Access to Optically Pure 4- and 5-Substituted Lactones:  A Case of Chemical−Biocatalytical Cooperation

Optically pure or highly enantiomerically enriched 4- and 5-substituted lactones are rather difficult to obtain. Chemical or enzymatic syntheses alone are not particularly successful. A combination of chemical catalysis and biocatalysis, however, provides a convenient route to a variety of these useful chiral compounds. In this paper we describe the synthesis of several optically pure 4- and 5-substituted lactones obtained via whole cell-catalyzed Baeyer−Villiger oxidations of highly enantiomerically enriched 3-alkyl cyclic ketones. Such chiral ketones are readily accessed by recently developed copper-catalyzed asymmetric conjugate reductions of the corresponding enones. A very high proximal regioselectivity and complete chirality transfer was obtained by employing biological Baeyer−Villiger oxidations, using recombinant E. coli strains that overexpress cyclopentanone monooxygenase (CPMO). A comparative study showed that CPMO gives superior results to those obtained with cyclohexanone monooxygenase (CHMO) catalyzed oxidations.