1Centro de Ciencias Medioambientales, CSIC, Madrid, Spain.
2Instituto de Productos Naturales y Agrobiología, CSIC, La Laguna, Spain.
As part of our ongoing search for bioactive natural products from endemic Compositae
species native to the Canary Islands, we have selected the plant Senecio heritieri
based on a previous bioactivity screening. A bioassay-guided fractionation of an extract of
this plant afforded the pentacyclic triterpene lupeol (1) as the major
component of a bioactive fraction with moderate feeding deterrency against Leptinotarsa
decemlineata and strong growth-disrupting effects on the lepidopteran Spodoptera
littoralis. To study the structure-activity relationships of 1, the
acetylated derivative 2 was prepared, and its biological activities compared
with 1 and the structurally-related betuline (3), its
acetylated derivative (4), and betulinic acid acetate (5).
A study of their antifeedant effects showed that 2 was the most active
antifeedant to L. decemlineata, followed by 1,
3 and 5, with 4 being inactive.
None of these compounds was toxic to this insect. Some were, however, toxic to the
lepidopteran (1>3
»5) reducing
larval consumption and growth when orally injected. Thus, the presence of an OAc group
in C-3 reduced postingestive toxicity (2 and 3)
except for 5, with a CO