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

Daniel J Nurco; Douglas E Conklin; Nathan S Shapiro; Elaine Tran

doi:10.1107/S1600536810050142

. 2010 Dec 18;67(Pt 1):o181–o182. doi: 10.1107/S1600536810050142

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

Daniel J Nurco ^a,^*, Douglas E Conklin ^a, Nathan S Shapiro ^a, Elaine Tran ^a

PMCID: PMC3050379 PMID: 21522687

Abstract

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.

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 ▶).

Experimental

Crystal data

2C₂₄H₂₉ClO₈·5H₂O
M _r = 525.98
Orthorhombic,
a = 9.8505 (4) Å
b = 13.5256 (6) Å
c = 39.3169 (17) Å
V = 5238.3 (4) Å³
Z = 8
Mo Kα radiation
μ = 0.20 mm⁻¹
T = 95 K
0.60 × 0.32 × 0.14 mm

Data collection

Bruker SMART APEXII diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ▶) T _min = 0.888, T _max = 0.972
75112 measured reflections
13416 independent reflections
12944 reflections with I > 2σ(I)
R _int = 0.031

Refinement

R[F ² > 2σ(F ²)] = 0.069
wR(F ²) = 0.172
S = 1.22
13416 reflections
673 parameters
12 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.92 e Å⁻³
Δρ_min = −0.60 e Å⁻³
Absolute structure: Flack (1983 ▶), 5967 Friedel pairs
Flack parameter: 0.07 (7)

Data collection: APEX2 (Bruker, 2007 ▶); cell refinement: SAINT (Bruker, 2007 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: SHELXTL (Sheldrick, 2008 ▶); software used to prepare material for publication: SHELXTL.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536810050142/hb5711sup1.cif

e-67-0o181-sup1.cif^{(40.1KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810050142/hb5711Isup2.hkl

e-67-0o181-Isup2.hkl^{(655.9KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O6—H6B⋯O13	0.83 (2)	2.02 (3)	2.784 (4)	154 (6)
O14—H14B⋯O8ⁱ	0.85 (2)	2.13 (3)	2.932 (4)	156 (6)
O17—H17A⋯O3ⁱⁱ	0.81 (4)	2.04 (5)	2.834 (4)	167 (8)
O17—H17B⋯O16ⁱⁱⁱ	0.80 (4)	2.31 (5)	3.082 (5)	161 (7)

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) .

Acknowledgments

We thank Annette G. E. Smith for collecting Ptilosarcus gurneyi sea pens using SCUBA techniques near Juneau, Alaska, and the Alaska Department of Fish and Game for providing a Scientific Collecting Permit allowing the collection. This work was supported in part by a UC Davis Jastro–Shields Grant.

supplementary crystallographic information

Comment

In subtidal waters along the west coast of North America lives the orange sea pen Ptilosarcus gurneyi (Gray). Sea pens are soft-bodied octocorals, cnidarians with 8-tentacled polyps, in the order pennatulacea. Like many acidacians, a single sea pen 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.

The structure (Figure 1) has two ptilosarcenone molecules and five disordered water molecules in the asymmetric unit. The γ-lactone and cyclohexenone rings are both trans-fused to the central cyclodecene ring. The cyclehexenone ring featured an α,β-unsaturated ketone with torsion angles between the conjugated carbonyl and alkene bonds of 0.6 (3)° and 7.4 (4)° for C11=C12—C13=O1 and C35=C36—C37=O9 in 1a and 1 b, respectively. The absolute configuration of ptilosarcenone was unambiguously determined with a Flack parameter of 0.06 (7) and revealed the following stereochemical assignments for 1a: C1(S), C2(S), cis-C3=C4, C6(S), C7(R), C8(R), C9(S), C10(S), cis-C11=C12, C14(R), C21(R), and 1 b: C25(S), C26(S), cis-C27=C28, C30(S), C31(R), C32(R), C33(S), C34(S), cis-C35=C36, C38(R), C45(R). The characterization of ptilosarcenone revealed similar absolute stereochemistry as found in structures of similar briaranes. Specifically, the absolute configuration at C6 (bearing a chlorine) and C8 (bearing a hydroxyl) was the same for ptilosarcenone as for briarein A (Burks, et al., 1977), briantheins V (Coval, et al., 1988), X (van der Helm et al., 1986), and Y (Grode, et al., 1983), 11-hydroxybrainthein (Gonzalez, et al., 2002), and juncin E (Hamann, et al., 1996). The ptilosarcenone C3=C4—C5=C19 and C27=C28—C29=C43 torsion angles between conjugated alkene bonds were 56.2 (5)° and 55.4 (6)° for 1a and 1 b. 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.