Asymmetric Synthesis of Enantiomerically Enriched a-Amino Acids Containing 2-Furyl- and 2-Thienyl-1,2,4-triazoles in the Side-Chain

An efficient method for the asymmetric synthesis of a-amino acids, containing furyl- and thiophenyl-substituted triazoles in their side-chain, is reported. The strategy relies on Michael addition of 3,4,5-substituted 1,2,4-triazoles to the C=C bond of chiral NiII complexes containing the Schiff base formed from dehydroamino acids (dehydroalanine and (E + Z)-dehydroaminobutyric acid) and from chiral auxiliaries, i. e. (S)-2-N-(N0-benzylprolyl)aminobenzophenone and (S)-2-N- (N0-2-chlorobenzylprolyl) aminobenzophenone. The reactions proceeded with good to very good diastereoselectivity. Hydrolysis of the diastereomeric mixtures of metal complexes afforded the enantiomerically pure a-amino acids with high enantiomeric excess (ee> 98%).