A triclinic polymorph of methyl (3R,3′S)-1′,1′′-dimethyl-2,2′′-dioxodispiro[indoline-3,2′-pyrrolidine-3′,3′′-indoline]-4′-carboxylate

In the title compound, C22H21N3O4, the central pyrrolidine ring adopts a C-envelope conformation with a C atom 0.6593 (13) Å displaced from the mean plane formed by the remaining ring atoms. The indoline ring systems (r.m.s. devisations of 0.0356 and 0.0547 Å) are almost perpendicular to the mean plane of the pyrrolidine ring, making dihedral angles of 89.7 (6) and 82.5 (6)°. The acetate group attached to the pyrrolidine ring assumes an extended conformation. In the crystal,N—H⋯O and C—H⋯O hydrogen bonds connect adjacent molecules, forming an infinite tape extending along [1-1-1]. The crystal packing is further consolidated by strong π–π interactions with a centroid–centroid distance of 3.2585 (8) Å. The title compound is a polymorph of previously reported monoclinic structure [Ganesh et al. (2012 ▶). Acta Cryst. E68, o2902–o2903].

In the title compound, C 22 H 21 N 3 O 4 , the central pyrrolidine ring adopts a C-envelope conformation with a C atom 0.6593 (13) Å displaced from the mean plane formed by the remaining ring atoms. The indoline ring systems (r.m.s. devisations of 0.0356 and 0.0547 Å ) are almost perpendicular to the mean plane of the pyrrolidine ring, making dihedral angles of 89.7 (6) and 82.5 (6) . The acetate group attached to the pyrrolidine ring assumes an extended conformation. In the crystal,N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds connect adjacent molecules, forming an infinite tape extending along [111]. The crystal packing is further consolidated by strong interactions with a centroid-centroid distance of 3.2585 (8) Å . The title compound is a polymorph of previously reported monoclinic structure [Ganesh et al. (2012). Acta Cryst. E68, o2902-o2903].

SubbiahPandi Comment
We have recently reported the structure of the title compound in the monoclinic system (Ganesh et al., 2012). Here we report the structural details of its triclinic polymorph.

Experimental
A mixture of 1 eq of (E)-methyl 2-(1-methyl-2-oxoindolin-3-ylidene) acetate, 1 eq of isatin and 1.5 eq of sarcosine dissolved in acetonitrile was refluxed at 353 K for 8 h. Upon completion of the reaction as determined with the aid of TLC, the reaction mixture was extracted with ethyl acetate and water. The product was dried and purified by column chromatography using ethyl acetate and hexane (1:9) as an elutent to afford the title compound in pure form. (Yield = 90%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of the title compound in ethyl acetate at room temperature.

Refinement
All H atoms were fixed geometrically and allowed to ride on their parent C atoms, with N-H = 0.86 Å and C-H distances in the range 0.93-0.98 Å with U iso (H) = 1.5U eq (methyl C) and 1.2U eq (non-methyl C/N).

Computing details
Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 2012); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009  The molecular structure of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are presented as small spheres of arbitrary radius. 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. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.7776 (