En route to a carbohydrate-based vaccine against Burkholderia cepacia

We report a very high yielding first total synthesis of trisaccharide 5, alpha-D-Rhap-(1 --> 3)-alpha-D-Rhap-(1 --> 4)-alpha-D-Galp, corresponding to the repeating unit 1 of an O-polysaccharide present in the lipopolysaccharide of clinical isolate of Burkholderia cepacia. The approach included two successive glycosylations, based on D-rhamnosyl trichloroacetimidate donors 12 and 14. The oligosaccharide 5 has been further functionalized by photochemical coupling or cross-metathesis with non-natural amino acid derivatives. Trisaccharidylamino acids 16 and 17 are now available, with the aim of preparing a novel synthetic carbohydrate-based vaccine.