Electrochemically Mediated Synthesis of Cyanated Heterocycles from α-amino Esters, Pyridine-2-carbaldehydes and NH4SCN as Cyano Group Source

The electrochemically mediated cyanation/annulation process with in situ cyanide ion generation from NH4SCN and multi-step oxidative construction of CN-functionalized heterocycles from easily available α-amino esters and pyridine-2-carbaldehydes has been discovered. Depending on the nature of the α-amino ester, 1-cyano-imidazo[1,5-a]pyridine-3-carboxylates, 3-alkyl- and 3-aryl-imidazo[1,5-a]pyridines-1-carbonitriles, and the first reported 4-oxo-4H-pyrido[1,2-a]pyrazine-1-carbonitriles were obtained. The electrosynthesis is carried out in an undivided electrochemical cell under constant current conditions. The success of the discovered electrochemical synthesis is based on the combination of two anodic processes: oxidation of SCN anion to CN anion and oxidation of C-N bonds to C=N bonds during heterocycle construction. Mechanistic studies based on CV measurements, and control experiments confirm the generation of [CN] species from NH4SCN with subsequent addition to an imine formed from α-amino esters and pyridine-2-carbaldehyde. Computational analysis suggests that for reactive intermediates from glycine esters, the subsequent 5-endo-trig cyclization leading to 1-cyano-imidazo[1,5-a]pyridine-3-carboxylates is more favourable and the 6-exo-trig cyclization leading to 4-oxo-4H-pyrido[1,2-a]pyrazine-1-carbonitriles is less favourable. For α-amino esters with alkyl or aryl substituents, both cyclization pathways are relatively thermodynamically possible. The leading 4-oxo-4H-pyrido[1,2-a]pyrazine-1-carbonitrile showed high fungicidal activity against phytopathogenic fungi.

Synthesis of pyrazino[1,2-b]indazoles via cascade cyclization of indazole aldehydes with propargylic amines

An efficient intermolecular annulation of indazole aldehydes with propargylic amines has been developed for the synthesis of pyrazinoindazoles under catalyst- and additive-free conditions. This straightforward methodology was found to feature a wide substrate scope, high atom economy and environmental advantages. The bioactivity results of these new pyrazino[1,2-b]indazoles showed that some of them exhibited significant antifungal activity.

A TEMPO promoted tandem reaction of 2-aminobenzophenones and benzylamines under electrochemical conditions

This study describes the efficient synthesis of quinazolines promoted by TEMPO via electro-catalysis with 2-aminobenzophenones and benzylamines. The method exhibited remarkable chemoselectivity under mild reaction conditions. A series of quinazolines could be obtained in moderate to good yields. In addition, control experiments were carried out to verify the reaction mechanism. Furthermore, the synthesis on the gram scale was conducted successfully to give the target product.

Copper-Catalyzed Oxidative [3 + 2] Cycloaddition of N-Alkyl Pyridinium Salts to Imidazo[1,2-a]pyridines

A copper-catalyzed reaction of pyridinium salts and trimethylsilyl cyanide (TMSCN) in the presence of diethyl phosphite is developed. This reaction, which allows the single-step construction of biologically important 2-cyanoimidazo[1,2-a]pyridine from readily available starting materials, is realized for the first time and is feasible at the gram scale. The scope of the protocol is demonstrated with 27 examples. A consecutive double cyanation and cyclization can be achieved in this one-pot process. TMSCN plays a dual role not only as the "CN" source but also as the coupling partner for the cyclization of imidazo[1,2-a]pyridines.

Paired Electrolysis Enabled Cyanation of Diaryl Diselenides with KSCN Leading to Aryl Selenocyanates

The first example of paired electrolysis-enabled cyanation of diaryl diselenides, with KSCN as the green cyanating agent, has been developed. A broad range of aryl selenocyanates can be efficiently synthesized under chemical-oxidant- and additive-free, energy-saving and mild conditions.

Metal-Free Synthesis of N-Heterocycles via Intramolecular Electrochemical C-H Aminations

N-heterocycles are key structural units in many drugs, biologically interesting molecules and functional materials. To avoid the residues of metal catalysts, the construction of N-heterocycles under metal-free conditions has attracted much research attention in academia and industry. Among them, the intramolecular electrochemical C-H aminations arguably constitute environmentally friendly methodologies for the metal-free construction of N-heterocycles, mainly due to the direct use of clean electricity as the redox agents. With the recent renaissance of organic electrosynthesis, the intramolecular electrochemical C-H aminations have undergone much progress in recent years. In this article, we would like to summarize the advances in this research field since 2019. The emphasis is placed on the reaction design and mechanistic insight. The challenges and future developments in the intramolecular electrochemical C-H aminations are also discussed.

I2-Mediated [3 + 2] annulation of methyl-azaarenes with alkyl 2-isocyanoacetates or amino acid ester hydrochlorides: selective synthesis of iodine-functionalized and non-iodine-functionalized fused imidazoles

An I2-mediated [3 + 2] annulation of methyl-azaarenes with alkyl 2-isocyanoacetates or amino acid ester hydrochlorides has been demonstrated. This strategy involves the C≡N cleavage of isocyanides and can selectively...