X-Ray structures of hymenoxon and hymenolane : pseudoguaianolides isolated from Hymenoxys odorata DC. (Bitterweed)

Abstract

The structures-of two sesquiterpene lactones isolated from Hymenoxys Odorata DC. have been determined by X-ray methods. In hymenoxon (1) the tricyclic skeleton consists of a six- and a five-membered ring fused on opposite sides of a seven-membered ring, which itself has a distorted twisted-boat conformation. The stereochemistry of hydroxy-groups attached to the six-membered ring is established. In hymenolane (2) the tricyclic skeleton consists of two five-membered rings fused on opposite sides of a seven-membered ring, again in a distorted twisted-boat conformation. The stereochemistry of three acetate groups attached to the carbocyclic five-membered ring is established. Both compounds crystallize in space group P2₁2₁2₁, with Z= 4 in unit cells of dimensions: (1)a= 7.500(2), b= 8.011 (4), c= 24.549(5)Å: (2)a= 8.963(3), b= 9.265(4), c= 26.292(11)Å. Structures were solved from diffractometer data by direct methods and refined by block-diagonal least squares to R 0.035 [(1), 902 reflections] and 0.043 [(2), 1 509 reflections].