Thioethers monothioacetals

Zn, (CH30)2CH2, BrCH2C02Et, 80-82% yield. Formation of the meth-oxymethyl thioether with dimethoxymethane avoids the use of the carcinogen chloromethyl methyl ether. The reaction forms an intermediate zinc thiolate, which then forms the monothioacetal. 

These results indicate that the order of reactivity toward acid is generally acetals > monothioacetals dithioacetals. The ease of removal of the latter two groups is determined by essentially the same factors that govern the removal of the thioethers namely, ease of protonation, the stability of the cation formed and the possibility of irreversible reactions which can shift the equilibria in favor of the cleavage products. On the basis of available data the S-monothioacetal derivatives of cysteine appear to react somewhat more cleanly than the S-trityl thioethers but are of similar reactivity whereas the S-dithioacetal derivatives react more nearly like the S-benzyl or S-f-butyl thioethers. 

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