Pyrimidine derivatives and related compounds. XLVI. Thermal and photochemical transformation of 5-substituted 6-azido-1,3-dimethyluracils into fused pyrimidines such as isoxazolo(3,4-d)pyrimidines, pyrazolo(3,4-d)pyrimidines, and pyrimido(4,5-d)(1,2,3)triazine.