Crystal structure and Hirshfeld surface analysis of pulcherrin J

The natural diterpene known as pulcherrin J was isolated from stem barks of medicinally important Caesalpinia pulcherrima (L.). The molecule is composed of a central core of three trans-fused cyclohexane rings and a near planar five-membered furan ring, along with an axially oriented cinnamate moiety and ahydroxyl substituent attached to the C-8 and C-9 positions of the steroid ring system, respectively. Hirshfeld surface analysis indicates that the most significant contacts in packing are H⋯H (67.5%), followed by C⋯H (19.6%) and H⋯O (12.9%).


Chemical context
Caesalpinia pulcherrima (L.) is one of the widely cultivated species of the genus Caesalpinia. It is an ornamental plant with attractive inflorescence in yellow, red, and orange, generally blooming in winter. Its small size and tolerability towards pruning allows it to be grown in groups to form a windbreak. It can also be used to create a center of attention for humming birds (Frisch et al., 2005). In addition to the ornamental value, C. pulcherrima has been known to exhibit cytotoxic (Promsawan et al., 2003;McPherson et al., 1986), antitubercular (Promsawan et al., 2003), antibacterial, antifungal (Ragasa et al., 2002), and leishmanicidal (Erharuyi et al., 2016) activities. The compounds isolated from C. pulcherrima are also reported to be active against DNA repair-deficient yeast mutant (Patil et al., 1997). The plants of genus Caesalpinia, including C. pulcherrima, are known to be a rich source of cassane-type diterpenoids. The literature reports the isolation of a number of cassane-type diterpenoids from the stems, and root barks, such as pulcherrimins A-F, and pulcherrins A-R (Erharuyi et al., 2017;Yodsaoue et al., 2011;Pranithanchai et al., 2009;Roach et al., 2003). In continuation of our work on the phytochemical investigation of medicinally important plants, we have isolated the crystalline pulcherrin J, a cassanetype diterpenoid, previously reported by Erharuyi and coworkers (Erharuyi et al., 2017). To the best of our knowledge, this is the first report of the the crystal structure and the Hirshfeld surface analysis of pulcherrin J.

Hydrogen bonding and Hirshfeld surface analysis
In the crystal, the molecules are connected by O2-H2AÁ Á ÁO1 i interactions to generate C(8) chains propagating in the [100] direction. (Table 1, Fig. 2). The Hirshfeld surface analysis (Spackman et al., 2009) indicates that the percentage contribution of HÁ Á ÁH interactions to the packing is 67.5% (Fig. 3). Other important interactions based upon the percentages are CÁ Á ÁH (19.6%) and HÁ Á ÁO (12.9%), as shown in the fingerprint plots, in which cyan dots indicate the percentage of the interaction over the total Hirshfeld surface (Fig. 4).

Comparison with reported literature
Structurally the title compound is similar to the reported isovouacapenol C (Fun et al., 2010b) with the difference that no hydroxy substituent occurs on ring B, while the benzoate moiety is replaced by a cinnamate moiety. The O-HÁ Á ÁO hydrogen bond is the most important contributor to the crystal packing of pulcherrin J, and other related structures such as isovouacapenol C and vouacapen-5a-ol (Fun et al., 2010a,b), all of which lead to chains in the crystal.  Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) x À 1 2 ; Ày À 1 2 ; Àz þ 1.

Figure 2
The crystal packing. H atoms involved in hydrogen bonding are shown.

Figure 1
The molecular structure, with displacement ellipsoids drawn at the 30% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in

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.