Novel Coumarin Derivatives Containing Oxazole/Isoxazole/Pyrazole Groups as Antifungal Agents: Design, Synthesis, Quantitative Structure–Activity Relationships, and Preliminary Mechanistic Insights

Abstract Plant pathogenic fungi pose a significant threat to global crop productivity and food security. Herein, we report the design and synthesis of 55 coumarin–oxazole, −isoxazole, and −pyrazole hybrids as novel antifungal agents. Most derivatives exhibited notable potency, with C1, C4, and C5 showing strong activity against Rhizoctonia solani (EC50 = 6.4, 5.9, and 8.6 μg/mL), while C2 displayed superior activity against Alternaria solani (EC50 = 4.5 μg/mL). Structure–activity relationship and density functional theory (DFT) analyses revealed that the pyrazole heterocycle and electron-withdrawing substituents were key determinants of potency. Mechanistic studies on C4 demonstrated that it induces morphological damage, mitochondrial dysfunction (reactive oxygen species (ROS) overproduction, mitochondrial membrane potential (MMP) dissipation), cell membrane disruption (propidium iodide (PI) fluorescence, nucleic acid leakage, malondialdehyde (MDA) elevation, increased conductivity), and interferes with ergosterol-associated pathways. These findings establish coumarin–azole hybrids as promising leads for antifungal development.