Structure, conformation and biological activity studies on rivularin, a new heliangolide isolated from Leptocarpha rivularis

Rivularin, a new heliangolide, was isolated together with leptocarpin as a minor constituent from Leptocarpha rivularis. Structural and conformational studies of this compound were based on elemental analysis. IR spectroscopy, ¹³C-NMR and ₁H-NMR experiments (LIS, NOESY and Py-d₅ spectra). Of the four most probable conformations for rivularin, the preferred one is the TC conformation. The ¹H-NMR data were compared with those obtained from the synthetic β-epimer 11,13-dihydroleptocarpin, prepared by either catalytic hydrogenation or sodium borohydride reduction of leptocarpin, in order to establish the correct structure and conformation in the heliangolide.