Crystal structures and conformational analyses of three pyranochromene derivatives

In the crystal structures of the three title pyran–chromene derivatives, (I)–(III), molecules are linked by C—H⋯O hydrogen bonds which generate molecular sheets parallel to the ab plane with (28) loops in (I), inversion dimers with (10) loops in (II) and chains along the b axis with (12) ring motifs in (III).

Chromene derivatives also play an important role in the production of highly effective fluorescent dyes for synthetic fibers, daylight-fluorescent pigments and electrophotographic and electroluminescent devices (Khairy et al., 2009). Against this background, the title compounds, (I), (II) and (III), were synthesized and we report herein on their crystal structures and molecular conformations.

Structural commentary
The molecular structures of compounds, (I), (II) and (III) are illustrated in Figs. 1, 2 and 3, respectively. All three compounds comprise a central pyran ring (B) fused with a chromene ring system (C+A). The central pyran ring (B) is fused with a second chromene ring system (E+F) in (I), a cyclohexene ring (E) in (II) and a pyrimidine ring (E) in (III); see scheme and Figs. 1-3. In compounds (I) and (II), a carboxylate side chain and a benzene ring (D) are attached to the central pyran ring (B), in adjacent positions, whereas in (III) there is a cabonitrile side chain and an ethyl-substituted benzene ring attached to the central pyran ring (B). The molecular structure of compound (I), showing the atom labelling. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The molecular structure of compound (II), with the atom labelling. The intramolecular C4-H4Á Á ÁO3 interaction, which generates an S(7) ring motif, is shown as a dashed line. Displacement ellipsoids are drawn at the 30% probability level.

Figure 3
The molecular structure of compound (III), with the atom labelling. The intramolecular C4-H4Á Á ÁO3 interaction, which generates an S(7) ring motif, is shown as a dashed line. Displacement ellipsoids are drawn at the 30% probability level.
In compounds (I) and (III), the central pyran rings (B) adopt half-chair conformations with puckering amplitudes Q = 0.5166 (15) Å , = 51.22 (17) In compound (I), the dihedral angle between the benzene ring (C) and the mean plane of the pyran ring (A -sofa conformation) of the chromene moiety is 14.95 (8) , whereas in (II) and (III) the same angles are 7.83 (7) and 6.42 (10) , respectively (the A rings here have half-chair conformations). The decrease in the value of the dihedral angle in compounds (II) and (III) is probably due to the intramolecular C-HÁ Á ÁO short contacts which generate S(7) ring motifs. The second coumarin ring system (E+F) is almost planar with the dihedral angle between the pyran and benzene rings being 3.73 (7) . Atom O4 deviates from the mean plane of this coumarin ring system by 0.111 (1) Å . The phenyl ring (D) is inclined to the mean plane of the central pyran ring (B), by 60.48 (8) .
In compound (II), the mean plane of the central pyran ring (B) makes dihedral angles of 22.63 (8) and 56.99 (9) with the mean plane of the six-membered carbocylic ring (E) and the phenyl ring (D), respectively. Atom O3 deviates from the mean plane of ring (E) by 0.199 (1) Å .

Supramolecular features
In compound (I), C-HÁ Á ÁO hydrogen bonds are present in which the carboxylate and chromene ring C atoms, C27 and C1, respectively, act as donors and the coumarin ring O atom, O4, acts as a single acceptor (Table 1). These hydrogen bonds link the molecules into R 3 4 (28) ring motifs, resulting in the formation of sheets parallel to the ab plane (Fig. 4). The sheets are linked by C-HÁ Á Á interactions, forming a three-dimensional framework (Table 1).

Figure 4
The crystal packing of compound (I), viewed along the c axis, showing the formation of two-dimensional molecular sheets running parallel to the ab plane. Dashed lines indicate the intermolecular C-HÁ Á ÁO interactions (Table 1). H atoms not involved in hydrogen bonding have been excluded for clarity.

Figure 5
The crystal packing of the title compound (II), viewed along the a axis, showing the formation of inversion dimers with the descriptor R 2 2 (10). Dashed lines indicate the intermolecular C-HÁ Á ÁO interactions (Table 2). H atoms not involved in hydrogen bonding have been excluded for clarity.

Figure 6
The crystal packing of the title compound (III), viewed along the a axis, showing the formation of adjacent R 2 2 (12) ring motifs which connect the inversion-related molecules into chains along [010]. Dashed lines indicate the intermolecular C-HÁ Á ÁO interactions (Table 3). H atoms not involved in hydrogen bonding have been excluded for clarity.

Synthesis and crystallization
Compound (I): A mixture of (E)-methyl 2-[(2-formylphenoxy)methyl]-3-phenylacrylate (0.296 g, 1 mmol) and 4-hydroxy-2H -chromen-2-one (0.162 g, 1 mmol) was placed in a round bottom flask and heated at 453 K for 1 h. After completion of the reaction as indicated by TLC, the crude product was washed with 5 ml of ethylacetate and hexane mixture (1:49 ratio) which successfully provided compound (I) as a colourless solid. Single crystals suitable for X-ray diffraction were prepared by slow evaporation of a solution of (I) in ethylacetate at room temperature. Compound (II): A mixture of (E)-methyl 2-[(2-formylphenoxy)methyl]-3-phenylacrylate (0.296 g, 1 mmol) and cyclohexane-1,3-dione (0.112 g, 1 mmol) was placed in a round bottom flask and heated at 453 K for 1 h. After completion of the reaction as indicated by TLC, the crude product was washed with 5 ml of ethylacetate and hexane mixture (1:49 ratio) which successfully provided the crude product of compound (II) as a colourless solid. Single crystals suitable for X-ray diffraction were prepared by slow evaporation of a solution of (II) in ethylacetate at room temperature.

Refinement
Crystal data, data collection and structure refinement details for compounds (I), (II) and (III) are summarized in Table 4. The positions of all of the H atoms were located in difference electron density maps. During refinement they were treated as riding atoms, with d(C-H) = 0.93, 0.96, 0.97 and 0.98 Å for aryl, methyl, methylene and methine H atoms, respectively, and with U iso (H)= 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms.

(I) Methyl 7-oxo-14-phenyl-1H,7H,14H-pyrano[3,2-c:5,4-c′]dichromene-14a(6bH)-carboxylate]
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.15 e Å −3 Δρ min = −0.20 e Å −3 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.

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.  (3) Atomic displacement parameters (Å 2 )

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.