摘要：

Values have been obtained for the stereoselective reduction by fermenting yeast of a series of ketones including all combinations of the substituents methyl, ethyl, n-propyl, n-butyl, and phenyl. Fermentation yields were improved by dispersing the insoluble ketone substrates on cellulose powder or Celite; isolation of highly purified optically active carbinols was made possible by use of gas chromatography. The enantiomorph of the carbinol produced in excess corresponded to the absolute s configuration, with one controversial exception, namely n-propyl-n-butylcarbinol. Stereoselectivity varied from values as high as 90% in the aromatic series to as low as 12 % for ethyl n-propyl ketone. From this it is concluded that the difference in steric requirements of the two groups attached to the carbonyl function is the important factor in determining the configuration of the predominate transition state and not the absolute preference of one group or the other for a particular site on the enzyme surface. The lack of 100% stereoselectivity during fermentative reduction of these ketones can be rationalized by assuming that the reductions are catalyzed by alcohol dehydrogenase as the only enzyme involved but that it is only partially stereoselective when acting on these unnatural substrates.

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