Total Synthesis of the Zwitterionic Trisaccharide Repeating Unit of Fusobacterium nucleatum ATCC 51191 O-antigen

Herein, we report an efficient total synthesis of Fusobacterium nucleatum ATCC 51191 zwitterionic trisaccharide repeating unit. Striking features of the target molecule are the presence of rare sugars, six nitrogen atoms dispersed over three sugars and functionalities like carboxylic acids and an amide. Challenges such as installation of 1,2-cis glycosidic linkage in AAT sugar, glycosylation using GlcNAcA donors with less reactive 4-OH of GlcNAcA, orthogonal late-stage amino acid coupling in the sterically hindered site, and synthesis of orthogonally protected rare amino sugars have been addressed. The total synthesis of target molecule accomplished via longest linear sequence of 22 steps in 2.3 % overall yield.