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

Abstract 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 (GlcNH 2 ) 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 GlcNH 2 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.