PURPOSE: To produce in high yield the high purity compd. by one pot reaction by mixing formic acid with a specified suspension consisting of a 4- aminopyridine deriv. and NaNO2, reducing and heating to cyclize, adding a NaOH soln. and separating it in a form of crystal.

CONSTITUTION: The formic acid is mixed under cooling with the suspension in which the 4-aminopyridine deriv. such as 2,4diamino-6oxodihydropyrimidine and the NaNO2 are dissolved in a formamide solvent. Then the mixture is reduced with sodium dithionite, etc., and the obtained compd. such as 2,4 diamino-5-formamino6-oxodihydropyrimidine is heated to cyclize, and then the formamide solvent is distilled off under a reduced pressure, and a concentrated NaOH soln. is added and the objective compd. such as crystalline disodium-2 amino-1,9dihydropurin-6-on useful to synthesize a pharmaceutical is separated by the one pot reaction.