Synthesis of benzyl 2-acetamido-3,6-di-O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-2-deoxy-α-d-galactopyranoside

Glycosylation of benzyl 2-acetamido-2-deoxy-3-O-(3,4-6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-α-d-galactopyranoside with 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl bromide in dichloromethane, in the presence of silver trifluoromethanesulfonate and 2,4,6-trimethylpyridine, afforded benzyl 2-acetamido-2-deoxy-3,6-di-O-(3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-α-d-galactopyranoside (5). Deacylation of 5 with hot hydrazine hydrate in ethanol, followed by peracetylation, furnished benzyl 2-acetamido-3,6-di-O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-d-glucopyranosyl)-4-O-acetyl-2-deoxy-α-d-galactopyranoside (7). The ¹H-n.m.r. spectra of both 5 and 7 were in accord with the structures assigned. Condensation of benzyl 2-acetamido-3-O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-d-glucopyranosyl)-2-deoxy-α-d-galactopyranoside (r) with 2-methyl-(3,4,6-tri-O-acetyl-1,2-dideoxy-α-d-glucopyrano)-[2,1-d]-2-oxazoline, and acetylation of the resulting intermediate, without isolation, also afforded, after column-chromatographic purification, the trisaccharide peracetate 7. Compound 4 was obtained by deacetylation of its 4,6-O-benzylidene derivative in hot, aqueous acetic acid. O-Deacetylation of 7 in methanolic sodium methoxide furnished the title trisaccharide (8). The structure of 8 was confirmed by ¹³C-n.m.r. spectroscopy.