A parallel solution-phase synthesis of substituted 3,7-diazabicyclo[3.3.1]nonanes

The parallel solution-phase synthesis of a series of building blocks and combinatorial libraries based on natural bispidine scaffold has been accomplished. Key reactions include catalytic hydrogenation of the (-)-cytisine heterocyclic system, followed by alkali-mediated ring cleavage. Using this approach, a series of new bispidine core building blocks for combinatorial synthesis with three points of diversity were effectively synthesized. The libraries from libraries were then obtained in good yields and purities using solution-phase acylation reactions. Obtained combinatorial libraries of 3,4,7-trisubstituted bispidines are potentially useful in the discovery of novel physiologically active compounds.     

Liquid-phase synthesis of combinatorial libraries based on 7-trifluoromethyl-substituted pyrazolo[1,5-a]pyrimidine scaffold

The parallel solution-phase synthesis of more than 2200 7-trifluoromethyl-substituted pyrazolo[1,5-a]pyrimidine and 4,5,6,7-tetrahydropyrazolo[1,5-a]pyrimidine carboxamides on a 50-100-mg scale has been accomplished. Key reactions include assembly of the pyrazolo[1,5-a]pyrimidine ring by condensation of 5-aminopyrazole derivatives with the corresponding trifluoromethyl-beta-diketones. The libraries from libraries were then obtained in good yields and purities using solution-phase acylation and reduction methodologies. Simple manual techniques for parallel reactions using special CombiSyn synthesizers were coupled with easy purification procedures (crystallization from the reaction mixtures) to give high-purity final products. The scope and limitations of the developed approach are discussed.     

Parallel liquid-phase synthesis of N-substituted 6-aminosulfonyl-2-oxo-1,2-dihydroquinoline-4-carboxamide and 6-aminosulfonylquinoline-4-carboxamide derivatives

Two efficient strategies for solution-phase parallel synthesis of libraries of quinoline derivatives are described. The first synthetic pathway features the Pfitzinger reaction of isatin with diethyl malonate and sulfochlorination of the resulting 2-oxo-1,2-dihydroquinoline-4-carboxylate followed by generation of sulfonamide library. The second strategy employs the unusual behavior of 5-sulfamoylisatins in Pfitzinger reactions, which results in formation of 6-sulfamoyl-4-carboxyquinolines instead of the anticipated 2-oxo-1,2-dihydroquinoline structures. The obtained carboxylates appeared to be convenient synthetic intermediates for the generation of the corresponding carboxamide libraries. Using these reagents, the parallel solution-phase synthesis of more than 500 substituted quinoline and 2-oxo-1,2-dihydroquinoline derivatives has been accomplished on the 50-100-mg scale. Simple manual techniques for parallel reactions using special CombiSyn synthesizers were coupled with easy purification procedures to give high-purity final products. The scope and limitations of the developed approaches are discussed.     

Multistep parallel synthesis of substituted 5-aminobenzimidazoles in solution phase

An efficient solution-phase parallel synthesis of multisubstituted 5-aminobenzimidazoles is described. The two fluorine atoms of 1,5-difluoro-2,4-dinitrobenzene (DFDNB) are sequentially and quantitatively replaced by nucleophiles. Simultaneous reduction of aromatic m-dinitro groups by Pd-C/HCOONH(4) results in 2,4,5-benzenetriamines, which are continuously condensed with aldehydes to successfully construct the benzimidazole ring without additional oxidants. The free aromatic amino group is further modified by anhydrides, isocyanates, isothiocyanates, and sulfonyl chlorides. All the reactions involved here are highly effective in giving the desired products at room temperature. Four diversity points are introduced in the final products.     

Efficient one-pot preparation of 5-substituted-2-amino-1,3,4-oxadiazoles using resin-bound reagents

A robust one-pot solution-phase synthesis of 2-amino-1,3,4-oxadiazoles directly from acylhydrazines and isothiocyanates is described. Commercially-available polymer-supported reagents help facilitate both cyclization and purification. This convenient method benefits from its broad applicability, ease and safety of reagent handling, simple product isolation, and the ability to perform multiple reactions in parallel fashion without need for purification. The details and scope of this reaction strategy are presented herein.     

Synthesis of heterocyclic compounds possessing the 4H-thieno[3,2-b]pyrrole moiety

A series of novel heterocyclic combinatorial libraries containing 4H-thieno[3,2-b]pyrrole, thieno[2',3':4,5]-pyrrol[1,2-d][1,2,4]triazine and thieno[2',3':4,5]pyrrolo[1,2-a]pyrazine heterocyclic moieties were obtained by parallel solution-phase synthesis. Key steps include different reactions of initial alkyl 4H-thieno[3,2-b]-pyrrole-5-carboxylates, such as alkylation with alkylating agents; transformation of the carboxylate group into different reactive functionalities, followed by reactions with electrophilic species; intramolecular cyclizations; and amide bond formation. Simple manual techniques for parallel reactions were coupled with easy purification procedures to give high-purity final products.     

Polymer-supported reagents and catalysts: recent advances in synthetic applications

The current surge in parallel array synthesis for the production of small molecule libraries has generated keen interest in the application of solid-supported reagents and catalysts in solution-phase chemistry. The strategy assimilates the advantages of product isolation and purification of solid-phase organic synthesis with the flexible choice of chemistry from the vast repertoire of solution-phase organic reactions. This review summarizes the significant recent advances in the application of polymer-bound reagents and catalysts in solution-phase synthesis of organic molecules. Multi-step reaction sequences employing sequential use of polymer-supported reagents are also discussed. In view of the earlier review publications on this topic, only the recent literature covering 1998 and 1999 is included.     

Parallel synthesis of 3-amino-4H-quinolizin-4-ones, fused 3-amino-4H-pyrimidin-4-ones, and fused 3-amino-2H-pyran-2-ones

On the basis of the enaminone methodology, libraries of 3-amino-4H-quinolizin-4-ones, fused 3-amino-4H-pyrimidin-4-ones, and fused 3-amino-2H-pyran-2-ones were synthesized by the solid-phase and by the solution-phase parallel synthesis. The solution-phase approach turned out to be advantageous over the solid-phase approach. The solution-phase synthesis afforded, in most cases, analytically pure products in high yields, whereas the solid-phase approach gave products in poor yields and in low purity.     

An efficient method for solution-phase parallel synthesis of 2-quinoxalinol salen Schiff-base ligands

A solution-phase parallel method for the synthesis of 2-quinoxalinol salen ligands was designed and optimized. The synthesis begins with commercially available 1,5-difluoro-2, 4-dinitrobenzene (DFDNB) and employs a sequence of five straightforward and high-yielding reaction steps. Simple laboratory techniques with low sensitivity to water or air for solution-phase parallel reactions were coupled with convenient workup and purification procedures to give high-purity and yield a small ligand library of 20 compounds. The final step, a Schiff-base condensation of an aldehyde with the diaminoquinoxaline results in a new category of ligands for metal coordination or of potential bioactivity, based on the skeleton 2,2'-(1E,1'E)-(quinoxaline-6,7-diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)diphenol. The approach described here is easily adaptable for parallel synthesis of a larger library.     

Straightforward entry to libraries of diversely substituted azinones by a consecutive aza-Michael/palladium-catalyzed functionalization strategy

A highly divergent, flexible, and conceptually simple sequence allowing the parallel solution-phase assembly of functionalized azinone libraries has been developed in a one-pot consecutive fashion. Structural decoration of in situ-generated heterocyclic aza-Michael adducts AB was accomplished by exploiting the diversity potential of Heck, Suzuki, Sonogashira, and Stille reactions.     

Three-step solution-phase combinatorial access to 1,2-disubstituted and 1,2,5-trisubstituted pyrroles from carboxylic esters

An efficient diversity-oriented strategy has been developed for the solution-phase parallel synthesis of di- and trisubstituted pyrrole libraries. Methyl esters 1 were effectively converted to 1,2-di- and 1,2,5-trisubstituted pyrroles 5 and 6 in three steps. Treatment of ester 1 with vinylmagnesium bromide in the presence of copper (I) cyanide yielded the corresponding homoallylic ketone 2, which was subjected to ozonolysis or Tsuji-Wacker oxidation to yield the respective cyclization precursors 3 and 4 after aqueous workup. Compounds 3 and 4 were condensed without further purification with a primary amine to afford the desired 1,2-di- or 1,2,5-trisubstituted pyrroles 5 and 6 in good yield and purity.     

SynCar: an approach to automated synthesis

The automation of all aspects of manual solution-phase synthesis into one integrated, efficient, and reliable system could be regarded as something of an unmet challenge in organic chemistry. The requirements for modern solution-phase libraries in mainstream drug discovery is typically 50-250 high-purity compounds on a 10-100-mg scale, whether for target class libraries or lead optimization, and short cycle time in combination with high capacity is critical. To achieve these goals, in a codevelopment between Aventis and Accelab GmbH, Kusterdingen, Germany, we designed a completely novel system of independent workstations connected by a shuttle transfer system produced by Montech, Derendingen, Switzerland. Seven modular workstations process four reactions on each shuttle in parallel, with the ability to perform synthesis (temperature control and liquid reagent handling), filtration, liquid-liquid extraction, evaporation, weighing, solid-phase extraction, and HPLC/MS analysis. The modular design enables the continuous loading of shuttles at any time, and each shuttle can have its own workflow. The design also allows easy expansion for future needs. The result is a combination of high flexibility and high throughput.     

Design and synthesis of screening libraries based on the muurolane natural product scaffold

The plant-derived natural product 14-hydroxy-6,12-muuroloadien-15-oic acid (1) was identified as a unique scaffold that could be chemically elaborated to generate novel lead- or drug-like screening libraries. Prior to synthesis a virtual library was generated and prioritised based on drug-like physicochemical parameters such as log P, log D(5.5), hydrogen bond donors/acceptors, and molecular weight. The natural product scaffold (1) was isolated from the endemic Australian plant Eremophila mitchellii and then utilised in the parallel solution-phase generation of two series of analogues. The first library consisted of six semi-synthetic amide derivatives, whilst the second contained six carbamate analogues. These libraries have been evaluated for antimalarial activity using a chloroquine-sensitive Plasmodium falciparum line (3D7) and several compounds displayed low to moderate activity with IC(50) values ranging from 14 to 33 μM.     

Synthesis of substituted 4-oxo-2-thioxo-1,2,3,4-tetrahydroquinazolines and 4-oxo-3,4-dihydroquinazoline-2-thioles

We have developed a liquid-phase synthesis of combinatorial libraries of new disubstituted 4-oxo-2-thioxo-1,2,3,4-tetrahydroquinazolines and trisubstituted 4-oxo-3,4-dihydroquinazoline-2-thioles. The former were prepared using two general procedures: (i) cyclization of substituted methyl anthranilates with isothiocyanates, or (ii) cyclization of substituted 2-(methylcarboxy)benzeneisothiocyanates with primary amines or hydrazines. 4-Oxo-3,4-dihydroquinazoline-2-thioles were prepared by S-alkylation of disubstituted 4-oxo-2-thioxo-1,2,3,4-tetrahydroquinazolines with alkyl or aryl halides. The hydrolysis of methyl benzimidazo[1,2-c]quinazoline-6(5H)-thione-3-carboxylate led to the corresponding acid. This acid was utilized in the synthesis of new benzimidazo[1,2-c]quinazoline-6(5H)-thione-3-carboxamide and S-substituted 6-mecaptobenzimidazo[1,2-c]quinazoline-3-carboxamide libraries.     

Solution-phase combinatorial libraries: modulating cellular signaling by targeting protein-protein or protein-DNA interactions

The high-throughput synthesis and screening of compound libraries hold tremendous promise for drug discovery and powerful methods for both solid-phase and solution-phase library preparation have been introduced. The question of which approach (solution-phase versus solid-phase) is best for the preparation of chemical libraries has been replaced by which approach is most appropriate for a particular target or screen. Herein we highlight distinctions in the two approaches that might serve as useful considerations at the onset of new programs. This is followed by a more personal account of our own focus on solution-phase techniques for the preparation of libraries designed to modulate cellular signaling by targeting protein-protein or protein-DNA interactions. The screening of our libraries against a prototypical set of extracellular and intracellular targets, using a wide range of assay formats, provided the first small-molecule modulators of the protein-protein interactions studied, and a generalized approach for conducting such studies.     

Functionalized Polymers-Emerging Versatile Tools for Solution-Phase Chemistry and Automated Parallel Synthesis

As part of the dramatic changes associated with the need for preparing compound libraries in pharmaceutical and agrochemical research laboratories, industry searches for new technologies that allow for the automation of synthetic processes. Since the pioneering work by Merrifield polymeric supports have been identified to play a key role in this field however, polymer-assisted solution-phase synthesis which utilizes immobilized reagents and catalysts has only recently begun to flourish. Polymer-assisted solution-phase synthesis has various advantages over conventional solution-phase chemistry, such as the ease of separation of the supported species from a reaction mixture by filtration and washing, the opportunity to use an excess of the reagent to force the reaction to completion without causing workup problems, and the adaptability to continuous-flow processes. Various strategies for employing functionalized polymers stoichiometrically have been developed. Apart from reagents that are covalently or ionically attached to the polymeric backbone and which are released into solution in the presence of a suitable substrate, scavenger reagents play an increasingly important role in purifying reaction mixtures. Employing functionalized polymers in solution-phase synthesis has been shown to be extremely useful in automated parallel synthesis and multistep sequences. So far, compound libraries containing as many as 88 members have been generated by using several polymer-bound reagents one after another. Furthermore, it has been demonstrated that complex natural products like the alkaloids (+/-)-oxomaritidine and (+/-)-epimaritidine can be prepared by a sequence of five and six consecutive polymer-assisted steps, respectively, and the potent analgesic compound (+/-)-epibatidine in twelve linear steps ten of which are based on functionalized polymers. These developments reveal the great future prospects of polymer-assisted solution-phase synthesis.     

Fluorous mixture synthesis of two libraries with hydantoin-, and benzodiazepinedione-fused heterocyclic scaffolds

Diversity-oriented synthesis (DOS) and fluorous mixture synthesis (FMS) are two aspects of combinatorial chemistry. DOS generates library scaffolds with skeletal, substitution, and stereochemistry variations, whereas FMS is a highly efficient tool for library production. The combination of these two aspects in solution-phase synthesis of two novel heterocyclic compound libraries is presented in this paper. Mixtures of different fluorous amino acids undergo [3 + 2] cycloadditions followed by postcondensation reactions. The mixtures are then demixed by fluorous HPLC. Fluorous tags are removed by cyclization to afford hydantoin- and benzodiazepinedione-fused heterocyclic compounds as individual, pure, and structurally defined molecules. The application of MS-directed HPLC purification and parallel four-channel LC/MS analysis further increases the efficiency of FMS.     

Combinatorial synthesis of natural product-like molecules using a first-generation spiroketal scaffold

Recently, significant attention has been focused on the synthesis small-molecule libraries based on natural product or natural product-like structures. In this paper, we report our initial studies on the use of the 1,7-dioxaspiro[5,5]undecane (spiroketal) moiety as a rigid-core template for elaboration using parallel synthesis techniques. The synthesis of a spiroketal scaffold that is reminiscent of the spiroketal subunits found in the spiroketal macrolide antibiotics will be described. Elaboration of three independently addressable functional groups on the scaffold using solution-phase parallel synthesis techniques led to the preparation of a small library of natural product-like compounds. These studies pave the way for evaluation of highly functionalized spiroketals in phenotypic assays and as prospective antagonists of protein-protein interactions.     

Syntheses of 1,3-Imidazoline-2-thione and 2-Phenylimino-1,3-thiazoline combinatorial libraries through different sequences of the same components

We have developed combinatorial libraries of new 1,3-imidazoline-2-thiones 5 and 2-phenylimino-1,3-thiazolines 7 by way of different reaction sequences of the same three components, gamma-chloroacetoacetanilides 1, amines 2, and isothiocyanates 3 in a parallel synthetic fashion. One of the building blocks, the gamma-chloroacetoacetanilides 1, was prepared by the sequential reaction of 4-methylene-oxetan-2-one (ketene dimer) with chlorine and various anilines. The condensation of 1 with amines gave dihydrofuran 4 intermediates that when reacted with 3 afforded the 1,3-imidazoline-2-thiones 5. On the other hand, reaction of 3 with 2 provided substituted thioureas 6 that were reacted with 1 to yield 2-phenylimino-1,3-thiazolines 7.     

Global transformation of OBOC combinatorial peptide libraries into OBOC polyamine and small molecule libraries

In "one-bead-one-compound" (OBOC) combinatorial chemistry, a compound-bead library with hundreds of thousands to millions of diversities can be rapidly generated such that each bead displays only one chemical entity. The highly efficient "libraries-from-libraries" approach involves the global transformation of a peptide library into many small molecule solution-phase mixture libraries, but this approach has never been successfully applied to OBOC libraries. Here we report a novel approach that allows us to combine these two enabling technologies to efficiently generate OBOC encoded small molecule bead libraries. By using a topologically segregated bilayer bead and a "ladder-synthesis" method, we can prepare peptide libraries with the peptide on the bead surface and a series of peptide ladders in the bead interior. Various global transformation reactions can then be employed to transform the starting peptide library into a variety of peptidomimetic libraries. During the transformation reactions, the peptide ladders in the bead interior are also transformed in a predictable manner. As a result, individual compound bead can be decoded by analyzing the hydrogen fluoride-released encoding tags with matrix-assisted laser desorption ionization Fourier transform mass spectrometry. Using this novel approach, a random encoded dipeptide library was prepared and subsequently transformed into polyamine and poly- N-acetylamine sublibraries. Random beads isolated from these sublibraries were reliably decoded.