Absolute configuration of micromelin

The title compound {systematic name: 7-methoxy-6-[(1R,2R,5R)-5-methyl-4-oxo-3,6-dioxabicyclo[3.1.0]hexan-2-yl]-2H-chromen-2-one}, C15H12O6, is a coumarin, which was isolated from the roots of Micromelum glanduliferum. There are two molecules in the asymmetric unit with slight differences in bond angles. In both molecules, the furan ring adopts a flattened envelope conformation. In the crystal, molecules are linked by weak C—H⋯O interactions into chains along the a axis. Aromatic π–π stacking interactions with centroid–centroid distances in the range 3.6995 (11)–3.8069 (11) Å and C⋯O short contacts [3.030 (2)–3.171 (3) Å] also occur.

Although Micromelum glanduliferum is one of Rutaceae plants, however, no phytochemical investigation has been reported.
As part of our continuing studies of the phytochemical constituents and bioactive compounds in Thai medicinal plants, we report herein the crystal structure of the title compound which was isolated from the roots of M. glanduliferum which were collected from Nongkai Province in the northeastern part of Thailand.
There are two crystallograpic independent molecules A and B in the asymmetric unit of (I), C 15 H 12 O 6 , (  (Allen et al., 1987). The absolute configuration at atoms C10, C11 and C13 or positions 1, 2 and 5 of the micromelin are R,R,R configurations.

Experimental
The chemical contents of the roots of M. glanduliferum (5.25 kg) were successively extracted with CH 2 Cl 2 over the period of 3 days at room temperature. Removal the solvent under reduced pressure provided CH 2 Cl 2 extract which were subjected to quick column chromatography (QCC) over silica gel and eluted with a gradient of n-hexane-EtOAc (100% n-hexane to 100% EtOAc) to provide nine fractions (A-I). Fraction F (6.23 g) was washed with n-hexane and recrystallized from CH 2 Cl 2 /CH 3 OH (1:4 v/v) to give colourless needles of the title compound. Mp 491.0-492.2 K (decompose).
supplementary materials sup-2 Refinement H atoms were placed in calculated positions with (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 0.79 Å from C5B and the deepest hole is located at 0.81 Å from C7B. 2632 Friedel pairs were used to determine the absolute configuration. The crystal is a twin with BASF = 0.497 (1).

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.