Isolation, characterisation and some synthesis studies on insecticidal natural products from Sri Lankan plants

Hemalal, Kirihettiliyanage Patrick (1998). Isolation, characterisation and some synthesis studies on insecticidal natural products from Sri Lankan plants. PhD thesis The Open University.



Four Sri Lankan plant species, Acronychia pedunculata (L.) Miq (Rutaceae), Pleurostylia opposita (Wall) Alston (Celastraceae), Alseodaphne semicarpifolia (Lauraceae) and Walsura piscidia Roxb. (Meliaceae) were investigated for insecticidal activity.

Acrovestone (1), was identified as the major insecticidal principle from A. pedunculata. New reactions conditions were developed for its synthesis from isovaleraldehyde and two key intermediates, acronylin and demethylacronylin. The latter intermediate was synthesised by optimisation of a previously reported method. This is the first report of the synthesis of acrovestone and recognition of its insecticidal activity.

A structure-activity relationship programme, based on synthetic analogues incorporating small but systematic structural changes, revealed that the number, position and size of substituents were all critical for activity. Optimum activity was associated with the presence of a single methoxy group and that only the central region was amenable to variation leading to increased activity. Recognition of the structural similarities between acrovestone and another class of insecticidal products, the kunzeins, prompted an extension of the SAR study. A series of novel ‘hybrid’ compounds incorporating structural features from both classes was synthesised and tested. In contrast to the conclusions from the systematic SAR study, this part of the study indicated that substantial variations could be tolerated in one segment of the acrovestone molecule leading to analogues with higher levels of activity than the individual parent compounds. Overall this study has demonstrated the advantage of combining a conventional SAR approach based on small but systematic structural changes with one using larger changes through synthesis of hybrid compounds from related natural products. Limited evaluation against other species indicated variation in the spectrum of activity with chemical structure.

Previously reported insecticidal compounds of P. opposita were re-isolated and structures reassigned by means of chemical degradation and spectroscopic methods. They were identified as sesquiterpene polyol ester alkaloids, compounds 2, 3 and 4 containing a previously unknown combination of 6 ,6 β-dihydroagarofuran core with an alkaloidal firagment.

Sesamin (5) was isolated as the insecticidal component from A. semicarpifolia. This is the first report of its isolation from the genus Alseodaphne.

7-Deacetyl-7α-hydroxyazadirone (6) was isolated as the insecticidal component from W. piscidia. This is the first report of its insecticidal activity and its isolation from the genus Walsura.

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