Sites of protonation in cardiotonic polyazaindolizines by NMR spectroscopy

Abstract

Abstract The p K a , values of six sulmazole analogues were measured spectrophotometrically. The major protonation sites for most of these bridgehead nitrogen heterocycles were determined by 1 H and 13 C 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 differences in basicity properties between these aryl analogues and the bridgehead heterocycles have been observed.