2-Ethyl-6-(2-pyridyl)-5,6,6a,11b-tetrahydro-7H-indeno[2,1-c]quinoline

The title compound, C23H22N2, was obtained using the three-component imino Diels–Alder reaction via a one-pot condensation between anilines, α-pyridinecarboxyaldehyde and indene using BF3·OEt2 as the catalyst. The molecular structure reveals the cis-form as the unique diastereoisomer. The crystal structure comprises one-dimensional zigzag ribbons connected via N—H⋯N hydrogen bonds. C—H⋯π interactions also occur.

Cg4 is the centroid of the C4-C9 ring.
In our preliminary studies of TAS-103 analogues, we have developed the synthesis (using the imino Diels Alder reaction) and studied the biological activity of the 6-α-pyridinyl-tetrahydro)indeno[2,1-c]quinolines (Kouznetsov et al., 2006). It was found that these compounds were active against MCF-7, H-460 and SF-268 cancer cell lines making them potential anti-cancer agents (Kouznetsov et al., 2006).
In order to obtain detailed information on its molecular conformation and the stereochemistry of the reaction, in this contribution, the molecular structure of the title compound, (I), is described. The structural analysis indicated (I) exists in the cis-form as a unique regio-and diastereo-isomer (Fig. 1). The tetrahydropyridine ring adopts a half-chair conformation and the indene ring displays an envelope configuration. The crystal packing of (I) consists of one-dimensional zigzag ribbons that run along the c direction and linked via N-H···N hydrogen bonding interactions ( Fig. 2 & Table 1).

Experimental
A mixture of aryl amine (3.6 mmol) and α-pyridinecarboxyaldehyde (4.0 mmol) in anhydrous CH 3 CN (15 ml) was stirred at room temperature for 30 min after which BF 3 .OEt 2 (3.6 mmol) was added. Over a period of 20 min, an acetonitrile solution (10 ml) of indene (4.0 mmol) was added dropwise. The resulting mixture was stirred at 343 K for 5 h. After completion of the reaction, as indicated by TLC, the reaction mixture was diluted with water (30 ml) and extracted with ethyl acetate (3 x 15 ml). The organic layer was separated and dried (Na 2 SO 4 ), concentrated in vacuo, and the resulting product was purified by column chromatography (silica gel, petroleum ether: EtOAc) to afford pure (I) as a colorless solid, mp 424-425 K (yield 43%). This compound was recrystallized by slow evaporation from the solvent mixture, hexane-ethyl acetate.

Refinement
All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.93 (aromatic) and 0.96 Å (methyl), and with U iso (H) = 1.5 (1.2 for aromatic-H atoms) times U eq (C). The low completeness ratio is due to the experimental setup whereby the equipment has a χ circle and an added area detector supplementary materials sup-2 (four-circle diffractometer modified with a CCD). This precludes the collection of some regions of reciprocal lattice space and lowers the completeness. In order to compensate, additional redundant data were measured.   0.0429 (9) 0.0609 (9) 0.0431 (9) 0.0008 (7) 0.0077 (7) −0.0047 (7) N2 0.0480 (10) 0.0581 (10) 0.0636 (11) 0.0033 (7)