The rational design and mechanism study of heterocyclic thioester fungicides based on the bioisosterism strategy

Aiming to develop highly efficient and environmentally friendly fungicides, this study designed and synthesized a series of heteroaromatic thioester compounds (C=O → C=S) using a bioisosterism strategy. In vitro antifungal assays revealed that most compounds exhibited enhanced antifungal activity to varying degrees after thionation. Among them, compound Bs6 showed the most potent inhibitory activity against Sclerotinia sclerotiorum (EC₅₀ = 0.283 mg/L), which was significantly superior to the commercial fungicide boscalid (EC₅₀ = 1.060 mg/L). In vivo experiments further validated its efficacy, as Bs6 markedly reduced lesion expansion on rape leaves. Structure-activity relationship analysis, supported by frontier molecular orbital and molecular electrostatic potential studies, indicated that the C=O → C=S substitution enhances electron delocalization and concentrates the negative electrostatic potential region. Meanwhile, the increased lipophilicity (clogP = 3.177) and reduced topological polar surface area (TPSA = 9.23) facilitate membrane permeability and intracellular accumulation, thereby explaining the superior activity of Bs6. Mechanistic investigations demonstrated that Bs6 disrupts the plasma membrane integrity of Sclerotinia sclerotiorum (S. sclerotiorum), leading to electrolyte leakage and mycelial damage. Furthermore, predictions from the ADMETlab 3.0 platform showed that compound Bs6 (C=S) has LC₅₀ values of 4.971 mg/L and 4.629 mg/L against Daphnia magna and fathead minnow, respectively, which are increased compared with those of the corresponding carbonyl analogue Bo6 (C=O), suggesting that thionation may improve the biocompatibility of the compounds. This study demonstrates that thioester modification is an effective strategy for optimizing ester fungicides and provides promising lead compounds for crop disease control.