7,7′,8,8′-Tetramethoxy-4,4′-dimethyl-3,3′-bicoumarin

In the crystal structure, the title compound, C24H22O8, lies on a twofold rotation axis and the asymmetric unit comprises one half-molecule. The dihedral angle formed by the coumarin unit with the symmetry-related part is 74.78 (14)°. One of the methoxy groups attached to the coumarin unit is considerably twisted, making an angle of 87.17 (17)° with respect to the coumarin unit; the other is twisted by 0.66 (19)°. No classical hydrogen bonds are found in the sturcture; only a weak C—H⋯π interaction and short intramolecular O⋯O contacts [2.683 (2)–2.701 (2) Å] are observed.

In the crystal structure, the title compound, C 24 H 22 O 8 , lies on a twofold rotation axis and the asymmetric unit comprises one half-molecule. The dihedral angle formed by the coumarin unit with the symmetry-related part is 74.78 (14) . One of the methoxy groups attached to the coumarin unit is considerably twisted, making an angle of 87.17 (17) with respect to the coumarin unit; the other is twisted by 0.66 (19) . No classical hydrogen bonds are found in the sturcture; only a weak C-HÁ Á Á interaction and short intramolecular OÁ Á ÁO contacts [2.683 (2)-2.701 (2) Å ] are observed.

Comment
Coumarins are a large group of naturally occurring oxygen heterocycles representing 2H-1-benzopyran-2-one derivative.
Many natural coumarins are reputed for their wide range of biological activites such as antibacterial (El-Agrody et al., 2001;Pratibha & Shreeya, 1999), antifungal (Shaker, 1996;El-Farargy, 1991), antioxidant (Yang et al., 2005), analgesic (Ghate et al., 2005), anti-inflammatory (Emmanuel-Giota et al., 2001) and antitumor (Nofal et al., 2000) properties. Bi and tri-coumarins are comparatively new groups which are widely spread in nature and their biological properties are also well known (Laakso et al., 1994). One of the characteristic pharmacological properties of coumarin derivatives is the anticoagulant action (Kennedy & Thornes, 1997). A large number of natural and semisynthetic coumarin and bicoumarin derivatives have been reported to demonstrate chemopreventive (Carlton et al., 1996) and anti-HIV (Zhou et al., 2000) activities. Keeping in view of these biological importance of coumarins and their dimers, we have synthesized the title compound (I) and report here its structure.
The asymmetric unit of (I) (Fig. 1 (Allen et al., 1987) and bond angles are normal.

Experimental
A mixture of 7,8-dimethoxy-4-methyl coumarin (2.20 g, 10 mmol) and manganese(III) acetate (0.774 g, 1 mmol) was stirred at room temperature, then 70% perchloric acid (0.8 g, 6 mmol) was added. The reaction mixture was heated under reflux at 114°C with stirring in the atmosphere of nitrogen for 3 h. The reaction mixture was cooled and diluted with 50 ml of benzene.
The benzene solution was washed with water and aq. NaHCO 3 , dried over anhydrous Na 2 SO 4 and left to evaporate. The residue showed two major compounds which were separated by column chromatography followed by preparative thin layer chromatography (Benzene: EtOAc, 9:1) into the title compound (I) (260 mg, 12%).

Refinement
All the hydrogen atoms were located from the Fourier map and allowed to refine freely. Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids and the atom numbering scheme. [Symmetry code: -x, y, -z + 1/2 to generate equivalent atoms].

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K. 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 Rfactors(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.