Studies of substituent effect on asymmetric epoxidation of chromenes by chiral dioxirane.

A series of 6- and 8-substituted chromenes has been investigated for asymmetric epoxidation using chiral ketone catalysts. Up to 93% ee was achieved. Higher ee's are obtained when substrates are substituted at the 6-position. The enhanced enantioselectivity is likely due to the beneficial interaction between the 6-substituent of the substrate and the N-aryl or alkyl group of the ketone catalyst.