The reaction of 2,4,6,8-tetrachloropyrimido[5,4-d]pyrimidine (I) with piperidine (A) at room temperature gives 2,6-dichloro-4,8-di(N-piperidino)pyrimido[5,4-d]pyrimidine (II), which is treated with diethanolamine (B) at 200 C to yield 2,6-bis(diethanolamino)-4,8-di(N-piperidino)pyrimido[5,4-d])pyrimidine (III) (1). The reduction of (III) with Zn in hot formic or acetic acid or in hot aqueous HCl gives 2,6-bis(diethanolamino)-8-(N-piperidino)-1,2,3,4-tetrahydropyrimido[5,4-d]pyrimidine (IV), which is then dehydrogenated with I2 - KI in water.