The absolute structure of ptilosarcenone 2.5-hydrate, a diterpenoid briarane from the orange sea pen Ptilosarcus gurneyi (Gray).

In the title compound, C24H29ClO8·2.5H2O, which contains two organic mol­ecules (A and B) and five heavily disordered water mol­ecules in the asymmetric unit, the γ-lactone ring and the cyclo­hexenone ring are both trans-fused to the central cyclo­decene ring. The cyclehexenone ring features an α,β-unsaturated ketone with torsion angles between the conjugated carbonyl and alkene bonds of 0.6 (3) and 7.4 (4)° for mol­ecules A and B, respectively. The ptilosarcenone torsion angles between conjugated alkene bonds are 56.2 (5) and 55.4 (6)° for A and B, respectively. In the crystal, the components are linked by O—H⋯O hydrogen bonds. The absolute configuration of ptilosarcenone was determined unambiguously and exhibits similar absolute stereochemistry to that found in the crystal structures of other octocoralline briaranes.