Absolute configuration of fibaruretin B

The title furanoditerpenoid, known as fibaruretin B (systematic name: 2β,3α-dihydroxy-2,3,7,8α-tetrahydropenianthic acid lactone), C20H24O7, was isolated from the roots of Arcangelisia flava. The absolute configurations at positions 2, 3, 4, 4a, 7, 9, 10a and 10b of fibaruretin B are S, R, S, R, S, S, S and S, respectively. In the crystal structure, the molecules are linked into infinite chains along the c axis by O—H⋯O hydrogen bonds and weak C—H⋯O interactions.


Related literature
For ring conformations, see: Cremer & Pople (1975). For bond-length data, see: Allen et al. (1987). For background to and activities of furanoditerpenoids, see: Ito & Furukawa (1969); Keawpradub et al. (2005); Moody et al. (2006); Nguyen-Pouplin et al. (2007); Su et al. (2008). For a related structure, see: Bakhari et al. (1998). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986 Table 1 Hydrogen-bond geometry (Å , ).  et al., 2006). They were found to possess biological properties such as anti-inflammatory and antimalarial activities. Arcangelisia flava (Menispermaceae), commonly called `Khaminkhruea' in the southern Thailand, is widely distributed from Hainan (China), Indo-China, southern peninsular Thailand, peninsular Malaysia, Sumatra, Java, Borneo, the Philipines, Sulawesi, the Northern Moluccas to New Guinea. It is an available medicinal plant used for the treatment of malaria, dysentery and as a tonic. The antimalarial, cytotoxic and antioxidant effects of this plant have been reported (Nguyen-Pouplin et al., 2007;Keawpradub et al., 2005). During the course of our study of bioactive compounds from medicinal plants, the title furanoditerpenoid compound, (I), which is known as fibaruretin B, was isolated for the first time from the roots of Arcangelisia flava (Menispermaceae) which were collected from Songkhla province in the southern part of Thailand. The absolute configuration of (I) was determined by making use of the anomalous scattering of Cu Kα radiation with the Flack parameter being refined to 0.03 (12).
The molecule of (I) has four fused rings consisting of one five-and three six-membered rings (A/B/C/D) ( In the crystal packing of (I) (Fig. 2), the molecules are linked into chains along the c axis through O-H···O hydrogen bonds and C-H···O weak interactions ( Fig. 2 and Table 1) and the crystal is stabilized by these interactions.

Experimental
The air-dried roots of Arcangelisia flava (Menispermaceae) (1.8 kg) were extracted with CH 2 Cl 2 (2 × 10 l) under room temperature. The combined extracts were concentrated under reduced pressure to give a yellow extract (30.1 g) which was supplementary materials sup-2 subjected to quick column chromatography over silica gel using solvents of increasing polarity from n-hexane to EtOAc to afford 10 fractions (F1-F10). Fraction F9 was further purified by column chromatography using CH 2 Cl 2 -EtOAc (6:4), yielding the title compound as a white solid (275.3 mg). Colorless needle-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. 539-540 K).

Refinement
Hydroxy H atom was located from the difference map and refined isotropically. The remaining H atoms were placed in calculated positions with C-H = 0.93 for aromatic, 0.96 Å for CH 3 , 0.97 for CH 2 and 0.98 Å for CH. 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 0.78 Å from H6B and the deepest hole is located at 1.51 Å from C10. 1098 Friedel pairs were used to determine the absolute configuration.
Outlier reflection (11 0) was omitted. Fig. 1. The structure of the title compound, showing 50% probability displacement ellipsoids and the atom-numbering scheme.

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. supplementary materials sup-9