Crystal structure and spectral characteristics of 2-methyl-3-chloro-9- hydroxypyrido[1,2-a]pyrimidin-4-one and bis(2-methyl-3-chloro-9-hydroxypyrido[1, 2-a]pyrimidin-4-onium) perchlorate

2-Methyl-3-chloro-9-hydroxypyrido[1,2-a]pyrimidin-4-one (I) and bis(2-methyl-3-chloro-9-hydroxypyrido[1,2-a]pyrimidin-4-onium) perchlorate (Ia) are synthesized and studied by X-ray diffraction and IR spectroscopy. It is shown that compound I exists in the molecular form in crystals and in the zwitterion form in solutions. The structural units of compound Ia are partially protonated centrosymmetric dimeric cations and perchlorate anions. The dimeric cation involves two molecules I in the zwitterion form. These molecules are linked together by a strong symmetric hydrogen bond formed with the participation of a proton located at the inversion center. The structures of a series of pyrido[1,2-a]pyrimidines are investigated and compared. It is demonstrated that the tautomeric equilibrium revealed in solutions of compounds of this class depends on the solvent type and pH of the medium. © 2004 MAIK "Nauka/Interperiodica".