Crystal structures of 2′-benzoyl-1′-(4-methylphenyl)-1,1′,2,2′,5′,6′,7′,7a′-octahydrospiro[indole-3,3′-pyrrolizin]-2-one and 2′-(4-bromobenzoyl)-1′-(2-chlorophenyl)-1,1′,2,2′,5′,6′,7′,7a′-octahydrospiro[indole-3,3′-pyrrolizin]-2-one

The chemical modifications in terms of changes in substituents in the title compounds have not affected the type nor strength of two defining intermolecular interactions present in both crystal structures.

In both compounds, the spiro-fused ring systems tend to be rigid by remaining nearly perpendicular to each other, whereas the remaining substituted rings appear to be more 'compromising' towards hydrogen-bonding requirements, irrespective of their intra-or intermolecular nature. As an example, the free rotation of the benzoyl group in (II) allows the formation of an intramolecular C-HÁ Á ÁO hydrogen bond (Table 2, last entry) while the interaction is absent in (I).
A significant difference between the two structures is observed in the deviation of benzoyl atom O2 from the leastsquares plane of the C15-C21 atoms: 0.593 (4) in (I) and 0.131 (3) Å in (II). The larger deviation in (I) appears to be the result of the participation of O2 in three very weak (but cooperative) intermolecular C-HÁ Á ÁO hydrogen bonds, all three coming from the same side of the plane (Table 1, three topmost entries and Fig. 3). In the structure of (II), instead, only two (competitive) C-HÁ Á ÁO bonds involving O2 occur, Displacement ellipsoid plot (50% probability level) of title compound (I), showing the atom-labelling scheme. H atoms have been omitted for clarity.

Supramolecular features
Even if the differences in the substituents produce differences in lattice types, space group, cell metrics, etc, these molecular modifications do not seem to affect the type nor strength of the two relevant N-HÁ Á ÁN and C-HÁ Á ÁO intermolecular hydrogen bonds defining the crystal structures (Tables 1 and  2), which can thus be considered as essential for the crystal structure layout. In particular, those bonds involving C7 and N1 link glide-related molecules into similar one-dimensional strings along the shortest cell axis (Figs. 5 and 6). As already discussed, the other, relatively weaker, intermolecular C-HÁ Á ÁO hydrogen bonds involving the benzoyl atom O2 as acceptors have a profound effect on the molecular conformation of the molecules. Finally, a close O1Á Á ÁBr1(Àx + 1 2 , y À 1 2 , Àz + 3 2 ) contact [d OÁ Á ÁBr = 3.192 (2) Å ] is present in structure (II), with no further significant ClÁ Á ÁCl, ClÁ Á ÁBr, BrÁ Á ÁBr or C-HÁ Á Á orinteractions present in either crystal structure.  The three C-HÁ Á ÁO bonds in (I) involving benzoyl O2 as acceptor (Table 1, top three entries) all from the same side of the plane.

Figure 4
The two C-HÁ Á ÁO bonds in (II) involving benzoyl O2 (Table 2, top two entries) on opposite sides of the benzoyl plane.

Figure 5
One-dimensional strings of molecules of (I), along the c axis.

Figure 6
One-dimensional strings of molecules of (II), along the b axis.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.53, update February 2014;Groom et al., 2016) for organic non-polymeric single-crystal structures revealed 27 structures of which only two bear a close relationship to the title compound (POXZIL and POXZOR; Fokas et al., 1998). There are no other direct analogues of the title compounds, either in coordinated or uncoordinated form. In POXZOR, the deviation of the benzoyl atom O2 from the plane containing the rest of the atoms of the group is about 0.465 Å , similar to the case in (I), but the quality of the H-atom treatment in POXZOR precluded any meaningful comparison.

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. Refinement. Refined as a 2-component perfect inversion twin.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) 0.31457 (6) 0.81610 (7) −0.0631 (2) 0.0509 (6)  N1 0.25279 (7) 0.96268 (9) −0.1968 (2) 0.0418 (6)     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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )