Synthesis of thymyl esters and their insecticidal activity against Spodoptera litura (Lepidoptera: Noctuidae)

Thymol, a natural phenolic monoterpene originating from Thymus vulgaris, is recognized as a safe and potent botanical insecticide to many insects. The structural modification of thymol into thymyl esters is a potential approach for the development of novel insecticides, which showed more toxicity than thymol. However, there are no reports on the insecticidal activity of thymyl esters to control Spodoptera litura.

Thymol was structurally modified into ten thymyl esters by esterification using a new reagent, PPh₃ /Br₃ CSO₂ Ph. The insecticidal activity of these compounds was examined against the second instars of Spodoptera litura using a topical application. Among the ten thymyl esters evaluated, thymyl cinnamate was the most toxic with LD₅₀  = 0.41 and 0.34 μg/larva after 24 and 48 h posttreatment, respectively. In addition, thymyl cinnamate-treated larvae showed increasing carboxylesterase and acetylcholinesterase activities in vivo experiment, whereas glutathione S-transferase activity showed no significant difference.

Thymyl cinnamate were first reported to exhibit toxicity against S. litura 2.41-2.46 fold more efficient than thymol. However, the detailed biochemical interactions are necessary for further development of novel insecticides. © 2019 Society of Chemical Industry.