Abstract: Thermolysis of 6-azidouracils in the presence of polyphosphoric acid leads
either to oxazolo[5,4-d]pyrimidine-5,7-diones (by reaction with benzoic acid 2a)
or to isoxazolo[3,4-d]pyrimidine-4,6-diones (by reaction with aliphatic carboxylic
acids). 5-Benzoyl-pyrimidinetriones could be shown to cyclize to isoxazolo
[5,4-d]pyrimidine-4,6-diones by chlorination with phosphorus pentachloride
and subsequent reaction with sodium azide.
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