X-ray crystallographic characterization of two polymorphs of 8-(2-methoxycarbonylamino-6-methylbenzyloxy)-2-methyl-3-(2- propynyl)-imidazo [1,2-a]pyridine.

Structural characterization of two polymorphs (Forms A and B) of 8-(2-methoxycarbonylamino-6-methylbenzyloxy)-2-methyl-3-(2-p ropynyl)-imidazo [1,2-a]pyridine (1) was accomplished by X-ray crystallographic analysis. Form A crystallized in the monoclinic space group C2/c, with a = 42.936 (14), b = 4.356 (1), c = 21.536 (6) A, beta = 109.92 (4) degrees, z = 8, and dcal = 1.275 g/cm3. Form B crystallized in the monoclinic space group P2(1)/c, with a = 4.367 (1), b = 38.214 (3), c = 11.253 (1) A, beta = 95.47 (2) degrees, z = 4, and dcal = 1.292 g/cm3. Some subtle conformational differences between the polymorphs were observed. Although the Form B crystal has a somewhat more compactly packed structure than Form A, this compactness is thought to be disadvantageous to conformational features and to the crystal structure of Form B. The intramolecular hydrogen bonding force between N(1) and NH(22) atoms of Form B is weaker than that of Form A, and the stacking force between the imidazopyridine rings of Form B may also be weaker than that of Form A. Thus, Form A is considered to be a more stable structure than Form B. In DSC analysis, Form B transformed to Form A. Based on these results, phase transformation from Form B to A occurs during a transient fluid state.