STEREOSELECTIVE REDUCTION OF THIOCARBONYL COMPOUNDS WITH BAKER YEAST

Alkyl esters of 3-thioxo-butanoic, -pentanoic and -hexanoic acid, 2-th ioxo-1-cyclopentanecarboxylic acid ethyl ester, and 2-octanethione hav e been reduced with baker's yeast to give optically active thiols. The reductions parallel those of the oxygen analogues with respect to rat e and diastereo- and enantioselectivity but, generally, the enantiomer ic excess (ee) values are smaller. The influence of experimental varia bles such as substrate concentration, physiological condition of the y east (resting/fermenting, fresh/dry/frozen), substrate modification, a nd addition of inhibitors on stereoselectivity has been studied. The f ormation of S-products was favoured by small substrate concentrations, use of dry or frozen yeast, and by addition of R-enzyme inhibitors (e thyl acrylate and biacetyl). The yields of thiol products are limited by extensive hydrolysis of the thioxo groups of all substrates used. I n order to make meaningful comparisons of enantiomeric preferences in baker's yeast reduction of different substrates it is suggested that t he maximum ee values reached below certain (substrate dependent) subst rate/yeast ratios are used.