Pyrimidines. 20. Novel reactions of 5‐cyano‐1,3‐dimethyluracil. A simple synthesis of pyrido[2,3‐ d ]pyrimidines

A reaction of 5‐cyano‐1,3‐dimethyluracil (1, R = CN) with acetone in base afforded 1,3,7‐trimethylpyrido‐[2,3‐ d ]pyrimidine‐2,4(1 H ,3 H )dione ( 9a ) in a moderate yield. From a reaction mixture of 1 (R = CN) with butanone, 1,3,6,7‐tetramethyl‐ and 7‐ethyl‐1,3‐dimethylpyrido[2,3‐ d ]pyrimidine‐2,4(1 H ,3 H )‐dione ( 9b and 9c , respectively) were isolated in low yields. When ethyl cyanoacetate or malononitrile was used in place of the ketone in the above reaction, 7‐amino‐6‐ethoxycarbonyl‐ and 7‐amino‐6‐cyano‐1,3‐dimethylpyrido[2,3‐ d ]‐pyrimidine‐2,4(1 H ,3 H )‐dione ( 14a and 14b , respectively) were obtained in quantitative yields. A plausible mechanism for the formation of bicyclic compounds is discussed.