Treatment of 5-nitro-6-(N-substituted-anilino) uracils (I) with the Vilsmeier reagent (dimethylformamide-phosphorus oxychloride) gave the corresponding 8-chloroisoalloxazines (8-chloroflavins) (II) in a single step. The conversion of I into II probably proceeds via the intermediate formation of the corresponding isoalloxazine 5-oxides (flavin 5-oxides). In fact, treatment of flavin 5-oxides with the Vilsmeier reagent also gave the 8-chloroflavins. Under the same conditions, 7-bromoflavin 5-oxides did not give the corresponding 7-bromo-8-chloroflavins, but the deoxygenated 7-bromoflavins were obtained. In this case, the Vilsmeier reagent acted as a reducing agent as well as a dehydrating and chlorinating agent. The 8-chloroflavins were converted into 8-(substituted-amino) flavins by treatment with appropriate amines.
