Probing for steric and electronic effects in diastereoselective dioxirane epoxidations compared to the oxygen transfer by peroxy acids

Article abstract of DOI:10.1002/jlac.199719970316

The spiro transition state for the oxygen transfer by dioxiranes is substantiated by the fact that no enhanced steric effects are observed when dioxiranes with alkyl groups of different size are employed, as manifested by the same (within the experimental error) diastereoselectivities in the epoxidation of 2-menthene and 1,3-dimethylcyclohexene for different dioxiranes. The π-facial selectivity (anti attack) in the epoxidation of the acetate and the methyl and trimethylsilyl ether derivatives of 2-cyclohexenol derives from steric interactions, whereas a pronounced electronic effect (electrostatic repulsion) is held responsible for the high anti selectivity of peroxides such as ascaridol and 3-hydroperoxycyclohexene. Quite generally, dioxiranes display only slightly higher diastereoselectivities than mCPBA in sterically controlled epoxidations of cycloalkenes. VCH Verlagsgesellschaft mbH, 1997.