Microporous Schiff base network polymer as an efficient catalyst for the synthesis of spirooxindole tetrahydro-β-carbolines via Pictet–Spengler reaction

Spirooxindole and tetrahydro-β-carboline are important chemical motifs that are widely found in natural products and drug molecules. Herein, we report an efficient, recoverable, nitrogen-rich melamine-based microporous Schiff base network (SNW) polymer as a catalyst for the Pictet–Spengler reaction of isatins with tryptamines. A series of desired spirooxindole tetrahydro-β-carbolines was achieved in excellent yields (up to 99%). Notably, the gram-scale experiment was achieved with 95% yield. And the SNW-CrCl₃ catalyst could be easily separated from the reaction mixture and reused at least 7 times with little reduction in reaction activity, implying that this catalytic method was effective and convenient for the synthesis of spirooxindole tetrahydro-β-carbolines. Finally, a possible mechanism was also proposed.