To improve the insecticidal activity
of (+)-nootkatone, a series
of 42 (+)-nootkatone thioethers containing 1,3,4-oxadiazole/thiadiazole
moieties were prepared to evaluate their insecticidal activities against Mythimna separata Walker, Myzus persicae Sulzer, and Plutella xylostella Linnaeus.
Insecticidal evaluation revealed that most of the title derivatives
exhibited more potent insecticidal activities than the precursor (+)-nootkatone
after the introduction of 1,3,4-oxadiazole/thiadiazole on (+)-nootkatone.
Among all of the (+)-nootkatone derivatives, compound 8c (1 mg/mL) exhibited the best growth inhibitory (GI) activity against M. separata with a final corrected mortality rate
(CMR) of 71.4%, which was 1.54- and 1.43-fold that of (+)-nootkatone
and toosendanin, respectively; 8c also displayed the
most potent aphicidal activity against M. persicae with an LD50 value of 0.030 μg/larvae, which was
closer to that of the commercial insecticidal etoxazole (0.026 μg/larvae);
and 8s showed the best larvicidal activity against P. xylostella with an LC50 value of 0.27
mg/mL, which was 3.37-fold that of toosendanin and slightly higher
than that of etoxazole (0.28 mg/mL). Furthermore, the control efficacy
of 8s against P. xylostella in the pot experiments under greenhouse conditions was better than
that of etoxazole. Structure–activity relationships (SARs)
revealed that in most cases, the introduction of 1,3,4-oxadiazole/thiadiazole
containing halophenyl groups at the C-13 position of (+)-nootkatone
could obtain more active derivatives against M. separata, M. persicae, and P. xylostella than those containing other groups.
In addition, toxicity assays indicated that these (+)-nootkatone derivatives
had good selectivity to insects over nontarget organisms (normal mammalian
NRK-52E cells and C. idella and N. denticulata fries) with relatively low toxicity.
Therefore, the above results indicate that these (+)-nootkatone derivatives
could be further explored as new lead compounds for the development
of potential eco-friendly pesticides.