The reaction of 6-hydroxyamino-1, 3-dimethyluracil (1) with Vilsmeier reagent (phosphorus oxychloride and dimethylformamide) or arylaldehydes afforded 5, 7-dimethyl-isoxazolo [3, 4-d] pyrimidine-4, 6 (5H, 7H)-dione (2) and its 3-aryl derivatives (10-17), respectively. The 3-arylisoxazolo [3, 4-d] pyrimidines (9, 10) underwent photorearrangement to yield the corresponding 2-aryloxazolo [4, 5-d] pyrimidines (19-20). Some reactions on 2 are also reported.