HETEROGENOUS PHASE AS A MEAN
IN COMBINATORIAL CHEMISTRY
S.G. Abdel-Hamid
Combinatorial chemistry is a rapid and inexpensive technique for the synthesis of
hundreds of thousands of organic compounds of potential medicinal activity. In the past few
decades a large number of combinatorial libraries have been constructed, and significantly
supplement the chemical diversity of the traditional collections of the potentially active
medicinal compounds. Solid phase synthesis was used to enrich the combinatorial chemistry
libraries, through the use of solid supports (resins) and their modified forms. Most f the new
libraries of compounds appeared recently, were synthesized by the use of solid-phase. Solidphase combinatorial chemistry (SPCC) is now considered as an outstanding branch in
pharmaceutical chemistry research and is used extensively as a tool for drug discovery within
the context of high-throughput chemical synthesis. The best general pure libraries synthesized
by the use of solid phase combinatorial chemistry (SPCC) may well be those of intermediate
complexity that are free of artifact-causing nuisance compounds.
Contents
1.
2.
Introduction ......................................................................................................... 1
Principle of Combinatorial Chemistry .............................................................. 1
2.1 Screening Issues ............................................................................................. 2
2.1.1
Test Mixture in Solution .................................................................... 3
2.1.2
Test Individual Compounds in Solution .......................................... 4
2.1.3
Test Compounds on the Beads .......................................................... 5
3. Combinatorial Synthesis in Solution ................................................................. 5
3.1 Modified Liquid Phase Combinatorial Synthesis ....................................... 6
3.1.1
Non-Peptidyl Library of Sulfonamides ............................................ 8
4. Combinatorial Synthesis on Solid Phase ........................................................... 8
4.1 Solid Supports................................................................................................ 9
4.1.1
Merrifield Resin ............................................................................... 10
4.1.1.1
Synthesis of a Library of 1,3-diols with Antioxidant Activity ....... 10
4.1.1.2
“Diversomers”: An Approach to Nonpeptide, Nonoligomeric
Chemical Diversity .......................................................................................... 10
4.1.2
Wang Resin ....................................................................................... 12
4.1.2.1
Generation of a Substituted Guanidine Library (43) ...................... 12
4.1.2.2
Synthesis of Peptidomimetic Inhibitors for the Hepatitis C Virus
NS3 Protease .................................................................................................... 13
4.1.2.3
Synthesis of Piperazinedione Combinatorial Library ..................... 14
4.1.3
Rink Resin ......................................................................................... 16
i
5.
6.
7.
4.1.3.1
Combinatorial Lead Optimization of a Neuropeptide FF
Antagonists ...................................................................................................... 16
4.1.3.2
Compounds that Activate the Mouse Melanocortin-1-Receptor
Identified by Screening a Small Molecule Library Based Upon the -turn .... 18
4.1.3.3
Characterization and Screening of 43000 Compounds
Tetrahydroisoquinoline Combinatorial Library ............................................... 18
4.1.3.4
Construction of a “100-Member DHP Library” ............................. 20
4.1.4
Tenta Gel Resin ................................................................................ 22
4.1.4.1
Identification of Novel Inhibitors of Matrix Metalloproteinases ... 24
4.1.4.2
Biological Screening of the Produced (MMPS) Inhibitors ............ 25
4.1.5
Ellman’s Dihydropyran Resin ........................................................ 28
4.1.5.1
Synthesis of a Small Library of Substituted Hydroxy Proline-Based
2,5-Diketopiperazine ........................................................................................ 28
4.1.5.2
Combinatorial Libraries in the Discovery of Aspartic Acid Protease
Inhibitors ........................................................................................................ 30
4.1.6
Trityl Chloride Polystyrol (TCP) Resin ......................................... 33
4.1.6.1
Synthesis of New Selective vB3 Integrin Antagonists ................. 33
4.1.6.2
Solid Phase Synthesis and Biological Evaluation of a Combinatorial
Library of Philanthotoxin Analogues............................................................... 35
4.1.7
Tetrafluorophenol TFP-Activated Resin ....................................... 38
4.1.7.1
Synthesis and Identification of a Potent Factor Xa Inhibitors ........ 39
4.1.8
Sasrin Resin ...................................................................................... 39
4.1.8.1
Orphan Nuclear Receptor FXR Agonists ....................................... 40
4.1.9
Polystyrene-based Selenenyl Bromide Resin ................................. 42
4.1.9.1
Nicolaou’s Natural Product-like Benzopyran Libraries ................. 43
4.1.9.2
A Novel Strategy for the Solid Phase Synthesis of Substituted
Indolines ........................................................................................................ 44
4.1.10
Miscellaneous Advanced Resins...................................................... 46
4.1.10.1 Alkyne Amino Ester Resin (193) ....................................................... 46
4.1.10.2 ROMP-Spheres ................................................................................. 48
4.1.10.3 Polymer-Supported Enantioenriched Allysilanes ............................. 49
4.1.10.4 Resin-Bound Iminophosphorane ...................................................... 51
4.1.10.5 3-Carboxypropane Sulfonamide-P.S Resin (226)............................. 52
4.1.10.6 Resin-bound S-methylisothiourea (232) ........................................... 53
4.1.10.7 Phoxime Resin ................................................................................... 53
4.1.10.8 Resin-bound -Sulfonated Ketones ................................................... 56
Miscellaneous Solid Support ............................................................................ 57
Conclusion .......................................................................................................... 59
References .......................................................................................................... 60
ii
1.
Introduction
The field of combinatorial chemistry has grown exponentially in the past
decade. It is driven by the profound expansion in our knowledge of molecular
components of biology, the thirst to identify antagonists and mimetics of these
biomolecules for use in biotechnology, and the realization that rational design of these
new molecules is not a perfect art.1,2,3,4 Diversity libraries offer the possibility of
matching the diversity of biological targets with the diversity of potential molecular
leads.5,6,7 Publications from both academic and industry on the synthesis and
application of chemical libraries continued at a rapid pace in 2000.8 Of the all new
libraries, published last decade 25% were accompanied by biological screening
results, 82% were prepared on solid phase, and 65% came from academic
laboratories.
2.
Principle of Combinatorial Chemistry
For over a hundred years the main task of the synthetic chemist has been the
directed synthesis of one specific product using solution chemistry. In achieving this,
chemists have usually worked on one reaction, on one substrate in one reaction vessel at a
time-with each reaction targeting just one product. Combinatorial chemistry, has focused on
technologies that have the potential to make large numbers of products, whether this is
through preparing many single compounds in parallel, or many compounds simultaneously
in mixtures, for example, if coupling monomer A with monomer B gives the product A-B
(Figure 1), combinatorial synthesis can take a range of monomers A1-An and react those
with B1-Bn and make any product combination.
This process may be carried out using solution or solid-phase chemistry but for
reasons of reaction yield and reaction purity, there has been a preferential focus on the use
1
Figure 1: Contrasting orthodox and combinatorial synthesis, the former generally produces only one
compound at a time, but combinatorial methods provide the potential to produce a range of products in
parallel.
of solid phase chemistry to give compound mixtures. The universal advantage of
combinatorial chemistry is that it is faster and thus more efficient than orthodox chemistry
and can give rise to hundreds, thousands or even millions of products (Figure 2). In solid
phase synthesis, the substrate is linked to a solid phase such as a resin bead, it is possible
to generate products that are physically separate, and thus reagent and by-product not
bound to the resin may be removed simply by washing. Furthermore, the varying
reactivity of a range of substrates may be readily accommodated by using several
equivalents of reagent in order to drive reaction to completion, and the excess may be
removed by washing the solid phase with an inert, volatile solvent.
2.1
Screening Issues
The sets of compounds produced by combinatorial chemistry are generally
referred to as libraries, which depending on how the solid-phase is handled, may be
either mixtures or individual compounds, when individual compounds are prepared,
the screening issues are generally the same as for the orthodox drug discovery. If one
considers the formation of a number of compounds on resin beads in a mixture there
will be a range of options for testing those compounds in a biological assays.
2
Figure 2: Synthesizing mixtures of compounds has been enhanced by the use of solid-phase
techniques.
2.1.1
Test Mixture in Solution
All the compounds are cleaved from the beads and tested in solution
(Figure 3) if the resin beads were intimately mixed, it is not possible to test the
product separately, but rather as a mixture. If activity in pharmacological screening is
observed it is not possible to say which compound or compounds are active. To
identify the most active component, it is necessary to resynthesize the compounds
individually and thereby find the most potent. This iterative process of resynthesis and
screening is one of the most simple and successful methods for identifying active
compounds from libraries.
3
Figure 3: The products of combinatorial synthesis may be cleaved form the beads and tested in
solution as a mixture of compounds.
Figure 4: Using the handle provided by resin beads, individual compounds can be separated prior to
cleavage from the solid-phase and screening.
2.1.2
Test Individual Compounds in Solution
A second method is to separate the beads manually into individual wells and
cleave the compounds from the solid phase (Figure 4). The invention of novel tagging
methods allows the rapid identification of compound structure through the use of an
encoding molecule, synthesized on the resin bead in parallel with the ligand molecule.
Methods vary, but amongst the most useful of the tags are peptides (Selectide),9 oligonucleotide (Affymax,10 Lerner and Brenner11) and halogenated aromatic molecules
that encode a “binary” sequence deciphered by electron capture capillary gas
chromatography.12 In these cases, the use of tagging methods has necessitated the
design of coding chemistry that can be assembled on the bead without interfering with
the chemistry of library synthesis.
4
2.1.3
Test Compounds on the Beads
This method for screening is testing on the beads, using a colourimetric or
fluorescent assay technique (Figure 5). If there are active compounds, the appropriate
beads can be selected by colour or fluorescence, “picked” out by micromanipulation
and the product structure, if a peptide, determined by sequencing on the bead. Non
peptide structures would need to be identified by one of the tagging methods
highlighted above. Screening on the bead may be an inappropriate method for drug
discovery, as the bead and linker present conformational restrictions that may prevent
binding to the receptor.
3.
Combinatorial Synthesis in Solution
Despite the focus on the use of solid-phase techniques for the synthesis of
combinatorial libraries, there have been many examples where libraries have
successfully been made and screened in solution.13-16a,b Indeed, some groups have
expressed a preference for solution libraries because there is no prior requirement to
develop workable solid-phase coupling and linking techniques. Reactions are pushed
Figure 5: Compounds synthesized on solid-phase can be tested whilst still attached to the resin bead.
5
to completion by the use of excess quantities of the reactive reagent, and are isolated
by solvent-solvent extraction. There is no further purification and thus they prefer to
describe these samples as reaction products.
Sullivan and coworkers17 described the identification of a novel series of
compounds that blocks the activation of two key transcription factors, activator protein-1
(AP-1) and nuclear factor-K binding (NFKB), Figure 6. These transcription factors,
regulate the expression of several critical pro-inflammatory proteins and cytokines and
represent attractive targets for drug discovery. Through the use of high-throughput
screening and solution phase parallel synthesis, a small library was formed and a very
potent inhibitor of NF–KB and AP-1 was identified which is 2-chloro-4-(trifluoromethyl)pyrimidine-5-N-(3,5-bis(trifluoromethyl)phenyl)carboxamide (17, Figure 6).
3.1
Modified Liquid Phase Combinatorial Synthesis18
Liquid phase combinatorial synthesis (L.P.C.S.) has been developed as an
alternative methodology for the construction of small molecule libraries wherein the
use of a soluble polymer support provides both the advantage of solid phase synthesis
and the benefits of a classical solution phase organic chemistry. The success of the
liquid phase method results from the use of polyethylene glycol monomethylether
(MeO-PEG) as the polymeric support. MeO-PEG solubility in a wide variety of
aqueous and organic solvents not only enables homogenous reactions under numerous
reaction conditions, but these solubility properties permit individual reactions steps to
be monitored by infrared spectroscopy and/or proton and carbon-13 nuclear magnetic
resonance spectroscopy. Furthermore, this monofunctional polymer exhibits a helical
structure that possesses a strong propensity to crystallize thus, as long as the polymer
remains unaltered during library synthesis then purification by crystallization can be
utilized at each step.
6
CF3
N
O
N
HN
Cl
RO
NaOR
THF/RT
Cl
Cl
Cl
Cl
O
RR NH
THF, RT
NH
O
NH
EtOH/H2 O
N
N
N
O
NH
N
N
F3 C
F3 C
F3 C
N
(1) n-BuLi
H
11
Cl
R
Cl
Cl
5% Pd/C, MgO
1
Cl
9: R = H
10: R = CH3
(2) CH3 I
1
Cl
2: R = N,N-dimethylamino
3: R = amino, 4: R = n-butylamino
5: R = aniline, 6: R-benzylamine,
7: R = cyclohexylamine 8: R = piperidyl
Cl
O
N
N
N
R
CF3
Cl
12: R = CH3
13: R = CH2 Ph
O
Cl
F3 C
R NH2
+
N
N
(1) Amberlyst A-21
EtOAc/sonication
O
R
NH
N
N
F3 C
(2) H2 O/sonication
Cl
Cl
14
15
F3 C
CF3
O
NH
O
R
N R1
N
N
screening
F3 C
F3 C
N
N
Cl
R2
17
16
Library of (28 members)
IC50 0.05 (M) for NF-K B and
0.05 (M) for AP-1
Figure 6: NF–KB and AP-1 inhibitors via substituted pyrimidines.
7
3.1.1
Non-Peptidyl Library of Sulfonamides
A new route for the synthesis of sulfonamides (22) was developed to allow
attachment of arylsulfonyl moiety to MeO-PEG and to provide for molecular diversity
(Figure 7). Attachment to the polymeric support was formed quantitatively by
reaction of 4(chloro-sulfonyl)phenylisocyanate (18) with MeO-PEG (19) in the
presence of catalytic amount of dibutyltinlaurate. Impressively, coupling occurred
without any competing participation of the sulfonyl chloride moiety. Splitting the
MeO-PEG-aryl sulfonyl chloride (20) among six reaction vials containing different
amines resulted in a small membered library of MeO-PEG-protected sulfonamides
(21). Hydrolytic cleavage of the urethane linkage under basic condition yielded the
final library of six membered (22) in analytically pure form in overall yields of 9597%. It should be noted that the overall success of sulfonamide synthesis 22 was
dependent on the pKa of the nucleophilic amine.
4.
Combinatorial Synthesis on Solid Phase
The techniques for solid phase synthesis (S.P.S.) are based extensively on the
pioneering work of Merrifield19 who was the first to utilize substituted resin as a solid
phase for the synthesis of peptides.
MeO-PEG
19
+
O
S Cl
O
O C N
cat. Dibutyltinlaurate
O
S Cl
O
Resin O HN
O
18
20
RNH 2
NH 2
NaOH
O
S NHR
O
Resin O HN
O
O S O
NHR
22
21
Figure 7: The use of LPCS to construct a combinatorial library of sulfonamides.
8
The use of solid supports for organic synthesis relies on three interconnected
requirements:
(a)
a cross-linked, insoluble, polymeric material that is inert to the conditions of
synthesis.
(b)
some means of linking the substrate to this solid phase that permit selective
cleavage of some or, all of the product from the solid support during synthesis
for analysis of the extent of reaction(s) and ultimately to give final product of
interest.
(c)
a chemical protection strategy to allow selective orthogonal protection and
deprotection of reactive groups in monomers (Figure 8).
4.1
Solid Supports
The earliest forms of resin used were partially cross-linked polystyrene beads
(the styrene is cross-linked with 1% divinyl benzene to give mechanical strength and
insolubility whilst still permitting the flexibility apparent during solvent swelling) in a
wide variety of sizes, prepared by light- or radical-catalyzed polymerization in
aqueous/organic mixture. Polymerization takes place in micro droplets giving beads
of an approximately spherical shape. The consistency of size is ensured by sieving.
Figure 8: Essential to the success of solid-phase synthesis is judicious choice of synthetically
compatible solid-phase, linker chemistry and protecting group strategy. In this example, functional
groups (FG1,3,5 and 6) on the monomers A, B and C are protected until the end of the synthesis.
Reactive groups on the monomers (FG2 and 4) involved in the coupling chemistry are also protected
until required.
9
R
R
+
Cl
NHBoc
O
NHBoc
O
O
O
Merrifield Resin
23
25
24
Figure 9: Loading of Merrifield chloromethyl resin with N-protected amino acids.
4.1.1
Merrifield Resin
The earliest form of polystyrene resin (Merrifield resin) used for peptide
synthesis was derivatized with a chloromethyl group (23) to which amino acids (24)
could be coupled by nucleophilic displacement (Figure 9). The resulting ester bond
was stable to the conditions of peptide synthesis and was cleaved to give carboxylic
acid product under vigorous acidic conditions (hydrogen fluoride).
4.1.1.1 Synthesis of a Library of 1,3-diols with Antioxidant Activity
Kurth and coworkers20,21 have described the synthesis of a library of 1,3-diols
with potential antioxidant activity by the use of a two-step aldol-reduction sequence
(Figure 10). The common precursors (26) are readily prepared from Merrifield resin
and can readily be converted to a zinc enolate that react smoothly with a range of
aldehydes. DIBAL reduction of the intermediate aldol products (27) effects
concomitant removal from the resin giving diols such as (28) (26% overall as a 7:5
threo:erythreo mixture) In all, 27 discrete compounds were prepared in this manner.
4.1.1.2 “Diversomers”: An Approach to Nonpeptide, Nonoligomeric Chemical
Diversity22
Solid phase chemistry, organic synthesis, and an apparatus for multiple,
simultaneous synthesis have been combined to generate libraries of organic
compounds (diversomers). Arrays of compounds were synthesized over two to three
steps incorporating diverse building blocks on a Merrifield solid support in a multiple,
10
simultaneous manner. The generality of this approach is illustrated by the synthesis of
dipeptides, hydantoin and benzodiazepines.
The construction of benzodiazepine library of forty members (32) was down
by treatment of five amino acid resins (29) with each of eight 2-aminobenzophenone
imines. This two steps synthesis created an array with five variations in step one and
eight variations in step two to generate forty different compounds (32), (Figure 11).
These compounds were screened in a competitive binding assay to ascertain
biological activity.
Ph
Cl
COO Na
O
Bu4 N Br
THF, reflux, 3 hr
R
O
23
26
o
(1) LDA, THF, 78 C
o
(2) ZnCl2 , 0 C
(3) CH3 O
CHO
OH
O
HO
o
OCH3
DIBAL C
Toluene
OH
O
OCH3
27
28
Library of 27 members.
Figure 10.
Figure 11: General route for synthesis of benzodiazepines.
11
Figure 12: General route for synthesis of hydantoins.
Applying the same method, the authors were able to construct a small library
of hydantoins (Figure 12) by deprotection, then treating each of the eight amino acid
resins (33) with each of five isocyanates (34) to give 35 which was treated with a
protic acid and refluxed to remove all protecting group, releasing the compound from
the resin and induce cyclization to give the hydantoin derivatives (36).
4.1.2
Wang Resin
Wang resin is one of the most commonly used solid supports in combinatorial
chemistry. It is a modified polystyrene resin.
4.1.2.1 Generation of a Substituted Guanidine Library (43)
A novel and efficient method for the synthesis of guanidine derivatives23
adapted by the following sequence, the wang resin (37) was activated with p-nitro
phenyl chloroformate to give activated resin (38). The product was treated with 1-Hpyrazole-1-carboxamide 39 to give resin bound pyrazole 40 which was deprotenated
and acetylated to give resin-bound guanidinylating agent (41). The latter was reacted
with different amines to give resin-bound disubstituted guanidine (42), which was
finally cleaved by trifluoroacetic acid to give 43 (Figure 13).
12
O
OH + Cl
O
NO2
O
O
DIPEA
10 hr
NO2
O
37
38
1H-pyrazole-1DIPEA carboxamide
39
N
O
R2R3NH/THF
R1
N
LiHMDS/THF
0-5oC
R1COCl
N
H
N
O
N
H
O
N
NH
40
41
NR2R3 O
O
O
O
N
H
N
O
N
R1
60:40
RT
NH
O
TFA: CH2Cl2
R1
N
H
NR2R3
43
42
Figure 13: Solid-phase synthesis of N-acyl-N-alkyl/aryl disubstituted guanidines.
4.1.2.2 Synthesis of Peptidomimetic Inhibitors for the Hepatitis C Virus NS3
Protease24
The NS3 serine protease enzyme for the hepatitis C virus (HCV) is essential
for viral replication. Intensive investigation in this area of research revealed that
compounds 44, 45 and 46 are very good lead compounds and that the existence of an
aromatic substituted to the C4 of proline unit via short linker leads to a significant
increase in its potency (Figure 14). This suggests the existence of a lipophilic pocket
at a distance from the binding site of the back bone. It was decided to explore this
pocket and optimize the interaction with some new inhibitors. A small library of
compound based on a peptidomimetic Scaffold 46 has led to the identification of
potent and highly selective inhibitors of the NS3 protease enzyme.
13
COOH
O
H
N
O
N
H
N
H
O
COOH
O
H
N
O
O CH2
COOH
O
N
H
44
O
N
H
COOH
H
N
O
O
O
N
N
H
O
O
N
H
COOH
H
N
O
O
N
O
O
45
N
H
COOH
46
Figure 14: Inhibitors of the Hepatitis C NS3 protease.
Automated peptide chemistry was used to synthesize the Wang-bound
tetrapeptide 48, having the (S)-absolute stereochemistry at the C4 of proline, as the
TBDMS protected Wang-bound silyl derivative 47 (Figure 15). Each coupling step
was carried out using 2 equivalents of Fmoc-protected amino acid, HOBt, and TBTU
in the presence of 4 equivalents of a base. Upon completion of all peptide elongation
steps, the N-terminus was capped with acetic acid and the TBDMS group was
removed with tetrabutylammonium fluoride. The resin-bound peptide small aliquots
(120 mg) were loaded into the 96-well block of the Advanced Chem Tech 396
synthesizer for production of Mitsunobu library.
4.1.2.3 Synthesis of Piperazinedione Combinatorial Library
Gordon and Steele25 have reported the synthesis of a prototype library of 1000
trisubstituted piperazinediones (58) (diketopiperazines, DKPs) by a sequence of three
key steps: a novel solid-phase reductive alkylation, acylation by a second amino acids
14
O
H3C
N
H
Ar
O
H
N
O R1
N
O
NH
O
O
O
O
(1) PPh3/DIAD
(5 eq.)
H3C
N
(2) Ar-OH or Ar-SH
H
(5 eq.)
THF/RT, 4 hr
R2
O
N
O
NH
O
O
O
49
47: R1 = OSi (CH3)2-tBu
48: R1 = OH
TBAF
O
H
N
When R2 = Br
Pd(PPh3)4
2 M Na2CO3
Ar"-B(OH)2
DME, 85oC, 15-18 h
Ar
O
H3C
H
N
N
H
O
O
R1
TFA 45%
CH 2Cl 2
N
O
NH
O
H3C
N
H
H
N
O
Ar"
O
N
O
NH
COO H
O
O
O
50
51
Library of 42 compounds with different substituents at C4 proline
screening
O
, IC50 = 5 M
The most potent inhibitors is 52 R =
N
O
O
IC50 = 6 , 54 R =
53, R =
CF 3
, IC50 = 2 M
Cl
Figure 15: Library synthesis of the hepatitis C NS3 protease inhibitors.
and cyclization to the DKP products (Figure 16). Amino acids on Wang resin 55,
were reductively alkylated with alkyl or aryl aldehydes using sodium triacetoxy
borohydride as the reductant, to give resin-bound secondary amines (56). The
acylation of secondary benzylic amines is very difficult and required a double
coupling procedure using PyBrOP to achieve good conversion to 57. TFA-mediated
cleavage of 57 from the support did not cause spontaneous cyclization and so a brief
15
R1
O
R1
ArCHO
NaBH (OAc)3 /CH2Cl2
sonicate then repeat
NH 2
O
O
Ar
N
H
O
56
55
R2
HOOC
PyBrOP
NHBoc i-Pr2NEt, CH2Cl2
OH
O
N
HN
R1
C9H19
screening
O
O
R1
Ar
(1) TFA 25oC
N
HN
O
(2) Toluene, reflux
O
Ar
N
O
SMe
59
NHBoc
O
2
R
R
58
2
57
Figure 16.
reflux in toluene effected closure to 58. Iterative screening and resynthesis of this
library have identified several DKPs with significant biological activity including 59
which has a high affinity for the neurokinin-2-receptor (IC50 = 31 nM).
4.1.3
Rink Resin
Polystyrene is completely hydrophobic in nature, whereas the growing peptide
chain is much more hydrophilic, and this difference induces a chain folding effect in
which peptide chain satisfies its own hydrogen-bond requirements rather than being
solvated and hence the synthetic pathway will not be completed. To solve this
problem another different solid phase such as Sheppard’s polyamide resin have been
developed as these polymers are hydrophilic like growing peptide chain itself and
both can be readily solvated by dipolar aprotic solvents (e.g. DMF or N-methyl
pyrrolidinone).
4.1.3.1 Combinatorial Lead Optimization of a Neuropeptide FF Antagonists26
The tripeptide Pro-Gln-Arg-NH2, derivatized at the secondary amino group of
proline residue with (5-dimethylamino)-1-naphthalenesulfonyl (dansyl-PQR-NH2)
16
(60), antagonize the central anti-opioid action of neuropeptide FF in animals after
systemic administration and therefore, is a therapeutic lead to treat opiate withdrawal.
For a combinatorial optimization to improve potency, libraries focused on the possible
replacement of the proline and glutamine residues of this lead compound were
obtained by a solid phase (using Rink amide, methoxybenzyl hydrazine resin) splitand-mix method with coded amino acids (excluding cysteine) as building blocks.
After screening for competitive binding against a radioiodinated neuropeptide
FF analogue, 5-(dimethylamino)-1-naphthalene sulfonyl-Gly-Ser-Arg-NH2 (61) (dansylGSR-NH2) has emerged as one of the compounds in the library with high affinity to
the NPFF receptor.
Compound 61 has a high affinity to the NPFF receptor and even with a
moderate increase compared to 60 in its predicted ability to penetrate the central
nervous system.
O
H3C
N
H3C
NH 2
O
O
S
N
O
N
H
H
N
O
NH 2
O
NH
Lead compound 60
H3C
N
H3C
O
S
O
O
H
N
N
H
HN
OH
H
N
NH 2
O
NH 2
O
NH
61
HN
NH 2
Since there are only two variable residues to manipulate, an approach
involving mixture based synthetic combinatorial libraries and positional scanning was
applied.27,28 Dansyl-OXR-NH2 and dansyl-XOR-NH2 combinatorial libraries were
17
prepared using 9-fluorenylmethyloxycarbonyl (Fmoc) chemistry and the split and mix
method.29
4.1.3.2 Compounds that Activate the Mouse Melanocortin-1-Receptor Identified
by Screening a Small Molecule Library Based Upon the -turn30
A library of 951 compounds based upon the -turn motif were examined for
their ability to stimulate the melanocortin-1-receptor from the screening process, two
compounds were identified to possess low micromolar agonist activity at the
mMC1R. Compound 69 possesses an EC50 value of 42.5  6.9 M and compound 70
possesses an EC50 value of 63.4  26.9 M. The results of the library screening
process are consistent with the hypothesis proposing that a -turn conformation
involving the melanocortin “Phe-Arg-Trp” core amino acids provides the key
recognition element31-33. This library was synthesized34,35 by using amino methylated
resin beads (62) coupled with S-acetyl-2-mercapto-2-methyl propionic acid to give 63
which was used as a starting material for the preparation of heterocyclic -turn
mimetics (Figures 17 and 18).
4.1.3.3 Characterization and Screening of 43000 Compounds Tetrahydroisoquinoline Combinatorial Library36,37,38
The isoquinoline library was prepared using 11 amino acids, 38 aldehydes and
51 amine nucleophiles. Library synthesis applied the divide, coupled and recombine
method, whereby 11 amino acids were separately coupled with MBHA resin to give
(71). The resin-bound derivatives were mixed and then split into 38 equal size pools.
Each mixed amino acid pool underwent imine formation with a single aldehyde (73),
and then reacted with homophthalic anhydride (74). The resulting 38 isoquinoline
pools (75) were again mixed and divided into equal portions for coupling to amino
nucleophiles to give (77), the substituted isoquinoline library (Fig. 19). This library
was tested in  and  opioid receptor binding assays, and in  receptor binding assay
18
and the greatest activity was observed in the  receptor binding assay for pool 291
containing adamantanemethylamine amides of the mixed isoquinolines. Deconvolution of pools active in these assays is in progress.
O
O
NH 2
O
O S
CH 3
HO C C CH 3
CH 3
+
S
PyBOP/DMF
12 hr/RT/i-Pr2EtN
CH 3
N
H H3C CH 3
62
63
(1) NaOCH3
3:1 THF/MeOH
(2) BtSS-(CH2)2-OMs
H
Ri + 3 N
MsO
S
S
CH 3
CH 3
O
H2N
R
S
S
i +3
CH 3
CH 3
O
NH
NH
65
64
1. Fmoc--amino acid
HATU, i-Pr2EtN
2. Piperidine
3. -haloacids, DICI, HOAt
Ri + 2 O
O
N
Ri + 3
Ri + 2 O
Ri + 1
N
H
(1) TCEP
S
S
Br
O
H3C
(2)
N CH3
CH 3
CH 3
N
N CH3
CH3
NH
N
H
O
67
N
Ri + 1
screening
S
Ri + 3
68
66
H
N
H2N
O
O
S
O
N
O
NH
S
N
N
NH
70
69
Figure 17: Synthetic route for the preparation of heterocyclic -turn mimetics.
19
Figure 18: Side chain building blocks used in the generation of the -turn library examined at the
melanocortin receptor subtypes.
The “libraries from libraries” concept involves the blanket transformation of
one library into a related library exhibiting significantly different physical properties.
The feasibility of diborane reduction of 4-carboxamidotetrahydroisoquinoline library
to provide a 4-aminomethyl tetrahydroisoquinoline library (Figure 20) is an excellent
example of this previously mentioned concept.
4.1.3.4 Construction of a “100-Member DHP Library”39
The dihydropyridine core nucleus has been found to be present in numerous
molecules spanning a wide range of bioactivity, the most successful examples being
20
O
R2
Resin R1 NHFmoc
(2) R2CHO (72)
71
O
R1
74
X-R 3
N
X-R 3
R2
screening
R1
O
O
R2
(1) HATU
R1
(2) R3 Nuc
TFA
N
N
73
O
O
O
(1) Piperidine
OH
R2
N
O
O
76
75
R1
Pool 291 showed the greatest activity
in the sigma receptor binding which contains
adamantanemethylamine amides of the
mixed isoquinolines.
77
Figure 19: Synthesis and construction of isoquinoline library.
R4
N 3
R
O
R2
N
R4
N
Diborane
R2
N
R1
R3
R1
O
Figure 20: Libraries from libraries, reduction to 4-aminomethyl isoquinoline.
the calcium channel blocker antihypertensive drugs such as Nifedipine, Nitrendipine
and Nimodipine.
Preparation of a large size DHP library was undertaken to screen for calcium
blockade activity as a test study, and for eventually screening against a range of
enzyme and receptor targets in high-throughput screening format. In the first step 10
different -keto esters representing an adequate degree of structural diversity were
employed, and the resulting enamines were pooled together. In the second step, a
21
NO2
CH3OOC
H3C
COOCH3
N
H
CH3
Figure 21: Nifedipine.
3-component heterocyclization was employed. Thus, methyl acetoacetate was chosen
as the 1,3-dicarbonyl component and held constant, diversity was then derived from
choice of 10 different aromatic aldehydes. The end result was a 100-member DHP
library derived from employing 10 -keto esters in the first step and 10 aromatic
aldehydes in the second step. The library is in the form of 10 different pools, with
each pool bearing a distinct single aryl group and 10 different R1/R2 substituents
derived from the 10 -keto esters employed for forming enamino esters (Figure 22).
Screening of the 10 pools for the calcium blockade activity after their
oxidation by human liver microsomes40 indicated that o-nitrobenzaldehyde and
o-fluorobenzaldehyde DHP pools are the most active pools. Deconvolution and
parallel synthesis of individual members of a pool was performed and rough IC50 (the
concentration of the compound which cause 50% calcium blockage) estimations were
made using the crude, unpurified samples. The compounds of interest were then
purified by HPLC, fully characterized, and used for precise IC50 value determinations
(Figure 23). In summary, both ethyl and isopropyl analogs of nifedipine possess
remarkable activities.
4.1.4
Tenta Gel Resin
Tenta gel resin consists of about 80% polyethylene glycol grafted to cross-
linked polystyrene and is often used in the synthesis of non peptide combinatorial
22
Figure 22: Design and construction of a 100 member DHP library.
23
Figure 23: Screening of “100 member” DHP library to identify calcium, blockade activity.
libraries. It is generally considered that the reaction milieu within this resin is more
closely related to ether and tetrahydrofuran and consequently, it has the potential for
compatibility with the large range of reactions that are currently being investigated for
compound library synthesis.
4.1.4.1 Identification of Novel Inhibitors of Matrix Metalloproteinases41
The discovery of novel series of heterocyclic matrix metalloproteinases
(MMPs) inhibitors was achieved through consolidation of combinatorial synthesis and
rational drug design42. Combinatorial libraries were prepared using Tenta gel resin
and evaluated for their ability to inhibit collagenase-1, stromelysin-1 and gelatinase-B
substrate hydrolysis. Recently, important approach has been reported for
identification of some novel inhibitors for combinatorial libraries based upon the
rational design of new inhibitor series. On the basis of published crystal structures of
24
succinyl hydroxamate inhibitors bound with various MMPs43, the pharmacophoric
model was compared with a number of internally generated heterocyclic scaffolds.
Acceptable overlap was obtained with 2,5-diketopiperazine (DKP) scaffold, DKP
libraries were constructed utilizing cysteine as the zinc ligand, and the remaining two
diversity sites were expected to interact with enzymes specificity pockets. Initial
libraries incorporated both L- and D- cysteine two different substitution patterns
DKP-I and DKP-II were investigated. DKP-I has a 584-members library (Figure 24)
was prepared via the incorporation of L- and D- cysteine, 19 aldehydes and 18 amino
acids, each library consisted of 36 pools, each defined by cysteine stereoconfiguration and amino acids R3. While, a 684-member DKP-II library (Figure 25)
was prepared similarly through the utilization of 19 amino acids, 18-aldehydes and
both isomers of cysteine, the library consisted of 36 pools, each defined by cysteine
stereo-configuration and aldehydes R2.
The synthesis of DKPs44 on a solid support (Tenta gel Resin), begins with
esterification of the solid support with an amino acid to give 79, followed by
reductive alkylation of the amino acid and acylation of the resulting secondary amine
80 to give 81. Deprotection of the N-alkylated dimmer followed by cyclative cleavage
of the linear precursor from the resin, yields a soluble DKP 82 (Figure 26).
4.1.4.2 Biological Screening of the Produced (MMPS) Inhibitors
A fluorogenic assay that was compatible with the efficient screening of the
DKP libraries against four targeted enzymes. Regarding DKP-I, the most active pool
D-cysteine and pyridylalanine at R3 having an IC50 of 15 M against collagenase-1.
Deconvolution of this pool identifies two KDPs, with low-micromolar activity
(Figure 27).
25
R2
HS
O
N
O
N
H
R3
Fig. 24: Diversity set for DKP-1.
Figure 25: Diversity set for DKP-II.
26
R1
(P) HN
COOH
(i) DIC, DMAP, DCM OR
DMFP, DIEA, DCM
(ii) Deprotection
HO
NH2
H
NaCNBH3, MeOH or
HOAC trimethyl orthoformate
R1
79
O
R2
R2
R3
N
HN
R2
O
78
R3
O
O
TFA
R1
Boc N
H
Toluene
O
82
N
O
R2
R3
O
O
(P) HN
HN
COOH
HATU, DIEA, DCM
R1
81
O
O
R1
80
Figure 26: Solid phase synthesis of diketopiperazine.
R2 (1-18)
N
HS
O
O
N
N
H
R2 = CH3 -(CH2)6 -
IC50 = 10M
R2 = CH3-(CH2)4 -
IC50 = 2 M
Pool IC50 = 15 M
Figure 27: DKP-I-Deconvolution results.
On the other hand, in case of DKPII, L-cysteine and p-anisaldehyde at R2 is
the most active pool, deconvolution of this pool indicated that variation of amino
acids R1 had significant effect on inhibition potency (Table 1). Compounds with
desired potencies were resynthesized, purified, fully characterized and assayed to
obtain accurate IC50 values.
The results obtained from biological data indicate that there is a great
correlation between the stereo-configuration of the drug and its biological activity, by
the mean of combinatorial synthesis, the four stereoisomers of DKP’s were
synthesized and assayed. The change in the configuration of any of the stereocenters
resulted in substantial loss of activity (Figure 28). The overall results of the obtained
data revealed that (S)-configuration is required for both amino acids and cysteine.
Table 1: IC50 values for purified DKPs.
27
O
HS
N
HN
R2
R1
O
IC50 (nm)
R1
R2
C6H11CH2
C6H11CH2
C6H11CH2
PhCH2
PhCH2
A
B
C
D
MMP-1
MMP-3
47
35
30
64
319
2910
2400
3800
4045
19000
121
87
79
2365
2365
Stromelysin
2.600
>40,000
>40,000
>40,000
Matrilysin
590
>40,000
4,800
>40,000
C6H11
2-Quindyl
p-CH3OC6H4
p-CH3OC6H4
n-Propyl
Collagenase-1
47
14,000
1,600
2,600
Gelatinase-B
113
>40,000
12,000
7,000
MMP-9
Figure 28: Dependence of IC50 values (nM) upon stereoconfiguration of DKP centers.
4.1.5
Ellman’s Dihydropyran Resin
Ellman’s resin is 3,4-dihydro-2H-pyran-2-ylmethoxymethylpolystyrene (DHP-HM
resin), Figure 29.
4.1.5.1 Synthesis of a Small Library of Substituted Hydroxy Proline-Based
2,5-Diketopiperazine
A general solid phase method45 for the synthesis of a new class of 2,5diketopiperazines (DKPs) containing the trans-4-hydroxy-1-proline amino acid
residue (Hyp) have been developed (Figure 30). An N-protected hydroxyproline
28
O
O
83
O
+
O
Cl
23
84
PPTS
85
RO
+ ROH
O
O
86
Figure 29: The linking and cleavage of alcohols from a dihydropyran-derivatised resin.
Figure 30: Synthesis of bicyclic piperazine library.
methylester was linked through the hydroxy function to the Ellman resin (84). The
synthesis procedures were conceived to enable a sequence of Hyp alkylation, Hyp-Nacylation, cyclization, and amide bond alkylation. Up to three different centers of
molecular diversity were introduced around the DKP scaffold. Highly functionalized
bicyclic compounds were obtained in good yield and purity. The alkylation of
hydroxyproline -CH was performed without control of the diastereoselectivity.
During the final alkylation of the backbone, amide bond epimerization at the -carbon
atoms of the two amino acid residue was observed.
29
4.1.5.2 Combinatorial Libraries in the Discovery of Aspartic Acid Protease
Inhibitors46
Aspartic acid proteases are a large distributed family of enzymes that play
pivotal roles in plants, vertebrates, fungi and retroviruses. The aspartic acid proteases
(contains two aspartic acid residues at the active site) catalyze the hydrolysis of amide
bonds with specificity for peptide bonds located between large hydrophobic residues.
A number of aspartic acid proteases are important pharmaceutical targets including
cathepsin D47, human t-cell leukemia48, renin49 and the human immunodeficiency
virus (HIV) protease50.
Potent inhibitors of these enzymes usually designed as an isostere that mimics
the geometry of the tetrahedral intermediate in place of the scissile bond of peptide
substrate51. Unfortunately, these inhibitors have limited therapeutic utility, due to the
poor oral availability and/or short circulating half-lives that result from their peptidic
nature. For this reason, there has been a tremendous amount of work towards the
development of aspartic acid protease inhibitors that display nonpeptide functionality
about the isostere of the tetrahedral intermediate. First, several orally available HIV-1
protease inhibitors that incorporate these isosteres have been identified (Figure 31),
including compounds that are currently in clinical trials for the treatment of HIV
infection. Second, solid-phase methods could be developed to display a wide range of
diverse functionality about these isosteres.
The scaffold 93 was first coupled to dihydropyran functionalized polystyrene
support. Displacement of tosyl alcohol (94) with either functionalized or unfunctionalized
primary or secondary amines, including amines found in known HIV-1 protease
inhibitors (Figure 32, 95a-h and 96i). After coupling of the primary amines, the resulting
secondary amine products 95a-g can be converted to ureas (96a-f) by reaction with
30
Figure 31: Examples of HIV-1 protease inhibitors.
(a) R1NH2, NMP, 80°C; (b) piperazine, NMP, 80°C; (c) N-tert-butyl-L-pipecolinamide, NMP, 95°C;
(d) R2NCO, ClCH2CH2Cl; (e) (i) OC (OCCl3)2, Et3N, cat. DMAP, THF, (ii) 4-(3-aminopropyl)morpholine, THF; (f) butyryl chloride, i-Pr2EtN, CH2Cl2.
isocyanates or by stepwise treatment with triphosgene followed by amine addition (96g),
the piperazine derivative (95h) was acylated with butyryl chloride to provide 96h.
Compounds 96a-i were converted into primary amines (97a-i), where (97a-c) were
31
(a) SnCl2:HSPh:Et3N (1:4:5), THF, (b) 3(S))-tetrahydrofuranylsuccinimidyl, i-Pr2EtN, CH2Cl2; (c) 95:5
TFA/H2O; (d) Fmoc amino acid, PyBOP, HOBt, i-Pr2EtN (3 equiv), DMF; (e) 20% piperidine in DMF;
(f) pentafluorophenyl ester of quinaldic acid, HOBt, Et 3N; DMF.
Figure 32: Synthesis of aspartic acid protease inhibitors.
Figure 33: (Hydroxyethyl)amine and (hydroxyethyl)urea derivatives synthesized on solid support.
Yields of analytically pure material after chromatography were determined from mass balance and
were based upon the initial loading of alcohol 93. Elemental analyses were all within 0.4% of
theoretical value.
acetylated with protected amino acids to give 98a-c which were further acylated to give
99a-c while compounds 97d-i were converted directly into 99d-i (Figure 32).
In summary, the synthetic pathway gave a good yields (47-86%) of molecules
incorporating the (hydroxyethyl)amine and (hydroxyethyl)urea isosteres after four to
six transformation on solid support (Figure 33).
32
4.1.6
Trityl Chloride Polystyrol (TCP) Resin
It contains a trityl based linker that can be straightforward loaded with amines
and other functionalities, also the eventual cleavage of products can be carried out
conveniently using dilute TFA.
4.1.6.1 Synthesis of New Selective vB3 Integrin Antagonists
The solid phase synthesis (using TCP resin) of a low molecular weight RGD
(Arg-Gly-Asp) mimetic library is described.52 On the basis of the lead compound
10053,54, a library of RGD mimetics consisting of four building blocks A-D (Figures
34-39), has been synthesized. Two sets of mimetics have been investigated. In the
first set, -amino acids have been used as carboxylic building block A1, whereas the
second set – the azacarba analogues – represents a novel peptidomimetic approach
with glutaric acids as carboxylic building block A2 (Figures 34, 35).
Activities of the compounds in inhibiting the interaction of ligands vitronectin and
fibrinogen, with isolated immobilized integrins vB3 and IIb3 were determined in a
screening assay.55 All guanidine-containing compounds show excellent affinities to the
v3 receptor in nanomolar range. To increase lipophilicity of RGD mimetics, the
Figure 34: Aza-RGD mimetics with various aromatic -amino acids (X = NH) or glutaric acids (X =
CH2) and different guanidine mimetics derived from compound 100.
33
Figure 35: Retrosynthetic analysis of the RGD mimetic library obtaining four different building
blocks: carboxylic acid A1 (-amino acids) and A2 (glutaric acids), B1 (aza-glycine) and B2 (hydrazine),
spacer C, and basic building block D.
guanidino moiety has been substituted by methyl amidine and aminopyridine as basic
building block D. Incorporation of the less basic aminopyridine led to RGD mimetics
with promising properties for oral bioavailability while keeping nanomolar activity.
Synthesis of building block A1 and A2 (Figure 36, 37).
Reagents: (a) NH4OAc, malonic acid, EtOH (74-88%); (b) Fmoc-Cl, NaHCO3, dioxane (76-98%); (c)
ethyl acetoacetate, piperidine (cat.) (42-85%); (d) 20 M KOH, 85°C (68-99%); (e) acetic anhydride
(62-89%); (f) N-Fmoc-hydrazine, THF (100%); (g) TCP resin, DIEA, CH2Cl2.
Figure 36.
34
Reagents: (a) TCP resin, DIEA, CH2Cl2; (b) 20% piperidine/DMF; (c) 5-(9H-fluoren-9-ylmethoxy)-1,3,4oxadiazol-2(3H)-one (108); (d) phosgene. (1.9 M solution in toluene), sat. NaHCO3, CH2Cl2 (85%).
Figure 37.
Reagents: (1) 100°C (23%); (b) 1 N NaOH (64%); (c) NaH, DMF, 80°C (10%).
Figure 38: Synthesis of building block C and D.
4.1.6.2 Solid Phase Synthesis and Biological Evaluation of a Combinatorial
Library of Philanthotoxin Analogues56
Philanthotoxins, a group of non competitive antagonist of ionotropic receptors,
are composed of long-chain polyamines connected to a relatively non polar head
group via an amide bond.57,58 The prototype structure is philanthotoxin-433 (124) has
35
Reagents: (a) 20% piperidine/DMF; (b) 3-(N-Fmoc-amino)benzoic acid, HATU, collidine, DMF; (c)
5-(4-methylpyridine-2-yl)aminopentanoic acid (113), HATU, collidine, DMF; (d) AcOH/TFE/CH2Cl2
(1:1:3); (e) N,N-bis-Boc-1-guanylpyrazole, CHCl3, 50°C; (f) 95% TFA/5% TIPS; (g) S-2-naphthylmethyl thioacetimidate hydrobromide, DIEA, NMP.
Figure 39: Synthesis of vB3 Integrin antigonist.
recently been highlighted by the observation that specificity of their antagonist action
on various classes of ionotropic receptor can be achieved by modification of the
polyamine portion of the molecule59. Thus, the natural toxin PhTX-433 (124) and its
synthetic analogue PhTX-343 (125) antagonize various types of nicotinic acetylcholine receptor (nAcRs)60 and ionotropic glutamate receptor (iGluRs)61 with similar
potency. On the other hand, the analogues 126 (PhTX-12) and 127 (4,9-dioxa
PhTX-12) in which the secondary amino groups are replaced by methylene groups or
oxygen atoms, exhibit enhanced activity at mammalian muscle type nAChR and
Torpedo nAChR while being inactive on several types of iGluR62 (Figure 40).
36
OH
O
NH
H
N
N
H
H
N
NH2
X
NH2
O
124 (S)-PhTX-433
OH
O
NH
X
N
H
O
125 rac PhTX-343 (X = NH)
126 rac PhTX-12 (X = CH2)
127 rac-4,9-dioxa-PhTX-12 (X = O)
Figure 40.
The latter selectivity is not obtained when the aromatic head group portion of
the philanthotoxin molecule is modified63. In previous structure activity investigation,
a considerable number of synthetic analogues of 125 containing the symmetrical
spermine moiety or closely related polyamine have been tested on iGluR and
nAChR64. The results of these investigations emphasize the importance of the
hydrophobic character of the head group. By contrast, no information about the
influence of the structure of the head group on the potency of philanthotoxin
analogues that lack inner basic sites (such as compound 126 and 127). This issue is
addressed in this study where a library of 18 members was constructed (Figure 41),
using (S)-tyrosine and (S)-3-hydroxyphenylalanine as amino acid components,
spermine, 1,12-dodecanediamine and 4,9-dioxa-1,12-dodecanediamine as amine
components, and butanoyl, phenyl acetyl and cyclohexyl acetyl as N-acyl groups. The
37
Figure 41: Synthesis of philanthotoxin analogues.
obtained compounds were tested electrophysiologically for their antagonist properties
on (nAChR) and on glutamate receptor (non-NMDAP).63,64
4.1.7
Tetrafluorophenol TFP-Activated Resin
Salvino and coworkers67 at Rhone-Poulenc Rorer (now Aventis) developed a
novel set of tetrafluorophenol (TFP)-activated resins for amine derivatization
(Figure 42). TFP resin 148 is prepared67 by coupling 4-hydroxy-2,3,5,6tetrafluorobenzoic acid 146 to amine polystyrene resin 147. The resin 148 is activated
either by acylation (148149) or sulfonylating (148150), the phenolic OH with
38
carboxylic or sulfonic acids, their anhydrides or acid chlorides. Resins 149 and 150
are suspended in DMF and reacted with an amine nucleophile to cleanly provide the
corresponding amide or sulfonamide derivative (151152,153). The major advantage
of using the TFP-activated resins over other known resins is that
19
F NMR
conveniently determines loading.67,68,69 This is important because it allows the amine
to be accurately used as limiting reagent thus negating the need to scavenge excess
amine (consumed) or acylating/sulfonylating reagent (remains resin bound). A wide
variety of carboxylic acid and sulfonic acids may be loaded onto the resin, creating a
reagent kit which can be used on demand for a derivatization campaign. Activated
TFP resins may be kept for years without decomposition. By simple distribution of
TFP resins 149 and 150 into 96-well plates, suspension of the resin in DMF and
addition of a limiting amount of amines, hundreds to thousands of amides and
sulfonamides can be rapidly generated. The derivatives are sufficiently pure (>85%)
for direct evaluation in biological screening.
4.1.7.1 Synthesis and Identification of a Potent Factor Xa Inhibitors70
By way of application, TFP-activated sulfonate resin was used to further
delineate the SAR in a series of factor Xa inhibitors (Figure 43). The library composed of 52 discrete compounds derived from the reaction of amine set 156-159 with
resin 150, furnished several submicromolar inhibitors including 161: IC50 = 15 nM.
4.1.8
Sasrin Resin
Sasrin resin is polystyrene attached to sasrin linker, it has an enhanced acid
sensitivity and able to release fully protected compounds (Figure 44).
(a)
Attachment of the acid to this resin achieved by using DCC/DMAP
(sometimes at reduced temperature).
(b)
Cleavage using 0.5-1% trifluoroacetic acid in DCM.71
39
Figure 42: Synthesis and application TFP-activated resins for amine derivatrization.
4.1.8.1 Orphan Nuclear Receptor FXR Agonists
Maloney and coworkers at Glaxo Welcome (now Glaxo-Smithkline) have
identified the first nonsteroid agonist for the orphan nuclear receptor FXR72. FXR is
believed to be involved in the regulation of bile acid and cholesterol homeostasis 73,74
(Figure 45). The only known ligands for the receptor are chendeoxycholic acid
(CDCA, 165) and the retenoic acid receptor agonist, TTNPB 166. Both 165 or 166 are
inadequate as pharmacological tools to study the orphan receptor because of the
interaction of 165 with bile acid binding and transport proteins and its metabolic
instability75 and because of the weak potency and poor selectivity of 166 (EC50 > 1
M)76,77. Glaxo’s lead, isoxazole 167 for FXR was identified from a combinatorial
40
Figure 43: Factor Xa inhibitors via TFP-activated sulfonate esters.
library of 1000 stilbine-containing carboxylic acid (unpublished results). Compound
167 was a weak FXR agonist (EC50 = 4.1 M) in a cell-based assay and enhancement
of the lead’s potency and selectivity was desired. Retrosynthetic library analysis of
167 led to its dissection into three sets of building blocks: vinyl substituted acids 168,
bromo/iodo-substituted phenols 169, and hydroxymethylisoxazole 170. Four olefins
were combined with five phenols via the Heck reaction to give, after two-step phenol
protected sequence, 20 stilbene carboxylic acids 171. The 20 templates were then
41
O
OH
O
O
O
+
DCC/DMAP
R
OH
OCH 3
OCH 3
Sasrin resin
162
R
O
164
163
Figure 44: Loading of Sasrin resin with acids.
loaded on to sasrin resin, the phenol deprotected, and coupled with 40 hydroxy methyl
isoxazoles (Mitsunobu Coupling reaction). Cleavage of the final products from resin
with TFA gave 600 discrete acids 175 on a 2-3 mg scale with > 80% purity. The acids
as obtained directly from cleavage were evaluated against FXR and 31 of them were
discovered with a cell based activity equal to that of CDCA at 50 M. Several of the
actives were resynthesized and purified on a 50 mg scale. Isoxazole 176 is a full
agonist with EC50 = 90 nM in CV-1 cell transfected with human FXR. In further
studies, 176 possessed an oral bioavailability of 10% in rat (t1/2 = 3.5 h) and upon a
7-day dosing in Fisher rats, a dose dependent lowering of serum triglycerides was
observed (ED50 = 20 mg/kg).
4.1.9
Polystyrene-based Selenenyl Bromide Resin
This resin was prepared by Nicolaou and coworkers.78 Recently they reported
a selenium-based approach for the solid-phase combinational synthesis of
benzopyran-containing natural products, utilizing a novel cycloloading strategy.
Given the versatility of this approach, it was sought to extend it toward the solid phase
synthesis of other heterocycles.
42
Figure 45: Orphan nuclear receptor FXR agonists.
4.1.9.1 Nicolaou’s Natural Product-like Benzopyran Libraries
A family of naturally occurring 2,2-dimethyl benzopyrans is the active
constituent in Cube resin, a century old botanical insecticide (Figure 46). The
mechanism of action is the disruption of oxidative phosphorylation (ATP synthesis)
43
which occurs through inhibition of mitochondrial NADH/ubiquinone oxidoreductase,
one of a cascade enzymes operating in the electron-transport system. As seen by
Nicolaou, the common structural feature or pharmacophore found throughout this
class is a benzopyran nucleus with a pendant electron-rich aromatic ring. By the use
of these natural products as a guide, a 52 member of 2,2-dimethylbenzopyrans with a
tethered (or bridged) aromatic ring was screened for oxidoreductase activity. The
salient methodology for constructing these compounds is the so-called “cycloloading”
strategy a clever solid-phase variety of intramolecular seleno-etherification reaction.
In this chemistry, selenyl bromide resin 181 is reacted with O-phenylphenols 180,
which under cycloloading i.e. simultaneous cyclic ether formation and resin
attachment. Because resin 182 is stable to Lewis and Brönsted acids, organometallics,
reducing agents, bases, electrophiles, HF pyridine, Pd and Ru catalysts and many
other reagents, broad structural diversification of the benzopyran nucleus is possible.
Cleavage occurs upon selenoxide formation (treatment with m-CPBA or H2O2) and
Syn elimination to yield 3,4-dihydrobenzopyrans. Several actives, for example, 185
(IC50 = 55 nM), were identified from library and biological activity was highly
dependent on the bridging elements. Five following libraries were then synthesized to
define the SAR and to optimize potency, ultimately yielding a family of potent
2,2-dimethylbenzopyran inhibitor (186: IC50 = 19 nM) with IC50 values against
NADH/ubiquinone oxidoreductase in the range 18-55 nM.
4.1.9.2 A Novel Strategy for the Solid Phase Synthesis of Substituted Indolines79
In this project, Nicolaou and his coworkers demonstrated that substituted
o-allyl anilines (187) might be cycloloaded onto a polystyrene-based selenenyl
bromide resin (181) via a 5-exo-trig cyclization to afford resin-bound indoline
44
Figure 46: Nicolaou’s natural product-like benzopyran libraries.
scaffolds (188). Elaboration of 188 would provide structures such as 189 that could be
tracelessly cleaved providing access to 1-methyl indolines (190), a structural class
from which several drug candidates have emerged including antineoplastic,
sulfonamides, 5-hydroxy tryptamine receptor antagonists (5-HT3)80 and muscarine
receptor agonist and antagonist81. Moreover, it was envisaged that the ability of this
45
Se
X
Functionalize
Se
X
Se
Functionalize
X
N
H
N
R1
N
X
188
189
191
Traceless
cleavage
SeBr
Cycloloading
R2
Cyclization
cleavage
181
NH2
X
X
CH3
N
X
N
R1
X
187
R2
Polycyclic indoline
192
190: 1-methyl indoline
Figure 47: General strategy for the solid-phase cycloloading, functionalization and cleavage of
substitute indolines.
selenium tether to generate a carbon-centered radical upon cleavage might also be
utilized to create additional complexity in the target structure concomitant with
release. Specifically, if an intramolecular radical acceptor could be positioned in
proximity to this radical (i.e. 191) then relatively complex polycyclic indolines 19282
(Figure 47), could be constructed.
4.1.10 Miscellaneous Advanced Resins
Solid phase synthesis has gained in popularity as a result of its relative ease of
automation and the emergence of high-throughput screening. Many efforts were
directed towards the development of many new advanced resins to meet the increased
demands in combinatorial chemistry83,84.
4.1.10.1 Alkyne Amino Ester Resin (193)85
Alkyne amino ester resin (193) was synthesized as depicted in (Figure 48).
The condensation of glycine ethyl ester. HCl (194) with benzophenone (195) gave
46
Schiff base (196).86 Subsequent alkylation with propargyl chloride gave the protected
amino ester 197 which, upon imine hydrolysis with aq. HCl and neutralization of the
resulting ammonium salt with aq. NaOH delivered 198. Boc-protection of the amino
moiety gave (199), followed by saponification and neutralization produced
Boc-protected amino acid (200) which was coupled with hydroxypropyloxymethylpolystyrene (from reaction of Merrifield resin with the alkoxide of 1,3-propanediol) in
the presence of DIC to give resin 193 (Figure 48).
This resin was used successfully to construct isoxazolothiohydantoin library of
19 members (Figure 49), using a Quest 210 Manual Synthesizer.
O
HClNH2CH2COOEt
O
Condensation
H2O
+
194
Ph
N C
EtO
Ph
196
195
NaH
R.T. HC C CH2Cl
THF/DMF
10:1
O
NHBoc
EtO
(Boc)2O
CH2Cl2
rt
O
O
NH2
EtO
199
1. aq. HCl
2. aq. NaOH
EtO
198
197
aq. NaOH, then
aq. HCl
O
HO
O
NHBoc
PS-O(CH2)3-OH
NHBoc
O
DIC, DMAP
CH2Cl2/DMF
4:1
200
193
Figure 48: Preparation of alkyne amino ester resin.
47
N C(Ph)2
4.1.10.2 ROMP-Spheres87
ROMP-spheres is a novel high-loading polymer support cross metathesis
between vinyl polystyrene and norbornene derivatives.88,89 Synthesis of this polymer
was achieved by adapting the following sequence (Figure 50).
O
O
NHBoc
O
NHBoc
O
R1CH2NO2, PhNCO
Et3N, THF
O
N
193
(1) THF/CH2Cl2
aq. HCl, then
aq. NaOH
(2) R2CHO, NaCNBH3,
ACOH
R1
201
R3
N
R3
NH
S
R2
O
N
O
O
O
N CH2R2
O
60oC
S
THF
R3NCS
NHCH2R2
O
O
N
O
N
N
R1
R1
204
203
202
Figure 49: Solid-phase synthesis of isoxazolothiohydantoin.
Cl
Me3S+I, n-BuLi, THF
PhCH=Ru(PCy3)2Cl2
CH2Cl2
Merrifield resin
23
205
P(Cy)3
Cl
Ru
Cl
P(Cy)3
206
(1)
CH2Cl2/5 h
X
(2)
O
(3) CH2Cl2
P(Cy)3
Cl
H2C Ru
Cl
P(Cy)3
n
X
208
207
Figure 50: Synthesis of ROMP-Sphere.
48
Ru
The immobilized catalyst (206) has been used to initiate a ring opening
metathesis polymerization of norbornene derivatives onto a polymer support to
prepare novel high loading resin for use in combinatorial chemistry.
4.1.10.3 Polymer-Supported Enantioenriched Allysilanes90
Commercially available PS-bound (PS: polystyrene) hydrosilane 209 was
chosen as a starting material of which transformation to chlorsilane 210 was achieved
by using 1,3-dichloro-5,5-dimethylhydantoin. Reaction of 210 with (diethylamino)
diphenylsilyllithium was carried out at room temperature for 10 h. The reaction
mixture was treated with acetyl chloride for conversion of the diethylamino group in
211 into chloride (212), (Figures 51). This P-S bound chlorodisilane (212) was
reacted with highly enantioenriched allylic alcohols 213a-c in the presence of
imidazole as a base in CH2Cl2. The resulting resins (214a-c) were treated with a
catalyst generated from Pd(acac)2/1,1,3,4,- tetramethylbutylisocyanide at 110°C in
toluene. Subsequently, the reaction mixture were treated with PhLi(for 214a) or n-BuLi (for
214b) at 0°C in THF to give PS-bound allylsilanes including 215a and 215b. For the
preparation of 215c bearing the w-hydroxy group on the allylic moiety, the THP protection
was removed with a catalytic amount of p-toluenesulfonic acid in EtOH/THF at 50°C.
With these PS-bound allylsilanes 215a and 215b in hands, the examination of
some reactions with aldehydes under typical reaction conditions using TiCl 4 at –78°C
(Table 2) was done.91
OH
Me
OH
Ph
(R)-213a 99.1% ee
Me
OH
Hex
(R)-213b 97.6% ee
49
Ph
OTHP
(R)-213c 99.0% ee
R
4 SiEt2
Si
Ph2
y
4 SiEt2X
b
209 (X = H)
210 (X = Cl)
211 (Y = NEt2)
212 (Y = Cl)
a
4 SiEt2
Ph2Si
O
d
CH3
214a R = Ph
214b R = n-Hex
214c R = (CH2)2O THP
c
(e) (f) For a, b
(e) (g) For c
R
4 Si
Et2
CH3
215 a R = Ph
b R = h-Hex
c R = (CH2)2-OH
(a) 1,3-Dichloro-5,5-dimethylhydantoin, CH2Cl2, room temperature, 5 h; (b) Ph2 (Et2N)SiLi, THF,
room temperature, 10 h; (c) AcCl, THF, room temperature, 4h; (d) 213a-c (1 equiv.), imidazole,
CH2Cl2, room temperature; (e) Pd (acac)2 (6 mol %), 1,1,3,3,-tetramethylbutylisocyanate, toluene;
110°C, 6 h; (f) PhLi or n-BuLi, THF, 0°C, 1 h; (g) TSOH, EtOH/THF (112), 50°C, 5 h.
Figure 51: Synthesis of Polymer-Supported Enantioenriched Allylsilanes.
Table 2: Allylation of aldehydes with PS-Bound Enantioenriched alkylsilanes 215a
and 215b.
R
R1CHO
TiCl4, CH2Cl2
4 Si
Et2
OH
R
Me
Aldehyde
Me
216a-d
(S)-215a (R = Ph, 99.1% ee
(S)-215b (R = Hex, 97.6% ee
Allylsilane (R)
R1
o
-78 C, 0.5-2 h
Product
Yield %
Syn: anti
% ee.
215a (Ph)
MeCHO
216a
(54)
96:4
98.6
215a
n-Hex CHO
216b
(44)
93:7
99.0
215a
i-PrCHO
216c
(40)
97:3
96.0
215b
i-PrCHO
216d
(34)
99:1
95.6
The reaction of PS-bound allylsilane 215c with aldehydes and acetal leads to
the asymmetric formation of disubstituted oxacycloheptanes.
50
Table 3: Asymmetric synthesis of disubstituted oxacycloheptenes 217 via reaction of
P-S-enantioenriched allylsilane 215c with aldehyde and acetal.
OH
O
RCHO
or RCH (OEt)2
Si
Et2
CH3
CH3
TMSOTF, CH2Cl2
-78oC, 2 h
217
(S) -215 c
99.0% ee
Allylsilane
Electrophile
R
Product (R)
% Yield
Trans:cis
% ee
215c
n-Hex-CHO
217a (n-Hex)
64
92:8
97.9
215c
n-Hex-CH(OEt)2
217a (n-Hex)
67
93:7
97.9
215c
Me-CHO
217a (Me)
70
91:9
97.2
215c
c-Hex-CHO
217c (c-Hex)
59
91:9
98.1
215c
Ph-CHO
217 d (Ph)
70
92:8
98.1
4.1.10.4 Resin-Bound Iminophosphorane92
The resin 221 was prepared from commercially available resin-bound
triphenylphosphine, which was suspended in dry CH2Cl2 placed within a
polypropylene tube, was evaluated and then sealed under N2. A solution of vinyl azide
220 (prepared by condensation of pyridine-2-carboxaldehyde 218 with ethylazidoacetate 219 in the presence of NaOEt in dry CH2Cl2)93, was injected into the
suspension. After shaking at room temperature for 2 h the supernatant was removed
and the resin was washed with dry CH2Cl2 and dried under vacuum (Figure 52).
This resin can be used for the synthesis of 1H-pyrido[1,2-c]pyrimidine
derivatives 224, 225, which has some inhibitory effects on leukocyte functions and
experimental inflammation94-96. The target compounds were synthesized as shown in
(Figure 53).
51
N
N
NaOEt
+ N3 CH2 COOEt
CHO
N3
218
220
219
Ph
N
Ph
P N
Ph
COOEt
PhPPh2
COOEt
CH2Cl2/N2
221
Figure 52: Synthesis of resin-bound iminophosphorane.
N
N
Ph
P N
Ph
Ph
Solid CO 2 or CS2
X
N
COOEt
Toluene
COOEt
225a X=O
225b X=S
ArNCO, CH2Cl2
221
RT
ArN
C
N
Ph
COOEt
+
N
COOEt
N Ar
O
222
N
N
P N
Ph
223
Ar
N
Ph
N
Ph
COOEt
O
P Ph
Ph
224
Figure 53: Synthesis of some pyrido[1,2-c]pyrimidine compounds.
4.1.10.5 3-Carboxypropane Sulfonamide-P.S Resin (226)97
Polyethylene glycol polystyrene HCl resin or Tenta gel was washed with
i-Pr2EtN and DMF several times until swelling was complete, Then 3-carboxypropanesulfonamide98,99, diisopropyl carbodiimide (DIPC) and 1-hydroxybenzotriazole
52
(HOBt) were added in the minimum amount of DMF just to cover the resin. After
stirring for 24 hr, the resin was extensively washed with DMF. This new resin (226) is
very stable in basic conditions needed for Fmoc cleavage during the synthesis of
peptides (Figure 54). Activation with diazomethane or iodoacetonitrile followed by
displacement with a suitable thiol produces the thioester in good to excellent yield.
The method is also compatible with Boc/benzyl chemistry.
4.1.10.6 Resin-bound S-methylisothiourea (232)100
The reaction of Fmoc-NCS in methylene chloride with the free amino group of
Rink amide MBHM resin (62) was rapid, reaching completion in 15 min, as indicated
by a negative ninhydrin test, the resin was then deprotected with 20% piperidine in
DMF and treated with methyl iodide to give S-methylisothiourea 232. Fmoc-protected
amino acids 233 (5 equiv.) were coupled to the resin using HOBT/HBTU/NMM (1:1:2)
to yield 234, and the resin was deprotected using 20% piperidine in DMF. The
intramolecular cyclization was accomplished, by heating the resin at 80°C for 24h in
DMSO. The resin was washed (DMF, CH2Cl2) and dried for 10 minutes under a stream
of nitrogen to give 235. Modest heating at (60°C) was required to cleave the
2-aminoimidazolone101 (236) from resin using 95% aqueous trifluoroacetic acid,
(Figure 55). Treatment of 232 with the oxazolones 237 and sodium ethoxide yielded the
unsaturated-2-aminoimidazolone 238. Cleavage of 238 with 95% aqueous trifluoracetic
acid yielded the unsaturated 2-amino imidazolone (239), (Figure 56).
4.1.10.7 Phoxime Resin102,103
Phosgenated-p-nitrophenyl (polystyrene) ketoxime104 is a new resin for solid phase
combinatorial synthesis. This resin is a novel, air stable, easily handled polymer-bound
phosgene equivalent. Unlike typical chloroformate, little to no decomposition was observed
on prolonged exposure to air. It can be synthesized as shown in Figure 57.
53
H
N
NH2
S O
O
O
H
N
Fmoc-aa, PyBOP
i-Pr2EtN
S
O
O
R
H
N
N
H
OO
Fmoc
227
226
SPPS
t-Bu
H
N
TMS-CHN2 (R1=CH3) or
I CH2CN, i-Pr2EtN (R1=CH2CN)
S
O
O
R
H
N
Peptide
N
H
O O
Boc
Boc
t-Bu
H
N
S
O
R1
N
O O
O
228
R
Peptide
N
H
Boc
Boc
229
HSR"/NaSPh
t-Bu
R
"
RS
O
Peptide
N
H
R
Boc
TFA, scavangers
RS
O
Boc
Peptide
N
H
231
230
Figure 54: Synthesis and uses of 3-carboxypropane sulfonamide-P.S-resin.
Isocyanates, because of their high reactivity with nucleophiles are utilized as
reagents to prepare small molecule combinatorial libraries. Phoxime resin provides a
route to thermolabile polymer-bound oxime carbamate which serves as latent
isocyanates. Phosgenated oxime resin has been intensively used in solid phase
combinatorial synthesis of many heterocyclic libraries including urea, 1,2,4-triazinedione, -ureidoacetamide and 3-aminohydantoin (Figure 58). Recently, a
convergent approach to solid phase synthesis in which two fragments of a molecule
are synthesized on independent supports and then condensed in a key resin-to-resin
54
NH2 OCH3
a
R1=H
NH
O
R
R1
OCH3
62
62
N
R1
NH
O
SCH 3 O
R3
N
Fm oc
H
N
R4 NH2
N
R1
OH
R3
R4
233
234
b
SCH3
232
c
R3
N
N
R1
R3
R4
R2
d
O
N
H
N
N
R1
R4
O
N
H
236
235
(a) (i) Fmoc-NCS, CH2Cl2 (ii) 20% piperidine in DMF, (iii) MeI, DMF (b) (i) HOBT, HBTU, NMM,
DMF (ii) 20% piperidine in DMF, (c) DMSO, 80°C (d) 95% aq. TFA, 60°C
Figure 55: Synthesis and applications of resin-bound-S-methylisothioure.
O
NH
N
R1
SCH3
O
+
Ph
232
R2
N
R1
N
N
H
R5
DMF, EtONa (2 equiv.)
N
100oC
237
R5
O
N
95% aqueous TFA
60oC
N
R1
N
H
R5
O
238
239
Figure 56: Synthesis of 5-unsaturated-2-aminoimidazolones.
transfer reaction. This approach has been utilized for the synthesis of amides and
ureas by transferring acyl groups and aminoacyl groups from p-nitrophenyl
(polystyrene) ketoxime resin to amino acid-functionalized Wang resin (Figure 58).
55
p-Nitrobenzoyl chloride
P
AlCl3/CH2Cl2
P
240
O
C
NO2
241
NH2OH.HCl
Pyridine
EtOH, 
NO2
O
N
O
Cl
O
C
Cl
Cl
CH2Cl2
P
P
C
N
NO2
OH
242
243
Kaiser Oxime resin
Figure 57. Synthesis of Phosgenated Oxime resin.
4.1.10.8 Resin-bound -Sulfonated Ketones105
Nicalaou described the preparation and utility of resin bound -sulfonated
ketones (Figure 59). Loading of -sulfonic ketones onto resin was readily
accomplished upon treatment of polystyrene sulfonic acid resin 254 (a solid-phase
version of toluenesulfonic acid) with epoxide (e.g., 255) in DCM at room temperature
for 24 h and subsequent oxidation with Dess-Martin periodinane. A one-pot entry into
258 could be achieved directly from olefins 253 via in situ olefin oxidation with
dimethyl dioxirane (DMDO), followed by the two-step addition/oxidation-sequence.
Resin 258 was remarkably stable and underwent a variety of nucleophilic cleavage
reactions generating -hydroxy-, -acyloxy-, -phenoxy, -alkoxy, -amino,
-anilino, and -thioaryl ketones. Reaction of 258 with bis nucleophiles including
thioamides, catechols, dithiols and diamines gave rise to a series of diverse fused
heterocyclic rings. Over 20 functionalizing cleavage options were reported.
56
Figure 58: Different chemical reactions to construct many libraries on solid support phosgenated
oxime resin.
5.
Miscellaneous Solid Support
Fueled by a rapidly growing interest in combinatorial chemistry using solid
phase synthesis, it is becoming obvious that the traditional solid supports cannot
always meet the requirements of this new field. Some of the recent research that has
been done to modify the solid support in combinatorial synthesis is enumerated.
57
Figure 59: Application of resin-bound -sulfonated ketones.
a)
Synthesis of three series of polyethyleneglycol polymers.106
b)
Novel -hydroxyethyl-polystyrene, solid support for solid phase organic
synthesis.107
c)
Resin effects in solid phase S(n) Ar and S(n) Z macrocyclization.108
d)
Polymer supported silylcyanide and silyl azide.109
58
e)
Exploring the scope of polyethyleneglycol for the Stille cross coupling
reaction.110
f)
Influence of resin cross-linking on solid-phase chemistry.111
g)
The use of Weinreb resin in the solid phase synthesis of some aldehyde and
ketone derivatives of unnatural amino acids and peptides.112
h)
Trityl isothiocyanate support for solid phase synthesis.113
i)
Preparation of polyethyleneglycol methacrylate polymer beads.114
j)
Polytetrahydrofuran cross-linked polystyrene resins for solid phase organic
synthesis.115
k)
Barcoded resins: A new concept for polymer-supported combinatorial
library.116
l)
Use of polymer-supported dialkylphosphinobiphenyl ligands for PalladiumCatalyzed Amination and Suzuki Reactions.117
m)
Preparation of polymer-bound ruthenium phosphine complex.118
n)
Preparation of polymer supported chiral sulfonamides.119
o)
Alkynyl alcohol anchored on trityl chloride resin for the construction of a
library of isoxazoles.120
p)
Polyethers: A solid-phase iterative approach.121
q)
Application of base cleavable safety catch linkers to solid phase library
production.122
r)
Solid phase synthesis supports are like solvents.123
6.
Conclusion
Of the all new libraries published in last decade, 80% were prepared on solid-
phase, which indicates the importance of this technique in combinatorial synthesis.
Solid-phase combinatorial chemistry (SPCC) is now a mature science and is used
59
extensively as a tool for drug discovery within the context of high-throughput
chemical synthesis. The best general pure libraries synthesized by the use of solid phase
combinatorial chemistry (SPCC) may well be those of intermediate complexity that are free
of artifact-causing nuisance compounds.
7.
References
1.
Gallop, M.A.; Barrett, R.W.; Dower, W.J.; Fodor, S.P.A.; Gordon, E.M.
Applications of Combinatorial Technologies to Drug Discovery 1.
Background and Peptide Combinatorial Libraries. J. Med. Chem. 1994, 37,
1233-1251.
2.
Terrett, N.K. Combinatorial Chemistry Online. Comb. Chem. J. 2004, 6(5),
107-120.
3.
Terrett, N.K. Combinatorial Chemistry Online. Comb. Chem. J. 2005, 7(12),
49-52.
4.
Dolle, R.E. Comprehensive Survey of Combinatorial Library Synthesis. J.
Comb. Chem. 2004, 6(5), 623-679.
5.
Gordon, E.M.; Barrett, R.W.; Dower, W.J.; Fodor, S.P.A., Gallop, M.A.
Applications of Combinatorial Technologies to Drug Discovery 2.
Combinatorial Organic Synthesis, Library Screening Strategies and Future
Directions. J. Med. Chem.,1994, 37, 1385-1401.
6.
Kamel, A.; Reddy, K.L.; Devaiah, V.; Shankaraia, N. reddy, D.R. Recent
Advances in the Solid-phase Combinatorial Synthesis, Strategies for the
Benzodiazepine Based Privileged Structures. Mini Rev. Med. Chem. 2006,
6(1), 53-59.
7.
Rupasinghe, N.C.; Spaller, R.M.; the Interplay between Structure-based
Design and Combinatorial Chemistry. Current Opinion in Chemical Biology
2006, 6(1), 63-59.
8.
Dolle, R.E. Comprehensive Survey of Combinatorial Library Synthesis. 2000.
J. Comb. Chem. 2001, 3(6), 477-517.
9.
Nikolaiev, V.; Stierandova, A.; Krchnak, V.; Seligmann, B.; Lam, K.S.;
Salmon, S.E.; Lebl, M. Peptide-Encoding for Structure Determination of
Nonsequenceable Polymers Within Libraries Synthesized and Tested on SolidPhase Supports. Pept. Res. 1993, 6, 161-170.
10.
Needels, M.C.; Jones, D.G.; Tate, E.H.; Heinkel, G.L.; Kochersperger, L.M.;
Dower, W.J.; Barrett, R.W.; Gallop, M.A. Generation and Screening of an
Oligonucleotide-Encoded Synthetic Peptide Library. Proc. Natl. Acad. Sci.
U.S.A. 1993, 90, 10700-10704.
60
11.
Brenner, S.; Lerner, R.A. Encoded Combinatorial Chemistry Proc. Natl. Acad.
Sci. U.S.A. 1992, 89, 5181-5183.
12.
Ohlmeyer, M.H.J.; Swanson, R.N.; Dillard, L.W.; Reader, J.C.; Asouline, G.;
Kobayashi, R.; Wigler, M.; Still, W.C. Complex Synthetic Chemical Libraries
Indexed With Molecular Tags. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 1092210926.
13.
An, H.; Haly, B.D.; Cook, P.D. “Discovery of Novel Pyridinopolyamines with
Potent Antimicrobial Activity: Deconvolution of Mixtures Synthesized by
Solution-phase Combinatorial Chemistry”. J. Med. Chem. 1998, 41, 706-716.
14.
An, H.; Cook, P.D. “Solution-phase Combinatorial Chemistry I. Synthesis of
Polyazacyclophane Scaffolds and Teritary Amine Libraries”. Tetrahedron
Lett. 1996; 37: 7333-7236.
15.
An, H.; Cummins, L.L.; Griffey, R.H.; Bharadwaj, R.; Haly, B.D.; Fraser,
A.S.; Wilson-Lingardo, L.; Risen, L.M.; Wyatt, J.R.; Cook, P.D. Solution
Phase Combinatorial Chemistry Discovery of Novel Polyaza-pyridinophanes
with Potent Antibacterial Activity by a Solution Phase Simultaneous Addition
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