Methyl 2-(2,2-dimethyl-3a,6a-dihydrofuro[3,2-d][1,3]dioxol-5-yl)-4-oxo-4H-chromene-3-carboxylate

In the title molecule, C18H16O7, the dioxolane ring adopts an envelope conformation with the dimethyl-substituted C atom as the flap. The furan ring is almost coplanar with the pyran ring, with a dihedral angle of 1.04 (10)° between the planes, and it makes a dihedral angle of 67.97 (11)° with the mean plane of the dioxolane ring. The latter makes a dihedral angle of 67.15 (10)° with the pyran ring. The O atom attached to the pyran ring deviates by −0.009 (1) Å. The crystal packing features C—H⋯O hydrogen bonds, forming a three-dimensional structure. The methoxycarbonyl atoms are disordered over two positions, with a refined occupancy ratio of 0.508 (18):0.492 (18).

In the title molecule, C 18 H 16 O 7 , the dioxolane ring adopts an envelope conformation with the dimethyl-substituted C atom as the flap. The furan ring is almost coplanar with the pyran ring, with a dihedral angle of 1.04 (10) between the planes, and it makes a dihedral angle of 67.97 (11) with the mean plane of the dioxolane ring. The latter makes a dihedral angle of 67.15 (10) with the pyran ring. The O atom attached to the pyran ring deviates by À0.009 (1) Å . The crystal packing features C-HÁ Á ÁO hydrogen bonds, forming a three-dimensional structure. The methoxycarbonyl atoms are disordered over two positions, with a refined occupancy ratio of 0.508 (18):0.492 (18).
In the title molecule, Fig. 1 The crystal packing is stabilized by intermolecular C-H···O hydrogen bonds, forming a three-dimensional structure (Table 1 and Fig. 2).

Experimental
Triethylamine (1.10 ml, 4 equiv) was added to a stirred solution of 4-hydroxycoumarin (0.32 g, 2 mmol) and The reaction mixture was heated at 343 -353 K for 24 h, and the progress of the reaction was monitored by TLC. After completion of the reaction, the solvent was evaporated in vacuum. The resulting residue was further purifed by flash column chromatography (ethyl acetate/hexane) on silica gel. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of the title compound in ethyl acetate at room temperature.
The restraints, SHELXL97 commands FLAT and SIMU, ensure chemically and physically reasonable parameters for the 0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and = 1.2U eq (C) for other H atoms.

Figure 1
The molecular structure of the title molecule, with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level.  The crystal packing of the title compound viewed along the a axis. C-H···O hydrogen bonds are shown as dashed lines (see Table 1 for details).  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.18 e Å −3 Δρ min = −0.19 e Å −3

Special details
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (