7,7′-Dihydroxy-4,4′-dimethyl-3,4-dihydro-2H,2′H-4,6′-bichromene-2,2′-dione

The title compound, C20H16O6, which contains one chiral centre, crystallizes as a racemate. The mean planes of the two coumarin units make a dihedral angle of 88.07 (2)°. The pyrone ring containing the chiral centre adopts a sofa conformation. In the crystal, four molecules are linked by O—H⋯O hydrogen bonds, forming a tetrameric ring with graph-set motif R 4 4(32). These tetramers are further linked by O—H⋯O hydrogen bonds into a three-dimensional network.


& Seshadri
The title compound, (I), Fig. 1, has one chiral carbon atom (the C11 atom). Both enantiomers are present in the crystal structure, forming a racemate.
In the molecule of (I), the mean planes of the two coumarin units make a dihedral angle of 88.07 (2). In one of the coumarin units, the the dihedral angle between the least-squares planes of the pyrone and benzene rings is 3.36 (6)°. In the other coumarin unit the pyrone ring adopts an envelope conformation and the dihedral angle with the aromatic ring is 13.23 (6)°.
In the crystal, the molecules are linked by O-H···O hydrogen bonds (Fig. 2, Table 2) forming rings with four molecules, graph-set motif R 4 4 (32), according to the Etter's graph-set theory (Etter et al., 1990), centered about inversion centres.
These rings are linked, with each molecule participating in two rings, forming a three-dimensional network. The structure is stabilized further by weak C-H···O hydrogen bonds.

Experimental
Polyphospharic acid was prepared by mixing orthophosphoric acid (15 mL) and phosphorus pentaoxide (23.5 g) followed by heating on a water bath for 1.5 hr. A catalytic amount of polyphosphoric acid (160 g) was added to resorcinol (11 g, 100 mmol) and ethyl acetoacetate (13 mL, 100 mmol) and was heated on a water bath (75-80 °C) for 20 min. with stirring.
The viscous mixture was then poured into ice cold water and the resulting solid (18 g, m.p. 180 °C) was crystallized with EtOH as shinning crystal (7 g), it was characterized as 7-hydroxy-4-methyl coumarin by comparison with authentic sample.
The mother liquor showed the presence of two bands I&II (TLC, silica-gel, benzene-ethylacetate 2:1) which was separated into individual compounds by preparative thin layer chromatography in the same solvent. The compound I was identified as 7-hydroxy-4-methyl coumarin while the compound II (m.p. 305 °C) was characterized as a novel coumarin dimer, (I), by IR, 1 H NMR, 13 C NMR & MS spectra. supplementary materials sup-2 Refinement All H atoms were located in a difference Fourier synthesis, placed in calculated positions and refined as riding on their parent atoms, using SHELXL97 (Sheldrick, 2008) defaults. Fig. 1. A plot of the title compound. Displacement ellipsoids are drawn at the 50% probability level.