Catalytic, asymmetric synthesis of α,α-disubstituted amino acids

Copper(I)-catalyzed asymmetric alkylation of α-imino-esters

Asymmetric alkylation of enolates is one of the most direct and important reactions to prepare α-chiral carbonyl compounds. Except for the classical methods that rely on the use of chiral auxiliaries, asymmetric catalysis emerged as a powerful tool, especially asymmetric phase-transfer catalysis. However, in the field of transition metal catalysis, only limited success with asymmetric alkylation of enolates was achieved. Hereby, we disclose a copper(I)-catalyzed asymmetric alkylation of α-imino-esters with various alkyl halides, including allyl bromides, propargyl bromide, benzyl bromides, α-bromo carbonyl compounds, and alkyl iodides. Both linear and cyclic α-imino-esters serve as competent pronucleophiles in the alkylation, which affords α-amino acid derivatives bearing either a trisubstituted or a tetrasubstituted stereogenic carbon center in high to excellent enantioselectivity. Control experiments indicate that the α-imino-ester is activated by a chiral copper(I)-phosphine complex through coordination, thus enabling facile deprotonation to provide a stabilized copper(I)-enolate in the presence of a mild base. Finally, the mildly basic nature allows the asymmetric alkylation of chiral dipeptides with excellent both chemo- and enantioselectivities.

Synthesis, Characterization, and Study of Catalytic Activity of Chiral Cu(II) and Ni(II) Salen Complexes in the α-Amino Acid C-α Alkylation Reaction

A new family of Cu(II) and Ni(II) salen complexes was synthesized and fully characterized through various physicochemical methods. Their catalytic activity was evaluated in the phase transfer C_α-alkylation reaction of the Schiff bases of D,L-alanine ester and benzaldehyde derivatives. It was found that the introduction of a chlorine atom into the ortho- and para-positions of the phenyl ring of the substrate resulted in an increase in both the chemical yield and the asymmetric induction (ee 66-98%). The highest enantiomeric excess was achieved in the case of a Cu(II) salen complex based on (S,S)-cyclohexanediamine and salicylaldehyde at -20 °C. The occurrence of a bulky substituent in the ligand present in the complexes led to a drastic decrease in ee and chemical yield. For instance, the introduction of bulky substituents at positions 3 and 5 of the phenyl ring of the catalyst resulted in a complete loss of the stereoselectivity control in the alkylation reaction.

Double nucleophilic addition to iminomalonate, leading to the synthesis of quaternary α-amino diesters and desymmetrization of the products

Alkylation of iminomalonate with Grignard reagents followed by oxidation and allylation gave symmetrical quaternary α-amino diesters in good yields. Subsequent desymmetrization of a diol derivative from these products was conducted via asymmetric carbamylation catalyzed by Cu-Bnbox to give chiral quaternary aminodiol mono-carbamates.

Diastereoselective synthesis of quaternary α-amino acids from diketopiperazine templates

Sequential enolate alkylations of (S)-N(1)-methyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one and (S)-N(1)-p-methoxybenzyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one proceed with excellent levels of diastereoselectivity (>90% de) affording quaternary alpha-amino acids in high enantiomeric excess (>98% ee) after deprotection and hydrolysis.

Recent approaches towards the asymmetric synthesis of α,α-disubstituted α-amino acids

The class of alpha,alpha-disubstituted alpha-amino acids has gained considerable attention in the past decades and continues doing so. The ongoing interest in biological and chemical properties of the substance class has inspired the development of many new methodologies for their asymmetric construction, which have not found their way into the general focus of organic chemistry yet. The aim of this review is to provide an overview of the developments in the field since 1998.

Cinchona alkaloid-based polymer-bound phase-transfer catalysts: Efficient enantioselective alkylation of benzophenone imine of glycine esters

Cross-linked polystyrene-bound and poly(ethylene glycol)-bound phase-transfer catalysts as well as homopolymers of cinchona alkaloid derivatives have been synthesised. Both soluble and insoluble polymers have been investigated. The enantioselective alkylation of N-diphenyl methylene glycine t-butyl ester has been successfully carried out in heterogeneous and homogeneous systems. High enantioselectivities (up to 96%) have been obtained. The polymer-bound catalysts have been easily recovered and conditions for efficient recycling have been studied.

Synthesis of (1R,2S)-1-Amino-2-vinylcyclopropanecarboxylic Acid Vinyl-ACCA) Derivatives: Key Intermediates for the Preparation of Inhibitors of the Hepatitis C Virus NS3 Protease

(1R,2S)-1-Amino-2-vinylcyclopropanecarboxylic acid (vinyl-ACCA) is a key building block in the synthesis of potent inhibitors of the hepatitis C virus NS3 protease such as BILN 2061, which was recently shown to dramatically reduce viral load after administration to patients infected with HCV genotype 1. We have developed a scalable process that delivers derivatives of this unusual amino acid in >99% ee. The strategy was based on the dialkylation of a glycine Schiff base using trans-1,4-dibromo-2-butene as an electrophile to produce racemic vinyl-ACCA, which was subsequently resolved using a readily available, inexpensive esterase enzyme (Alcalase 2.4L). Factors that affect diastereoselection in the initial dialkylation steps were examined and the conditions optimized to deliver the desired diastereomer selectively. Product inhibition, which was encountered during the enzymatic resolution step, initially resulted in prolonged cycle times. Enrichment of racemic vinyl-ACCA through a chemical resolution via diastereomeric salt formation or the use of forcing conditions in the enzymatic reaction both led to improvements in throughput and the development of a viable process. The chemistry described herein was scaled up to produce multikilogram quantities of this building block.

Mechanistic studies on the asymmetric alkylation of amino ester enolates using a copper(ii)salen catalyst

Hammett data indicate that the asymmetric alkylation of enolates catalysed by copper(II)salen complex 1, proceeds by an asynchronous S(N)2 reaction and that the role of the catalyst is to enhance the nucleophilicity of the enolate.

Synthetic applications of aliphatic unsaturated α-H-α-amino acids

This article provides an overview of the literature concerning synthetic applications of unsaturated aliphatic amino acids in the period May 2000 to December 2004.