Further synthetic approaches to flavines (isoalloxazines). A new synthesis of riboflavine

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Abstract
Dehydrative cyclization of 5-nitro-6-(N-substituted anilino) uracils (2) with concentrated sulphuric acid gave the corresponding flavine 5-oxides, which were converted into flavines. Treatment of 5-amino-6-(N-substituted anilino) uracils (5)[prepared by hydrogenation of the nitro-derivatives (2)] with diethyl azodiformate (DAD) or nitrosobenzene in indirect sunlight or under irradiation with a sun-lamp gave the corresponding flavines. The reaction mechanism has been demonstrated by the isolation of the intermediate 5-imino-6-(N-substituted anilino) uracils. Autoxidation of the amino-derivatives (5) also gave flavines; by this method riboflavine and related compounds were obtained. Treatment of the nitro-compounds (2) with DAD in the presence of bases such as triphenylphosphine and pyridine gave flavines; in this case DAD becomes the source of N-5 of the flavine ring system.

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