Helikauranoside a, a new bioactive diterpene.

A new ent-kaurane glucoside, named helikauranoside A (4), was isolated from the aerial parts of Helianthus annuus L. together with three known ent-kaurane-type diterpenoids: (-)-kaur-16-en-19-oic acid (1), grandifloric acid (2), and paniculoside IV (3). The structure of 4 was determined by using a combination of 1D (1H-NMR and 13C-NMR) and 2D (COSY, HSQC, and HMBC) NMR techniques. Bioactivity spectra of isolated compounds were tested by using the etiolated wheat coleoptile bioassay in aqueous solutions at concentrations ranging from 10(-3) to 10(-6)M. Helikauranoside A (4) was the most active (-84%, 10(-3)M; -56%, 10(-4)M). These results suggest that this new compound may be involved in defense mechanisms of H. annuus.