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

Synthesis of a core heptasaccharide asparagine N-4-(alpha-D-mannopyranosyl-(1 --> 6)-[(alpha-D-mannopyranosyl)-(1 --> 3)]-[(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-(1 --> 2)]-(beta-D-mannopyranosyl)-(1 --> 4)-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-(1 --> 4)-[(alpha-L-fucopyranosyl)-(1 --> 6)]-2-acetamido-2-deoxy-beta-D-glucopyranosyl)-L-asparagine (1a) found from CHO glycosylation mutant cell LEC 1 4 is described. The structure of la is highly novel in terms of the presence of an extra GlcNAc residue linked to the 2 position of beta-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-alpha-D-mannopyranoside was adopted for this particular purpose, in which afforded beta-mannoside p-methoxyphenyl 2,3-O-(p-methoxybenzylidene)-4,6-O-(1,1,3,3-tetraisopropyl)disiloxanylidene-beta-D-mannopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-phthalimido-beta-D-glucopyranoside stereoselectively in 75% yield. (C) 2000 Elsevier Science Ltd. All rights reserved.