Crystal structure of olivetolic acid: a natural product from Cetrelia sanguinea (Schaer.)

The packing in olivetolic acid is similar to that in resorcinolic acid.

The title compound, C 12 H 16 O 4 (systematic name: 2,4-dihydroxy-6-pentylbenzoic acid) is a natural product isolated from C. sanguinea (Schaer.) and is reported to have various pharmacological activities. The molecule is approximately planar (r.m.s. deviation for the non-H atoms = 0.096 Å ) and features an intramolecular O-HÁ Á ÁO hydrogen bond. In the crystal, each olivetolic acid molecule is connected to three neighbours via O-HÁ Á ÁO hydrogen bonds, generating (101) sheets. This crystal is essentially isostructural with a related resorcinolic acid with a longer alkyl chain.

Chemical context
Monoaromatic compounds from lichens have attracted a great interest in the pharmaceutical field due to their potential pharmacological activities such as antibacterial, antifungal, cytotoxic, and photoprotective activities (Gianini et al.,2008: Stocker-Wö rgö tter, 2008Ismed et al., 2012). The title compound, C 12 H 16 O 4 , is a derivative of alkyl resorcinolic acid which is commonly found in certain species of lichens (Gomes et al., 2006).

Structural commentary
The title compound ( Fig. 1) crystallizes with monoclinic metric symmetry and adopts a roughly planar conformation (r.m.s. deviation = 0.093 Å ). All bond distances, angles and dihedral angles appear to be usual except the bond angle of C6-C5-C12 [124.61 (13) ] compared to the mean value and their standard deviation of selected 24 similar structures reported in Cambridge Structural Database (CSD, Version 5.37, Update 2 Feb 2016; Groom et al., 2016). In this case, the deviating bond angle may be a result of the strong intramolecular O2-H2Á Á ÁO3 interaction. ISSN 2056-9890

Supramolecular features
In the crystal, each molecule is connected with three others (Fig. 2): O1 acts as an O-HÁ Á ÁO hydrogen bond donor while O2 is an O-HÁ Á ÁO acceptor, forming a C 1 1 (6) infinite chain. In addition, an O4-H4Á Á ÁO3 carboxylic acid homodimer synthon is observed, generating an R 2 2 (8) loop. Together, these hydrogen bonds construct a layered architecture propagating in the (101) plane. Details of the hydrogen bonds are given in Table 1.
Interestingly, the title compound showed isostructurality with alkyl resorsinolic acid derivatives with longer alkyl chain of 6-n-pentadecyl-2,4-dihydroxy-benzoic acid (Gadret et al., 1975;refcode: PDCHBZ10). Both structures exhibited extremely similar hydrogen bond in resorsinolic acid shown in Fig. 3a and 3b. Both crystal structures consist of a hydrophilic layer of the resorcinol acid moiety with hydrogen-bonding interactions, and a hydrophobic layer of normal alkyl chains.

Crystallization
Crystallization of the title compound was conducted by dissolving 700 mg of the isolate in an ethyl acetate-hexane solvent mixture (1:1). The solution was kept for one week at room temperature yielding colourless needles of the title compound.

Figure 1
The molecular structure of the title compound, showing 50% probability displacement ellipsoids.
The arrows indicate the one-dimensional hydrogen-bond chains involving resorsinolic acid. Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: PROCESS-AUTO (Rigaku, 1998); program(s) used to solve structure: SHELXS2014 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.