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.

In the title compound, C 24 H 29 ClO 8 Á2.5H 2 O, which contains two organic molecules (A and B) and five heavily disordered water molecules in the asymmetric unit, the -lactone ring and the cyclohexenone ring are both trans-fused to the central cyclodecene ring. The cyclehexenone ring features an ,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-HÁ Á ÁO 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.

Related literature
In the 1970's, two diterpenoid briaranes, ptilosarcone and ptilosarcenone, were purified from Ptilosarcus gurneyi (Wekell 1974;Wratten et al. 1977;Wekell 1978) and other octocorals have yielded similar compounds (Sung et al. 2002). In the presence of water or alcohol, ptilosarcone eliminates butyric acid, forming ptilosarcenone. Ptilosarcenone has also been found in extracts of Tochuina tetraquetra, a Tritoniid nudibranch that preys upon Ptilosarcus gurneyi (Williams & Andersen, 1987). For the structure of ptilosarcenone determined from a mostly complete room-temperature dataset, see: Hendrickson (1990); Hendrickson & Cardellina (1986). Sea pens of the species Ptilosarcus gurneyi were collected near Juneau, Alaska (Smith, 2006) at depths of 5 to 10 m. For extraction and purification methods used, see: Wekell (1974). For related structures, see: Burks et al. (1977); Coval et al. (1988); Gonzalez et al. (2002); Grode et al. (1983); Hamann et al. (1996); van der Helm et al. (1986). For scientific background, see: Nurco (2008 can be made up of thousands of specialized individuals each providing a specific function. They are also colonial organisms, measuring up to 1 meter in height and tending to live in large groups containing sometimes thousands of individual sea pens. The habit of a sea pen is to anchor into soft substrate of the ocean's bottom by its basal polyp. In the 1970's, two diterpenoid briaranes were purified from Ptilosarcus gurneyi, ptilosarcone and ptilosarcenone (Wekell 1974;Wratten et al. 1977;Wekell 1978). Other octocorals have yielded similar compounds (Sung et al. 2002). In the presence of water or alcohol ptilosarcone eliminates butyric acid, forming ptilosarcenone. Ptilosarcenone has also been found in extracts of Tochuina tetraquetra, a Tritoniid nudibranch that preys upon Ptilosarcus gurneyi (Williams & Andersen 1987). A crystal structure of ptilosarcenone from a room temperature dataset, mostly but not entirely complete, was reported in a dissertation (Hendrickson 1990) and referenced as unpublished data (Hendrickson & Cardellina 1986) but has not appeared in the peer-reviewed literature or the Cambridge Structural Database. Herein, we report a new crystallographic investigation of ptilosarcenone conducted with a low temperature dataset and featuring more favorable calculated results than the previous structure.
These values are similar to the analogous torsion angles found in the above mentioned compounds which were 69.0°, 57.0°, 57.3°, 48.7°, 48.5°, and 70.3° respectively, as ordered above.

Experimental
Sea pens of the species Ptilosarcus gurneyi were collected near Juneau, Alaska (Smith, 2006) at depths of 5 to 10 meters using SCUBA techniques. Ptilosarcenone was purified from them via published procedures (Wekell, 1974). Crystals of ptilosarcenone were grown in a one half-dram vial from a binary solvent system using ethanol as the good solvent and water as the poor solvent.

Refinement
Hydrogen atoms bonded to C atoms were generated by their idealized geometry and refined with a riding model, with C-H distances in the range 0.95 -1.00 Å, and U iso equal to 1.2U eq or 1.5U eq (methyl) of the bonded atom. Hydroxyl H6b and H14b were found on a difference map and refined with a restrained bond length of 0.84 (2) Å. H17a and H17b, bound to water molecule O17, were found in a difference map and refined with a restrained bond length of 0.84 (5) Å. The remaining water oxygen sites were severely disordered, and hydrogen atoms for these O atoms could not be located. For water oxygen atoms O18 through O28, isotropic thermal parameters were fixed at 0.036Å 2 , and their occupancies were restrained to sum to equal four water molecules.
Figures Fig. 1. The molecular structure of both organic molecules in the asymmetric unit (1a and 1 b) drawn with 35% probability thermal ellipsoids.  (4)