A STUDY OF ENANTIOSELECTIVE REDUCTION OF PARASUBSTITUTED 2-METHYL-CINNAMALDEHYDES BY BAKERS-YEAST

Reduction of a series of para-substituted 2-methyl-cinnamaldehydes (1- 7) at 20 degrees C and at pH = 2-3 afforded S-enantiomers of saturated alcohols 8-14 in 20-80% yield and 75 - greater than or equal to 99% e nantiomeric excess (e.e.); at 30 degrees C, lower yields and e.e.s wer e obtained. Relative rates of the formation of allylic alcohols 15-21, catalyzed by alcohol dehydrogenase (ADH), correlate with the Hammett sigma(+) values of para-substituents, revealing that a more efficient delocalization of the positive charge on carbonylic carbon slow down t he reduction rates, whereas no correlation of the electronic propertie s of the substituents with the rate of C=C double bond reduction, cata lyzed by enoate reductase, is observed. On reduction of 3 by dried yea st in (H2O)-H-2, alpha,beta-carbon atoms in 10a bear H-2 atoms, in acc ordance with the previously reported hydrogenation of selectively H-2- labeled cinnamic aldehyde and cinnamic alcohol. The accumulated data i ndicate that the mechanism of the enone C=C bond reduction that compri ses nucleophilic attack of the hydride ion species on the beta-carbon in the first step, followed by enantioselective protonation on the alp ha-carbon atom.