Epoxidations enole benzoates

Armstrong has developed a-fluoro-W-ethoxycarbonyltropinone 54 as an asymmetric catalyst for the enantio-selective epoxidation reaction 1-phenylcyclohexene oxide is produced in 97% yield and 69% ee (Scheme 20) <1998CC621>. Analogously, the use of a-acetoxy-8-oxabicyclo[3.2.1]octan-3-one 55 results in 71% yield and 98% ee (Scheme 20) <2001TA2779>. Tropinone catalyst 54 gives only low levels of enantioselectivity in the asymmetric epoxidation of dihydronaphthalene and an enol benzoate (Scheme 20) <2002JOC8610>. 

C in dichloromethane. Solvent effects have been observed. Thus treatment of enol ether (53) with MCPBA in ether resulted in isolation of the benzoate (54). This was considoed to arise as a result of the increased nucleophilicity of the residual carboxylic acid in ether over that in dichloromethane. Isolation of the silyloxy epoxide by an analogous ethereal oxidation suggests periugrs that the 1,4-silyl migration is intrinsically less facile in this solvent. Generally however the process is efficiem and simple substrates are readily oxygenated (Scheme 11). 

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