Dithioacetals demercaptalation

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. 

Benzyl 1-thio-lactoside and ethyl lactoside, prepared from lactose dibenzyl dithioacetal116 by the preceding methods, are necessarily pyranosides (owing to the substitution on 0-4). Stevens and coworkers310 prepared ethyl 2,3 4,5-di-0-isopropylidene-l-thio-/3-D-glucoseptanoside by partial demercaptalation of 2,3 4,5-di-O-isopropylidene-D-glucose diethyl dithioacetal. 

The pentoses may be converted by the conventional sequence (5) of dithioacetal formation, acetylation, and demercaptalation into the respective aldehydo-oentose 2,3,4,5-tetraacetates, which afford by Wittig chain-extension (2) the desired dienophiles. Thus aldehydo-D-arabinose 2,3,4,5-tetraacetate reacted with Ph3PCHC02Me in boiling benzene to give 90% of the pure, crystalline E-unsaturated sugar derivative 6, and the corresponding enantiomer was likewise prepared from L-arabinose. 

Ethyl 2-amino-2-deoxy-l-thio-a- and -jS-o-arabinopyranoside have been obtained by direct ethanethiolation of 2-amino-2-deoxy-D-arabinose, whereas ethyl 2-amino-2-deoxy-l-thio-a- and -jS-D-arabinofuranoside were prepared by partial demercaptalation of a 2-amino-2-deoxy-D-arabinose diethyl dithioacetal derivative with either mercuric chloride or bromine (see Scheme 10). Demer-... 

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