Synthesis of a novel asparagine-linked heptasaccharide structure via p-methoxybenzyl-assisted β-mannosylation Original Research Article

Synthesis of a core heptasaccharide asparagine N4-{α-d-mannopyranosyl-(1→6)-[(α-d-mannopyranosyl)-(1→3)]-[(2-acetamido-2-deoxy-β-d-glucopyranosyl)-(1→2)]-(β-d-mannopyranosyl)-(1→4)-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-(1→4)-[(α-l-fucopyranosyl)-(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl}-l-asparagine (1a) found from CHO glycosylation mutant cell LEC 14 is described. The structure of 1a is highly novel in terms of the presence of an extra GlcNAc residue linked to the 2-position of β-linked mannose. The synthesis was performed using p-methoxybenzyl-assisted intramolecular aglycon delivery as the key transformation. 4,6-O-TIDPS-protected thiomannoside methyl 2-O-p-methoxybenzyl-4,6-O-(1,1,3,3-tetraisopropyl)disiloxanylidene-3-O-trimethylsilyl-1-thio-α-d-mannopyranoside was adopted for this particular purpose, which afforded β-mannoside p-methoxyphenyl 2,3-O-(p-methoxybenzylidene)-4,6-O-(1,1,3,3-tetraisopropyl)disiloxanylidene-β-d-mannopyranosyl-(1→4)-3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-d-glucopyranoside stereoselectively in 75% yield.