Chemoenzymatic Synthesis and Antigenicity Evaluation of an O‐Antigen Octadecasaccharide from <i>Helicobacter pylori</i>

Helicobacter pylori infection represents a major global health challenge, characterized by high prevalence, significant association with gastric cancer, and rising antibiotic resistance. Carbohydrate-based vaccines targeting the O-antigen of lipopolysaccharide (LPS) present a promising alternative to conventional antimicrobial therapies. To explore the immunogenicity of LPS O-antigen from clinical isolate H. pylori SS1, we report an integrated chemoenzymatic strategy for the first synthesis of its octadecasaccharide O-antigen and related fragments for antigenicity evaluation. Our strategy features modular chemical synthesis of a decasaccharide precursor containing a high-carbon sugar (D,D-Hep) residue, a unique oligomeric β1,2-linked ribofuranosyl tetrasaccharide motif and a switchable glucosamine (GlcNH₂) residue through stereoconvergent [6 + 4] assembly, followed by protecting-group-controlled enzymatic elongation to precisely install hybrid Lewis antigen moiety (Le^y-Le^x) in a site-specific fucosylation manner to afford the target octadecasaccharide bearing five challenging 1,2-cis-glycosidic linkages. Chemical stereoselective construction of 1,2-cis-glucosidic and 1,2-cis-fucosidic linkages was accomplished by reagent-controlled glycosylation and 4-O-acyl remote participation, respectively. Enzymatic site-specific installation of the remaining three 1,2-cis-fucosidic linkages was achieved using two robust fucosyltransferases and a strategically designed GlcNH₂ residue. Glycan microarray-based screening of the synthetic O-antigen and its subunits with H. pylori-infected patient sera identified an undecasaccharide as a simpler and key epitope for vaccine development.