Polymer-Supported Phosphoric-Acid Catalysed Enantioselective Pictet-Spengler Cyclisation for the Synthesis of Quaternary Tryptolines in Batch/Continuous Flow

The enantioselective Pictet-Spenlger reaction of tryptamines and diketo compounds catalyzed by a polymer-supported phosphoric acid (TRIP) has been accomplished for the first time in continuous flow. Using this approach, a diverse library of quaternary tryptolines has been synthesized in 36–95% yield with 39–99% enantiomeric excess. The scalability of the reaction has been demonstrated in continuous flow, with a residence time of only 24 minutes. The robust immobilized catalyst has been recycled and reused multiple times in batch and flow. The synthesis of the chiral precursors of Tadalafil, the Iboga-type alkaloid (+)-Tabertinggine and antimalarial spiroindolinones has been achieved in batch and continuous-flow.