over Na2SO4 and
evaporated. Flash chromatography (Hexane:EtOAc, 7:3) of the residue afforded 2.96 g
(0.01 mmol, 52%) of alcohol 5.33 as colorless oil: 1H NMR (CDCl3, 300 MHz) δ 4.75
(t, J = 4.5 Hz, 1H), 3.98 (m, 2H), 3.89 (m, 2H), 3.41 (m, 1H), 2.82 (bs, 1H), 1.73 (m,
2H), 1.51 (m, 2H), 1.44 (m, 2H) 0.97 (t, 3H).
O
O
O
H
OH
5.30
DMP, DCM
90%
O
H
5.31
O
1-[1,3]-Dioxolan-2-yl-pentan-3-one 5.31. To a solution of alcohol 5.30 (0.10 g, 0.39
mmol) in CH2Cl2 (16 mL) at 0 °C was added dropwise of Dess-Martin periodinane
(0.25 g, 0.59 mmol, 1.5 equiv) in CH2Cl2 (34 mL) during 2 m. The mixture was stirred
for 2 h at room temperature and was poured into saturated NaHCO3 (100 mL)
containing Na2S2O3 (25.0 g). The mixture was stirred for 15 m, the phases were
separated. The organic phase was washed with saturated aqueous NaHCO3 (30 mL),
water (35 mL) and concentrated under reduced pressure to give oil crude. Flash
chromatography (Hexane:EtOAc, 6:4) of the residue afforded 0.089 g (0.35 mmol,
90%) of ketone 5.31 as colorless oil: 1H NMR (CDCl3, 400 MHz) δ 4.85 (t, J = 4.3
Hz, 1H), 3.90 (m, 2H), 3.79 (m, 2H), 2.49 (t, J = 7.3 Hz, 2H), 2.39 (q, J = 7.4 Hz, 2H),
1.92 (m, 2H), 1.00 (t, J = 7.1 Hz, 3H); 13C NMR (CDCl3, 75 MHz) δ 210.6 (s), 103.5
(d), 65.1 (t×2), 36.2 (t), 36.0 (t), 27.9 (t), 8.0 (q).
105
O
O
O
TsOH, acetone-H2O
reflux, 75%
H
O
H
O
5.25
5.31
4-Oxo-hexanal 5.25. Ketone (0.01 g, 0.04 mmol) and p-toluenesulfonic acid
monohydrate (3.74 g, 0.02 mmol, 0.5 equiv) in 1.6 mL of acetone/water (1:1) was
reflux for 2-3 h. After cooling, the reaction mixture was extracted with CH2Cl2 (3 × 50
mL). The combined organic extracts was washed with saturated NaHCO3, dried
MgSO4 and concentrated under reduced pressure. Flash chromatography (100%
CH2Cl2) of the residue afforded 0.003 g (0.03 mmol, 75%) of 5.25 as colorless oil: 1H
NMR (CDCl3, 300 MHz) δ 9.76 (s, 1H), 2.71 (m, 4H), 2.45 (q, J = 7.4 Hz, 2H), 1.03
(t, J = 5.0 Hz, 3H); 13C NMR (CDCl3, 75 MHz) δ 209.7 (s), 201.1 (d), 37.9 (t), 36.2
(t), 34.6 (t), 8.2 (q).
NH2
N
H
5.15a
Cl
1. 2.1 eq 5.25, DCM, -78°C, 2h
2. 6 eq TFA, 0°C, 2h
30%
N
H
N
5.32
7,11-Diethyl-5,12-dihydro-6H-6a,12-diaza-indeno[1,2-a]fluorine 5.32.
Chlorotryptamine 5.15a (0.55 g, 2.84 mmol) was dissolved in 10 mL of dry CH2Cl2,
aldehyde (0.67 g, 5.95 mmol, 2.1 equiv) and MgSO4 (674 mg, 2.1 equiv) were added
at -78 °C. The mixture was stirred at room temperature for 2 h and added TFA (1.31
mL, 17.0 mmol, 6 equiv) at 0 °C. The reaction mixture was stirred for 2 h, and
extracted with CH2Cl2. The phases were separated, and the organic phase was washed
with saturated aqueous Na2CO3 and brine. Dichloromethane was dried with MgSO4,
filtered and concentrated under reduced pressure to give green oil. Flash
chromatography (Hexane:EtOAc, 19:1) of the residue afforded 0.27 g (1.39 mmol,
30%) of 5.34: 1H NMR (CDCl3, 400 MHz) δ 7.91 (bs, 1H), 7.49 (dt, J = 7.2, 1.64 Hz,
1H), 7.33 (dd, J = 7.4, 1.6 Hz, 1H), 7.15-1.28 (m, 3H), 7.03 (dd, J = 6.3, 0.7 Hz, 1H),
106
4.30 (ddd, J = 8.1, 5.8, 4.6 Hz, 2H), 3.16 (ddd, J = 8.1, 5.8, 4.6 Hz, 2H), 3.01 (q, J =
7.58 Hz, 2H), 2.82 (q, J = 7.4 Hz, 2H), 1.34 (t, J = 5.9 Hz, 6H); 13C NMR (CDCl3, 75
MHz) δ 142.6 (s), 137.0 (s), 136.3 (s), 134.8 (s), 127.8 (s), 127.7 (s), 122.9 (d), 121.9
(s), 121.5 (d), 120.9 (d), 118.4 (d), 118.2 (d), 111.2 (d), 109.3 (s), 107.2 (d), 96.6 (s),
43.3 (t), 26.8 (t), 25.0 (t), 20.3 (t), 15.3 (q), 13.2 (q).
O
O
O
5.33
1. 6 eq TEA
MeOH, 2h
2. PCC, NaOAc
DCM, rt, 3h
85% in 2 steps
OMe
H
O
5.35
4-Oxo-butyric acid methyl ester 5.35. 5.0 g (58.1 mmol) of γ-Butylrolactone was
added to 100 mL of MeOH and 48.5 mL (34.8 mmol, 6 equiv) of triethylamine was
added. The reaction was stirred at room temperature for 2 h and concentrated under
reduced pressure. The resulting crude methyl ω-hydroxy ester 1.00 g (8.47 mmol) was
taken up in CH2Cl2 (10 mL) and sodium acetate (0.21 g, 2.54 mmol, 0.3 equiv) and
pyridinium chlorochromate (PCC) (2.74 g, 12.7 mmol, 1.5 equiv) were added. After
stirring at room temperature for 1.5 h, Et2O (300 mL) was added. The reaction mixture
was filtered through florisil, and the filtrate was concentrated at reduced pressure.
Flash chromatography (Hexane:Et2O, 3:7) of the residue afforded aldehyde 5.35 in
0.84 g. 7.20 mmol, 85% yield: 1H NMR (CDCl3, 300 MHz) δ 9.80 (s, 1H), 3.71 (s,
3H), 2.56 (dd, J = 7.2, 1.2 Hz, 2H), 2.41 (dd, J = 7.3, 1.3 Hz, 2H).
107
O
NH2
1. 1.5 eq 5.35, DCM, 0°C
2. 6 eq TFA, 0°C, 2h
Br
N
H
5.15b
HN
O
HN
NH
O
N
O
OMe
5.37
5.36
column
H:EtOAc:MeOH(NH3)
6:3.5:0.5
70%
Spirooxindole 5.37. To dissolved bromotryptamine 5.15b (0.40 g, 1.68 mmol) in 10
mL dry CH2Cl2, was added ester aldehyde (0.29 g, 2.51 mmol, 1.5 equiv) and MgSO4
(303 mg, 1.5 equiv) under nitrogen. The reaction was stirred for 2 h and cool down to
0 °C, then added TFA (1.15 g, 10.1 mmol, 6 equiv). The mixture was stirred at room
temperature for 2 h and extracted with CH2Cl2. The phases were separated, and the
organic
phase
was
washed
with
saturated
aqueous
Na2CO3
and
brine.
Dichloromethane was dried with MgSO4, filtered and concentrated under reduced
pressure to give foam solid. Flash chromatography (CH2Cl2:MeOH(NH3), 19:1) of the
residue afford 5.36 followed by flash chromatography (Hexane:EtOAc:MeOH(NH3),
6:3.5:0.5) of 5.36 gave 0.27 g (1.11 mmol, 70%) of spiro 5.37:
5.36: 1H NMR (CDCl3, 300 MHz) δ 8.68 (bs, 1H), 7.21 (dt, J = 7.5 Hz, 1H), 7.15 (d, J
= 8.9 Hz, 1H), 7.52 (t, J = 7.5 Hz, 1H), 6.93 (d, J = 8.9 Hz, 1H), 3.6 (m, 1H), 3.50 (s,
3H), 3.37 (m, 1H), 3.38 (m, 1H), 2.50 (m, 1H), 2.35 (m, 1H), 2.24 (m, 1H), 2.18 (m,
1H), 1.44 (m, 2H); 13C NMR (CDCl3, 100 MHz) δ 81.9 (s), 174.1 (s), 140.8 (s), 132.3
(s), 128.4 (d), 124.5 (d), 123.0 (d), 110.4 (d), 68.7 (d), 58.6 (s), 52.1 (q), 46.3 (t), 38.9
(t), 32.2 (t), 26.4 (t).
5.37: mp 202-204 °C; 1H NMR (CDCl3, 400 MHz) δ 8.68 (bs, 1H), 7.30 (t, J = 7.7 Hz,
1H), 7.08 (t, J = 7.45 Hz, 1H), 7.01 (d, J = 7.9 Hz, 1H), 6.93 (d, J = 7.4 Hz, 1H), 4.44
(dt, J = 5.3, 2.7 Hz, 1H), 3.94 (q, J = 11.6, 2.6 Hz, 1H), 3.94 (t, J = 10.5 Hz, 1H), 2.83
(q, J = 9.9, 3.1 Hz, 1H), 2.30 (m, 2H), 2.05 (m, 1H), 1.30 (m, 2H); 13C NMR (CDCl3,
108
100 MHz) δ 178.5 (s), 175.9 (s), 140.9 (s), 130.4 (s), 129.1 (d), 124.1 (d), 123.7 (d),
110.7 (d), 69.5 (d), 56.3 (s), 41.2 (t), 38.1 (t), 34.2 (t), 19.5 (t).
O
H
5.38
pyrrolidine, K2CO3
overnight, rt
40%
N
5.39
1-Pyrrolidino-butene 5.39. A mixture of pyrrolidine (80.0 g, 1.12 mol, 2 equiv) and
K2CO3 (77.8 g, 0.56 mol, 1 equiv) were stirred at 0 °C under nitrogen, freshly distilled
butyraldehyde 5.38 (41.5 g, 0.56 mol, 1 equiv) was added dropwise. After one night
stirring at room temperature, the reaction mixture was diluted with benzene (150 mL)
and filtrated. Vacuum distillation of the residue gave 1-pyrrolidine-butene 5.39 (bp. 35
°C, 0.6 mmHg) as colorless oil in 55.4 g (0.44 mol, 40%): 1H NMR (CDCl3, 300
MHz) δ 6.20 (d, J = 13.6 Hz, 1H), 4.19 (dt, J = 13.5, 6.7 Hz, 1H), 2.98 (bt, 4H), 2.02
(dt, J = 14.2, 6.7 Hz), 1.86 (bt, 2H), 0.99 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 100
MHz) δ 135.6 (d), 101.4 (d), 49.6 (t×2), 25.2 (t×2), 24.1 (t), 16.7 (q).
N
5.39
CO2CH3 , CH3CN
0°C to rt, 6h;
reflux, 40h;
AcOH, reflux, 8h
54%
CO2CH3
H
O
5.40
4-Formyl-hexanoate methyl ester 5.40. A mixture of 1-pyrrolidino-butene 5.39 (15.0
g, 0.12 mmol) and acetonitrile (75 mL) was stirred at 0 °C under nitrogen. A solution
of methyl acrylate 13.5 mL (0.14 mmol, 1.2 equiv) in 25 mL acetonitrile was added
dropwise. The stirring was continued during 6 h at room temperature, and then 40 h
under refluxing. Then 7.5 mL acetic acid in 50 mL water was added in small portions.
The refluxing was continued for another 8 h. The mixture was allowed to cool and
saturated with saturated NaCl. The water phase was isolated and extracted with ether.
The organic phase are gathered and distilled (bp. 65-68 °C, 0.6 mmHg) to give 14.0 g
(0.11 mol, 54%) of 5.40 as colorless oil: 1H NMR (CDCl3, 300 MHz) δ 9.58 (s, 1H),
109
3.65 (s, 3H), 2.33 (m, 2H), 2.24 (m, 1H), 1.95 (m, 1H), 1.78 (m, 1H), 1.67 (m, 1H),
1.56 (m, 1H), 0.93 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 75 MHz) δ 204.9 (d), 173.9
(s), 52.1 (q), 46.3 (d), 31.8 (t), 23.5 (t), 22.2 (t), 9.9 (q).
CO2CH3
H
O
5.40
OH
, p-TsOH
HO
benzene, reflux, 1d
Dean-Stark trap
83%
CO2CH3
O
O
5.41
4-[1,3]Dioxolan-2-yl-hexanoic acid methyl ester 5.41. A mixture of aldehyde methyl
ester 5.40 (6.90 g, 43.7 mmol), 40 mL of dry benzene, 13.0 mg (0.07 mmol, 1.6 equiv)
of p-toluenesulfonic acid and 2.71 g (43.7 mmol, 1 equiv) of ethylene glycol was
purged with nitrogen for 1 h and the clear, colorless reaction mixture was then heated
at reflux with vigorous stirring for 24 h. During this time water was collected in a
Dean-Stark trap. After the orange-colored reaction mixture was allowed to cool to
room temperature, the benzene was removed under reduced pressure at 40 °C to leave
a dark red liquid. This was then taken up into 70 mL of dichloromethane and washed
with NaHCO3 (25 mL) and NaCl (25 mL), dried, filtered and concentrated to get a
deep red liquid. The residue was purified via distillation (bp. 95 °C, 0.6 mmHg) to
yield 7.32 g (0.04 mol, 83%) of acetal ester 5.41 as colorless oil: 1H NMR (CDCl3,
300 MHz) δ 4.80 (d, J = 4.0 Hz, 1H), 3.90 (m, 2H), 3.80 (m, 2H), 3.58 (s, 3H), 2.43 (t,
J = 8.0 Hz, 2H), 1.58 (m, 1H), 1.57 (m, 1H), 1.56 (m, 1H), 1.52 (m, 1H), 1.32 (m,
1H), 0.93 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 100 MHz) δ 174.7 (s), 106.8 (d), 65.3
(t), 65.2 (t), 51.9 (q), 42.7 (d), 32.4 (t), 24.1 (t), 22.5 (t), 11.8 (q).
CO2CH3
O
O
5.41
1M LiAlH4, THF
0°C to reflux, overnight
85%
OH
O
O
5.42
4-(1,3-Dioxolan-2-yl)-1-hexanol 5.42. With stirring 1.48 mL of a 1 M solution of
LiAlH4 (1.48 mmol, 1 equiv) in THF was added dropwise over 1 h at 0 °C, to a
solution of acetal ester (0.30 g, 1.48 mmol) in 2 mL THF under nitrogen. The mixture
110
was then heated at reflux for overnight and cooled to 0 °C and 0.058 mL of water in
0.29 mL THF was added dropwise with rapid stirring. Over 1 h, 0.058 mL of 15 %
NaOH was then added at 0 °C, followed by 0.175 mL of water. The mixture was then
filtered and the solids were washed with 10 mL of ether. Concentrating the filtrate at
42 °C under vacuum, solution of the concentrate was dissolved in 10 mL of ether,
washing with 1.16 mL 5% NaOH and brine and concentrated under reduced pressure.
The residue was purified via distillation (bp. 100-103 °C, 0.6 mmHg) to yield 0.22 g
(1.26 mmol, 85%) of alcohol 5.42 as colorless oil: 1H NMR (CDCl3, 400 MHz) δ 4.82
(d, J = 3.9 Hz, 1H), 3.90 (m, 2H), 3.80 (m, 2H), 3.65 (dd, J = 12.1, 6.1 Hz, 2H), 1.65
(m, 2H), 1.61 (m, 2H), 1.43 (m, 2H), 0.96 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 100
MHz) δ 107.0 (d), 65.3 (t×2), 63.5 (t), 43.0 (d), 30.8 (t), 24.8 (t), 22.6 (t), 11.8 (q).
O
OH
O
O
PCC, NaOAc
3h, quant
H
O
5.41
O
5.27
4-(1,3-Dioxolan-2-yl)hexanal 5.27. To 0.96 g (4.45 mmol, 1.5 equiv) of PCC, 0.07 g
(0.89 mmol, 0.3 equiv) of sodium acetate and 10 mL dichloromethane, stirred
vigorously, was added 0.52 g (2.97 mmol) of acetal alcohol in 5 mL dry CH2Cl2. The
reaction mixture was stirred at room temperature for 3 h. Ether was added; the reaction
mixture was filtered and concentrated under reduced pressure. Flash chromatography
(CH2Cl2:Et2O, 19:1) of the residue afforded aldehyde 5.27 in quantitative yield (0.51
g, 2.96 mmol): 1H NMR (CDCl3, 400 MHz) δ 9.80 (t, J = 1.6 Hz, 1H), 4.79 (d, J = 4.0
Hz, 1H), 3.98 (m, 2H), 3.87 (m, 2H), 2.56 (td, J = 7.6, 1.6 Hz, 2H), 1.83 (m, 1H), 1.73
(m, 1H), 1.63-153 (m, 2H), 1.40 (m, 1H), 0.98 (t, J = 7.5 Hz, 3H); 13C NMR (CDCl3,
100 MHz) δ 203.2 (d), 106.8 (d), 65.3 (t), 65.2 (t), 42.7 (d), 42.3 (t), 22.7 (t), 21.1 (t),
11.8 (q).
111
1. 1.5 eq 5.27, DCM, 0°C
2. 6 eq TFA, 0°C, 2h
20%
Br-tryptamine
5.15b
N
H
N
5.43
3-Ethyl-1,2,6,7,12,12b-hexahydro-indolo[2,3-a]quinolizine
5.43.
To
dissolved
bromotryptamine (0.40 g, 1.68 mmol) in 10 mL dry CH2Cl2, was added acetal
aldehyde (0.43 g, 2.51 mmol, 1.5 equiv) and MgSO4 (303 mg, 1.5 equiv) under
nitrogen. The reaction was stirred for 2 h and cool down to 0 °C, then added TFA
(0.95 g, 8.40 mmol, 6 equiv). The mixture was stirred at room temperature for 2 h,
extracted with CH2Cl2. The phases were separated, and the organic phase was washed
with saturated aqueous Na2CO3 and brine. Dichloromethane was dried with MgSO4,
filtered and concentrated under reduced pressure to give foam solid. Flash
chromatography (CH2Cl2:MeOH(NH3), 19:1) of the residue afforded 0.08 g (0.36
mmol, 20%) of 5.43: mp 214-216 °C; 1H NMR (CDCl3, 400 MHz) δ 7.50 (d, J = 7.7
Hz, 1H), 7.39 (d, J = 8.2 Hz, 1H), 7.24 (dd, J = 14.3, 7.1 Hz, 1H), 7.15 (dd, J = 14.2,
7.1 Hz, 1H), 6.85 (s, 1H), 4.15 (dd, J = 8.8 Hz, 1H), 3.42 (m, 1H), 3.10 (m, 1H), 2.91
(m, 1H), 2.80 (m, 1H), 2.48 (m, 2H), 2.22 (q, J = 7.4 Hz, 2H), 2.05 (m, 2H), 1.18 (t, J
= 7.4 Hz, 3H);
C NMR (CDCl3, 100 MHz) δ 137.4 (s), 136.2 (s), 127.4 (s), 125.1
13
(s), 122.2 (d), 120.4 (d), 118.6 (d), 118.2 (d), 109.6 (d), 109.4 (s), 54.6 (d), 43.4 (t),
32.8 (t), 31.5 (t), 31.1 (t), 22.5 (t), 14.1 (q).
O
O
O
BnBr, NaH
THF, Bu4NI, 95%
H
O
H
OBn
OH
5.29
5.44a
2-(3-Benzyloxy-pentyl)-[1,3]dioxolane 5.44a. To a suspension of NaH (0.04 g, 1.56
mmol, 4 equiv) in dry CH2Cl2 was added a solution of acetal alcohol (0.10 g, 0.39
mmol) in dry THF, followed by addition of benzyl bromide (0.27 g, 1.56 mmol, 4
equiv) and tetrabutyl ammonium iodine (0.58 g, 1.56 mmol, 4 equiv) at 0 °C. The
resulting mixture was stirred overnight at room temperature. The reaction mixture was
112
diluted by saturated NH4Cl and Et2O. The organic layer was washed with brine and
dried over MgSO4, filtered and concentrated under reduced pressure. Flash
chromatography (Hexane:EtOAc, 9:1) of the residue afforded 0.12 g (0.34 mmol,
95%) of 5.44a as colorless oil: 1H NMR (CDCl3, 400 MHz) δ 7.31 (m, 5H), 4.91 (t, J
= 4.28 Hz, 1H), 4.55 (d, J = 1.6 Hz, 2H), 3.98 (m, 2H), 3.89 (m, 2H), 3.41 (m, 1H),
1.72 (m, 2H), 1.64 (m, 2H), 1.44 (m, 2H), 1.00 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3,
100 MHz) δ 139.0 (s), 128.8 (d×2), 128.3 (d×2), 127.8 (d), 105.1 (d), 79.5 (d), 71.1 (t),
65.3 (t×2), 30.1 (t), 27.9 (t), 26.3 (t), 9.9 (q).
O
O
O
H
TsOH, acetone-H2O
reflux, 85%
H
OBn
OBn
5.44a
5.44
4-Benzyloxy-hexanal 5.44. The benzyl ether (0.094 g, 0.003 mmol) and ptoluenesulfonic acid monohydrate (0.476 g, 0.002 mmol, 0.7 equiv) was dissolved in
1.6 mL of acetone:water (1:1) and reflux overnight. After cooling, the reaction mixture
was extracted with CH2Cl2 (3 × 50 mL). The reaction mixture was extracted with Et2O
(3 × 10 mL). The combined ether extracts was washed with saturated NaHCO3, dried
MgSO4 and concentrated under reduced pressure. Flash chromatography 100%
CH2Cl2 of the residue afforded 0.047 g (0.23 mmol, 85%) of 5.44 as colorless oil: 1H
NMR (CDCl3, 400 MHz) δ 9.82 (s, 1H), 7.3-7.5 (m, 5H), 4.57 (d, J = 11.5 Hz, 1H),
4.46 (d, J = 11.5 Hz, 1H), 3.40 (m, 1H), 2.55 (tm, J = 7.2 Hz, 2H), 1.94 (m, 1H), 1.85
(m, 1H), 1.67 (m, 1H), 1.57 (m, 1H), 0.98 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 100
MHz) δ 203.0 (s), 139.0 (s), 128.8 (d×2), 128.3 (d×2), 128.0 (d), 79.5 (d), 71.3 (t),
40.5 (t), 26.6 (t), 26.3 (t), 9.9 (q).
113
O
NH2
N
H
HN
NH
OBn
1. 1.5 eq 5.44, DCM, 0°C
2. 6 eq TFA, 0°C, 2h
50%
Cl
5.15a
5.45
Spirooxindole 5.45. To dissolved chlorotryptamine (0.14 g, 0.72 mmol) in 10 mL dry
CH2Cl2, was added aldehyde (0.22 g, 1.08 mmol, 1.5 equiv) and MgSO4 (129 mg, 1.5
equiv) under nitrogen. The reaction was stirred for 2 h and cool down to 0 °C, then
added TFA (0.49 g, 4.32 mmol, 6 equiv). The mixture was stirred at room temperature
for 2 h, extracted with CH2Cl2. The phases were separated, and the organic phase was
washed with saturated aqueous Na2CO3 and brine. Dichloromethane was dried with
MgSO4, filtered and concentrated under reduced pressure to give foam solid. Flash
chromatography (Hexane:EtOAc:MeOH(NH3), 5:4.5:0.5) of the residue afforded 0.12
g (0.36 mmol, 50%) of 5.45 as white foaming solid: 1H NMR (CDCl3, 400 MHz)
δ 9.59 (bs), 7.30-7.64 (m, 9H), 4.31 (m, 1H), 4.20 (m, 1H), 3.48 (m, 1H), 3.42-3.35
(m, 2H), 3.13 (m, 1H), 2.49 (m, 1H), 2.09 (m, 1H), 1.55 (m, 1H), 1.43-1.09 (m, 4H),
0.75 (t, 3H); 13C NMR (CDCl3, 100 MHz) δ 181.1 (s), 140.8 (s), 139.1 (s), 130.9 (s),
128.9 (d), 128.7 (d×2), 128.2 (d), 128.1 (d), 127.8 (d), 125.2 (d), 123.4 (d), 109.9 (d),
79.9 (d), 70.9 (t), 67.9 (d), 57.7 (s), 45.5 (t), 38.4 (t), 30.4 (t), 26.5 (t), 26.3 (t) 9.8 (q).
O
NH2
N
H
1. 1.5 eq , DCM, 0°C
2. 6 eq TFA, 0°C, 2h
50%
Br
O
O
H
5.15b
H
HN
NH
O
O
O
HN
NCHO
THF, 0°C
10m, 85%
5.46
5.20
N-Formyl-spirooxindole 5.46. To dissolved bromotryptamine (0.50 g, 2.09 mmol) in
10 mL dry CH2Cl2, was added aldehyde (0.27 g, 3.14 mmol, 1.5 equiv) and MgSO4
(377 mg, 1.5 equiv) under nitrogen. The reaction was stirred for 2 h and cool down to
0 °C, then added TFA (1.43 g, 12.5 mmol, 6 equiv). The mixture was stirred at room
temperature for 2 h, extracted with CH2Cl2. The phases were separated, and the
organic
phase
was
washed
with
saturated
aqueous
Na2CO3
and
brine.
114
Dichloromethane was dried with MgSO4, filtered and concentrated under reduced
pressure to give foam solid. Flash chromatography (Hexane:EtOAc:MeOH(NH3),
5:4.5:0.5) of the residue afforded 0.40 g (1.64 mmol, 50%) of 5.20 as white foaming
solid: To a solution of crude amine (0.05 g, 0.2 mmol) in 2 mL THF, was added
excess of acetic-formic anhydride at 0 °C. The reaction mixture was stirred for 10 m at
room temperature and concentrated under reduced pressure. Flash chromatography
(Hexane:EtOAc:MeOH(NH3), 5:4.5:0.5) of the residue afforded 0.047 g (0.17 mmol,
85%) of N-formamide 5.46: major rotamer: 1H NMR (CDCl3, 400 MHz) δ 9.18 (bs,
1H), 8.32 (s, 1H), 7.3-7.2 (m, 2H), 7.07 (dd, J = 6.6, 4.7 Hz, 1H), 7.00 (d, J = 6.7 Hz,
1H), 4.16 (t, J = 7.0 Hz, 1H), 3.98 (m, 1H), 3.83 (m, 1H), 2.45 (dd, J = 9.4 Hz, 1H),
2.11 (dd, J = 7.9, 3.1 Hz, 1H), 1.53 (dd, J = 13.1, 6.5 Hz, 1H), 1.34 (dd, J = 13.2, 6.5
Hz, 1H), 1.27 (dd, J = 13.2, 6.5 Hz, 1H), 0.86 (d, J = 6.35 Hz, 3H), 0.69 (d, J = 6.35
Hz, 3H); major rotamer:
C NMR (CDCl3, 100 MHz) δ 179.7 (s), 161.4 (d), 141.0
13
(s), 129.6 (s), 129.1 (d), 124.8 (d), 123.1 (d), 110.9 (d), 62.0 (d), 57.3 (s), 43.3 (t), 40.8
(t), 34.7 (t), 25.3 (d), 23.2 (q), 22.8 (q); minor rotamer: 13C NMR (CDCl3, 100 MHz)
δ 181.3 (s), 162.8 (d), 141.1 (s), 129.7 (s), 129.1 (d), 125.3 (d), 122.6 (d), 110.9 (d),
60.0 (d), 56.7 (s), 45.7 (t), 40.1 (t), 35.4 (t), 25.3 (d), 23.2 (q), 22.8 (q).
O
O
HN
NH
OBn
H
O
O
O
HN
NCHO
OBn
THF, 0°C, 10m
quant
5.45
5.47
N-Formyl-spirooxindole 5.47. To a solution of amine (0.065 g, 0.178 mmol) in 2 mL
THF, was added acetic-formic anhydride 0.25 mL at 0 °C. The reaction mixture was
stirred for 1-2 h at room temperature and concentrated under reduced pressure. Flash
chromatography (Hexane:EtOAc:MeOH(NH3), 5:4.5:0.5) of the residue afforded
0.068 g (0.17 mmol, 90%) of N-formamide 5.47: IR (KBr) : 3021, 2952, 2910, 2169,
1721, 1652, 1618, 1470, 1379 cm-1; major: 1H NMR (CDCl3, 400 MHz) δ 8.85 (bs,
1H), 8.36 (s, 1H), 7.00-7.35 (m, 9H), 4.32 (d, J = 11.6 Hz, 1H), 4.32 (d, J = 11.6 Hz,
115
1H), 4.03 (m, 2H), 3.77 (m, 1H), 3.17 (m, 1H), 2.43 (m, 1H), 2.09 (m, 1H), 1.89 (m,
1H), 1.59-1.24 (m, 4H), 1.12 (m, 1H), 0.74 (t, 3H);
13
C NMR (CDCl3, 100 MHz)
δ 179.5 (s), 161.6 (d), 140.8 (s), 139.0 (s), 129.4 (s), 129.3 (d), 128.8 (d×2), 128.1
(d×2), 128.0 (d), 124.9 (d), 123.2 (d), 110.9 (d), 79.3 (d), 70.9 (t), 64.0 (d), 57.1 (s),
43.5 (t), 35.1 (t), 28.9 (t), 27.6 (t), 26.2 (t), 9.6 (q); HRFABMS m/z calcd for
C24H28O3N2 [M+H]+: 392.4800; found: 393.2000.
O
HN
O
NCHO
OBn
HN
NCHO
OH
H2, Pd/C
MeOH, quant
5.47
5.48
Alcohol 5.48. A mixture of 5.47 (0.02 g, 0.05 mmol) and 10 % palladium carbon (0.06
g) in 8 mL absolute MeOH, was placed under a balloon of hydrogen. After 6 h, the
reaction mixture was filtered over the pad of celite. The solution was concentrated
under reduced pressure. Flash chromatography (CH2Cl2:MeOH, 19:1) of the residue
afforded alcohol 5.48 in quantitative yield (15 mg, 0.05 mmol): IR (KBr) : 3427, 3240,
2957, 2927, 2873, 2169, 1716, 1648, 1470, 1387 cm-1; major: 1H NMR (CDCl3, 300
MHz) δ 8.60 (bs), 8.36 (s), 7.29 (dd, J = 8.8, 7.4 Hz, 1H), 7.10 (dd, J = 6.8, 7.5 Hz,
1H), 7.06 (d, J = 7.4 Hz, 1H), 6.97 (d, J = 7.5 Hz, 1H), 4.07 (t, J = 7.0 Hz, 1H), 3.98
(m, 1H), 3.76 (m, 1H), 3.38 (m, 1H), 2.42 (m, 1H), 2.10 (m, 1H), 1.82 (m, 1H), 1.591.2 (m, 3H), 1.16 (m, 1H), 1.27 (q, 2H), 0.74 (t, 3H);
13
C NMR (CDCl3, 75 MHz)
δ 179.3 (s), 161.6 (d), 140.7 (s), 129.3 (d), 129.3 (s), 124.9 (d), 123.3 (d), 110.9 (d),
79.9 (d), 64.0 (d), 57.1 (s), 43.4 (t), 35.0 (t), 32.8 (t), 30.3 (t), 28.1 (t), 9.9 (q);
HRFABMS m/z calcd for C17H22O3N2 [M+H]+: 302.2100; found: 303.0000.
116
O
O
HN
HN
NCHO
OH
NCHO
O
PCC, NaOAc
DCM, quant
salacin
1.17
5.48
Salacin 1.17. To 0.010 g (0.049 mmol, 1.5 equiv) of PCC and 0.001 g (0.01 mmol, 0.3
equiv) of NaOAc and 2 mL dry CH2Cl2, stirred vigorously, was added 0.01 g (0.03
mmol) of alcohol in 1 mL of dry CH2Cl2. The reaction mixture was stirred at room
temperature for 3 h. Ether was added; the reaction mixture was filtered and
concentrated under reduced pressure. Flash chromatography (CH2Cl2:MeOH, 19:1) of
the residue afforded ketone 1.17 in quantitative yield (9.0 mg, 0.3 mmol): IR (KBr) :
3250, 2950, 2921, 2852, 2169, 1714, 1651, 1632, 1470, 1386 cm-1; major: 1H NMR
(MeOH-d4, 400 MHz) δ 8.31 (bs, 1H), 7.32 (dd, J = 11.4, 7.2 Hz, 1H), 7.25 (d, J = 7.2
Hz, 1H), 7.10 (dd, J = 11.4, 7.6 Hz, 1H), 7.01 (d, J = 7.8 Hz, 1H), 4.05 (dd, J = 6.6
Hz, 1H), 4.03 (dd, J = 8.2, 8.6 Hz, 1H), 3.67 (dd, J = 12.3, 7.8 Hz, 1H), 2.39 (m, 1H),
2.32 (m, 2H), 2.30 (q, J = 7.3 Hz, 2H), 2.22 (m, 1H), 2.16 (m, 1H), 1.66 (m, 1H), 0.97
(t, J = 7.3 Hz, 3H); 13C NMR (MeOH-d4, 100 MHz) δ 210.8 (s), 180.3 (s), 162.7 (d),
141.9 (s), 129.0 (d), 128.8 (s), 125.0 (d), 122.6 (d), 110.5 (d), 64.0 (d), 57.1 (s), 43.4
(t), 35.0 (t), 32.8 (t), 30.3 (t), 28.1 (t), 9.9 (q); HRFABMS m/z calcd for C17H20O3N2
[M+H]+: 300.0000; found: 301.1536.
7.6 Almazole C and D
MeO2C
O
CN
N
H
4.24
1.2 eq
MeO2C
1.2 eq DBU, 10h
70%
N
NC
O
N
H
6.11
Methyl 5-indolyloxazole-4-carboxylate 6.11. To a stirred solution of indole-3carbonyl-nitrile 4.24 (1.00 g, 5.90 mmol) in 50 mL of THF at 0 °C was dropwise
117
added isocyanate (0.65 mL, 7.10 mmol, 1.2 equiv) and DBU (1.1 mL, 7.1 mmol, 1.2
equiv). The mixture was stirred at room temperature for 10 h and concentrated. The
resulting residue was dissolved in 50 mL ethyl acetate, and concentrated under
reduced pressure. Flash chromatography (Hexane:EtOAc, 3.2) of the residue afforded
0.97 g (4.01 mmol, 70%) of oxazole methyl ester 6.11: mp 236-238 °C; IR (KBr) :
3300, 1699, 1571, 1416, 1289 cm-1; 1H NMR (DMSO-d6, 300 MHz) δ 12.45 (bs, 1H),
8.64 (d, J = 2.9 Hz, 1H), 8.45 (s, 1H), 8.04 (d, J = 7.3 Hz, 1H), 7.52 (d, J = 7.7 Hz,
1H), 7.19 (td×2, J = 7.6 Hz, 2H), 3.84 (s, 3H); 13C NMR (DMSO-d6, 75 MHz) δ 163.5
(s), 154.9 (s), 149.6 (d), 136.9 (s), 130.9 (d), 125.6 (s), 123.5 (d), 122.6 (s), 121.9 (d),
121.3 (d), 113.3 (d), 103.0 (s), 52.4 (q).
MeO2C
O
N
H
6.11
O
N
MeOH/HCl
60°C, 3d
seal-tube, 76%
NH2
N
H
CO2Me
6.10
β-Oxotryptophan methyl ester 6.10. Methyl 5-indolyloxazole-4-carboxylate 6.11
(0.80 g, 3.29 mmol) was dissolved in the mixture of methanol (88 mL) and
hydrochloric acid (8 mL). The mixture was stirred at 60 °C for 3 d. Methanol was
removed by evaporation and the reaction mixture was purified by column
chromatography using a 19:1-9:1 gradient of CH2Cl2:MeOH(NH3) as eluent to give
brown solid β-oxotryptophan methyl ester (0.58 g, 2.51 mmol, 76%): free base 6.10:
IR (KBr) : 3342, 3277, 1736, 1654, 1521, 1452, 1421, 1242 cm-1; 1H NMR (DMSOd6, 400 MHz) δ 12.1 (bs, 1H), 8.47 (s, 1H), 8.17 (d, J = 2.0 Hz, 1H), 7.49 (d, J = 2.1
Hz, 1H), 7.23 (m, 2H), 5.03 (s, 1H), 3.67 (s, 3H), 2.38 (bs, 2H); 13C NMR (DMSO-d6,
100 MHz) δ 190.7 (s), 172.7 (s), 137.5 (s), 136.7 (d), 126.5 (s), 124.1 (d), 123.0 (d),
122.1 (d), 115.1 (s), 113.2 (d), 61.3 (d), 53.4 (q).
118
O
O
O
NH2
N
H
Cl
1.5 eq TEA, 30m
80-90%
4.25
H
N
O
N
H
6.12
N-[2-(1H-Indole-3-yl)-2-oxo-ethyl]-butyramide 6.12. To a stirred solution of βoxotryptamine 4.25 (0.30 g, 1.72 mmol) in 20 mL of THF was added triethylamine
(0.18 mL, 2.5 mmol, 1.5 equiv) and butyryl chloride (0.20 g, 1.90 mmol, 1.1 equiv) at
23 °C. The mixture was stirred for 30 m, and concentrated. The residual product was
dissolved in 100 mL of ethyl acetate, and washed with brine (2 × 50 mL) and dried
(MgSO4). Upon filtration and evaporation under vacuum, indole-oxo-alkylamide 6.12
crystallized from the solution in 0.33-0.38 g (1.35 mmol, 80-90%): mp 228-230 °C; IR
(KBr) : 3320, 3190, 1653, 1626, 1543, 1516, 1463 cm-1; 1H NMR (DMSO-d6, 400
MHz) δ 11.97 (bs, 1H), 8.40 (d, J = 3.1 Hz, 1H), 8.16-8.13 (m, 1H), 8.08 (bt, J = 5.7
Hz, 1H), 7.48-7.44 (m, 1H), 7.24-7.15 (m, 2H), 4.44 (d, J = 5.7 Hz, 2H), 2.16 (t, J =
7.3 Hz, 2H), 1.55 (q, J = 7.3 Hz, 2H), 0.89 (t, J = 7.3 Hz, 3H); 13C NMR (DMSO-d6,
100 MHz) δ 190.4 (s), 172.3 (s), 136.4 (s), 133.5 (d), 125.4 (s), 122.8 (d), 121.8 (d),
121.1 (d), 114.1 (s), 112.1 (d), 45.6 (t), 37.2 (t), 18.7 (t), 13.6 (q); HRMS m/z calcd for
C14H16N2O2 [M++1]: 244.1213; found: 245.0000.
O
N
H
N
POCl3
O
O
N
H
6.12
overnight
85-90%
N
H
6.13
3-(2-Propyl-oxazol-5-yl)-1H-indole 6.13. A mixture of 6.12 (0.10 g, 0.40 mmol) in 5
mL of POCl3 was stirred at 23 °C. The reaction was allowed to proceed overnight and
concentrated. The residual product was dissolved in 50 mL of ethyl acetate, washed
with saturated aqueous NaHCO3 (50 mL), brine (2 × 50 mL) and dried (MgSO4).
Upon filtration and evaporation under reduced pressure, alkyl oxazole indole 6.13
crystallized from the solution in 0.07-0.08 g, 0.32 mmol, 85-90% yield: mp 161-163
119
°C; IR (KBr) : 3128, 2950, 1632, 1571, 1452, 1439, 1122 cm-1; 1H NMR (DMSO-d6,
400 MHz) δ 11.52 (bs, 1H), 7.82 (bd, J = 7.7 Hz, 1H), 7.72 (d, J = 2.6 Hz, 1H), 7.46
(bd, J = 7.8 Hz, 1H), 7.28 (s, 1H), 7.19 (bt, J = 7.7 Hz, 1H), 7.13 (bt, J = 7.7 Hz, 1H),
2.76 (t, 2H), 1.76 (m, 2H), 0.97 (t, 3H); 13C NMR (DMSO-d6, 100 MHz) δ 161.5 (s),
147.2 (s), 136.3 (s), 123.5 (s), 122.9 (d), 122.1 (d), 120.0 (d), 119.4 (d), 119.0 (d),
112.0 (d), 103.9 (s), 29.3 (t), 20.1 (t), 13.5 (q); HRMS m/z calcd for C14H14N2O
[M++1]: 227.0246; found: 277.0000.
N
HO
O
O
NH2 1.2 eq
6.6
N
H
4.25
O
1.2 eq DEPC, TEA
rt, 12h, 85%
H
N
N
H
N
O
prealmazole
6.7
1-Benzyl-3-[2-(1H-indole-3-yl)-2-oxo-ethyl]-1-isopropyl-urea 6.7. To a stirred
solution of β-oxotryptamine 4.25 •HCl (0.50 g, 1.3 mmol) in 100 mL of THF was
added triethylamine (0.27 mL, 1.95 mmol, 1.5 equiv) at 0 °C, and the solution was
stirred for 30 m. The mixture was added N,N-dimethyl-L-phenylalanine (0.27 g, 1.4
mmol, 1.2 equiv) diethyl pyrocarbonate (0.17 mL, 1.40 mmol, 1.2 equiv), and stirred
at 23°C under nitrogen. After 12 h, the reaction was concentrated under vacuum. The
residual product was dissolved in 100 mL ethyl acetate, washed with brine (2 × 50
mL), and dried (MgSO4). Filtration and evaporation afford a residue which was
chromatographed over silica gel, and eluted with DCM:MeOH 39:1 to give
prealmazole C 6.7 in 0.85 g, 2.43 mmol, 85% yield: mp 141-143 °C; 1H NMR
(acetone-d6, 300 MHz) δ 11.15 (bs, 1H), 8.35 (d, J = 3.2 Hz, 1H), 8.32-8.26 (m, 1H),
7.72 (bs, 1H), 7.56-7.50 (m, 1H), 7.33-7.11 (m, 7H), 4.65 (dd, J = 18.0, 5.5 Hz, 1H),
4.50 (dd, J = 18.0, 4.9 Hz, 1H), 3.45 (dd, J = 7.6, 5.8 Hz, 1H), 3.16 (dd, J = 13.7, 7.6
Hz, 1H), 2.92 (dd, J = 13.7, 5.8 Hz, 1H), 2.39 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ
189.4 (s), 173.4 (s), 140.2 (s), 136.8 (s), 131.9 (d), 129.6 (d×2), 128.8 (d×2), 126.6 (s),
120
125.7 (d), 124.3 (d), 123.3 (d), 122.4 (d), 115.7 (s), 112.2 (d), 71.6 (d), 46.4 (t), 42.8
(q×2), 33.7 (t); HRMS m/z calcd for C21H23N3O2 [M++1]: 349.1792; found: 305.0000.
O
H
N
O
O
N
H
prealmazole
6.7
N
N
POCl3, 60°C
2d, 50%
N
N
H
almazole C
1.46
{1-[5-(1H-Indol-3-yl)-oxazol-2-phenyl-ethyl}-dimethyl-amine 1.46. A mixture of
6.7 (0.10 g, 0.29 mmol) in 5 mL of POCl3 was stirred at 60 °C. The mixture was
stirred for 2 d and concentrated. The residual product was dissolved in 50 mL of ethyl
acetate, washed with saturated aqueous NaHCO3 (50 mL), brine (3 × 50 mL) and dried
(MgSO4). Upon filtration and evaporation under vacuum, 1.46 was crystallized from
the solution. The crystal was filtrated and washed with hexane and ether to give 1.46
in 0.047 g, 0.14 mmol, 50% yield: mp 115-117 °C; 1H NMR (CDCl3, 300 MHz) δ
8.48 (bs, 1H), 7.84-7.81 (bd, J = 7.3 Hz, 1H), 7.52 (d, J = 2.6 Hz, 1H), 7.45-7.43 (bd,
J = 5.7 Hz, 1H), 7.21 (s, 1H), 7.32-7.14 (m, 7H), 4.09 (dd, J = 9.6, 5.5 Hz, 1H), 3.42
(dd, J = 13.5, 9.6 Hz, 1H), 3.24 (dd, J = 13.5, 5.5 Hz, 1H), 2.43 (s, 6H);
13
C NMR
(CDCl3, 75 MHz) δ 160.4 (s), 148.3 (s), 138.7 (s), 136.7 (s), 129.6 (d×2), 128.8 (d×2),
124.5 (s), 123.4 (d), 122.6 (d), 121.3 (d), 120.3 (d), 120.0 (d), 112.1 (d), 106.0 (s),
65.1 (d), 42.3 (q×2), 37.5 (t); HRMS m/z calcd for C21H21N3O [M++1]: 331.1686;
found: 322.0000.
121
N
HO
O
NH2
1.2 eq
6.6
O
CO2Me 1.2 eq DEPC, TEA
rt, 12h, 60%
N
H
6.10
H
N
O
N
H
N
O
CO2Me
6.14
1-Benzyl-3-[2-(1H-indole-3-yl)-2-oxo-ethyl]-1-isopropyl-urea 6.14. To a stirred
solution of β-oxotryptophan methyl ester 6.10•HCl (0.20 g, 0.86 mmol) in 20 mL of
THF was added triethylamine (0.36 mL, 2.5 mmol, 3 equiv) at 0 °C, and the solution
was stirred for 20 m. The mixture was added N,N-dimethyl-L-phenylalanine (0.19 g,
1.03 mmol, 1.2 equiv) and diethyl pyrocarbonate (0.16 mL, 1.03 mmol, 1.2 equiv),
and stirred at 23 °C under nitrogen. After 12 h, the reaction was concentrated under
reduced pressure. The residual product was dissolved in 100 mL ethyl acetate, washed
with brine (2 × 50 mL), and dried (MgSO4). Filtration and evaporation afforded a
residue. Flash chromatography (CH2Cl2:MeOH, 39:1) gave 0.02 g (0.05 mmol, 60%)
of prealmazole C 6.14: IR (KBr) : 3366, 3250, 2955, 1751, 1654, 1647, 1508, 1498,
1244 cm-1; major diastereomer 1H NMR (MeOH-d4, 300 MHz) δ 12.15 (s, 1H), 8.69
(d, J = 7.7 Hz, 1H), 8.26 (d, J = 1.8 Hz, 1H), 8.10 (bd, J = 7.7 Hz, 1H), 7.50 (bd, J =
7.7 Hz, 1H), 7.27-7.01 (m, 9H), 5.90 (d, J = 7.7 Hz, 1H), 3.64 (s, 3H), 3.59 (s, 1 H),
3.51 (dd, J = 8.6, 5.6 Hz, 1H), 2.95 (dd, J = 13.48 Hz, 1H), 2.74 (dd, J = 13.4, 8.6 Hz,
1H), 2.30 (s, 6H).
MeO2C
O
N
H
H
N
O
CO2Me
6.14
N
N
POCl3
60°C, 5d
53%
O
N
N
H
6.15
Almazole D methyl ester 6.15. A mixture of prealmazole D 6.14 (0.13 g, 0.3 mmol)
in 5 mL of POCl3 was stirred at 60 °C for 5 d. The reaction mixture was concentrated
and the resulting residue was dissolved in 50 mL of EtOAc, washed with sat. NaHCO3
122
(3 × 50 mL), brine (2 × 50 mL) and dried MgSO4. Filtration and evaporation afforded
a residue. Flash chromatography of the resulting residue using a 19:1 and 39:1
gradient of CH2Cl2:MeOH as eluent gave 6.15 in 0.65 g (1.67 mmol, 53%): mp 236238 °C; IR 3333.1, 3031.1, 2935.18, 2380.5, 2345.8, 1690.1, 1588.3, 1458.4, 1212.6
cm-1; 1H NMR (MeOH-d4, 400 MHz) δ 8.73 (s, 1H), 8.14 (m, 1H), 7.4 (m, 1H), 7.297.20 (m, 2H), 7.35-7.14 (m, 5H), 4.23 (dd, J = 10.7, 5.1 Hz, 1H), 3.96 (s, 3H), 3.48 (t,
1H), 3.30 (t, 1H), 2.47 (s, 6H);
C NMR (MeOH-d4, 100 MHz) δ 162.78 (s), 158.1
13
(s), 155.3 (s), 137.4 (s), 136.4 (s), 130.0 (d), 128.7 (d×2), 128.2 (d×2), 126.3 (d), 125.0
(s), 122.6 (d), 121.4 (d), 120.9 (d), 120.3 (d), 111.7 (d), 102.6 (s), 64.7 (d), 50.8 (q),
41.0 (q×2), 36.7 (t); HRMS calcd for C23H25N3O3 [M++1]: 391.4600; found: 392.2000.
MeO2C
HOOC
N
N
O
N
1N NaOH, MeOH
2d, rt, 90%
O
N
N
H
N
H
6.16
6.15
Almazole D carboxylic acid 6.16. To a stirred solution of almazole D methyl ester
6.15 (0.11 g, 0.281 mmol) in 3.3 mL of MeOH and 1N NaOH solution (0.43 mL) was
added at room temperature and the mixture was stirred under nitrogen. After 2 d,
addition of H2O (10 mL) and 2N HCl (0.74 mL) until pH ~ 2 and stirred for 10 m
white solid were precipitate. The white solid was filtrated and gave 6.16 in 95 mg,
0.25 mmol, 90% yield: mp 228-230 °C; IR 3333.1, 3031.1, 2935.18, 2380.5, 2345.8,
1690.1, 1588.3, 1458.4, 1212.6 cm-1; 1H NMR (MeOH-d4, 400 MHz) δ 8.70 (s, 1H),
7.87 (dd, J = 7.4 Hz, 1H), 7.46 (dd, J = 7.4 Hz, 1H), 7.29-7.16 (m, 7H), 5.17 (t, J = 8.1
Hz, 1H), 3.65 (d, J = 8.2 Hz, 2H), 3.11 (s, 6H);
13
C NMR (MeOH-d4, 100 MHz) δ
163.8 (s), 156.9 (s), 151.9 (s), 136.8 (s), 134.1 (s), 131.3 (d), 129.3 (d×2), 129.1 (d×2),
127.9 (d), 125.3 (s), 123.6 (s), 123.1 (d), 121.5 (d), 120.7 (d), 112.1 (d), 102.4 (s), 64.0
(d), 40.8 (q×2), 35.1 (t); HRMS m/z calcd for C22H22N3O3 [M++1]: 376.4260; found:
376.0000.
123
HOOC
NaO2C
N
1.5 eq NaOH, D2O
O
N
O
N
N
H
N
N
H
6.16
6.3
Almazole D 6.3. To a solution of 6.16 in D2O, 1.5 eq NaOH was added at room
temperature and gave almazole D: IR 3333.1, 3031.1, 2935.18, 2380.5, 2345.8,
1690.1, 1588.3, 1458.4, 1212.6 cm-1; 1H NMR (MeOH-d4, 400 MHz) δ 8.70 (s, 1H),
8.04 (d, J = 7.4 Hz, 1H), 7.43 (d, J = 7.4 Hz, 1H), 7.20-7.07 (m, 7H), 4.13 (dd, J =
10.9, 4.7 Hz, 1H), 3.46 (dd, J = 13.0, 10.9 Hz, 1H), 3.28 (d, J = 13.0, 4.9 Hz, 1H),
2.48 (s, 6H);13C NMR (MeOH-d4, 100 Hz) δ 169.9 (s), 158.1 (s), 153.0 (s), 139.0 (s),
137.8 (s), 130.4 (s), 130.3 (d×2), 129.7 (d×2), 129.4 (d), 127.8 (d), 126.8 (s), 123.4
(d), 121.8 (d), 121.7 (d), 112.9 (d), 105.1 (s), 66.2 (d), 42.6 (q×2), 38.3 (t).
124
GENERAL CONCLUSION
The research described in this dissertation presents results on the studies
toward the total synthesis of bromopyrrole alkaloids; nagelamide A, D and agelastatin
D and indole alkaloids, dragmacidin A-C, salacin and almazoles.
We have achieved the synthesis of nagelamide A and D via the oxidative
dimerization of 2-aminoimidazole 6 and 7. The method provides a rapid entry into the
synthesis of nagelamides without the use of protecting groups on the nitrogen. The
first total synthesis of nagelamide A and D could be completed in 8 and 6 steps
starting from ornithine.
Our approach toward agelastatin D demonstrated that the ABD-ring system
could be derived from β–functionalization of linear imidazolone. The studies carried
out in the course of this dissertation have set in place a major portion of the ABD-ring
core of agelastatin D structure; thus requiring only the construction of the C-ring by
making a one-carbon bridge.
In the synthesis of bisindole alkaloids, a short synthetic strategy for
dragmacidin A, B and C was accomplished by involving the dimerization of
oxotryptamines to give bis(indolyl)pyrazines, which upon selective reduction and
methylation with sodium cyanoborohydride in acetic acid or formic acid afforded the
target piperazine natural products as the key steps.
The application of the interrupted Pictet-Spengler cyclization involving
halotryptamine spirocyclization with aldehydes having various functionalities has
been investigated. The methodology appears to work well with aldehydes containing
alcohol or ester groups but not with ketones or protected aldehydes. Furthermore, we
have completed the synthesis of salacin via halotryptamine spirocyclization.
We have completed a short synthesis of almazole C in 5 steps and almazole D
in 7 steps starting from indole via a peptide coupling and Gabriel-Robinson
cyclization with chiral, nonracemic keto amides as the key steps. In addition, a
convenient preparation of β–oxotryptophan methyl ester, an important tryptophan-
125
based synthon, has been developed. We have revised the structure of almazole D as 5(3-indolyl)oxazole.
126
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131
APPENDIX
1
N
H
O
CCl3
2.23
H NMR 300 MHz
CDCl3
Br
Br
132
13
N
H
O
CCl3
2.23
C NMR 100 MHz
CDCl3
Br
Br
133
1
CO2Me
NH2
2.28a•2HCl
H NMR 300 MHz
DMSO-d6
H2N
134
1
N
H
N
NH2
2.25•2HCl
H NMR 400 MHz
DMSO-d6
H2N
135
13
N
H
N
NH2
2.25•2HCl
C NMR 100 MHz
DMSO-d6
H2N
136
1
N
H
N
NH2
2.26
H NMR 400 MHz
DMSO-d6
H2N
137
13
N
H
N
NH2
2.26
C NMR 100 MHz
DMSO-d6
H2N
138
1
N
H
N
N
H
N
NH2
2.28•3HCl
H NMR 300 MHz
DMSO-d6
H2N
NH2
139
13
N
H
N
N
H
N
NH2
2.28•3HCl
C NMR 75 MHz
DMSO-d6
H2N
NH2
140
1
N
H
N
H
N
Br
NH2
H
N
2.29•2HCl
H NMR 300 MHz
DMSO-d6
H2N
N
HN
O
Br
141
13
H2N
N
H
N
Br
NH2
H
N
2.29•2HCl
C NMR 75 MHz
DMSO-d6
N
H
N
HN
O
Br
142
1
N
H
N
NH2
N
H
N
NH2
2.30•4HCl
H NMR 300 MHz
DMSO-d6
H2N
NH2
143
13
N
H
N
NH2
N
H
N
NH2
2.30•4HCl
C NMR 75 MHz
DMSO-d6
H2N
NH2
144
1
N
H
N
NH2
N
H
N
NH2
2.31•4HCl
H NMR 300 MHz
DMSO-d6
H2N
NH2
145
1
H2N
N
N
NH
N
NH2
2.32•4HCl
H NMR 400 MHz
DMSO-d6
H2N
NH2
146
13
H2N
NH
N
NH2
2.32•4HCl
C NMR 75 MHz
DMSO-d6
H2N
N
N
NH2
147
Br
1
Br
O
N
H
NH
N
H
N
Br
NH2
H
N
2.2•2HCl
H NMR 400 MHz
DMSO-d6
N
H
H2N
N
HN
O
Br
148
Br
Br
13
N
H
O
N
H
NH
N
H
N
Br
NH2
H
N
2.2•2HCl
C NMR 75 MHz
DMSO-d6
H2N
N
HN
O
Br
149
Br
N
H
O
N
H
NH
N
H
N
Br
NH2
H
N
HMBC of 2.2•2HCl
DMSO-d6
Br
H2N
N
HN
O
Br
150
Br
Br
1
O
N
H
NH
N
H
N
Br
NH2
H
N
2.31a•2HCl
H NMR 300 MHz
DMSO-d6
N
H
H2N
N
HN
O
Br
151
Br
Br
13
O
N
H
NH
N
H
N
Br
NH2
H
N
2.31a•2HCl
C NMR 75 MHz
DMSO-d6
N
H
H2N
N
HN
O
Br
152
1
N
H
N
NH2
N
H
N
NH2
2.33•2HCl
H NMR 300 MHz
DMSO-d6
H2N
NH2
153
13
H2N
NH2
N
H
N
NH2
2.33•2HCl
C NMR 75 MHz
DMSO-d6
N
H
N
NH2
154
Br
Br
1
O
N
H
NH
N
H
N
Br
NH2
H
N
1.5•2HCl
H NMR 300 MHz
DMSO-d6
N
H
H2N
N
HN
O
Br
155
Br
Br
13
O
N
H
NH
N
H
N
Br
NH2
H
N
1.5•2HCl
C NMR 75 MHz
DMSO-d6
N
H
H2N
N
HN
O
Br
156
Br
N
H
O
N
H
NH
N
H
N
Br
NH2
H
N
HMBC of 1.5•2HCl
DMSO-d6
Br
H2N
N
HN
O
Br
157
1
N
H
NH2
3.23•HCl
H NMR 400 MHz
DMSO-d6
O
H
N
158
13
N
H
NH2
3.23•HCl
C NMR 100 MHz
DMSO-d6
O
H
N
159
1
N
H
OMe
NH2
3.33
H NMR 400 MHz
DMSO-d6
O
H
N
160
13
N
H
OMe
NH2
3.33
C NMR 100 MHz
DMSO-d6
O
H
N
161
1
N
H
NH2
3.34
H NMR 300 MHz
DMSO-d6
O
H
N
162
13
N
H
NH2
3.34
C NMR 100 MHz
DMSO-d6
O
H
N
163
O
1
N
H
H
N
H
N
3.32
H NMR 400 MHz
DMSO-d6
N
H
O
Br
Br
164
O
13
N
H
H
N
H
N
3.32
C NMR 100 MHz
DMSO-d6
N
H
O
Br
Br
165
O
1
N
H
O
H
N
3.35
H NMR 300 MHz
DMSO-d6
N
H
H
N
Br
Br
166
O
13
N
H
O
H
N
3.35
C NMR 100 MHz
DMSO-d6
N
H
H
N
Br
Br
167
O
1
N
H
H
N
N
H
O
H
N
3.36
H NMR 300 MHz
DMSO-d6
OMe
Br
Br
168
O
13
N
H
H
N
N
H
O
H
N
3.36
C NMR 100 MHz
DMSO-d6
OMe
Br
Br
169
O
1
N
H
H
N
N
H
Br
O
H
N
3.37
H NMR 400 MHz
DMSO-d6
OMe
Br
Br
170
O
13
N
H
H
N
N
H
Br
O
H
N
3.37
C NMR 100 MHz
DMSO-d6
OMe
Br
Br
171
O
1
N
H
H
N
N
H
Br
O
H
N
3.38
H NMR 300 MHz
DMSO-d6
O
Br
Br
172
O
13
N
H
H
N
N
H
Br
O
H
N
3.38
C NMR 100 MHz
DMSO-d6
O
Br
Br
173
1
Br
N
O
NH
NH
O
3.39
H NMR 400 MHz
MeOH-d4
Br
Br
HN
174
13
Br
N
O
NH
NH
O
3.39
C NMR 100 MHz
MeOH-d4
Br
Br
HN
175
1
Br
Br
H MeO
N
H
N
MeO
N
H
H
N
O
3.45
H NMR 300 MHz
DMSO-d6
Br
O
176
Br
H MeO
N
H
N
MeO
N
H
H
N
3.45
C NMR 75 MHz
DMSO-d6
Br
13
Br
O
O
177
Br
Br
Br
H MeO
N
H
N
MeO
N
H
H
N
NOE of 3.45
DMSO-d6
O
O
178
1
Br
N
O
NH
NH
O
3.46
H NMR 400 MHz
DMSO-d6
Br
Br
HN
179
13
Br
N
O
NH
NH
O
3.46
C NMR 100 MHz
DMSO-d6
Br
Br
HN
180
1
CN
4.24
H NMR 300 MHz
DMSO-d6
N
H
O
181
13
CN
4.24
C NMR 75 MHz
DMSO-d6
N
H
O
182
1
NH2
4.25•HCl
H NMR 300 MHz
DMSO-d6
N
H
O
183
13
NH2
4.25•HCl
C NMR 75 MHz
DMSO-d6
N
H
O
184
Br
1
NH2
4.26
H NMR 300 MHz
DMSO-d6
N
H
O
185
13
Br
NH2
4.26
C NMR 100 MHz
DMSO-d6
N
H
O
186
Br
NOE of 4.26
DMSO-d6
N
H
O
NH2
187
Br
1
NH2
4.16
H NMR 400 MHz
DMSO-d6
N
H
O
188
13
Br
NH2
4.16
C NMR 100 MHz
DMSO-d6
N
H
O
189
Br
NOE of 4.16
DMSO-d6
N
H
O
NH2
190
1
4.27
H NMR 400 MHz
DMSO-d6
N
H
N
N
H
N
191
13
4.27
C NMR 100 MHz
DMSO-d6
N
H
N
N
H
N
192
Br
1
4.28
H NMR 400 MHz
DMSO-d6
N
H
N
N
H
N
Br
193
Br
13
4.28
C NMR 100 MHz
DMSO-d6
N
H
N
N
H
N
Br
194
Br
1
4.17
H NMR 400 MHz
DMSO-d6
N
H
N
N
H
N
Br
195
Br
13
4.17
C NMR 100 MHz
DMSO-d6
N
H
N
N
H
N
Br
196
1
4.29
H NMR 300 MHz
DMSO-d6
N
H
N
H
H
N
H
N
197
13
4.29
C NMR 100 MHz
DMSO-d6
N
H
N
H
H
N
H
N
198
1
H
N
4.35
H NMR 300 MHz
DMSO-d6
N
H
N
Me
Me
N
199
13
H
N
4.35
C NMR 75 MHz
DMSO-d6
N
H
N
Me
Me
N
200
Br
1
4.37
H NMR 400 MHz
DMSO-d6
N
H
N
H
H
N
H
N
Br
201
Br
13
4.37
C NMR 100 MHz
DMSO-d6
N
H
N
H
H
N
H
N
Br
202
Br
1
4.39
H NMR 400 MHz
DMSO-d6
N
H
N
H
H
N
H
N
Br
203
Br
13
4.39
C NMR 100 MHz
DMSO-d6
N
H
N
H
H
N
H
N
Br
204
1
Cl
5.15a
H NMR 300 MHz
DMSO-d6
N
H
NH2
205
1
Br
5.15b•HBr
H NMR 300 MHz
DMSO-d6
N
H
NH2
206
13
Br
5.15b•HBr
C NMR 100 MHz
DMSO-d6
N
H
NH2
207
1
O
O
OH
5.30
H NMR 300 MHz
CDCl3
H
208
1
O
O
O
5.31
H NMR 400 MHz
CDCl3
H
209
13
O
O
O
5.31
C NMR 75 MHz
CDCl3
H
210
1
O
5.25
H NMR 300 MHz
CDCl3
H
O
211
13
O
5.25
C NMR 75 MHz
CDCl3
H
O
212
1
N
5.32
H NMR 400 MHz
CDCl3
N
H
213
13
N
5.32
C NMR 75 MHz
CDCl3
N
H
214
N
NOE of 5.32
CDCl3
N
H
215
1
O
OMe
5.35
H NMR 300 MHz
CDCl3
H
O
216
1
O
NH
O
OMe
5.36
H NMR 300 MHz
CDCl3
HN
217
13
O
NH
O
OMe
5.36
C NMR 100 MHz
CDCl3
HN
218
1
O
N
O
5.37
H NMR 400 MHz
CDCl3
HN
219
13
O
N
O
5.37
C NMR 100 MHz
CDCl3
HN
220
1
5.39
H NMR 300 MHz
CDCl3
N
221
13
5.39
C NMR 100 MHz
CDCl3
N
222
1
O
5.40
H NMR 300 MHz
CDCl3
H
CO2CH3
223
13
O
5.40
C NMR 75 MHz
CDCl3
H
CO2CH3
224
1
O
5.41
H NMR 300 MHz
CDCl3
O
CO2CH3
225
13
O
5.41
C NMR 100 MHz
CDCl3
O
CO2CH3
226
1
O
5.42
H NMR 400 MHz
CDCl3
O
OH
227
13
O
5.42
C NMR 100 MHz
CDCl3
O
OH
228
1
O
H
5.27
H NMR 400 MHz
CDCl3
O
O
229
13
O
H
5.27
C NMR 100 MHz
CDCl3
O
O
230
1
N
5.43
H NMR 400 MHz
CDCl3
N
H
231
13
N
5.43
C NMR 100 MHz
CDCl3
N
H
232
1
O
O
OBn
5.44a
H NMR 400 MHz
CDCl3
H
233
13
O
O
OBn
5.44a
C NMR 100 MHz
CDCl3
H
234
1
OBn
5.44
H NMR 400 MHz
CDCl3
H
O
235
13
OBn
5.44
C NMR 100 MHz
CDCl3
H
O
236
1
O
NH
OBn
5.45
H NMR 400 MHz
CDCl3
HN
237
13
O
NH
OBn
5.45
C NMR 100 MHz
CDCl3
HN
238
1
NCHO
5.46
H NMR 400 MHz
CDCl3
HN
O
239
13
NCHO
5.46
C NMR 100 MHz
CDCl3
HN
O
240
NCHO
NOE of 5.46
CDCl3
HN
O
241
1
NCHO
OBn
5.47
H NMR 400 MHz
CDCl3
HN
O
242
13
O
NCHO
OBn
5.47
C NMR 100 MHz
CDCl3
HN
243
1
O
NCHO
OH
5.48
H NMR 300 MHz
CDCl3
HN
244
13
O
NCHO
OH
5.48
C NMR 75 MHz
CDCl3
HN
245
1
O
NCHO
O
1.17
H NMR 400 MHz
MeOH-d4
HN
246
13
O
NCHO
O
1.17
C NMR 100 MHz
MeOH-d4
HN
247
NCHO
O
NOE of 1.17
MeOH-d4
HN
O
248
1
O
N
6.11
H NMR 300 MHz
DMSO-d6
N
H
MeO2C
249
13
O
N
6.11
C NMR 75 MHz
DMSO-d6
N
H
MeO2C
250
1
CO2Me
NH2
6.10
H NMR 400 MHz
DMSO-d6
N
H
O
251
13
CO2Me
NH2
6.10
C NMR 100 MHz
DMSO-d6
N
H
O
252
1
H
N
O
6.12
H NMR 400 MHz
DMSO-d6
N
H
O
253
13
H
N
O
6.12
C NMR 100 MHz
DMSO-d6
N
H
O
254
1
O
6.13
H NMR 400 MHz
DMSO-d6
N
H
N
255
13
O
6.13
C NMR 100 MHz
DMSO-d6
N
H
N
256
1
H
N
O
N
6.7
H NMR 300 MHz
acetone-d6
N
H
O
257
13
H
N
O
N
6.7
C NMR 100 MHz
CDCl3
N
H
O
258
1
N
1.46
H NMR 300 MHz
CDCl3
N
H
O
N
259
13
N
1.46
C NMR 75 MHz
CDCl3
N
H
O
N
260
1
O
CO2Me
H
N
N
6.14
H NMR 300 MHz
DMSO-d6
N
H
O
261
1
O
CO2Me
H
N
N
6.15
H NMR 400 MHz
MeOH-d4
N
H
O
262
13
O
N
N
6.15
C NMR 100 MHz
MeOH-d4
N
H
MeO2C
263
1
O
N
N
6.16
H NMR 400 MHz
MeOH-d4
N
H
HOOC
264
13
O
N
N
6.16
C NMR 100 MHz
MeOH-d4
N
H
HOOC
265
1
O
N
N
6.3
H NMR 400 MHz
MeOH-d4
N
H
NaO2C
266
13
O
N
N
6.3
C NMR 100 MHz
MeOH-d4
N
H
NaO2C
267