Nitrogen bridgehead compounds, part 49. Synthesis and stereochemistry of 9-aminotetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones.

Abstract

The 9-phenylaminotetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones (4)–(9), synthesized from the 9-bromo compounds (1)–(3), displayed imine–enamine tautomerism. In solution (if R²≠ H) the equilibrium mixtures contain both cis- and trans-imines. The enamine form is stabilized by increasing polarity of the solvent and by increasing the electron-withdrawing effect of substituent R¹. Owing to 1,3-allylic strain, in the derivatives where R²= Me the imine form is energetically less favoured than in the derivatives with R²= H. The chemical structures of the synthesized products were studied by u.v., ¹H and ¹³C n.m.r. spectroscopy.