Recent Development in Self-Promoted N-Glycosylation

Anomeric sets of glycosyl trichloroacetimidates have been used as glycosyl donors together with carbamate protected sulfonamides for the synthesis of protected N-glycosides. The reaction is self-promoted and hence no additives or catalysts are added. 1,2-Trans axial trichloroacetimidates are stereo-specifically substituted, whereas the selectivity drops when having equatorial trichloroacetimidates. NMR studies suggest that the decrease in stereo-specificity is partly due to an anomerisation of the trichloroacetimidiate. Mechanistic studies based on NMR furthermore suggest a SN1 type reaction. The N-carbamate and the N-sulfonyl groups are furthermore shown to be orthogonally cleaved allowing further functionalization of the N-glycosides.