Design, Synthesis, and Fungicidal Activity of Novel Thiosemicarbazide Derivatives Containing Piperidine Fragments

In order to discover novel eco-friendly lead compounds for plant pathogenic fungi control, a series of benzaldehyde thiosemicarbazide derivatives with a piperidine moiety have been designed and synthesized. Fungicidal activities of all the synthesized compounds were evaluated in vitro. The results indicated that all the title compounds exhibited moderate to good fungicidal activities. Compound 3b displayed excellent activities against Pythium aphanidermatum, Rhizoctonia solani, Valsa mali, and Gaeu-mannomyces graminsis, with EC(50) values lower than 10 μg/mL. Especially, in the case of Pythium aphanidermatum, its activity (EC(50) = 1.6 μg/mL) is superior to the commercial azoxystrobin (EC(50) = 16.9 μg/mL) and close to fluopicolide (EC(50) = 1.0 μg/mL). Initial structure–activity relationship (SAR) analysis showed that the heterocyclic piperidine group can influence the biological activities of the title compounds significantly. The fungicidal activity of compounds with piperidine is better than that of compounds without piperidine. The highly-active compound 3b, with its simple structure and easy synthetic route, is worthy to be further studied as a new lead fungicide.