Nitrogen bridgehead compounds. Part 29. Tautomerism and Z–E isomerism of ethyl 9-aminomethylene-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidine-3-carboxylates and their homologues

Abstract

1 H and 13C n.m.r. studies have proved that ethyl 9-dimethylaminomethylene-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidine-3-carboxylates (2)–(5) and the corresponding pyrrolo homologue (6) exist in the form of E-isomers whereas the azepino[1,2-a]pyrimidine derivative (7) appears as an equilibrium mixture of Z-and E-isomers. On the basis of 15N chemical shifts an analogous tautomeric structure has been established for the 9-dimethylaminomethylene derivatives (2)–(7) and the ethyl 9-arylaminomethylene-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidine-3-carboxylates (8)–(12) as well. 15N Shifts were sensitively affected by the Z–E isomerism and structural changes in remote parts of the molecule, too. Protonation of compounds (2)–(12) takes place on the N(1) atom, forming a 1,6,7,8-tetrahydro structure.