Synthesis of 2,5-dihydroxy-3-(indol-3-yl)benzoquinones by acid-catalyzed condensation of indoles with 2,5-dichlorobenzoquinone

Three methods for the conjugate addition of indoles to 2,5-dichlorobenzoquinone have been developed. A wide variety of indoles substituted with halogen, alkyl, alkoxy, and aryl groups participate in anaerobic condensation reactions promoted by HCl, H2SO4, or CH3CO2H. The hydroquinone product is partially oxidized by excess dichlorobenzoquinone and fully converted to the 2,5-dichloro-3-(indol-3-yl)benzoquinone targets by DDQ or Ag2CO3 oxidation. 2,5-Dihydroxy-3-(indol-3-yl)benzoquinones can be obtained from the dichlorides by alkaline hydrolysis. The rotational characteristics of the biaryl bond created in these reactions have been examined by theoretical and spectroscopic methods.