A tandem highly stereoselective FeCl3-promoted synthesis of a bisindoline: synthetic utility of radical cations in heterocyclic construction

A conceptually distinctive stereoselective construction of the novel dimer, N-[N′-acetyl-7,7′-bis-(3,4-dimethoxy-phenyl)-7,8,7′,8′-tetrahydro-N′H-[8,8′]biindolyl-N-yl]-ethanone (bisindoline) is described below. These structures, which include 7-(3,4-dimethoxyphenyl)-indoline (veratryl indoline), were obtained by the tactical combination of palladium-catalysed coupling which produced 10-acetamido-3,4-dimethoxystilbene , followed by FeCl3 induced oxidative cyclization/dimerization. All new structures were fully characterized by 1- and 2D NMR spectroscopy, (proton, carbon-13, COSY, HMBC, HMQC) and mass spectrometry. Configurational assignments were further supported by semi-empirical AM1 calculations. Mechanistic interpretations, consistent with our results, are discussed.