Methyl 2-(3a,8a-dimethyl-4-oxodeca-hydro-azulen-6-yl)acrylate.

The title compound, C(16)H(24)O(3), was synthesized from ilicic acid, which was isolated from the aerial part of Inula viscosa- (L) Aiton [or Dittrichia viscosa- (L) Greuter]. The asymmetric unit contains two independent mol-ecules, in each of which the seven-membered ring shows a chair conformation, whereas the five-membered ring presents disorder. In the two molecules, three C atoms in the five-membered ring are disordered over two positions with site-occupancy factors of 0.53/0.47 and 0.83/0.17. The dihedral angle between the two rings is different in the two mol-ecules [31.7 (3) and 47.7 (7)°]. The crystal structure is stabilized by weak inter-molecular C-H⋯O hydrogen-bond inter-actions.