Synthesis of the trisaccharide repeating unit related to Klebsiella 012 serotype

Synthesis of the trisaccharide, allyl α-l-rhamnopyranosyl-(1→3)-2-acetamido-2-deoxy-β-d-glucopyranosyl-(1→4)-α-l-rhamnopyranoside related to O-chain glycans isolated from the deaminated LPSs of Klebsiella pneumoniae serotype 012, was achieved through condensation of suitably synthesized disaccharide, allyl 4,6-O-benzylidene-2-deoxy-2-phthalimido-β-d-glucopyranosyl-(1→4)-2,3-di-O-benzoyl-α-l-rhamnopyranoside and donor, ethyl 2,3,4-tri-O-acetyl-1-thio α-l-rhamnopyranoside starting from l-rhamnose and d-glucosamine hydrochloride. The trisaccharide can be utilized for the synthesis of neoglycoconjugates for use as a synthetic vaccine by coupling it with a suitable protein after deprotection. Various regio- and stereoselective protecting group strategies have been carefully considered, as protecting groups can influence the reactivity of the electrophile and nucleophile in glycosylation reactions on the basis of steric and electronic requirements.