Cleavage of mercaptals

Mercuric chloride mercuric oxide Cleavage of mercaptals... 

Addition of methyllithium to the lactone 1219, followed by reduction with sodium borohydride in refluxing ethanol, afforded, almost quantitatively, ellipticine (228). Reaction of the compound 1219 with the lithio derivative of formaldehyde diethylmercaptal, and reduction with sodium borohydride in refluxing ethanol, led to the mercaptal 1221. Cleavage of the mercaptal 1221 with bis(trifluoroacetoxy) iodobenzene [Phl(OCOCF3)2] in aqueous acetonitrile gave the 11-formyl derivative, which was reduced with sodium cyanoborohydride (NaBHsCN) to 12-hydroxyellipticine (232) (710,711) (Scheme 5.202). The same group also reported the synthesis of further pyiido[4,3-fc]carbazole derivatives by condensation of 2-substituted indoles with 3-acetylpyridine (712). 

Cleavage of streptomycin with ethyl mercaptan and hydrogen chloride yielded streptidine and a new derivative of streptobiosamine, ethyl thio-streptobiosaminide hydrochloride diethyl mercaptal. Treatment of methyl streptobiosaminide hydrochloride dimethyl acetal with ethyl mercaptan and hydrogen chloride readily yielded the same mercaptal hydrochloride. It thus was apparent that the relationship between these two compounds resided simply in the replacement of methoxy by thio-ethoxy groups on the same carbon skeleton. Acetylation of the mercaptal yielded two anomeric forms of ethyl tetraacetylthiostreptobiosaminide diethyl mercaptal. ... 

Photosensitized oxygenation of a-ketoketene mercaptals has been described and these give dioxetanols, which subsequently collapse to carboxylic acids. Photo-oxidation of the mesoionic compounds (63) and (64) gives ring-cleavage products via endoperoxides and irradiation of tetrathiatetracene and of its selenium analogue in halocarbon solvents leads to radical cation salts. 

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