Conformational behavior of peracetylated ß-D-mannopyranosyl methanesulfonamide: implications for the mechanism of sulfonamidoglycosylation of carbohydrate derivatives.

The conformational behavior of 2,3,4,6-tetra-O-acetyl-ß-D-mannopyranosyl methanesulfonamide has been investigated from a combined theoretical and experimental point of view. The study of the conformational space of the glycosyl sulfonamide revealed that the ß anomer is thermodynamically more stable than the α one. This fact suggests that the synthesis reaction could take place mainly under thermodynamic control as the main experimental product is the ß-anomeric form of the sulfonamide. Several intramolecular hydrogen bonds were found in the stable conformers of the N-mannopyranosyl sulfonamide under study. A relationship was found to exist between them and the relative stability of the conformers. A detailed analysis of geometrical parameters shed light into the nature of the solid state structure of the novel 2,3,4,6-tetra-O-acetyl-ß-D-mannopyranosyl methanesulfonamide in terms of exo- and endo-anomeric effects and antiperiplanar relationships. NBO calculations confirmed those findings. Calculated (1)H and (13)C NMR chemical shifts support previous findings concerning configuration and conformation assignments of the title sulfonamide. Finally, an explanation of the stereochemical outcome of sulfonamidoglycosylations, was given in terms of exo- and endo-anomeric effects and steric factors.