Heteroaromatic Synthesis via Olefin Cross-Metathesis: Entry to Polysubstituted Pyridines

Schiff-base Polymer Immobilized Ruthenium for Efficient Catalytic Cross-coupling of Secondary Alcohols with 2-amino- and γ-aminobenzyl Alcohols to Give Quinolines and Pyridines

A Schiff-base porous polymer has been impregnated with ruthenium trichloride for acceptor-free dehydrogenation coupling (ADC) of secondary alcohols with γ-amino- and 2-aminobenzyl alcohols to give pyridines and quinolines. This heterogenous catalyst exhibited high catalytic efficiency over repeated cycles with wide functional group tolerance.

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.

One-Pot Sequential Hydroamination Protocol for N-Heterocycle Synthesis: One Method To Access Five Different Classes of Tri-Substituted Pyridines

Tri-substituted pyridines are important scaffolds that can be found in a plethora of commercially available drugs. A one-pot general method for the selective synthesis of less explored/challenging patterns of tri-substituted pyridines is described. Hydroamination of alkynes with commercially available N-triphenylsilylamine generates N-silylenamines. These in situ generated N-silylenamines, upon reaction with α,β-unsaturated carbonyl compounds and subsequent oxidation, furnish 25 examples of selectively substituted 2,4,5-, 2,3,4-, 3,4,5-, 2,3,5-, and 2,3,6-trisubstituted pyridines in up to 78% yield. The reaction features high functional group compatibility providing an expeditious and general approach for the assembly of selectively substituted tri-substituted pyridine derivatives. The robustness and practicality of the reaction have been demonstrated in a gram-scale reaction.

De Novo Synthesis of Polysubstituted 3-Hydroxypyridines Via “Anti-Wacker”-Type Cyclization

We report an efficient method to prepare polysubstituted 3-hydroxypyridines from amino acids, propargyl alcohols, and arylboronic acids. The process involves Pd(0)-catalyzed anti-selective arylative cyclizations of N-propargyl-N-tosyl-aminoaldehydes with arylboronic acids (“anti-Wacker”-type cyclization), oxidation of the resulting 5-substituted-3-hydroxy-1,2,3,6-tetrahydropyridines to 3-oxo derivatives, and elimination of p-toluenesulfinic acid. This method provides diverse polysubstituted 3-hydroxypyridines, whose hydroxy group can be further substituted by a cross-coupling reaction via a triflate.

Design, Synthesis and Evaluation of Novel Derivatives of Curcuminoids with Cytotoxicity

Curcumin and curcuminoids have been discussed frequently due to their promising functional groups (such as scaffolds of α,β-unsaturated β-diketone, α,β-unsaturated ketone and β'-hydroxy-α,β-unsaturated ketone connected with aromatic rings on both sides) that play an important role in various bioactivities, including antioxidant, anti-inflammatory, anti-proliferation and anticancer activity. A series of novel curcuminoid derivatives (a total of 55 new compounds) and three reference compounds were synthesized with good yields using three-step organic synthesis. The anti-proliferative activities of curcumin derivatives were examined for six human cancer cell lines: HeLaS3, KBvin, MCF-7, HepG2, NCI-H460 and NCI-H460/MX20. Compared to the IC50 values of all the synthesized derivatives, most α,β-unsaturated ketones displayed potent anti-proliferative effects against all six human cancer cell lines, whereas β'-hydroxy-α,β-unsaturated ketones and α,β-unsaturated β-diketones presented moderate anti-proliferative effects. Two potent curcuminoid derivatives were found among all the novel derivatives and reference compounds: (E)-5-hydroxy-7-phenyl-1-(3,4,5-trimethoxyphenyl)hept-1-en-3-one (compound 3) and (1E,4E)-1,7-bis(3,4,5-trimethoxyphenyl)hepta-1,4-dien-3-one (compound MD12a). These were selected for further analysis after the evaluation of their anti-proliferative effects against all human cancer cell lines. The results of apoptosis assays revealed that the number of dead cells was increased in early apoptosis and late apoptosis, while cell proliferation was also decreased after applying various concentrations of (E)-5-hydroxy-7-phenyl-1-(3,4,5-trimethoxyphenyl)hept-1-en-3-one (compound 3) and (1E,4E)-1,7-bis(3,4,5-trimethoxyphenyl)hepta-1,4-dien-3-one (compound MD12a) to MCF-7 and HpeG2 cancer cells. Analysis of the gene expression arrays showed that three genes (GADD45B, SESN2 and BBC3) were correlated with the p53 pathway. From the quantitative PCR analysis, it was seen that (1E,4E)-1,7-bis(3,4,5-trimethoxyphenyl)hepta-1,4-dien-3-one (compound MD12a) effectively induced the up-regulated expression of GADD45B, leading to the suppression of MCF-7 cancer cell formation and cell death. Molecular docking analysis was used to predict and sketch the interactions of the GADD45B-α,β-unsaturated ketone complex for help in drug design.

Synthesis of Highly Functionalized Pyridines: A Metal-Free Cascade Process

Herein, we report a new process for the synthesis of highly functionalized pyridines based on a tandem Pummerer-type rearrangement, aza-Prins cyclization, and elimination-induced aromatization. This formal [5+1] cyclization provides pyridines in good yields with easily accessible starting materials. The synthetic potential of our new method is further demonstrated in the modification of the frameworks of BINOL and some natural products.

Facile one-pot synthesis of ketones from primary alcohols under mild conditions

One-pot transformation of primary alcohols to ketones was achieved by using dibromoisocyanuric acid and Fe₂O₃. This synthetic method provides a variety of ketones in high yield under mild reaction conditions.

One-Pot Amberlyst 15-Controlled Cyclocondensation of Piperidines and Arylaldehydes: Synthesis of 3,5-Diarylmethylpyridines

Amberlyst 15-controlled one-pot easy-operational intermolecular cyclocondensation of substituted piperidines with arylaldehydes provides diversified 3,5-diarylmethylpyridines in high to excellent yields under refluxing toluene conditions. The uses of various acidic solid supports and reaction solvents are investigated for facile and efficient transformation. A plausible mechanism has been proposed.

Improved synthesis of 2,4,6-trialkylpyridines from 1,5-diketoalkanes: the total synthesis of Anibamine

Many pyridine syntheses have been developed to date. In this study, we focused on pyridine synthesis with 1,5-diketone derivatives and hydroxylamine. Treatment of simple 1,5-diketoalkanes and hydroxylamine in basic or acidic conditions gave aldol adducts without any pyridine compounds. However, by screening the reaction conditions, we found that acidic conditions produced via the formation of oxime intermediates derived from 1,5-diketoalkanes allowed the formation of the corresponding pyridine derivatives. This is the first example of 2,4,6-trialkylpyridine synthesis from quite simple 1,5-diketoalkanes. In order to demonstrate the utility of the reaction, we demonstrated the synthesis of pyridine derivatives and the total synthesis of a 6-substituted pyridyl-natural product, anibamine. This was achieved by following the above methodology using a reported compound as the starting material to give the product in 12% yield.

Pd-Catalyzed regioselective synthesis of 2,6-disubstituted pyridines through denitrogenation of pyridotriazoles and 3,8-diarylation of imidazo[1,2-a]pyridines

Synthesis of 2,6-disubstituted pyridines from pyridotriazoles through palladium-catalyzed aerobic oxidative denitrogenative reactions has been described.

l-Proline catalyzed one-pot synthesis of polysubstituted pyridine system incorporating benzothiazole moiety via sustainable sonochemical approach

The efficient, highly convenient synthesis of polysubstituted pyridine derivatives was established via the reaction of N-(benzothiazol-2-yl)-2-cyanoacetamides with an assortment of arylidene malononitriles and arylidene ethyl cyanoacetates in the presence of l-proline as an efficient organocatalyst for this type of ultrasonic-mediated Michael addition. The mechanistic pathway and factors affecting this reaction were also established. The main characteristics of this procedure are high yields, use of a cost-effective catalyst, and easy work-up and purification.

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.

Diversity in the Syntheses of Muscone, Muscopyridine and Civetone

Syntheses of muscone, muscopyridine and civetone since 1997 have been reviewed.

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.

Sequential Michael addition/retro-Claisen condensation of aromatic β-diketones with α,β-unsaturated esters: an approach to obtain 1,5-ketoesters

A K₂CO₃-catalyzed one-pot protocol involving sequential C–C bond formation and cleavage of aromatic β-diketones with α,β-unsaturated esters is developed to obtain 1,5-ketoesters, proceeding smoothly under mild conditions in up to 98% isolated yield.

Selected synthetic strategies to cyclophanes

In this review we cover various approaches to meta- and paracyclophanes involving popular reactions. Generally, we have included a strategy where the reaction was used for assembling the cyclophane skeleton for further functionalization. In several instances, after the cyclophane is made several popular reactions are used and these are not covered here. We included various natural products related to cyclophanes. To keep the length of the review at a manageable level the literature related to orthocyclophanes was not included.

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.

Regioselective Pd-Catalyzed Synthesis of 2,3,6-Trisubstituted Pyridines from Isoxazolinones

Substituted pyridines are prevalent heterocycles of fundamental importance. Their efficient regioselective preparation is often still a challenge despite a large number of reported synthetic methodologies. In this letter we report an operationally simple approach that makes use of readily accessible isoxazolinones. The protocol involves a Pd(II)-catalyzed C-regioselective 1,4-addition to vinylketones, followed by a Pd(0)-catalyzed transformation, which is assumed to proceed via vinylnitrene-Pd intermediates. Both hydrogen and air are necessary for the pyridine formation step and could be employed at ratios above the upper explosive limit thus avoiding a safety issue. This new strategy allows an effective, scalable and practical access to various previously unknown 2,3,6-trisubstituted pyridines.

Ammonium iodide-promoted cyclization of ketones with DMSO and ammonium acetate for synthesis of substituted pyridines

A convenient and efficient method has been developed for the construction of substituted pyridines via ammonium iodide-promoted cyclization of ketones with DMSO and ammonium acetate, employing DMSO as the source of C4 or C6.

Formation and Direct Detection of Non-Conjugated Triplet 1,2-Biradical from β,γ-Vinylarylketone

Laser flash photolysis of 2-methyl-1-phenylbut-3-en-1-one (1) conducted at irradiation wavelengths of 266 and 308 nm results in the formation of triplet 1,2-biradical 2 that has λmax at 370 and 480 nm. Biradical 2 is formed with a rate constant of 1.1 × 107 s–1 and decays with a rate constant of 2.3 × 105 s–1. Isoprene-quenching studies support the notion that biradical 2 is formed by energy transfer from the triplet-excited state of the ketone chromophore of 1. Density functional theory calculations were used to verify the characterization of triplet biradical 2 and validate the mechanism for its formation. Thus, it has been demonstrated that intramolecular sensitization of simple alkenes can be used to form triplet 1,2-biradicals with the two radical centres localized on the adjacent carbon atoms.

Metal-free aerobic one-pot synthesis of substituted/annulated quinolines from alcohols via indirect Friedländer annulation

Metal-free one-pot aerobic synthesis of highly functionalized/annulated quinolines is devised in excellent yields from viable alcohols via indirect Friedländer annulation.

De novo synthesis of multisubstituted aryl amines using alkene cross metathesis

The olefin cross-metathesis reaction allows rapid access to 1,5-dicarbonyl intermediates which, upon treatment with a primary or secondary amine, allow the synthesis of a range of multisubstituted carbocyclic aryl amines. This de novo arene synthesis yields nonclassical substitution patterns in a regioselective and predictable approach that is compatible with several functional groups.

Synthesis of fully-substituted pyridines and dihydropyridines in a highly chemoselective manner utilizing a multicomponent reaction (MCR) strategy

An efficient protocol has been developed for the synthesis of pyridines and 1,4-dihydropyridines based on chemoselective multicomponent reactions.

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.