Crystal structure and computational study of 3,4-dihydroxy-3-hydroxymethyl-9-methyl-6-methylidene-3a,4,5,6,6a,9,9a,9b-octahydroazuleno[4,5-b]furan-2,8(3H,7H)-dione

The cyclopentane ring displays a twist conformation and the γ-lactone ring has an envelope conformation while the cycloheptane ring adopts a twist-chair conformation. O—H⋯O hydrogen bonds link the molecules, forming a three-dimensional network. A comparison between the structural parameters obtained by X-ray structure analysis and theoretical calculations give a satisfactory agreement.


Supramolecular features
In the crystal, neighbouring molecules are connected by O-HÁ Á ÁO hydrogen bonds (Table 1; Fig. 2), forming a three dimensional network.

Theoretical calculations
According to the results of a quantum mechanical calculation using the CNDO approximation (Pople et al., 1970), the charges at atoms O1, O2, O3, O4, O5 and O6 are À0.270, À0.241, À0.261, À0.255, À0.243 and À0.268 e À , respectively. The total energy and dipole moment of the title molecule are The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level.
energy levels are À12.5301 and 3.7741 eV, respectively. In addition, a geometrical optimization calculation of the title compound was performed using MOPAC PM3 (Stewart, 1985). The spatial disposition of the atoms of the title molecule calculated with PM3 is shown in Fig. 3. The net charges at atoms O1, O2, O3, O4, O5 and O6 are À0.225, À0.304, À0.340, À0.318, À0.287 and À0.307e À , respectively. The total energy and dipole moment of the title molecule are À3848.31 eV and 3.305 Debye. The HOMO and LUMO energy levels are À10.3738 and 0.5350 eV, respectively. In the calculations, the molecule was assumed to be isolated and in an absolute vacuum therefore resulting in calculated bond lengths, bond angles and torsion angles that are greater than those observed experimentally. The PM3 method gives the lowest values for the HOMO and LUMO energy levels and the dipole moment.

Synthesis and crystallization
Centaurea polypodiifolia Boiss. (1.0 kg) was extracted with methanol (3 Â 5L), filtered, and the solvent removed in vacuo to obtain the crude material which was dissolved in water (333 K) and extracted with ethyl acetate. The organic phase was separated by separator funnel and the solvent was removed by reduced pressure to yield the extract (10 g). The extract was subjected to silica gel (60, GF 254 ) column chromatography (2.5 cm Â 60 cm). A hexane/ethyl acetate mixture (6:4 v/v) was used as eluent. 24 fractions of 250 mL were collected. After checking by thin layer chromatography, 6-8 fractions were combined and crystallized in methanol to give suitable crystals of the title compound on slow evaporation of the solvent (yield: 10 mg). 13

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bound to oxygen atoms were found in a difference Fourier map and allowed to ride on their parent atoms, with O-H = 0.82 Å and with U iso = 1.5 U eq (O). H atoms bound to carbon atoms were placed in idealized positions and allowed to ride on their parent atoms, with C-H = 0.93-0.98 Å , and with U iso = 1.2 U eq (C). One outlier (1 0 1) was omitted in the last cycles of refinement.

Figure 3
Spatial view of the molecule of the title compound calculated using the PM3 method.  (Farrugia, 2012); software used to prepare material for publication: PLATON (Spek, 2009).