Crystal structure of dibromomethoxyseselin (DBMS), a photobiologically active pyranocoumarin

The title compound, a bromo derivative of pyranocoumarin, possesses photobiological activity. It was formed by bromination of seselin by using NBS in MeOH at room temperature. In the crystal, molecules are linked by π–π stacking interactions and weak C—H⋯O interactions, forming layers parallel to (001).


Chemical context
The title compound is a substituted product of seselin containing two bromine atoms and a methoxy group. This class of pyranocoumarins have an absorption band in the near-UV region due to the presence of extended conjugated double bonds and exhibit photomutagenic (Appendino et al., 2004) and photocarcinogenic properties to bind with the purin base of DNA in a living cell to yield photoadducts (Conforti et al., 2009). Based on the properties of these molecules, they are employed for the treatment of numerous inflammatory skin diseases such as atopic dermatitis and the pigment disorders vitiligo and psoriasis on exposure to ultra violet (UV) radiation in photodynamic therapy (PDT). It has also been found that as a result of their strong ability for absorption of UV radiation, they are utilized as photoprotective agents to prevent the absorption of harmful UV radiation by the skin in the form of a variety of sun-screening lotions widely used in dermatological applications in the cosmetic and pharmaceutical industries (Chen et al., 2007(Chen et al., , 2009). In addition to these activities, antiproliferative activity and photo-toxicity of related coumarin molecules has been reported against numerous cancer cell lines such as HL60, A431 (Conconi et al., 1998). Inhibited proliferation in the human hepatocellular carcinoma cell line has also been reported (March et al., 1993). Recently, this type of molecule has been connected as a spacer with porphyrin moieties to obtain a scaffold-type macromolecule (molecular nanotweezers) and has been employed to study the interaction (host-guest interaction) with fullerenes ISSN 2056-9890 such as C 60 and C 70 Ghosh et al., 2014) in supramolecular chemistry and material science. Molecular tweezers containing a coumarin moiety showed better quantum yield and fluorescence absorption as a result of the presence of the extended conjugated enone of pyranocoumarin. As part of our ongoing studies in this area, we herein describe the synthesis and structure of the title molecule.

Structural commentary
The title molecule ( Fig. 1) is composed of three different types of rings viz. benzene, pyran and dihydropyran. The benzopyran ring system C1/C5-C12/O2 is essentially planar with a maximum deviation of 0.044 (2) Å for atom O2. The dihydropyran ring C1-C5/O1 is in a half-chair conformation and atoms C2 and C3 deviate by À0.385 (4) and 0.280 (4) Å from the plane through the other four essentially planar atoms (mean deviation 0.003 Å ), which makes a dihedral angle of 4.6 (2) with the benzopyran ring system. The relative stereochemistry at atoms C3 and C4 is R/S and S/R.

Figure 1
The molecular structure of the title compound, showing the atom labelling and displacement ellipsoids drawn at the 50% probability level Part of the crystal structure with weak C-HÁ Á ÁO hydrogen bonds shown as dashed lines. Only the H atoms involved in hydrogen bonds are shown.

Synthesis and crystallization
The title compound is a colourless solid substance formed on bromination of the naturally occurring seseline isolated from the methanol extract of T. stictocarpum by means of column chromatography over SiO 2 gel with gradient elution by using a mixture of the binary solvents hexane and ethyl acetate. The bromination was conducted using NBS in methanol at room temperature with continuous stirring by means of mechanical stirrer over a period of 12 h. The reaction product was worked up by the usual method to yield crude product, which was then purified by solvent elution to yield the title compound. A colourless prism-shaped crystal was obtained after recrystallization (Â3) from ethyl acetate:hexane (1:4) at room temperature by slow evaporation of the solvents.

Refinement
Crystal data, data collection and structure refinement details are summarized in   software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 &gt; 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. Rfactors 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.