Design, Synthesis, and Structure–Activity Relationship of Novel Spiropyrimidinamines as Fungicides against Pseudoperonospora cubensis

Harmful fungus and the developed resistance to available fungicides seriously threaten the yield and quality of crops; thus, the search for new, highly efficient, and resistance-overcoming fungicides remains a quite urgent goal of agricultural scientists. In this study, a series of novel spiropyrimidinamine derivatives were designed and synthesized by employing the intermediate derivatization method (IDM). Their structures were identified by ¹H NMR, elemental analyses, and MS spectra. The structure of compound 5 was further confirmed by X-ray diffraction. Bioassays indicated that a number of the title compounds exhibited some fungicidal activities against Pseudoperonospora cubensis. Especially, compound 5 displayed excellent activity (EC₅₀ = 0.422 mg/L), significantly higher than those of the commercialized fungicides cyazofamid, flumorph, and diflumetorim. The structure-activity relationship was also discussed. It was concluded that compound 5 with super fungicidal potency and a novel structure is a promising agrochemical fungicide candidate for further development.