Synthesis of a 1,3 β-glucan hexasaccharide designed to target vaccines to the dendritic cell receptor, Dectin-1

• Synthesis of a β1,3 glucan as a tether glycoside for attachment to conjugate vaccines.
• A synthetic hexasaccharide ligand for Dectin-1 a dendritic cell C-type lectin.
• Dendritic cell C-type lectins can serve as receptors of targeting vaccines for improved antigen presentation.
• Selective debenzoylation of a β1,3 disaccharide at 4,6 and 4′,6′ positions.
• Hexasaccharide synthesis by sequential 2+2 and 2+4 glycosylation reaction with a disaccharide thioglycoside.

Transformation of 3-O-benzyl-1,2:5,6-di-O-isopropylidene-α-d-glucofuranose into 2,4,6-tri-O-benzoyl-3-O-benzyl glucopyranosyl imidate proceeded efficiently via crystalline benzyl and per-benzoylated derivatives. This imidate glycosylated di-O-isopropylidene-α-d-glucofuranose in high yield and glycosylation of the disaccharide after removal of the 3′-O-benzyl ether afforded the β1,3 linked trisaccharide in excellent yield. Di- and trisaccharides imidates were readily prepared from the furanose terminated glycosylation products but both were unreactive in glycosylation reaction with the debenzylated di- and trisaccharide alcohols. The 3′-O-benzyl perbenzoylated disaccharide pyranose derivative could be selectively debenzoylated and converted to the corresponding perbenzoylated 4,6:4′,6′-di-O-benzylidene derivative. Lewis acid catalyzed glycosidation gave the selectively protected disaccharide ethylthioglycoside in good overall yield. Glycosidation of this thioglycoside donor with 5-methoxycarbonylpentanol gave the disaccharide tether glycoside and after catalytic removal of benzyl ether the resulting disaccharide alcohol was glycosylated by the thioglycoside in a 2+2 reaction to yield a tetrasaccharide. Repetition of selective deprotection of the terminal 3-O-benzyl ether followed by glycosylation by the disaccharide thioglycoside gave a protected hexasaccharide. Hydrogenolysis of this hexasaccharide followed by transesterification and second hydrogenolysis to remove a residual benzyl group gave the target hexasaccharide glycoside 1 as a Dectin-1 ligand functionalized to permit covalent attachment to glycoconjugate vaccines and thereby facilitate improved antigen processing by dendritic cells.

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