o-Benzenedisulfonimide as a reusable acid catalyst for an easy, efficient, and green synthesis of tetrahydroisoquinolines and tetrahydro-β-carbolines through Pictet–Spengler reaction

The synthesis of tetrahydroisoquinolines and tetrahydro-β-carbolines, using the Pictet–Spengler reaction, was carried out in the presence of a catalytic amount of o-benzenedisulfonimide, which worked as a Brønsted acid organocatalyst. The reaction conditions were mild and green and good target product yields were achieved. The catalyst was easily recovered and purified, ready to be used in further reactions with economic and ecological advantages.