The Tetracyclines
Baran Lab
Cl HO Me
H
The tetracycline family of antibiotics
D. W. Lin
NMe2
H
H
NMe2
NMe2
HO Me OH
H
H
OH
O
HO
OH
O
O
OH
H
O
HO
OH
O
O
Cl
HO
OH
H
H
O
HO
OH
O
OH
OH
OH
tetracycline
(achromycin)
1953
O
HO
OH
O
O
H
OH
O
HO
Me
OH
O
HO
OH
OH
O
rolitetracycline
(reverin)
1958
O
N
OH
O
HO
O
limecycline
(tetralysal)
1961
NMe2
O
H
H
O
O
NMe2
OH
O
NH2
O
OH
O
HO
OH
O
O
minocycline
(minocin)
1972
H
NMe2
H
OH
O
H
N
H
N
OH
HO
NH2
N
H
OH
O
HO
OH
O
O
t-butylglycylamidominocycline
(tigilcycline)
1993
(Phase III clinical trials in progress)
OH
H
N
O
OH
NMe2
Semisynthetic derivatives on the market
OH
OH
OH
methacycline
(rondomycin)
1965
O
demethylchlortetracycline
(declomycin)
1957
NMe2
HO Me OH
H
H
OH
NMe2
H
Me Me
NMe2
HO Me OH
H
H
O
NH2
doxycycline
(vibramycin)
1967
NH2
O
OH
NH2
NMe2
NH2
OH
HO
Me
oxytetracycline
(terramycin)
1949
NMe2
O
OH
clomocycline
(megaclor)
1963
NH2
chlortetracycline
(aureomycin)
1948
HO Me
H
OH
OH
NH2
OH
NMe2
H
H
N
The natural products
Cl HO Me
H
OH
H
OH
Cl HO Me
8
HN
4
H2N
CO2H
9
7
D
10
OH
I. Chopra, M. Roberts. Microbiol. Mol. Biol. Rev. 2001, 65, 232.
6
OH
5a
C
11
O
5
NMe2
4a 4 3 OH
A 2
B
12 12a
HO
1
O
O
NH2
Notation
1
The Tetracyclines
Baran Lab
D. W. Lin
NMe2
HO Me OH
H
H
Discovery and The Dawn of Semisynthetic Antibiotics
OH
NH2
Cl HO Me
H
H
OH
NMe2
O
HO
OH
O
O
terramycin
OH
NH2
OH
O
HO
OH
O
O
aureomycin
Bayer Pharmaceuticals
Benjamin Duggar
University of Missouri
The first tetracycline antibiotic
discovered, aureomycin was
isolated in 1948 from a Missouri
soil sample by Lederle
Laboratories. The Lederle team
was led by Benjamin Duggar - a
consultant who was a 73-year-old
emeritus professor of botany who
had recently retired from the
University of Missouri! As Jie
Jack Li cracks, "Your greatest
discovery could happen in your
retirement."
The Nobel Prize Committee
R. B. Woodward
About the same time as the Lederle discovery of aureomycin, Pfizer was
scouring the globe for new antibiotics. Soil samples were collected from
jungles, deserts, mountaintops, and oceans. But ultimately terramycin was
isolated in 1949... from a soil sample collected on the grounds of a factory in
Terre Haute, Indiana, owned by Pfizer!
From the beginning, terramycin was a molecule enveloped in controversy. It
was the subject of the first mass-marketing campaign by a modern
pharmaceutical company. Pfizer advertised the drug heavily in medical
journals, eventually spending twice as much on marketing as it did to discover
and develop terramycin. Still, it turned Pfizer - then a small company - into a
pharmaceutical giant.
About Lederle Labs:
Lederle Labs was founded in
1902 in an old farmhouse on the
Pearl River in New York.
Aureomycin was one of many
lifesaving products developed by
Lederle, including vaccines for
polio and smallpox. It is now a
part of Wyeth Pharmaceuticals.
Ebay
Pfizer and R.B. Woodward collaborated to determine the structure of
terramycin, succeeding for the most part in 1952 (JACS 1953, 75, 5455). The
stereochemistry at C4a was revised after X-ray crystallography and NMR
studies in the 1960's (JACS 1965, 87, 134; JACS 1963, 85, 851).
Ad for aureomycin as additive in cattle feed
J. J. Li. "A History of Drugs and Their Discoverers." Pfizer Intranet Magazine. March-April 2004.
2
The Tetracyclines
Baran Lab
D. W. Lin
Now, Back to Actual Science
Big Pharma Behaving Badly: In 1955, Conover
discovered that hydrogenolysis of aureomycin gives a
deschloro product that is just as active as the original
product. This proved for the first time that chemicallymodified antibiotics could have biological activity. Within a
few years, a number of semisynthetic tetracyclines had
entered the market, and now most antibiotic discoveries are
of novel active derivatives of older compounds.
Biosynthesis and Biological Activity: Tetracyclines are polyketide antibiotics,
biosynthesized in a fashion similar to that of fatty acids, erythromycin and a host of
other antibiotics. Tetracyclines are produced naturally by Streptomyces
aureofaciens (T. Nakano, et al. Biosci. Biotechnol. Biochem. 2004, 68, 1345.).
Tetracyclines bind to the bacterial ribosome, preventing the binding of aminoacyltRNA to the ribosomal A site. This prevents bacterial protein translation (I. Chopra,
M. Roberts. Microbiol. Mol Biol. Rev. 2001, 65, 232.).
Conover's discovery, however, provoked further controversy
for tetracycline. Pfizer became embroiled in a patent
dispute with American Cyanamid, which owned the rights to
aureomycin (the starting material for Conover's procedure
to make tetracycline). Pfizer and American Cyanamid
eventually settled the dispute out of court when they
realized that neither company held truly exclusive rights to
the drug, and agreed to cooperate on selling the drug in
order to drive off competitors trying to enter the tetracycline
market. At one point, Pfizer employed a private decective
to tap the phones of Bristol-Meyers, a competitor seeking to
enter the tetracycline market! Bristol-Meyers agreed to
overlook this brazen act in exchange for a share of the
tetracycline market. Eventually five companies colluded in
order to maintain artificially high prices for tetracycline.
However, the Federal Trade Commission stepped in after
several years, finding Pfizer and company guilty of patent
fraud and anti-trust violations, and broke up the monopoly.
The Challenge to Synthetic Chemists: Muxfeldt and colleagues outline the basic
obstacles to achieving a total synthesis of any of the natural tetracyclines:
Stereochemical Complexity. There are up to five contiguous asymmetric centers
(terramycin) which must be established.
About.com
Lloyd Conover
Pfizer
Chemical Sensitivity. For the 6-methyl-6-hydroxy tetracyclines, mild acid rapidly
catalyzes dehydration, ketalization and a retro-aldol to produce the lactone below.
Mildly basic conditions results in deprotonation of the C5 and C6 hydroxyls,
initiating a cascade of events which leads to decomposition of the molecule.
Finally, the C4 stereocenter is readily epimerized upon exposure to acetic acid or
aqueous buffers.
NMe2
HO Me OH
H
H
Legal issues aside, for this discovery Lloyd Conover is now in the American
Inventors' Hall of Fame, alongside Thomas Edison and the Wright brothers.
OH
O
NMe2
H HO Me
H
OH
O
HO
OH
O
O
HO
OH
O
OH
O
HO
Conover, L.H. 1955. U.S. Patent No. 2,699,054.
OH
O
OH
O
OH
NH2
OH
O
O
base
NMe2
HO Me OH
H
H
OH
O
OH
O
O
OH
NH2
MeOH/
dioxane
NMe2
O
NMe2
H2, Pd/C
NH2
OH
H
acid
acid
M. Mintz. "Golden Ox of Antitrust." The Nation 14 April 1969, Vol. 208, Issue 15.
pp. 467-468.
H
H
NH2
U.S. Federal Trade Commission, "Anticipating the 21st Century: Competition
Policy in the New High-Tech, Global Marketplace".
Cl HO Me
H
Me
OH
NH2
OH
O
HO
OH
O
COOH
HO
COOH
Me
O
H. Muxfeldt, et al. J. Am. Chem. Soc. 1979, 101, 689.
3
The Tetracyclines
Baran Lab
Woodward's First Total Synthesis of a Biologically-Active Tetracycline, 6Demethyl-6-deoxytetracycline.
L.H. Conover, et al. J. Am. Chem. Soc. 1962, 84, 3222. (Initial communication)
R.B. Woodward. Pure Appl. Chem. 1963, 6, 561. (A personal account)
J.J. Korst, et al. J. Am. Chem. Soc. 1968, 90, 439. (Full article)
(i)
O
O
OMe
NaH
DMF
CO2Me
(ii)
NaH
DMF
55%
O
CO2Me
OMe
OMe
CO2Me
OMe
OMe
(i) H2, 200 psi
Pd/C
AcOH, 30 oC
O
OMe
Cl
Triton-B
dioxane-MeOH
50-70 oC
88%
CO2Me
MeO
OMe O
H2SO4
MeOH/CHCl3
reflux
66%
OMe O
Chlorination blocks the para
position, forcing condensation onto
the more hindered ortho position.
OMe O
OMe O
MeOH is essential to suppressing the kinetically
favorable intramolecular condensation and
permitting the intermolecular condensation with
the oxalate prior to formation of the desired
tricycle. In the absence of MeOH, Woodward
observed formation of the intramolecular product
first, followed by condensation onto the oxalate
to form the five-membered ring shown:
OH
Cl
CO2Me
OH
O
OMe OH
O
Cl
AcOH/HCl
H2 O
90 oC
73%
OH
OMe O
CO2Me
OH
intermolecular condensation outcompetes intramolecular!
OH
O
OMe O
(i) NHMe2, -10 oC;
(ii) NaBH4, -70 oC
O
OMe O
OH
CO2nBu
H
CO2nBu
Cl
Cl
CO2H
CO2Me
then NaH (4 eq.),
then MeOH (1 eq.),
rt --> 80 oC
45%
CO2Me
(ii) I2, AcOH;
then Cl2 in AcOH
(iii) HF, neat
63% over 3 steps
OMe
(2 eq.)
CO2Me
OMe
(i) NaOH
H2 O
100 oC
O
Cl
Cl
Cl
CO2Me
CO2Me
CO2Me
O
OMe
O
(i) AcOH/
H2SO4
(ii) H2SO4
MeOH/CHCl3
44%
CO2Me
(ii) H2SO4
MeOH/CHCl3
93%
O
OMe
O
Cl
OMe
O
D. W. Lin
OH
Cl
H
Mg(OMe)2
toluene
reflux
52%
H
NMe2
CO2nBu
69%
OH
OMe O
OH
The thermodynamically more favorable diastereomer is formed
exclusively in this step, with the carboxyamino substituent assuming
an equatorial position and thus establishing the cis relationship of
the bridgehead hydrogens. Ketone reduction is also stereoselective.
4
The Tetracyclines
Baran Lab
Cl
H
H
NMe2
Cl
CO2nBu
Cl
H
H
H
NMe2
(i)
(ii)
OH
O
Mg(OEt)
H
DMF/MeOH
120 oC
15 min
OMe OH
H
H
OMe O
NMe2
OH
EtO2C
OH
NH
t
Bu
No acylation of the enols by the
chloroformate was observed.
NMe2
O
O
OMe O
H
H
OH
NMe2
OH
EtO
O
O
NtBu
Observe the classic Woodward master
stroke. Despite the presence of four
O
enolates, we observe only one of two
plausible intramolecular condensation
CONHtBu events. The other event is impossible since
the enolate double bond cannot rotate to
bring the amide into position for cyclization.
NMe2
O
48% HBr
CONH2
OH
OH
OH
OH
OH
OH
15% bsm from A;
30% of A recovered
H
O2
CeCl3
O
O
NHtBu
A
OMe OH
H
EtO
NaH
OH
DMF-MeOH
glycine-NaOH buffer,
pH = 10
15 min
Cl
H
20 min
H2, Pd/C
Et3N
91%
O
O
O
CONHtBu
OH
CO2H
OMe O
OMe O
CO2H
OMe O
H
O
NMe2
H
H
H
NMe2
O
toluene
reflux
90%
OH
Zn dust
formic acid
1 min
81%
H
TsOH
OH
OMe O
H
NMe2
D. W. Lin
OH
O
H
OH
OH
NMe2
OH
CONH2
O
Mixture of epimers at C4
This was a difficult step to optimize - Woodward himself noted dryly that "the case
at hand was by no means the smoothest we had encountered." Competitive
hydroxylation at C11a was also observed, as well as rapid decomposition of the
product under prolonged reaction conditions, forcing Woodward and colleagues
to halt the reaction prematurely.
CaCl2
BuOH-H2O, pH = 8.5
ethanolamine buffer
reflux, 10 min
6% over 2 steps,
10% recovered SM
Thermodynamic equilibration
to the desired epimer.
6 H
OH
O
H
OH
OH
NMe2
OH
CONH2
O
6-desmethyl-6-deoxytetracycline
This was the first total synthesis of a tetracycline with all the requisite
functionality for full antibiotic activity. Note, however, that this is not the total
synthesis of a tetracycline natural product. Substituents at the C6 position are
missing.
5
The Tetracyclines
Baran Lab
D. W. Lin
Shemyakin: The First Total Synthesis of a Tetracycline Natural Product
OH
OH
LiAl(OtBu)3H
86%
64%
OH
OAc
O
OH
Me
BnBr
OH
O
OBn
MeMgBr
6 eq
BnO
OH
H
H
OBn
KOH/MeOH
OH
H
OH
BnO
OH
OAc
BnO
OH
Me
OBn
OH
O
(i) 0.1 N KOH, THF-H2O
O
(ii)
O
N
O
Me
OEt
H
NPhth
N
OEt
CO2Et
(ii) MeI, Ag2O
H
H
NH2
H
CO2Et
O
(i)
OH
Zn
CO2Et dust
AcOH
O
OBn
85%
H
O
NO2
O
OAc
Me
CO2Et
OH
EtOH
OAc
H
NO2
H
OEt
HCl
74%
K2CO3
54%
OAc
H
O
O
H
H
H
BF3.OEt2
O
OBn
Me
O
+
O 2N
HO Me
H
Et3NH+
THF
Note that the Lieb. Ann. reference cites a number of obscure Russian journals.
The JOC reference, however, illustrates Shemyakin's approach to the tricyclic
precursor produced below.
O
O
HO Me
H
A.I. Gurevich, et al. Tet. Lett. 1967, 8, 132.
M.N. Kolosov, S.A. Popravko, M.M. Shemyakin. Lieb. Ann. 1963, 668, 86.
B.-M.G. Gaveby, J.C. Huffmann, P. Magus. J. Org. Chem. 1982, 47, 3779.
O
OMe O
PCl5 in DMF, then
OH
Me
H
CO2H
HO Me
H
O
NPhth
EtO
O
Me
NH2
H
NPhth
Mg(OEt)
Jones reagent
EtO2C
OBn
60%
OBn
O
OH
OMe O
OH
OBn
CONH2
OMe O
Notice Shemyakin adopting the Woodward approach to ring A.
6
The Tetracyclines
Baran Lab
D. W. Lin
HO2C
Me
H
NPhth
Na+
S
CONH2
EtO2C
OBn
O
OH
Me
H
HN
OH
DMSO
OMe O
CONH2
OBn
Me
H
OH
(ii) MeI in THF
OH
O
OH
This intercepts a degradation product which had
previously been elaborated into tetracycline.
Me
OH
O2 over Pt
CONH2
OH
H
O
OH
OH
NMe2
(i) HBr-AcOH
OH
OMe O
Et3N
THF
rt, 8 hr
A.I. Gurevich, M.G. Karapetyan,
M.N. Kolosov. Khim. Prirodn.
Soedin., Akad. Nauk Uz.SSR
1966, 141.
(i) O2, hν
3,4-benzopyrene (cat.)
benzene
NMe2
OH
(ii) H2, Pd/C
CONH2
Mechanism? Answer on Slide 16.
O
M. Schach von Wittenau. J. Org. Chem. 1964, 29, 2746.
HO Me
H
OH
O
H
OH
OH
NMe2
OH
CONH2
O
tetracycline
7
The Tetracyclines
Baran Lab
D. W. Lin
Ph
Muxfeldt's Total Synthesis of 6-Desmethyl-6-deoxytetracycline
Cl
N
Ph
Cl
O
H. Muxfeldt, W. Rogalski. J.Am. Chem. Soc. 1965, 87, 933. (Communication)
H. Muxfeldt, E. Jacobs, K. Uhlig. Chem. Ber. 1962, 95, 2901. (Prep of precursors)
HCl
O
Cl
Cl
Br
CO2Me
CO2Me
CO2Me
NaOMe
MeOH
MeO2C
CO2Me
CO2Me
OMe
OMe
Cl
Cl
CO2H
CO2H
OMe
MeO
85%
over three steps
OMe O
O
Cl
CO2Me
MeO
O
O
LiAlH4
benzene-Et2O
0 oC
94%
OH
MeO
O
CHO C6H5
CN Li(EtO) AlH
3
N
MeO
O
O
benzene-Et2O
0 oC
O
MeO
64%
O
O
O
taut.
O
CONHtBu
O
Ph
Cl
O
N
O
O
O
CONHtBu
MeO2C
MeO
O
O
CONHtBu
MeO2C
MeO
O
O
O
N
O
Ph
O
N
H
N
MeO2C
MeO
O
Cl
Cl
Ph
Cl
Cl
Ph
NaH (2 eq.)
THF-Et2O
35 oC, 24 hr
(ii) NaCN
NaI (cat.)
DMF-H2O
92%
O
O
O
NHtBu
(i) MsCl
pyridine
97%
CO2H
Pb(OAc)4 (cat.)
Ac2O
O
Cl
HN
O
Ph
O
taut.
CONHtBu
MeO2C
MeO
O
O
MeO
MeO2C
Cl
Cl
THF
O
O
(i) H2SO4
MeOH
CO2H
95%
(ii) ethylene glycol
TsOH, benzene
91%
H3PO4
80 oC
quant.
(i) NaOH
(ii) pyrolysis,
160 oC
N
CONHtBu
MeO2C
MeO
O
O
Now the stage is set for the second cyclization in this magnificent transformation.
Only one equivalent of NaH used so far!
8
The Tetracyclines
Baran Lab
D. W. Lin
O
Cl
HN
Ph
Cl
O
MeO
O
MeO
O
Cl
MeO
NaH
CONHtBu
MeO2C
MeO
O
O
O
O
H. Muxfeldt, et al. J. Am. Chem. Soc. 1968, 90, 6534.
H. Muxfeldt, et al. J. Am. Chem. Soc. 1979, 101, 689.
CONHtBu
Terramycin is a much more difficult target than the prototypical tetracyclines
discussed previously - Woodward and Muxfeldt avoided many of the problems
outlined earlier with by targeting a structure without the troublesome C5 and C6
substituents, while Shemyakin targeted a tetracycline which did not have the
C6 hydroxyl. Here Muxfeldt and colleagues (including a young Edwin Vedejs!)
tackle those problems head-on! Sadly, this is reported in a posthumous
communication from the Muxfeldt laboratories.
O
Cl
NHBz
O
Muxfeldt's Last Hurrah: Total Synthesis of Terramycin
NHBz
NHBz
O
OH
CONHtBu
CONHtBu
MeO
O
O
OH
O
82% from the starting aldehyde
isolated as mixture of epimers
at C4
Muxfeldt thus effects the construction of the A and B rings in a single step! The only
problem, unfortunately, is the failure to control C4 stereochemistry.
Cl
NH2
OH
(i) Me3OBF4
(ii) HBr/AcOH,
100 oC
CONH2
OH
O
OH
OH
H2, Pd/C, H2CO
Me
Et3N, MeOH
O
OH
OH
O
HO
NMe2
NMe2
O
H
OH
O
(i) deprotects the benzoyl amide; (ii) deprotects the remaining functional groups.
OH
(i) Ac2O, H2SO4
0 oC
83%
(ii) 1-acetoxybutadiene
benzene, reflux
60%
O
OH
OH
CONH2
CONH2
OH
O
OH
OH
O
H
Me
Me
O
(i) acetone,
CuSO4
84%
(ii) Ac2O,
NaOAc
95%
AcO
Here they intercept an intermediate from the Woodward synthesis. They also report
hydroxylation with O2 over platinum (Angew. Chem. Intl. Ed. Eng. 1962, 1, 157).
O
AcO
O
H
(ii) NaOH
84%
H
Me
Me
H
O
Me
H
KClO3
OsO4 (cat.)
50 oC
O
89%
AcO
O
H
Me
O
H
(i) MeMgI,
-65 oC
82%
OAc
Me
OH
O
6-Demethyl-6-deoxytetracycline
O
Pb(OAc)4
40 oC
Me
O
OH
AcO
O
Me
H
H
O
O
O
Mixture of cis-diols
9
The Tetracyclines
Baran Lab
Me
Me
AcO
O
Me
O
O
xylenes
reflux
52% over
two steps
O
H
Me
DBU-AcOH,
piperidine
(cat.)
O
H
O
Me
Me
Me
H
O
CHO
H
O
AcO
O
Me
O
O
H
(i) O3
-50 oC
(i)
Me
H
MeO
H
N
Me
O
O
silica gel,
deactivated
N
Me
O
MOMO
O
S
Ph
Me
N
O
Me
H
Ph
O
S
O
MOMO
O
B
C
NH2
O
H
Ph
O
N
S
O
O
O
MeO
Me
H
O
MOMO
NH2
Me
O
S
Me
H
O
H
HN
MOMO
O
Me
Ph
OH
O
H
O
THF, reflux
2h
O
Me
O
OH
O
S
Me
H
O
H
HN
CONH2
MeO2C
MOMO
O
Ph
OH
+
CONH2
27%
N
Pb2(OH)(OAc)3
77%
O
BuLi (1.0 eq), -78 oC,
then
Me Me
The thermodynamically more favorable epimer is obtained exclusively.
Me
O
Me
Me
O
NH2
MeO
Coupling of B and C:
CHO
70-80%
MOMO
O
MeO
O
91%
Me
O
NH2
(ii) NaH, then
MOMCl
90%
Me
H
O
MeOH
33%
OMe
conc. HCl
Mixture of C5 epimers
Me
NH3
O
O
85%
HO
O
CHCl3
60%
CHO
O
O
(ii) H2O
68%
CHO
H
O
Preparation of C:
Me
Me
2:1
CH2Cl2 :
0.5 N NaCO3
in H2O
CHO
AcO
Me
D. W. Lin
O
Mixture of diastereomers at C4, C4a
Once again, Muxfeldt employs his beautiful method for forming the A and B
rings in a single step. And once again, there is little stereocontrol - all four
possible epimers at C4 and C4a are formed in solution. Fortunately, the desired
diastereomer readily crystallizes. The reason for employing the thiazolanone
rather than the oxazolanone employed before will become clear shortly...
10
The Tetracyclines
Baran Lab
Me
Me
MOMO
O
O
S
Me
H
H
O
H
HN
Me
9:1
AcOH :
H2 O
Ph
OH
reflux,
6 min
90%
CONH2
OH
Me
O
HO
O
O
S
Me
H
H
O
H
HN
S
Ph
OH
O
No epimerization at C4 observed!
S
Me
(i) P(OEt)3, NaH, O2
DMF-THF-H2O
15 min
HO
OH
OH HN
H
H
O
OH
OH
Ph
OH
CONH2
O
12%
desired C12a
hydroxylated
product
+
(ii) 0.01 N HCl
in MeOH
rt, 1.5 h
Me
Me
HO
O
O
S
Me
H
OH
O
H
HN
OH
H
OH HN
H
HO
O
OH
OH
(i) MeI in THF
(ii) 0.17 N HCl
OH
in THF-H2O, 1.5 h
Ph
Me
OH
H
OH
CONH2
O
HO
O
OH
H
OH
NH3 Cl
OH
CONH2
O
While the oxazolone substrate could also be carried to this step, the resulting
benzoyl amide could not be deprotected at this stage, nor could any other
amide devised, without decomposition. By contrast, deprotection conditions
for the thioamide proved to be sufficiently gentle.
Me
Me2SO4
(i-Pr)2NEt
23% from
thioamide
HO
OH
O
H
OH
OH
NMe2
H
OH
OH
CONH2
O
terramycin
Ph
OH
CONH2
OH
Me
CONH2
OH
D. W. Lin
47%
C11a
hydroxylated
byproduct
O
This concludes an elegant synthesis which assembles the A and B rings in a
single step. Unfortunately, Muxfeldt and colleagues never satisfactorily address
the issues of controlling the C4 and C4a stereocenters, nor do they improve
upon Woodward's solution to the C12a hydroxylation problem.
+
14% recovered SM
(i) hydroxylates the molecule; (ii) cleaves the acetonide. Unfortunately,
hydroxylation occurs principally at C11a. In a fortunate accident, however, it turned
out that the acetonide could not be cleaved unless the C12a hydroxyl was present.
Thus separation of the desired deprotected product from the undesired major
product was quite facile by polyamide chromatography.
11
The Tetracyclines
Baran Lab
Stork: Controlling the C4, C4a Stereocenters
O
G. Stork, et al. J. Am. Chem. Soc. 1996, 118, 5304.
Here Gilbert Stork and colleagues take a completely different approach in order
to achieve stereocontrol at the C4 and C4a centers.
O
O
O
OH
Me
OH
O
O
OH
n-Bu3SnH,
AIBN
Me
Me
SH
BF3.OEt2
0 oC, 15 min
88%
OH
O
Me
O
O
OAllyl
S
S
OH
O
OH
S
O
N , then
O
Me
NaHMDS, then
the above tricycle
O
O
O
HO
S
OH
Me2N
O
, then
O
the dithiane
92%
Stork postulates a ketene intermediate
formed from the mixed anhydride.
O
Me
O
Transketalization, followed
by hydrolysis to aldehyde.
OH
O
Me
4a
O
5a
H
MeO2C
O
NMe2
H
O
Pd(PPh3)4
N
95% from
the tricycle
OBn
O
NMe2
Me
Bu3SnOCH3
O
H
H
OH
NMe2
O
O
MeO2C MeO C
2
OTMS OTMS
H
5a
60 oC
97%
Mild
reagent
for
OBn
lactone cleavage
TMSCN
KCN
18-crown-6
OH
N
Me
OAllyl
CHO
(ii) 5% aq. HCl
quant.
5a
4a
O
OH
O
O
Here Stork exploits the stereochemistry of the tricycle to
direct conjugate addition to the more accessible face.
Observe that the C5a and C4a stereocenters are now set.
TFA anhydride, then
(i) PhI(OTFA)2,
MeOH
92%
O
OAllyl
MeO2C
OH
O
HS
OH
O
OBn
MeO2C
∆
90%
O
O
Me
mol. sieves
benzene
0 oC --> rt
2.5 h
97%
O
O
OEt
Br
N,N-dimethylaniline, ∆
98%
OAllyl
OAllyl
45% overall yield from start
of the synthetic sequence!
OEt
OEt
Br
piperidine (11 eq)
AcOH (40 eq)
CHO
OH
This sets the C6 stereocenter. Now watch Stork use this
stereocenter to bootstrap his way through the molecule...
Br
O
O
(ii) ∆
100%
100%
OH
O
O
OH
Me
(i) MeMgBr
78%
Me
D. W. Lin
N
OBn
O
H
NMe2
4a
MeO2C MeO C
2
O
N
OBn
This protects the C6 and
C10 hydroxyls, and sets
the stage for the
remaining cyclization
steps.
12
The Tetracyclines
Baran Lab
Me
H
H
KH
(25 eq)
NMe2
O
O
N
MeO2C MeO C
2
OTMS OTMS
Me
O
H
BnO
H
Me
H
H
NMe2
O
-78 --> 0 oC,
3 h, then
0 --> 50 oC,
30 min
O
N
MeO2C
OTMS OTMS
NMe2
Me
OH
H
O
H
BnO
NMe2
N
N
MeO
O
O
O
A Note on C12a Hydroxylation: This intercepts an intermediate which has
been hydroxylated at the C12a position according to literature reports,
completing in principle the formal synthesis of tetracycline. However, Stork
and colleagues were unable to successfully apply any of the C12a
hydroxylation methods previously reported. The presence of the C4
dimethylamino substituent seems to interfere with the hydroxylation. Clearly
a satisfactory solution to the C12a hydroxylation problem is still needed...
O
O
O
D. W. Lin
BnO
OH
O
OH
OH BnO
59%
The protecting group scheme permits formation of the A ring first, followed by in
situ deprotection and cyclization of the B ring to complete the basic tetracycline
framework. Previous studies had indicated that failure to protect the C11 ketone
resulted in formation of a BCD tricycle for which conditions to complete A ring
cyclization could not be found.
H2
Pd black
Me
OH
H
H
NMe2
OH
94%
CONH2
OH
O
OH
OH
12a-deoxytetracycline
13
The Tetracyclines
Baran Lab
D. W. Lin
Tatsuta: Asymmetric Total Synthesis of Natural (-)-Tetracycline
OBn
K. Tatsuta, et al. Chem. Lett. 2000, 647.
Here Tatsuta and colleagues not only produce an asymmetric total synthesis, but
they also take a very different approach to the synthetic problem, constructing
the A and B rings first and exploiting the carbohydrate chiral pool for starting
materials. And as a bonus, they solve the C12a hydroxylation problem as well!
(i) DMSO
DCC, Py-TFA
97%
OTBS
O
HO
BnO
CbzHN
OMe
(ii) Ph3PCH3Br
BuLi/THF,
-78 oC --> rt
91%
O
CbzHN
H 2C
Se
O
BnO
CbzHN
CH2
O
BnO
CbzHN
OMe
(ii) HgCl2
THF-H2O
67%
(i) MsCl, Et3N
0 oC, 15 min
82%
O
OH
CbzHN
NHCbz
t
Bu
Bu
CHO
BnO
CbzHN
OH
(ii) DBU, -30 oC
quant.
In addition to eliminating to the enone, (ii) also
epimerizes to the thermodynamic diastereomer.
OH
H
O
OBn
170 oC
72%
Me
Me
H
SeCN
(i) BnBr
BaO/Ba(OH)2
84%
HO
BnO
BnO
t
O
NHCbz
OMe
O
Me OH
H
OBn
OH
OBn
Me
H
NHCbz
OBn
OMe OH
O
OH
TMSCHN2
i-Pr2NEt
H
NHCbz
OBn
O
LDA, -40 oC,
15 min
80%
OMe
BnO
CH2
OH
BnO
NO2
OMe
OBn
OTMS
PBu3 (cat.)
90%
NO2
BH3.THF,
0 --> 45 oC;
then H2O2,
NaOH/THF
69%
(ii)
BnO
CrO3, H2SO4
0 oC, 10 min
85%
NHCbz
H
(i) HCl-MeOH
93%
OTBS
H 2C
O
OMe OH
O
(i) BBr3
-78 oC
15 min
OH
OBn
Me
(ii) H2, Pd/C
Boc2O, Et3N
92% over
OBn
two steps
H
OMe OH
Me
H
O
SOCl2
Et3N
-30 oC
10 min
90%
NHBoc
OH
OH
OH
NHBoc
OH
72%
OMe OH
O
OMe OH
Attempts to directly oxidize this 1,3-diol to the 1,3-dicarbonyl failed, requiring
the following detour of sequential alcohol oxidations.
14
The Tetracyclines
Baran Lab
Me
OMe OH
NHBoc
H
OH
Me
Br2
(Bu3Sn)2O
O
NHBoc
H
OH
O
OH
Br
mol. sieves
-78 oC, 15 min
OMe OH
85%
OMe OH
D. W. Lin
O
N
N
N
N
O O OMe
N
H
H
Me
H
Dess-Martin
periodinane
Zn, AcOH
2 min
O
Me
Br
15 min
91%
Me
H
O
H
NHBoc
O
OMe OH
Me
Dess-Martin
periodindane
15 min
62% over
two steps
H
OMe OH
NHBoc
O
O
TsN
Ph
OH
OH
O
CN
O
O
OMe OH
O
OMe
Me
H
NHBoc
Ph
Et3N
-78 oC, 30 min
60%
O
OMe OH
O
OH
O
Here Tatsuta et al. employ DMDO to achieve the desired
hydroxylation. They also achieve enantioselectivity by
exploiting the chiral boron catalyst which Corey developed
for enantioselective aldols and Diels-Alder reactions. Note
the fantastic yield!
(ii)
H
0 --> rt
88%
(i) H2NOH.HCl
Et3N, 30 min
OH
Me
OOH
H
Me
(ii) H2CO,
HCOOH
80 oC, 1 h
80%
BBr3
OH
NTs
(i) H3PO4
100 oC,45 min
68%
Me
OH
Cl
B
NH2
H
O O OMe
OMe OH
OMe OH
NHBoc
OH
O
OH
OMe OH
H
O
OH
NMe2
OH
CONH2
O
NMe2
OH
CONH2
O
O2, hν
ν
meso-tetraphenylporphyrin (cat.)
10 min
75%
NMe2
OH
O
N
N
N
N
80%over 2 steps
Mechanism?
OH
O
O
OH
CONH2
O
Here Tatstuta et al apply a protocol developed by Wassermann, Lu and
Scott for hydroxylating anhydrotetracyclines. Provide a mechanism for this
reaction, and rationalize the stereospecific nature of this reaction.
H. Wassermann, T.-J. Lu, A.I. Scott. J. Am. Chem. Soc. 1986, 108, 4237.
15
The Tetracyclines
Baran Lab
O
H
O
O
H
H
H
O
H
O
H
Myers' Rapid Asymmetric Access to Analogs of Tetracycline
H
H
H
NMe2
O
H
OH O
O
O
O
H
O
H
H
OH
OOH
O
H
O
OH
NMe2
OH
H2/Pt
Me
OH
H
H
O
N
H
NMe2
OH
H
A. eutrophus B9
12a
CO2H 75%, >95% ee
This bacterial-catalyzed reaction
can be run on a 90 g scale!
OH
O
OH
OH
OH
O
CO2Me
OH
O
TBSO
O
73%
OBn
N
BnO
TBSO
OH
O
NMe2
H
O
B
TBSO
OBn
OTBS O
Me
N
N
O
O
Li
Me
OH
NMe2
N
O
2. TFA, CH2Cl2
60%
CO2H
NMe2
OTBS
1. LiOTf,
toluene, 60 oC
O
83%
Notice how Myers begins with installation of the
troublesome C12a hydroxyl group, and then
proceeds to build the molecule around it!
(-)-tetracycline
There are many elegant features to this synthesis. Tatsuta and colleagues
mimic Stork's Diels-Alder approach to establishing stereochemistry, but employ
it to define the troublesome C4a stereocenter immediately. They construct the
central tetracycline scaffolding in just three steps from simple precursors. And
they solve the C12a hydroxylation problem with a very mild oxidant in the
presence of a chiral catalyst, and introduce the C6 hydroxyl stereospecifically
at a very late stage of the synthesis.
mCPBA
EtOAc
THF, -78 oC
70%
CONH2
CO2H
OH
1. TMSCHN2
2. TBSOTf, Et3N
TBSO
CONH2 62%
O
In an extraordinary report, Myers and colleagues present a highly efficient and
enantioselective method for accessing the tetracyclines.
NMe2
O
H
OH O
N
H
M.G. Charest, C.D. Lerner, J.D. Brubaker, D.R. Siegel, A.G. Myers. Science 2005,
308, 395.
H
Wassermann, Lu and Scott invoke a formal ene reaction. The orbital
alignment requirements dictate that only the axial hydrogen can participate
in the reaction, inducing hydroperoxidation on the upper face of the
molecule and thus ensuring the proper stereochemistry at C6.
Me
D. W. Lin
HO
TBSO
A
OH
O
N
OBn
21% over 7 steps
Here Myers closes the ring and sets the C4 amine stereochemistry. Myers
compares this key ring-closing step to a Sommelet-Hauser rearrangement, where
the amine initially undergoes an intramolecular SN-prime epoxide ring opening,
followed by ylide formation and finally a [2,3] sigmatropic rearrangement. TFA
selectively deprotects the allylic alcohol. Notice the remarkable yield so far!
16
The Tetracyclines
Baran Lab
Now Myers takes his key intermediate A and converts it into two fragments: B,
which will go on to form C6-deoxy analogs of tetracycline, and C, which will go
on to form analogs with the normal C6-oxygenation.
N
OH
TBSO
NMe2
O
then
O2
S
OBn
O
A
O
NH2
TBSO
OH
H
OBn
O
74%
NO2
1. HCl, MeOH
2. IBX, DMSO
3. TBSOTf, 2,6-lutidine
O
N
N
H
O
OTBS
OBn
With these fragments in hand, Myers now can install the C and D rings, and he
proceeds to do so in a fashion that allows for analogs of tetracycline with deepseated structural modifications to the D ring.
O
Me
NMe2
O
N
66%
O
C
NMe2
H
PPh3, DEAD;
O
HO
H
N
NMe2
H
BnO2CO
D. W. Lin
B
CO2Ph
OBn
O
BocO
Me
LDA, TMEDA,
THF, -78 oC;
H
H
O
N
HO
TBSO
OH
OH
N
PhS
TBSO
OH
O
OBn
A
Me
Me
1.
N
Cl
OH
H
NMe2
O
S O
O2
2. P(OMe)3, MeOH
70 oC
76%
N
TBSO
OH
O
OBn
1. BnO2CCl, DMAP
2. TBAF, HOAc
3. IBX, DMSO
4. TBSOTf, Et3N
OH
H
1. HF, MeCN
2. H2, Pd, THF/MeOH
OH
OBn
O
OTBS
H
NMe2
OH
NH2
90%
OH
Cl
O
Me
O
87%
OBn
O
O
NMe2
H
1. CBr4, PPh3
2. PhSH, Et3N
H
OBn
NMe2
N
then C,
-78 oC -> 0 oC
79%
OTBS
NMe2
O
O
OH
OH
O
O
(-)-doxycycline
18 steps, 8.3%
85%
17
The Tetracyclines
Baran Lab
D. W. Lin
With this strategy, Myers and colleagues are able to synthesize a number of
remarkable analogs of tetracycline:
Me
Me
H
H
B
NMe2
OH
NH2
CO2Ph
OBoc
OH
O
OH
OH
O
O
(-)-6-deoxytetracycline
Me
Me
N
B
B
N
O
OH
H
H
O
OH
O
O
NMe2
OH
OH
O
O
NMe2
H
H
B
OH
NH2
CO2Ph
O
CH2Br
OH
NH2
CO2Ph
CH2Br
OH
NH2
O
Me
NMe2
HN
CO2Ph
OBn
N
H
H
OH
OH
O
H
H
B
O
NMe2
OH
NH2
CO2Ph
OMe
OMe O
OH
OH
O
O
18
The Tetracyclines
Baran Lab
Addendum: Tetracycline Tidbits
D.H.R. Barton spent over a decade tinkering with tetracycline, but never completed
a total synthesis of the molecule. Over the course of this work, however, he
discovered some interesting chemistry (naturally).
Barton and colleagues (including a young Steven Ley!) also discovered the
utility of phenylseleninic anhydride for the deprotection of dithianes. This led to
their applying this reagent in a variety of transformations:
O
Photocyclizations of acetals onto enones:
Me
hν
ν
H
O
H
ArCO2H
O
AcO
H
O
O
O
Ph
Ph
PhSe
R'
O
O
O
R
O
SePh
Me
R
Me
R'
H
O
Ph
O
O
O
H
AcO
H
D.H.R. Barton, D.J. Lester, S.V. Ley. J. Chem. Soc. Perkin Trans. I 1980, 2209.
In his book Reason and Imagination, Barton concludes his chapter on the
tetracyclines with the following perspective on the role of academic research in
synthetic chemistry today:
OH
O
O
Me
O
O
O
D. W. Lin
O
Ph
"Just as the studies on the bitter principles [a class of natural products]
convinced me that X-ray crystallography was a superior procedure for structure
determination, the major effort on tetracycline synthesis convinced me that this
sort of work should be left to Industrial friends who have the money and the
resources to finish any multi-step synthesis, if it is economically justified. So it
is the originality in the reactions and the reagents and any new principles
that finally justify academic effort in synthesis. We are far away from the
Woodwardian dogma of completely planned synthesis."
D.H.R. Barton, et al. J. Chem. Soc. Perkin Trans. I 1976, 503.
19