Crystal structures of hibiscus acid and hibiscus acid dimethyl ester isolated from Hibiscus sabdariffa (Malvaceae)

The isolation and crystal structures of the title compounds from Hibiscus sabdariffa (Malvaceae) are described. Hibiscus acid dimethyl sulfoxide monosolvate forms a two-dimensional hydrogen-bonded motif, while hibiscus acid dimethyl ester (Z′ = 2) forms a one-dimensional hydrogen-bonded motif.


Chemical context
Lactone acid producing plants, including Hibiscus sabdariffa (Malvaceae), have been documented to have significant potential in the traditional treatment of various diseases. H. sabdariffa Linn is a species of hibiscus from the Malvaceae family, commonly known as 'Karkade' or 'red sorrel'. It is used in traditional medicine in the form of herbal teas or cold drinks for its hypotensive and diuretic effects and to lower body temperature and blood viscosity (Ali et al., 2005;Da-Costa-Rocha et al., 2014). Little attention has been paid to organic acids from H. sabdariffa, specifically hibiscus acid. However, studies have documented the activity of hibiscus acid and hibiscus acid methyl ester. These report an inhibitory effect against enzymes, such as -amylase and -glucosidase (Hansawasdi et al., 2000(Hansawasdi et al., , 2001. As these compounds are not available commercially and to enable a study of their biological activities, we report on the extraction of hibiscus acid and hibiscus acid dimethyl ester from H. sabdariffa (Malvaceae), and on their purification and characterization. The crystal structures of the acid, as the dimethyl sulfoxide monosolvate, (I), and the diester, (II), are reported herein.

Structural commentary
The crystal structures of the 1:1 dimethyl sulfoxide (DMSO) solvate of hibiscus acid, (I), and of hibiscus acid dimethyl ester, (II), are shown in Figs. 1 and 2. The COOR (R = H or Me) groups lie in equatorial positions on their rings and the absolute configuration of both species is confirmed by the Flack parameter values (Parsons et al., 2013), for arbitrarily ISSN 2056-9890 named atoms in (I) [C2(R),C1(S), 0.00 (4)] and both arbitrarily named equivalent atoms in (II) [C3(R),C4(S) and C11(R),C12(S), 0.08 (17)] ( Table 1). The absolute configuration found thus agrees with that originally proposed by Boll et al. (1969) for hibiscus acid. The structure of garcinia lactone, an epimer of hibiscus acid, has been reported (Mahapatra et al., 2007). The comparable molecular geometries of (I) and its epimer are similar. The five-membered ring of (I) adopts an envelope conformation, with the OH-bearing C2 atom 0.582 (6) Å out of the plane defined by the other four atoms.
The structure of (II) contains two crystallographically independent molecules (A and B) (Z 0 = 2), whose molecular geometries differ only by small deviations in torsion angles, for example, C3-C5-O5-C6 in A is 175.1 (4) , whilst the equivalent angle in B (C11-C13-O12-C-14) is 180.0 (4) . As with structure (I), the five-membered rings adopt envelope conformations, with the OH-bearing C atoms lying out of the plane of the other four atoms, here by 0.505 (5) and 0.530 (5) Å for molecules A and B, respectively.

Figure 1
The molecular structure of compound (I), with the atom labelling and 50% probability displacement ellipsoids.

Supramolecular features
Despite containing two carboxylic acid functionalities, the structure of (I) does not feature the classic R 2 2 (8) carboxylic acid dimer motif. Instead, each of the three potential hydrogen-bond donors of the acid molecule form interactions with a total of three separate neighbouring molecules (Fig. 3). The H atom of the carboxylic acid group (O3-H) adjacent to the ether forms a bifurcated hydrogen bond that is accepted by the ROH and C O functions (i.e. O4 i and O6 i ) of one neighbour, whilst the other two donors, the second carboxylic acid (O5-H) and the hydroxy group (O4-H), form hydrogen bonds with atoms O8 ii and O8 of DMSO solvent molecules, respectively (Table 2). These interactions combine to give a two-dimensional hydrogen-bonded layered structure, with DMSO and acid layers alternating along the c-cell direction (Fig. 4).
Both independent molecules in the structure of (II) donate single hydrogen bonds through their OH groups, but only one molecule (A) acts as a hydrogen-bond acceptor (O3-HÁ Á ÁO4 i and O10-HÁ Á ÁO2 ii ; Table 3). That a total of four carbonyl O atoms do not act as acceptors is probably related to the low ratio of classic hydrogen-bond donors to acceptors in this The crystal packing of compound (I), viewed along the a axis. Table 3 Hydrogen-bond geometry (Å , ) for (II).

Figure 2
The molecular structures of the two independent molecules comprising the asymmetric unit of (II), with the atom labelling and 50% probability displacement ellipsoids. Table 2 Hydrogen-bond geometry (Å , ) for (I). (4) 167 (5) compound. In (II), the hydrogen bonding combines to give a four-molecule-wide one-dimensional ribbon of linked molecules that propagates parallel to the a axis (Fig. 5).  (Glusker et al., 1972) and of the diastereomer mentioned previously (Mahapatra et al., 2007) have been reported. The closest relative of (II) to have been structurally described is a derivative with additional OH and Me substituents on the fivemembered ring (Evans et al., 1997).

Synthesis and crystallization
Dried H. sabdariffa calyces were crushed to a powder (500 g) and extracted in a Soxhlet apparatus using 2500 ml each of hexane, ethyl acetate and methanol. The methanol extract was dried and concentrated at 313 K by rotatory evaporation, yielding about 125 g (25%) of crude extract. The methanol extract (2 g) was dissolved in about 2 ml of methanol and subjected to gel filtration chromatography (GFC) using a glass column packed with a wet slurry of 30 g of Sephadex LH20 in methanol. Vials were collected (5 ml each) after elution with 100% methanol, which led to isolation of pure hibiscus acid (0.5%). Crystals of (I) were obtained by recrystallisation from DMSO.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. For all structures, C-bound H atoms were placed in their expected geometrical positions and treated as riding, with C-H = 0.95-0.99 Å and U iso (H) = 1.5U eq (C) for methyl C atoms and 1.2U eq (C) for the other H atoms. The absolute configuraion was determined for the molecules in both acid (I) for arbitrarily named atoms [C2(R),C1(S), Flack parameter 0.00 (4) (Altomare et al., 1993); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2008); 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq S1 0.90564 ( (17) 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.