Total Synthesis Of Core Oligosaccharide Antigens Of Helicobacter Pylori Lipopolysaccharide

Helicobacter pylori is a gram-negative pathogen that infects about half of the global population.Long-term carriage of the infection has risk of developing chronic gastritis,gastric ulcers,duodenal unlcers and even gastric cancer.H.pylori is estimated to be responsible for5.5% of all cancer cases and more than 60% of gastric cancers.In 1994,Word Health Organization(WHO)classified H.pylori as definite I human carcinogen.The eradication rate has fallen down by treatment with proton pump inhibitor and antibiotics due to the increased in antimicrobial resistance of H.pylori.It is of great significance to develop vaccines for H.pylori treatment based on specific H.pylori carbohydrate antigens.The chemical synthetic pure and well-defined carbohydrate antigens of H.pylori are required for identifing the immunogenic epitopes and developing carbohydrate-based vaccines.The core oligosaccharides of H.pylori lipopolysaccharide and the diphosphate heptose intermediate were chosen as the synthetic targets.The synthetic experience of core oligosaccharides of H.pylori lipopolysaccharide would provide important references for synthesis of other complex glycans.1.Chemical synthesis of core undecasaccharide of H.pylori lipopolysaccharide and its fragments.Based on lipopolysaccharide core oligosaccharide of H.pylori,core undecasaccharide,outer core octasaccharide,outer core pentasaccharide,α-(1→6)-triglucoside,inner core trisaccharide and phosphorylated inner core trisaccharide equipped aminopentyl linker were designed.D,DHeptoside building blocks were synthesized via key Swern oxidation,Wittig olefination and dihydroxylation reactions from D-mannose.The D,D-heptoside converted to L,D-heptoside efficiently via Mitsunobu reaction.The 3-deoxy-oct-2-ulopyranosidsonate(Kdo)building block was prepared via key elimination and addition reactions from D-arabinose.A synergistic glycosylation strategy was developed for selectively constructing α-glucosidic bonds by taking advantage of acyl remote participation group,solvent effect and temperature effect.The anomeric ethylthio group of galactoside acceptor was replaced with O-tertbutyldimethylsilyl(OTBS)group to avoid intermolecular migration during glycosylation,significantly improving the glycosylation yield.The steric hidrince during assembly of outer core octasaccharide and core undecasaccharide was overcome by fine-tuned assembly strategies.The inner core trisacchairde at the reduced end of undecasaccharide reduced the reactivity of primary hydroxyl group significantly of heptoside.The phosphate group installed onto L,D-heptoside building block at the first-stage of phosphorylated inner trisaccharide assembly and was stable under glycosylation and protecting-deprotecting conditions.Eighteen different protecting groups were used in a systematic protection strategy and nineteen different building blocks were prepared for synthesis of conserved core undecasaccharide and outer core octasaccharide.Outer core pentasaccharide,α-(1→6)-trisglucoside,inner core trisaccharide and phosphorylated inner core trisaccharide were also synthesized to construct the fragments library of core oligosaccharide.2.Chemical synthesis of 1β,7-biphosphate-3-O-aminamyl-D,D-heptose and its derivativeBased on a unique anomeric β-phosphate of biphosphate-heptose H.pylori antigen,1β,7-biphosphate-D,D-heptose with O3-aminopentyl linker and its monophosphate derivative 7-phosphate-α-O-aminopentyl-D,D-heptose were designed.The aminopentyl linker was selectively installed at O3 position of heptoside building block under dibutyltin oxide and tetrabutylamine iodide(TBAI)conditions and the yield was improved from 41% to 95% by replacing tert-butyldiphenylsilyl(TBDPS)group with small naphthylmethylene(Nap)group at C7 position of heptoside.The anomeric β-phosphate of heptoside was obteined by NISmediated phosphorylation and the β ratio was improved from 14% to 29% by lowering the reaction temperature.The 1β,7-biphosphate-3-O-aminamyl-D,D-heptose and 7-phosphate-α-O-aminamyl-D,D-heptose were prepared from simple D-mannose via 20 steps and 19 steps respectively.