Organocatalytic Synthesis of Highly Functionalized Pyridines at Room Temperature

Organocatalytic access to 3-pyridylphosphonates from vinyl phosphonates and aldehydes

The preparation of 3-phosphorylated pyridines has remained the most challenging compared to the corresponding 2- or 4-functionalized pyridines. Herein, an unprecedented amino-organocatalyzed direct synthesis of 3-pyridylphosphonates from vinylphosphonates and aldehydes has been achieved. This allows access to a wide range of multi-substituted phosphorylated pyridines in excellent yields under transition metal-free conditions.

Zn(II)-Catalyzed Multicomponent Sustainable Synthesis of Pyridines in Air

Herein, we report a Zn(II)-catalyzed solvent-free sustainable synthesis of tri- and tetra-substituted pyridines using alcohols as the primary feedstock and NH₄OAc as the nitrogen source. Using a well-defined air-stable Zn(II)-catalyst, 1a, featuring a redox-active tridentate azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^a), a wide variety of unsymmetrical 2,4,6-substituted pyridines were prepared by three-component coupling of primary and secondary alcohols with NH₄OAc. Catalyst 1a is equally compatible with the four-component coupling. Unsymmetrical 2,4,6-substituted pyridines were also prepared via a four-component coupling of a primary alcohol with two different secondary alcohols and NH₄OAc. A series of tetra-substituted pyridines were prepared up to 67% yield by coupling primary and secondary alcohols with 1-phenylpropan-1-one or 1,2-diphenylethan-1-one and NH₄OAc. The 1a-catalyzed reactions also proceeded efficiently upon replacing the secondary alcohols with the corresponding ketones, producing the desired tri- and tetra-substituted pyridines in higher yields in a shorter reaction time. A few control experiments were performed to unveil the mechanistic aspects, which indicates that the active participation of the aryl-azo ligand during catalysis enables the Zn(II)-complex to act as an efficient catalyst for the present multicomponent reactions. Aerial oxygen acts as an oxidant during the Zn(II)-catalyzed dehydrogenation of alcohols, producing H₂O and H₂O₂ as byproducts.

Pyridine vs DABCO vs TBAB in Annulations of δ-Acetoxy Allenoates with Thioamides Leading to Dihydrothiophene, Thiopyran, and Thiazole Scaffolds

The same δ-acetoxy allenoates and thioamides, under DABCO, pyridine, or tetra-n-butyl ammonium bromide (TBAB) catalysis, undergo distinctly different annulations giving chemoselective routes to dihydrothiophene, thiopyran, or thiazole motifs. Thus, using pyridine in [3 + 2] annulation, dihydrothiophenes are obtained as essentially single diastereomers. By contrast, under DABCO catalysis, allenoates deliver thiopyran motifs in good to high yields through 6-exo-dig cyclization. In the thiazole forming [3 + 2] annulation, tetra-n-butyl ammonium bromide (TBAB) facilitates addition-elimination and 5-exo-trig cyclization in which β- and γ-carbons of allenoates participate to deliver thiazole cores exclusively with a Z-isomeric exocyclic double bond. A possible rationale for these observations is delved into.

Synthesis of Unsymmetrically Tetrasubstituted Pyrroles and Studies of AIEE in Pyrrolo[1,2-a]pyrimidine Derivatives

Pyrroles are among the most important heterocycles in pharmaceuticals and agrochemicals. Construction of pyrrole scaffolds with different substituents and free NH group however, is challenging. Herein, a metal-free method for...

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.

An Isoxazole Strategy for the Synthesis of Fully Substituted Nicotinates

A four-step quasi one-pot procedure for the preparation of fully substituted nicotinates from ketone enamines and 4-methylideneisoxazol-5-ones has been developed. The reaction sequence involves (1) reaction of 4-methylideneisoxazol-5-ones with ketone enamines with the formation of isoxazole-5-ols, (2) their O-methylation with diazomethane, (3) hydrogenative cleavage of the O-N bond in 5-methoxyisoxazoles under action of Mo(CO)₆/H₂O and simultaneous isomerization and condensation of the formed enamines, with the formation of dihydropyridines, and (4) aromatization of the latter.

Sustainable Four-Component Annulation for the Synthesis of 2,3,4,6-Tetraarylpyridines

A one-pot, four-component annulation of 2,3,4,6-tetraarylpyridines from aromatic aldehydes, methyl ketones, diaryl ethanones, and ammonium acetate is described. The reaction features high functional group compatibility in air under solvent-free conditions without any additive and only water as the nontoxic byproduct, providing a strategy for the facile, economical, and eco-friendly construction of multiaryl-substituted pyridines from simple and readily available reactants.

Construction of CF3-Containing Tetrahydropyrano[3,2-b]indoles through DMAP-Catalyzed [4+1]/[3+3] Domino Sequential Annulation

A [4+1]/[3+3] domino sequential annulation reaction of o-aminotrifluoroacetophenone derivatives and β'-acetoxy allenoates enabled by DMAP has been reported. A variety of CF₃-containing tetrahydropyrano[3,2-b]indoles were obtained as a single diastereomer in high yields (≤98%) under mild conditions. The reaction can build one C-N bond, one C-C bond, and one C-O bond sequentially in a single step. The synthetic utility was demonstrated with gram-scale reactions and various transformations of the products.

A theoretical review for novel Lewis base amine/imine-catalyzed reactions

Recent advances in computational investigations of Lewis base amine/imine-catalyzed reactions have been systematically summarized and reviewed for the first time.

Asymmetric synthesis of benzothiazolopyrimidines with high catalytic efficiency and stereoselectivity under bifunctional phosphonium salt systems

Bifunctional phosphonium salt-mediated formal [4+2] annulation towards chiral benzothiazolopyrimidine compounds with excellent yields and stereoselectivities.

A simple, tandem approach to the construction of pyridine derivatives under metal-free conditions: a one-step synthesis of the monoterpene natural product, (−)-actinidine

(−)-Actinidine, along with various diversely substituted pyridines were synthesized from simple starting materials, including natural products, using a mild and metal-free method.

Facile Synthesis of Pyridines from Propargyl Amines: Concise Total Synthesis of Suaveoline Alkaloids

A general and efficient protocol was developed for the synthesis of polysubstituted pyridines from propargyl amines and unsaturated carbonyl compounds through a tandem condensation/alkyne isomerization/6π 3-azatriene electrocyclization sequence. This process was found to be applicable to a wide range of readily available substrates (30 examples, up to 95 % yield) and could be readily performed on a preparative (20 g) scale. By taking advantage of this method for late-stage pyridine incorporation, we successfully completed the collective total synthesis of suveoline, norsuveoline, and macrophylline.

Carbon-nitrogen bond cleavage of pyridine with two molecular substituted allenoates: access to 2-arylpyrimidin-4(3H)-one

A DABCO-catalyzed annulation reaction of pyridin-2-amine and substituted allenoates has been disclosed. This strategy allows for the ring-opening of a pyridine ring system and the formation of two new rings including a pyrimidinone ring and a benzene ring in an efficient manner.

Copper-Catalyzed Ring-Expansion Cascade of Azirines with Alkynes: Synthesis of Multisubstituted Pyridines at Room Temperature

The first intermolecular ring-expansion cascade of azirines with alkynes for the synthesis of pyridines, enabled by a copper/triethylamine catalytic system via simultaneous generation and utilization of yne-enamine and skipped-yne-imine intermediates, is reported. Experimental as well as computational mechanistic studies revealed that the role of triethylamine is crucial in deciding the reaction pathway toward the pyridine products. This process offers a novel, one-step, direct, and practical strategy for the rapid construction of highly substituted pyridines under exceedingly mild conditions, and an installed alkyne functionality.

NHC-Catalyzed Hetero-Diels-Alder Reaction of Allenoate with Chalcone: Synthesis of Polysubstituted Pyranyl Carboxylate

An NHC-catalyzed hetero-Diels-Alder and isomerization process of chalcones with allenoates was discovered, which furnished highly functionalized multisubstituted pyranyl carboxylates successfully. This method features a convergent assembly, mild reaction conditions, moderate to good yields, and high atom economy.

Synthesis of polysubstituted pyridines from oxime acetates using NH4I as a dual-function promoter

An NH₄I dual-function promoter enables pyridine synthesis through oxime N–O bond reduction and subsequent condensation reactions.

Divergent synthesis of hydropyridine derivatives via nitrogen-containing Lewis base mediated regioselective [4 + 2] cyclizations

A nitrogen-containing Lewis base mediated regioselective [4 + 2] cyclization strategy between allenic esters and α,β-unsaturated ketimines was discovered.

Acid-catalyzed tandem reaction for the synthesis of pyridine derivatives via CC/C(sp3)–N bond cleavage of enones and primary amines

2,4,6-Trisubstituted pyridines can be generated from an acid-catalyzed tandem reaction via CC/C(sp³)–N bond cleavage of enones and primary amines.

Metal-Free Synthesis of Fully Substituted Pyridines via Ring Construction Based on the Domino Reactions of Enaminones and Aldehydes

An unprecedented domino reaction involving primary enaminones/enaminoesters and aldehydes has been developed for the synthesis of fully substituted pyridines. The construction of the products has been accomplished via the cascade generation of two C-C and one C-N bond by simply using TfOH as a promoter.

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.

Theoretical investigations toward TMEDA-catalyzed [2 + 4] annulation of allenoate with 1-aza-1,3-diene: mechanism, regioselectivity, and role of the catalyst

A computational study on the reaction between allenoate and 1-aza-1,3-diene catalyzed by TMEDA has been performed using the DFT method.

Understanding the mechanisms, regioselectivies and enantioselectivities of the DMAP-catalyzed [2 + 4] cycloaddition of γ-methyl allenoate and phenyl(phenyldiazenyl)methanone

DMAP-catalyzed [2 + 4] cycloaddition reaction.

Approach to thiazole-containing tetrahydropyridines via Aza–Rauhut–Currier reaction and their potent fungicidal and insecticidal activity

Insecticidal and fungicidal active thiazole-containing tetrahydropyridine derivatives with accurately predicted 3D QSAR model againstAphis LaburniKaltenbach and predicted potential anti-fungus target of fumarate reductase without cross resistance were synthesized.

Base-Promoted N-Pyridylation of Heteroarenes Using N-Propargyl Enaminones as Equivalents of Pyridine Scaffolds

N-Pyridylation of various N-heteroarenes, including N-heteroarene-containing peptides, was achieved using N-propargyl enaminones (isolated or generated in situ from propargyl amine and propynones) as masked polysubstituted pyridine cores. This metal-free procedure proceeds under mild reaction conditions and generates 1 equiv of H2O as the sole byproduct.

Amine-catalyzed tunable reactions of allenoates with dithioesters: formal [4+2] and [2+2] cycloadditions for the synthesis of 2,3-dihydro-1,4-oxathiines and enantioenriched thietanes

The chemoselective [4+2] vs. [2+2] cycloaddition between allenoates and dithioesters can be controlled by switching the nucleophilic amine catalyst. The two modes of cyclizations represent the first example of controllable and chemoselective annulations between allenoates and dienophiles catalyzed by amine. These cyclizations are useful in offering a divergent synthesis of sulfur-containing heterocycles. On the basis of this investigation, it can be realized that dithioesters with a vicinal electron-withdrawing group can react not only like a Michael acceptor but also as a ketone or imine.

(n)Bu3P-catalyzed desulfonylative [3 + 2] cycloadditions of allylic carbonates with arylazosulfones to pyrazole derivatives

Highly efficient (n)Bu3P-catalyzed desulfonylative [3 + 2] cycloadditions of allylic carbonates with arylazosulfones were developed for the synthesis of pyrazole derivatives. The reactions proceed smoothly under mild conditions to generate corresponding annulation products in good to excellent yields.