Synthesis of pentacyclic pyrrolo[3,4-a]carbazole-1,3(2H)-diones via an intermolecular Diels–Alder, intramolecular carbonyl-ene reaction strategy

Abstract

Pentacyclic pyrrolo[3,4-a]carbazole-1,3(2H)-diones are accessed via two key synthetic steps, an intermolecular Diels–Alder (D–A) reaction between an N-protected 3-vinyl-1H-indole and N-methyl-maleimide, and a Lewis acid-catalysed intramolecular carbonyl-ene cyclisation reaction. Cyclopentyl- or cyclohexyl-containing scaffolds can be formed through variation in the length of the alkyl tether, whilst the observed stereospecificity of carbonyl-ene cyclisation supports a concerted mechanism.