Bioglycerol-derived carbon−SO3H as a recyclable catalyst for the synthesis of tetrahydro-β-carbolines

A variety of aldehydes undergo smooth coupling with N-tosyltryptamine in the presence of carbon−SO3H in ethanol at 80 °C to furnish the corresponding tetrahydro-β-carbolines in excellent yields with high selectivity. The coupling of tryptamine with isatins affords the respective spiro-tetrahydro-β-carbolines in good yields. The use of a recyclable solid acid catalyst makes this method simple, convenient, and cost-effective.