STEREOCHEMICAL CONTROL IN MICROBIAL REDUCTION .18. MECHANISM OF STEREOCHEMICAL CONTROL IN THE DIASTEREOSELECTIVE REDUCTION WITH BAKERS-YEAST

Reduction of a variety of 2-alkyl-3-oxobutanoates with bakers' yeast yields the corresponding L-(3S)-hydroxy esters with exclusive enantioselectivity, while the diastereoselectivity, syn/anti ratio, varies depending on the structure of the alkoxyl moiety. In order to elucidate the mechanism for this stereochemical control, oxidoreductases that reduce 2-alkyl-3-oxobutanoate have been isolated from the cells of bakers' yeast and purified. Two dominant competing 2-alkyl-3-hydroxybutanoate oxidoreductases have been obtained: one of them reduces 2-alkyl-3-oxobutanoates stereoselectivity yielding the corresponding L-syn-hydroxy esters, whereas the other affords a mixture of L-syn- and L-anti-hydroxy esters. The experimental results on stereoselectivity have nicely been simulated using kinetic parameters elucidated.