Dithioacetals reaction with vinylsilanes

Trost has reported the use of dithioacetals as initiators in vinylsilane-terminated cyclizations (Scheme 11). Reaction of the dimethyl thioketal (26) with dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) led to the formation of the allylic sulfide (28) via a 2,3-sigmatropic rearrangement of the initially formed sulfonium salt (27a), which may have been catalyzed by complexation with excess DMTSF. 

Competition experiments between two ir-bonded nucleophiles within the same molecule were studied in an attempt to identify reaction parameters and factors responsible for regioselectivity. These experiments, summarized in heme 12, demonstrated that the dithioacetal (initiating carbocation) is in competition with two nucleophilic functional groups within the same molecule, a silyl enol ether and a vinylsilane. In this instance, when the thioacetal (29) was treated with DMTSF, complete chemoselectiv-... 

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