Synthesis of blood-group substances. Part 8. A synthesis of the branched trisaccharide 2-acetamido-2-deoxy-4-O-(α-L-fucopyranosyl)-3-O-(β-D-galactopyranosyl)-D-glucopyranose
“…While 1 H NMR at 600 MHz showed no signal corresponding to the 2-NH expected in the wanted trisaccharide, an exchangeable signal coupling to H-4 of the glucosamine unit was identified at 3.90 ppm supporting the presence of the free 4-OH group. In accordance with the few examples of N -methyl glycosyl acetimidates reported by Sinaÿ et al a signal assigned to the rhamnosyl H-1‘ ‘ was found at 6.08 ppm. In addition, an HMBC experiment showed a long-range correlation between this signal and a quaternary carbon at 162 ppm, which was assigned to the imidate carbon CN.…”
supporting
confidence: 90%
“…To assess its cross reactivity with Le a , we have embarked on the preparation of analogue 2 in which the fucosyl residue is replaced by a rhamnosyl unit. Thus, our synthetic scheme involved the rhamnosylation of the known 4 disaccharide 4 which, along with other similar glycoside derivatives, were successfully fucosylated to prepare Le a trisaccharide analogues. − 1 Blood group antigens and analogue: Le a ( 1 ), Le a analogue ( 2 ), and Le a Le x ( 3 ).…”
[structure: see text] Rhamnosylation in mild conditions of a disaccharide containing N-acetylglucosamine afforded the imidate 6 while at higher temperature and concentration of promoter trisaccharide 7 was isolated. The kinetic imidate 6 was independently rearranged in 50% yield to the thermodynamic trisaccharide 7. Comparative NMR studies of 7 in CDCl(3) and DMSO-d(6) suggest the formation of a nonchair conformation in CDCl(3). The structure of 7 was confirmed through the independent synthesis of the N-acetylacetamido trisaccharide 11.
“…While 1 H NMR at 600 MHz showed no signal corresponding to the 2-NH expected in the wanted trisaccharide, an exchangeable signal coupling to H-4 of the glucosamine unit was identified at 3.90 ppm supporting the presence of the free 4-OH group. In accordance with the few examples of N -methyl glycosyl acetimidates reported by Sinaÿ et al a signal assigned to the rhamnosyl H-1‘ ‘ was found at 6.08 ppm. In addition, an HMBC experiment showed a long-range correlation between this signal and a quaternary carbon at 162 ppm, which was assigned to the imidate carbon CN.…”
supporting
confidence: 90%
“…To assess its cross reactivity with Le a , we have embarked on the preparation of analogue 2 in which the fucosyl residue is replaced by a rhamnosyl unit. Thus, our synthetic scheme involved the rhamnosylation of the known 4 disaccharide 4 which, along with other similar glycoside derivatives, were successfully fucosylated to prepare Le a trisaccharide analogues. − 1 Blood group antigens and analogue: Le a ( 1 ), Le a analogue ( 2 ), and Le a Le x ( 3 ).…”
[structure: see text] Rhamnosylation in mild conditions of a disaccharide containing N-acetylglucosamine afforded the imidate 6 while at higher temperature and concentration of promoter trisaccharide 7 was isolated. The kinetic imidate 6 was independently rearranged in 50% yield to the thermodynamic trisaccharide 7. Comparative NMR studies of 7 in CDCl(3) and DMSO-d(6) suggest the formation of a nonchair conformation in CDCl(3). The structure of 7 was confirmed through the independent synthesis of the N-acetylacetamido trisaccharide 11.
Oligosaccharide synthesis has gained major attention in the last two decades particularly in the total synthesis of biologically interesting molecules. Carbohydrates contain many stereocenters and are often branched oligomers. The preparation of these complex methods is time consuming and mostly carried out by specialized laboratories. The synthesis of oligosaccharides is challenging because every glycosylation creates a new stereogenic center. Each carbohydrate unit has up to four sites for chain elongation, which exhibit similar reactivity. The strategic choice of protection for the hydroxyl and amino groups is essential
The use of polymeric support can simplify the synthesis. Glycosylation efficiency can be increased using one coupling partner (either the glycoside nucleophile or glycoslyating agent) or other reagent in excess. Use of polymeric supports results in shortened process times since intermediate purification steps are not required. This chapter deals with the choice of carbohydrate monomers or dimers including the protecting groups and an anomeric leaving group, the choice of polymer support, and the selection of a proper linker resistant to all relevant reaction conditions. The choice of carbohydrate building blocks requires an appropriate protecting group strategy. It is important to consider whether the carbohydrate is connected to the support via the anomeric position or another hydroxyl group.
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