Thioacetals 1-thioglycosides

In- the preceding section the reductive desulfurization of some substances which are both thioglycosides and thioacetals has been mentioned here the hydrogenolysis of the simple 1-thioglycosides will be discussed. 

Little preparative importance attaches to conversion of organic sulfides into chloro compounds The fission of compounds R-S-R by chlorine, bromine, cyanogen bromide, and methyl iodide, with replacement of C-S by C-halogen bonds has been reviewed by Tarbell and Harnish.1287 The action of 1 mole of bromine on thioacetals in an anhydrous medium (ether) at room temperature affords bromo hemiacetals (a-bromo sulfides) and a sulfenyl bromide, and bromo sugars analogously from ethyl thioglycosides. 

In some cases the addition of electrophilic alkenes, such as 4-allyl-1,2-dimethoxybenzene, to the reaction mixture can improve the outcome of the reaction, particularly when run in the presence of thioglycosides, by scavenging a by-product of the reaction, phenylsulfenyl tritlate. Non-carbohydrate thioacetal sulfoxides also undergo similar acetalation reactions when treated with Tf20 (eq71).i ... 

Also, thioacetic acid can be used as nucleophile in this approach, to give anomeric thioacetates, which, in a two step-procedure, can be selectively S-deacylated and alkylated to give thioglycosides, a sequence often used in the synthesis of thio-oligosaccharides [25]. 

Indirect formation of thioglycosides can also be performed from glycosyl halides. Reaction with thiourea yields a pseudothiouronium salt, which (as the thioacetate above) can be hydrolyzed under mild, selective conditions to give the anomeric thiol, which then can be alkylated in an efficient, non-smelling procedure to produce... 

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