Design, selective synthesis and biological activities evaluation of novel thiazol-2-ylbenzamide and thiazole-2-ylbenzimidoyl chloride derivatives

To promote the development and exploitation of novel antifungal agents, a series of thiazol-2-ylbenzamide derivatives (3A-3V) and thiazole-2-ylbenzimidoyl chloride derivatives (4A-4V) were designed and selective synthesis. The bioassay results showed that most of the target compounds exhibited excellent in vitro antifungal activities against five plant pathogenic fungi (Valsa mali, Sclerotinia scleotiorum, Botrytis cinerea, Rhizoctonia solani and Trichoderma viride). The antifungal effects of compounds 3B (EC50 = 0.72 mg/L) and 4B (EC50 = 0.65 mg/L) against S. scleotiorum were comparable to succinate dehydrogenase inhibitors (SDHIs) thifluzamide (EC50 = 1.08 mg/L) and boscalid (EC50 = 0.78 mg/L). Especially, compounds 3B (EC50 = 0.87 mg/L) and 4B (EC50 = 1.08 mg/L) showed higher activity against R. solani than boscalid (EC50 = 2.25 mg/L). In vivo experiments in rice leaves revealed that compounds 3B (86.8 %) and 4B (85.3 %) exhibited excellent protective activities against R. solani comparable to thifluzamide (88.5 %). Scanning electron microscopy (SEM) results exhibited that compounds 3B and 4B dramatically disrupted the typical structure and morphology of R. solani mycelium. Molecular docking demonstrated that compounds 3B and 4B had significant interactions with succinate dehydrogenase (SDH). Meanwhile, SDH inhibition assay results further proved their potential as SDHIs. In addition, acute oral toxicity tests on A. mellifera L. showed only low toxicity for compounds 3B and 4B to A. mellifera L. populations. These results suggested that these two series of compounds had merit for further investigation as potential low-risk agricultural SDHI fungicides.