Asymmetric epoxidation of trans-olefins via chiral dioxiranes: A possible contribution of axial approaches in the case of tri- and tetrasubstituted alpha-fluoro cyclohexanones

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