1,4a,7-Trimethyl-7-vinyl-1,2,3,4,4a,4b,5,6,7,8,10,10a-dodecahydrophenanthrene-1-carboxylic acid

The title compound, also known as isopimaric acid, C20H30O2, was isolated from slash pine rosin. There are two unique molecules in the unit cell. The two cyclohexane rings have classical chair conformations. The cyclohexene ring represents a semi-chair. The molecular conformation is stabilized by weak intramolecular C—H⋯O hydrogen-bonding interactions. The molecules are dimerized through their carboxyl groups by O—H⋯O hydrogen bonds, forming R 2 2(8) rings.


Comment
The title compound has been isolated from slash pine rosin. It was identified as isopimaric acid on the basis of the comparison of its physical and spectral data with literature values (Baldwin et al., 1958;Harris et al., 1948). Isopimaric acid exhibits a wide range of biological activities such as antibacterial activity (Smith et al., 2005), BK channels activity (Russo et al., 2007).
Although much attention has been paid to the bioactivities of isopimaric acid, the crystal structure of the title compound has not yet been reported. In this work, we describe the crystal structure of the title compound (I).
The molecular structure of (I) is shown in Fig. 1 and its crystal packing in Fig.2. The atoms of C11, C12 in the cyclohexene ring and the atoms C10, C14 in the conjoint cyclohexane ring are in the same plane. The atoms of C11, C12 and the atoms of C16, C17 in the cyclohexane ring are in another plane. The dihedral angle of the two planes is 134.4 degree. The three methyl groups attached to the cyclohexane rings are in axial positions and in the same direction. The molecular conformation is stabilized by C-H···O intra-molecular hydrogen bonding interactions ( Table 1). The molecules are dimerized through their carboxyl groups by O-H···O hydrogen bonds, forming R 2 2 (8) rings (Fig. 2).

Experimental
A mixture of slash pine rosin (150 g), acetone (300 ml) and 2-amino-2-methyl-1-propanol (0.1 mol) was stirred at room temperature for 2 h and then filtrated. The residue was recrystallized with 95% ethanol. The crystal obtained from the solution was acidified by 5% hydrochloric acid solution and then dissolved in ether. The solution was washed with water until it was neutral, dryed with sodium sulfate and then concentrated. The residue was recrystallized with acetone and the title compound was obtained as colourless solid.

Refinement
All H atoms were placed geometrically with C-H = 0.93-0.98 Å and O-H = 0.82 Å, and included in the refinement in riding motion approximation with U iso (H) = 1.2 or 1.5U eq of the carrier atom. Because of no atom heavier than Si present in the structure, in the absence of significant anomalous dispersion effects, so Friedel pairs were averaged.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.
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.