Sites of Protonation in Cardiotonic Polyazaindolizines by NMR Spectroscopy

Article abstract of DOI:10.1002/mrc.1260290511

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 1H and 13C NMR methods.The arylsubstituted 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 differences in basicity properties between these aryl analogues and the bridgehead heterocycles have been observed.KEY WORDS Polyazaindolizines Protonation 13C NMR 1H NMR pK values