Asymmetric Weitz-Scheffer epoxidation of isoflavones with hydroperoxides mediated by optically active phase-transfer catalysts.

The asymmetric Weitz-Scheffer epoxidation of the isoflavones 3, mediated by the cinchonine- and cinchonidine-derived phase-transfer catalysts (PTCs) 1, affords the enantiomerically enriched isoflavone epoxides 4 with ee values of up to 98% in nearly quantitative yields. With the appropriately configured PTC 1, both enantiomers of the isoflavone epoxides may be obtained by using the commercially available cumyl hydroperoxide 2b as oxidant. Methylation of the hydroxy functionality in the most effective PTC (1b) reduces significantly the enantioselectivity of the isoflavone epoxidation as illustrated for the substrate 3c. This fact indicates the pivotal role of the hydroxy group for enantioselective control, which is rationalized in terms of a hydrogen-bonded aggregate between the ether-oxygen atom of isoflavone 3 and the phase-transfer catalyst 1. The present attractive and convenient method should be useful for the preparation of optically active epoxides of the biologically relevant isoflavone structure.