Synthesis of a Branched Heptose- and Kdo-Containing Common Tetrasaccharide Core Structure of <i>Haemophilus </i><i>i</i><i>nfluenzae</i> Lipopolysaccharides via a 1,6-Anhydro-<scp>l</scp>-<i>g</i><i>lycero</i>-β-<scp>d</scp>-<i>m</i><i>anno-</i>heptopyranose Intermediate

Abstract

The synthesis of a common tetrasaccharide core structure of Haemophilus influenzae lipopolysaccharides, β- d -glucopyranosyl-(1→4)-[ l - glycero -α- d - manno -heptopyranosyl-(1→3)]- l - glycero -α- d - manno -heptopyranosyl-(1→5)-3-deoxy-α- d - manno -octulopyranoside, and the trisaccharide β- d -glucopyranosyl-(1→4)-[ l - glycero -α- d - manno -heptopyranosyl-(1→3)]- l - glycero -α- 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 -β- d - manno -heptopyranose intermediate to diminish the steric crowding between the 3- and 4-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 acceptors 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.