C-5-substituted antifeedant silphinene sesquiterpenes from Senecio palmensis.

The new sesquiterpenes, 5alpha-senecioyloxysilphinen-3-one (5), 5alpha-tigloyloxysilphinen-3-one (7), and 3beta-hydroxy-5alpha-angeloyloxysilphinene (8), and the known compounds (6S)-2,10-bisaboladien-1-one (1), 6,7-epoxy-3(15)-caryophyllene (2), 6,7-epoxy-2,9-humuladiene (3), 5alpha-angeloyloxysilphinen-3-one (4), and 5alpha-acetoxysilphinen-3-one (6) were isolated from bioactive fractions of Senecio palmensis. The structures of these compounds were established by spectroscopic analysis and chemical evidence. The semisynthetic analogues silphinen-3,5-dione (9), 5alpha-hydroxysilphinen-3-one (10), 5beta-hydroxysilphinen-3-one (11), 5beta-acetoxysilphinen-3-one (12), and 5beta-isobutyryloxysilphinen-3-one (13) were generated to carry out a structure-activity study on the antifeedant action of these molecules against several divergent insect species.