Design, synthesis, and antibacterial activities of conformationally constrained kanamycin A derivatives

A series of conformationally constrained kanamycin A derivatives with a 2'-hydroxyl group in ring I and a 5-hydroxyl group in ring II tethered by carbon chains were designed and synthesized. Pivotal 5,2'-hydroxyl groups were exposed, and the kanamycin A intermediate was synthesized from 5, 2', 4″, 6″-di-O-benzylidene-protected tetraazidokanamycin A. Cyclic kanamycin A derivatives with intramolecular 8-, 9-, 10-, and 11-membered ethers were then prepared by cesium carbonate mediated Williamson ether synthesis or a ring-closing metathesis reaction. The kanamycin A derivatives were assayed against both susceptible and resistant bacterial strains. Although no derivative showed better antibacterial activities than kanamycin A, the antibacterial activities of these cyclic kanamycin A derivatives indeed varied with the length of the bridge. Moreover, different variations of activities were observed between the susceptible and resistant bacterial strains. More tightly constrained derivative 2 with a one-carbon bridge showed better activity than the others against susceptible strains, but it was much less effective for resistant bacterial strains than derivative 3 with a two-carbon bridge and derivative 6 with an unsaturated four-carbon bridge.