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

In the title compound, C(24)H(29)ClO(8)·2.5H(2)O, which contains two organic mol-ecules (A and B) and five heavily disordered water mol-ecules in the asymmetric unit, the γ-lactone ring and the cyclo-hexenone ring are both trans-fused to the central cyclo-decene ring. The cyclehexenone ring features an α,β-unsaturated ketone with torsion angles between the conjugated carbonyl and alkene bonds of 0.