Carbon-oxygen bonds glycosylation

In a different approach to the preparation of glycosides, the nucleophilic substitution takes place at the carbon atom of the aglycone rather than at the anomeric carbon (Scheme 4.53). In contrast to the methods discussed in Section 4.3, there is no scission of the glycosyl-oxygen bond in these reactions. Instead, the R-X bond is cleaved. 

Carbohydrate derivatives, in which one or more of the oxygen atoms bonded directly to the carbon skeleton have been replaced by sulfur, are termed thiosugars. The placement of the sulfur atom at the anomeric position constitutes a special case, because thioglycosides, alkyl, aryl and heterocyclic, occupy a very important place as versatile glycosyl donors in glycosidation methodology. Anomeric thiocarbonyl compounds, on the contrary, have been less explored, although their potential and scope is likely to be similar. 

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