Bakers' yeastreduction of 1-chloro-2,4-alkanediones 1a afforded 1-chloro-2-hydroxy-4-alkanones 2a regioselectively with low opticalpurities. Application of inhibitors and heat-treatment of bakers' yeast enhanced the opticalpurities toward the S enantiomer (88–91% ee). Organic solvents added in small amounts were also found to enhance the S selectivity significantly. Highopticalpurities of 94–96%