Ring closure reactions of azidouracils to oxazolo- and isoxazolopyrimidines

Thermolysis of 6-azidouracils 1 in the presence of polyphosphoric acid leads either to oxazolo5,4-d]pyrimidine-5,7-diones 5 (by reaction with benzoic acid 2a ) or to isoxazolo3,4-d]pyrimidine-4,6-diones 7 (by reaction with aliphatic carboxylic acids 2b,c ). 5-Benzoylpyrimidinetriones 12 could be shown to cyclize to isoxazolo5,4-d]pyrimidine-4,6-diones 15 by chlorination with phosphorus pentachloride and subsequent reaction with sodium azide.