Enantioselective synthesis and stereoselective rearrangements of enol ester epoxides.

Enol esters can be epoxidized with high enantioselectivities using the fructose-derived chiral ketone 1 as catalyst and Oxone as oxidant. A detailed study of enantiomerically enriched enol ester epoxides has revealed that the acid-catalyzed rearrangement can proceed through two distinct pathways, one with retention of configuration and the other with inversion. The competition between the two pathways is highly dependent upon the nature of the acid catalyst. A strong acid favors retention of configuration and a weak acid favors inversion of configuration. Under thermal conditions, these epoxides rearrange highly stereoselectively with inversion of configuration. Either enantiomer of an alpha-acyloxy ketone can be formed from one enantiomer of an enol ester epoxide by judicious choice of reaction conditions.