1,5-Hydrogen atom transfer of α-iminyl radical cations: a new platform for relay annulation for pyridine derivatives and axially chiral heterobiaryls

The exploitation of new reactive species and novel transformation modes for their synthetic applications have significantly promoted the development of synthetic organic methodology, drug discovery, and advanced functional materials. α-Iminyl radical cations, a class of distonic ions, exhibit great synthetic potential for the synthesis of valuable molecules. For their generation, radical conjugate addition to α,β-unsaturated iminium ions represents a concise yet highly challenging route, because the in situ generated species are short-lived and highly reactive and they have a high tendency to cause radical elimination (β-scission) to regenerate the more stable iminium ions. Herein, we report a new transformation mode of the α-iminyl radical cation, that is to say, 1,5-hydrogen atom transfer (1,5-HAT). Such a strategy can generate a species bearing multiple reactive sites, which serves as a platform to realize (asymmetric) relay annulations. The present iron/secondary amine synergistic catalysis causes a modular assembly of a broad spectrum of new structurally fused pyridines including axially chiral heterobiaryls, and exhibits good functional group tolerance. A series of mechanistic experiments support the α-iminyl radical cation-induced 1,5-HAT, and the formation of several radical species in the relay annulations. Various synthetic transformations of the reaction products demonstrate the usefulness of this relay annulation protocol for the synthesis of significant molecules.

Synthesis of Phosphorus(V)-Substituted Six-Membered N-Heterocycles: Recent Progress and Challenges

Heterocycles functionalized with pentavalent phosphorus are of great importance since they include a great variety of biologically active compounds and pharmaceuticals, advanced materials, and valuable reactive intermediates for organic synthesis. Significant progress in synthesis of P(O)R2-substituted six-membered heterocycles has been made in the past decade. This review covers the synthetic strategies towards aromatic monocyclic six-membered N-heterocycles, such as pyridines, pyridazines, pyrimidines, and pyrazines bearing phosphonates and phosphine oxides, which were reported from 2012 to 2022.

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.

Nature-inspired remodeling of (aza)indoles to meta-aminoaryl nicotinates for late-stage conjugation of vitamin B3 to (hetero)arylamines

Despite the availability of numerous routes to substituted nicotinates based on the Bohlmann–Rahtz pyridine synthesis, the existing methods have several limitations, such as the inevitable ortho-substitutions and the inability to conjugate vitamin B₃ to other pharmaceutical agents. Inspired by the biosynthesis of nicotinic acid (a form of vitamin B₃) from tryptophan, we herein report the development of a strategy for the synthesis of meta-aminoaryl nicotinates from 3-formyl(aza)indoles. Our strategy is mechanistically different from the reported routes and involves the transformation of (aza)indole scaffolds into substituted meta-aminobiaryl scaffolds via Aldol-type addition and intramolecular cyclization followed by C–N bond cleavage and re-aromatization. Unlike previous synthetic routes, this biomimetic method utilizes propiolates as enamine precursors and thus allows access to ortho-unsubstituted nicotinates. In addition, the synthetic feasibility toward the halo-/boronic ester-substituted aminobiaryls clearly differentiates the present strategy from other cross-coupling strategies. Most importantly, our method enables the late-stage conjugation of bioactive (hetero)arylamines with nicotinates and nicotinamides and allows access to the previously unexplored chemical space for biomedical research.

Vitamin B3 derivatives display a range of biological activities. Here, the authors report the synthesis of meta-aminoaryl nicotinates, derivatives of vitamin B3, and their late-stage conjugation with (hetero)arylamines, ultimately expanding the chemical space for biomedical research.

NH4OAc-Promoted Domino Route to Hydroxyarylated Unsymmetrical Pyridines under Neat Conditions

A new metal-, oxidant-, and solvent-free ecofriendly domino method has been established for modular synthesis of a diverse range of medicinally promising hydroxyarylated unsymmetrical pyridines in good to high chemical yields with an excellent regioselectivity. This domino process involves a range of N-sulfonyl ketimines as C,N-binucleophiles, enolizable ketones, and aromatic/heteroaromatic aldehydes using ammonium acetate as an ideal promoter under neat conditions, which creates two new C-C bonds and one C-N bond. Notably, the neutral reaction conditions are mild enough to tolerate a range of functionalities and cover a variety of substrates, thus bestowing a powerful avenue to access tri- and tetrasubstituted pyridines including carbo- and heterocyclic fused ones. Interestingly, a practical, scalable, and high-yielding synthesis of pyridylphenol derivatives was successfully accomplished by our unique method.

Synergistic promotion by intramolecular hydrogen bonding: a bi-functionally catalyzed cascade reaction for the synthesis of enantiopure chromenopyrrolidines

An effective organocatalytic asymmetric cascade reaction of o-hydroxy aromatic aldimines and β,γ-unsaturated-α-ketoesters was developed.

DABCO- and DBU-promoted one-pot reaction of N-sulfonyl ketimines with Morita-Baylis-Hillman carbonates: a sequential approach to (2-hydroxyaryl)nicotinate derivatives

An intriguing DABCO-catalyzed and DBU-promoted one-pot synthesis of an important class of (2-hydroxyaryl)pyridine derivatives bearing a carboxylate or a nitrile group suitably placed at C3 position of the aza-ring has been achieved in acceptable chemical yields with a broad functional group tolerance. This sequential C-C/C-N bond making process proceeds through a regioselective allylic alkylation/aza-Michael reaction between MBH carbonates derived from an acrylate/acrylonitrile and N-sulfonyl ketimines as C,N-binucleophiles catalyzed by DABCO, followed by elimination of SO2 under the influence of base and subsequent aromatization in an open atmosphere.

N-Silylenamines as Reactive Intermediates: Hydroamination for the Modular Synthesis of Selectively Substituted Pyridines

A modular and selective synthesis of mono-, di-, tri-, tetra-, and pentasubstituted pyridines is reported. Hydroamination of alkynes with N-silylamine using a bis(amidate)bis(amido)titanium(IV) precatalyst furnishes the regioselective formation of N-silylenamines. Addition of α,β-unsaturated carbonyls to the crude mixtures followed by oxidation affords 47 examples of pyridines in yields of up to 96%. This synthetic route allows for the synthesis of diverse pyridines containing variable substitution patterns, including pharmaceutically relevant 2,4,5-trisubstituted pyridines, using this one-pot protocol.

Recent developments in functionalization of acyclic α-keto amides

This review describes the synthetic utility of α-keto amides to synthesize various important molecules via mono, dual and triple functionalization reactions.

Domino reaction of cyclic sulfamidate imines with Morita–Baylis–Hillman acetates promoted by DABCO: a metal-free approach to functionalized nicotinic acid derivatives

A series of 4,6-diarylnicotinates have been prepared in good to excellent yields via a domino reaction of cyclic sulfamidate imines with MBH acetates in 2-MeTHF promoted by DABCO under an O₂ atmosphere.

Multicomponent reactions: A simple and efficient route to heterocyclic phosphonates

Multicomponent reactions (MCRs) are one of the most important processes for the preparation of highly functionalized organic compounds in modern synthetic chemistry. As shown in this review, they play an important role in organophosphorus chemistry where phosphorus reagents are used as substrates for the synthesis of a wide range of phosphorylated heterocycles. In this article, an overview about multicomponent reactions used for the synthesis of heterocyclic compounds bearing a phosphonate group on the ring is given.

Facile Diversity-Oriented Synthesis of Polycyclic Pyridines and Their Cytotoxicity Effects in Human Cancer Cell Lines

A three-component cascade method has been developed for the direct synthesis of polysubstituted pyridines. This strategy provides a very convenient route to pyridines using a variety of β-bromo-α,β-unsaturated aldehydes, 1,3-diketones, and ammonium acetate without any additional catalyst or metal salt under mild conditions. A variety of β-ketoesters and 4-hydroxycoumarin were also used instead of 1,3-diketones for the diverse synthesis of polycyclic pyridines. One of the synthesized pyridines has been unambiguously established by a single crystal XRD study. All of the synthesized pyridine derivatives were evaluated for their antiproliferative properties in vitro against the human cancer cell lines HeLa, Me180, and ZR751. Compounds 4{4,1} and 4{2,4} showed significant cytotoxicity in the human breast cancer cell line ZR751 and cervical cancer cell line Me180, respectively, and a few other compounds were found to have moderate activities.

Solvent-free aminocarbonylation of iodobenzene in the presence of SILP-palladium catalysts

The recyclable palladium catalysts were used for selective mono- or double carbonylation under solvent-free conditions and in DMF, respectively.

Multicomponent synthesis of pyridines via diamine functionalized mesoporous ZrO2 domino intramolecular tandem Michael type addition

A new and straightforward synthetic method was developed for the facile synthesis of heterocycle-fused pyridine derivatives in aqueous media from Knoevenagel condensation between an aromatic aldehyde and an active methylene compound.

A mild and facile synthesis of polyfunctionalized pyridines: merging three-component cyclization and aerobic oxidation by amine/metal catalysts

A mild and facile synthesis of polyfunctionalized pyridines from NH₄OAc, β,γ-unsaturated α-ketoesters, and ketones/aldehydes has been reported through tandem three-component cyclization and aerobic oxidation using the combination of amine and metal catalysts.

Organocatalyzed oxidative N-annulation for diverse and polyfunctionalized pyridines

A variety of polyfunctionalized pyridines were synthesized from readily available ketones with α,β-unsaturated aldehydes and ammonium acetate under a mild organocatalyst.

A heavy metal- and oxidant-free, one-pot synthesis of pyridines and fused pyridines based on a Lewis acid-catalyzed multicomponent reaction

Pyridines and fused pyridines are accessible by a combination of a Lewis acid-catalyzed multicomponent reaction and aromatization involving loss of a 2-furylmethyl chain.

Synthesis of alkynyl/alkenyl-substituted pyridine derivatives via heterocyclization and Pd-mediated Sonogashira/Heck coupling process in one-pot: a new MCR strategy

The present MCR strategy opens new avenues in the development of i) a diversity-oriented new cyanopyridine based compound library and ii) new chemical entities other than the present reported molecules.

One-pot access to a library of structurally diverse nicotinamide derivatives via a three-component formal aza [3 + 3] cycloaddition

The three-component formal [3 + 3] aza-annulation between chalcones, β-ketoamides, and ammonium acetate in the presence of CAN as a Lewis acid affords good to excellent yields of highly substituted nicotinamides or their fused derivatives. This transformation leads to the formation of one C-C and two C-N bonds in a single synthetic operation and involves up to five individual steps.

Highly diastereo- and enantioselective organocatalytic michael addition of α-ketoamides to nitroalkenes

The first organocatalytic enantio- and diastereoselective conjugate addition of α-ketoamides to nitroalkenes has been achieved using a bifunctional amino thiourea catalyst. In this new approach, the substrate amide proton plays a critical role in the formation of the Michael anti-adducts in high yields and high stereoselectivities. To illustrate the high synthetic potential of this methodology, the diastereo- and enantioselective synthesis of a hexasubstituted cyclohexane via a Michael-Michael-Henry cascade reaction is described.