Design, synthesis and biological evaluation of imidazo[5,1‐b]thiazole derivatives: antifungal activity and mechanism of action

Abstract BACKGROUND The aggravation of plant diseases, compounded by the problems of fungicide overuse, has created an urgent need for safe and potent antifungal agents. This context has drawn considerable attention to the imidazothiazole scaffold, a structure frequently found in both natural products and synthetic pharmaceuticals. RESULTS A series of novel compounds featuring imidazothiazole‐based heterocyclic frameworks were designed and synthesized. The in vitro antifungal efficacy of these target compounds was assessed against six fungal pathogens. Among them, compounds A5 and E1 displayed the most potent inhibitory activity against Valsa mali , with median effective concentration (EC 50 ) values of 1.31 and 1.34 mg L −1 , respectively, which significantly outperformed the positive control boscalid (EC 50 = 13.91 mg L −1 ). In vivo trials demonstrated that compound A5 showed remarkable protective and curative effects, achieving 53.14% (protective) and 68.44% (curative) at a concentration of 50 mg L −1 , comparable to boscalid (59.16% and 64.63%). Preliminary mechanistic insights were gained through a combination of confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and physiological and biochemical assays. Acute toxicity assessments classified compound A5 as low‐toxic to zebrafish, earthworms, and worker bees. CONCLUSION This study reports that the imidazothiazole derivative A5 exhibits potent in vitro activity against V. mali and in vivo efficacy comparable to a commercial fungicide. Additionally, compound A5 shows low environmental toxicity, positioning it as a promising lead candidate for the development of next‐generation, low‐risk fungicides. © 2026 Society of Chemical Industry.