We developed a safe, simple, inexpensive, and environmentally benign method for preparing N(5)-ethylated flavinium organocatalysts without using hazardous reagents or inert conditions as previously required. 5-Ethyl-3-methyllumiflavinium cation was prepared from its reduced form by NaNO -free aerobic oxidation, which was subsequently extracted onto commercial cation-exchange resins under NaClO -free conditions. The resulting resin-immobilized flavinium salts were found to be effective organocatalysts for aerobic oxidation reactions.
Isoalloxazine ring systems are found in flavin cofactors in nature, and the simulation of their redox catalyses is an important task for developing sustainable catalytic oxidation reactions. Although 5‐ethyl‐4a‐hydroxyisoalloxazines are among the most promising candidates as catalyst for such purposes, the use of them for laboratorial as well as industrial synthetic chemistry has so far been quite limited presumably due to the lack of their preparation methods readily, safely, and inexpensively available. In this communication, we introduce an environmentally benign and practical preparation of 5‐ethyl‐4a‐hydroxy‐3,7,8,10‐tetramethylisoalloxazine (1EtOH) from 3,7,8,10‐tetramethylisoalloxazine (1), in which conventional synthetic requirements, including (i) operations under inert conditions, (ii) risky or expensive chemicals, and (iii) isolation of labile intermediates, have all been dissolved. In addition, we have presented that 1EtOH could be an effective catalyst for Baeyer–Villiger oxidation as well as sulfoxidation with molecular oxygen (O2) as a terminal oxidant under suitable conditions, which is the first report on aerobic oxygenations catalyzed by 5‐alkyl‐4a‐hydroxyisoalloxazines.
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