Synthesis of the docosanasaccharide arabinan domain of mycobacterial arabinogalactan and a proposed octadecasaccharide biosynthetic precursor.

Two major components of the cell wall in mycobacteria, including Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), are polysaccharides containing arabinofuranose residues. In one of these polysaccharides, arabinogalactan, this arabinan domain consists of three identical motifs of 22 arabinofuranose residues, which are in turn attached to an underlying galactofuranan backbone. Recent studies have proposed that this docosanasaccharide motif, and a structurally related arabinan present in another cell wall polysaccharide, lipoarabinomannan, are biosynthesized from a common octadecasaccharide precursor. To facilitate the testing of this hypothesis, we report here the first total syntheses of these 18- and 22-residue oligosaccharides both functionalized with an aminooctyl linker arm. The route to the target compounds involved the preparation of four tri- to heptasaccharide building blocks possessing only benzoyl protecting groups that were coupled in a highly convergent manner via glycosyl ...