Racemic 2′-hydroxy-4′,4′-dimethylpyran-1,5-dihydroxyxanthone monohydrate

The title xanthone (systematic name: 3,6,11-trihydroxy-1,1-dimethyl-2,3-dihydrochromeno[2,3-f]chromen-7-one monohydrate), known as pruniflorone N, crystallized as a monohydrate, C18H16O6·H2O. The three ring systems of the xanthone skeleton are approximately coplanar, with an r.m.s. deviation of 0.0270 (1) Å from the plane through the 14 non-H atoms. The O atoms of the two hydroxy substituents on the benzene rings also lie close to this plane, with deviations of 0.019 (1) and 0.070 (1) Å. The 2′-hydroxy-4′,4′-dimethylpyran ring is disordered over two positions with a 0.798 (3):0.202 (3) site-occupancy ratio. An intramolecular O—H⋯O hydrogen bond generates an S(6) ring motif. In the crystal, the xanthone and water molecules are linked into a three-dimensional network by O—H⋯O hydrogen bonds and weak C—H⋯O interactions. π–π interactions, with centroid–centroid distances of 3.5982 (7), 3.6081 (7) and 3.6456 (7) Å, are also observed.


Comment
Xanthones are reported to exhibit various biological and pharmacological properties (Obolskiy et al., 2009) such as antibacterial , antifungal (Gopalakrishnan et al., 1997), anti-inflammatory (Boonnak, Khamthip et al., 2010) and anti-cancer (Ho et al., 2002) activities. We have previously reported several isolated xanthones and their biological activities Boonnak, Khamthip et al., 2010). Among these compounds, the title xanthone (I), which is also known as pruniflorone N, showed antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) with a MIC value of 9.37 µg mL -1 . Compound (I) crystallized out in the centrosymmetric Pbca space group indicating that the extracted material was a racemate, Figure 1.

The crystal packing of (I) is stabilized by intermolecular O-H···O hydrogen bonds and weak C-H···O interactions
( Table 1). The xanthone and water molecules are linked into a three dimensional network by these interactions (Fig. 3).

Experimental
The green fruits of C. formosum ssp. pruniflorum (5.00 kg) were 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 crude CH 2 Cl 2 extract (31.42 g), which was subjected to QCC (Quick Column Chromatography) on silica gel using hexane as a first eluent and then increasing the polarity with acetone to give 14 fractions (F1-F14). Fraction F10 was separated by QCC eluting with a gradient of supplementary materials acetone-hexane to give 17 subfractions (F10A-F10Q). Subfractions F10N was separated by CC and eluted with gradient of EtOAc-hexane to obtain 8 subfractions (F10N1-F10N8). Subfraction F10N6 was separated by CC and eluted with CHCl 3 to give the title compound as a yellow solid (5.3 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.

Refinement
Hydroxy H atoms were located from the difference maps and refined isotropically. The remaining H atoms were placed in calculated positions with d(C-H) = 0.93 Å for aromatic, 0.98 for CH, 0.97 for CH 2 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 2′-hydroxy-4′,4′-dimethylpyran is disordered over two sites with refined site occupancies of 0.798 (3) and 0.202 (3). All disordered atoms were subjected to similarity restraints. The same U ij parameters were used for atom pairs C12A/C12B, C13A/C13B, C18A/C18B, C19A/C19B and O5A/O5B.

Figure 1
The chemical transformation that yields the title compound.     (Cosier & Glazer, 1986) operating at 100.0 (1) K. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.