Crystal and molecular structures of three anxiolytic pyrazolopyridines

The crystal structures of three pyrazolopyridines, ethyl 4-amino-6-ethyl-1- pentyl-1H-pyrazolo[3,4-b]pyridinium-5-oate chloride hydrate C16H25N4O2+. Cl -. H2O (1), ethyl 4-amino-6-isopropyl-1-pentyl-1H-pyrazolo[3,4-b]pyridinium -5-oate chloride C17H27N4O2+. Cl- (2), and 4-amino-1-pentyl-1H-pyrazolo[3,4 -b]pyridine-5-N-(2-propenyl)carboxamide C15H21N5O (3) have been determined by X-ray structure analysis of single crystals. The ethyl and isopropyl sub stituents in compounds 1 and 2, respectively, which are in ortho position r elative to the ester group, cause a twist of the O=C-O-C plane with respect to the plane of pyrazolopyridine ring system. Ln the absence of that steri c hindrance, in compound 3, the intramolecular N(amine)-H ... O hydrogen bo nd closes the nearly planar six-membered ring. The coplanarity of the ester or amide plane with the plane of ring system is probably a necessary condi tion for the significant anxiolytic action. In both cations, the protonatio n takes place at the pyridine nitrogen atom. In the crystal structures of s alts there are separate layers of anions and cations. In the free base, int ermolecular hydrogen bonds make infinite chains of molecules.