4-Substituted-α,α-diaryl-prolinols improve the enantioselective catalytic epoxidation of α,β-enones

To seek novel metal-free organic catalysts for epoxidation with high stereoselectivity, a series of 4-substituted-alpha,alpha-diaryl-prolinols were synthesized in four steps from trans-4-hydroxyl-L-proline. These prolinol derivatives catalyzed the asymmetric epoxidation of alpha,beta-enones to give the corresponding chiral epoxides in good yields and high enantioselectivities under mild reaction conditions. Studies of substituent effects on enantioselectivity revealed that steric bulk and electronic effect promoted higher enantioselectivity, and prolinol 8a was found to be the best catalyst until now.