Crystal structure of 6,6′-dimethyl-2H,2′H-3,4′-bichromene-2,2′-dione

In the title compound, the dihedral angle between the two coumarin units is 52.37 (19)°, showing a gauche arrangement across the C—C bond which links the two ring systems. In the crystal, C—H⋯O hydrogen bonds connect centrosymmetrically-related molecules into dimers.

In view of the above cited activities of directly linked coumarin dimers, the present work reports the synthesis under metal-free conditions of a new 4-3 0 bicoumarin and its structure.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The packing viewed along the b axis (Fig. 2) shows the existence of intermolecular C-HÁ Á ÁO hydrogen bonds between the carbonyl O4 of one coumarin moiety and the aromatic H8 of the second unit (Table 1), which has also been observed in a 3-5 0 bicoumarin (Fun et al., 2009). The two coumarin rings exhibit an s-trans arrangement across the C4-C11 bond for the two double bonds viz. C3 C4 and C11 C12. The non-planar nature of the bi-heterocyclic system is revealed through the torsion angles C3-C4-C11-C12 [À52. 37 (19) ] and C10-C4-C11-C19 [À59.32 (17) ], which almost corresponds to a gauche conformation.

Synthesis and Crystallization
6-Methylcoumarin 4-acetic acid (0.01 mol) and 5-methylsalicylaldehyde (0.01 mol) were taken in a round-bottomed flask containing (1.5 eq) NaH and 3 ml of acetic anhydride. The flask, fitted with a guard tube, was stirred for 1.5 h. The progress of the reaction was monitored by TLC, the solid that separated was filtered off and washed with diethyl ether and The molecular structure of the title compound, showing the atomlabelling scheme and with displacement ellipsoids drawn at the 50% probability level.

Figure 2
A packing diagram of the title compound, viewed along the b axis.
Dashed lines indicate C-HÁ Á ÁO hydrogen bonds andinteractions. H atoms not involved in hydrogen bonding have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).
again with 5% NaHCO 3 to remove unreacted 6-methylcoumarin 4-acetic acid. Then the solid was dried and recrystallized from ethanol. Crystals suitable for diffraction studies were obtained through slow evaporation from a DMF solution.

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.