6-Hydroxysalvinolone1

The title compound {systematic name: 5,6,10-trihydroxy-7-isopropyl-1,1,4a-trimethyl-2,3,4,4a-tetrahydrophenanthren-9(1H)-one}, C20H26O4, is a diterpenoid which was isolated from the roots of Premna obtusifolia. The molecule has three fused six-membered rings; the cyclohexane ring is in a twisted-boat conformation and the cyclohexene ring adopts a sofa form. Intramolecular O—H⋯O hydrogen bonds generate two S(5) ring motifs. In the crystal, molecules are linked into infinite one-dimensional chains along the [001] direction by O—H⋯O hydrogen bonds and weak C—H⋯O interactions.


Comment
During the course of our studies on the chemical constituents and bioactive compounds from Thai medicinal plants, the title diterpenoid compound (I) known as 6-hydroxysalvinolone (Topcu & Ulubelen, 1996) or 14-deoxycoleon U (Fraga et al., 2005;Hueso-Rodríguez et al., 1983), was isolated from the roots of Premna obtusifolia. The previous report (Fraga et al., 2005) shows that this compound exhibits insecticidal activity. We report herein the crystal structure of (I).
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). The molecules are linked into infinite one dimensional chains along the [0 0 1] direction (Fig. 2).

Experimental
The air-dried roots of Premna obtusifolia (4.5 kg) were extracted with CH 2 Cl 2 (2 x 20 L) at room temperature. The combined extracts were concentrated under reduced pressure to afford a dark yellow extract (40.5 g) which was subjected to quick column chromatography (QCC) over silica gel using solvents of increasing polarity from n-hexane to EtOAc to afford 12 fractions (F1-F12). Fraction F7 was further purified by QCC using hexane-EtOAc (9.5:0.5), yielding the title compound (212.0 mg). Colourless block-shaped single crystals of the title compound suitable for x-ray structure determination were recrystallized from CH 2 Cl 2 /CH 3 OH (1:1, v/v) after several days.

Refinement
Hydroxy H atoms attached to O1 and O4 were located from the difference map and isotropically refined. The remaining H atoms were placed in calculated positions with d(O-H) = 0.82 Å and d(C-H) = 0.93 Å for aromatic, 0.98 for 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 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 0.67 Å from C12 and the deepest hole is located at 0.47 Å from H1O4. A total sup-2 of 3037 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. Intramolecular hydrogen bonds are shown as dashed lines. 5,6,10-trihydroxy-7-isopropyl-1,1,4a-trimethyl-2,3,4,4a-tetrahydrophenanthren-9(1H)-one

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.