Isolation, characterisation and synthesis of insecticidal natural products of the Myrtaceae family
New insecticidal natural products are required to find compounds with higher intrinsic activities to lower field application rates, and with novel modes of action to combat insect pest species which have developed resistance to current commercial insecticides. Using a taxonomic approach, studies on plants of the Myrtaceae family led to the isolation and characterisation of a range of insecticidal natural products 1 - 9 (figure 1). These compounds are all structurally related as they contain a tetramethylcyclohexenedione group, attached to either a terpene or a phloroglucinol moiety. Seven of the nine compounds (1 - 6, 9) are novel. For compounds 7 and 8, no synthesis had been previously reported. The compounds are active against a range of insect species, although in general they are less active than commercial natural products. Further tests show some of the compounds are potent antifeedants. Synthesis of seven of the nine natural products (1 - 5, 7, 8) by short, convergent, stereospecific and high yielding routes was achieved. The synthetic routes were devised to mimic the postulated biosynthesis of the compounds. Reaction of syncarpic acid, an aldehyde and pyrrollidine formed a Mannich base, which on elimination gave a key alkylidene intennediate. This key intermediate was reacted, with either terpenes in Diels-Alder reactions or with phloroglucinols in aromatic alkylation reactions, to synthesise seven of the natural products.