SITES OF PROTONATION IN CARDIOTONIC POLYAZAINDOLIZINES BY NMR-SPECTROSCOPY

The pK(a) values of six sulmazole analogues were measured spectrophotometrically. The major protonation sites for most of these bridgehead nitrogen heterocycles were determined by H-1 and C-13 NMR methods. The aryl-substituted imidazo[1,2-a]pyrimidine (4), 8-methoxyimidazo[1,2-a]pyrazine (6), imidazo[1,2-b]pyridazine (9) and imidazo[1,2-b][1,2,4]triazine (11) undergo protonation at the imidazo nitrogen. The imidazo[1,2-a]pyrazine (5) protonates mainly at N-7. In some cases difference in basicity properties between these aryl analogues and the bridgehead heterocycles have been observed.