Preparation of disaccharides having a beta-D-mannopyranosyl group from N-phthaloyllactosamine derivatives by double or triple SN2 substitution.

Condensation of 1,2,3,4,6-penta-O-acetyl-beta-D-galactopyranose with methyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-beta-D-glucopyranoside, followed by alkaline methanolysis, gave a derivative of lactosamine that has an unsubstituted beta-D-galactopyranosyl group. Tributyltin oxide-mediated allylation gave a good yield of the 3'-O-allyl (9) and a poor yield of the 3',6'-di-O-allyl ether (8). Protection of 9 at O-6' was achieved by reductive opening of the 4',6'-O-anisylidene derivative, to give the 6'-O-(4-methoxybenzyl) ether 15. Conversion of 8 and 15 to their 2',4'-bis(trifluoromethanesulfonates), followed by SN2 reaction with benzoate, gave the corresponding beta-D-mannopyranosyl disaccharides. However, the model methyl 3-O-allyl-beta-D-galactopyranoside and 9 were converted into beta-D-mannopyranosyl derivatives in better yield (52-55%) by a one-pot, triple SN2 substitution of the tris(trifluoromethanesulfonates).