A synthesis of alloxazine 5-oxides by the reaction of 6-anilinouracils with nitric acid and 4-phenylurazole

The treatment of 6-anilinouracils with a mixture of nitric acid and 4-phenylurazole (which is virtually a mixture of nitrous acid and 4-phenyl-1, 2, 4-triazoline-3, 5-dione (PTAD)) in dioxane gave the corresponding alloxazine 5-oxides exclusively. This reaction involves the nitrosative cyclization of 6-anilinouracils by nitrous acid, followed by the dehydrogenation of the hydroxylamine intermediates with PTAD. 6-Anilinouracils react with PTAD to give the corresponding Michael-type adducts, 6-anilino-5-(4-phenylurazol-1-yl) uracils.