SYNTHESES OF LINEAR TETRASACCHARIDE, HEXASACCHARIDE, AND OCTA-SACCHARIDE FRAGMENTS OF THE I-BLOOD GROUP ACTIVE POLY-(N-ACETYL-LACTOSAMINE) SERIES - BLOCKWISE METHODS FOR THE SYNTHESIS OF REPETITIVE OLIGOSACCHARIDE SEQUENCES

N-Phthaloylation of lactosamine gave various glycosyl donors (β-chloride, β-trichloroacetimidate) and glycosyl acceptors (3′,4′-diol). Coupling of the chloride with a methyl β-d-glycoside led to the tetrasaccharide fragment, β-d-Galp-(1→4)-β-d-GlcpNac-(1→3)-β-d-Galp-(1→4)-β-d-GlcpNAcOMe. Acetolysis of the protected tetrasaccharide, followed by treatment with hydrogen chloride, gave a tetrasaccharide chloride which was coupled with the methyl β-glycoside of lactosamine. A hexasaccharide fragment, [β-d-Galp-(1→4)-β-d-GlcpNAc-(1→3)]2-β-d-Galp-(1→4)-β-d-GlcpNAcOMe, was thus obtained by this ("n + 1") method. A more efficient ("n + n") method was applied for the synthesis of an octasaccharide fragment, [β-d-Galp-(1→4)-β-d-GlcpNAc-(1→3)]3-β-d-Galp-(1→4)-β-d-GlcpNAcOMe (38), where di- and tetra-saccharide intermediates having a 3,4-O-isopropylidene-β-d-galactopyranosyl nonreducing terminal group and a benzyl β-d-glycoside group were precursors, either as glycosyl donors (β-trichloroacetimidates) or glycosyl acceptors (3,4-diols as nonreducing terminal groups). Thus, doubling the length of the repetitive oligosaccharide sequence could be efficiently accomplished at each glycosylation step. © 1990.