Synthesis of a Conjugation-Ready, Phosphorylated, Tetrasaccharide Fragment of the O‑PS of <i>Vibrio cholerae</i> O139

A new pathway to the tetrasaccharide α-Col<i>p</i>-(1→2)-4,6-<i>P</i>-β-d-Gal<i>p</i>-(1→3)-[α-Col<i>p</i>-(1→4)]-β-d-Glc<i>p</i>NAc-1-(OCH₂CH₂)₃NH₂ has been developed. Glycosylation of 8-azido-3,6-dioxaoctyl 4,6-<i>O</i>-benzylidene-2-deoxy-2-trichloroacetamido-β-d-glucopyranoside with 3,4,6-tri-<i>O-</i>acetyl-2-<i>O</i>-bromoacetyl-α-d-galactopyranosyl bromide afforded the β-linked disaccharide. Debromoacetylation followed by reductive opening of the benzylidene acetal afforded the disaccharide diol acceptor. Halide-assisted glycosylation with 2,4-di-<i>O</i>-benzyl-α-colitosyl bromide gave the 1,2-<i>cis-</i>coupling product. Deacetylation followed by regioselective phosphorylation gave isomeric (<i>R,S</i>)-(P)-4^II,6^II-cyclic phosphates, which were globally deprotected by one-step catalytic (Pd/C) hydrogenation/hydrogenolysis. The target tetrasaccharide, obtained in high overall yield, is amenable for conjugation to proteins