Total Synthesis of (±)-Hirsutine: Application of Phosphine-Catalyzed Imine-Allene [4+2] Annulation

The total synthesis of the indole alkaloid hirsutine has been achieved, with a key step being the application of our phosphine-catalyzed [4 + 2] annulation of an imine with ethyl alpha-methylallenoate. From commercially available indole-2-carboxaldehyde, the target was synthesized in 14 steps and 6.7% overall yield.