ent-5α,3,15-Dioxodolabr-4(18)-ene-16,18-diol

The title compound, C20H30O4, is a dolabrane diterpenoid isolated from Ceriops tagal, in which one of the three fused cyclohexane rings adopts a half-chair conformation and the other two are in the standard chair conformations. The hydroxymethylidene substituent is attached to the half-chair cyclohexane. An intramolecular O—H⋯O hydrogen bond generate an S(6) ring motif. In the crystal, molecules are arranged into screw chains along the [001] direction. The crystal is stabilized by O—H⋯O hydrogen bonds and weaker C—H⋯O interactions.

The title compound, C 20 H 30 O 4 , is a dolabrane diterpenoid isolated from Ceriops tagal, in which one of the three fused cyclohexane rings adopts a half-chair conformation and the other two are in the standard chair conformations. The hydroxymethylidene substituent is attached to the half-chair cyclohexane. An intramolecular O-HÁ Á ÁO hydrogen bond generate an S(6) ring motif. In the crystal, molecules are arranged into screw chains along the [001] direction. The crystal is stabilized by O-HÁ Á ÁO hydrogen bonds and weaker C-HÁ Á ÁO interactions.
Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009). During the course of our studies on the chemical constituents and bioactive compounds from Thai medicinal plants, the title dolabrane diterpenoid compound (I), which is known as Tagalsin S (Hu et al., 2010), was isolated from the the stem barks of C. tagal. We have also previously reported the crystal structures of two diterpenoid compounds isolated from the same plant (Chantrapromma et al., 2007;Fun et al., 2006). We herein report the crystal structure of (I).
The molecule of the title compound contains a fused three-ring system A/B/C (Fig. 1). The A/B ring junction is cisfused and B/C is trans-fused. The cyclohexane ring A adopts half-chair conformation with puckering parameters Q = 0.539 (3) Å, θ = 111.0 (3)° and φ = 92.5 (4)°, rings B and C are in standard chair conformations (Cremer & Pople 1975).
In the crystal structure ( Fig. 2), the molecules are arranged into screw chains along the [0 0 1] direction and the adjacent chains are further linked by weak C-H···O interactions ( Table 1). The crystal packing of (I) is stabilized by intermolecular O-H···O hydrogen bonds and weak C-H···O interactions ( Fig. 2 and Table 1).

Experimental
The air-dried and crushed stem barks of C. tagal (4.8 kg) were extracted with methylene chloride and then concentrated in vacuo to give a residue (17.4 g). This residue was subjected to quick column chromatography over silica gel using solvents of increasing polarity from hexane through 50% acetone/hexane. The eluates were collected and combined, based on TLC, to give 20 fractions (F1-F20). Fraction F14 was further purified by repeated quick column chromatography with CH 2 Cl 2 /acetone (9:1 v/v) yielding title compound (30.4 mg). Colorless block-shaped single crystals of the title compound suitable for x-ray structure determination were recrystallized from hexane/CH 2 Cl 2 (1:1, v/v) after several days, Mp. 395-396 K.

Refinement
All H atoms were placed in calculated positions with d(O-H) = 0.82 Å and d(C-H) = 0.93 Å for aromatic and CH, 0.97 for CH 2 and 0.96 Å for CH 3 atoms. The U iso values were constrained to be 1.5U eq of the carrier atom for hydroxy and supplementary materials sup-2 methyl H atoms and 1.2U eq for the remaining H atoms. A rotating group model was used for the methyl groups. The highest residual electron density peak is located at 0.18 Å from H1B and the deepest hole is located at 0.50 Å from O2. A total of 2024 Friedel pairs were merged before final refinement as there is no large anomalous dispersion for the determination of the absolute configuration. Fig. 1. The structure of (I), showing 50% probability displacement ellipsoids and the atomnumbering scheme. Hydrogen bonds was drawn as dash line.