A new strategy for the stereoselective synthesis of unnatural α-amino acids

A new method for the synthesis of racemic non-proteinogenic α-amino acids has been developed, which involves (i) hetero-Diels-Alder addition of ethyl 2-nitrosoacrylate to electron rich alkenes such as enol ethers, enamines and allylsilanes, (ii) NaCNBH₃ reduction of the CN bond in the oxazines thus generated, the stereochemistry of the products being controlled by epimerisation of the thermodynamically less stable isomer to the more stable one, (iii) protection of the N-H group as N-Boc and (iv) finally, N-O bond cleavage of both free and protected products to give proline or bis-homoserine derivatives, respectively. An example with concomitant reduction of the carboxylate group, resulting in the formation of the respective amino alcohol is reported. Applying this methodology to a homochiral enol ether, the protected parent d-proline was prepared in enantiomerically pure form, whereas the asymmetric synthesis of the respective bis-homoserine was unsuccessful.     

A new enantioselective synthesis of β-amino acids

Enantioselective hydrogenation of some α,β-unsaturated nitriles and their corresponding methyl esters bearing a phthalimidomethyl substituent at the α-carbon using Rh-DuPHOS catalysts afforded β-amino acid precursors with modest e.e.s of up to 48%. Hydrogenation of the α,β-unsaturated methyl esters using a Ru-BINAP catalyst gave higher e.e.s of up to 84%. Method development for the determination of the enantiomeric excesses of these derivatives using chiral HPLC is also reported.     

Asymmetric synthesis of anti-β-hydroxy-α-amino acids

The stereoselective aldol addition reactions of chiral haloacetate enolates is reported. The conversion of these adducts to enantiomerically pure anti-β-hydroxy-α-amino acids is demonstrated (Eq 2).     

Oxazoline-mediated highly stereoselective synthesis of α,β-substituted-β-aminoalkanamides, potential precursors of unnatural β-amino acids

The reaction of α-lithiated-2-alkyl-2-oxazolines 1-Li with aliphatic, aromatic and heteroaromatic N-cumyl nitrones results in the stereoselective formation of N-cumyl-1,6-dioxa-2,9-diazaspiro[4,4]nonanes 3 which equilibrate with the hydroxylamino derivatives 4. Such equilibrating mixtures can be easily transformed into β-amino alkanamides 5 under reductive conditions, whereas acidic hydrolysis with trifluoroacetic acid (TFA) furnishes high yields of β-phenylamino alkanamides 6 via a cumene hydroperoxide-type rearrangement. Derivatives 5 and 6 provide a backbone of potentially useful unnatural β^2,2,3-amino acids.     

Iodolactonization of 3-amino-4-pentenoic acid: a stereoselective synthesis of syn-γ-hydroxy-β-amino acids

The reaction of (S)-3-N-Cbz-4-pentenoic acid with iodine in acetonitrile in the presence of AgOTf gave the cis-iodo-lactone 6 in excellent yield and in a highly diastereoselective manner. The substitutions of the iodine in 6 by different Grignard reagents in the presence of CuI and the subsequent conversions into the functionalized syn-γ-hydroxy-β-amino acids have been investigated. By the above reaction sequence, (3S,4S)-3-amino-4-hydroxy-5-cyclohexyl pentanoic acid was synthesized with high enantioselectivity.     

A protocol for an asymmetric synthesis of γ-amino acids

A new and practical method for the asymmetric synthesis of γ-amino acids from β,γ-butenolides by an in situ esterification, condensation, and reduction in a one-pot procedure is described. This method is quite general for the preparation of both enantiomers of aryl or aliphatic γ-amino acids in high yields. These γ-amino-acid derivatives were also shown to be versatile synthetic intermediates for further transformations by their conversion to γ-lactams, δ-amino alcohols, and hydrolysis products in high yields with no racemization.     

A modular synthesis of dithiocarbamate pendant unnatural α-amino acids

Unnatural α-amino acids containing dithiocarbamate side chains were synthesized by a one-pot reaction of in situ generated dithiocarbamate anions with sulfamidates. A wide range of these anions participated in the highly regio- and stereo-selective ring opening of sulfamidates to produce the corresponding dithiocarbamate pendant α-amino acids in high yields.     

Asymmetric synthesis of a homochiral differentially protected pseudo-meso bis-β-amino acid scaffold

A strategy for the asymmetric synthesis of a homochiral differentially protected pseudo-meso bis-β-amino acid scaffold utilising the conjugate addition of homochiral lithium amides and subsequent selective deprotection is demonstrated.     

Absolute asymmetric synthesis of tertiary α-amino acids

Frozen: the spontaneous crystallization of an achiral compound in a chiral conformation is used as the unique source of chirality in an absolute asymmetric synthesis of tertiary amino acids. The dynamic axial chirality of tertiary aromatic amides is frozen in a crystal and is responsible for the stereoselectivity of the deprotonation/alkylation. α-Amino acid derivatives are synthesized in up to 96 % ee.     

A new stereoselective synthesis of sterically constrained uncommon α,α′-dialkylated α-amino acids. Part 2

The alkylation of the diastereomeric mixture of the chiral morpholinone derivatives 5 and 6 occurs with good yield and a prevalence of the cis isomer. Cleavage of the alkylated intermediates 7b,c yields enantiomerically pure sterically constrained uncommon α,α′-dialkylated α-amino acids. The absolute configuration of the new stereocentres has been assigned on the basis of the ¹H NMR spectra and NOE measurements. A model to explain the observed diastereoselection is proposed.     

A simple approach for the synthesis of new pyrimidinyl α-amino acids

A simple synthetic method for the preparation of optically active pyrimidinyl α-amino acids is presented. A nucleophilic ipso-substitution reaction between 2-(benzylsulfonyl)-4-isopropoxypyrimidines and a nucleophilic side chain of several protected natural α-amino acids is investigated to obtain new pyrimidin-2-yl α-amino acids. A detailed optimisation study of this reaction is discussed. Moreover, the selective O-alkylation of 2-(benzylsulfanyl)-4(3H)pyrimidinones with a hydroxylic side chain of some natural α-amino acids under Mitsunobu conditions is studied as a method to prepare new pyrimidin-4-yl α-aminoesters.     

A new general, practical and enantioselective synthesis of α-amino acids

In recent years, considerable efforts have been expended to the development ofenantioselective synthesis of α-amino acids and the use of α-amino acids as chiralbuilding blocks for the synthesis of complex molecules. Numerous unnatural aminoacids which are often the characteristic units of biologically active peptides, have alsobeen discovered. Herein we describe a convenient and general access to natural andunnatural α-amino acids which are based on the use of the chiral building blocks,(S)-1a-e and (R)-1a-e from kinetic resolution of α-furfuryl amines (Scheme 1).     

A general and efficient stereoselective synthesis of γ-azido-tetrahydrofuran carboxylic acids from glycals

An efficient, direct and general synthesis of enantiopure γ-azido-tetrahydrofuran carboxylic acid monomers (5-8) from commercially available glycals, suitable to design peptidomimetic oligomers with predisposed conformation, is described. The single crystal X-ray study of 8 showed that the compound crystallized in orthorhombic space group. The crystal-packing showed the presence of weak intermolecular C-H?O and aliphatic C-H?π interactions.     

Asymmetric synthesis of α,β-substituted γ-amino acids via conjugate addition

The first conjugate addition reaction of organocuprates to N-enoyl oxazolidinone where a N-protected γ-nitrogen atom and an α-methyl group are present into α, β-unsaturated system is described. This reaction gave anti-products in moderate yields and high diastereomeric ratios. The anti-products have two contiguous stereogenic centers, one formed by the conjugate addition reaction and the other by a diastereoselective protonation reaction. The removal of chiral oxazolidinone moiety and N-deprotection of amino group furnished chiral α, β-disubstituted γ-amino acids.     

Asymmetric synthesis of β-amino acids through application of chiral sulfoxide

This paper describes asymmetric synthesis of β-aminophenylpropionic acid through application of a homochiral sulfoxide auxiliary. High kinetically controlled (3R,2S,R_S)-diastereoselectivity (−60°C) is achieved during addition of the lithium enolate of tert-butyl (+)-(R)-p-toluenesulfinylacetate to substituted N-(benzylidene)toluene-4-sulfonamides 2a–2d. The reductive cleavage of adduct 3a with sodium amalgam yielded tert-butyl 3-(toluene-4-sulfonamido)-3-phenylpropionate 5a, which was subjected to ester hydrolysis and subsequent detosylation with sodium in liquid ammonia to yield (S)-β-aminophenylpropionic acid in good yield and high 91% e.e.     

Samarium iodide-mediated Reformatsky reactions for the stereoselective preparation of β-hydroxy-γ-amino acids: synthesis of isostatine and dolaisoleucine

The synthesis of β-hydroxy-γ-amino acids via SmI(2)-mediated Reformatsky reactions of α-chloroacetyloxazolidinones with aminoaldehydes is reported. Diastereoselective coupling is demonstrated to depend on the absolute configuration of the Evans chiral auxiliary employed in the reaction, allowing erythro or threo products to be obtained selectively. The potential utility of the methodology is exemplified by the facile synthesis of biologically relevant N-Boc-isostatine (2b) and N-Boc-dolaisoleucine (3c).     

Stereoselective,nonracemic synthesis of ω-borono-α-amino acids

ω-Unsaturated α-amino acids are synthesized through condensation of allyl and propargyl bromides or of 9-bromoundecene with a Ni(II) complex of the Schiff base derived from glycine and BPB. Hydroboration with Ipc₂BH followed by oxidation with acetaldehyde affords enantiomerically pure ω-borono-α-aminocarboxylic acids.