Facile One-Pot Direct Arylation and Alkylation of Nitropyridine N-Oxides with Grignard Reagents

First total synthesis of caerulomycin K: a case study on selective, multiple C-H functionalizations of pyridines

Caerulomycins, natural alkaloids with antimicrobial properties, have been previously synthesized starting with highly pre-functionalized building blocks or requiring many functional group manipulations. In this work, we report the first total synthesis of caerulomycin K, a diversely trifunctionalized pyridine readily assembled in three steps exploiting the recent advancements in the C-H activation of N-heterocycles.

Chemo- and Regioselective Alkylation of Pyridine N-Oxides with Titanacyclopropanes

While titanacyclopropanes are used to react mainly with ester, amide, and cyano to undergo cyclopropanation, herein they react preferentially with pyridine N-oxide to accomplish C2-H alkylation beyond these functionalities with double regioselectivity. After being pyridylated at the less hindered C-Ti bond, the remaining C-Ti bond of titanacyclopropanes can be further functionalized by various electrophiles, allowing facile introduction of complex alkyls onto the C2 of pyridines. Its synthetic potential has been demonstrated by late-stage diversification of drugs.

Analogy of the Reactions of Aromatic and Aliphatic π-Electrophiles with Nucleophiles

The aim of this essay is to disclose the similarity of a great variety of reactions that proceed between nucleophiles and π-electrophiles-both aromatic and aliphatic. These reactions proceed via initial reversible addition, followed by a variety of transformations that are common for the adducts of both aliphatic and aromatic electrophiles. We hope that understanding of this analogy should help to expand the scope of the known reactions and inspire the search for new reactions that were overlooked.

Synthesis of Bis-heteroaryls Using Grignard Reagents and Pyridylsulfonium Salts

Herein are reported ligand-coupling reactions of Grignard reagents with pyridylsulfonium salts. The method has wide functional group tolerance and enables the formation of bis-heterocycle linkages, including 2,4′-, 2,3′-, and 2,2′-bipyridines, as well as pyridines linked to pyrimidines, pyrazines, isoxazoles, and benzothiophenes. The methodology was successfully applied to the synthesis of the natural products caerulomycin A and E.

Installing the “magic methyl” – C–H methylation in synthesis

Following notable cases of remarkable potency increases in methylated analogues of lead compounds, this review documents the state-of-the-art in C–H methylation technology.

Copper-Catalyzed ortho-Functionalization of Quinoline N-Oxides with Vinyl Arenes

An efficient copper-catalyzed regioselective C-H alkenylation and borylative alkylation of quinoline N-oxides with vinyl arenes in the presence of pinacol diborane has been developed. The reaction proceeds through the borylcupration of the vinyl arenes followed by nucleophilic attack of the resulting alkyl copper species to the quinoline N-oxides. Benzoquinone and KO^t Bu were identified as the necessary additives at the second step of the reaction that are crucial for the success of the reaction. A wide range of C2-functionalizaed quinolines were obtained with good functional group tolerance, which may find utilities in pharmaceuticals and synthetic chemistry.

Visible-Light-Promoted C2 Selective Arylation of Quinoline and Pyridine N-Oxides with Diaryliodonium Tetrafluoroborate

A protocol of visible-light-promoted C2 selective arylation of quinoline and pyridine N-oxides, with diaryliodonium tetrafluoroborate as an arylation reagent, using eosin Y as a photocatalyst for the construction of N-heterobiaryls was presented. This methodology provided an efficient way for the synthesis of 2-aryl-substituted quinoline and pyridine N-oxides. This strategy has the following advantages: specific regioselectivity, simple operation, good functional group tolerance, and high to moderate yields under mild conditions.

Pyridinium salts: from synthesis to reactivity and applications

This review highlights the pyridinium salts in terms of their natural occurrence, synthesis, reactivity, biological properties, and diverse applications.

Trapping of Transient Zwitterionic Intermediates by N-Acylpyridinium Salts: A Palladium-Catalyzed Diastereoselective Three-Component Reaction

We developed a palladium-catalyzed diastereoselective three-component reaction of N-aryl diazoamides, pyridine, and chloroformate, which proceeded through trapping of transient zwitterionic intermediates by in situ formed N-acylpyridinium salts in a regioselective 1,4-addition fashion. This reaction can rapidly provide a library of biologically relevant 4-(2-oxoindolin-3-yl)-1,4-dihydropyridine derivatives in high yields (up to 98%) with moderate to excellent diastereoselectivities (up to >95:5 dr) under mild reaction conditions.

Transition-metal-free dehydrogenation coupling of pyridinium through a self-promoted hydride transfer process

An intramolecular dehydrogenation coupling between phenol and pyridinium through a unique substrate self-promoted hydride transfer process has been developed.

Base-promoted, deborylative secondary alkylation of N-heteroaromatic N-oxides with internal gem-bis[(pinacolato)boryl]alkanes: a facile derivatization of 2,2′-bipyridyl analogues

A base-promoted, secondary alkylation of N-heteroaromatic N-oxides using internal gem-bis[(pinacolato)boryl]alkanes as alkylation reagents is achieved to form deoxygenated secondary alkylated N-heteroarenes.

Direct, catalytic, and regioselective synthesis of 2-alkyl-, aryl-, and alkenyl-substituted N-heterocycles from N-oxides

A one-step transformation of heterocyclic N-oxides to 2-alkyl-, aryl-, and alkenyl-substituted N-heterocycles is described. The success of this broad-scope methodology hinges on the combination of copper catalysis and activation by lithium fluoride or magnesium chloride. The utility of this method for the late-stage modification of complex N-heterocycles is exemplified by facile syntheses of new structural analogues of several antimalarial, antimicrobial, and fungicidal agents.

Reductive couplings of 2-halopyridines without external ligand: phosphine-free nickel-catalyzed synthesis of symmetrical and unsymmetrical 2,2'-bipyridines

An unexpectedly facile synthetic approach for symmetrical and unsymmetrical 2,2'-bipyridines through the Ni-catalyzed reductive couplings of 2-halopyridines was developed. The couplings were efficiently catalyzed by 5 mol % of NiCl2·6H2O without the use of external ligands. A variety of 2,2'-bipyridines including caerulomycin F have been efficiently synthesized.

Palladium-catalyzed oxidative CH/CH cross-coupling of pyridine N-oxides with five-membered heterocycles

Using Ag₂CO₃ as an additive, we developed the Pd-catalyzed intermolecular C–H/C–H cross-coupling of pyridine N-oxides with five-membered heterocycles such as 1-benzyl-1,2,3-triazoles, thiophens and furans.