Insecticidal natural products from Sri Lankan plants
This thesis describes investigations of the insecticidal compounds of three Sri Lankan plants, Pleurostylia opposita (Wall) Alston (Celastraceae), Aegle marmelos Correa (Rutaceae) and Excoecaria agallocha Linn. (Euphorbiaceae). After establishing the insecticidal activity of the extracts of three plants, separation of compounds was achieved by bio-assay directed chromatography, and the compounds were characterised by NMR especially 2D experiments, mass, UV and IR spectroscopy. Three new macrocyclic sesquiterpene polyol ester alkaloids 1, 2 and 3 containing a novel 6,7 ring skeleton have been identified from the petroleum ether-ethyl acetate extract of the stem bark of Pleurstylia opposita. They are homologues of known macrocyclic sesquiterpene alkaloids which contains a 6,6 ring system based on the ß-dihydroagarofuran core. The extra methylene has been shown by NMR to be in the 8 position of the 6,7 ring. Observation of insecticidal activity in the Pleurostylia genus is novel. The previously reported 20-hydroxylupane-3- one has also been identified from the active extract and shown to be noninsecticidal. Two new insecticidal compounds 6 and 7 have been identified from the petroleum ether-ethyl acetate extract of the stem bark of Aegle marmelos. They are shown to be protolimonoids, and are senecioate ester analogues of the known isovalerate esters of C-21-ß and C-21-α glabretal which were also isolated. Compounds 8 and 9 were also found to be insecticidal against mustard beetles and houseflies. Separation and characterisation of epimeric mixtures were first achieved for the di p-nitrobenzoate derivative. Previously recorded epoxyaurapten, marmesin, marmin and lupeol have been isolated from the active extract of Aegle marmelos and shown to be noninsecticidal. Five compounds 24-28 have been isolated from the ethyl acetate extract of the stem bark of Excoecaria agallocha. They all have the previously reported daphnane diterpenoid orthoester skeleton, and differ only in the orthoester alkyl side chain. Two of them 24 and 27 are novel. Insecticidal activity has not been recorded previously for extracts of Excoecaria species, nor for the daphnane orthoesters.