Novel Citral-thiazolyl Hydrazine Derivatives as Promising Antifungal Agents against Phytopathogenic Fungi.

To develop new antifungal agents against phytopathogenic fungi, a series of citral-thiazolyl hydrazine derivatives were designed, synthesized, and characterized by FT-IR, ¹ H NMR, ¹³ C NMR, and HRMS. Antifungal activity results showed that most synthetic compounds exhibited broad-spectrum antifungal activities against six phytopathogenic fungi in vitro . Notably, compounds b and c15 exhibited remarkable antifungal activity against Colletotrichum gloeosprioides , Rhizoctonia solani , Phytophthora nicotianae var. nicotianae , Diplodia pinea , Colletotrichum acutatum , and Fusarium oxysporum f. sp. niveum , which were all superior to the positive control tricyclazole. Structure-activity relationship (SAR) studies demonstrated that introducing electron-withdrawing groups such as F on the benzene ring exhibited outstanding antifungal activities against all the tested fungi. Furthermore, compound b could effectively control rice sheath blight and showed higher curative activities against R. solani than validamycin·bacillus in vivo . In addition, the in vitro cytotoxicity results indicated that compound b possessed moderate cytotoxicity activity, and all citral-thiazolyl hydrazine derivatives exhibited lower or no cytotoxicity to the LO2 and HEK293 cell lines. In addition, the acute oral toxicity test showed that compound b had moderate toxicity (level II) with an LD ₅₀ value of 310 mg/kg bw (95% confidence limit: 175-550 mg/kg bw). Finally, a preliminary action mechanism study showed that causing obvious malformation of mycelium and increasing cell membrane permeability are two of the potential mechanisms by which compound b exerts antifungal activity. The present work indicates that some of these derivatives may serve as novel potential fungicides, and compound b is expected to be the leading structure for the development of new antifungal agents.