Pseudoguaianolides : a stereoselective approach to 8(α)-oxygenated helenanolides : X-ray structural determination of a synthetic intermediate

The synthesis of perhydroazulenic lactone, a potential precursor for several helenanolides is described. Its structure is determined by X-ray diffraction of the corresponding alchohol

(±)-Decahydro-9-hydroxy-9-methylspiro[azuleno[4,5-b]furan-6(2H),2'-[1, 3]dioxolan]-2-one (3aα, 6aα, 9α, 9aα, 9bβ)

C15H22O5, monoclinic, P21/n, a = 14.549 (7), b: 10.618 (4), c = 9.007 (5) Å, β = 90.38 (5)°, V = 1391.4 (1.1) Å3, Dx = 1.35 Mg m-3 for Z = 4. The structure has been solved by direct methods (MULTAN) and refined to a final R value of 0.055 for 1476 observed reflections. The stereochemistry of the title compound is as depicted in (1) for one enantiomer ; the relative configuration of the asymmetric carbons can be specified as C(1)R, C(4)R, C(5)S, C(5)S, C(7)S. The lactone ring adopts a conformation intermediate between an envelope conformation with C(7) at the flap and a twist form with pseudo-diad axis through C(12). The cyclopentane and seven-membered rings both possess Cs(m) symmetry with the pseudo-mirror plane bisecting the C(1)-C(5) fusion bond