Nitrogen bridgehead compounds. Part 6. Ring transformation. Part 3. Thermal cyclization of diethyl 2-(2-pyridylaminomethylene)-succinates and -glutarates

Abstract

The pyridylsuccinates (1) and pyridylglutarates (2), with various substituents on the pyridine ring, were cyclized in Dowtherm A at 250 °C. The succinates cyclized in two competing reaction routes, giving rise to pyrido[1,2-a]pyrimidines (route A) and N-pyridylpyrrolinones (route B). The ratio of the two products varied with the nature of the substituent and its position. The N-pyridylpyrrolinones proved to be mixtures of the desmotropes (5) and (6) which were separated. The pyridylglutarates (2) gave only the pyrido[1,2-a]pyrimidines (4). The 6-substituted pyrido[1,2-a]pyrimidines underwent a ring-transformation reaction at or above 250 °C, forming the corresponding 1,8-naphthyridines (8) and (9).