Strong influence of the trifluoromethyl group on the chemoselectivity of [3+2]-cycloadditions of thiocarbonyl S-methanides with α,β-unsaturated ketones

Abstract

The in situ-generated reactive thiocarbonyl S-methanides were reacted with fluorinated enones. The type of the obtained [3+2]-cycloadduct depends strongly on the location of the activating CF3 group. In the case of enones containing the CF3CH=CH moiety, the [3+2]- cycloaddition occurs chemo- and regioselectively onto the C=C bond to give trifluoromethylated tetrahydrothiophene derivatives. On the other hand, enones containing the CF3–C=O unit react as carbonyl dipolarophiles leading to trifluoromethylated 1,3- oxathiolanes also in a chemo- and regioselective manner. These are the first reported reactions of thiocarbonyl S-methanides with α,β-unsaturated ketones.