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

The asymmetric unit of the title compound, C(17)H(22)O(5)·0.25H(2)O [systematic name: 2-hy­droxy-2,2a,6,9a-tetra­methyl-2a,4a,5,6,6a,9a,9b,9c-octa­hydro-2H-1,4-dioxadi­cyclo­pent[cd,f]azulene-3,9-dione 0.25-hydrate], a natural product isolated from Helenium amarum, contains two independent tenulin mol­ecules and half a water mol­ecule of crystallization situated on a twofold rotation axis. The hy­droxy group of the hemiketal moiety is in a β-position. In the crystal, each water mol­ecule inter­acts with four tenulin mol­ecules via O—H⋯O hydrogen bonds. The two independent tenulin mol­ecules (A and B) differ only in the character of their participation in hydrogen bonding. Specifically, while A is an acceptor of O(water)—H⋯O(A) and a donor of O(A)—H⋯O(B) hydrogen bonds, mol­ecule B is an acceptor of the latter hydrogen bond and the donor of an O(B)—H⋯O(water) hydrogen bond. In the crystal, these O—H⋯O hydrogen bonds link the tenulin and water mol­ecules into layers parallel to the ac plane.