Total Synthesis of an <i>Aeromonas hydrophila</i> Pt679 O-Antigen Pentasaccharide Repeat Unit Containing β- <scp>l</scp> -Rha, β- <scp>d</scp> -QuiNAc, and α- <scp>d</scp> -Fuc3NAc

The total synthesis of the complex pentasaccharide repeating unit related to the O-polysaccharide from <i>Aeromonas hydrophila</i> Pt679 is reported. Suitably protected derivatives of the rare β-d-QuiNAc and α-d-Fuc3NAc units are synthesized from d-glucosamine hydrochloride and 1,2;5,6-di-<i>O</i>-isopropylidene-α-l-allofuranose, respectively, and used for stereoselective glycosylation. The 1,2-<i>cis</i> rhamnosidic linkage is successfully accomplished using picoloyl-mediated hydrogen bond-assisted aglycon delivery (HAD method). The target pentasaccharide in the form of its 2-aminoethyl glycoside is ready for further conjugation through the terminal amine functionality without hampering the reducing end stereochemistry.