Brasilixanthone1

The title xanthone [systematic name: 5,13-dihydroxy-3,3,10,10-tetramethyl-3H-dipyrano[3,2-a:2′,3′-i]xanthen-14(10H)-one], C23H20O6, was isolated from the roots of Cratoxylum formosum ssp. pruniflorum. There are two molecules (A and B) in the asymmetric unit, which show chemical but not crystallographic inversion symmetry. The xanthone skeleton in both molecules is approximately planar, with an r.m.s. deviation of 0.0326 (9) Å for molecule A and 0.0355 (9) Å for molecule B from the plane through the 14 non-H atoms. The pyran rings in both molecules adopt sofa conformations. Intramolecular O—H⋯O hydrogen bonds generate S(5) and S(6) ring motifs. Viewed onto the bc plane, the crystal structure resembles a herringbone pattern. Stacks of molecules are stabilized by π–π interactions with centroid–centroid distances of 3.600 (5) Å. The crystal structure is further stabilized by weak C—H⋯O and C—H⋯π interactions.

Compound (I) crystallizes with two independent molecules (A and B) per asymmetric (Fig. 1). The conformations of molecule A differ from those observed in molecule B in which the two chromene rings in A pucker in the opposite direction from those in B (Fig. 1). In both molecules, the three ring system [C1-C13/O1] are essentially planar with the r.m.s. deviation of 0.0326 (9) and 0.0355 (9) Å, respectively for A and B from the plane through all 14 non-hydrogen atoms of the three rings and with a maximum deviation of -0.085 (9) Å (for A) and +0.081  (Table 1) involving O3A and O3B hydroxy O atoms generate S(6) whereas the one involving O5A and O5B atoms generate S(5) ring motifs ( Fig. 1) (Bernstein et al., 1995). There are weak intramolecular C-H···O interactions in the crystal structure, [C16A-H16A···O2A and C16B-H16B···O2B], which generate two S(6) ring motifs. The bond distances in (I) are comparable to those in a related structure (Fun et al., 2006).

Experimental
The air-dried roots of C. formosum ssp. pruniflorum (5.00 kg) was extracted with CH 2 Cl 2 (2 x 20 L, for a week) at room temperature and was further evaporated under reduced pressure to afford a deep green crude CH 2 Cl 2 extract (58.87 g), which was subjected to QCC (Quick Column Chromatography) on silica gel using n-hexane as a first eluent and then increasing the polarity with acetone to give 12 fractions (F1-F12). Fractions F8-F11 were combined and separated by QCC eluting with 30% EtOAc-n-hexane to give 8 subfractions (F8A-F8H). Subfractions F8E and F8F were combined and then separated by QCC and eluted with 30% EtOAc-n-hexane to obtain 20 subfractions (F8E1-F8E20). Subfraction F8E10-F8E12 were combined and then separated by QCC and eluted with a gradient of CH 2 Cl 2 -n-hexane to give 12 subfractions (F8E10A-F8E10L).
Subfraction FR8E10B was further purified by CC (Column Chromatography) and eluted with 5% acetone-n-hexane to give the title compound as yellow solid (4.5 mg). Yellow needle-shaped single crystals of the title compound suitable for x-ray structure determination were recrystallized from acetone/CH 3 OH (9.5:0.5, v/v) after several days (M.p. 478-480 K).

Refinement
All H atoms were placed in calculated positions with d(O-H) = 0.82 Å and d(C-H) = 0.93 Å for aromatic and CH, and 0.96 Å for CH 3 atoms. The U iso 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 0.85 Å from O2A and the deepest hole is located at 0.76 Å from O1A. A total of 3445 Friedel pairs were merged before final refinement as there is no large anomalous dispersion for the determination of the absolute structure.  sup-3 5,13-dihydroxy-3,3,10,10-tetramethyl-3H-dipyrano[3,2-a:2',3'-i]xanthen-14(10H)-one sup-4 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.