STRUCTURE OF HYMENORATIN

Hymenoratin, (3a-alpha,4a-beta,6-alpha,7a-alpha,8-alpha,9a-alpha)-decahydro-5,6-dihydroxy-4a,8-dimethyl-3-methyleneazuleno[6,5-b]furan-2(3H)-one, C15H22O4, M(r) = 266.34, monoclinic, P2(1), a = 13.525 (7), b = 7.549 (4), c = 13.646 (7) angstrom, beta = 90.02 (4)-degrees, V = 1393.2 (11) angstrom 3, Z = 4, D(x) = 1.270 g cm-3, lambda(Mo K-alpha) = 0.70930 angstrom, mu = 0.849 cm-1, F(000) = 576, T = 225 K, R = 0.046 for 2838 unique observed reflections, I greater-than-or-equal-to 3-sigma(I). The sesquiterpene lactone has its seven-membered ring cis fused to the alpha-methylene-gamma-lactone ring. Conformational detail of the two crystallographically independent molecules differs only slightly. The cycloheptane ring conformation is a slightly twisted boat; DELTA-C2 values are 4.5 and 5.1. Individual torsion angles in the cycloheptane ring agree within +/- 8-degrees. The alpha-methylene-gamma-lactone ring adopts an envelope conformation with C(7) at the flap of the unprimed molecule, but a flatter conformation with C(8) out-of-plane in the primed molecule. All hydroxyl donors participate in the hydrogen-bonding scheme, but not all possible acceptors. A hydrogen bond between the two molecules of the asymmetric unit is found [O3'...O4, 2.688 (3) angstrom]. Symmetry related molecules are linked [O4'...O3', 2.684 (2) angstrom] and molecules translated along the b axis are linked [O3...O4', 2.894 (3) angstrom; O4'...O4, 2.751 (3) angstrom]. One hydroxyl thus acts as a double acceptor (O4) and one (O3) does not accept and hydrogen bonds. Likewise, the lactone carbonyl is not hydrogen bonded. An additional close intramolecular contact [O3'...O4', 3.070 (3) angstrom] is present. It is the disparate hydrogen-bonding interactions in which each molecule participates which principally distinguishes one from the other.