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Five trisaccharide derivatives designed for detailed exploration of the acceptor specificity of glycosyltransferases involved in termination of N-acetyllactosamine-type structures have been synthesized: beta-D-Gal p-(1--4)-beta-D-Glc pNAc-(1--2)-alpha-D-Man p-(1--0)(CH2)7CH3 (1), 4-deoxy-beta-D-Gal p-(1--4)-beta-D-Glc pNAc-(1--2)-alpha-D-Man p-(1--O)(CH2)7CH3 (2), 4-O-methyl-beta-D-Gal p-(1--4)-beta-D-Glc pNAc-(1--2)-alpha-D-Man p-(1--O)(CH2)7CH3 (3), 4-deoxy-4-fluoro-beta-D-Gal p-(1--4)-beta-D-Glc pNAc-(1--2)-alpha-D-Man p(1--O)(CH2)7CH3 (4), and beta-D-Glc p-(1--4)-beta-D-Glc pNAc-(1--2)-alpha-D-Man p-(1--O)(CH2)7CH3 (5). A general disaccharide acceptor octyl 3,4,6-tri-O-benzyl-2-O-(3,6-di-O-benzyl-2-deoxy-2-phthalimido-beta-D -glucopyranosyl)-alpha-D-mannopyranoside was synthesized by condensation of 4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-alpha, beta-D-glucopyranosyl trichloroacetimidate with octyl 3,4,6-tri-O-benzyl-alpha-D-mannopyranoside, followed by deacetylation. 2,3,4,6-Tetra-O-acetyl-alpha-D-galactopyranosyl trichloroacetimidate and 2,3,4,6-tetra-O-acetyl-alpha-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-alpha-D-xylo-hexopyranosyl trichloroacetimidate and ethyl 2,3,6-tri-O-acetyl-4-deoxy-1-thio-alpha, beta-D-xylo-hexopyranoside; for 3, ethyl 2,3,6-tri-O-acetyl-4-O-methyl-1-thio-alpha, beta-D-galactopyranoside; and for 4, 2,3,6-tri-O-acetyl-4-deoxy-4-fluoro-alpha-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. |
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