(E)-2-Hydroxy-4-methoxy-3-(3-methylbut-2-enyl)-6-styrylbenzoic acid

The title compound, C21H22O4, also known as cajanine, features an intramolecular O—H⋯O hydrogen bond between the adjacent carboxy and hydroxy groups. The benzene rings make an interplanar angle of 175.4 (2)°. In the crystal, molecules are linked by pairs of O—H⋯O hydrogen bonds, forming inversion dimers.

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Cajanine is a good drug candidate because of its wide range of pharmacological activities, which include antitumor The crystal structure of the title compound is reported here. In this crystal, the two benzene rings are not in the same plane, and the interplanar angle between them is 175.4 (2) °. A strong intramolecular O-H···O hydrogen bond is formed between the carboxyl group and a hydroxy group.

Refinement
Hydrogens were generated geometrically.  The title molecule with the atom-numbering scheme. Ddisplacement parameters are shown at the 30% probability level.

Figure 2
Packing of the title molecules viewed along the a direction.

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 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.