1Centro de Ciencias Medioambientales, CSIC, Madrid, Spain.
2Instituto de Productos Naturales y Agrobiología, CSIC , La Laguna, Spain.
3UDI Fitopatología, Facultad de Biología, Universidad de La Laguna, Tenerife, Spain.
As part of our ongoing search for bioactive natural products from endemic Boraginaceae species native to the Canary Islands, we have selected the plant Echium wildpretti based on a previous bioactivity screening. A bioassay-guided fractionation of an extract of this plant afforded the pentacyclic triterpene glutinol (1) as the major component of a bioactive fraction with strong feeding deterrency against Leptinotarsa decemlineata. To study the structure-activity relationships of 1, two derivatives were prepared (2 and 3) and their biological activities compared with the parent compound 1 and the natural triterpenes beta-amirine (4), uvaol (5), its acetylated derivatives 6 and 7, anagadiol (8) and germanicol (9). A study of their antifeedant effects showed that 1 was the most active antifeedant, followed by 4, 5 and 2. None of these compounds was toxic to this insect. Thus, an -OH (1, 4 and 5) or ketone substituent (5) in C-3 determined their antifeedant effects except in the presence of an unsaturated D-ring (8 and 9). None of these compounds deterred the lepidopteran Spodoptera littoralis. Some were however toxic to this insect (5>2>3> 4) reducing its larval consumption and growth when orally injected. We did not observe any correlation between deterrency and toxicity for these compounds.