Methyl 2-acetamido-2-(1-acetyl-3-hydroxy-2-oxoindolin-3-yl)propanoate

In the title isatin compound, C16H18N2O6, the pyrrolidine ring adopts an envelope conformation and is inclined at a dihedral angle of 7.31 (5)° with respect to the benzene ring. The acetyl group is disordered over two positions with refined occupancies of 0.503 (4) and 0.497 (4). These groups make dihedral angles of 12.6 (6) and 19.6 (7)° with the pyrrolidine ring. In the crystal structure, intermolecular C—H⋯O hydrogen bonds link neighbouring molecules into infinite chains along the b axis. These chains are further interconnected by intermolecular O—H⋯O hydrogen bonds into two-dimensional arrays parallel to the bc plane. Weak intermolecular C—H⋯π interactions further stabilize the crystal structure.

Cg1 is the centroid of the C1-C6 benzene ring.

Comment
Isatin (1H-indole-2,3-dione) was first discovered by Erdmann and Laurent in 1841 (Popp, 1975). Isatin and its derivatives are versatile molecules and possess a wide range of activities, especially in the biological and pharmaceutical fields. They are also basic structural units and important synthetic precursors of many naturally occuring alkaloids (Shvekhgeimer, 1996).
Recently, a number of isatin-based compounds are reported as inhibitors of caspase-3 and caspase-7 (Chu et al., 2007).
Photoreactions of N-acetylisatin with various species have also been of research interest (Zhang et al. 2004). Due to the importance of the isatin derivatives, the crystal structure of the biologically active title compound is reported in this paper.

Experimental
The title compound was obtained in the reaction between N-acetylisatin and 2,4-dimethyl-5-methyloxy-oxazole. The compound was purified by flash column chromatography. X-ray quality single crystals of the title compound were obtained from slow evaporation of a solution of chloroform and petroleum ether (1:3; v:v). M.p. 431-434 K.

Refinement
Atoms H1O1 and H1N1 were located from difference Fourier map and allowed to refine freely. All other hydrogen atoms were placed in their calculated positions, with C-H = 0.93 or 0.96 Å, and refined using a riding model, with U iso (H) = 1.2 or 1.5U eq (C). The acetyl group is disordered over two positions with a refined occupancy ratio of 0.503 (4):0.497 (4).

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1)K.
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.