Treatment of 3-methyl-6-phenoxyuracils with the Vilsmeier reagent gave the corresponding 5-formyl-3-methyl-6-phenoxyuracils. Dehydrative cyclization of the above 5-formyluracils with polyphosphoric acid gave 3-methyl-2H-chromeno [2, 3-d] pyrimidine-2, 4 (3H)-diones (3- methyl-5-deaza-10-oxaflavins). These 5-deaza-10-oxaflavins showed strong oxidizing power in oxidizing benzyl alcohol even under neutral conditions to give benzaldehyde, ...