Investigating the Diels-Alder reactivity of the natural pyrethrins

Pyrethrum-based insecticides have remained ideal for domestic and agricultural pest control due to the efficacy and environmentally favourable qualities of the active ingredients, the natural Pyrethrins. However, the propensity of the natural Pyrethrins to degrade with heat, light and oxygen affects their long-term storage and subsequent effectiveness as an insecticide. Whilst a number of processing and storage countermeasures have been implemented, efforts have recently been made to synthetically modify the pyrethrin scaffold with particular focus on functionality that is sensitive to these degradative processes. This work describes an investigation of the Diels-Alder reactivity of pyrethrin I and II and its potential to produce stabilised pyrethrin derivatives. Initial exploration into normal electron demand Diels-Alder cycloadditions of pyrethrin I and II found that the reactions did not proceed even under forcing conditions. However, a series of pyrethrin derivatives were obtained from the inverse electron demand Diels Alder reaction employing these natural products as dienophiles and electron-deficient 1,2,4,5-tetrazines as dienes.