Brown Seaweed-Derived Alginic Acid: An Efficient and Reusable Catalyst for Pictet-Spengler Reaction to Access Tetrahydro-β-Carboline and Tetrahydroisoquinoline Frameworks

Pictet-Spengler reaction (PSR) is an efficient and reliable synthetic tool for the construction of tetrahydro-beta-carbolines (THBCs) and tetrahydroisoquinolines (THIQs) moieties. Herein, we reported a brown seaweed-derived biocompatible alginic acid as an alternative and effective reusable catalyst for PSR. This protocol was found suitable for various aliphatic and aromatic aldehydes with electron-donating and withdrawing groups resulting in good to excellent yields. The utility of this sustainable protocol highlighted by synthesizing different THBCs and THIQs including a natural spirooxindole (+/-)-coerulescine (9a) and its derivative 9b. The reusability of catalyst was also examined, and no significant deterioration in the reaction yields was observed till the 6th cycle.

Automated summary

The Pictet-Spengler reaction is an efficient synthetic tool for constructing tetrahydro-beta-carbolines and tetrahydroisoquinolines, with a brown seaweed-derived alginic acid reported as an effective reusable catalyst. The protocol shows good yields with various aliphatic and aromatic aldehydes, and demonstrates sustainable synthesis of different compounds including a natural spirooxindole. The catalyst was found to be reusable with no significant deterioration in reaction yields observed up to the 6th cycle.