Abstract 5-Arylaminomethylene compounds such as 5-arylaminomethylenepyrimidine-2, 4, 6-triones 2 or 2-phenylaminomethylenephenalene-1, 3-dione 13 cyclize by thermolysis via migration of the arylamino group to pyrimido [4, 5-b] quinoline-2, 4-diones 6 or 7-oxo-7H- naphtho [1, 8-bc] acridine 15, respectively. 2-Phenylaminomethylenecyclohexane-1, 3-dione 8 cyclizes to 9, 9-dimethyl-9, 10-dihydrophenanthridin-7 (8H)-one 11 without ...