Abstract
Cantharidin is a natural compound of novel structure with ideal insecticidal activity. However, the relationship of structure to insecticidal activity of cantharidin and its derivatives has not been ever clarified. To explore what determines the insecticidal activity structurally of cantharidin-related compounds, two series target compounds 6 and 7 were synthesized by replacing the anhydride ring of norcantharidin with an aromatic amine or fatty amine with different electron density, respectively. The structures of these compounds were characterized by 1H NMR, 13C NMR and HRMS-ESI. A bioassay showed that compounds 6 (a–m) lacked any larvicidal activity against Plutella xylostella; whereas their ring-opened partners 7 (a–m) provided a variety of larvicidal activities against P. xylostella, and compound 7f indicated the highest larvicidal activity with LC50 value of 0.43 mM. The present work demonstrated that the form of the compound (cyclic or ring-opened) or their ability to hydrolyze facilely was the key to determine whether it exhibits larvicidal activity. Moreover, it revealed that the improvement of insecticidal activity required a reasonable combination of both aliphatic amide and aromatic amide moieties, and the type of substituent Y on the aniline ring was critical.
Highlights
The blister beetle Mylabris phalerata (Coleoptera: Meloidae) has been used for centuries as a traditional Chinese medicine
As shown in Scheme 1, the target analogs 6 were synthesized by the reaction of norcantharidin 3 and various primary amines in the presence of triethylamine as the binding acid agent [15], and the analogues 7 were synthesized by replacing the anhydride ring of norcantharidin 3 with an aromatic amine or fatty amine of different electron density [16]
These results indicated that compounds 2, 4 and 7f displayed comparable larvicidal activity with their corresponding lead compound against P. xylostella at 500 μg mL−1
Summary
The blister beetle Mylabris phalerata (Coleoptera: Meloidae) has been used for centuries as a traditional Chinese medicine. Cantharidin 1 (Figure 1), the active ingredient in the blister beetle toxin, has served as a valuable drug against several kinds of cancer. Among the novel applications of cantharidin and its analogs were preliminary studies of the anti-insect activity of this natural product. Cantharidin derivatives possess different structures and are likely to have either higher or lower insecticidal effects. Our study is designed to explore the relationships between the structures and insecticidal activities of cantharidin analogs. Two series of anhydride-modified cantharidin analogs were designed and synthesized with norcantharidin 3 (Figure 1) as the parent compound. 6b, 6g, 6m, 7a, 7b, 7e and 7f, seven derivatives, were new compounds Their insecticidal activities were evaluated against the pre-third-instar larvae of Plutella xylostella, in vivo
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