Methyl 2-bromo-2-butenoate(Methyl (Z)-2-bromocrotonate) yields (S)-2-bromobutanoic acid by baker′s yeast fermentation. It undergoes biotransformation catalyzed by old yellow enzyme to yield methyl (S)-2-bromobutanoate, a key intermediate for the synthesis of chiral drugs. It is a biannelating reagent which participates in double Michael-additions.