Crystal structure of 6-(4-chlorophenyl)-6a-nitro-6a,6b,8,9,10,12a-hexahydro-6H,7H-spiro[chromeno[3,4-a]indolizine-12,11′-indeno[1,2-b]quinoxaline]

The title compound crystallized with two independent molecules (A and B) in the asymmetric unit. They differ essentially in the conformation of the pyrrolidine and cyclopentene rings; respectively, twisted and flat in molecule A, but envelope and twisted in molecule B.

The title compound, C 35 H 27 ClN 4 O 3 , crystallized with two independent molecules (A and B) in the asymmetric unit. In both molecules, the pyran and pyridine rings adopt envelope and chair conformations, respectively. The conformation of the pyrrolidine and cyclopentene rings differ in the two molecules; twisted and flat, respectively, in molecule A, but envelope and twisted, respectively, in molecule B. In both molecules, there is a C-HÁ Á ÁN intramolecular hydrogen bond present. In both molecules, the oxygen atoms of the nitro groups are disordered as is the chlorine atom in molecule B. In the crystal, the B molecules are linked by C-HÁ Á ÁO hydrogen bonds, forming -B-B-B-chains along [010], and by C-HÁ Á Á interactions. The A and B molecules are also linked by a number of C-HÁ Á Á interactions, resulting in the formation a supramolecular three-dimensional structure. In molecule A, the nitro group oxygen atoms are disordered over two positions with refined occupancy ratios of the nitro group oxygen atoms O3A and O4A in 0.59 (2):0.41 (2) while in molecule B one of the nitro O atoms is disordered over two positions with a refined occupancy ratio of 0.686 (13):0.314 (13) and the chlorine atoms is disordered over two positions with a refined occupancy ratio of 0.72 (3):0.28 (3).

Chemical context
Nitrogen-containing heterocyclic compounds are reported to possess a diverse range of biological activities such as antimicrobial, antitumor and anti-inflammatory (Syed Abuthahir et al., 2019;Thirunavukkarsu et al., 2017) properties. Spiro compounds are often encountered in many pharmacologically active alkaloids (NizamMohideen et al., 2009c;Cravotto et al., 2001). The cornerstone for cycloaddition reactions, nitrones are excellent spin-trapping and highly versatile synthetic intermediates (Bernotas et al., 1996;NizamMohideen et al., 2009b). Highly substituted spiro compounds result from the 1,3-dipolar cycloaddition of exocylic olefins with nitrones; these spiro compounds have also been transformed into complex heterocycles (Hossain et al., 1993;NizamMohideen et al., 2009a). Recognizing the importance of such compounds in drug discovery and our ongoing research into the construction of novel heterocycles has prompted us to investigate the title compound: we report herein the synthesis and the crystal structure.). ISSN 2056-9890

Structural commentary
The molecular structures and conformation of the two independent molecules, A and B, in the asymmetric unit are shown in Figs. 1 and 2, respectively. In both molecules, the oxygen atoms of the nitro groups are disordered as is the chlorine atom in molecule B. For all further discussion only the major components of the disordered atoms will be considered. The View of the molecular structure of molecule A of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level. The intramolecular C-HÁ Á ÁN contact (Table 1) is shown as an orange dashed line.

Figure 2
View of the molecular structure of molecule B of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level. The intramolecular C-HÁ Á ÁN contact (Table 1) is shown as an orange dashed line.

Figure 3
A view of the molecule overlay of inverted molecule B (red) on molecule A (blue), with an r.m.s. deviation of 0.208 Å . The H atoms have been omitted for clarity. bond lengths and angles are close to those reported for similar compounds (Devi et al., 2013a,b;Syed Abuthahir et al., 2019). In both molecules there is an C-HÁ Á ÁN intramolecular hydrogen bond present enclosing an S(7) ring motif ( Fig. 1 and Table 1 The overall conformation of the two molecules is very similar, as seen from the molecule overlay figure (Fig. 3), calculated and drawn using Mercury (Macrae et al., 2008). The essential difference concerns the conformations of the pyrrolidine (N1/C12/C13/C21/C22) and cyclopentene (C1/C2/ C10-C12) rings. In molecule A the former ring has an envelope conformation with atom C12A as the flap, while in molecule B this ring has a twisted conformation on the N1B-C12B bond. The cyclopentene ring is twisted on the C12A-C1A bond in molecule A but is flat in molecule B. The pyran rings (O2/C19/C14/C13/C21/C20) in both molecules have envelope conformations with atom C20 as the flap, and the piperidine rings in both molecules (N1/C22-C26) have chair conformations.

Supramolecular features
In the crystal, the B molecules are linked by C-HÁ Á ÁO hydrogen bonds, forming chains propagating along the b-axis direction (Table 1 and  The crystal packing of the title compound viewed along the c axis. The interlinking of B molecules via C-HÁ Á ÁO hydrogen bonds (see Table 1  by C-HÁ Á Á interactions, and are linked to the A molecules by C-HÁ Á Á interactions (Table 1). The result of these intermolecular interactions is the formation of a supramolecular three-dimensional structure (Table 1 and Fig. 4).

Synthesis and crystallization
To a solution of indenoquinoxalinone (1.0 mmol) and pipacolinic acid (1.5 mmol) in dry toluene, was added 2-(4chlorophenyl)-3-nitro-2H-chromene (1 mmol) under a nitrogen atmosphere. The solution was refluxed for 20 h in a Dean-Stark apparatus to give the corresponding cycloadduct. After completion of the reaction, as indicated by TLC, the solvent was evaporated under reduced pressure. The crude product obtained was purified by column chromatography using hexane/EtOAc (6:4) as eluent (yield 86%). Colourless blocklike crystals of the title compound were obtained by slow evaporation of a solution in ethanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically and constrained to ride on their parent atoms: C-H = 0.93-0.98 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms.