Synthesis of the Zwitterionic Repeating Unit of the O‑Antigen from <i>Shigella sonnei</i> and Chain Elongation at Both Ends

<i>Shigella sonnei</i> O-antigen features a zwitterionic disaccharide repeat encompassing two rare monosaccharides. The synthesis of the <b>AB</b> repeat and of trisaccharides <b>ABA</b>′ and <b>B</b>′<b>AB</b>, which validates chain elongation at either end, is reported. All targets were synthesized using a postglycosylation oxidation strategy in combination with imidate chemistry. Precursors to residue <b>A</b> were obtained from l-glucose. The AAT (<b>B</b>) donor and acceptor were obtained from d-glucosamine. A one-step Pd­(OH)₂/C-mediated deprotection provided the propyl glycoside targets