Asymmetric epoxidation of various olefins catalyzed by fructose- and glucose-derived ketones
Date
2009
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Abstract
Numerous laboratories have studied dioxirane- and oxaziridinium-mediated epoxidations during the past two decades. Many chiral ketone and iminium salt catalysts, bearing a wide variety of structural features, have been investigated. Out of the systems studied a fructose-derived ketone has been proven to be one of the most general and practical catalysts. This catalyst epoxidizes trans- and trisubstituted olefins in good yield and enantioselectivity, and it has been employed in the syntheses of many complex molecules. In efforts to expand the substrate scope a series of glucose-derived, oxazinolidine-bearing ketones were reported to be excellent catalysts for the epoxidation of conjugated cis-olefins. The stereodifferentiation in the epoxidation transition state originates from the attraction between the N-substituent of the oxazolidinone and the Rπ substituent on the olefin. The existence of this interaction was supported by the observation that 6-substituted chromenes were epoxidized with higher enantioselectivities than 8-substituted chromenes. Using this glucose-derived ketone system, substituted chiral styrene oxides could be obtained in 80-92% ee. Fluoroolefins were investigated as epoxidation substrates with several fructose- and glucose-derived ketone catalysts. A fluorine substituent was found to improve enantioselectivity in some cases but was detrimental to enantioselectivity in others. The substrate scope of a diacetate-containing ketone was expanded. High enantioselectivities were obtained for the epoxidation of trans- and trisubstituted olefins, and cis-olefins bearing a bulky substituent. The optical rotations of the resulting cis-epoxides were opposite to those obtained using glucose-derived ketones. 1,1-Disubstituted terminal olefins were epoxidized in good enantioselectivities with a glucose-derived morpholinone ketone. From the absolute configuration of the resulting epoxides, the major transition state appears to be a planar-like transition state. Also studied was a glucose-derived dimethylmorpholinone ketone that has the combined features of several of the previously studied ketones. This catalyst epoxidizes trans- and trisubstituted olefins in high enantoselectivities, but compared to the oxazolidinone-containing ketones gives slightly lower enantioselectivities with cis- and 1,1-disubstituted olefin substrates. Lastly, the epoxidation transition state model was studied using 18O-labeled ketone catalysts, and the results support the currently accepted transition state model.
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Subject
dioxirane
epoxidation
fructose
glucose
ketones
olefins
organic chemistry