Dehydrogenation of 6-azaquinazoline derivatives. Formation of unexpected quinonediimine intermediates

Abstract

2,6-Disubstituted 5,6,7,8-tetrahydropyrido[4,3-d] pyrimidin-4(3H)-one (6-azaquinazoline) derivatives 7a–e were synthesized from N-substituted 3-methoxycarbonyl-4-piperidones 5a, b and amidines 6a–c. Compounds 7a–d and the debenzylated derivatives 8a–c underwent dehydrogenation in xylene or in nitrobenzene in the presence of a palladium–carbon catalyst, furnishing products 9a, b and d or 10a–c, respectively. It was found that the formation of the two types of products, 9 or 10, from the same molecules depends on the substituents at positions 2 and 6, and on the inert or oxidative character of the solvent used. The quinonediimine forms 9a, b can be considered to be intermediates of the transformation 7a, b→10a, b.