Milbemycin derivatives: modification at the C-5 position

Protection of the hydroxyl group at the C-5 position of milbemycin A4 and D was carried out to investigate the influence of the C-5 hydroxyl group on the anthelmintic potency of these derivatives. Moreover, the hydroxyl group was converted into amide groups as bioisosters. Biological activities of these derivatives were measured against Nippostrongylus brasiliensis in vitro, and minimal concentrations which induce 100% immotility in worms were determined for each derivative. Biological testing revealed that the hydroxyl group at C-5 is a structural requirement for retaining anthelmintic activity.