Reactions of the Dithioacetal Group

Replacement of Alkylthio Groups by Action of Mineral and Lewis Acids 

The course of reactions with mercury(II) chloride is subject to control by experimental conditions, and by substitutive and stereochemical constraints within the particular dithioacetal undergoing hydrolysis. Much of the synthetic versatility of dithioacetals derives from this control, and various aspects have been reviewed in detail in this Series6-8 and elsewhere.261 

At neutral pH, hydrolysis is extremely slow, but the reaction proceeds to completion with minimal interference from side reactions as an increase in the acidity of the medium not only accelerates the rate of hydrolysis but also introduces competing reactions and displaces the equilibrium towards the dithioacetal, the optimal conditions for the demercaptalation reaction are a compromise176 between the rate and the yield of the reaction. The identity of the thiol also influences the rate of mercury(II) chloride-catalyzed hydrolysis, diphenyl dithioacetals reacting30 very readily and ethylene dithioacetals slowly188,235 or not at all,72 the reverse of their relative rates of formation. 

The aspects of synthetic utility are reversed between aldehydo  

D-furanoside (after acetylation, 144, 32%) and the free sugar (142, 24%) are formed in almost equal amounts, minor components being the starting material (12%), the l-thio-/3-D-pyranoside 143 (4%), and the 1-thio-jS-D-furanoside (after acetylation, 145, 3%). 

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