Asymmetric epoxidation of ( Z)-enol esters catalyzed by titanium(salalen) complex with aqueous hydrogen peroxide

Abstract

Titanium(salalen) complex 1 was an effective catalyst for asymmetric epoxidation of enol esters. Although ( E)-enol esters were reluctant to proceed, ( Z)-enol esters underwent asymmetric epoxidation to give the epoxides in high yields with high enantioselectivity ranging from 86 to >99% ee in the presence of aqueous hydrogen peroxide as the stoichiometric oxidant. Complete enantioselectivity was observed in the reaction of ( Z)-3,3-dimethylbut-1-en-1-yl 4-methoxybenzoate. The obtained epoxide was readily transformed into the corresponding 1,2-diol by reduction with lithium borohydride without erosion of the high enantiomeric excess.