Stereoselectivity in reactions of metal complexes VIII. Asymmetric synthesis of some amino acids by stereoselective transamination of aliphatic keto acids in mixed ligand copper(II)-Schiff-base complexes

Optically active alanine, valine and leucine were obtained by a transamination reaction between pyridoxamine and the corresponding α-keto acid in the presence of a Cu²⁺-complex with the tridentate ligand 2,6-bis(3S)-3-phenykl-2-azabutyl]pyridine. In each case the amino acid with (R)-configuration was formed preferentially, and the maximum enantiomeric excesses were 54% (alanine), 48% (leucine) and 29% (valine). The stereoselectivity of the reaction is discussed in terms of the possible structure and the stability of the intermediate Cu²⁺-ketimine-ligand complex.