Abstract

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.