SYNTHESIS OF HALOGENATED 4H-PYRIDO[1,2-a]PYRIMIDIN-4-ONES

Halogenated 4H-pyrido[1,2-a]pyrimidin-4-one were synthesized by thermal cyclization and decarboxylation of isopropylidene (2-pyridylamino)methylenemalonates, prepared from 2-aminopyridines and isopropylidene methoxymethylenemalonate formed in situ. Instead of 4H-pyrido[1,2-a]pyrmidin-4-ones, the 6-chloro and 6-bromo derivatives afforded mixtures of 7-halo-1,4-dihydro-1,8-naphthyridin-4-ones and 1-(6-halo-2-pyridyl)-3[(6-halo-2-pyridylamino)methylene]-1,2,3,4-tetrahydropyridine-2,4-diones. The latters formed from N-(2-pyridyl)iminoketenes, the common intermediates of 4H-pyrido[1,2-a]pyrimidin-4-one and 1,8-naphthyridin-4-ones, via a a "head-to-tail" [4+2] cycloaddition. 3-Halo-4H-pyrido[1,2-a]pyrimidin-4-ones were obtained from 4H-pyrido[1,2-a]pyrimidin-4-one with N-halosuccinimides. The structures of the new compounds were characterized by means of H-1 NMR and C-13 NMR examinations.