Synthesis of 4H-Pyrido[1,2-a]pyrimidin-4-ones

Significance

A straightforward synthesis of the 4H-pyrido[1,2-a]pyrimidin-4-one skeleton in good to excellent yields is reported. It features the innovative use of tosylisocyanate as a carbonyl dication equivalent in reactions with pyridyl and picolyl ketimines. Among several isocyanates tested, tosylisocyanate was shown to give the best yields of products, which may be attributed to the leaving group ability of the tosylamide moiety. Two equivalents of isocyanate are needed, one equivalent undergoing a reaction with amine to give ditosyl urea.