In situ vinylindole synthesis. Diels-Alder reactions with maleic anhydride and maleic acid to give tetrahydrocarbazoles and carbazoles

Abstract

Tetrahydrocarbazoles have been prepared in one-flask syntheses from indoles, ketones, and maleic anhydride, with acid catalysis. The reactions involve a condensation of the indole with the ketone or aldehyde, follwed by an in situ trapping of the vinylindole in a Diels-Alder reaction with maleic anhydride. Depending on which indole is used, this may or may not be followed by a double bond isomerization. If the double bond isomerization doesn't occur, the adduct may or may not be acylated at the nitrogen by maleic anhydride. 1,2,3,4-Tetrahydrocarbazole-1,2-dicarboxylic anhydrides have been hydrolyzed to the 1,2-dicarboxylic acids in good yields. The corresponding 1,2-dicarboxylic acids have been regioselectively decarboxylated at the 1-position in excellent yields. New results with the use of maleic acid as the dienophile in this reaction are also discussed.