Heating of 6-arylamino-1, 3-dimethyluracils (IIa-e) with one-carbon reagents (dimethylformamide dimethylacetal, dimethylformamide-phosphorus oxychloride, and triethyl orthoformate) afforded 1, 3-dimethylpyrimido [4, 5-b] quinoline-2, 4 (1H, 3H)-diones (Va-e) via the intramolecular cycloaddition of azahexatrienes. Similarly, treatment of IIa with arylaldehydes provided 5-aryl-1, 3-dimethylpyrimido [4, 5-b] quinoline-2, 4 (1H, 3H, 5H, 10H)-diones (VIIa-e), which were subsequently dehydrogenated with either thionyl chloride or diethyl azodicarboxylate to give 5-aryl-1, 3-dimethylpyrimido [4, 5-b] quinoline-2, 4 (1H, 3H)-diones (Xa-e). The reaction of 6-arylamino-1, 3-dimethyl-4-N-phenylcytosines (XIIa-c) with dimethylformamide dimethylacetal to give 4-arylimino-1, 3-dimethylpyrimido [4, 5-b] quinoline-2 (1H, 3H)-ones (XIIIa-c) is also described.