The Search for New Insecticidal and Fungicidal Compounds from Plants

Abstract

For several years, our research group at Sumitomo has been conducting an extensive search for new biologically active natural products. Some of the isolated natural products could be looked upon as prototype models for synthetic research to develop new agrochemicals. In this paper, the followings are described as examples of our research: (i) The extract of black pepper fruits (Piper nigrum L.) exhibited strong insecticidal activity against several pests. A new amide, N-isobutyl-1, 1-(3, 4-methylenedioxyphenyl)-2E, 4E, 10E-2, 4, 10-undecatrienamide (pipercide), as well as two structurally related amides, were isolated as insecticidal principles. From the results of synthetic studies, N-isobutyl-12-(3-trifluoromethylphenoxy)-2E, 4E-2, 4-dodecadienamide was found to have especially potent activity. This amide, as well as the amides from pepper plants, exhibited notable paralyzing effects and lethal activity against pyrethroid-resistant houseflies. Electro-physiological studies using the central nerve cord of the American cockroach demonstrated that these amides are neurotoxic compounds. (ii) The chloroform extract of the stems and leaves of May apple (Podophyllum peltatum L.) showed fungicidal activity against rice blast. Three new 2-pyrones (podoblastin A, B and C) were isolated as fungicidal components and their structures were determined by synthesis. Fungicidal activities against rice blast were greatly improved by structural modifications, and 3-(2-oxodecanyl)-4-hydroxy-6-n-propyl-5, 6-dihydropyran-2-one was selected as the most potent compound. The preventive effect of this compound against rice blast was most successful, however the curative and systemic effects require further improvement. (iii) A biphenyl (3, 5-dimethoxy-4-hydroxybiphenyl, aucuparin), as well as a new dibenzofuran (2, 4-dimethoxy-3-hydroxydibenzofuran, eriobofuran), were isolated as phytoalexins from fungi-infected loquat trees (Eriobotrya japonica L.). The structure of eriobofuran was determined by synthesis. Eriobofuran exhibited fungicidal activity against Pestalotia funerea (phytopathogenic fungi to loquat), however it did not show any activity against several fungi which are non-pathogenic to loquat. The biological significance of the self-induction of the phytoalexins in the loquat tree was considered in relation to plant resistance against fungal attack.