Synthesis of a tri-, a penta-, and a hepta-saccharide containing terminal N-acetyl-β-D-lactosaminyl residues, part of the ‘complex-type’ carbohydrate moiety of glycoproteins
Silver trifluoromethanesulphonate-promoted condensation of 3,6-di-O-(2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-2-deoxy-2-phthalimido-β-D-glucopyranosyl bromide with benzyl 3-O-benzyl-4,6-O-benzyl-idene-α-D-mannopyranoside, benzyl 3,6-di-O-benzyl-α-D-mannopyranoside, and benzyl 3,6-di-O-(3,4,6-tri-O-benzyl-α-D-mannopyranosyl)-2,4-di-O-benzyl-α-D-mannopyranoside gave the protected tri-, penta-, and heptasaccharide derivatives in 66, 72, and 56% yields, respectively. The free oligosaccharides were obtained after exchanging of the 2-deoxy-2-phthalimido-groups for 2-acetamido-2-deoxy-groups and deblocking.