Potential allelopathic sesquiterpene lactones from sunflower leaves

Isolation, structure elucidation and allelopathic bioassays of 13 sesquiterpene lactones, four of them new, from the sunflower cultivar VYP(R) are described. Structures of the lactone assigned as 1,2-anhydrido-4,5-dihydroniveusin A, previously isolated from capitate glandular trichomes of Helianthus annuus and of annuithrin are corrected. The effect of a series of aqueous solutions at 10(-4)-10(-9) M of the sesquiterpene lactones was studied on the root and shoot lengths of Lactuca sativa var. nigra, Lepidium sativum, Lycopersicon esculentum, seedlings (dicotyledons) and Hordeum vulgare and Triticum estivum (monocotyledons). The guaianolides generally had no effect on the germination and growth of L. sativum and L. esculentum, except for C-10 epimers 8 beta-angeloyloxycumambranolide and annuolide G where inhibitory effects were found on the shoot length of L. esculentum. Both exhibit similar activity profiles, the most persistenty active compound on dilution was 8 beta-angeloyloxycumambranolide with an alpha-orientated hydroxyl group at C-10. The conformational changes due to functionalization within germacranolides influence principally root and shoot growth. Heliangolides have greater effect on root and shoot length of dicotyledon species, presumably due to conformational flexibility and the presence of electrophylic groups. Copyright (C) 1996 Elsevier Science Ltd