Exploratory N‐Protecting Group Manipulation for the Total Synthesis of Zwitterionic <i>Shigella sonnei</i> Oligosaccharides

International audienceShigella sonnei surface polysaccharides are well-established protective antigens against this major causeo fd iarrhoeal disease.T hey also qualify as unique zwitterionic polysaccharides (ZPSs)f eaturing ad isaccharide repeating unit made of two 1,2-trans linked rare aminodeoxy sugars, a 2-acetamido-2-deoxy-l-altruronic acid (l-AltpNAcA)a nd a2acetamido-4-amino-2,4,6-trideoxy-d-galactopyranose (AAT). Herein,t he stereoselective synthesis of S. sonnei oligosaccharides comprising two, three and four repeating units is reported for the first time. Severals ets of up to seven pro-tectingg roups wereexplored,s hedding light on the singular conformational behavior of protected altrosamine and altruronic residues. Ad isaccharide buildingb lock equipped with three distinct N-protectingg roups and featuring the uronate moiety already in place was designed to accomplish the iterative high yieldingg lycosylation at the axial 4-OH of the altruronate componenta nd achieve the challenging full deprotection step. Key to the successfulr oute was the use of a diacetyls trategy whereby the N-acetamido group of the l-AltpNAcAism asked in the form of an imide