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.