How cell physiology affects enantioselectivity of the biotransformation of ethyl 4-chloro-acetoacetate with Saccharomyces cerevisiae

Saccharomyces cerevisiae (baker's yeast) reduces ethyl 4-chloro-acetoacetate enantioselectively to (R)- or (S)ethyl 4-chloro-3-hydroxybutyrate depending on the reaction conditions and the physiological state of the yeast cells. The (S)-enantiomer is obtained under batch conditions with resting cells (55%, enantiomeric excess [ee]), and 4-chloro-acetate fed-batch actively metabolising yeast affords the (R)-isomer (54%, ee). The enantioselective reduction of the substrate is accompanied by competing enzyme actions. Of the metabolites formed from the substrate, chloroacetone and the target compound (R)-ethyl 4-chloro-3-hydroxybutyrate emerged as most important effectors of enantioselectivity of the microbial reduction. As a minor side-reaction, an aerobic reductive dehalogenation of the substrate was observed. The unusual high enantiopurity of the dehalo-product (S)-ethyl 3-hydroxybutyrate confirms the stereodirecting effect of chloroacetone impressively. Hence, with S. cerevisiae either enantiomer can be obtained by variation of reaction conditions. The yeast further turned out to be a promising biocatalyst for dehalogenations.