6-Hydroxy-1,2-dihydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one

The molecule of the title compound, C11H9NO2, is essentially planar [r.m.s. deviation of the non-H atoms = 0.056 (1) Å]. In the crystal, strong O—H⋯O hydrogen bonds form zigzag chains along the b axis. The molecules form stacks along the a axis due to π–π interactions, the shortest distance between the centroids of the benzene and pyridinone rings being 3.6146 (7) Å.

The molecule of the title compound, C 11 H 9 NO 2 , is essentially planar [r.m.s. deviation of the non-H atoms = 0.056 (1) Å ]. In the crystal, strong O-HÁ Á ÁO hydrogen bonds form zigzag chains along the b axis. The molecules form stacks along the a axis due tointeractions, the shortest distance between the centroids of the benzene and pyridinone rings being 3.6146 (7) Å .
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: YK2087).

Andreeva Comment
By now the most convenient method of obtaining ethyl esters of N-substituted 4-hydroxy-2-oxo-1,2-dihydroquinoline-3carboxylic acids which are widely used in synthesis of the biologically active substances is condensation of the corresponding secondary anilines with triethyl methanetricarboxylate (Kutyrev & Kappe, 1997;Jönsson et al., 2004;Ukrainets et al., 2006;2010;2011). The method is efficient and gives higher yields. However, as it turned out, in such reactions specific by-products of the same type are also formed besides the targeted esters -(usually 2%-6% by HPLC).

Experimental
As shown in Fig. 1, Indoline (1) (11.2 ml, 0.1 mol) was added dropwise with stirring to triethyl methanetricarboxylate (2) (63.3 ml, 0.3 mol) heated to 488 K, at such a rate that the temperature of the reaction mixture was maintained within ±5 K of the initial temperature. The ethanol eliminated during the reaction was distilled through a suitable still-head. After adding all the indoline, the reaction mixture was kept at the same temperature for 30 min, after which it was cooled. The excess of triethyl methanetricarboxylate was removed in vacuo. To the residue was added 50 ml of xylene. The insoluble solid quinolin-4-one (3) was filtered off, washed with hexane, and dried. Yield: 0.77 g (4.1%). M.p. 577-579 K (DMF).

Refinement
The H atom of hydroxyl group was located from electron density difference map and refined isotropically. The methylene and aromatic H atoms were placed in calculated positions and refined in the riding model approximation with C-H = 0.97 Å for methylene and C-H = 0.93 Å for aryl H atoms with U iso (H) = 1.2U eq (C).    The O-H···O hydrogen bonds in the crystals of the title compound (shown by dashed lines). Symmetry code: (i) -x, y+1/2, -z+1/2.