Crystal structure of 2-(2,3-dimethoxynaphthalen-1-yl)-3-hydroxy-6-methoxy-4H-chromen-4-one

In the title compound, C22H18O6, the dimethoxy-substituted naphthalene ring system is twisted relative to the 4H-chromenon skeleton by 88.96 (3)°. The two methoxy substituents are tilted from the naphthalene ring system by 1.4 (4) and 113.0 (2)°, respectively. An intramolecular O—H⋯O hydrogen bond closes an S(5) ring motif. In the crystal, pairs of O—H⋯O hydrogen bonds form inversion dimers with R 2 2(10) loops and C—H⋯O interactions connect the dimers into [010] chains.

Supporting information for this paper is available from the IUCr electronic archives (Reference: FF2142).

S1. Introduction
Flavonols, such as Quercetin, Azaleatin and Kaempferol, are a class of flavonoids that have a 3-hydroxyflavone backbone. Because of their wide spectrum of biological activities (Burmistrova et al. 2014, Dias et al. 2013, variety of flanonols have been isolated from natural sources and synthesized (Bendaikha et al. 2014;Prescott et al. 2013). In addition, they have been used as fluorescent probes for sensing and imaging due to their dual fluorescence. The fluorescence of flavonols has been shown to be related to the angle between the 4H-chromene-4-one moiety and the attached aromatic ring (Klymchenko et al. 2003). Our research project has been focused on development of novel flavonols which show broad range of biological activities (Lee et al. 2014), therefore the title compound was synthesized and its crystal structure was determined. A starting material, chalcone (III), was prepared by the previously reported methods (Yong et al. 2013). Flavonol was obtained by oxidative cyclization of the chalcone (III) with H 2 O 2 in alkaline methanol medium (Fig. 3). In the title compound, C 22 H 18 O 6 , angle between the dimethoxy-substituted naphthalene ring and the 4H-chromenon skeleton is 88.96 (3)°, which shows they are almost orthogonal each other. In our previous report on flavonol (Yoo et al., 2014), the angle between 4H-chromenon and benzene ring is 5.2 (4)°. The methoxy groups in naphthalene ring at C12 and C13 are tilted from naphthalene ring by 1.4 (4)° and 113.0 (2)°, respectively. Methoxy group at C12 (meta position) lies almost in the same plane of naphthalene ring. Methoxy group at C13 (ortho position), however, is twisted away from the plane of naphthalene ring. An intramolecular O-H···O hydrogen bond closes S(5) ring motif. In the crystal, pairs of O-H···O hydrogen bonds form inversion dimer with graph-set notation R 2 2 (10) and C -H···O interactions connect the dimers into [010] chains. Examples of structures of flavonols have been published (Narita et al., 2015;Serdiuk et al., 2013).
To a cooled reaction mixture was added 2 ml of 50% (w/v) aq. KOH solution and stirred at room temperature for 20 h. At the end of the reaction, ice-water was added to the mixture and acidified with 3 N HCl (pH = 3-4). The precipitation was filtered under vacuum and washed with methanol to give chalcone compound III (yield: 48%, m.p: 407-408 K). The chalcone compound (III, 1 mmol, 364 mg) was dissolved in 6 ml of methanol and 4 ml of THF. The reaction was cooled in a water-ice bath (2-4 °C) and a cold solution of 16% sodium hydroxide (1 ml) was added with stirring. After 10 min, to the reaction mixture was added 2 ml of 35% H 2 O 2 . The end point of reaction was monitored by TLC. After completion of reaction, the reaction mixture was acidified with 3 N HCl (pH = 4-5). The pale yellow precipitate obtained was filtered and washed with ethanol to give the titled compound (66%). Recrystallization in the ethanol solvent gave crystals

S2.1. Refinement
The H atoms were placed at calculated positions and refined as riding with C-H = 0.95 A [U iso (H) = 1.2 U eq (C)].

Figure 1
Molecular structure of the title compound, showing the atom-labelling scheme and with displacement ellipsoids drawn at the 50% probability level.  Synthetic scheme for the title compound. Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > 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. Symmetry codes: (i) −x, −y+1, −z; (ii) x, −y+1/2, z+1/2; (iii) x, y−1, z; (iv) −x, y−1/2, −z+1/2.