A cyclodextrin-modified ketoester for stereoselective epoxidation of alkenes

Abstract

A β-cyclodextrin-modified ketoester 2 was prepared by covalent attachment of a reactive ketone moiety to β-cyclodextrin. Treatment of 2 with Oxone as terminal oxidant would produce CD-substituted dioxirane, which can effect stereoselective alkene epoxidation. The 2-mediated (S)-α-terpineol epoxidations proceeded to give terpineol oxides in high yields, and the stereoselectivities (i.e., cis-/trans-epoxide ratio) decreased from 2.5:1 to 1:1.2 with increasing steric bulkiness of the terpenes. This steric-dependent stereoselectivity can be understood based on different binding geometries of the 2/terpene inclusion complexes according to the 1H NMR titration and 2D ROESY experiments. Enantioselective epoxidation of styrenes has also been achieved with 2 as catalyst (20-50 mol %) in aqueous acetonitrile solution, and up to 40% ee was obtained in 4-chlorostyrene epoxidation at 0 °C. Similar enantioselectivities were also obtained for the 2-mediated epoxidation of 1,2-dihydronaphthalene (37% ee), 4-chlorostyrene (36% ee), and trans-stilbene (31% ee).