Total Synthesis of O43 Tetrasaccharide Repeating Unit.

is a pathogen responsible for the deadly pandemic - cholera. The glycans present on the surface of various strains of are considered as potential vaccine candidates. The tetrasaccharide repeating unit (RU) of O43 is decorated with less-explored rare deoxy amino sugars like d-quinosamine and d-viosamine, along with a rare amino acid, -acetyl-l-allothreonine. Herein, we report a detailed account of the total synthesis of O43 tetrasaccharide RU. In our earlier attempt, while a one-pot assembly of trisaccharide was successful, the final coupling with a fully functionalized d-viosamine donor was low yielding. The successful route involved employing the Fmoc-protected d-viosamine building block as a donor and a late-stage amide bond formation of the tetrasaccharide.