Structural Studies of the O -Acetyl-Containing O-Antigen from a Shigella flexneri Serotype 6 Strain and Synthesis of Oligo­saccharide Fragments Thereof

Abstract : Extensive analysis by NMR spectroscopy of the delipidated lipopolysaccharide of Shigella flexneri serotype 6 strain MDC 2924‐71 confirmed the most recently reported structure of the O‐antigen repeating unit as {→4)‐β‐D‐GalpA‐(1→3)‐β‐D‐GalpNAc‐(1→2)‐α‐L‐Rhap3Ac/4Ac‐(1→2)‐α‐L‐Rhap‐(1→}, and revealed the non‐stoichiometric acetylation at O‐3C/4C. Input from the CASPER program helped to ascertain the fine distribution of the three possible patterns of O‐acetylation. The non‐O‐acetylated repeating unit (ABCD) corresponded to about 2/3 of the population, while 1/4 was acetylated at O‐3C (3AcCDAB), and 1/10 at O‐4C (4AcCDAB). Di‐ to tetrasaccharides with a GalpA residue (A) at their reducing end were synthesized as their propyl glycosides following a multistep linear strategy relying on late‐stage acetylation at O‐3C. Thus, the 3C‐O‐acetylated and non‐O‐acetylated targets were synthesized from common protected intermediates. Rhamnosylation was most efficiently achieved by using imidate donors, including at O‐4 of a benzyl galacturonate acceptor. In contrast, a thiophenyl 2‐deoxy‐2‐trichloroacetamido‐D‐galactopyranoside precursor was preferred for chain elongation involving residue B. Final Pd/C‐mediated deprotection ensured O‐acetyl stability. All of the target molecules represent parts of the O‐antigen of S. flexneri 6, a prevalent serotype. Non‐O‐acetylated oligosaccharides are also fragments of the Escherichia coli O147 O‐antigen.