Fluorous mixture synthesis of 4-alkylidene cyclopentenones via a rhodium-catalyzed [2+2+1] cycloaddition of alkynyl allenes

Fluorous mixture synthesis was used to prepare a library of 4-alkylidene cyclopentenones starting from a mixture of four alpha-amino acid derivatives tagged with different fluorous benzyl carbamates ((F)CBz) of varying fluorine content. The amino acids were converted to the corresponding propargyl esters and then subjected to an ester-enolate Claisen rearrangement to give a mixture of allenic amino esters. The allenes were then split four ways and propargylated with different propargyl bromides to give four mixtures of alkynyl allenes. The 4-alkylidene cyclopentenones were formed by a formal [2+2+1] cycloaddition of the alkynyl allenes using catalytic [Rh(CO)2Cl]2 under CO atmosphere. Demixing by fluorous preparative HPLC, removal of the fluorous benzyl carbamates, and then exposure to HCl/ether gave the hydrochloride salts of 16 compounds as diastereomeric mixtures in 69-99% purity. Thus, after just 26 chemical steps, a library of 16 cyclopentenones was prepared by using fluorous mixture synthesis. By comparison, the same library would have required 112 steps if each compound were made individually by parallel 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.     

Solution-phase parallel synthesis of a library of delta(2)-pyrazolines

A parallel synthesis of a library (80 members) of 2-pyrazolines in solution phase is described. The 2-pyrazoline core was accessed through the [3 + 2] cycloaddition of nitrilimines with enoyl oxazolidinones. The cycloaddition provided two regioisomers, the major product being the C regioisomer. The oxazolidinone moiety was further reduced to the primary alcohol, producing another library of 5-hydroxymethyl-2-pyrazolines. The Lipinski profiles and calculated ADME properties of the compounds are also reported.     

Target cum flexibility: synthesis of C(3′)-spiroannulated nucleosides

We report a simple strategy for the synthesis of a collection of C(3′)-spirodihydroisobenzo-furannulated nucleosides featuring a [2+2+2]-cyclotrimerization as the key reaction. The cyclotrimerization reactions are facile with the unprotected nucleosides having a diyne unit. When both alkynes of the diyne are terminal, the regioselectivity is poor. However, when one of the terminal alkynes is additionally substituted, the cyclotrimerizations are highly diastereoselective. Since the key bicycloannulation is the final step, this strategy provides flexibility in terms of the alkynes and is thus amenable for the synthesis of a focussed small molecule library.     

Synthesis of 2H-indazoles by the [3 + 2] dipolar cycloaddition of sydnones with arynes

A rapid and efficient synthesis of 2H-indazoles has been developed using a [3 + 2] dipolar cycloaddition of sydnones and arynes. A series of 2H-indazoles have been prepared in good to excellent yields using this protocol, and subsequent Pd-catalyzed coupling reactions can be applied to the halogenated products to generate a structurally diverse library of indazoles.     

Chemoselective high-throughput purification mediated by solid-supported reagents: its application to the first 6, 9-disubstituted purine library synthesis

A new application of solid-supported reagents was developed to separate the alkylated N7/N9 regioisomers derived from commercially available 2-amino-6-chloropurine. Simple filtration through an alumina/H+ pad or scavenging by AG/Dowex-50W-X8 resin provides diverse N9 regioisomers selectively in moderate yields with high purities (>90%). This purification method can be conveniently used in a high-throughput format and facilitates the synthesis of a purine library without laborious regioisomer separation and aqueous work-up. The first library synthesis of 6,9-disubstituted purines is reported using the combination of this novel separation method in conjunction with polymer-supported reagents.     

Exploration of structure--activity relationships among foldamer ligands for a specific protein binding site via parallel and split-and-mix library synthesis

We describe the use of parallel and split-and-mix library synthesis strategies for exploration of structure-activity relationships among peptidic foldamer ligands for the BH3-recognition cleft of the anti-apoptotic protein Bcl-xL. This effort began with a chimeric (alpha/beta+alpha)-peptide oligomer (composed of an alpha/beta-peptide segment and an alpha-peptide segment) that we previously identified to bind tightly to the target cleft on Bcl-xL. The side chains that interact with Bcl-xL were varied in a 1000-member one-bead-one-compound library. Fluorescence polarization (FP) screening identified four new analogues with binding affinities similar to that of the lead compound but no analogues with enhanced affinity. These results suggested that significant improvements in affinity were unlikely in this series. We then used library synthesis to examine backbone variations in the C-terminal alpha-peptide segment of the lead compound. These studies provided an opportunity for direct comparison of parallel and split-and-mix synthesis formats for foldamer libraries with respect to synthetic variability and assay sensitivity. We found that compounds from both the parallel and one-bead-one-compound libraries could be reliably screened in a competition FP assay without purification of library members. Our findings should facilitate the use of combinatorial library synthesis for exploration of foldamers as inhibitors of protein-protein interactions.     

Fluorous Synthesis of Hydantoin-, Piperazinedione-, and Benzodiazepinedione-Fused Tricyclic and Tetracyclic Ring Systems

Fluorous proline derivatives generated from one-pot, three-component [3+2] cycloaddition of azomethine ylides are employed for different post-condensation reactions to form hydantoin-, piperazinedione-, and benzodiazepinedione-fused tricyclic and tetracyclic ring systems. The high synthetic efficiency is achieved by conducting fast microwave reactions and easy fluorous-solid phase extractions for reaction mixture purifications. Methods developed for these novel drug-like heterocyclic compounds can be applied to diversity-oriented library synthesis.     

Stereocontrolled synthesis of a complex library via elaboration of angular epoxyquinol scaffolds

We have accomplished the synthesis of a complex chemical library via elaboration of angular epoxyquinol scaffolds with distinct skeletal frameworks. The key strategy involves highly stereocontrolled [4 + 2] Diels-Alder cycloadditions of chiral, nonracemic epoxyquinol dienes to generate the scaffolds. Further scaffold diversification involves hydrogenation, epimerization, dehydration, and condensation of the carbonyl group with alkoxyamine and carbazate building blocks. Further elaboration of the scaffolds also provided new skeletal frameworks using hydroxyl-directed Diels-Alder cycloaddition and reductive N-N bond cleavage. The overall process afforded 244 highly complex and functionalized compounds. Preliminary biological screening of the library uncovered six compounds which showed significant inhibition of Hsp 72 induction.     

Synthesis of 4‐Vinyl‐1H‐1,2,3‐triazoles on Solid Supports via Polystyrene‐Bound But‐3‐ynyl Selenide

A facile method for solid‐phase organic synthesis of 1‐substituted‐4‐vinyl‐1,2,3‐triazoles from polystyrene‐supported but‐3‐ynyl selenide has been developed. This sequential [3+2] cycloaddition and oxidation–elimination reactions could be carried out under mild reaction conditions with straightforward operation and good yield and purity of the products, and broad scope of substrates, and could be applied in this reaction system in generation of a small library of title compounds.     

Click chemistry on solid phase: parallel synthesis of N-benzyltriazole carboxamides as super-potent G-protein coupled receptor ligands

The click chemistry-based backbone amide linker 1 was employed for an efficient and practical parallel synthesis of 1,2,3-triazole carboxamides when 1,3-dipolar cycloaddition was exploited for both the construction of a compound library and the functionalization of the resin. A three-step solid-phase-supported sequence included reductive amination by N-phenylpiperazinyl-substituted alkylamines, N-acylation by alkynoic acids, and azide-alkyne [3 + 2] cycloaddition. In most cases, cleavage under acidic conditions yielded the final products in excellent purities. A focused library of 60 target compounds was screened for G-protein coupled receptor binding employing eight biogenic amine receptors. Radioligand displacement experiments indicated a number of hit compounds revealing excellent receptor recognition when the methyl-substituted N-benzyltriazoles 29, 40, and 42 exhibited superior affinities for the alpha1 subtype (K(i) = 0.056-0.058 nM).     

Synthesis of spirohydantoins and spiro-2,5-diketopiperazines via resin-bound cyclic alpha,alpha-disubstituted alpha-amino esters

A seven-step solid-phase synthesis of spirohydantoins and an eight-step solid-phase synthesis of spiro-2,5-diketopiperazines is reported. Key intermediate in the synthesis of both compound libraries is the resin-bound cyclic alpha,alpha-disubstituted alpha-amino ester, which can be obtained after selective homogeneous reduction of the aliphatic nitro ester using tin(II) chloride dihydrate. Nitro ester, in turn, is synthesized by a high-pressure-assisted [4 + 2] cycloaddition of resin-bound nitro alkene and butadiene, whereas nitro alkene is obtained by a Knoevenagel condensation of resin-bound nitro acetate with an imine. Novel spirohydantoins are obtained by isocyanate coupling with the resin-bound amino ester 5, followed by cyclization cleavage using a base. Novel spiro-2,5-diketopiperazines are obtained by PyBOP coupling of a Fmoc-protected amino acid with resin-bound amino ester, followed by Fmoc deprotection and an acid-assisted cyclization cleavage. After preparation of seven different resin-bound alpha,alpha-disubstituted alpha-amino esters, a 7 x 8 compound library of spirohydantoins was synthesized using eight different isocyanates, and a 7 x 8 compound library of spiro-2,5-diketopiperazines was synthesized using eight different Fmoc amino acids.     

Design and parallel solid-phase synthesis of ring-fused 2-pyridinones that target pilus biogenesis in pathogenic bacteria

A new method for the solid-phase synthesis of enantiomerically enriched highly substituted ring-fused 2-pyridinones 13 has been developed. The synthesis mediates introduction of substituents at two positions in the 2-pyridinone ring in a diverse manner and is suitable for parallel synthesis. (19)F NMR spectroscopy was used as a tool to monitor each of the five steps in the reaction sequence. The optimized conditions thus obtained were then used to prepare a library of 20 2-pyridinones with high yields. The library members were chosen from a statistical multivariate design to ensure diversity and reliable data for structure-activity relationships. Screening of the library against the bacterial periplasmic chaperone PapD was performed using surface plasmon resonance. Three new 2-pyridinones with a higher affinity for the chaperone PapD than the previous best 13[10,1] were found, and important structural features could be deduced.     

Solid-phase synthesis development of a thymidinyl and 2′-deoxyuridinyl Ugi library for anti-bacterial agent screening

A solid-phase synthesis has been developed to make a thymidinyl and 2′-deoxyuridinyl Ugi library starting from 5′-azidonucleosides loaded onto polystyrene butyl diethylsilane (PS-DES) resin. Library synthesis development including: Ugi reaction optimization; selection of building blocks and optimization to 96-well filter plate synthesis are reported. A 1344 member library was synthesized for anti-bacterial screening.     

Efficient synthesis of a beta-peptide combinatorial library with microwave irradiation

The predictable relationship between beta-amino acid sequence and folding has inspired several biological applications of beta-peptides. For many such applications, it would be desirable to prepare and screen beta-peptide libraries. However, standard peptide synthesis protocols are not efficient enough to support a library approach for many types of beta-peptides. We recently optimized the solid-phase synthesis of beta-peptides using microwave irradiation, and we have now adapted this approach to synthesis on polystyrene macrobeads. We rapidly prepared a high-quality beta-peptide combinatorial library via a split-and-mix strategy. This library was screened in search of beta-peptide antagonists of the p53-MDM2 protein-protein interaction.     

Practical solid-phase parallel synthesis of Delta(5)-2-oxopiperazines via N-acyliminium ion cyclization

A practical solid-phase strategy for the synthesis of Delta(5)-2-oxopiperazines via N-acyliminium ion cyclization has been developed. A key step in the library synthesis is tandem acidolytic cleavage with subsequent in situ iminium formation, followed by stable enamide transformation. This approach is exemplified by the preparation of a 192-member pilot library using bromoacetal resins without further purification.     

A highly automated, polymer-assisted strategy for the preparation of 2-alkylthiobenzimidazoles and N,N'-dialkylbenzimidazolin-2-ones

A multistep, polymer-assisted solution phase strategy for the highly automated (auto-PASP) synthesis of 2-alkylthiobenzimidazole and N,N'-dialkylbenzimidazolin-2-one libraries is presented. The approach incorporates in-line purification techniques to afford library products directly with high purities and is exemplified by the preparation of a 96-member 2-alkylthiobenzimidazoline library 1[1-12,1-8] and a 72-member N,N'-dialkylbenzimidazolin-2-one library 9[1-12,1-6].     

Novel isomenthone-derived 1,3-diol ligands identified through parallel synthesis and screening catalyse an asymmetric aldol reaction

A library of new (+)-isomenthone-derived 1,3-diol ligands, containing 5 contiguous stereocentres (3 fixed, 2 variable), was prepared in 2 steps by parallel synthesis. Evaluation in parallel identified several different ligands from the library for catalysing a Mukaiyama aldol reaction with 87-90% e.e.     

A solid-phase, library synthesis of natural-product-like derivatives from an enantiomerically pure tetrahydroquinoline scaffold

With the goal of developing a library synthesis of tetrahydroquinoline-derived natural-product-like small molecules, a practical synthesis of enantiomerically pure tetrahydroquinoline scaffold was achieved. An asymmetric aminohydroxylation reaction was the key step in this strategy. This scaffold was further immobilized onto the solid support for the library generation. The library was obtained from three diversity sites: (i) acylation of the hydroxyl group (R(1)), (ii) coupling of the Fmoc-protected amino acid to the amino group (R(2)), and (iii) amidation of the N-terminal amine group (R(3)).     

A solution- and solid-phase approach to tetrahydroquinoline-derived polycyclics having a 10-membered ring

With the goal of library generation using a polycyclic derivative 5 having an enamide functional group, a simple and practical, enantioselective synthesis of tetrahydroquinoline derivative 2 was achieved. The phenolic hydroxyl group in compound 2 was utilized as an anchoring site for solid-phase synthesis. The ring closing metathesis approach yielded the desired polycyclic product 5 on solid phase in five steps (overall 40% yield). Compound 5 is a novel scaffold for the library generation of natural product-like polycyclics having a functionalized medium ring for obtaining a new class of small molecules to be utilized as chemical probes.