Combinatorial chemistry using polymer-supported reagents

101

Combinatorial chemistry using polymer-supported reagents Stephen W Kaldor* and Miles G Siegel During the past several years, the field of combinatorial chemistry has expanded to include not only solid- and solution-phase methods for expedited compound synthesis, but also hybrid approaches that span these two extremes. In particular, polymer-supported reagents have emerged as useful combinatorial chemistry tools for the discovery and optimization of new pharmaceutical leads.

relationship studies, or SARs), purification issues have impeded progress in this area. For this reason, chemists are now investigating hybrid solid/solution-phase synthesis techniques involving polymer-supported reagents which combine the purification advantages of SPOS with the flexibility of solution-phase synthesis. In this review, we cover the literature from the past couple of years on the use of polymer-supported reagents for the construction of small molecule libraries.

Addresses

Lilly Research Laboratories, Lilly Corporate Center, Eli Lilly and Company, Indianapolis, IN 46285, USA ~e-mail: [email protected] -e-mail: [email protected]

Figure 2 (a)

Current Opinion in Chemical Biology 1997, 1:101-106

http://biomednet.com/elecref/13675931001 00101 © Current Biology Ltd ISSN 1367-5931

R1NH2

H 2 R1.NvR

+ R2CHO

Abbreviations

PA-Sc-TAD polyallylscandium-trifylamide ditriflate solid-phaseorganic synthesis

(b)

SPOS

~1-10

.•1-10 / \

Introduction T h e construction of small molecule libraries by solid-phase organic synthesis (SPOS) is now a well established practice in the pharmaceutical industry [1 ]. These libraries are used to identify molecular structures which may possess important therapeutic properties. Combinatorial chemists can readily synthesize libraries in excess of 100000 members using mcthodology unique to the solid phase, such as mix and split synthesis and solid-support tagging. Pharmaceutical companies have recently begun to examine complementary solution-phase methods for high speed synthesis [1].

/ \

1-10

Br

~ O

4

(c) RI-IO.~cI

N02 OH

Pyddine

C H2CI2

~ ~ - - ~ O

NO2 R1-10

O O RR,N..JJ.RI.10 or ROfl.Rl.t °

RR'NH or ROH C H3CN, Et3N 70°C

Figure 1

( a ) Polymer supported stoichiometric

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A-B + ~ x

filte~

A-B

\

(>1 eq) (b)

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supportedcatalyticreagent co. 199?' Current Opinion in Chemical Biology

A + B

(C) Polymer A + B

~--x ~ (<1 eq)

filter ~

A-B + ~ - X

A-B

Synthesis reactions involving polymer-supported reagents. Polymer

supportedscavengingreagent ~

A-B +

side

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" AB

Current Opinionin Chemical Biology

is shown as a shaded sphere. (a) Use of a polymer-supported borohydride for reductive amination. (b) Phenol alkylation with polymer-supported phenoxides. (c) Use of polymer-supported active esters for the discovery of a novel herbicide.

Polymer-supported reagent classes.

Although solution-phase chemistry has proven useful for lead generation and lead optimization (structure activity

Synthesis using polymer-supported reagents is distinct from traditional SPOS ('Merrifield synthesis') in that the purpose of the solid phase is to retain undesired or unre-

102

Combinatorial chemistry

acted

materials-the

desired

product

is deposited

directly

Amberlites

IRA-400

borohydride

resin

for

use

in the

into the solution phase and is never linked covalently to the polymer. We have divided polymer-supported reagents

generation of amine libraries by reductive amination (Fig. Za). This reagent effectively reduces aldimines to

into three classes: stoichiometric covalent or ionic scavengers

secondary amines in high yield and, although the boron is linked ionically to the resin, no residual boron appears in the products when the reaction is conducted under neutral

reagents, (Fig. 1).

catalysts, and Stoichiometric

reagents and catalysts participate in the formation of product, while scavengers are typically added post-reaction solely for removal of impurities. Each of these classes has been

used

for accelerated

synthesis

of small

molecule

applied

libraries.

Stoichiometric

conditions. Liu and Vederas [S] have recently reported a polymer-supported version for the oxidation of primary and secondary alcohols, although to date this has not been

polymer-supported

reagents

Stoichiometric polymer-supported reagents have been employed in synthesis for many years (see [2,3] for representative reviews covering polymer-supported reagents in use before the advent of combinatorial chemistry). Their application to small molecule library construction, however, is a relatively recent phenomenon. Their use tends to be limited to solution reactions that are known to proceed in good yield with well-defined side products, since the only impurities removed from solution at the end of the reaction are those that remain bound to the polymer. Our definition of polymer-supported reagents includes ionically or covalently linked activating reagents (e.g. carbodiimides) and preactivated reagents (e.g. active esters, oxidants, reductants). Polymer-supported oxidants and reductants are well known in organic synthesis, and their application to library generation has been demonstrated recently. Kaldor [4**] examined the commercially available reductant

to library

generation.

Polymer-supported reagents have also been used to generate ether libraries via phenol alkylation. Parlow [6”] has generated a series of preactivated phenoxides bound ionically to a polymer support (Fig. Zb). These polymer-supported phenoxides are isolated and then treated with a limiting amount of alkyl halide to generate the corresponding phenolic ether. Parlow further demonstrated the synthesis of phenolic ether mixtures by reacting Amberlitea IRA-900 with a mixture of ten different phenols to generate a polymer-supported mixture of ten different phenoxides. He then treated this resin with a-butyl bromide to form an equimolar mixture of ten different phenolic ethers [6”]. Parlow and Normansell [7*] have synthesized amide and ester libraries in solution using a preactivated polymersupported acylating agent. In this scheme, polymer supported o-nitrophenol reacts with a mixture of ten acid chlorides to create a resin containing a mixture of ten carboxylic acids activated as their o-nitrophenyl

Figure 3

(a)

P-EDC

P-EPC

(b)

Amide coupling agents. (a) Reagents for carboxylic acid actwatlon: 1-(3.dimethylaminopropyl)-3-ethylcarbodiimide (EDC), a traditional peptide coupling reagent, and Its polymer-supported variants P-EDC and P-EPC. (b) The use of P-EPC, a polymer-supported reagent, In amide bond formation.

Combinatorial chemistry using polymer-supported reagents Kaldor and Siegel

103

Figure 4

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~/"

[~i+20

(TOEP,

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N-N=-(N -

B i+20

/

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S

n

I © 1997 Current Opinion in Chemical Biology

Construction of a library of ~-turn mimetics. Polymer-supported guanidine serves a dual role as base for promotion of macrocyclization and ion exchange resin for removal of TCEP and TCEP oxide. esters. This resin is isolated and then heated with a discrete amine or alcohol to give a well-defined mixture of ten amides or esters in solution (Fig. 2c). T h e amide or ester mixtures are submitted directly to a high throughput herbicidal assay. Compound 1 (Fig. 2c), which displayed considerable herbicidal activit> was identified by this approach. In a similar vein, a very recent and thorough study on the use of a polymer-supported version of N-hydroxybenzotriazole (HOBT) for carboxylic acid activation has been reported [8]. Amide synthesis has also been conducted using in situ activation with polymer-supported carbodiimides. Peptide chemists recognized many years ago that if one links a standard coupling agent such as a carbodiimide (Fig. 3a) to a resin, the by-product urea will be retained on the solidphase and can be readily removed by filtration [9]. In addition, excess carboxylic acid will remain covalently bound to the resin. Desai and Stramiello [10] reported the synthesis of polymer-supported 1-(3-dimethylaminopropyl)-3ethylcarbodiimide (EDC), dubbed P-EDC, and its application to rapid amide preparation. Kaldor has developed a similar reagent, polymer-supported 1-(3-pyrrolidinylpropyl)3-ethylcatbodiimide (P-EPC; unpublished results) (Fig. 3a), and has used it extensively in library production and rapid analogue synthesis (Fig. 3b). Polymer-supported reagents may be used in conjunction with SPOS for the post-cleavage modification of SPOSderived libraries. As an example, Ellman and co-workers [11] synthesized a straight chain pseudopeptide using traditional solid-phase synthesis. After the pseudopeptides were reductively cleaved from the resin, polymer-supported guanidine was utilized as a general base to effect macmcyclization, generating a library of !3-turn mimetics (Fig. 4). T h e polymer-supported guanidine plays a dual role as an ionic scavenger to remove tris(2carboxyethyl)-phosphine (TCEP) and T C E P oxide (see below). T h e potential for the post-cleavage diversification of solid-phase libraries using polymer-supported reagents, however, remains unexploited.

An intriguing feature of polymer-supported reagents is their inability to interact with other polymer-supported reagents. This feature allows for the use of reagents which would otherwise be mutually exclusive in a given reaction. In one elegant example, Parlow [12] performed three synthetic transformations in one vessel using three reagents which would have been incompatible in solution. As shown in Figure 5, sec-phenethyl alcohol is oxidized to acetophenone using poly(4-vinylpyridinium dichromate). Acetophenone is then brominated with perbromide on Amberlyst®A-26, and the resulting o~-bromoketone alkylates Amberlite®IRA-900 4-chloro-l-methyl-5(trifluoromethyl)-lH-pyrazol-3-ol. T h e final product is obtained in 48% overall yield, as compared to the yield of 42% obtained by performing the three reactions in discrete vessels. Figure 5

OH

(4-chloro- 1-met hyl-5-(t dfluoromethyl)- 1H -pyrazol-3-ol), poty(4-vinylpyridiniu m dichromate}, perbromide on Ambeltyst® A-26, cyclohexane, 65°C

1997 Current Opinion in Chemical Biology

O

~NN_ 48%

F3

Simultaneous multistep synthesis using polymer-supported reagents.

Polymer-supported catalytic reagents Despite their rich history, polymer-supported catalysts have yet to receive extensive attention as tools for combinatorial library production. Several recent literature reports, however, suggest that this area has considerable potential. Kobayashi and Nagayama [13"] have prepared polyallylscandium trifylamide ditriflate (PA-Sc-TAD) and used this partially soluble polymer to catalyze a number of organic reactions. This catalyst is water-tolerant, is readily recovered from reaction mixtures by precipitation with hexane, and can be used repeatedly without any loss

Combinatorial chemistry

104

in activity. As shown in Figure 6a, quinolines can be prepared in a three-component coupling reaction employing PA-Sc-TAD as catalyst. A 15-member quinoline library was synthesized as a demonstration exercise, and the authors pointed to the potential for the preparation of a million-member library using commercially available starting materials and appropriate automation [13"].

Figure 6 (a)

NH2

R (b)

R6

R2

H

NHR2 PA-Sc-TAD RI~.__Nu RICHO+ R2NH2+ MeaSi-Nu

wherePA-Sc'TAD= I CH2-CC~IN~Tfn Sc(OTf)2 © 1997 Current Opinion in Chemical Biology

Various multicomponent coupling reactions catalyzed by polyallylscandium trifylamide ditriflate. (a) Preparation of tetrahydroquinolines with multiple points of diversity. (b) Mannich reaction catalysis.

Kobayashi et al. [14] have also reported on the use of PA-Sc-TAD to catalyze three-component coupling reactions between amines, aldehydes, and silylated nucleophiles (Fig. 6) [14]. After hexane precipitation, filtration, and concentration, almost pure products are generally obtained. T h e efficient and highly convergent synthesis of a 24 compound library consisting of ]3-amino ketones, ]3-amino esters, and or-amino nitriles in yields ranging from 74-96% is described in their initial communication [14]. A stream of literature continues to appear on the synthesis and use of polymer-supported reagents in noncombinatorial applications (see [15-17]). For example, Jang [17] has applied a polymer-supported palladium catalyst with success in Suzuki cross-coupling reactions. Given the attention the Suzuki reaction and related palladium-catalyzed processes have recently received in the context of library synthesis using polymer-supported substrates, Jang's methodology may prove to be a viable alternative to the established SPOS methodology for Suzuki couplings. In general, it appears likely that polymer-supported catalysts will see increased use for library production.

Polymer-supported scavenging reagents Polymer-supported scavengers constitute the third category of polymer-supported reagents. These are used to entrain impurities upon completion of solution-phase

reactions, either covalently or ionically. Both types of scavenging have been utilized for library production. We [4"] have developed a series of polymer-supported nucleophiles and electrophiles for the selective removal of reaction impurities. Use of these reagents facilitates the clean solution-phase production of small molecule libraries. T h e choice of scavenger depends on the nature of both the impurity and the desired product (Fig. 7a). Covalent scavengers that arc selective for the removal of electrophiles in the presence of nonelectrophiles, nucleophiles in the presence of nonnucleophiles, and primary amines in the presence of secondary amines have all been developed [4°']. T h e use of covalent scavengers in a reaction sequence is illustrated in Figure 7b. Libraries created using covalent scavengers can be useful tools for pharmaceutical lead discovery. Kaldor et al. [18"] synthesized a library of 4000 ureas as ten-compound mixtures and screened the entire library for antirhinoviral activity. Resynthesis of active mixtures as discrete compounds led to the identification of two compounds which possess potent activity against human rhinovirus-14 (HRV-14) [18"]. Purification of solution phase reaction mixtures using ionic scavengers has also been demonstrated. Workers at Signal Pharmaceuticals [19"] examined a series of anion exchange resins as bases in the acylation of amines with excess acid chlorides. When the reaction went to completion, the excess acid chlorides were hydrolyzed to the corresponding acids. T h e acids were then selectively removed from solution by ionic interaction with the exchange resin, leaving the product amides in high purity. We have examined the utility of cation exchange chromatography for the purification of amine derived libraries [20"]. Amines can be functionalized efficiently in solution under standard reaction conditions, then the reaction mixture can be placed directly over a cation exchange column to produce high purity libraries. This process has been automated to further increase the ease of library construction. The power of this approach lies in its potential generality: nearly any ionizable molecule can be rapidly and efficiently separated from nonionizable impurities.

Conclusions It is becoming increasingly clear that solid-phase and solution-phase syntheses represent but two extremes within a palette of methodologies available to the combinatorial chemist. Synthesis using polymer-supported reagents occupies an intermediate position within this continuum, and offers the practitioner unique advantages. Although a limited number of examples have been reported which point to the utility of these reagents for pharmaceutical lead generation and optimization, medicinal chemists are only beginning to appreciate their

Combinatorial chemistry using polymer-supported reagents Kaldor and Siegel

105

Figure 7

~

(a)

NH2 ,

>

selective removal of electrophiles in the presence of non-electrophiles

selective removal of 1o or 2 ° amines or other nucleophiles in the presence of acylated or non-nucleophilic amines

0

~

Jl~H

i

:,

selective removal of 1o amines in the presence of 2 ° amines

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R1.NH2

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~I~'H filter

+ RI"NH2

~

H RI'Iq~R2

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1N.~.N,.R3 R a2.,."J H

+ R 3_ N=C=O

~

NH2 filter

O R 1..N..JJ..N.R 3 a 2..,J ~.' 1997 Current Opinion in Chemical Biology

Polymer-supported scavenging reagents. (a) Selectivities of solid-supported covalent scavengers. (b) A multistep synthesis of trisubstituted ureas conducted with polymer-supported scavengers. Here an aldehyde is combined with excess primary amine in methanol to generate an imine. Reduction of the imine with solid supported borohydride (vide supra) gives a mixture of secondary amine product and primary amine starting material. Addition of a polymer-supported aldehyde results in selective removal of the primary amine via formation of an imine. The secondary amine product isolated after filtration is then treated in a second step with an excess of isocyanate to give a mixture of urea product and isocyanate starting material. Addition of the polymer-supported nucleophile aminomethylated polystyrene results in selective removal of the electrophilic isocyanate to give clean urea product.

potential. We project that combinatorial chemists will use polymer-supported reagents with increasing regularity to conduct one- to three-step synthetic sequences. Given the parallels to automated SPOS (liquid and resin handling, filtration, and evaporation), automation of synthetic protocols involving polymer-supported reagents should become routine. Hybrid synthetic sequences will also be explored which alternate between polymer-bound and solutionphase intermediates. For example, after conducting a muhistep sequence using SPOS, a compound could be cleaved from the polymeric support to reveal a reactive functional group which subsequently could be derivatized using a polymer-supported reagent. We also project that polymer-supported reagents will prove particularly valuable for the construction of libraries derived from compounds which are not readily attached to a polymeric support (e.g. high throughput screening leads derived

from natural product sources or historical compound archives). As more polymer-supported reagents, catalysts, and scavengers become available, their importance as complementary tools to SPOS in drug discovery will increase.

References and r e c o m m e n d e d

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: • of special interest • ,, of outstanding interest 1,

Thompson LA, EIIman JA: Synthesis and applications of small molecule libraries. Chem Rev 1996, 96:555-600.

2.

Akelah A, Sherrington DC: Application of functionalized polymers in organic synthesis. Chem Rev 1981, 81:557-587.

3.

Sherrington DC, Hodge P (Eds): Syntheses and Separations Using Functional Polymers. Chichester: Wiley; 1988.

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Combinatorial chemistry

4. •°

Kaldor SW, Siegel MG, Fritz JE, Dressman BA, Hahn PJ: Use of solid supported nucleophiles and electrophiles for the purification of non-peptide small molecule libraries. Tetrahedron Lett 1996, 37:7193-7196, Polymer-supported scavengers are used to expedite amine alkylations and acylations. Multistep sequences are performed which also incorporate the use of stoichiometric polymer-supported reagents. These solid-supported scavengers are particularly advantageous for the construction of nonpeptide libraries in a parallel array format. 5.

Liu Y, Vederas JC: Modification of the Sworn oxidation: use of stoichiometric amounts of an easily separable, recyclable, and odorless sulfoxide that can be polymer-bound. J Org Chem 1996, 61 :?856-7859.

Parlow JJ: The use of anion exchange resins for the synthesis of combinatorial libraries containing aryl and heteroaryl ethers. Tetrahedron Lett 1996, 37:5257-5260. One of the first reports on the use of ionically bound polymer-supported reagents for combinatorial library production.

13. ••

Kobayashi S, Nagayama S: A new methodology for combinatorial synthesis. Preparation of diverse quinoline derivatives using a novel polymer-supported scandium catalyst. J Am Chem Soc 1996, 118:8977-8978. One of the first reports on the use of a polymer-supported catalyst for combinatorial library production. The catalysis of the multicomponent heteroDiels-Alder reaction leads to a library of molecules containing three points of diversity in a single solution-phase reaction. 14.

Kobayashi S, Nagayama S, Busujima T: Polymer scandiumcatalyzed three-component reactions leading to diverse amino ketone, amino ester, and amino nitrile derivatives. Tetrahedron Lett 1996, 37:9221-9224.

15.

Han H, Janda KJ: Soluble polymer-bound ligand-accelerated catalysis: asymmetric dihydroxylation. J Am Chem Soc 1996, 118:7632-7633.

16.

Kamahori K, Ire K, Itsuno S: Asymmetric Diels-Alder reaction of methacrolein with cyclopentadiene using polymer-supported catalysts: design of highly enantioselective polymeric catalysts. J Org Chem 1996, 61:8321-8324.

17.

Jang SB: Polymer-bound palladium-catalyzed cross-coupling of organoboron compounds with organic halides and organic triflates. Tetrahedron Lett 1997, 38:1793-1796.

6. o.

'7. •

Parlow JJ, Normansell JE: Discovery of a herbicidal lead using polymer-bound activated esters in generating a combinatorial library of amides and esters. Mol Divers 1995, 1:266-269. Equimolar mixtures of amides and esters are synthesized using polymersupported nitrophenyl esters as acylating agents and a herbicidal lead is identified through iterative screening and deconvolution. 8.

Pop IE, Deprez BP, Tartar AL: Versatile acylation of N-nucleophiles using a new polymer-supported 1hydroxybenzotriazole derivative. J Org Chem 1997, 62:2594-2603.

9.

Wolman Y, Kivity S, Frankel M: The use of polyhexamethylenecarbodiimide, an insoluble condensing agent, in peptide synthesis. J Chem Soc Chem Commun 1967:629-630.

10.

Desai MC, Stramiello LMS: Polymer bound edc (p-edc): a convenient reagent for formation of an amide bond. Tetrahedron Lett 1993, 37:7685-7688.

11.

Virgilio AA, Schurer SC, EIIman JA: Expedient solid-phase synthesis of putative ~-turn mimetics incorporating the i + 1, i + 2, and i + 3 sidechains. Tetrahedron Lett 1996, 37:6961-6964.

12.

Parlow JJ: Simultaneous multistep synthesis using polymeric reagents. Tetrahedron Lett 1995, 36:1395-1396.

18. •

Kalder SW, Fritz JE, Tang J, McKinney ER: Discovery of antirhinoviral leads by screening a combinatorial library of ureas prepared using covalent scavengers. Bioorg Med Chem Lett 1996, 6:3041-3044. Covalent scavenging resins are used to construct ten-compound equimolar urea mixtures. Mixture screening in a whole cell assay and iterative deconvolution provide antiviral leads with low cytotoxicities. 19. •

Gayo LM, Suto M J: Ion-exchange resins for solution phase parallel synthesis of chemical libraries. Tetrahedron Lett 1997, 36:513-516. Basic ion-exchange resins are used for synthesis and purification of various amide derivatives in parallel array format. 20. •

Siegel MG, Hahn PJ, Dressman BA, Fritz JE, Grunwell JR, Kaldor SW: Rapid purification of small molecule libraries by ion exchange chromatography. Tetrahedron Lett 1997, 38:3357-3360. Amines are functionalized in traditional solution-phase reactions, then rapidly purified by ion exchange chromatography to yield pure products. The generic purification sequence is applicable to a variety of reactions, and is amenable to automation with commercially available equipment.