A cyclodextrin-modified ketoester for stereoselective epoxidation of alkenes.
A beta-cyclodextrin-modified ketoester 2 was prepared by covalent attachment of a reactive ketone moiety to beta-cyclodextrin. Treatment of 2 with Oxone as terminal oxidant would produce CD-substituted dioxirane, which can effect stereoselective alkene epoxidation. The 2-mediated (S)-alpha-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 (1)H 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 degrees 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).
Chan
Wing-Kei
WK
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
Yu
Wing-Yiu
WY
Che
Chi-Ming
CM
Wong
Man-Kin
MK
D001665:Binding Sites; D002240:Carbohydrate Sequence; D003505:Cyclodextrins / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D008969:Molecular Sequence Data; D013237:Stereoisomerism; D013729:Terpenes / Q000737:chemistry
D003505:Cyclodextrins; D007659:Ketones; D013729:Terpenes