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.

Thermolysis and photolysis of 6-azido-1, 3-dimethyluracils possessing certain substituents (formyl, benzoyl, hydrazonomethyl, phenyl, and benzyl groups) at the 5-position provided new methods for the preparation of fused pyrimidines. Irradiation and heating of 5-formyl-and 5-benzoyl-6-azidouracils (3 and 7) led to the formation of isoxazolo [3, 4-d] pyrimidines (4 and 8). 2-(Substituted amino) pyrazolo [3, 4-d] pyrimidines (14) were conveniently prepared by the reaction of 6-chloro-5-hydrazonomethyl-1, 3-dimethyluracils (12) with sodium azide, followed by thermolysis of the resulting 6-azides. Upon thermolysis and photolysis of 5-phenyl-and 5-benzyl-6-azidouracils (16 and 19), pyrimido [4, 5-b] indole (17) and pyrimido [4, 5-b] quinoline (10) were obtained, respectively. Cyclization of 3 using triethyl phosphite and triphenylphosphine resulted in the formation of a new ring system, pyrimido [4, 5-d] [1, 2, 3] triazine (5).