Alkenes, epoxidation with dioxiranes

Trifluoromethyl ketones that are frequently used for mediating alkene epoxidation with dioxiranes are subject to degradation. A stabilized form is made by covalent anchoring to silica. ... 

Enantioselective epoxidation of unfunctionalized alkenes was until recently limited to certain ds-alkenes, but most types of alkenes can now be successfully epoxi-dized with sugar-derived dioxiranes (see Section 9.1.1.1) [2]. Selective monoepox-idation of dienes has thus become a fast route to vinylepoxides. Functionalized dienes, such as dienones, can be epoxidized with excellent enantioselectivities (see Section 9.1.2). 

The dioxirane epoxidation of a prochrral alkene will produce an epoxide with either one new chirality center for terminal alkenes, or two for internal aUcenes. When an optically active dioxirane is nsed as the oxidant, expectedly, prochiral alkenes should be epoxi-dized asymmetrically. This attractive idea for preparative purposes was initially explored by Curci and coworkers in the very beginning of dioxirane chemistry. The optically active chiral ketones 1 and 2 were employed as the dioxirane precursors, but quite disappointing enantioselectivities were obtained. Subsequently, the glucose-derived ketone 3 was used, but unfortunately, this oxidatively labile dioxirane precursor was quickly consumed without any conversion of the aUcene . After a long pause (11 years) of activity in this challenging area, the Curci group reported work on the much more reactive ketone... 

Dimethyldioxirane oxidizes acyclic vinylsilanes at room temperature to the corresponding epoxides 166 in excellent yield (equation 141)255. Allylic oxidation is found in appreciable amounts when cyclic vinylsilanes are used. It is interesting to note that simple alkenes react faster with dioxirane than vinylsilanes. The trend appears to be reversed when MCPBA is employed as the oxidant. 

Less regularly used reagents are tert-hu y hydroperoxide, tf/Y-butyllithium, ozone. dioxiranes, fluorine/water/acetonitrile, or A, A -diethylhydroxylamine. Alkenes carrying a donor substituent can also be epoxidized with pcracids. Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclcclivity. ... 

Epoxidation of alkene 216 with A-sulfonyloxaziridine 217 gave /ra r-epoxide 218 and m-epoxide 219 in 64% and 15% yield, respectively <1999TL8637>. Use of methyl(trifluoromethyl)dioxirane exhibited no selectivity. 

Enantioselective epoxidation of pro-chiral alkenes with dioxiranes generated from optically active ketones affords optically active epoxides with enantiomeric excesses (ee) in the 9-12.5% range <84CC155>. In the late 1980s values up to 24% have been observed when employing 4,4,4-trifluoro-3-phenyl-3-methoxybutan-2-one as the ketone <89ACR205>. 

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