5,6-Dimethyl-1,2,9,10-tetrahydropyrano[3,2-f]chromene-3,8-dione

The title molecule, C14H14O4, lies on a twofold rotation axis that bisects the central benzene ring, with only one half-molecule in the asymmetric unit. The pyranone systems adopt distorted twist- boat conformations, with the two methylene C atoms displaced by 0.537 (1) and 0.163 (2) Å from the best-fit plane through the remaining five C and O atoms (r.m.s. deviation = 0.073 Å). In the crystal, bifurcated C—H⋯(O,O) hydrogen bonds link pairs of adjacent molecules in an obverse fashion, stacking molecules along c. These contacts are further stabilized by very weak π–π interactions between adjacent benzene rings with centroid–centroid distances of 4.1951 (4) Å. Additional C—H⋯O contacts link these stacks, giving a three-dimensional network.

The title molecule, C 14 H 14 O 4 , lies on a twofold rotation axis that bisects the central benzene ring, with only one halfmolecule in the asymmetric unit. The pyranone systems adopt distorted twist-boat conformations, with the two methylene C atoms displaced by 0.537 (1) and 0.163 (2) Å from the best-fit plane through the remaining five C and O atoms (r.m.s. deviation = 0.073 Å ). In the crystal, bifurcated C-HÁ Á Á(O,O) hydrogen bonds link pairs of adjacent molecules in an obverse fashion, stacking molecules along c. These contacts are further stabilized by very weakinteractions between adjacent benzene rings with centroid-centroid distances of 4.1951 (4) Å . Additional C-HÁ Á ÁO contacts link these stacks, giving a three-dimensional network.
The asymmetric unit of the title compound contains only one half of the molecule, which lies on a twofold rotation axis that bisects the central benzene ring (Fig 1) The pyranone ring systems adopt distorted twist boat conformations, with the C2 and C3 methylene carbon atoms displaced by 0.537 (1) and 0.163 (2) Å respectively from the best fit plane through C1/(O1)/O2/C5/C4 which has an r.m.s. deviation 0.073 Å. A search of the Cambridge Database (Allen, 2002) revealed no comparable compounds, with or without substitution on the benzene ring. However we have previously reported closely related chroman-2-one derivatives without a second pyranone ring system , Cameron et al., 2011.
Bond lengths in the structure are not unusual (Allen et al., 1987) and are comparable to those in the chroman-2-one compounds mentioned previously.

Experimental
The title compound was obtained as a by-product from a Friedel-Crafts type addition reaction of 2,3-dimethylhydroquinone with acrylic acid during the synthesis of 6-hydroxy-7,8-dimethylchroman-2-one (Lecea et al., 2010). Following work-up according to the literature, X-ray quality crystals of (I) were obtained from dichloromethane solution.

Refinement
All H-atoms bound to carbon were refined using a riding model with d(C-H) = 0.99 Å, U iso =1.2U eq (C) for methylene and 0.98 Å, U iso = 1.5U eq (C) for CH 3 H atoms.

Figure 4
Overall packing of (I) with hydrogen bonds drawn as dashed lines. 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.