Total Synthesis Of Serotype O2 And O6 Lipopolysaccharide O-antigens Of Helicobacter Pylori

Helicobacter pylori,a gram-negative pathogenic bacterium infecting more than 50% of the world’s population,is strongly associated with chronic gastritis,peptic ulcer and gastric cancer.Although antimicrobial therapies to combat H.pylori infections have been successful,antibiotic resistance and high reinfection rates result in an overall treatment failure rate of>20%.Therefore,it is imperative to develop new treatment approaches.To date,no H.pylori vaccine is available.Since lipopolysaccharide（LPS）is the major components of the outer membrane of H.pylori,its O-antigen part is considered to be potential antigens for anti-bacterial vaccine development.Synthesizing well-defined and pure oligosaccharide antigen is essential for the immunogenicity investigation and assessment of its structure-activity relationships.Here,total synthesis of O-antigens from Helicobacter pylori serotype O2 and O6 was achieved and will form the basis for developing carbohydrate-based vaccines and diagnostics.1.Total synthesis of Helicobacter pylori serotype O6 tridecasaccharide O-antigenBased on H.pylori serotype O6 O-antigen tridecasaccharide[α-L-Fucp-（1→2）-β-D-Galp-（1→4）-[α-L-Fucp-（1→3）]-β-D-GlcNAcp-（1→3）-β-D-Galp-（1-[→3）-DD-Hepp-1-]₅→6)-DD-Hepp-（→2）-[DD-Hepp-1-]₂→],target tridecasaccharide equipped with an anomeric linker was designed.Three manno-heptoside building blocks were synthesized starting from common D-mannose in overall yield of 10%,9%and 12%respectively.The conversion of six-carbon mannose to seven-carbon heptose was achieved by swern oxidation and wittig-olefination reaction.A diversity-oriented synthesis of three manno-heptosides started with the same heptoside was successfully.Subsquently,a diversity-orientation synthesis strategy has been employed for the facile fabrication of three crucial manno-heptosides started with the same heptoside.D-Galactosepentaacetate served as the starting material to access two galactose building blocks in total of 8 steps with 36%yield and 12 steps with 29%yield separately.D-Glucosamine building block was synthesized starting from D-glucosamine in a total of 7steps and 30%yield.For the assembly of DD-heptoglycan withα-（1→2）-、α-（1→3）-andα-（1→6）-linkages,a[3+2+3]coupling strategy from the reducing to the nonreducing end was adopted.The successful synthesis of DD-heptan formed the basis for studying its biological activity.An assembly sequence planning resulted in a[8+3+2]synthesis of tridecasaccharide,which may serve as reference for the syntheses of other sterically hindered glycans.Finally,based on the Le^y tetrasaccharide unit,DD-heptoglycan unit and different glycosidic linkages,six oligosaccharides related to the H.pylori O6 O-antigen were designed and chemically synthesized.2.Total synthesis of Helicobacter pylori serotype O2 O-antigen oligosaccharidesBased on H.pylori serotype O2 O-antigen disaccharide repeating unit[→2)-α-D-Glcp-（1→3）-α-D-Glcp-(1→],the uniqueα-（1→2）-andα-（1→3）-linked oligoglucosides equipped with an anomeric linker were designed,including di-,tetra-and hexasaccharides.Based on the remote acyl participation strategy,two building blocks with acyl groups at both O3 and O6 positions were successfully prepared.One was with levulinyl（Lev）at O3 positions and Ac at O6 positions in a total of 7 steps and 46%yield,and another one was with Bz at O3positions and Ac at O6 positions in a total of 7 steps and 51%yield.Using the above two glucose building blocks,we adopted a[2+2]coupling strategy to successful synthesis of the uniqueα-（1→2）-andα-（1→3）-linked oligoglucosides,including di-,tetra-and hexasaccharides.Due to the steric hindrance of 3-OH is less than the 2-OH on1,2-cis-glucose,it’s encouraged to employ disaccharide unit for the efficient formation of the more challengingα-（1→2）-glucosidic linkage at the monosaccharide stage.To achieve the coupling ofα-（1→3）-glucoside,the synergistic remote participation effects of acyl groups at the O3 and O6 positions on disaccharide unit was exploited,and finally,the O-antigen oligosaccharide chain was successfully extended.Acyl remote participation and solvent effects were found to counteract duringα-stereoselective glucosylations for the first time.The challenging synthesis of several O-antigen glycans based on assembly sequence planning which may serve as reference for the syntheses of other sterically hindered long-chain glycans.Furthermore,all synthetic O-antigen glycans were equipped with a linker at the reducing end,allowing for conjugation and surface immobilization to enable immunological investigations.