Synthesis and Antifungal Activity of Novel Myrtenal Hydrazide, Carboxamide, and Acylthiourea Derivatives as Potential Laccase Inhibitors ^†

To discover novel laccase inhibitors as potential fungicides, three series of myrtenal hydrazide, carboxamide, and acylthiourea derivatives were designed, synthesized, and assayed for their antifungal property. The majority of these target compounds demonstrated significant in vitro antifungal activity toward Monilinia fructicola, Sclerotinia sclerotiorum, Valsa mali, and Botryosphaeria dothidea. Notably, compounds 3d and 3l exhibited potent inhibition to M. fructicola with median effective concentration (EC₅₀) values of 0.307 and 0.465 mg/L, respectively, which were comparable to carbendazim (EC₅₀ = 0.199 mg/L). In vivo experiments conducted on pear fruits indicated that 3d possessed exceptional protective and curative effects. The morphological study through scanning electron microscopy observation showed that 3d caused substantial damage to the mycelial structure of M. fructicola. Additionally, during in vitro laccase inhibition assays, compound 3d displayed remarkable inhibition toward laccase, achieving a half-maximal inhibitory concentration value of 4.86 µM, significantly stronger than the positive control cysteine. These findings suggest that the target myrtenal derivatives hold great promise as lead compounds for the development of laccase-targeting fungicides.