1-{3-[(4-Oxopiperidin-1-yl)carbonyl]benzoyl}piperidin-4-one

Two independent molecules comprise the asymmetric unit in the title compound, C18H20N2O4. One of the molecules exhibits disorder in one of its 4-piperidone rings, which is disposed over two orientations [site occupancy of the major component = 0.651 (5)]. The first independent molecule and the minor component of the second disordered molecule are virtually superimposable. The central four C atoms in the major component of the disordered molecule have an opposite orientation. All the 4-piperidone rings have a chair conformation. The carbonyl groups in each molecule have approximate anti conformations [O=C⋯C=O = 146.2 (2) and −159.9 (2)°]. The 4-piperidone rings lie to opposite sides of the central benzene ring in both molecules. In the crystal, molecules are linked by C—H⋯O interactions. The crystal studied was found to be a non-merohedral twin (twin law −1 0 0, 0 1 0, 0 − 1/2 − 1), the fractional contribution of the minor component being approximately 11%.


Comment
The synthesis of N-substituted-4-piperidones is subject of continuing interest owing to their importance as synthetic building blocks in medicinal chemistry, in particular for the synthesis of pharmacologically active agents (Dyakov et al., 1991;Scherer et al., 1993). In continuation of recent structural studies on N-substituted-4-piperidones (Vijayakumar et al., 2010, Rajesh et al., 2010, the title compound, (I), was investigated.
Two independent molecules comprise the asymmetric unit of (I). One molecule is ordered, Fig. 1, and the other is disordered, Fig. 2. In the disordered molecule, one 4-piperidone ring is disordered over two positions. The major component of the disorder has the central four carbon atoms of the N4-bound 4-piperidone ring in an opposite orientation to that found in the ordered molecule. This is emphasized in Fig. 3 which shows the superimposition of the ordered molecule upon the inverted disordered molecule. The minor component of the disordered ring has a conformation similar to that observed in the ordered molecule, Fig. 1. All 4-piperidone rings have a chair conformation. The carbonyl groups bound to the central benzene ring are almost anti as seen in the O═ C···C═O torsion angles of 146.2 (2) and -159.9 (2) ° for the two independent molecules, respectively. In each case, the 4-piperidone rings lie to opposite sides of the central benzene ring.
The most significant intermolecular contacts in the crystal structure are of the type C-H···O, Table 1, and these consolidate the crystal packing, Fig. 4.

Experimental
To a suspension of 4-piperidone hydrochloride monohydrate (1 mmol) in benzene (20 ml) was added triethyl amine (3 mmol), followed by thorough stirring for 15 min. To that, isophthaloyl dichloride (0.5 mmol) dissolved in benzene (20 ml) was added drop wise with stirring, followed by refluxing for 7 h. The progress of the reaction was monitored by TLC.
After the completion of the reaction, excess solvent was removed under reduced pressure. The crude products obtained were purified by column chromatography using an ethyl acetate/petroleum ether mixture (1:1). The sample (50 mg) was crystallized in 1:1 mixture of chloroform/methanol (5 + 5 ml) to yield colourless blocks of (I); m.pt. 415-417 K.

Refinement
The C-bound H atoms were geometrically placed (C-H = 0.93-0.97 Å) and refined as riding with U iso (H) = 1.2U eq (C).
Disorder in the N4-bound 4-piperidone ring was resolved over two positions. The major component (anisotropic refinement) had a site occupancy factor = 0.651 (5); the minor component was refined isotropically. The N-C and C-C distances in the disordered rings were refined with distance restraints 1.46±0.005 and 1.51±0.005 Å, respectively. For the treatment of

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(F 2 ) is used only for calculating Rfactors(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.