Modular Approach to Triazole-Linked 1,6-alpha-D-Oligomannosides to the Discovery of Inhibitors of Mycobacterium tuberculosis Cell Wall Synthetase

Aiming at developing inhibitors of mannosyltransferases, the enzymes that participate in the biosynthesis of the cell envelope of Mycobacterium tuberculosis, the synthesis of a range of designed triazole-linked 1,6-oligomannosides up to a hexadecamer has been accomplished by a modular approach centered on the Cu(I)-catalyzed azide-alkyne cycloaddition as key process. The efficiency and fidelity of the cycloaddition are substantiated by high yields (76-96%) and exclusive formation of the expected 1,4-disubstituted triazole ring in all oligomer assembling reactions. Key features of oligomers thus prepared are the anomeric carbon-carbon bond of all mannoside residues and the 6-deoxymannoside capping residue. Suitable bioassays with dimer, tetramer, hexamer, octamer, decamer, and hexadecamer showed variable inhibitor activity against mycobacterial alpha-(1,6)-mannosyltransferases, the highest activity (IC50 = 0.14-0.22 mM) being registered with the hexamannoside and octamannoside.