Tenulin 0.25-hydrate, a sesquiterpene lactone isolated from Helenium amarum

The asymmetric unit of the title compound, C17H22O5·0.25H2O [systematic name: 2-hy-droxy-2,2a,6,9a-tetra-methyl-2a,4a,5,6,6a,9a,9b,9c-octa-hydro-2H-1,4-dioxadi-cyclo-pent[cd,f]azulene-3,9-dione 0.25-hydrate], a natural product isolated from Helenium amarum, contains two independent tenulin mol-ecules and half a water mol-ecule of crystallization situated on a twofold rotation axis. The hy-droxy group of the hemiketal moiety is in a β-position. In the crystal, each water mol-ecule inter-acts with four tenulin mol-ecules via O-H⋯O hydrogen bonds. The two independent tenulin mol-ecules (A and B) differ only in the character of their participation in hydrogen bonding. Specifically, while A is an acceptor of Owater-H⋯O A and a donor of O A -H⋯O B hydrogen bonds, mol-ecule B is an acceptor of the latter hydrogen bond and the donor of an O B -H⋯Owater hydrogen bond. In the crystal, these O-H⋯O hydrogen bonds link the tenulin and water mol-ecules into layers parallel to the ac plane.