Thioglycosides from glycosyl halides

Due to the interest in thioglycosides, the methods of their synthesis from glycosyl halides with thio-lates or by S-alkylation of 1-thioaldoses have been complemented by new methods. ... 

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... 

IV. In situ Generation of Glycosyl Halides from Thioglycosides. 184... 

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

This method works well for the preparation of methyl D-arabinofuranosides, and an excellent preparation is available [68] that can be used to prepare multigram quantities of the product (Fig. 8). Treatment of D-arabinose (23) with acidic methanol at room temperature provides a 1 1 a (3 mixture of methyl arabinofuranosides (24). Benzoylation provides 25, again as a mixture of anomers however, the a isomer can be selectively crystallized from ethanol. With either 24 or 25 in hand, it is possible to prepare, in 1-3 steps, the glycosyl donors shown in Figure 9. These include glycosyl acetates 26 [69] and 27 [70,71], glycosyl halides 28 [68,72], 29 [49], 30 [73], 31 [73], and thioglycosides 32-34 [55,74-76]. 

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