Pictet–Spengler reactions catalyzed by Brønsted acid-surfactant-combined catalyst in water or aqueous media

A Transition Metal-Free Strategy for Dihydrobenzazepinoindole via KI-Mediated Oxidative Pictet-Spengler Reaction

A transition-metal-free synthesis of dihydrobenzazepinoindoles has been developed using a potassium iodide (KI)-mediated oxidative Pictet-Spengler reaction under mild conditions. While the classical Pictet-Spengler cyclization is well-established, its oxidative variant remains underexplored. This method offers a sustainable and efficient approach, providing operational simplicity, broad substrate scope, and enhanced environmental compatibility. The reaction tolerates various functional groups, enabling access to structurally diverse dihydrobenzazepinoindole derivatives. The synthesized compounds were fully characterized using 1H NMR, 13C NMR, DEPT-135, IR, and mass spectrometry, confirming their structural integrity. The scalability of this method has been demonstrated through a successful large-scale synthesis. A plausible reaction mechanism has been proposed, shedding light on the role of KI in the oxidative cyclization. This study underscores the potential of KI as an environmentally benign promoter and presents a practical alternative to traditional metal-mediated methodologies for the synthesis of nitrogen-containing heterocycles.

A facile and efficient method for the synthesis of crystalline tetrahydro-β-carbolines via the Pictet-Spengler reaction in water

A facile and efficient synthesis of tetrahydro-β-carbolines (tryptolines) in one step from tryptamine and aldehydes, in an environmentally friendly water solvent, has been investigated. This convenient and clean synthesis of various tryptolines was facilitated by l-tartaric acid, a natural compound, to obtain the desired products as clear crystals. Among the four crystalline products, the most substituted tryptoline 2 showed the best inhibitory activity against EJ cells and the least cytotoxicity, with an LC₅₀ value of 1.49 mg/mL, against brine shrimp larvae.

Selective construction of alkaloid scaffolds by alcohol-based direct and mild aerobic oxidative Pictet–Spengler reactions

Tetrahydro-β-carboline and β-carboline alkaloid scaffolds can be selectively obtained by direct aerobic oxidative Pictet–Spengler reactions of tryptamines with alcohols using TBN/TEMPO as the catalysts and oxygen as the oxidant under mild conditions.

Simple and efficient synthesis of tetrahydro-β-carbolines via the Pictet–Spengler reaction in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)

1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) can act as both the solvent and the catalyst to effectively promote the Pictet–Spengler reaction.

Synthesis of biologically active pyridoimidazole/imidazobenzothiazole annulated polyheterocycles using cyanuric chloride in water

An efficient and mild protocol for rapid access to N-fused polyheterocycles via Pictet–Spengler type 6-endo cyclization by cyanuric chloride in aqueous medium has been developed.

Heterogeneous Catalysts in Pictet-Spengler-Type Reactions

Several solid catalysts were evaluated as an alternative for 1,2,3,4-tetrahydroisoquinoline synthesis by means of the Pictet-Spengler reaction. The reaction catalysed by a mixed oxide (Mg and Al) led to the best yield and good regioselectivity; using an Al-pillared bentonite led to good yields and total regioselectivity. The results revealed no direct relationship between catalyst acidity and yield.

Ultrasound-enhanced one-pot synthesis of 3-(Het)arylmethyl-4-hydroxycoumarins in water

3-(Aryl)methyl-4-hydroxycoumarins were produced in good to excellent yields by reaction between 4-hydroxycoumarin and (hetero)aromatic aldehydes in the presence of Hantzsch 1,4-dihydropyridine (HEH) which works as an hydride donor (i.e., in a sequential Knoevenagel-reductive Michael addition). The sonochemical-assisted procedure (method B) provides an improved and accelerated conversion when compared to conventional silent reactions (method A). Experiments carried out according to method B showed that the reaction could be more efficiently run in the absence of organic solvents, at 30-40°C in open vessel, without the need of an excess HEH and with simplified work-up and separation procedures.