Crystal structure and Hirshfeld surface analysis of the naturally occurring cassane-type diterpenoid, 6β-cinnamoyl-7β-hydroxyvouacapen-5α-ol

The title compound, a cassane-type diterpenoid, was isolated from a medicinally important plant, Caesalpinia pulcherrima (Fabaceae). In the molecule, the three cyclohexane rings are trans-fused and adopt chair, chair and half chair conformations.


Chemical context
Caesalpinia pulcherrima (Fabaceae) is a decorative evergreen plant widely used for the treatment of various illnesses (Roach et al., 2003). It is commonly known as Gulmohor, Krishnachura and Mayirkonnai, respectively, in Hindi, Bengali and Tamil. Herbalists in the Amazon tropical rain forest have long known some of the medicinal uses of C. pulcherrima, known locally as ayoowiri (Patel et al., 2010). The plant is also known to be used for the treatment of inflammation, earache, muscular and sore pain and cardiovascular disorders and as an antimalarial, vermifuge and antipyretic agent (Patel et al., 2010;Roach et al., 2003). The natural constituents commonly known as cassane-type diterpenoids extracted from C. pulcherrima have been reported by Pranithanchai et al. (2009) and Rao et al. (2005). Cassane-type diterpenoids represent a class of pharmaceutically important natural products having various biological activities. The current study deals with the isolation, single-crystal X-ray diffraction and Hirshfeld surface analysis of the title compound, a naturally occurring cassanetype diterpenoid.

Supramolecular features
In the crystal, the molecules are linked via O-HÁ Á ÁO hydrogen bonds (O5-H5Á Á ÁO2 i ; symmetry code as in Table 1), forming chains along the b-axis direction (Fig. 2). The chains are further linked into a double-chain structure through C-HÁ Á Á interactions (C3-H3AÁ Á ÁCg1 ii ; symmetry code as in Table 1) involving the furan ring.

Hydrogen bonding and Hirshfeld surface analysis
The Hishfeld surface mapped over d norm (McKinnon et al., 2004;Spackman & Jayatilaka, 2009) for the title compound is depicted in Fig. 3. The red areas on the surface indicate short contacts as compared to the sum of the Van der Waals (vdW) radii, while the blue indicate long contacts and white area indicate contacts with distances equal to the sum of the vdW radii. The red highlighted area shows the O-HÁ Á ÁO hydrogen bond, which is responsible for connecting molecules to each other. The contribution of the HÁ Á ÁH contacts to the crystal packing is 65.5%, and CÁ Á ÁH, OÁ Á ÁH and CÁ Á ÁO contributions are 18.7, 14.5 and 0.3%, respectively. The Hirshfeld surface mapped over electrostatic potential (Spackman et al., 2008) is shown in Fig. 4. The red region indicates atoms having potential to be hydrogen-bond acceptors with negative electrostatic potential, while the blue shows potential to be hydrogen-bond donors with positive electrostatic potential.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structures of the title compound, showing atom-labelling scheme with displacement ellipsoids drawn at the 50% probability level. The intramolecular C-HÁ Á ÁO interaction is shown as a dashed line.

Isolation and crystallization
The powdered stem bark (2.5 kg) of C. pulcherrima was extracted with methanol (7.5 l) by cold maceration for four days, followed by filteration and concentration using a rotary evaporator under reduced pressure at 228 K to obtain the crude plant extract (200 g). The crude extract was further fractionated by silica gel chromatography first using n-hexane (9.4 l) and then with increasing polarities of solvents [nhexane:ethylacetate (1:1) (12.5 l), ethyl acetate (8.2 l), ethyl acetate:methanol (1:1) (13 l) and finally with methanol (7 l

Figure 4
Electrostatic potential surface generated incorporated with Hirshfeld surface for compound (I).

Data collection and Refinement
Crystal data, refinement results are summarized in Table 2. All H atoms were placed geometrically (C-H = 0.95-1.00 Å and O-H = 0.84 Å ) and were refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (O). Since a partial racemic twin of the crystal was suggested from a Flack parameter of 0.17 (7) (Parsons et al., 2013), a twin treatment was adopted in the final refinement. The BASF parameter refined to 0.0 (2). It is, therefore, uncertain whether the crystal used was an inversion twin or not. Two-dimensional fingerprint plots for compound (I).