(E)-Isopentyl 3-(3,4-dihydroxyphenyl)acrylate

The title compound, C14H18O4, a derivative of caffeic acid, has an E configuration about the C=C bond. The benzene ring is almost coplanar with the C=C—C(O)—O—C linker [maximum deviation = 0.050 (2) Å], making a dihedral angle of only 4.53 (2)°. In the molecule, the adjacent hydroxy groups form an O—H⋯O interaction. In the crystal, molecules are linked by O—H⋯O hydrogen bonds, generating a chain propagating in the [110] direction.

Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: PLATON (Spek, 2009 Caffeic acid esters are a component of propolis (a vegetable resin) and are reported to have a broad spectrum of biological effects, such as, anti-tumour, antioxidant, and anti-inflammatory activities (Uwai et al., 2008;Buzzi et al., 2009). The resin itself has been used as a cation-exchange resin for heterogeneous catalyst (Feng et al., 2011). This prompted us to synthesize a series of caffeic acid esters to investigate their properties better (Wang et al., 2011). Herein, we report on the crystal structure of the title compound, the isopentyl derivative of caffeic acid.
The title molecule has an E configuration about the C7═C8 bond ( Fig. 1). The benzene ring with the C7═C8-C9 linker is almost coplanar, with a root mean square deviation from the mean plane of 0.005 Å. All bond lengths and angles are in very close agreement with those found in similar caffeic acid structures (Xia et al., 2004(Xia et al., , 2006, and in the pentyl derivative of caffeic acid (Wang et al., 2011).
In the crystal, the hydroxy groups contribute to intermolecular O-H···O interactions (Table 1), that link the molecules into ribbons extending in the [110] direction (Fig. 2). On the other hand, the intramolecular O-H···O H-bond also contributes to the stability of the molecular configuration ( Fig. 1 and Table 1).

Experimental
The synthesis follows the method of (Wang et al., 2011). Esterification of caffeic acid with hexyl alcohol was performed in a column (inner diameter = 15 mm, length = 200 mm). A cation exchange resin CD-552 particles (5 g) molecular sieve (5 g) and glass beads of 2 mm in diameter were packed into the middle of the reactor. In a reaction mixture tank, 9 g of caffeic acid was mixed with 100 ml hexyl alcohol. The reaction mixture was supplied to the reaction column at a aret of 10.0 ml/h. The reaction continued at 353 K for 24 h. The solvent was then removed under reduced pressure. The residue was extracted with ethyl acetate three times and filtered. The filtrate was washed successively with dilute saturated aqueous NaHCO 3 solution, saturated aqueous NaCl, then dried over MgSO 4 , and evaporated. The residue was recrystallized from ethanol to give the title compound as colourless crystals (Yield 5.2 g; 57.7%).

Refinement
The OH and C-bound H-atoms were included in calculated positions and treated as riding atoms: O-H = 0.82 Å, C-H = 0.93, 0.98, 0.97 and 0.96 Å for CH(aromtic), CH, CH 2 , and CH 3 H-atoms, respectively, with U iso (H) = k × U eq (O,C), where k = 1.5 for OH and CH 3 H-atoms, and k = 1.2 for all other H-atoms.

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.