Human acrosin is an attracting target for the development of novel male contraceptives. Scaffold hopping was used to optimize the isoxazolecarbaldehyde human acrosin inhibitors and extend their structure–activity relationships. Four kinds of scaffolds, namely benzimidazole, benzothiazole, 3H-indazole, and 5-phenyl-1H-pyrazole, were designed and synthesized. Most of the synthesized compounds showed potent human acrosin inhibitory ...