Total synthesis of murrayanine involving 4,5-dimethyleneoxazolidin-2-ones and a palladium(0)-catalyzed diary insertion

A new total synthesis of the natural carbazole murrayanine (1) was developed by using the 4,5-dimethyleneoxazolidin-2-one 12 as starting material. The latter underwent a highly regioselective Diets - Alder cycloaddition with acrylaldehyde (= prop-2-enal; 13) to give adduct 14 (Scheme 3). Conversion of this adduct into diarylamine derivative 9 was carried out via hydrolysis and methylation (Scheme 4). Differing from our previous synthesis, in which such a diarylamine derivative was transformed into 1 by a Pd^II- stoichiometric cyclization, this new approach comprised an improved cyclization through a more efficient Pd⁰-catalyzed intramolecular diaryl coupling which was applied to 9, thus obtaining the natural carbazole 1 in a higher overall yield.