Glycosyl halides thioglycosides preparation

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... 

The application of diazo coupling is somewhat limited by the availability of the p-aminophenyl glycosides, particularly of those of the oligosaccharides. Their precursors, the p-nitrophenyl glycosides, are usually obtained by the condensation of a per-O-acetylated glycosyl halide with p-nitrophenol in ethanol (Michael reaction)19,20 or by the reaction of a per-O-acetylated sugar with p-nitrophenol in the presence of a Lewis acid catalyst (Helferich reaction).21 p-Nitrobenzyl 1-thioglycosides have also been prepared by the condensation of the... 

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]. 

A number of methods are available for preparing thioglycosides, those utilising per-(9-acetylated carbohydrates or acetylated glycosyl halides as the starting material being the most popular (for a summary of methods available see Table 4.1) [3,6,10]. 

Since Fisher and Delbriick s work, alkyl and aryl 1-thioglycosides of aldoses have been prepared by a large variety of methods peracylated glycosyl halides by reaction... 

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