Synthesis of 4-Trifluoromethylpyrimido[4,5-c]pyridazine-5,7-diones from Uracils

The reaction of 6-hydrazinouracils (1) with 3-aryl-1,1,1-trifluoropropane-2,3-dione monohydrates (2) in refluxing ethanol in the presence of p-toluenesulfonic acid gave regioselectively 3-aryl-4-trifluoromethyl-5,6,7,8-tetrahydropyrimido[4,5-c]pyridazine-5,7-diones (4a-e) in moderate yields. The location of a trifluoromethyl group at the C4 position was elucidated on the basis of the chemical transformation of the derivatives.