Cyclization of 5-cyano-6-cyanoimino-3,4-dihydropyridin-2(1H)-ones with Amines

The cyclization of 5-cyano-6-cyanoimino-3,4-dihydropyridin-2(1H)-ones with amines, leading to pyrido[2,3-d]pyrimidines, is studied. Four factors play an important role on the direction of cyclization: (1) the planarity of the reaction area; (2) the cyclization proceeds by the nucleophilic attack of an amidine onto an electrophilic group which can be either a cyano group or an alkylamidine; (3) the basicity of the amine plays an important role, the ionization of the substrate making more difficult the formation of the aforementioned amidine which is needed for the cyclization; and (4) the nucleophilic attack and the tautomerization of the cyclization product are processes which can occur practically simultaneously, so that the electronic movement involved in the tautomerization process coincides with the one which happens in the nucleophilic attack, both processes promoting each other.