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

is the second most common etiologic pathogen responsible for childhood acute diarrhea. An anti- vaccine is still eagerly awaited due to the increasing drug resistance of this pathogen. The O-antigen is a promising vaccine target. To identify the immune epitopes of the glycan, the first total synthesis of serotype 10 O-antigen tetrasaccharide containing a ()-4,6--pyruvyl ketal was completed. The 1,2--β-glycosylation & C2 epimerization and conformational locking strategies facilitated the construction of two 1,2--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 ()-4,6--pyruvyl ketal was introduced at the final stage due to its influence on the glycosylation stereospecificity and efficiency. In addition, ()-4,6--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 ()-4,6--pyruvyl ketal is an essential structural feature of this antigen for designing carbohydrate-based vaccines against serotype 10. The comparison of the ()-4,6--pyruvylated glycan and its ()-epimer will set an example for biological evaluation of other bacterial glycans containing pyruvyl ketals.