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61. Ring closure reactions of azidouracils to oxazolo- and isoxazolo-pyrimidines (Organic azides in heterocyclic synthesis, 20)
Dang V. T., Stadlbauer W., J. Heterocyclic Chem. 33 (1996) 1025-1030

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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