This article describes a highly effective catalytic asymmetric epoxidation method for olefins using potassium peroxomonosulfate (Oxone, Dupont) as oxidant and a fructose-derived ketone (1) as catalyst. High enantioselectivies have been obtained for trans-disubstituted and trisubstituted olefins which can bear functional groups such as tributylsilyl ether, acetal, chloride, and ester. The enantiomeric excesses for cis-olefins and terminal olefins are not high yet. The current epoxidation shows that the catalyst efficiency is enhanced dramatically upon raising the pH. Mechanistic studies show that the epoxidation mainly proceeds via a spiro transition state, which provides a model for predicting the stereochemical outcome of the reaction. The planar transition state is likely to be the main competing pathway. The extent of the involvement of the planar mode is subject to the steric effect of the alkyl groups on the olefins.

Epoxides are very important chiral building blocks for the synthesis of enantiomerically pure complex molecules. 1 Asymmetric epoxidation of olefins presents a powerful strategy for the synthesis of enantiomerically enriched epoxides. Great success has been achieved in the epoxidation of allylic alcohols 2 and unfuctionalized cis-olefins. 3 However, the epoxidation of trans-olefins bearing no allylic alcohol group with high enantiomeric excess still remains a challenging problem. 4 It was desirable to explore alternative systems for a solution. Among many other powerful methods for the epoxidation of olefins, 5 dioxiranes are remarkably versatile oxidation reagents, and their use as epoxidation reagents has risen to particular prominence. 6,7 The reaction is rapid and requires a simple workup. An important feature associated with dioxiranes is that they can be generated in situ from Oxone (potassium peroxomonosulfate) and a ketone, 8 which provides opportunities for asymmetric epoxidation when a chiral ketone is used.However, progress in the area of dioxirane-mediated asymmetric epoxidation has been limited. 9 The enantiomeric excess (ee) has been low (9-20%). Since dioxiranes have two reacting sites, it is crucial to limit possible competing approaches. Recently, some progress has been made in this regard. Yang reported an intriguing C 2 symmetric cyclic chiral ketone for asymmetric epoxidation. 10 An 87% ee was obtained in one case, although the ee values for most cases were low (5-50%).Herein we wish to report our efforts in the area of asymmetric epoxidation. We are utilizing ketones containing the following general features: (1) the stereogenic centers are close to the reacting center, resulting in efficient stereochemical communication between substrates and the catalyst; (2) the presence of a fused ring and a quaternary center R to the carbonyl group minimizes the epimerization of the stereogenic centers; (3) one face of the catalyst is sterically blocked to limit the possible competing approaches. Ketone 3 has these desirable structural features, and is readily prepared from very inexpensive Dfructose ($15/kg) by ketalization (acetone, HClO 4 , 0°C, 53%) and oxidation (PCC, rt, 93%). 11Initial studies involving ketone 3 in the epoxidation of transstilbene revealed that while the yield of stilbene epoxide increased with the reaction time, the enantiomeric excess decreased. Upon examination, we determined that ketone 3(1) For a review, see: Besse, P.; Veschambre, H. Tetrahedron 1994, 50, 8885-8927. (2) For a review, see: Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 4.1.(3) For leading references, see: (a) Collman, J. P.; Zhang, X.; Lee, V. b) References 7b and c and references cited therein. (9) (a) Curci, R.; Fiorentino, M.; Serio, M. R. J. Chem. Soc., Chem. Commun. 1984, 155-156. (b) Curci, R.; D'Accolti, L.; Fiorentino, M.; Rosa, A.Table 1. Asymmetric Epoxidation of Representative Olefins Catalyzed by Ketone 3 a aAll reactions wer...

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Chiral ketones have been shown to be effective organocatalysts for asymmetric epoxidation of olefins with broad substrate scope. High enantioselectivity has been obtained for a wide variety of trans and trisubstituted olefins, as well as a number of cis olefins, with encouragingly high ee's for some terminal olefins. The stereochemical outcome of the reaction can be rationalized by a spiro transition state model.

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