Vieillardiixanthone B

The title compound [systematic name: 1,5-dihydroxy-3,6-dimethoxy-4-(2-methylbut-3-en-2-yl)-9H-xanthen-9-one], C20H20O6, is a xanthone, which was isolated from the roots of Cratoxylum formosum ssp. pruniflorum. The three rings in the molecule are approximately coplanar, with an r.m.s. deviation of 0.0372 (2) Å for the plane through the 14 non-H atoms. The O atoms of the two hydroxy substituents also lie close to this plane with deviations of 0.0669 (2) and 0.1122 (2) Å, respectively. The 1,1-dimethyl-2-propenyl substituent is in a (−)-anticlinal conformation. Intramolecular O—H⋯O hydrogen bonds generate S(5) and S(6) ring motifs. In the crystal, molecules are linked into infinite chains along [010] by O—H⋯O hydrogen bonds and weak C—H⋯O interactions. π–π interactions with centroid–centroid distances of 3.6172 (10) and 3.6815 (10) Å are also observed.

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 previously reported the isolation of xanthones from the Cratoxylum formosum ssp. pruniflorum. We found that several isolated xanthones showed antibacterial, antifungal and cytotoxic activities (Boonnak, Karalai et al., 2006;2007;2009). Further isolation of materials from the roots of this plant resulted in the title xanthone known as vieillardiixanthone B (Hay et al., 2008). It was tested against both Gram-positive and Gram-negative bacteria i.e. Bacillus subtilis, Staphylococcus aureus Pseudomonas aeruginosa. Our results showed that the title compound has no antibacterial action against these pathogens.
Herein we report the crystal structure of the title xanthone (I).

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 separated by QCC and eluted with 30% EtOAc-n-hexane to obtain 20 subfractions (F8E1-F8E20). Subfractions F8E10-F8E12 were combined and separated by QCC and eluted with a gradient of CH 2 Cl 2 -n-hexane to give 12 subfractions (F8E10A-F8E10L). Subfracsup-2 tion F8E10D was separated by CC (Column Chromatography) eluting with 10% acetone-n-hexane to give 8 subfractions (F8E10D1-F8E10D8). Subfraction F8E10D5 was further purified by CC and eluted with a gradient of CH 2 Cl 2 -n-hexane to give the title compound as a yellow solid (3.5 mg). Yellow block-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. 486-488 K).

Refinement
Hydroxy H atoms attached to O3 and O5 and H atoms attached to C18 and C19 were located from the difference map and refined isotropically. The remaining H atoms were placed in calculated positions with d(C-H) = 0.93 Å for aromatic 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 0.72 Å from C15 and the deepest hole is located at 0.72 Å from C5. Fig. 1. The structure of (I), showing 50% probability displacement ellipsoids and the atomnumbering scheme. Intramolecular hydrogen bonds are shown as dashed lines. 1,5-dihydroxy-3,6-dimethoxy-4-(2-methylbut-3-en-2-yl)-9H-xanthen-9-one

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.