Total Syntheses of Conjugation-Ready Trisaccharide Repeating Units of Pseudomonas aeruginosa O11 and Staphylococcus aureus Type 5 Capsular Polysaccharide for Vaccine Development

Pseudomonas aeruginosa belongs to the group of three "critical priority" multi-drug-resistant pathogens listed by WHO and is responsible for severe and often deadly infections such as bloodstream infections and pneumonia. Staphylococcus aureus is also a "high priority" pathogen which is a major cause of serious nosocomial infections such as bacteremia, sepsis, and endocarditis. Owing to their ability to adapt resistance to almost any antibiotics, vaccines against these pathogens are urgently required. These pathogens express structurally unique and densely functionalized glycans on their surfaces which are absent on the host cells. Such carbohydrate antigens are valuable targets for the development of glycoconjugate vaccines and diagnostics. Here, we report the first total synthesis of the conjugation-ready trisaccharide repeating unit of Pseudomonas aeruginosa O11 via a highly stereoselective and efficient assembly of a rare L-fucosamine- and D-fucosamine-containing 1,2-cis-linked disaccharide motif and its regioselective glycosylation at O3. A systematic study was conducted for the notoriously difficult glycosylation with the most unreactive axial 4-OH of the rare disaccharide, and the successful outcome was utilized to accomplish the total synthesis of an aminopropyl linker-attached trisaccharide repeating unit of Staphylococcus aureus capsular polysaccharide type 5, which is also a potential antigen for immunotherapy and vaccine development. The judicious selection of protecting groups and reaction conditions allowed the stereoselective assembly and selective functional group interconversions to access the structurally complex linker-attached trisaccharide repeating units, which are valuable tools for immunological evaluation and vaccine development. The strategy is useful for the synthesis of other structurally related complex glycans.