Conformational isomers in (2RS,3SR,6SR)-3-ethyl-2,6-diphenyl-1-phenylacetyl-4-piperidone determined from synchrotron data at 120 K

In the title compound, C27H27NO2, the conformations of the two independent mol­ecules differ in the shape of the piperidone rings, the orientation of the N-acyl substituents and the axial/equatorial disposition of the ethyl and phenyl groups. The mol­ecules are linked into complex chains by four independent C—H⋯O hydrogen bonds.

In the title compound, C 27 H 27 NO 2 , the conformations of the two independent molecules differ in the shape of the piperidone rings, the orientation of the N-acyl substituents and the axial/equatorial disposition of the ethyl and phenyl groups. The molecules are linked into complex chains by four independent C-HÁ Á ÁO hydrogen bonds.

Comment
In piperidone derivatives containing N-chloroacetyl or N-ethoxycarbonyl substituents, the piperidone rings generally adopt distorted boat conformations (Ponnuswamy et al., 2002;Nallini et al., 2003). This study of the title compound, (I) (Figs. 1 and 2), was undertaken to investigate the effect of an N-phenylacetyl substituent upon the molecular conformation of the piperidone ring.
Compound (I) forms extremely small crystals of rather indifferent quality, and synchrotron radiation was necessary to obtain usable diffraction data. It crystallizes with Z 0 = 2 in the space group P2 1 /c (Figs. 1 and 2): molecules 1 and 2 contain atoms N11 and N21, respectively. The conformations of the two independent molecules are significantly different. Firstly, the ring-puckering parameters (Cremer & Pople, 1975) for the piperidone rings are, for the atom sequences Nn1-Cn2-Cn3-Cn4-Cn5-Cn6, = 94.0 (8) and ' = 254.6 (8) when n = 1, and = 88.3 (7) and ' = 124.1 (7) when n = 2, indicating an almost ideal twist-boat conformation in molecule 2, but a conformation midway between boat and twist-boat for molecule 1. Secondly, the orientation of the N-acyl substituent is entirely different in the two molecules, as indicated by the leading torsion angles (Table 1). Finally, in molecule 1, the substituents at C12 and C13 are axial and that at C16 is equatorial; in molecule 2 the substituents at C22 and C23 are equatorial and that at C26 is axial (Figs. 1 and 2, and Table 1). The two molecules are thus conformational isomers and this in itself precludes the possibility of any additional crystallographic symmetry.
The molecules are linked into rather complex chains by a combination of four C-HÁ Á ÁO hydrogen bonds (Table 2). Atoms C22 and C28 in the type 2 molecule at (x, y, z) both act as hydrogen-bond donors to atom O24 in the type 2 molecule at (x, 3 2 À y, 1 2 + z), so forming a C(4)C(5)[R 1 2 (7)] chain of rings (Bernstein et al., 1995) along [001]. This chain is formed by type 2 molecule only and the type 1 molecules are pendent from it. Atoms C13 and C222 at (x, y, z) act as hydrogen-bond donors respectively to atoms O24 and O14 at (1 À x, 1 À y, 1 À z), so forming a non-centrosymmetric R 2 2 (11) ring (Fig. 3). Antiparallel pairs of these chains, related to one another by inversion, are weakly linked by a single C-HÁ Á Á(arene) interaction (Table 2).

Experimental
The title compound was prepared by the condensation of equimolar quantities of phenylacetyl chloride and 3-ethyl-2,6-diphenylpiperidin-4-one in anhydrous benzene, in the presence of triethylamine as the base. Crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of a solution in aqueous ethanol (m.p. 368-371 K).  The RSS enantiomer of molecule 1 of compound (I), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 20% probability level.

Figure 2
The RSS enantiomer of molecule 2 of compound (I), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 20% probability level.

Figure 3
Stereoview of part of the crystal structure of compound (I), showing the formation of a [001] chain of rings containing only type 2 molecules, with type 1 molecules pendent from the chain. For the sake of clarity, H atoms not involved in the motifs shown have been omitted. Table 1 Selected torsion angles ( ).