An efficient access to protected disialylated glycohexaosyl threonine present on the leukosialin of activated T-lymphocytes Original Research Article

The total synthesis of the threonine-linked core 2 class disialylated hexasaccharide in a completely protected form was accomplished for the first time. The l-threonine conjugate, N-(9-fluorenylmethoxycarbonyl)-O-{(5-acetamido-4,7,8,9-tetra-O-benzyl-3,5-dideoxy-d-glycero-α-d-galacto-2-nonulopyranosylonic acid)-(2→3)-(2,6-di-O-benzyl-β-d-galactopyranosyl)-(1→4)-2-acetamido-2-deoxy-3,6-di-O-benzyl-β-d-glucopyranosyl-(1→6)-[(5-acetamido-4,7,8,9-tetra-O-benzyl-3,5-dideoxy-d-glycero-α-d-galacto-2-nonulopyranosylonic acid)-(2→3)-2,6-di-O-benzyl-β-d-galactopyranosyl-(1→3)]-2-acetamido-2-deoxy-α-d-galactopyranosyl-(1d→4c:1f→4e)-dilactone}-l-threonine allyl ester was synthesized via stereocontrolled glycosylations employing readily accessible monosaccharidic blocks; t-butyldiphenylsilyl-2-azido-2-deoxy-3,6-di-O-benzyl-β-d-glucopyranose, N-(9-fluorenylmethoxycarbonyl)-O-(2-azido-6-O-t-butyldimethylsilyl-2-deoxy-α-d-galactopyranosyl)-l-threonine allyl ester, 8, 9 and N-(9-fluorenylmethoxycarbonyl)-O-(2-azido-4,6-O-benzylidene-3-O-chloroacetyl-2-deoxy-α-d-galactopyranosyl)-l-threonine allyl ester. For the introduction of the amino acid, the azide group was used to temporarily mask the amino group of GalNAc so as to obtain an α-glycosidic linkage without participation from the C-2 substituent. The threonine was attached to the sugar unit at the monosaccharide stage to avoid loss of oligosaccharide at a later stage. The Fmoc and allyl ester protected amino acid at the reducing end facilitates efficient glycopeptide synthesis on solid-phase support.