Observations on the activation of methyl thioglycosides by iodine and its interhalogen compounds

Treatment of 'armed' methyl thiogalactosides with iodine in the absence of an acceptor alcohol results in thioglycoside epimerisation, whereas there i s no effect on the corresponding 'disarmed' methyl thioglycosides. In contr ast, iodine-hexamethyldisilane (which generates iodotrimethylsilane in situ ) brings about epimerisation of 'disarmed' thioglycosides, ultimately givin g rise to the corresponding cc-glycosyl iodides on extended exposure. Cross -over experiments show the former iodine-promoted epimerisation process to be intermolecular, whereas the latter iodine-hexamethyldisilane-promoted ep imerisation is intramolecular. Treatment of the same methyl thiogalactoside s with iodine monobromide gives rise to the thermodynamically favoured alph a-glycosyl bromides, whereas reaction with iodine monochloride initially gi ves the kinetic beta-glycosyl chlorides, which slowly epimerise to the ther modynamic alpha-linked products. Differences in the outcome of thioglycosid e activation by I-I, I-Br and I-CI suggest there may be scope for influenci ng the stereochemical course of thioglycoside-based glycosylation reactions through careful choice of promoter. (C) 2000 Elsevier Science Ltd. All rig hts reserved.