Ethyl 8-amino-6-bromoimidazo[1,2-a]pyridine-2-carboxylate

There are two independent molecules in the asymmetric unit of the title compound, C10H10BrN3O2, which are linked by N—H⋯O and C—H⋯O hydrogen bonds. The fused ring systems in both molecules are nearly planar with maximum deviations of 0.001 (3) and 0.029 (3) Å. All non-H atoms of the first molecule are approximately co-planar whereas in the second molecule, the ethyl group is almost perpendicular to the imidazo[1,2-a]pyridine system, the C—O—C—C torsion angles in the carboxylic acid ethyl group being −179.8 (4) and 112.1 (5)°, respectively.

In this work, we report a novel and efficient method for the synthesis of 6-bromo-8-amino-imidazo[1,2-a]pyridine via the treatment of ethyl bromopyruvate with 5-bromo-2,3-diaminopyridine in the presence of NaHCO 3 in ethanol at reflux in 65% yield (scheme1).
The Plot of the two molecules building the asymmetric unit of the 8-Amino-6-bromo-imidazo[1,2-a]pyridin-2-carboxylic acid ethyl ester is shown in Fig.1. The two fused five and six-membered rings belonging to each molecule are nearly planar with the maximum deviation of -0.001 (3)Å from C7 and 0.029 (3)Å from C17. The dihedral angle between them is 26.06 (11)°. The ethyl group is almost perpendicular to the imidazo[1,2-a]pyridine system, in the second molecule, as indicated by the torsion angle C18-O3-C19-C20 of 112.1 (5)° (see Fig.2). The first molecule is approximately planar and the torsion angle C-O-C-C is in the range of -179.8 (4)°. In the crystal, the molecules are linked by intermolecular N-H···O and C-H···O hydrogen bonds building a three dimensionnal network (Table 1).

Experimental
A mixture of ethyl bromopyruvate (0.3 ml; 2.35 mmol), 5-bromo-2,3-diaminopyridine (0.5 g, 2.35 mmol) and NaHCO 3 (0.22 g, 2.35 mmol) in ethanol was stirred at reflux for the appropriate time. After completion of the reaction, as indicated by TLC, The solution was extracted with dichloromethane and the organic layer was dried over anhydrous Na 2 SO 4 . Evaporation of the solvent followed by recrystallization in hexane afforded yellow crystals of the title compound.

Refinement
H atoms were located in a difference map and treated as riding with C-H = 0.93 Å, 0.97, Å, 0.96 Å, and 0.86 Å for aromatic, methylene, methyl and -NH respectively. All H atoms with U iso (H) = 1.2 U eq (aromatic, methylene, -NH) and U iso (H) = 1.5 U eq (methyl). H atoms attached to amino groups were located in difference Fourier map and their coordinates were initially refined using N-H restraints (0.86Å with s.u. of 0.01) with U iso (H) = 1.2 U eq (N). In the last cycles of refinement, they were treated as riding on their parent N atoms.

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 > 2σ(F 2 ) is used only for calculating Rfactors(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.