Fmoc/Oxazolidinone Orthogonality Enables the Total Synthesis of a Structurally Complex, Conjugation-Ready Trisaccharide Repeating Unit of Vibrio cholerae O100.

Herein, we report the total synthesis of a structurally complex trisaccharide repeating unit of Vibrio cholerae O100 via an orthogonal amine protecting group strategy. This approach involves the preparation of rare amino sugars, and regioselective glycosylation followed by post-glycosylation amidation using dHh (3,5-dihydroxyhexanoyl) and Hb (3-hydroxybutanoyl). A systematic investigation of post-glycosylation amidation revealed that the oxazolidinone protecting group is an efficient orthogonal protecting group compatible with Fmoc. Using this orthogonality, we accomplished the total synthesis of the target trisaccharide through a 26-step linear sequence, achieving an overall yield of 2.6%. Furthermore, the synthesized trisaccharide repeating unit is equipped with an aminopropyl linker at the reducing end, which in turn can be conjugated to carrier proteins for subsequent immunological studies. Overall, this approach establishes a versatile platform for accessing complex glycans bearing multiple orthogonal nitrogen functionalities and holds significant potential for applications in the study of bacterial glycoconjugates and immunological investigations of pathogenic bacteria.