Redetermination and absolute configuration of pruniflorone M monohydrate

The title xanthone known as pruniflorone M (systematic name: (2R)-5,10-dihydroxy-2-hydroxymethyl-1,1-dimethyl-1H-furo[2,3-c]xanthen-6-one), crystallized in a monohydrate form, C18H16O6·H2O. It was isolated from the green fruits of Cratoxylum formosum ssp. pruniflorum. The structure of the title compound has been reported previously [Boonnak et al. (2010 ▶). Aust. J. Chem. 63, 1550–1556], but we report here the absolute configuration determined using Cu Kα radiation. There are two crystallograpically independent molecules in the asymmetric unit, which differ slightly in the bond angles. The hydroxymethyl substituents at position 2 of the furan rings of both pruniflorone M molecules adopt R configurations. In both molecules, the three rings of the xanthone skeleton are approximately coplanar, with an r.m.s. deviation of 0.0124 (2) Å for one molecule and 0.0289 (2) Å for the other, and the furan ring adopts an envelope conformation. In the crystal, molecules of pruniflorone M and water are linked into a two-dimensional network by O—H⋯O hydrogen bonds and weak C—H⋯O interactions. The crystal structure is further consolidated by π–π interactions with centroid–centroid distances in the range 3.5987 (13)–3.7498 (14) Å. Short C⋯C [3.378 (3) Å] and O⋯O [2.918 (3) Å] contacts are also observed.

The title xanthone known as pruniflorone M (systematic name: (2R)-5,10-dihydroxy-2-hydroxymethyl-1,1-dimethyl-1H-furo[2,3-c]xanthen-6-one), crystallized in a monohydrate form, C 18 H 16 O 6 ÁH 2 O. It was isolated from the green fruits of Cratoxylum formosum ssp. pruniflorum. The structure of the title compound has been reported previously [Boonnak et al. (2010). Aust. J. Chem. 63, 1550-1556], but we report here the absolute configuration determined using Cu K radiation. There are two crystallograpically independent molecules in the asymmetric unit, which differ slightly in the bond angles. The hydroxymethyl substituents at position 2 of the furan rings of both pruniflorone M molecules adopt R configurations. In both molecules, the three rings of the xanthone skeleton are approximately coplanar, with an r.m.s. deviation of 0.0124 (2) Å for one molecule and 0.0289 (2) Å for the other, and the furan ring adopts an envelope conformation. In the crystal, molecules of pruniflorone M and water are linked into a two-dimensional network by O-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á ÁO interactions. The crystal structure is further consolidated byinteractions with centroidcentroid distances in the range 3.5987 (13)-3.7498 (14)
During the course of our research on the chemical constituents and bioactive compounds from the green fruits of Cratoxylum formosum ssp. pruniflorum, which were collected from Pha Yao province in the northern part of Thailand, the title xanthone (I) known as pruniflorone M (Boonnak et al., 2010) was isolated. The previous report showed that (I) possess nitric oxide inhibitory activity (Boonnak et al., 2010). The absolute configuration of (I) was determined by making use of the anomalous scattering of Cu Kα X-radiation with the Flack parameter being refined to 0.06 (19). We report herein the crystal structure of (I).
There are two crystallograpically independent molecules A and B in the asymmetric unit of (I), C 18 H 16 O 6 .H 2 O, ( Fig.   1) with the same conformation but with slight differences in bond angles. In the structure of (I), the three ring system  (Table 1) generate S(6) ring motifs (Bernstein et al., 1995). The bond distances in (I) are within normal ranges (Allen et al., 1987) and comparable to the related structures (Boonnak et al., 2006;Boonnak, Fun et al., 2007). The hydroxymethyl substituents at position 2 (on atoms C15A and C15B ) of the furan rings of both pruniflorone M molecules adopt R configurations.
In the crystal packing of (I) (Fig. 2), the molecules of pruniflorone M and water are linked into a two dimensional network by O-H···O hydrogen bonds and weak C-H···O interactions (Table 1). π···π interactions were also observed with centroid···centroid distances: Cg 1 ···Cg 5 v = 3.7453 (13) Å; Cg 1 ···Cg 6 vi = 3.6847 (13)  The green fruits of C. formosum ssp. pruniflorum (5.00 kg) were extracted with CH 2 Cl 2 (2x20 L, for a week) successively at room temperature and were further evaporated under reduced pressure to afford the crude CH 2 Cl 2 extracts (31.42 g). The crude extract was further 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 acetone-hexane to give 17 subfractions (F10A-F10Q). Subfractions F10N was further separated by CC and eluted with a gradient of EtOAc-hexane to give 8 subfractions (F10N1-F10N8). Subfraction F10N2 was further separated by CC and eluted with CHCl 3 to give the title compound as yellow powder (28.0 mg). Yellow block-shaped single crystals of the title compound suitable for x-ray structure determination were recrystallized from CHCl 3 by the slow evaporation of the solvent at room temperature after several days, Mp. 508-510 K.

Refinement
All H atoms were placed in calculated positions with (O-H) = 0.82-1.06 Å for OH, (C-H) = 0.93 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 1.34 Å from O6B and the deepest hole is located at 0.51 Å from O6B. 2102 Friedel pairs were used to determine the absolute configuration. There is no pseudo-symmetry observed in the crystal structure.   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.