Diastereoselective singlet oxygen ene reaction (Schenck reaction) and diastereoselective epoxidations of heteroatom-substituted acyclic chiral olefins: A mechanistic comparison

The directing propensity of allylic and homoallylic heteroatom substituents on the diastereoselective singlet oxygen ene reaction (Schenck reaction) of acyclic, chiral olefins has been investigated. These extensive stereochemical results offer evidence for our proposed mechanism of the diastereoselective singlet oxygen ene reaction. Thus, provided a substrate possesses sufficient 1,3-allylic strain to populate preferentially the appropriate conformation, the photooxygenation is directed three-selectively by intramolecular hydrogen bonding in the threo-A exciplex of allylic alcohols, while electron-accepting or bulky substituents result in the erythro-configurated products. A detailed comparison of the diastereoselectivities for the singlet oxygen ene reaction with those of the epoxidation by m-chlorobenzoic acid (mCPBA) and dioxirane of several judiciously chosen substrates showed that both oxygen transfer processes are influenced similarly by allylic and homoallylic substituents. Thus, it is shown that the acyclic, allylic alcohol la is epoxidized highly three-selectively by methyl(trifluoromethyl)dioxirane due to hydrogen bonding in the transition state and that bulky and/or electron-accepting substituents direct all three oxidants erythro-selectively. These pronounced steering effects on the diastereoselectivity of oxygen transfer processes (photooxygenation and epoxidation) should be valuable for the rational design of stereocontrolled oxyfunctionalizations by O-1(2), mCPBA, and dioxiranes.