Asymmetric Weitz-Scheffer epoxidation of conformationally flexible and fixed enones with sterically demanding hydroperoxides mediated by optically active phase-transfer catalysts

The asymmetric Weitz-Scheffer epoxidation of alpha,beta -enones 1a-d with h ydroperoxides 2 and mediated by cinchonine-and cinchonidine-derived phase-t ransfer catalysts (PTCs)3 affords the optically active epoxides 4 with mode rate to very good e.e.s and in near quantitative yields. For the conformati onally flexible enone 1b, the enantioselectivity decreases with more steric ally demanding tertiary hydroperoxides, while an opposite trend is observed for the rigid s-cis-enone I. With the bulky cumyl hydroperoxide 2c and the PTC 3c, the enone 1 was converted to the epoxide 4c with the highest enant ioselectivity (95% e.e.) so far observed for PTC-catalyzed epoxidations usi ng hydroperoxides as oxygen sources. (C) 2001 Elsevier Science Ltd. All rig hts reserved.