Total Synthesis of the Conjugation-Ready Tetrasaccharide Repeating Unit of Multidrug-Resistant Pathogen <i>Pseudomonas aeruginosa</i> NCTC 8505 O-Antigen

Herein, we report the first total synthesis of the conjugation-ready tetrasaccharide repeating unit of the O-antigen from <i>Pseudomonas aeruginosa</i> NCTC 8505. This work features a novel synthetic route for the incorporation of the α-linked l-galactosaminuronic acid (l-GalpNAcA) moiety in oligosaccharide synthesis. The target molecule poses several challenging elements, including synthesis of rare sugar units, such as d-bacillosamine and l-GalpNAcA, inherently poor nucleophilicity of the axial 4-hydroxyl group of l-galactosamine, and its demanding stereoselective couplings.