7,7′,8,8′-Tetramethoxy-4,4′-dimethyl-3,5′-bichromene-2,2′-dione

In the title molecule, C24H22O8, the mean planes of the two coumarin units are inclined to each other at a dihedral angle of 79.93 (3)°. The attached methoxy groups form torsion angles of 7.65 (19) and 78.36 (14)° with respect to one coumarin unit, and angles of 9.01 (16) and 99.08 (11)° with respect to the other coumarin unit. In the crystal structure, weak intermolecular C—H⋯O hydrogen bonds connect pairs of molecules to form dimers, generating R 2 2(16) and R 2 2(18) rings; the dimers are linked by further weak intermolecular C—H⋯O hydrogen bonds, forming extended chains. Additional stabilization is provided by weak C—H⋯π interactions.

In the title molecule, C 24 H 22 O 8 , the mean planes of the two coumarin units are inclined to each other at a dihedral angle of 79.93 (3) . The attached methoxy groups form torsion angles of 7. 65 (19) and 78.36 (14) with respect to one coumarin unit, and angles of 9.01 (16) and 99.08 (11) with respect to the other coumarin unit. In the crystal structure, weak intermolecular C-HÁ Á ÁO hydrogen bonds connect pairs of molecules to form dimers, generating R 2 2 (16) and R 2 2 (18) rings; the dimers are linked by further weak intermolecular C-HÁ Á ÁO hydrogen bonds, forming extended chains. Additional stabilization is provided by weak C-HÁ Á Á interactions.

Comment
Coumarins are a large group of naturally occurring oxygen heterocycles representing 2H-1-benzopyran-2-one derivatives.
Many natural coumarins are reputed for their wide range of biological activites such as antibacterial (El-Agrody et al., 2001;Pratibha et al., 1999), antifungal (Shaker, 1996El-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). Bi and tri-coumarins are a comparatively new group of compounds which are widespread in nature and their biological properties are also well known (Laakso et al., 1994). One of the characteristic pharmacological properties of coumarin derivatives is anticoagulant action (Kennedy et al., 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 herein its crystal structure.
The molecular structure of the title compound is shown in Fig .1. In crystal structure of (I) molecules are linked by weak intermolecular C-H···O hydrogen bonds to form R 2 2 (16) and R 2 2 (18) rings (Bernstein et al., (1995). The two coumarin units are essentially planar with the maximum deviation from planarity of 0.0665 (11) Allen et al. (1987) and bond angles are normal.
The crystal packing is illustrated in Fig. 2. In addition C-H···π interactions help stabilize the crystal structure.

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
H atoms were positioned geometrically [C-H = 0.93-0.96 Å] and refined using a riding model with U iso (H) = 1.2U eq (C) and 1.5U eq (methyl C). A rotating-group model was used for the methyl groups. Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids and the atom numbering scheme.