N’-Aryl α,β-Unsaturated Fatty Acid Hydrazides with Broad Antifungal Activity: Design, Bioactivity, and Action Mechanism

As part of our continuing investigation on hydrazide antifungal agents, a series of N'-aryl hydrazide derivatives of α,β-unsaturated fatty acids were designed and explored for their inhibitory activity on various phytopathogenic fungi and their mechanism. Most of the compounds showed excellent activity, with average EC₅₀ values of 0.76-5.79 μg/mL on A. solani, A. brassicae, A. alternate, C. gloeosporioides, and P. oryzae. Among N'-substituted phenyl hydrazide derivatives of senecioic acid, 4-fluoro (5a), 4-bromo (5g), 3-cholor-4-fluoro (5s) and 3,4-dicholor (5w) derivatives displayed average EC values of 0.76-0.87 μg/mL on the five fungi, far superior to 2.99 μg/mL of kresoxim-methyl, a commercial fungicide. Especially, 5s was able to completely inhibit the growth in vitro of all the fungi at 20 μg/mL and gave almost the same control effect as the positive fungicide boscalid against the infection of tomato fruits by A. solani at 200 μg/mL. Meanwhile, 5s did not display a visible effect on plant seed germination at 200 μg/mL and seedling growth at 100 μg/mL, nor significant cytotoxicity on normal L929 mouse fibroblasts at 1.0 mg/mL. Scanning electron microscopy, transmission electron microscopy, and fluorescence microscopy studies revealed that 5s could destroy the mycelial morphology and the structure of the cell membranes, increase the ROS level in cells, alter the MMP, and cause cell death. Additionally, 5s could also strongly inhibit the activity of SDH in A. solani, with an IC₅₀ value of 3.54 μg/mL, superior to 4.46 μg/mL of the positive SDHI boscalid. Molecular docking showed that the 4-fluoro group of the benzene ring plays an important role in the binding of 5s to SDH. The SAR indicated that the presence of a 4-halogen atom on the benzene ring is able to dramatically improve the antifungal activity. The N'-aryl hydrazides of α,β-unsaturated fatty acids emerged as novel and promising antifungal lead compounds.