The synthesis of optically active α-methyl β-hydroxy esters I by means of microbiological reduction of the corresponding β-keto esters II was carried out. Benzyl 2-methyl-3-oxobutyrate 1 was found to be reduced by a variety of yeasts to the α-methyl β-hydroxy esters with (2R, 3S)- and (2S, 3S)-configurations (2 and 3, respectively), and by carrying out screening experiments, yeasts which give each product with high optical purity were isolated. Moreover, the absolute configuration and the optical purity of the reduction products were found to be determinable from the 400 MHz nuclear magnetic resonance spectra of the (+)-α-methyoxy-α-trifluoromethylphenylacetyl esters of the alcohols produced.
