Structure, Conformation and Absolute Configuration of New Antifeedant Dolabellanes from Trichilia trifolia

The structures of three new dolabellane diterpenoids, (1R,3E,7Z,11S,12S)-dolabella-3,7,18-trien-17-oic acid (1), (1R,3E,6R,7Z,11S,12S)-dolabella-3,7,18-trien-6,17-olide (2) and (1R,3S,4R,7Z,11S,12S)-3-hydroxydolabella-7,18-dien-4,17-olide (3), isolated from the wood of Trichilia trifolia, were elucidated by 1D and 2D NMR spectroscopy. The stereochemistry of 1 and 2 was confirmed by X-ray diffraction analysis, while that of 3 was ascertained from NOESY data. Comparison between experimental and calculated 1H–1H vicinal coupling constants and the analysis of molecular mechanics structures revealed that the 11-membered ring of 1 and 2 exists in a conformational equilibrium in solution, while in 3 this ring possesses a more rigid structure. The absolute configuration of 3 was established from its Cotton effects. Dolabellanes 1–3 caused significant feeding reduction by the rice weevil Sitophilus oryzae.