Coumarin-Based Antifungal Leads: Design, Synthesis, Mechanistic Insights, and Synergistic Activity

To address the challenge of fungicide resistance in plant pathogenic fungi, coumarin derivatives were designed. Two series of coumarin derivatives, including 3-amide and thienocoumarin ether derivatives, were designed and synthesized through structural modification of the coumarin scaffold to explore their antifungal potential. Their antifungal activities were evaluated using the mycelial growth rate method, with half-maximal effective concentration (EC₅₀) values determined. The effects of compound IVd on membrane integrity and morphology were assessed by measuring electrolyte leakage, malondialdehyde (MDA) content, and scanning electron microscopy (SEM). Synergistic effects were quantified using Wadley's method by calculating the synergistic ratio (SR). Molecular docking studies were conducted to investigate interactions with potential targets (CYP51, CYP51B, and SDH). Among the derivatives, IVd showed the most potent antifungal activity, with EC₅₀ values of 88.5 µg/mL against Valsa mali, slightly surpassing azoxystrobin (EC₅₀ = 92.8 µg/mL). Treatment with IVd caused membrane disruption and hyphal deformation. A 2:1 combination ratio of IVd with azoxystrobin or fluconazole demonstrated synergistic effects (SR > 1.5). Molecular docking suggested favorable interactions with the investigated targets, supporting its multitarget activity. Overall, these findings highlight IVd as a promising antifungal scaffold for investigation and provide evidence for its potential use in combination strategies against plant pathogenic fungi.