Design and synthesis of chiral ketones for catalytic asymmetric epoxidation of unfunctionalized olefins

A series of C₂ symmetric chiral ketones were designed and synthesized for catalytic asymmetric epoxidation of unfunctionalized olefins. Among those ketones screened, (R)-7, (R)-9, and (R)-10 were found to be highly efficient catalysts for epoxidation of trans-stilbenes with enantioselectivities in the range of 84-95%. Convincing evidence was provided for a spiro transition state of dioxirane epoxidation. Through the ¹⁸O-labeling experiment, chiral dioxiranes were found to be the intermediates in chiral ketone catalyzed epoxidation reactions.