Resin-bound Intermediates Research Articles

Solid-phase synthesis of fluorescent analogues of Park’s nucleotide, lipid I and lipid II

Solid-phase modular synthesis of fluorescent analogues of Park’s nucleotide, lipid I and II, is described. The Fmoc protected lysine side chain of resin-bound intermediates were selectively modified by various fluorescent groups. This is the first example of solid-phase synthesis of these compounds. This synthetic strategy allows the preparation of various fluorescent probes for investigating the mechanisms of bacterial cell-wall biosynthesis.

Fluorous-Phase Approach to α-Hydroxytropolone Synthesis.

α-Hydroxytropolones (αHTs) are troponoids that demonstrate inhibition against an array of therapeutically significant targets, making them potential drug leads for several human diseases. We have utilized a recently discovered one-pot three-component oxidopyrylium cycloaddition in a solid-supported synthesis of αHTs. Though the procedure is time efficient and generates assay-ready molecules, the system suffers from low yields and an inability to perform reaction modifications on resin-bound intermediates. In order to combat these issues with the solid-phase platform, we incorporated fluorous tags into our synthetic route. Through the implementation of fluorous phase chemistry, we demonstrate a substantial increase in the overall yield of αHTs, as well as an ability to execute metal-catalyzed cross coupling and amide coupling on fluorous tagged intermediates. We also show that tagged molecules can be separated from nonfluorous impurities, and vice versa, by utilizing fluorous liquid-liquid and solid-phase extractions. Hence, these proof-of-principle investigations describe the viability of a fluorous phase approach to αHT synthesis and its potential to serve as a combinatorial technique to produce structurally diverse substrates.

3-Alkyl-3-(alkylamino)indolin-2-ones via Base-Mediated C-Arylation of 2-Nitrobenzenesulfonamides.

Resin-bound intermediates prepared from polymer-supported amino acid esters, 2-nitrobenezenesulfonyl chlorides, and alcohols were used to synthesize 3-alkyl-3-(alkylamino) indolin-2-ones. The key step of the reaction sequence was the formation of a quaternary carbon via the base-mediated C-arylation of 2-nitrobenzenesulfonamides. The cleavage of the acyclic precursors from the resin and subsequent reduction of the nitro group by Zn in acetic acid triggered the spontaneous cyclization of the arylated compounds to indolinones. The synthesis was carried out using simple commercially available building blocks under mild conditions and provided the 3,3-disubstituted indolinone derivatives with good overall yields however, the arylation reaction resulted in the epimerization of the quaternary carbon.

Solid-Phase [2+2+2] Cycloaddition of Resin-Bound Dipropargylamine with Alkynes

The resin-bound dipropargylamine 3 was prepared by treatment of commercially available trityl chloride resin 1 with dipropargyl­amine 2 using N,N-diisopropylethylamine as a base. The microwave-assisted iridium-catalyzed [2+2+2] cycloaddition of 3 with several alkynes 4, followed by TFA cleavage and catch-release purification with basic alumina of resin-bound intermediates 5, gave the corresponding salt-free isoindoline derivatives 6 in 50-70% yields.

Method for the Solid-Phase Parallel Synthesis of a 6-Alkylamino-2-(functionalized-aminomethyl)-2H-1-benzopyran Library

A 2000-member library of benzopyran analogues was prepared by using a solid-phase synthesis protocol. Polymer-bound 6-alkylaminobenzopyrans 7 were synthesized as part of a first generation diversification step by employing reactions of a variety of alkyl halides with the amine 6. Transformations of the resin-bound intermediates 8 formed in this manner by reactions with acid halides, sulfonyl chlorides, and isocyanates leads to introduction of the second level of diversification found in the series of 6-alkylamino-2-(functionalized-aminomethyl)-2-methyl-2H-1-benzopyran analogues 11 and 12.

Construction of a 2,6-Difunctionalized 2-Methyl-2H-1-benzopyran Library by Using a Solid-Phase Synthesis Protocol

[reaction: see text] A library containing 1200 analogues of 2,6-difunctionalized 2-methyl-2H-1-benzopyran was constructed by using a solid-phase synthesis protocol. Polymer-bound 6-amido-, 6-sulfonamido-, and 6-uredo-functionalized 2-hydroxymethyl-2-methylbenzopyrans 10 were prepared as part of a first-generation diversification step by employing reactions of respective acid halides, sulfonyl chlorides, and isocyanates with the amine precursor 7. Transformations of the resin-bound intermediates 10 by reactions with alkyl and acid halides were then used to produce a diverse series of 2,6-difunctionalized 2-methyl-2H-1-benzopyran analogues 12 and 14.

Solid-Phase, Multicomponent Reactions of Methyleneaziridines: Synthesis of 1,3-Disubstituted Propanones

[reaction: see text] Polymer-supported methyleneaziridines undergo ring opening by Grignard reagents under copper catalysis to yield metalloenamines which are alkylated in situ to yield ketimines. Filtration and washing of these Merrifield resin-bound intermediates prior to hydrolysis provides the corresponding 1,3-disubstituted propanones in a high state of purity without recourse to column chromatography.

Synthesis of a 2, 4, 8-trisubstituted pyrimidino[5, 4-d]pyrimidine library via sequential SNAr reactions on solid-phase.

A solid-phase synthesis of 2, 4, 8-substituted pyrimidino[5, 4-d]pyrimidines involving three controlled S(N)Ar reactions has been developed. Exploration of different heterocyclic starting materials and resin-bound intermediates is highlighted. The preferred method starts with the treatment of resin-bound anilines with 2, 4, 8-trichloropyrimidino[5, 4-d]pyrimidine. This intermediate is subsequently treated with various amines in two steps to yield the final products. The scope of each diversity step was determined and a library of 16, 000 compounds was synthesized.

Alpha-Ketocarbonyl peptides: a general approach to reactive resin-bound intermediates in the synthesis of peptide isosteres for protease inhibitor screening on solid support.

alpha-Ketocarbonyl peptides were generated from peptide precursors on solid support via a metal-ion-catalyzed transamination. The reaction proceeded to completion within 2 h with glyoxylate as electrophile and copper(II) ions as catalyst in an aqueous acetate buffer at pH 5.5-6.0. The variety of naturally occurring alpha-amino acid substrates gave rise to a diverse set of differentially functionalized ketones. The highly reactive terminal ketocarbonyls were prone to aldol-type dimerization and could be transferred into stable moieties by oxime formation, reduction to the alcohol, or reductive amination, respectively. The alpha-ketocarbonyl peptides were efficient in nucleophilic addition of C-nucleophiles such as phosphono-ylides and allylsilanes.

Multistep synthesis of 2,5-diketopiperazines on different solid supports monitored by high resolution magic angle spinning NMR spectroscopy.

The solid-phase synthesis of 2,5-diketopiperazines containing the trans-4-hydroxy-L-proline amino acid residue (Hyp) was performed on Ellman polystyrene, polyoxyethylene-polyoxypropylene (POEPOP), polystyrene-polyoxyethylene NovaSyn, and Wang resins, respectively. The reaction pathway allowed the introduction of different functional groups around the bicyclic scaffold in a combinatorial approach, and it generated mixtures of isomers. A detailed characterization of the single reaction steps by high resolution magic angle spinning (HRMAS) NMR spectroscopy was performed. The NMR spectral resolution of the resin-bound intermediates and final products was greatly influenced by the polymer matrix. The POEPOP resin permitted to obtain HRMAS NMR spectra with a resolution comparable with that of the spectra of the molecules in solution. Moreover, configurational and conformational isomers formed during the solid-phase reaction steps could be detected and easily assigned. Therefore, the combination of the HRMAS NMR technique with the use of nonaromatic resins may become an extremely powerful tool in solid-phase organic synthesis. This approach will allow the monitoring of multistep reactions and the conception of on-bead structural studies either on small molecules or on natural and/or synthetic oligomers.

Solid-phase synthesis of Nα-benzyl- Nα-cinnamyl lysine and glutamic acid derivatives

Several variations of a solid-phase strategy for the synthesis of N α-benzyl- N α-cinnamyl lysine and glutamic acid derivatives are presented. Starting from the corresponding N α-Fmoc amino acids on Wang resin, reductive alkylation using nitrocinnamaldehyde or a substituted benzaldehyde was followed by nucleophilic displacement of a substituted benzyl halide or nitrocinnamyl bromide to provide resin-bound intermediates. Diversity was added by reduction of the nitro group and derivatization of the resulting aminocinnamyl moiety with a variety of acylating or sulfonylating reagents. Using an orthogonal protecting group strategy, N ε-Dde-protected lysine derivatives were further functionalized at the side-chain amino group prior to cleavage from resin. This method allows for the preparation of analog libraries having up to four points of diversity.

Solid-phase synthesis of γ-lactams, γ-lactones and cyclobutane derivatives from common resin-bound intermediates

Resin-bound cyclobutanone iminium salts, prepared on the solid-phase using the [2+2]-keteneiminium olefin cycloaddition reaction, have been transformed into a number of distinct structural classes including γ-lactams, γ-lactones, cyclobutylamines and cyclobutanols.

Vinyl benzenes as dienes in mild solid-phase Diels-Alder reactions

Wang resin-bound intermediates derived from Fmoc-L-phenylalaninal and Fmoc-L-valinal, and a resin supported Horner-Wadsworth-Emmons reaction, were treated with cinnamaldehyde derivatives, acetic acid, and borohydride to give secondary amines which were subsequently benzoylated to afford various derivatives of 3. Heating 3 at 95 degrees C induced cycloaddition reactions and produced 4 as the major product. Compounds 3 which were derived from 4-methoxycinnamaldehyde were more reactive, but did not give 4 and 4-7g. The direct cleavage of 3b using TFA led to the isolation of cycloaddition-demethylation product 10. The derivative of 3, which contained an electron-withdrawing nitro group on the phenyl ring, produced a single diastereomer of 4. The Diels-Alder cycloaddition between two electron-deficient counterparts showed similar reactivity to that of the reactions which have a normal complementary electron-demand.

Semi-automated high throughput combinatorial solid-phase organic synthesis.

A semi-automated technique for massive parallel solid-phase organic synthesis based on a "split only" strategy is described. Two different types of purpose-oriented reaction vessels are used. The initial steps are performed in domino blocks, and the resin-bound intermediates then split into wells of a micro plate for the last combinatorial step. The domino block is a reaction block for manual and semi-automatic parallel solid-phase organic synthesis that simplifies liquid exchange and integrates common synthetic steps. The synthesis in micro plates does not use any filter for separation of resin beads from the supernatant liquid, and allows high throughput parallel synthesis on solid phase to be performed. This technique, documented on examples of diverse disubstituted benzenes, includes the use of gaseous cleavage in the last synthetic step and allows the synthesis of thousands of compounds per day in mg quantities.

Solid-phase synthesis of substituted 1-phenyl-2-aminomethyl-benzimidazoles and 1-phenyl-2-thiomethyl-benzimidazoles

A solid-phase route to the title compounds is described. Resin-bound intermediate 2, derived from 4-fluoro-3-nitrobenzoic acid, was reacted with substituted anilines then reduced with SnCl 2/NMP to form o-diphenyl diamines 4. Reaction with bromoacetic anhydride, nucleophilic displacement with amines or thiols in DMSO and subsequent cleavage/cyclization of the resin-bound intermediates 6 with neat TFA gave substituted 1-phenyl-2-aminomethyl-benzimidazoles and 1-phenyl-2-thiomethyl-benzimidazoles 7 in good yield and purity.

Solid Phase Synthesis of beta-Peptoids: N-Substituted beta-Aminopropionic Acid Oligomers.

A solid-phase organic synthesis method has been developed for the preparation of N-substituted-beta-aminopropionic acid oligomers or beta-peptoids 1. Treatment of polymer-bound 4-(benzyloxy)benzyl acrylate 2 with primary amines afforded N-substituted beta-alanines 3. Polymer loadings and product conversions were determined by direct cleavage of resin-bound materials and measurement by (1)H NMR with an internal standard. The NMR method was used to establish loading of all resin-bound intermediates including acrylic acid. Acylation with acryloyl chloride followed by Michael addition of primary amines to the acrylamide allowed preparation of di-beta-peptoids. By a linear set of seven reactions, trimeric N-benzyl-beta-aminopropionic acid was prepared in 67% overall yield. Single-bead FT-IR microspectroscopy was used to acquire spectra of the resin bound mono-beta-peptoids, di-beta-peptoids, and acrylamide intermediates. A combinatorial library of defined mixtures of tri-beta-peptoids was prepared by mixing equimolar amounts of the mono-beta-peptoid resins and carrying them through two sequences of the acylation-Michael addition. The identity of a sample mixture was determined by LC-MS analysis of the cleavage product.