Acid-catalyzed addition of indoles to hydroxyquinones

Hydroxyquinones react smoothly with substituted indoles in the presence of a catalytic amount of p-toluenesulfonic acid to afford a variety of addition products. The main products are either the corresponding p-quinone or the corresponding hydroquinone, resorcinol and benzenetriol mono- and diindolyl derivatives, depending on the nature of the hydroxyquinone and the substituents on the indole ring.