Isocyanate acting as a carbonyl precursor: Pyridyl group-assisted formation of 4H-pyrido[1,2-a]pyrimidin-4-ones from ketimines and isocyanates

By the reactions of ketimines bearing a pyridyl or a picolyl group on a nitrogen atom of the imine moiety with tosylisocyanate, 4H-pyrido[1,2-a] pyrimidin-4-one derivatives could be obtained in quantitative yields. In these reactions, tosylisocyanate acts as a carbonyl precursor. The pyridyl or picolyl group is a key functional group because it is not only the constituent structure of the 4H-pyrido[1,2-a]pyrimidin-4-one framework but also the promoter of the formation of a ketene intermediate.