Dithioacetals carbon-sulfur bond cleavage

Photochemical, carbon-sulfur bond cleavage is also observed in compounds containing sulfur in oxidation states higher than that which exists in sulfides and in dialkyl dithioacetals. For example, the irradiation of the sulfoxide 47 in methanol produces109 a 58% yield of galacti-tol (52). Even though homolysis of the carbon-sulfur bond does occur in 47, it is unlikely that 52 results from a simple, carbon-sulfur bond-cleavage, as such a reaction predicts products that were not observed... 

Allylsilanes react with alkoxymethyl and phenylthiomethyl chlorides in the presence of Lewis acids (Scheme 53), and dithioacetals, similarly, where substantial stereoselection can be achieved with large aryl groups on the sulfur atoms (Scheme 54). Monothioacetals react selectively with cleavage of the carbon-sulfur bond when tin(IV) chloride is used as the Lewis acid (Scheme 55). ° ... 

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. ... 

The dithioacetal group is readily converted to the disulfide bridge by oxidation to sulfoxide, followed by treatment with acid to effect the carbon-sulfur bond fission. The p-methoxybenzene group is essential for this cleavage and facilitates resonance stabilization of the incipient carbonium ion. 

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