Iodine/TBHP-Mediated One-Pot Multicomponent Protocol for Tandem C-N and C-S Bond Formation To Access Sulfenylimidazo[1,5-a]pyridines via C-H Functionalization

Interrupted borrowing hydrogen strategy enabled aminomethylation and direct cross-dehydrogenative coupling strategy enabled dicarbonylation reactions of imidazo[1,5-a]pyridines

Herein, we disclose the development of novel aminomethylation and dicarbonylation reactions of imidazo[1,5-a]pyridines. The developed aminomethylation strategy involves a Pd-catalyzed interrupted borrowing hydrogen strategy by utilizing MeOH as the methylene source. A wide variety of imidazo[1,5-a]pyridines and secondary amines were explored for the developed strategy. The established imidazo[1,5-a]pyridine dicarbonylation strategy involves a catalyst/additive-free direct cross-dehydrogenative coupling reaction between imidazo[1,5-a]pyridines and 2-oxoaldehydes.

Azolopyrimidine-Based Thioethers: Synthesis via Cross-Dehydrogenative C-S Coupling and In Silico Evaluation of Anti-SARS-CoV-2 Activity

Azoloazine derivatives are known as promising small molecules that are potentially able to counteract a broad spectrum of RNA viruses including SARS-CoV-2. However, a pool of synthetic pathways to provide convenient structural modification of such compounds without de novo construction of the heterocyclic scaffold is rather limited so far. This work proposes an approach to the direct C(sp2)-H functionalization of azolopyrimidine substrates with aromatic thiol residues, mediated by the iodine/persulfate reagent system. The reported herein sulfenylation protocol has afforded a series of previously undescribed azolopyrimidine-based thioethers obtained in yields of up to 87 %. Applicability of the approach to the selenium-centered synthons has been demonstrated as well. Besides, the in silico study with regard to the achieved cross-coupling products has suggested the possible affinity to the SARS-CoV-2 main protease (Mpro), as follows from the conducted pharmacophore search and the molecular docking experiments. As a result, the developed synthetic transformation is expected to be of utility in the design of novel antiviral agents based on small azaheterocyclic molecules.

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.

C-H Functionalization of Imidazo[1,5-a]pyridines: A Metal-Free Approach for Methylene Insertion to Access C(sp2)-C(sp3)-H-C(sp2) Bond Formation

Formaldehyde has been used as a solvent and a source of carbon to insert a methylene group for bridging two imidazo[1,5-a]pyridine molecules without using any metal catalysis. This strategy has been extended on other alkyl-, aryl-, and heteroaryl aldehydes as well. This C(sp2)-C(sp3)-H-C(sp2) bond forming reaction proceeds via C(sp2)H functionalization of imidazo[1,5-a]pyridine and was applied on a wide range of substrates offering moderate to good yields of methylene-bridged/inserted bis-imidazo[1,5-a]pyridines. Most importantly, as an application, the bis-heteroarene product has been demonstrated as a ligand. The ligand-like behavior of bis-imidazo[1,5-a]pyridines has been demonstrated as an extension of current methodology. This reaction works well at the gram scale level.

Iodine-Catalyzed Radical C-H Amination of Nonaromatic Imidazole Oxides: Access to Cyclic α-Aminonitrones

A straightforward cross-dehydrogenative coupling approach to incorporate alicyclic amino residues into the structure of model cyclic aldonitrones, 2H-imidazole oxides, is reported. The elaborated C(sp2)-H functionalization is achieved by employing cyclic amines in the presence of the I2-tert-butyl hydroperoxide (TBHP) reagent system. As a result, a series of 19 novel heterocyclic derivatives were obtained in yields of up to 97%. A mechanistic study involving electron paramagnetic resonance spectroscopic experiments allowed the radical nature of the reaction to be confirmed. In particular, the envisioned mechanistic rationale comprises N-iodination of a cyclic amine, followed by N-I bond homolysis of the resulting intermediate and subsequent amination of the nitrone moiety via the newly generated nitrogen-centered radical.

Relay Copper-Catalyzed Synthesis of Imidazo[1,5-a]pyridine Scaffolds from Phenylalanine and Halohydrocarbon

An efficient and straightforward strategy to synthesize imidazo[1,5-a]pyridine compounds from phenylalanine and halohydrocarbon has been successfully developed. The protocol features a relay copper-catalyzed reaction involving intermolecular C-O coupling and intramolecular C-N cyclization, providing an approach to access a diverse range of imidazo[1,5-a]pyridine derivatives with unique aza quaternary carbon centers.

Palladium-Catalyzed Direct Esterification via C-H Bond Activation of Aldehydes

A concise and highly efficient synthesis method of direct esterification of aldehydes via Pd-catalyzed C-H bond activation of aldehyde group has been developed. The strategy avoids the preoxidation step of aldehyde or use of condensing agents in ester synthesis, which is not only applicable to various alcohols but also suitable for the esterification of phenolics which are usually difficult to be esterified. The methodology has the significant advantages of broad substrate scope, mild reaction conditions, and nonrequirement of additional oxidants.