N1,N10-Ethylene-bridged flavinium salts derived from l-valinol: synthesis and catalytic activity in H2O2 oxidations

Three chiral N1,N10-ethylene-bridged flavinium salts with a stereogenic centre derived from l-valinol are prepared and investigated as oxidation catalysts. These salts efficiently catalyse chemoselective H2O2 oxidation of sulfides to sulfoxides and the oxidation of 3-phenylcyclobutanone to the corresponding lactone at room temperature. The flavinium salts react with hydrogen peroxide to form flavin-10a-hydroperoxide, which is the agent responsible for oxidation of the substrate.