Saturated heterocycles. Part 88. Synthesis of a new ring system: dipyrido[1,2-a:4,3-d]pyrimidin-11-one derivatives

The synthesis of 1,2,3,4-tetrahydro-11H-dipyrido[1,2-a:4,3-d]pyrimidin-11-one derivatives (6a–s), a new class of ‘pyracridones’, i.e. 2-azapyracridones, was performed by the condensation of 3-methoxycarbonyl-4-piperidones (5) and 2-aminopyridines in polyphosphoric acid. Catalytic reduction of compounds (6) or ring closure of (5) with 2-iminopiperidine was found to give the 1,2,3,4,6,7,8,9-octahydro-11H-dipyrido[1,2-a:4,3-d]pyrimidin-11-ones (7a)–(7c). The seven-membered ring C homologue derivatives (7d), (7e) have also prepared by the latter method.