Utilization of Natural Coumarin Resources for Developing Fungicide Candidates: Diversity-Oriented Structural Design, Synthesis, and Biological Assessment of Novel Osthole Derivatives

Osthole, a natural coumarin with fungicidal and insecticidal properties, represents a privileged scaffold for agrochemical discovery. A series of osthole derivatives featuring 1,3,4-oxadiazole, amine, quinazolinone, carbamate, or halogen substituents were synthesized and evaluated for antifungal potential. Compounds 4e and 4i strongly inhibited spore germination of <i>Botrytis cinerea</i>, and their IC₅₀ values (2.6 and 2.1 μg/mL, respectively) were significantly lower than those of osthole and difenoconazole. Compounds 2, 4i, 10, and 14 inhibited mycelial growth of <i>Alternaria solani</i> and <i>B. cinerea</i> more effectively than osthole. In vivo tests indicated that 4e outperformed both osthole and hymexazol against <i>A. solani</i>. SAR analysis indicated that introducing an amine moiety enhanced antifungal potency. Both 4i and osthole significantly reduced Chitinase activity in the tested fungi. Cytotoxicity assays revealed low to moderate toxicity toward HaCaT cells, suggesting acceptable mammalian safety. These findings support osthole derivatives as promising fungicidal candidates and inform the design of coumarin-based agrochemicals.