STRUCTURE OF THE NATURALLY-OCCURRING SESQUITERPENE LACTONE 8-EPIISOLIPIDIOL

Decahydro-4,8-dihydroxy-3,9-dimethyl-6-methyleneazuleno[4,5-b]furan-2(3H)-one, C15H21O4, M(r) = 265.3, orthorhombic, P2(1)2(1)2(1), a = 5.755 (1), b = 13.891 (2), c = 17.511 (2) angstrom, V = 1399.9 (4) angstrom 3, Z = 4, D(x) = 1.25 Mg m-3, lambda-(Cu K-alpha) = 1.54178 angstrom, mu = 0.65 mm-1, F(000) = 572, T = 295 K, R = 0.055, wR = 0.070 for 1685 observed reflections. The three-ring guaianolide system contains an exo-methylene-cycloheptane moiety cis-fused with the cyclopentane [at C(1)-C(5)] and trans-annelated with the alpha-methyl-gamma-lactone [at C(6)-C(7)]. The two five-membered rings are in envelope conformations while the seven-membered ring is about midway between chair and twist-chair forms. The secondary hydroxyl groups are at C(3) and C(8), and are both beta-orientated; they participate in intermolecular hydrogen bonds, O(8) both as a donor and an acceptor, and O(3) as a donor only.