Synthesis of modified mannose oligosaccharides as potential inhibitors of mycobacterial lipoarabinomannan biosynthesis

Infections by Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), have reemerged as a public health threat in recent years. The resurgence in TB has sparked renewed interest in identifying new antibiotics that can be used to treat this disease, which claims nearly three million lives worldwide each year. This disease has been difficult to treat and can be attributed in part to the unusual structure of the cell wall of the organism, which presents a formidable barrier to the passage of drugs into the organism. In order to identify new anti-tuberculosis agents, our research goals are two-fold: The first objective is to synthesize oligosaccharides, which are potential substrates for the enzymes (mannosyltransferases) involved in the biosynthesis of one of the polysaccharides (mannosylated lipoarabinomannan, Man-LAM) found in the cell wall complex that surrounds the organism. The second objective is to prepare mannose-based disaccharides (type A and B) which are potential inhibitors of the LAM biosynthesis. Such compounds are believed to be excellent lead candidates for new anti-tuberculosis agents. During the synthesis of oligosaccharide substrates, a key step is the stereoselective synthesis of octyl β-arabinofuranoside via a S_N2 type displacement reaction of octanol with a benzyl-substituted arabinofuranosyl chloride. Selective deprotection followed by sequential addition of thioglycosides, afforded the oligosaccharide fragments of Man-LAM. In the synthesis of the disaccharide inhibitors, our strategy was to proceed via a common intermediate leading to more than one target. We first developed a methodology for the synthesis of type A mannose disaccharides, which are modified at C-2 ′ of the mannose ring. We then made use of the intermediates used in the preparation of type A disaccharides in the synthesis of type B disaccharides, which contain an additional modification at C-6′ of the ring. These compounds will play a significant role in understanding the carbohydrate-enzyme interactions and in turn, can be useful as lead compounds for drug design. Additionally, we also synthesized potential suicide inhibitors, which could function by forming a covalent bond with the active site of the enzyme. We characterized the target oligosaccharides by 800 MHz NMR spectroscopy to ensure purity of such compounds for biological studies. Biological investigations on type A disaccharides were performed in collaboration, which suggested that 4 out of 5 compounds in type A class are substrates of the enzyme α(1→6)-ManT, but none inhibited the enzyme. Biological studies on type B disaccharides are in progress, as are studies with α(1→2)-ManT.