The synthesis of pyrimido[4,5-d] pyridazines

The synthesis of pyrimido4,5-d]pyridazin-2-one-4-thione (II), pyrimido4,5-d]pyridazine-2,4-dithione (VI), 4-aminopyrimido4,5-d]pyridazine-2-thione (III), 2-aminopyrimido4,5-d]-pyridazin-4-one (X), 2-methylpyrimido4,5-d]pyridazin-4-one (XII), and pyrimido4,5-d]-pyridazin-4-one (XIII) are reported together with several new pyridazine intermediates.