Lewis Acid Catalyzed Nenitzescu Indole Synthesis

A novel method for Lewis acid catalyzed Nenitzescu indole syntheses of 5-hydroxyindoles bearing different substituents in positions 1 )Alk, Bn, Ar), 2 )Me, Et, Ph), and 3 )COOEt, COMe, CONHPh) as well as tricyclic derivatives are reported. The method is simple, rapid, efficient, and allows preparation of hydroxyindoles from 1,4-benzoquinone and enamines in good to excellent yields with the use of low-polar solvents in the presence of weak Lewis acids catalysts. The formation of 5-hydroxyindoles under such mild conditions is explained in terms of a non-redox mechanism.