Synthesis and Biological Evaluation of Novel Benodanil-Heterocyclic Carboxamide Hybrids as a Potential Succinate Dehydrogenase Inhibitors

In order to discover new antifungal agents, twenty novel benodanil-heterocyclic carboxamide hybrids were designed, synthesized, and characterized by (1)H NMR and HRMS. In vitro, their antifungal activities against four phytopathogenic fungi were evaluated, as well as some of the target compounds at 50 mg/L demonstrated significant antifungal activities against Rhizoctonia solani. Especially, compounds 17 (EC(50) = 6.32 mg/L) and 18 (EC(50) = 6.06 mg/L) exhibited good antifungal activities against R. solani and were superior to the lead fungicide benodanil (a succinate dehydrogenase inhibitor, SDHI) (EC(50) = 6.38 mg/L). Furthermore, scanning electron microscopy images showed that the mycelia on treated media with the addition of compound 17 grew abnormally as compared with the negative control with tenuous, wizened, and overlapping colonies, and compounds 17 (IC(50) = 52.58 mg/L) and 18 (IC(50) = 56.86 mg/L) showed better inhibition abilities against succinate dehydrogenase (SDH) than benodanil (IC(50) = 62.02 mg/L). Molecular docking revealed that compound 17 fit in the gap composed of subunit B, C, and D of SDH. Furthermore, it was shown that the main interaction, one hydrogen bond interaction, was observed between compound 17 and the residue C/Trp-73. These studies suggested that compound 17 could act as a potential fungicide to be used for further optimization.