Design, Synthesis, and Antifungal Activities of Novel Carboxamides Derivatives Bearing a Chalcone Scaffold as Potential SDHIs

Design and semisynthesis of novel oleanolic acid-based tertiary amide derivatives as promising antifungal agents against phytopathogenic fungi

To further explore and discover natural products-based antifungal agents, seventeen tertiary amide-oleanolic acid hybrids were designed and synthesized, and structurally confirmed by 1H NMR, 13C NMR, HRMS, and melting point. Bioassay results illustrated that derivative 4 k exhibited prominent in vitro inhibitory activity against the mycelium growth of Gaeumannomyces graminis and Valsa mali with the EC50 values of 41.77 and 43.96 μg/mL, respectively. Meanwhile, the structure-activity relationships were also summarized. Moreover, in vivo control efficacy demonstrated that derivative 4 k displayed remarkable curative effect (CE) against V. mali at 200 μg/mL with the value of 52.6%, evidently superior to that of the positive control carbendazim (41.5%). Besides, derivative 4 k also exhibited good CE against Botrytis cinerea at 200 μg/mL with the value of 33.0%. Scanning electron microscope analysis initially indicated that derivative 4 k may exert its antifungal effect by leading to abnormal morphology on the mycelium surface, resulting in the aberrant hypha growth.

Novel Pyrazole-4-Carboxamide Derivatives Containing Oxime Ether Group as Potential SDHIs to Control Rhizoctonia solani

In the search for novel succinate dehydrogenase inhibitor (SDHI) fungicides to control Rhizoctonia solani, thirty-five novel pyrazole-4-carboxamides bearing either an oxime ether or an oxime ester group were designed and prepared based on the strategy of molecular hybridization, and their antifungal activities against five plant pathogenic fungi were also investigated. The results indicated that the majority of the compounds containing oxime ether demonstrated outstanding in vitro antifungal activity against R. solani, and some compounds also displayed pronounced antifungal activities against Sclerotinia sclerotiorum and Botrytis cinerea. Particularly, compound 5e exhibited the most promising antifungal activity against R. solani with an EC50 value of 0.039 μg/mL, which was about 20-fold better than that of boscalid (EC50 = 0.799 μg/mL) and 4-fold more potent than fluxapyroxad (EC50 = 0.131 μg/mL). Moreover, the results of the detached leaf assay showed that compound 5e could suppress the growth of R. solani in rice leaves with significant protective efficacies (86.8%) at 100 μg/mL, superior to boscalid (68.1%) and fluxapyroxad (80.6%), indicating promising application prospects. In addition, the succinate dehydrogenase (SDH) enzymatic inhibition assay revealed that compound 5e generated remarkable SDH inhibition (IC50 = 2.04 μM), which was obviously more potent than those of boscalid (IC50 = 7.92 μM) and fluxapyroxad (IC50 = 6.15 μM). Furthermore, SEM analysis showed that compound 5e caused a remarkable disruption to the characteristic structure and morphology of R. solani hyphae, resulting in significant damage. The molecular docking analysis demonstrated that compound 5e could fit into the identical binding pocket of SDH through hydrogen bond interactions as well as fluxapyroxad, indicating that they had a similar antifungal mechanism. The density functional theory and electrostatic potential calculations provided useful information regarding electron distribution and electron transfer, which contributed to understanding the structural features and antifungal mechanism of the lead compound. These findings suggested that compound 5e could be a promising candidate for SDHI fungicides to control R. solani, warranting further investigation.