Peramivir is the cyclopentane derivative designed with a guanidino group replacing the C-4 hydroxyl group of Neu5Ac2en in the active site, with negatively charged carboxylate group and a n-butyl side chain. 1-ethylpropylamide, diethylamide, dipropylamide and 4-morpholinylamide groups  all showed excellent inhibitory activity. Changing the cyclopentane ring to a pyrrolidine ring showed high inhibitory activity as well.