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Acta Cryst. (2013). E69, o1237-o1238  [ doi:10.1107/S1600536813018369 ]

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

K. S. Knight, C. T. Smith, T. G. Waddell and B. Noll

Abstract: The asymmetric unit of the title compound, C17H22O5·0.25H2O [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 [beta]-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 Owater-H...OA and a donor of OA-H...OB hydrogen bonds, molecule B is an acceptor of the latter hydrogen bond and the donor of an OB-H...Owater hydrogen bond. In the crystal, these O-H...O hydrogen bonds link the tenulin and water molecules into layers parallel to the ac plane.

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