Enantioselective BINOL-Phosphoric Acid Catalyzed Pictet−Spengler Reactions of <i>N</i>-Benzyltryptamine

Optically active tetrahydro-β-carbolines were synthesized via an (R)-BINOL-phosphoric acid-catalyzed asymmetric Pictet−Spengler reaction of N-benzyltryptamine with a series of aromatic and aliphatic aldehydes. The tetrahydro-β-carbolines were obtained in yields ranging from 77% to 97% and with ee values up to 87%. The triphenylsilyl-substituted BINOL-phosphoric acid proved to be the catalyst of choice for the reaction with aromatic aldehydes. For the aliphatic aldehydes, 3,5-bistrifluoromethylphenyl-substituted BINOL-phosphoric acid was identified as the best catalyst.