Stereochemical Control in Microbial Reduction. 9. Diastereoselective Reduction of 2-Alkyl-3-oxobutanoate with Bakers’ Yeast

Bakers’ yeast reduces esters of 2-alkyl-3-oxobutanoic acid (CH3COCHRCO2R′; R=methyl, ethyl, propyl, propargyl, and allyl) into the corresponding (S)-hydroxy esters with exclusive stereoselectivity, while the configuration at the 2-position of the hydroxy esters is either S (anti) or R (syn) depending on the structure of the alkoxyl group in the carboalkoxyl moiety of the ester. Oftenly, the stereoselectivity with respect to the 2-position is not satisfactory. In general, the reduction of t-butyl esters exerts predominancy in the products, whereas that of 1,1-dimethylpropyl esters exerts the syn predominancy. A marked difference between these two esters in diastereoselectivity is discussed from the view point of plausible conformations of the esters.