Synthesis of a branched heptose- and Kdo-containing common tetrasaccharide core structure of Haemophilus influenzae lipopolysaccharides via 1,6-anhydro-L-glycero-β-D-manno-heptopyranose intermediate

The synthesis of a common tetrasaccharide core structure of Haemophilus influenzae lipopolysaccharides, beta-D-glucopyranosyl-(1-->4)-[L-glycero-alpha-D-manno-heptopyranosyl-(1-->3)]-L-glycero-alpha-D-manno-heptopyranosyl-(1-->5)-3-deoxy-alpha-D-manno-octulopyranoside and the trisaccharide beta-D-glucopyranosyl-(1-->4)-[L-glycero-alpha-D-manno-heptopyranosyl-(1-->3)]-L-glycero-alpha-D-manno-heptopyranoside is described. The oligosaccharides are synthesized as glycosides of a bifunctional spacer, 2-(4-aminophenyl)ethanol, to allow the subsequent formation of immunogenic glycoconjugates, which will be evaluated as well-defined glycoconjugate vaccine candidates. The syntheses of the 3,4-branched structures were accomplished using a 1,6-anhydro-L-glycero-beta-D-manno-heptopyranose intermediate to diminish the steric crowding between the 3- and 1-substituent. This intermediate was effectively synthesized from a mannose precursor via a stereoselective one-carbon elongation using a Barbier reaction (which was found to be more convenient than a Grignard reaction) and anhydro bridge formation through an internal glycosylation of a 6-O-trimethylsilylated ethyl thioheptoside using NIS/TfOH as a promoter. The 3- and 4-substituent were readily introduced into the 1,6-anhydro intermediate by glycosylation reactions using thioglycosides as donors and NIS/TfOH as a promoter, a task which has not been possible using accepters with equatorial 3,4-substituents. Acetolysis of the anhydro bridge followed by conversion into the ethyl thioglycoside afforded a trisaccharide donor, which, in NIS/TfOH-promoted couplings to the spacer and to a Kdo acceptor followed by deprotection, efficiently gave the two target compounds.