Asymmetric Epoxidation oftrans-Olefins via Chiral Dioxiranes: A Possible Contribution ofaxialApproaches in the case of Tri- and Tetrasubstituted α-Fluoro Cyclohexanones

Abstract

During the asymmetric epoxidation of stilbene and methyl p-methoxycinnamate with chiral dioxiranes (derived from 2,2′,5-tri- and 2,2′,5,5′-tetrasubstituted cyclohexanones with an axial fluorine at C2) a 26 to 30% increase in ee was observed upon desymmetrization of the axial face of the dioxirane (through disubstitution at C5 or introduction of a fluorine on the equatorial iPr-substituent located at C5), which suggests a contribution of the axial approach of the olefin to the dioxirane (in addition to the main equatorial approach). The two ketones 5a and 6a, which do not undergo Baeyer− Villiger oxidation, can be used in substoichiometric amounts and are fully recovered after the reactions, provide epoxides in high yield (75−90%) and high (95:5) to satisfying (83:17 to 87:13) enantiomeric ratios.