Anti-Selective Epoxidation of Methyl α-Methylene-β-tert-butyldimethylsilyloxycarboxylate Esters. Evidence for Stereospecific Oxygen Atom Transfer in a Nucleophilic Epoxidation Process

Methyl α-methylene-β-tert-butyldimethylsilyloxycarboxylate esters are found to undergo diastereoselective epoxidation in the presence of potassium tert-butoxide−tert-butyl hydroperoxide to form anti products. In an effort to better understand mechanistic details of the transformation and the basis of diastereoselectivities observed, we studied the epoxidation of substrates with α-methylene groups containing (trans) deuterium labels and discovered that oxygen-atom transfer proceeds with ≥95% stereospecificity in all cases examined. These and other experiments suggest that the mechanism of epoxidation is not distinguishable from a concerted process.