Absolute configuration of vouacapen-5α-ol

The title compound, C20H30O2, {systematic name: (4aR,6aS,7R,11aS,11bR)-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2-b]furan-4a-ol}, is a cassane furanoditerpene which was isolated from the roots of Caesalpinia pulcherrima. The absolute configurations at positions 4a, 6a, 7, 11a and 11b are R, S, R, S and R, respectively. The molecule has four-fused rings consisting of three cyclohexane rings and one furan ring. The three cyclohexane rings are trans-fused. Two cyclohexane rings are in chair conformations, while the third is in an envelope conformation. In the crystal structure, the molecules are linked by intermolecular O—H⋯O hydrogen bonds into a zigzag chain along the a axis. A short O⋯O contact [3.0398 (14) Å] is also present.


was isolated from
Caesalpinia pulcherrima (L.) Swartz, a small tree which has been used as ornamental (Smitinand & Larson, 2001), abortifacient and emmenagogue purposes. We previously reported the absolute configuration of a cassane furanoditerpene namely isovouacapenol C (Fun et al., 2010) which was isolated from the same plant. Herein the absolute configuration of another cassane furanoditerpene was determined by making use of the anomalous scattering of Cu Kα radiation with the Flack parameter being refined to 0.0 (2) and its crystal structure is reported.
The molecule of the title compound ( Fig. 1) is constructed from the fusion of three cyclohexane rings and a furan ring.

Experimental
The air-dried roots of C. pulcherrima (6.3 kg) were extracted with CH 2 Cl 2 (2 × 2.5 L) for 5 days at room temperature. The combined extracts were concentrated under reduced pressure to afford a dark brownish extract (75.3 g) which was further purified by quick column chromatography (QCC) over silica gel using hexane as eluent and increasing polarity with EtOAc and MeOH to afford 16 fractions (F1-F16). Fraction F2 (5.9 g) was further purified by QCC with hexane-CH 2 Cl 2 (1:4), yielding the title compound as white solid (50.2 g). Colorless block-shaped single crystals of the title compound suitable for x-ray structure determination were recrystallized from CH 2 Cl 2 by the slow evaporation of the solvent at room temperature after several days (m.p. 371-373 K).
supplementary materials sup-2 Refinement Hydroxy H atoms were located in a difference map and refined isotropically. The remaining H atoms were placed in calculated positions with (C-H) = 0.98 for CH, 0.97 for CH 2 and 0.96 Å for CH 3 atoms. The U iso (H) values were constrained to be 1.5U eq of the carrier atom for methyl H atoms and 1.2U eq for the remaining H atoms. A rotating group model was used for the methyl groups. The highest residual electron density peak is located at 0.73 Å from C4 and the deepest hole is located at 1.32 Å from C17. 1202 Friedel pairs were used to determine the absolute configuration. Fig. 1. The structure of (I), showing 50% probability displacement ellipsoids and the atomnumbering scheme.  (4aR,6aS,7R,11aS,11bR)-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2b]

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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.