ASYMMETRIC HYDROGENATIONS OF N-PYRUVOYL-(S)-AMINO ACID-ESTERS

N- left bracket (R)-Lactoyl right bracket -(S)-amino acid isobutyl esters were obtained with a diastereoisomeric purity up to 34%, through the catalytic hydrogenations of N-pyruvoyl-(S)-amino acid (alanine, valine, and leucine) isobutyl esters over palladium on charcoal. It was found that a linear correlation exists between the dielectric constant of the solvent and the diastereoisomeric purity of the product obtained by catalytic hydrogenation. The ratio of N- left bracket (S)-lactoyl right bracket -(S)-amino acid isobutyl ester to N- left bracket (R)-lactoyl right bracket -(S)-amino acid isobutyl ester increased with the decrease of the dielectric constant of the solvent. This solvent effect could be explained by the chelation mechanism. The temperature effect on the asymmetric catalytic hydrogenation was also studied.