Prepd by boiling alloxantin with ammonium chloride: Whler, Liebig, Ann. 26, 241 (1838); by reduction of 5-nitrobarbituric acid with tin and HCl: Hartman, Sheppard, Org. Synth. coll. vol. II, 617 (1943); from barbituric acid, sodium nitrite and sodium hydrosulfite: Koppel, Robins, J. Am. Chem. Soc. 80, 2751 (1958).