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 DMD of chiral oxazolidine-substituted alkenes, equipped with a free urea NH functionality and a conformationally fixed double bond, proceed in high like diastereoselectivity (up to >95:5); also a high regioselectivity was found for the (1)O(2) ene reaction. Capping of the free NH functionality by methylation erases this like selectivity for both oxidants and significantly reduces the regioselectivity in the ene reaction. These data demonstrate effective hydrogen bonding between the remote urea NH functionality and the oxidant that favors the like attack on the C-C double bond. For (1)O(2), the hydrogen bonding in the exciplex results in preferred hydrogen abstraction from the alkyl group cis to the directing urea functionality.
Adam
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Contribution from the Institut für Organische Chemie, Universität Würzburg, D-97074 Würzburg, Germany. adam@chemie.uni-wuerzburg.de
Peters
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Peters
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Schambony
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