A simple, modular synthesis of substituted pyridines

Copper-catalysed electrophilic carboamination of terminal alkynes with benzyne looked at through the computational lens

A detailed computational mechanistic study of the copper-catalysed three-component-type electrophilic carboamination of terminal alkynes with benzyne and an archetypal O-benzoylhydroxylamine electrophile is presented. Probing various plausible pathways for relevant elementary steps and scrutinising performance degradation pathways, with the aid of a reliable computational protocol applied to a realistic catalyst model combined with kinetic analysis, identified the pathways preferably traversed in productive catalysis. It entails rapid alkynylcupration of in situ generated benzyne to deliver the arylcopper nucleophile that undergoes amination with the O-benzoylhydroxylamine electrophile to afford copper benzoate. Umpolung-enabled electrophilic amination favours a multistep SN2-type oxidative addition/N-C bond-forming reductive elimination sequence involving a short-lived formal {P^P}CuIII carboxylate amido aryl intermediate. SN2-type displacement of the benzoate leaving group at the arylcopper nucleophile, which represents the catalyst resting state, is predicted to be the turnover limiting step. Alkynolysis transforms copper benzoate back to catalytically competent alkynylcopper. The computational probe of a wider range of substrates reveals that only severely ring-strained C6-arynes, C6-cycloalkynes and electron-deficient cyclopropenes featuring a highly reactive C≡C linkage could replace benzyne. Moreover, strict control of stationary benzyne concentration is indispensable for electrophilic carboamination to ever become achievable.

Copper-mediated [3 + 2] oxidative cyclization of oxime acetate and its utility in the formal synthesis of fentiazac

A new protocol for the direct synthesis of 2-aminothiazole has been developed from oxime acetate and readily available sodium thiocyanate using a copper catalyst. The present transformation has good functional group tolerance. Various thiazoles were smoothly synthesized in good to excellent yields. The applicability of the present method has been extended to the formal synthesis of the non-steroidal and anti-inflammatory drug, fentiazac via the Sandmeyer reaction and Suzuki coupling.

Six-Membered Aromatic Nitrogen Heterocyclic Anti-Tumor Agents: Synthesis and Applications

Cancer stands as a serious malady, posing substantial risks to human well-being and survival. This underscores the paramount necessity to explore and investigate novel antitumor medications. Nitrogen-containing compounds, especially those derived from natural sources, form a highly significant category of antitumor agents. Among these, antitumor agents with six-membered aromatic nitrogen heterocycles have consistently attracted the attention of chemists and pharmacologists. Accordingly, we present a comprehensive summary of synthetic strategies and clinical implications of these compounds in this review. This entails an in-depth analysis of synthesis pathways for pyridine, quinoline, pyrimidine, and quinazoline. Additionally, we explore the historical progression, targets, mechanisms of action, and clinical effectiveness of small molecule inhibitors possessing these structural features.

A Chemoselective Polarity-Mismatched Photocatalytic C(sp3 )-C(sp2 ) Cross-Coupling Enabled by Synergistic Boron Activation

We report the development of a C(sp3 )-C(sp2 ) coupling reaction using styrene boronic acids and redox-active esters under photoredox catalysis. The reaction proceeds through an unusual polarity-mismatched radical addition mechanism that is orthogonal to established processes. Synergistic activation of the radical precursor and organoboron are critical mechanistic events. Activation of an N-hydroxyphthalimide (NHPI) ester by coordination to boron enables electron transfer, with decomposition leading to a nucleofuge rebound, activating the organoboron to radical addition. The unique mechanism enables chemoselective coupling of styrene boronic acids in the presence of other alkene radical acceptors. The scope and limitations of the reaction, and a detailed mechanistic investigation are presented.

Gold-Catalyzed Amine Cascade Addition to Diyne-Ene: Enantioselective Synthesis of 1,2-Dihydropyridines

With the well-documented chemical and biological applications, piperidine and pyridine are among the most important N-heterocycles, and a new synthetic strategy, especially one with an alternative bond-forming design, is of general interest. Using the gold-catalyzed intermolecular condensation of amine and diyne-ene, we report herein the first example of enantioselective 1,2-dihydropyridine synthesis through a formal [3+2+1] fashion (up to 95 % yield, up to 99 % e.e.).

One-Pot Knoevenagel/Imination/6π-Azaelectrocyclization Sequence for the Synthesis of Disubstituted Nicotinonitriles

We report on a copper-catalyzed three-component reaction for the synthesis of disubstituted nicotinonitriles using 3-bromopropenals, benzoylacetonitriles, and ammonium acetate (NH₄OAc). The Knoevenagel-type condensation of 3-bromopropenals with benzoylacetonitriles gives δ-bromo-2,4-dienones that contain strategically placed functional groups that react with the ammonia generated in situ to give the corresponding azatrienes. These azatrienes can then be transformed into trisubstituted pyridines under the reaction conditions via a reaction sequence involving 6π-azaelectrocyclization and aromatization.

anti-Selective synthesis of β-boryl-α-amino acid derivatives by Cu-catalysed borylamination of α,β-unsaturated esters

A copper-catalysed regio- and diastereoselective borylamination of α,β-unsaturated esters with B₂pin₂ and hydroxylamines has been developed to deliver acyclic β-boryl-α-amino acid derivatives with high anti-diastereoselectivity (up to >99 : 1), which is difficult to obtain by the established methods. A chiral phosphoramidite ligand also successfully induces the enantioselectivity, giving the optically active β-borylated α-amino acids. The products can be stereospecifically transformed into β-functionalised α-amino acids, which are of potent interest in medicinal chemistry.

Unified Total Synthesis of the Limonoid Alkaloids: Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. The materials produced in this study were evaluated for cytotoxicity, anti-oxidant effects, anti-inflammatory action, PTP1B and Nlrp3 inflammasome inhibition, which led to compelling anti-inflammatory activity and anti-oxidant effects being discovered.

Copper-catalyzed [3+3] annulation of ketones with oxime acetates for the synthesis of pyridines

A novel and efficient copper-catalyzed strategy for the synthesis of diverse pyridines through the [3+3] annulation of ketones with oxime acetates has been reported. It is very convenient to obtain various unsymmetrical 2,6-diarylpyridines by changing different substrates. The rare copper-catalyzed direct difunctionalization of saturated ketones for the synthesis of nitrogen heterocycles is developed. This protocol has excellent functional group tolerance, readily available raw materials, high chemoselectivity and broad substrate scope.

Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH₄OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.

Copper-Catalyzed Reactions of Alkenyl Boronic Esters via Chan-Evans-Lam Coupling/Annulation Cascades: Substrate Selective Synthesis of Dihydroquinazolin-4-ones and Polysubstituted Quinolines

Copper-catalyzed cascade cyclization reactions between alkenyl boronic esters and N-H-based nucleophiles have been established, providing new approaches for one-pot assembly of azacycles. Following the Chan-Evans-Lam C-N couplings, the cyclization processes occur via divergent pathways based on the utilized substrates, affording hydroamination product dihydroquinazolin-4-ones or aromatization product quinolines. Via this one-pot C-N coupling/annulation cascade, the target substituted azacycles can be obtained in moderate to good yields in each case.

Cationic palladium(ii)-catalyzed synthesis of substituted pyridines from α,β-unsaturated oxime ethers

An efficient method for the synthesis of multi-substituted pyridines from β-aryl-substituted α,β-unsaturated oxime ethers and alkenes via Pd-catalyzed C–H activation has been developed. The method, using Pd(OAc)₂ and a sterically hindered pyridine ligand, provides access to various multi-substituted pyridines with complete regioselectivity. Mechanistic studies suggest that the pyridine products are formed by Pd-catalyzed electrophilic C–H alkenylation of α,β-unsaturated oxime followed by aza-6π-electrocyclization. The utility of this method is showcased by the synthesis of 4-aryl-substituted pyridine derivatives, which are difficult to synthesize efficiently using previously reported Rh-catalyzed strategies with alkenes.

An efficient method for the synthesis of multi-substituted pyridines from α,β-unsaturated oxime ethers via cationic Pd(ii)-catalyzed C–H activation has been developed.

Synthesis of Substituted Pyridines via Formal (3+3) Cycloaddition of Enamines with Unsaturated Aldehydes and Ketones

An organocatalyzed, formal (3+3) cycloaddition reaction is described for the practical synthesis of substituted pyridines. Starting from readily available enamines and enal/ynal/enone substrates, the protocol affords tri- or tetrasubstituted pyridine scaffolds bearing various functional groups. This method was demonstrated on a 50 g scale, enabling the synthesis of 2-isopropyl-4-methylpyridin-3-amine, a raw material used for the manufacture of sotorasib. Mechanistic analysis using two-dimensional nuclear magnetic resonance (NMR) spectrometry revealed the transformation proceeds through the reversible formation of a stable reaction off-cycle species that precedes pyridine formation. In situ reaction progress kinetic analysis and control NMR studies were employed to better understand the role of FeCl₃ and pyrrolidine hydrochloride in promoting the reaction.

Recent Advances in Synthesis of Multiply Arylated/Alkylated Pyridines

Multiply aryl/alkyl-substituted pyridines are some of the untapped synthetic targets because of the challenge in regioselectively introducing less polar aryl/alkyl groups at the desired positions in the pyridine framework. Interestingly, the importance of this family of compounds has increased annually, particularly in biological and materials engineering applications. The syntheses of such pyridines have been extensively reported, but there is a lack of comprehensive review articles. Hence, this review discusses recent advances by grouping reaction patterns that generally deliver tri-, tetra-, and penta-aryl/alkyl pyridines.

Synthesis of Stable N-H Imines with a Benzo[7,8]indolizine Core and Benzo[7,8]indolizino[1,2-c]quinolines via Copper-Catalyzed Annulation of α,β-Unsaturated O-Acyl Ketoximes with Isoquinolinium N-Ylides

A copper-catalyzed annulation of α,β-unsaturated O-acyl ketoximes with isoquinolinium N-ylides has been developed for the concise synthesis of stable N-H imines with a benzo[7,8]indolizine core. When β-(2-bromoaryl)-α,β-unsaturated O-acyl ketoximes are used as the starting materials, a cascade cyclization occurs to afford the benzo[7,8]indolizino[1,2-c]quinolines.

Hydroamination, Aminoboration, and Carboamination with Electrophilic Amination Reagents: Umpolung-Enabled Regio- and Stereoselective Synthesis of N-Containing Molecules from Alkenes and Alkynes

Nitrogen (N) is ubiquitously found in bioactive molecules, pharmaceutical agents, and organic functional materials. Accordingly, development of new C-N bond-forming catalysis has been one of the long-standing research subjects in synthetic organic chemistry. In this Perspective, recent advances in highly selective amination reactions with electrophilic amination reagents are described: by taking advantage of the concept of nitrogen umpolung, otherwise challenging aminofunctionalizations, such as hydroamination, aminoboration, and carboamination, of readily available feedstock-like alkenes and alkynes are possible, giving densely functionalized complex and often chiral alkylamines with high selectivity. The scope, limitations, and reaction mechanism are briefly summarized.

Regioselective Synthesis of Spirocyclic Pyrrolines via a Palladium-Catalyzed Narasaka-Heck/C-H Activation/[4+2] Annulation Cascade Reaction

A novel palladium-catalyzed spirocyclization through sequential Narasaka-Heck, C-H activation and [4+2] annulation has been developed. In this reaction, cheap and readily available 2-chlorobenzoic acid or ethyl phenylpropiolate was employed as...

Synthesis of Poly-Substituted Pyridines via Noble-Metal-Free Cycloaddition of Ketones and Imines

An eco-friendly and noble-metal-free formal [4+2] cycloaddition reaction was developed for the efficient synthesis of biologically interesting poly-substituted pyridines from easily available ketones and imines, whereby two sequential C-C bonds are formed. The given approach features a unique synthetic strategy of imines and ketones with wide substrate scope, good functional group tolerance, mild conditions and operational simplicity, which represents a more direct pathway to synthesize poly-substituted pyridines than traditional methods.

Chemoselective [3 + 2] annulation of oxime acetate with 2-aryl-3-ethoxycarbonyl-pyrroline-4,5-dione: an entry to pyrrolo[2,3-b]pyrrole derivatives

A novel chemoselective [3 + 2] annulation reaction of easily accessible ketoxime acetate with 2-aryl-3-ethoxycarbonyl pyrroline-4,5-dione has been developed for the synthesis of unknown pyrrolo[2,3-b]pyrrole frameworks. This method involves copper-mediated N-O bond cleavage followed by the formation of carbon-carbon and carbon-nitrogen bonds. This operationally simple protocol provides broader functional group compatibility and good yields.

An umpolung-enabled copper-catalysed regioselective hydroamination approach to α-amino acids

A copper-catalysed regio- and stereoselective hydroamination of acrylates with hydrosilanes and hydroxylamines has been developed to afford the corresponding α-amino acids in good yields. The key to regioselectivity control is the use of hydroxylamine as an umpolung, electrophilic amination reagent. Additionally, a judicious choice of conditions involving the CsOPiv base and DTBM-dppbz ligand of remote steric hindrance enables the otherwise challenging C-N bond formation at the α position to the carbonyl. The point chirality at the β-position is successfully controlled by the Xyl-BINAP or DTBM-SEGPHOS chiral ligand with similarly remote steric bulkiness. The combination with the chiral auxiliary, (-)-8-phenylmenthol, also induces stereoselectivity at the α-position to form the optically active unnatural α-amino acids with two adjacent stereocentres.

The copper-catalyzed synthesis of dihydrooxazoles from α,β-unsaturated ketoximes and activated ketones

The first copper-catalyzed [3+2]-type condensation reaction of α,β-unsaturated ketoximes with activated ketones has been described for the synthesis of dihydrooxazoles, especially trifluoromethyl-decorated dihydrooxazoles. Notable features of this method include its broad substrate scope, good functional group tolerance, and simple operation.

Experimental and Computational Studies of Palladium-Catalyzed Spirocyclization via a Narasaka-Heck/C(sp3 or sp2)-H Activation Cascade Reaction

The first synthesis of highly strained spirocyclobutane-pyrrolines via a palladium-catalyzed tandem Narasaka-Heck/C(sp³ or sp²)-H activation reaction is reported here. The key step in this transformation is the activation of a δ-C-H bond via an in situ generated σ-alkyl-Pd(II) species to form a five-membered spiro-palladacycle intermediate. The concerted metalation-deprotonation (CMD) process, rate-determining step, and energy barrier of the entire reaction were explored by density functional theory (DFT) calculations. Moreover, a series of control experiments was conducted to probe the rate-determining step and reversibility of the C(sp³)-H activation step.

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C−N bond‐forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Palladium(II)-Catalyzed Substituted Pyridine Synthesis from α,β-Unsaturated Oxime Ethers via a C-H Alkenylation/Aza-6π-Electrocyclization Approach

An efficient synthetic method for multisubstituted pyridines from β-aryl-substituted α,β-unsaturated oxime ethers and alkenes via Pd-catalyzed C-H activation has been developed. Systematic optimization of catalyst ligands revealed that sterically hindered pyridines increased the reactivity. Mechanistic studies suggested that the products are formed by Pd-catalyzed β-alkenylation of α,β-unsaturated oxime followed by aza-6π-electrocyclization. Various oximes and alkenes could be utilized to afford multisubstituted pyridines with complete regioselectivity. The usefulness of this methodology was showcased by the synthesis of 4-aryl-substituted pyridine derivatives, which are difficult to access with previously reported Rh-catalyzed approaches with alkenes.

Substrate-oriented selectivity in the Mg-mediated conjugate addition of bromoform to electron-deficient alkenes

The Mg-mediated conjugate addition of bromoform to a variety of electron-deficient alkenes has been investigated. In the case of nitrodienes and dibenzylideneacetones, tribromomethylated products were isolated, whereas spiro-cyclopropanated products were obtained with cyclic dibenzylideneketones and 3-olefinic oxindoles. The spiro-cyclopropyl ketones derived from cyclic dibenzylideneketones were successfully transformed into fused furans via the Cloke-Wilson rearrangement.

Tandem Reaction Approaches to Isoquinolones from 2-Vinylbenzaldehydes and Anilines via Imine Formation-6π-Electrocyclization-Aerobic Oxidation Sequence

Two distinctive transition-metal-promoted aerobic oxidation protocols have been developed for the synthesis of isoquinolones from 2-vinylbenzaldehydes and aniline derivatives. Thus, the one-pot tandem reaction sequence of imine formation, thermal 6π-electrocyclization, followed by either Cu(OAc)₂-mediated or Pd(OAc)₂-catalyzed aerobic oxidation protocol allowed the ready access to isoquinolone derivatives. The control experiments revealed that the 1,2-dihydroisoquinoline intermediates from the 6π-electrocyclization of 1-azatrienes were aerobically oxidized to isoquinolones in the presence of either Cu(OAc)₂ or Pd(OAc)₂ catalyst.

Synthesis of Highly Substituted Pyridines via [4 + 2] Cycloadditions of Vinylallenes and Sulfonyl Cyanides

A convergent strategy for the synthesis of multisubstituted pyridines is described. Vinylallenes combine with commercially available arylsulfonyl cyanides in Diels-Alder cycloadditions to generate isopyridine cycloadducts that are converted to pyridines upon further heating or addition of a base. The 2-sulfonylpyridine products undergo nucleophilic aromatic substitution reactions with oxygen and carbon nucleophiles to provide access to a variety of highly substituted pyridines.