Structural characterization of analgesic isothiazolopyridines of Mannich base type; X-ray analysis of 2-[(4-phenylpiperazin-1-yl)ethyl]- and 2-[(4-methylpiperazin-1-yl)methyl]-4,6-dimethylisothiazolo[5,4-b]pyridin-3(2H)-ones

The crystal and molecular structures of the title 2-[(4-phenylpiperazin-1-yl)ethyl], 6, and 2-[(4-methylpiperazin-1-yl)methyl], 7, derivatives of isothiazolo[5,4-b]pyridine were determined. The molecular packing in 6 is influenced by C–H…X (X = N, O) hydrogen bonds and π…π interactions of the pairs of isothiazolopyridine rings belonging to inversion related molecules. The crystal structure of 7 contains the net of O–H…N and C–H…O intermolecular hydrogen bonds. Moreover, isothiazole and pyridine rings show significant stacking with the shortest π…π distances of 3.453 Å. The conformations of the molecules 6 and 7 were compared with those observed in the crystals of related analgesic 4-arylpiperazine (2, 3) and 4-arylpiperidine (4, 5) derivatives of isothiazolopyridine of Mannich base type. Additionally, the computational investigations using semi-empirical AM1 and RHF/6-31G∗∗ ab initio methods are performed within series 2–7 in order to find correlation between geometrical and electronic parameters of the molecules and their analgesic action. Results of the theoretical calculations show that the charge distribution on the piperazine N atoms is correlated with conformation of the (4-arylpiperazin-1-yl)methyl side chain and analgesic action of isothiazolopyridines analyzed.