The crystal structures of 6′-(4-chlorophenyl)- and 6′-(4-methoxyphenyl)-6a′-nitro-6a′,6b′,7′,9′,10′,12a′-hexahydro-2H,6′H,8′H-spiro[acenaphthylene-1,12′-chromeno[3,4-a]indolizin]-2-one

The conformations of the title compounds, (I) and (II), are very similar. The pyran rings adopt envelope conformations, the piperidine rings have chair conformations and the pyrrolidine rings adopt twist conformations. Intra- and intermolecular C—H⋯O hydrogen bonds occur. Compound (II) crystallizes with two independent molecules in the asymmetric unit which are linked by C—H⋯O hydrogen bonds.


Structural commentary
The molecular structure of compound (I) is shown in Fig. 1, while the molecular structures of the two independent mol-ecules, A and B, of compound (II) are shown in Figs. 2 and 3, respectively; they are in fact enantiomers. The bond lengths and angles in all three molecules are very similar. In (II), the methoxybenzene group of molecule B is positionally disordered and only the major component will be taken into consideration concerning the conformation of the molecule. The structural overlap of compound (I) on the major component of molecule B of compound (II) is shown in Fig. 4. The two molecules have an r.m.s. deviation of 0.212 Å . The molecular overlap of inverted molecule B of compound II (major component) on molecule A is shown in Fig. 5. Here the r.m.s. deviation is 0.297 Å and it can be seen that the major difference between the two molecules concerns the orientation of the 4-methoxyphenyl group. In all three molecules (I and IIA and IIB) the pyran rings have envelope conformations with the methylene C atom C21 as the flap. The piperidine rings adopt chair conformations, while the pyrrolidine rings adopt twist conformations on the N1-C12 bond (N1A-C12A in IIA and N1B-C12B in IIB).

Figure 2
View of the molecular structure of molecule A of compound (II), with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level. The intramolecular C-HÁ Á ÁO hydrogen bond (see Table 2) is shown as a dashed line. molecule A and 36.1 (2) for molecule B in (II), and is inclined to the piperidine ring mean plane (N1/C22-C26) by 9.9 (2) in (I), and 11.1 (2) in molecule A and 13.1 (2) in molecule B of compound (II). The mean planes of the pyran and piperidine ring are inclined to each other by 29.1 (2) in (I), and 33.5 (2) in molecule A and 36.2 (2) in molecule B of compound (II). Full details of the puckering parameters and lowest displacement asymmetry parameters are given in the supporting information. The keto atom O1 deviates from the mean plane of the plane of the acenaphthylene ring system (C1-C12) by 0.070 (2) Å in (I), and by 0.049 (2) and 0.162 (1) Å , respectively, in molecules A and B of compound (II). Chlorine atom Cl1 deviates by 0.109 (2) Å from the plane of the benzene ring (C27-C32) in (I). It can be seen that the conformations and the values of the dihedral angles in all three molecules of the title compounds are very similar. The bond lengths and angles are also close to those reported for similar compounds (Devi et al., 2013a,b).

Supramolecular features
For both compounds, the crystal structure is stabilized by intermolecular C-HÁ Á ÁO hydrogen bonds (Tables 1 and 2). In (I), the C-HÁ Á ÁO hydrogen bonds link adjacent molecules, forming chains propagating along the b-axis direction. The chains are linked by C-HÁ Á ÁCl hydrogen bonds, forming layers parallel to the (101) plane; see Table 1 and Fig. 6. Within the layers there are C-HÁ Á Á interactions present (Table 1)  The structural overlay of compound (I) on the major component of molecule B of compound (II).

Figure 5
The molecular overlay of inverted molecule B (major component) on molecule A of compound (II).

Figure 3
View of the molecular structure of molecule B of compound (II), with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level. The intramolecular C-HÁ Á ÁO hydrogen bond (see Table 2) is shown as a dashed line. Table 2 Hydrogen-bond geometry (Å , ) for (II).
In compound (II), the interlinking of A and B molecules via C-HÁ Á ÁO hydrogen bonds generates four-membered units ( Table 2 and Fig. 7). These are linked by C-HÁ Á Á interactions, forming a three-dimensional supramolecular structure (Table 2 and Fig. 8).
In both compounds, the piperidine ring has a chair conformation, as do the title compounds. In AFONEQ, the pyran ring has a envelope conformation, as do the title compounds, while in FIDCOM the pyran ring has a planar conformation. In these two compounds, the pyrrolidine ring adopts an envelope conformation, while in the title compounds these rings have twisted conformations. The bond lengths and bond angles are very similar to those reported here for the title compounds.

Synthesis and crystallization
To a solution of acenaphthoquinone (1.0 mmol) and piperidine-2-carboxylic acid (1.5 mmol) in dry toluene, was added 2-(4-chlorophenyl)-3-nitro-2H-chromene (1 mmol) for (I), and 2-(4-methoxyphenyl)-3-nitro-2H-chromene (1 mmol) for (II), under a nitrogen atmosphere. The solutions were refluxed for 18 h in a Dean-Stark apparatus to give the cycloadducts. After completion of the reactions as indicated by TLC, the solvent was evaporated under reduced pressure. The crude products obtained were purified by column chromatography using hexane/EtOAc (8:2) as eluent (yield 89%). Colourless block-like crystals of the title compounds, suitable for X-ray diffraction analysis, were obtained by slow evaporation of solutions in ethanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds the H atoms were positioned geometrically and constrained to ride on their A partial view along the b axis of the crystal packing of compound (II). The A (blue) and B (red) molecules are linked via C-HÁ Á ÁO hydrogen bonds (dashed lines; see Table 2).

Figure 6
A view normal to plane (101) of the crystal structure of (I), showing the C-HÁ Á ÁO and C-HÁ Á ÁCl hydrogen bonds (dashed lines; see Table 1).

Figure 8
A view along the b axis of the crystal packing of compound (II). The A (blue) and B (red) molecules are linked via C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions (dashed lines; see Table 2). For clarity, H atoms not involved in the various intermolecular interactions have been omitted.

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.