Design, Synthesis and Fungicidal Activity of N-(5-Phenyl-1,3,4-Thiadiazol-2-yl)-N-((1-Phenyl-1<i>H</i>-1,2,3-Triazol-4-yl) Methyl) Benzamide Derivatives

Objective: Plant pathogens are serious hazards to agricultural production, but existing drugs have problems such as increased resistance of pathogenic bacteria and environmental pollution, and there is a need to find high-efficiency, low-toxicity fungicides with novel chemical structures. Methods: 5-phenyl-1,3,4-thiadiazol-2-amine was used as the starting material, and 20 new thiadiazole-linked compounds were designed and synthesized. Their in vitro inhibitory activities against plant pathogenic fungi were tested using a mycelium growth inhibition assay, and the relationship between the antifungal activity of the target compounds and their molecular structures was preliminarily discussed. Results: Bioassay results revealed that most of the title compounds exhibited excellent in vitro fungicidal activity, in which compound D16 showed the highest activity against Rhizoctonia solani (EC 50 = 0.0028 μmol·L −1 ), D12 against Botrytis cinerea (EC 50 = 0.0024 μmol·L −1 ), and D5 against Stemphylium lycopersici and Curvularia lunata (EC 50 = 0.0105 μmol·L −1 , 0.005 μmol·L −1 ). The structure activity relationship (SAR) was also discussed and the results are of great significance for the structural optimization and development of more effective thiadiazole antibacterial compounds. Conclusion: The results of this study are important for structural optimization and development of more potent thiadiazole antimicrobial compounds.