Efficient control of the diastereoselectivity and regioselectivity in the singlet-oxygen ene reaction of chiral oxazolidine-substituted alkenes by a remote urea NH functionality: Comparison with dimethyldioxirane and m-chloroperbenzoic acid epoxidations

The singlet-oxygen ene reaction and the epoxidation by DMI) of chiral oxazo lidine-substituted alkenes, equipped with a free urea NH functionality and a conformationally fixed double bond, proceed in high like diastereoselecti vity (up to >95:5); also a high regioselectivity was found for the O-1(2) e ne reaction. Capping of the free NH functionality by, methylation erases th is like selectivity for both oxidants and significantly reduces the regiose lectivity in the ene reaction. These data demonstrate effective hydrogen bo nding between the remote urea NH functionality and the oxidant that favors the like attack on the C-C double bond. For O-1(2), the hydrogen bonding in the exciplex results in preferred hydrogen abstraction from the alkyl grou p cis to the directing urea functionality.