Chemical Synthesis and Antigenicity Evaluation of Shigella dysenteriae Serotype 10 O-Antigen Tetrasaccharide Containing a ( S )-4,6- O -Pyruvyl Ketal.

Shigella is the second most common etiologic pathogen responsible for childhood acute diarrhea. An anti- Shigella vaccine is still eagerly awaited due to the increasing drug resistance of this pathogen. The Shigella O-antigen is a promising vaccine target. To identify the immune epitopes of the glycan, the first total synthesis of Shigella dysenteriae serotype 10 O-antigen tetrasaccharide containing a ( S )-4,6- O -pyruvyl ketal was completed. The 1,2- trans -β-glycosylation & C2 epimerization and conformational locking strategies facilitated the construction of two 1,2- cis -β-glycosidic linkages. The reactivities of both the glycosyl donor and acceptor were improved by adding electron-donating benzyl groups, enabling an efficient assembly of the tetrasaccharide. The ( S )-4,6- O -pyruvyl ketal was introduced at the final stage due to its influence on the glycosylation stereospecificity and efficiency. In addition, ( R )-4,6- O -pyruvylated and nonpyruvylated tetrasaccharides and three further fragments were synthesized. Glycan microarray screening revealed that the tetrasaccharide repeating unit is the key antigenic epitope of the O-antigen. Moreover, the ( S )-4,6- O -pyruvyl ketal is an essential structural feature of this antigen for designing carbohydrate-based vaccines against S. dysenteriae serotype 10. The comparison of the ( S )-4,6- O -pyruvylated glycan and its ( R )-epimer will set an example for biological evaluation of other bacterial glycans containing pyruvyl ketals.