A reinvestigation of the reaction between 5,6-dihydroxyindoles and quinones.

5,6-Dihydroxyindoles– react readily with 1,4-naphthoquinone to give mainly deep-violet monoadducts identified as – by chemical and spectral analysis. When an excess of 1,4-naphthoquinone is used, a more complex reaction takes place leading to the dinaphthocarbazole rather than the diadduct , as previously reported. The same product () is also obtained by reaction of with the 2,2'-dinaphthyl-1 ,4,1 ',4' -diquinone () in 30% yield. Interestingly, when 5,6-dihydroxyindoles are allowed to react with p-benzoquinone under similar conditions, no addition products are formed (UV and TLC evidence), owing to the tendency of the indoles to undergo oxidative coupling with formation, besides melanin-like products, of a complex mixture of oligomers. In the case of , reduction and subsequent acetylation of the reaction mixture, has allowed the isolation of two oligomers, identified as and