Nitrogen bridgehead compounds. Part 64. Protonation of 9-formyltetrahydropyrido[1,2-a]pyrimidin-4-ones and their analogues

Abstract

As shown by ¹H, ¹³C, and ¹⁵N n.m.r. spectroscopy, protonation of 9-formyltetrahydropyrido[1,2-a] pyrimidin-4-ones takes place at the formyl oxygen atom. As a result, the enamine tautomer, predominant in the base form, is transformed into a protonated enolimine tautomer having an exocyclic double bond. ¹⁵N N.m.r. shifts reflect not only the state of the tautomeric equilibria but also the state of Z/E isomerism and the effect of remote substituents.