CBr as an Efficient Halogen Bond Donor Catalyst in Pictet-Spengler Cyclizations: A Metal-Free Approach to the Synthesis of Benzazepinoindoles and Tetrahydro-β-carbolines.

Indole-based alkaloids, such as benzazepinoindoles and tetrahydro-β-carbolines, hold profound significance in synthetic organic chemistry due to their diverse biological properties. These structurally intricate molecules are efficiently accessed through the well-established Pictet-Spengler cyclization, a reaction that entails the condensation of amines with carbonyl compounds, followed by an intramolecular cyclization. This transformation was effectively catalyzed by tetrabromomethane (CBr), an effective metal-free organocatalyst that facilitates high-yielding reactions while accommodating a broad substrate scope.