Synthesis of Hex p-(1 → 4)-β-d-Glc pNAc-(1 → 2)-α-d-Man p-(1 → O)(CH2)7CH3 probes for exploration of the substrate specificity of glycosyltransferases: Part I, Hex = β-d-Gal, 4-deoxy-β-d-Gal, 4-O-methyl-β-d-Gal, 4-deoxy-4-fluoro-β-d-Gal, or β-d-Glc

Five trisaccharide derivatives designed for detailed exploration of the acceptor specificity of glycosyltransferases involved in termination of N-acetyllactosamine-type structures have been synthesized: β-d-Gal p-(1 → 4)-β-d-Glc pNAc-(1 → 2)-α-d-Man p-(1 → O)(CH2)7CH3 (1), 4-deoxy-β-d-Gal p-(1 → 4)-β-d-Glc pNAc-(1 → 2)-α-d-Man p-(1 → O)(CH2)7CH3 (2), 4-O-methyl-β-d-Gal p-(1 → 4)-β-d-Glc pNAc-(1 → 2)-α-d-Man p-(1 → O)(CH2)7CH3 (3), 4-deoxy-4-fluoro-β-d-Gal p-(1 → 4)-β-d-Glc pNAc-(1 → 2)-α-d-Man p-(1 → O)(CH2)7CH3 (4), and β-d-Glc p(1 → 4)-β-d-Glc pNAc-(1 → 2)-α-d-Mann p-(1 → O)(CH2)7CH3 (5). A general disaccharide acceptor octyl was synthesized by condensation of 4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-α,β-dglucopyranosyl trichloroacetimidate with octyl 3,4,6-tri-O-benzyl-α-d-mannopyranoside, followed by deacetylation. 2,3,4,6-Tetra-O-acetyl-α-d-galactopyranosyl trichloroacetimidate and 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl trichloroacetimidate were used as the glycosyl donors in the syntheses of 1 and 5. The modified galactosyl derivatives required subtle anomeric activation. Suitable donors for 2 turned out to be 2,3,6-tri-O-acetyl-4-deoxy-α-d-xylo-hexopyranosyl trichloroacetimidate and ethyl 2,3,6-tri-O-acetyl-4-deoxy-1-thio-α,β-d-xylo-hexopyranoside; for 3, ethyl 2,3,6-tri-O-acetyl-4-O-methyl-1-thio-α,β-d-galactopyranoside; and for 4, 2,3,6-tri-O-acetyl-4-deoxy-4-fluoro-α-d-galactopyranosyl trichloroacetimidate. It was concluded that thioglycosides were most appropriate for stereoselective coupling of activated synthons (carrying deoxy or O-methyl groups), whereas trichloroacetimidates gave high yields with deactivated (fluorine-containing) synthons.