Anomeric radicals glycosyl halides

With all the anomeric activating groups useful in the generation of sugar-derived free radicals, glycosyl halides have been the most exploited. In the remainder of this section, various reactions involving glycosyl halides will be discussed. Additionally, the stereochemical consequences of these reactions will be analyzed. 

A convenient preparation of 2-deoxy sugars [204] from glycosyl halides is based on the rapid and irreversible migration of an acyloxy group from C-2 to the anomeric center upon generation of the glycosyl radicals. This is a thermodynamically controlled reaction because it restores a d-a-d arrangement. 

Bromine can be introduced directly into particular ring positions of certain sugar derivatives by free radical processes which can show high selectivity and efficiency. For example, peracety-lated glucose 90 is selectively brominated at the C-5 position by NBS yielding 91 however, in the case of glycosyl halides, such as tetra-O-acetyl- -D-glucopyranosyl chloride 92, anomeric substituted product 93 is mainly formed (O Scheme 47) [79,80]. 

Anomeric halides follow the typical reactivity order F < Cl < Br < I for nucleophilic substitutions. They have been used in stereoselective O-glycosylation, nucleophilic displacement, and carbanion as well as in radical reactions. 

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