Sulfonium fluoroborate, dimethyl catalyst

Allylstannanes are much better nucleophiles for dithioacetals than allylsilanes, and dimethyl(methyl-thio)sulfonium fluoroborate is a particularly good catalyst in this reaction. The reaction has been used tellingly in a macrocyclization (Scheme 56). ° Monothioacetals also react with allylstannanes, with cleavage of either the carbon-oxygen or the carbon-sulfur bond, depending upon the choice of Lewis acid (Scheme 57).Sugar thioacetals react in the same way (Scheme 58), but with the opposite anomeric stereoselectivity from that of the corresponding radical chain substitution. ... 

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