A one-step preparation of pyrido[3,4-d]pyrimidine ring system by reaction of 5-formyl-1,3,6-trimethyl-pyrimidine-2,4(1H,3H)-dione with primary amines

6,7-Dihydropyrido3,4-d]pyrimidine-2,4(1H,3H)-diones were obtained in high yields from the reaction of 5-formyl-1,3,6-trimethylpyrimidine-2,4(1H,3H)-dione ( 1 ) and primary amines. For this pyridopyrimidine synthesis the following reaction pathway is proposed; the 1,5]-hydrogen shift of 1 gives a 5,6-dihydro-5,6-dimethylenepyrimidine-2,4(1H,3H)-dione intermediate. The cycloaddition reaction of the intermediate with aldimines from 1 and the primary amines affords 5,6,7,8-tetrahydropyrido3,4-d]pyrimidine-2,4(1H,3H)-diones, which are dehydrated to the final products.