Enzymatic diastereo- and enantioselective synthesis of alpha-alkyl-alpha,beta-dihydroxyketones

An enzymatic strategy for the preparation of optically pure alpha-alkyl-alpha,beta-dihydroxyketones is reported. Homo- and cross-coupling reactions of alpha-diketones catalyzed by acetylacetoin synthase (AAS) produce a set of alpha-alkyl-alpha-hydroxy-beta-diketones (30-60%, ee 67-90%), which in turn are reduced regio-, diastereo-, and enantioselectively to the corresponding chiral alpha-alkyl-alpha,beta-dihydroxyketones (60-70%, ee >95%) using acetylacetoin reductase (AAR) as catalyst. Both enzymes are obtained from Bacillus licheniformis and used in a crude form. The relative syn stereochemistry of the enantiopure alpha,beta-dihydroxy products is assigned by NOE experiments, whereas their absolute configuration is determined by conversion of the selected 3,4-dihydroxy-3-methyl-pentan-2-one to the natural product (+)-citreodiol.