| Summary: | 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 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 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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