Crystal structure and Hirshfeld surface analysis of 3-oxours-12-ene-27a,28-dioic acid (quafrinoic acid)

The title compound, known as quafrinoic acid, is a pentacyclic triterpene isolated from Nauclea Pobeguinii. The compound is composed of five fused six-membered rings and the molecular conformation is stabilized by intramolecular C—H⋯O interaction, forming S6 and S8 rings.

The title compound, C 30 H 44 O 5 , is a pentacyclic triterpene isolated from the Cameroonian medicinal plant Nauclea Pobeguinii and known as quafrinoic acid. The molecule is composed of five fused six-membered rings, four of which adopt a chair conformation and one a half-chair conformation. Intramolecular C-HÁ Á ÁO hydrogen-bond interactions exist, which generate S6 and S8 rings. In the crystal, molecules are linked by pairs of O-HÁ Á ÁO hydrogen bonds, linking R 2 2 (8) rings into chains running parallel to the a axis; these chains are further connected into layers parallel to the ab plane by C-HÁ Á ÁO hydrogen bonds. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from HÁ Á ÁH (79.4%) and OÁ Á ÁH (20.4%) interactions.

Chemical context
Nauclea is a well-known genus of the Rubiaceae family consisting of 35 species of which ten are distributed in tropical Africa, Asia and Australia (Chen & Taylor, 2011). Several specimens of this genus, including Nauclea pobeguinii, are largely used in traditional medicine in Africa. During the last decade, many studies have been carried out on N. pobeguinii to explore its medicinal potential and promising results have made it an attractive target for researchers. The 80% ethanolic stem bark extract of N. pobeguinii has been successfully used in clinical trials for the treatment of uncomplicated malaria . The plant is also reported to have cytotoxic, anti-cancer (Kuete et al., 2015) and anti-diabetic properties (Agnaniet et al., 2016). The phytochemical investigations of N. pobeguinii have led to the isolation of monoterpene indole alkaloids, triterpenes and phenolic compounds (Kuete et al., 2015;Xu et al., 2012;Zeches et al., 1985). In a continuation of our phytochemical investigation of Cameroonian medicinal plants, we have examined the stem bark of N. pobeguinii and isolated quafrinoic acid. Although the atomic connectivity of quafrinoic acid has already been determined by spectroscopic methods (Ajaiyeoba & Krebs, 2003), we report herein the single crystal X-ray diffraction structure and Hirshfeld surface analysis of quafrinoic acid for ISSN 2056-9890 the first time.

Supramolecular features
In the crystal, molecules are linked into chains parallel to the a axis through pairs of O-HÁ Á ÁO hydrogen bonds, forming R 2 2 (8) rings. These chains are further connected into layers parallel to the ab plane by C-HÁ Á ÁO hydrogen bonds (Table 1; Fig. 2).

Figure 2
The crystal packing of the title compound viewed down the a axis. Only H atoms involved in hydrogen bonding are shown.

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 30% probability level. Dashed lines indicate intramolecular hydrogen bonds. H atoms not involved in hydrogen bonding have been omitted.
weakest intermolecular contact for the cohesion of the structure is OÁ Á ÁO, found to contribute only 0.4%. The graphical representation of the Hirshfeld surface (Fig. 4) suggests the locations of intermolecular contacts. These contacts are represented by conventional mapping of d norm on molecular Hirshfeld surfaces as shown in Fig. 3. The HÁ Á ÁH contribution for the crystal packing is shown as a Hirshfeld surface twodimensional fingerprint plot with red dots (Wolff et al., 2012).
The d e (y axis) and d i (x axis) values are the closest external and internal distances (Å ) from a given points on the Hirshfeld surface contacts (Fig. 4). Two-dimensional fingerprint plot analysis of (a) all interactions, (b) HÁ Á ÁH contacts, (c) OÁ Á ÁH contacts and (d) OÁ Á ÁO contacts. The outline of the full fingerprint plots is shown in grey. d i is the closet internal distance from a given point on the Hirshfeld surface and d e is the closest external contact.    (17) The purified compound was recrystallized by slow evaporation of a methanol solution at room temperature.

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. 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 > 2sigma(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.