Crystal structure of (E)-2-(furan-2-ylmethylidene)-2,3,4,9-tetrahydro-1H-carbazol-1-one

The title compound crystallized with two conformationally very similar independent molecules (A and B) in the asymmetric unit. In the crystal, the individual molecules are linked by pairs of N—H⋯O hydrogen bonds, forming A–A and B–B inversion dimers, with (10) rings.


Chemical context
Natural products comprising a carbazole skeleton linked to another heterocycle have received significant attention due to the promising antitumor properties of several of their naturally occurring representatives (Knö lker & Reddy, 2002). Numerous total syntheses of these compounds have been reported that use a variety of structural modification methods for annelating heterocyclic systems to carbazole frameworks. This rapidly growing class of heteroaryl-condensed carbazoles has continued to attract attention because of their broad spectrum of useful biological activities that extend well beyond the antitumor properties of the naturally occurring carbazole derivatives that originally spiked the interest of researchers (Knö lker & Reddy, 2002). Most heteroaryl carbazoles reported contain a heteroaryl moiety fused with a carbazole moiety; however, there are few reports where the heteroaryl unit is substituted with a carbazole unit (Sridharan et al., 2008). We have reported the synthesis of 1-oxo-2-arylidene-2,3,4,9-tetrahydrocarbazoles from potential precursors of the 2,3,4,9-tetrahydrocarbazole-1-one type and these synthons were utilized to derive a diverse variety of heteroannelated carbazoles (Sridharan et al., 2008;Sridharan & Rajendra Prasad, 2011;Archana et al., 2010a,b;Thiruvalluvar et al., 2013). Herein, we report on the crystal structure of one such compound, synthesized by the base-initialized reaction of 2,3,4,9-tetrahydrocarbazol-1-one with furan-2-carbaldehyde.

Structural commentary
The title compound, crystallizes with two independent molecules (A and B) in the asymmetric unit (Fig. 1). The conformations of the two molecules are similar, as can be seen in Fig. 2, which shows the molecular overlay of molecule B inverted on molecule A (r.m.s. deviation = 0.082 Å ). The cyclohexene rings of the tetrahydrocarbazole moieties have half-chair conformations in both molecules. The mean plane of the tetrahydrocarbazole moiety (r.m.s. deviations are 0.087 and 0.072 Å for molecules A and B, respectively) is inclined to the furan ring by 12.89 (14) in molecule A, and 12.09 (14) in molecule B.

Supramolecular features
In the crystal, the individual molecules are linked by pairs of N-HÁ Á ÁO hydrogen bonds forming A-A and B-B inversion dimers, with R 2 2 (10) ring motifs, which is the main motif that facilitates packing (Table 1 and Fig. 3). The individual dimers stack alternately along the a-axis direction, as shown in Fig. 3. The stacks are connected by C-HÁ Á Á interactions, forming layers parallel to the ab plane ( Fig. 4 and Table 1).

Figure 3
Crystal packing of the title compound, viewed along the b axis, showing the hydrogen bonded A-A and B-B inversion dimers, with R 2 2 (10) ring motifs. The N-HÁ Á ÁO hydrogen bonds are shown as dashed lines (see Table 1; molecule A blue, molecule B red).

Figure 1
The molecular structure of the two independent molecules (A and B) of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.
with 25 ml of a 5% ethanolic potassium hydroxide solution and stirred for 6 h at room temperature. The product precipitated as a yellow crystalline mass, which was filtered off and washed with 50% ethanol. A further crop of condensation product was obtained on neutralization with acetic acid and dilution with water. The product was recrystallized from ethanol to yield the title compound as yellow plate-like crystals (yield 1.17 g, 89%; m.p. 492-494 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The NH H atoms, H1A and H2B, were located in a difference-Fourier map and freely refined. The remaining H atoms were placed in calculated positions, with C-H bond distances of 0.95 Å (aromatic H), and 0.99 Å (methylene H), and refined as riding with U iso (H) = 1.2U eq (C). Reflections 002 and 100 were obstructed by the beam stop and omitted from the refinement.

Funding information
We are grateful to the UGC, New Delhi, India, for the award of a Major Research Project Grant (No. 31-122/2005  Crystal packing of the title compound, viewed along the c axis, showing the N-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions (blue dashed lines; see Table 1). Only the H atoms involved in these interactions have been included; A molecules are blue and B molecules are red.  Computer programs: SMART and SAINT-Plus (Bruker, 2003), SHELXTL (Sheldrick, 2008), Mercury (Macrae et al., 2008), SHELXL2017 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

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.