Synthesis of 4,7-indoloquinones from indole-7-carbaldehydes by Dakin oxidation

Dakin oxidation of 4,6-dimethoxyindole-7-carbaldehydes, using hydrogen peroxide and hydrochloric acid, results in the synthesis of 6-methoxy-4,7-indoloquinones. The starting materials are readily available through the activation of the indole C7 position by the presence of the methoxy groups. Structural variation is possible through functionalities at C2 and C3. A 2,2′-di-indolylmethane-7,7′-dicarbaldehyde can also be oxidized to a bisindoloquinone under these conditions. The related oxidation of indole-7-carbaldehydes with m-chloroperbenzoic acid or preferably magnesium monoperphthalate instead leads to oxidation at C2 to give indol-2-ones.