Glycosyl halides substrates

Glycosyl halides, a very important group of carbohydrate derivatives, are commonly prepared14 from per-O-acylated sugars by reaction with hydrogen halides or halides of aluminum or titanium. The selection of the method depends mainly on the anomeric configuration of the substrate, the kind of its O-acyl groups, and the stability of the product to be prepared. 

In the second example of the utility of glycosyl halides as substrates for the formation of C-arylglycosides, Allevi, et al.,89 demonstrated conditions utilizing silver triflate as the catalyst. This study, summarized in Scheme 2.4.13, proceeded in only 40% yield with total p selectivity. Thus, the developing trend, compared to that in the formation of C-allylglycosides is that C-arylglycosidations have a tendency to allow the formation of product mixtures highly enriched with p anomers. 

In addition to the use of substrates similar to those shown in Figure 5.1.1, glycosyl halides have found much utility in the chemistry of glycosidic radicals. 

The syntheses of a,a-thiotrehalose (203) and a-o-glucopyranosyl 1-thio-a-D-mannopyranose (204) as substrate analogs for trehalase were achieved more than twenty years ago [14]. This study was the first to note that the condensation of a thiolate or a glycosyl halide proceeds in HMPA without any neighboring group participation (Scheme 40). 

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