Nitrogen bridgehead compounds. Part 51 Autoxidation of 9-aminotetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones. Synthesis and stereochemistry of 9-amino- and 9-hydroxy-6,7-dihydro-4H-pyrido[1,2-a]-pyrimidin-4-ones

In solution the 9-phenylaminotetrahydro-4H-pyrido1,2-a]pyrimidin-4-ones 1-5 were oxidized into the 9-aminodihydro compounds 15-19 by atmospheric oxygen at ambient temperature. Autoxidation is most probably a free-radical chain process, which takes place with ground-state triplet oxygen via the radical cation of the enamine form. The 9-aminodihydro derivatives were also prepared from 9,9-dibromo compounds 10 and 11 and from 9-hydroxydihydro compounds 12-14 . The 9-hydroxydihydro derivatives, obtained from the 9-amino compounds 16, 19 and 21 by acidic hydrolysis, showed a solvent-dependent and R¹ substituent-dependent oxo-enol tautomerism. The enol form was stabilized by electron-withdrawing R¹ groups and a polar solvent. However, for the 9-aminodihydropyrido1,2-a]pyrimidines 15-26 only the enamine tautomer (E) could be identified independently of the substituent and the solvent. The chemical structures of the synthesized products were studied by uv, ir, ¹H- and ¹³C-nmr spectroscopy.