Crystal structure of 7,7-dimethyl-6-methylidenetricyclo[6.2.1.01,5]undecane-2-carboxylic acid

In the title compound, C15H22O2, both five-membered rings display an envelope conformation whereas the six-membered ring displays a chair conformation. In the crystal, pairs of O—H⋯O hydrogen bonds between carboxylic groups link molecules, related by a twofold rotation axis, into supramolecular dimers.


S1. Comment
Inula graveolens have consistently been the subject of research interest (Chiappini & Fardella, 1980;Rustaiyan et al., 1987). Our interest is the extracts from aerial parts of Algerian species such as stems, flowers and leaves. The asymmetric unit of the crystal structure consists of a single molecule (Fig. 1). Both five-membered rings display an envelope conformation (with C4 and C8 as the flap atoms) whereas the six-membered ring displays a chair conformation.
The structure consists of pairs of molecules linked by the classic dimeric carbocylic acid hydrogen bonding interaction ( Fig 2). Structures of some related compounds have been reported (Turner et al., 1980;Harlow & Simonsen, 1977;Dastlik et al., 1992).

S2. Experimental
The air-dried aerial parts of inula graveolens (500 g) were extracted with acetone/Et 2 O (1:1) at room temperature. The solution was filtered off and concentrated under reduced pressure to give a pale yellow gum (9 g). The gum was subjected to successive column chromatography (silica gel) and TLC (silica gel, PF254). Eleven fractions were obtained.
Fraction 9 gave a material which crystallized as colourless crystals with a melting point of 450 K.

S3. Refinement
H atoms were positioned geometrically and refined using a riding model with U iso (H) constrained to be 1.2 times U eq for the atom it is bonded to (except for methyl groups where it was 1.5 times with free rotation about the C-C bond).

sup-2
Acta Cryst. (2015). E71, o94 Crystal packing in the structure with H atoms omitted and hydrogen bonds shown as dotted lines. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.15 e Å −3 Δρ min = −0.15 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(F 2 ) is used only for calculating R-factors(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.