Total Synthesis of β-d-ido-Heptopyranosides Related to Capsular Polysaccharides of Campylobacter jejuni HS:4

The 6-deoxy-β-d-ido-heptopyranoside related to the capsular polysaccharides of C. jejuni HS:4 is very remarkable, owing to the unique, multifaceted structural features that have been combined into one molecule, which include (1) the rare ido-configuration, (2) the unusual 7-carbon backbone, and (3) the challenging β-(1→2)-cis-anomeric configuration. Two distinct strategies toward the total synthesis of this interesting target are reported. The first involved establishment of the β-d-idopyranosyl configuration from β-d-galactopyranosides, prior to a C-6-homologation extending the d-hexose to the desired 6-deoxy-d-heptose. However, this approach encountered difficulties due to the significantly reduced reactivity of the 6-position of the β-d-idopyranosides, so instead a second strategy was employed, which involved first carrying out a 6-homologation on the less flexible d-galactopyranose, followed by a very successful conversion to the desired β-d-ido-configuration found in the target heptopyranoside (2). This report is the first successful synthesis of the 6-deoxy-β-d-ido-heptopyranoside, which could possess interesting immunological properties.