Epoxidation of p-methoxycinnamates using chiral dioxiranes derived from new trisubstituted halogenated cyclohexanones: Enhanced efficiency of ketoneshaving an axial halogen

[GRAPHICS] Epoxidation of p-methoxycinnamates using substoichiometric amounts of chira l and enantiopure dioxiranes generated in situ from chiral trisubstituted c hloro and fluoro cyctohexanones showed that ketones with an axial Cl or F w ere more efficient than ketones with an equatorial Cl or F, that increasing the steric hindrance in the alpha -position (isopropyl instead of methyl) decreased the efficiency, and that the alpha -fluoro ketone 5a (axial fluor ine) was the most efficient (82% conversion and 46% ee in (-) epoxide).