The study, published in the journal Discover Chemistry, describes a zinc triflate-catalyzed version of the Pechmann condensation carried out entirely without solvent. Led by Hemant S. Chandak of G S Science, Arts and Commerce College in Khamgaon, the team showed that a catalytic amount of zinc triflate, a Lewis acid salt of zinc and triflic acid, can drive the condensation of activated phenols with beta-ketoesters to deliver coumarin derivatives in good to excellent yields. The protocol is operationally simple: phenol, beta-ketoester and ten mole percent of the catalyst are combined in a screw-capped vial, heated, and the product is isolated by straightforward extraction and chromatography. No volatile organic solvent is needed during the reaction itself, which is precisely where much of the environmental benefit lies.

The Pechmann condensation, first reported by Hermann von Pechmann in 1884, remains the most direct route to the coumarin framework. It marries a phenol with a beta-ketoester in a single pot, forming the coumarin ring through transesterification, electrophilic aromatic substitution and dehydration. The catch has always been the conditions. Classical versions of the reaction rely on harsh Bronsted acids such as concentrated sulfuric acid or trifluoroacetic acid, which corrode equipment, generate hazardous waste and can degrade sensitive substrates. Later refinements introduced solid acids like Nafion resin and sulfated zirconia, ionic liquids, and Lewis acids and nanocatalysts ranging from aluminum chloride to magnetically recoverable sulfonated composites. Each approach trades one drawback for another, whether in cost, toxicity, catalyst preparation or waste generation.

Chandak’s group began their optimization with a surprise failure. Guided by literature on Bronsted acid catalysis, they first tried l-proline nitrate for the reaction of resorcinol with ethyl acetoacetate, but no desired product formed at all. The authors attribute this to excessive protonation of the phenolic oxygen under strongly acidic conditions, which suppresses the nucleophilicity of the aromatic ring and stalls the electrophilic substitution step that lies at the heart of the Pechmann reaction. That dead end pushed the team toward Lewis acid catalysis, where activation happens at the carbonyl of the beta-ketoester rather than by flooding the phenol with protons.

Zinc emerged as the obvious candidate. Zinc-based catalysts have become increasingly popular as substitutes for more expensive or hazardous transition metals, and zinc triflate in particular offers high thermal stability, low toxicity, ready availability and affordability. The team first tested zinc oxide nanoparticles, which produced only sluggish conversion and low yields over prolonged reaction times. Zinc triflate, by contrast, delivered a moderate 42 percent yield after 240 minutes at room temperature, and the yield climbed to 65 percent when the temperature was raised to 60 degrees Celsius. At 80 degrees Celsius the reaction reached excellent yields in a shorter time, while pushing further to 100 degrees Celsius actually lowered the isolated yield to 76 percent, presumably through side reactions or product decomposition. The sweet spot, the researchers concluded, is 80 degrees Celsius.

Catalyst loading proved equally forgiving. Reducing the amount of zinc triflate from 20 mole percent to 10 mole percent had no adverse effect, with the model product, 7-hydroxy-4-methylcoumarin, isolated in 92 percent yield. Dropping further to 5 mole percent left conversion incomplete at 84 percent, so 10 mole percent became the standard. The team then interrogated the role of the reaction medium, screening ethanol, tetrahydrofuran, dimethyl sulfoxide, acetonitrile, chloroform and toluene under otherwise optimized conditions. Tetrahydrofuran managed only 66 percent conversion, and the remaining solvents performed substantially worse. Neat conditions, with no solvent at all, gave the best result of 92 percent. The pattern is chemically telling: coordinating solvents compete with the beta-ketoester for binding to the zinc center, effectively poisoning the catalyst, so removing solvent both eliminates waste and maximizes catalytic efficiency.

With the optimized conditions in hand, the researchers surveyed a library of activated phenols and beta-ketoesters. Resorcinol-derived substrates bearing one or two hydroxyl groups furnished coumarins such as 7-hydroxy-4-methylcoumarin and 7,8-dihydroxy-4-methylcoumarin, the latter known as dihydroxyesculetin’s close relative, in yields of 91 to 92 percent. Other electron-rich phenols gave products in the 68 to 84 percent range. A consistent trend emerged when methyl and ethyl acetoacetate were compared: the ethyl ester reliably delivered higher yields, which the authors attribute to more favorable transesterification and cyclization kinetics for the ethyl ester under Lewis acidic conditions. The protocol also tolerated the bulkier ethyl benzoylacetate, producing aryl-substituted coumarins at a higher reaction temperature of 120 degrees Celsius, demonstrating genuine tolerance for steric and electronic variation.

The method has clear limits, and the authors are candid about them. Electron-deficient phenols such as 4-nitrophenol and 2-chloro-4-nitrophenol failed to react, underscoring that the Pechmann condensation remains fundamentally dependent on electron-rich aromatic rings. Electron-withdrawing substituents drain electron density from the ring, weakening its response to the electrophilic substitution and cyclization steps that build the coumarin core. Even so, the successful substrates include molecules of real functional significance. The team efficiently prepared 4-methylumbelliferyl acetate and 4-methylumbelliferyl benzoate, compounds prized as fluorescent probes for detecting acetylcholinesterase inhibitors, an application relevant to drug screening and toxicology. That the same green protocol can serve both routine scaffold synthesis and specialized probe chemistry gives the method practical reach beyond the academic benchmark reaction.

Mechanistically, the picture is coherent with well-documented Pechmann chemistry. Zinc triflate coordinates to the carbonyl oxygen of the beta-ketoester, sharpening its electrophilicity and enabling transesterification with the phenolic hydroxyl. Enolization of the ketoester follows, and the resulting enol attacks the activated aromatic ring in an intramolecular electrophilic aromatic substitution, forging the key carbon-carbon bond. Cyclization and dehydration then close the lactone ring to deliver the coumarin. Because the Lewis acid activates the carbonyl rather than the phenol, the reaction avoids the nucleophilicity-quenching problem that doomed the Bronsted acid trial at the outset, and the triflate counterion keeps the zinc center soluble and reactive in the neat melt of the two substrates.

The broader significance of the work lies in its alignment with green chemistry principles. Solvent-free synthesis avoids volatile organic compounds, reduces waste, minimizes environmental impact and frequently shortens reaction times while improving yields, and this study demonstrates all of those advantages in a single transformation. The catalyst itself is inexpensive, easy to prepare from zinc carbonate and triflic acid, and simple to handle, and the reaction setup requires nothing more exotic than a screw-capped vial and a heated stirrer. For laboratories producing coumarin libraries for pharmaceutical screening, fluorescent sensor development or industrial applications, the protocol offers a route that is faster, cleaner and gentler than the sulfuric acid baths of the nineteenth century that still linger in many teaching labs. As the authors conclude, the combination of low catalyst loading, moderate temperatures, broad substrate scope and the complete avoidance of reaction solvents makes zinc triflate-catalyzed Pechmann condensation a compelling template for sustainable heterocycle synthesis, one that other chemists will likely adapt to related ring-forming reactions in the years ahead.

Subject of Research: Zinc triflate-catalyzed solvent-free Pechmann condensation for the synthesis of coumarin derivatives

Article Title: Zinc triflate catalyzed solvent-free synthesis of coumarins

Article References: Kale, S. S., Dhurve, N. K., Tanveer, S., Ali, M., Ingle, A. B., Shelke, N. S., Dharamkar, R. R., & Chandak, H. S. (2026). Zinc triflate catalyzed solvent-free synthesis of coumarins. Discover Chemistry, 3(1), Article 441. https://doi.org/10.1007/s44371-026-00893-1

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00893-1

Keywords: coumarins, Pechmann condensation, zinc triflate, solvent-free synthesis, green chemistry, Lewis acid catalysis, beta-ketoesters, phenols, fluorescent probes, heterocyclic chemistry, sustainable synthesis, Discover Chemistry

The compounds at the center of the work, tetrahydrobenzo[b]pyrans and related 2-amino-3-cyano-4H-pyrans, belong to a family of oxygen-containing ring systems with an impressive portfolio of biological activities. Published studies have linked members of this class to anticancer, antimicrobial, antioxidant, anti-inflammatory, antiviral, anticoagulant, antidiabetic, analgesic, and spasmolytic effects. Beyond pharmacology, these fused pyran frameworks serve as versatile intermediates for constructing more elaborate heterocyclic architectures and functional organic materials. That combination of therapeutic relevance and synthetic utility explains why finding faster, greener ways to make them has become one of the busiest corners of modern heterocyclic chemistry.

The reaction the team exploited is a classic of multicomponent chemistry. An aromatic aldehyde, malononitrile, and a cyclic 1,3-dicarbonyl compound such as dimedone, or alternatively ethyl acetoacetate, are combined in a single flask. The domino sequence that follows proceeds through a Knoevenagel condensation, in which the aldehyde and malononitrile form an activated alkene, followed by a Michael addition of the enolized dicarbonyl compound, and finally an intramolecular cyclization that closes the pyran ring. Because three or more starting materials merge in one operation, multicomponent reactions of this type offer excellent atom economy, operational simplicity, high bond-forming efficiency, short reaction times, low energy consumption, and minimal waste, all hallmarks of green chemistry.

What has historically held this transformation back is the catalyst. Most reported protocols rely on Brønsted acids, Lewis acids, transition-metal complexes, ionic liquids, deep eutectic solvents, metal-organic frameworks, carbon-based materials, or recyclable magnetic nanocatalysts. Many of these systems deliver good yields, but they often demand laborious, multistep catalyst preparation, carry high costs, resist large-scale recycling, or require elevated temperatures, long reaction times, and environmentally undesirable organic solvents. The Iranian team’s answer was to strip the catalyst out entirely and let the solvent do the work.

HFIP is an unusual liquid. It is highly polar, weakly nucleophilic, and possesses an exceptional capacity to donate hydrogen bonds, along with very high ionizing power. Those properties allow it to stabilize cationic intermediates and polar transition states, activate carbonyl compounds, and facilitate proton transfers, all without being consumed or incorporated into the product. In recent years HFIP has attracted growing attention as a medium for cyclizations, electrophilic additions, carbon-hydrogen functionalization, oxidative transformations, and multicomponent processes, and mounting evidence suggests it can reduce or even eliminate the need for conventional acid catalysts. What had been missing, the authors note, was a demonstration of HFIP acting alone as the promoter for this particular pyran synthesis.

The optimization experiments tell a striking story. Using benzaldehyde, malononitrile, and dimedone as the model reaction, the researchers varied the amount of HFIP and the temperature. At room temperature with 0.30 milliliters of HFIP, the isolated yield reached 87 percent in just ten minutes. Raising the temperature to 55 degrees Celsius cut the time to five minutes and pushed the yield to 92 percent. Adding more HFIP brought no further benefit. Most tellingly, when HFIP was removed altogether, only a trace of product appeared after thirty minutes under otherwise identical conditions. Because the yield and rate climb with HFIP concentration, the authors interpret the effect as promotional rather than catalytic, and they are careful not to claim a catalytic turnover cycle without dedicated kinetic evidence.

The mechanistic picture the team offers, explicitly framed as a plausible hypothesis rather than experimentally confirmed proof, rests on hydrogen bonding. HFIP can hydrogen-bond to the carbonyl oxygen of the aldehyde, increasing the electrophilicity of the carbonyl carbon and thereby accelerating the initial Knoevenagel condensation with malononitrile. The same hydrogen-bonding network is thought to stabilize the charged intermediates that appear during the subsequent Michael addition and the intramolecular cyclization, allowing the entire domino sequence to run quickly and cleanly at relatively low temperature. The authors acknowledge that no specific kinetic experiments or comparative studies with alternative hydrogen-bond-donating solvents were performed, so the separate contribution of hydrogen bonding cannot yet be distinguished from other solvent effects.

With the optimized conditions in hand, the researchers surveyed a representative set of aromatic aldehydes bearing both electron-donating and electron-withdrawing substituents. Methoxy, chloro, and nitro groups were all tolerated. When dimedone served as the dicarbonyl component, the tetrahydrobenzo[b]pyran products were isolated in 91 to 95 percent yields over reaction times of roughly 90 to 100 minutes. Swapping dimedone for ethyl acetoacetate delivered the corresponding 2-amino-3-cyano-4H-pyran derivatives in 83 to 94 percent yields. The authors are candid about the limits: heteroaromatic aldehydes, sterically hindered substrates, and more electronically extreme cases were not systematically examined, so the results demonstrate reliable compatibility within the tested set rather than a universally broad substrate scope.

How does the HFIP protocol stack up against the competition? The team compared its results with representative literature procedures, including magnetic nanocatalysts operating under microwave irradiation or grinding, ammonium hydroxide in water, and urea in aqueous ethanol. For the benchmark products 4a through 4c, the HFIP method delivered 92 to 95 percent isolated yields at 55 degrees Celsius with reaction times similar to or shorter than most of the reference procedures, some of which required reflux, extended stirring, or elaborate catalyst synthesis. The comparison rests on reaction time and isolated yield alone, since parameters such as catalyst loading, solvent quantity, and recycling efficiency are not uniformly reported across studies. The clear advantage of the new protocol is its formulation: a homogeneous medium, a one-pot operation, mild temperature, and no added metal or acid catalyst, with products isolated by simple crystallization from ethanol rather than chromatography.

The authors also weigh the environmental ledger honestly. HFIP is a fluorinated solvent, and although it is readily distilled and was recovered by distillation for reuse in subsequent runs in the laboratory, the study did not experimentally evaluate recovery efficiency, energy costs, or solvent losses at scale. The environmental burden of fluorinated compounds, occupational hazards, and the energy required for solvent recovery all must be counted against the benefits of catalyst elimination, fast reactions, and high atom economy. The team concludes that the method is a valuable addition to the synthetic toolbox for the substrates tested, but that future work should expand the substrate scope, probe the mechanism with kinetic and comparative solvent studies, and rigorously assess HFIP recovery. For now, the message is clear and likely to resonate across medicinal chemistry and green synthesis alike: sometimes the most powerful catalyst in the flask is the solvent itself.

Subject of Research: HFIP-promoted catalyst-free one-pot multicomponent synthesis of tetrahydrobenzo[b]pyran and 2-amino-3-cyano-4H-pyran derivatives

Article Title: HFIP-promoted Lewis acid-free one-pot synthesis of pyran derivatives under mild conditions

Article References: Nouri, F. Z., Maleki, B., Baharfar, R., Alinezhad, H., & Ashrafi, S. S. (2026). HFIP-promoted Lewis acid-free one-pot synthesis of pyran derivatives under mild conditions. Results in Chemistry, 31, Article 103961. https://doi.org/10.1016/j.rechem.2026.103961

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103961

Keywords: HFIP, multicomponent reaction, pyran synthesis, green chemistry, hydrogen bonding, Knoevenagel condensation, Michael addition, heterocyclic chemistry, fluorinated solvents, catalyst-free synthesis, tetrahydrobenzo[b]pyrans, organic synthesis

The tetrazole ring at the heart of these molecules is a five-membered structure built from one carbon atom and four nitrogen atoms, and it has long occupied a privileged position in medicinal chemistry. Its most celebrated property is that it acts as a bioisostere of the carboxylic acid group, meaning it can stand in for that common acidic motif while conferring better metabolic stability, tuned lipophilicity, hydrogen-bonding capacity and membrane permeability. The ring’s track record in the clinic is substantial: the β-lactam antibiotics cefotetan and cefotiam, the oxazolidinone antibiotic tedizolid phosphate, the antihypertensive losartan, the antiplatelet drug cilostazol, the leukotriene antagonist tomelukast, the anticonvulsant cenobamate and the antifungal candidate TAK-456 all carry a tetrazole unit that contributes to receptor recognition, binding affinity and pharmacokinetic behaviour. That pedigree made the scaffold a natural choice for a team hunting structurally novel antibacterial agents capable of sidestepping the resistance mechanisms—target modification, enzymatic degradation, reduced membrane permeability and active efflux—that steadily erode the value of conventional antibiotics.

The synthetic strategy is as much a story as the molecules themselves. Multicomponent reactions are prized in drug discovery because they stitch together three or more building blocks in one operation, delivering structural diversity with operational simplicity and high atom economy. The Ugi–azide variant used here condenses four partners: an aldehyde, a primary aromatic amine, trimethylsilyl azide and tert-butyl isocyanide. In the reported protocol, 2-(4-chloro-2-formylphenoxy)acetonitrile reacts with a range of substituted anilines, trimethylsilyl azide and tert-butyl isocyanide in anhydrous methanol at ambient temperature. The mechanism unfolds in a cascade: the aldehyde and amine first form an imine, or Schiff base, while the silyl azide undergoes methanolysis to release hydrazoic acid. The isocyanide then attacks the protonated imine to generate a nitrilium ion, the azide anion strikes the nitrilium carbon to form an azido-imidoyl intermediate, and an intramolecular 1,3-dipolar [3+2] cycloaddition closes the tetrazole ring. The bulky tert-butyl group steers the reaction toward N-1 regioselective ring formation, delivering the desired 1,5-disubstituted α-aminotetrazoles cleanly.

Solvent choice proved decisive. In a systematic screen using compound 5a as the model substrate, the non-polar solvent toluene was completely ineffective, while chlorinated solvents such as dichloromethane and chloroform managed only trace-to-poor conversions of 28 percent or less even after 48 to 72 hours. Polar aprotic solvents including tetrahydrofuran, acetonitrile and dimethylformamide gave moderate yields of 32 to 58 percent with incomplete product precipitation. Ethanol and isopropanol performed better but still required two days. Methanol emerged as the clear winner, delivering an 85 percent yield within 18 hours, accompanied by spontaneous, clean precipitation of the product. Under the optimized conditions—one millimole scale in eight millilitres of anhydrous methanol at room temperature for 18 to 24 hours—all ten derivatives were isolated in excellent yields of 84 to 88 percent simply by washing the precipitated solid with ice-cold methanol and anhydrous hexane, avoiding chromatography altogether.

Structural confirmation rested on a battery of spectroscopic techniques. Fourier-transform infrared spectra showed broad N–H stretching between 3311 and 3398 inverse centimetres, a nitrile absorption near 2230 inverse centimetres and tetrazole C=N vibrations between 1600 and 1660 inverse centimetres. Proton NMR spectra recorded at 500 megahertz in deuterated DMSO displayed the diagnostic methine proton as a doublet between 5.86 and 6.46 parts per million, the tert-butyl singlet at roughly 1.7 parts per million and the phenoxy methylene singlet near 5.2 parts per million. Carbon-13 NMR placed the quaternary tetrazole carbon at 152 to 154 parts per million, and mass spectrometry confirmed molecular ion peaks matching the calculated formulae for every compound in the series.

The biological evaluation covered four bacterial strains—Streptococcus pyogenes, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa—and two fungal species, Aspergillus clavatus and Candida albicans, using minimum inhibitory concentrations determined by the agar dilution method against the reference antibiotics gentamicin and ketoconazole. Compound 5c, bearing an ortho-fluorophenyl substituent, stood out as the most potent antibacterial agent, with MIC values ranging from 39.67 to 170 micrograms per millilitre across strains and small error bars indicating excellent reproducibility. Against Staphylococcus aureus, 5c reached 52.33 micrograms per millilitre, the lowest value in the series, while compounds 5b, 5i and 5j also performed respectably. The antifungal picture was far less encouraging: activity against A. clavatus and C. albicans was limited and generally far weaker than ketoconazole, suggesting the scaffold is selective for bacterial targets rather than a broad-spectrum antimicrobial.

The structure–activity relationship that emerged from the data is clear and chemically intuitive. Electron-withdrawing halogen substituents, particularly fluorine placed at the ortho position, enhance antimicrobial potency, presumably by increasing lipophilicity and strengthening interactions with microbial targets, whereas bulky substituents diminish activity. The authors rank the series as 5c greater than 5b, 5i and 5j, followed by 5a, 5g, 5d, 5e, 5f and 5h. Notably, compound 5d showed substantial variability against A. clavatus, with a value of 266.67 plus or minus 213.85 micrograms per millilitre, producing a non-significant difference from ketoconazole that the authors attribute to large variation among replicate observations rather than genuine equivalence.

To probe mechanism, the team docked all ten compounds into penicillin-binding protein 2a, the enzyme that renders methicillin-resistant Staphylococcus aureus, or MRSA, impervious to β-lactam antibiotics and is therefore a clinically validated target for anti-MRSA drug development. Using AutoDock Vina against the crystal structure with PDB identifier 3ZG5, every compound showed strong predicted binding free energies between −7.1 and −7.9 kilocalories per mole, all exceeding the −6.6 kilocalories per mole calculated for gentamicin. Compound 5f, carrying a para-nitrophenyl group, achieved the highest affinity at −7.9 kilocalories per mole, forming hydrogen bonds with lysine 215 and aspartate 367, while compound 5a ranked second at −7.8 and additionally formed a notable halogen bond with aspartate 367. Three residues—lysine 215, proline 370 and glutamate 379—were engaged consistently across the series, marking them as critical contacts for future optimization. The authors candidly note that docking scores and experimental MIC values correlated only partially, with the discrepancy for 5f likely reflecting membrane permeability and efflux effects that docking calculations cannot capture.

Computational ADME profiling using the pkCSM platform rounded out the assessment and delivered a largely encouraging pharmacokinetic forecast. All ten compounds were predicted to achieve high human intestinal absorption of 90.9 to 100 percent, dramatically better than gentamicin’s predicted 19.2 percent, pointing to favourable oral bioavailability for a class of molecules that could otherwise require injection. Aqueous solubility was moderate, and every compound was predicted to inhibit P-glycoprotein, an intriguing property that could suppress efflux-mediated resistance in bacteria. Distribution predictions showed low blood–brain barrier penetration, a desirable trait for peripherally acting antibacterial agents, alongside moderate-to-high plasma protein binding. Metabolically, all compounds were substrates of CYP3A4 but not CYP2D6, a favourable safety attribute, although predicted inhibition of CYP2C19, CYP2C9 and CYP3A4 flags a need for careful drug–drug interaction evaluation during preclinical development. Excretion profiles indicated moderate-to-low systemic clearance, consistent with a prolonged duration of antibacterial action.

Taken together, the study positions the α-aminotetrazole scaffold as a promising, drug-like antibacterial pharmacophore that merits focused medicinal chemistry optimization and eventual in vivo evaluation. The one-pot Ugi–azide synthesis, with its excellent yields, mild conditions and purification by simple washing, offers medicinal chemists an efficient platform for rapidly exploring analogues around the active ortho-halogenated leads, particularly compound 5c. With MRSA still driving prolonged hospitalizations, rising mortality and escalating healthcare costs worldwide, and with the global burden of bacterial antimicrobial resistance estimated in the hundreds of thousands of deaths annually, even modest advances in scaffold design carry weight. The Indian team’s work demonstrates how a century-old multicomponent reaction, a clinically proven heterocycle and modern computational screening can converge to generate credible new candidates in the fight against resistant bacteria—while honestly acknowledging that the distance from a docking score to a bedside drug remains long and demanding.

Subject of Research: Synthesis of disubstituted α-aminotetrazole derivatives via the Ugi–azide reaction and their antibacterial activity, molecular docking against MRSA PBP2a, and ADME profiling

Article Title: Synthesis of novel disubstituted tetrazole scaffolds via the Ugi TMS-N3 reaction and their antimicrobial evaluation molecular docking and ADME profiling

Article References: Gohil, R., Khoyanee, A., Ramani, H., & Khunt, R. (2026). Synthesis of novel disubstituted tetrazole scaffolds via the Ugi TMS-N3 reaction and their antimicrobial evaluation molecular docking and ADME profiling. Discover Chemistry, 3(1), Article 560. https://doi.org/10.1007/s44371-026-01018-4

Image Credits: AI Generated

DOI: 10.1007/s44371-026-01018-4

Keywords: tetrazole, Ugi-azide reaction, multicomponent reaction, antimicrobial resistance, MRSA, PBP2a, molecular docking, ADME profiling, medicinal chemistry, antibacterial agents, structure-activity relationship, heterocyclic chemistry

The appeal of carbazole begins with its electrons. Because the fused ring system is rigid, planar and richly π-conjugated, electron density can delocalize across all three rings, giving carbazole derivatives high fluorescence efficiency, efficient charge transport and excellent thermal stability. The nitrogen atom in the central ring further enhances charge-transfer behaviour, and derivatives that form charge-transfer complexes display red-shifted absorption and emission. These photophysical virtues explain why carbazole-containing polymers and small molecules have become staples of organic light-emitting diodes, organic field-effect transistors, photovoltaic cells and sensors. As hole-transport materials, they even suppress crystallization in small-molecule devices, extending operational lifetimes. More surprisingly, carbazole-based microporous polymers, built by exploiting the reactive hydrogen atoms at the 3- and 6-positions of the ring, can adsorb carbon dioxide at ambient temperature thanks to the electron-withdrawing nature of the nitrogen atom, opening an unexpected environmental application for this optical workhorse.

The medicinal story is equally compelling. The planar tricyclic framework facilitates π-π stacking and hydrogen bonding with DNA, enzymes and receptor proteins, allowing carbazole derivatives to interact with a striking range of biological targets. Compounds built on this scaffold have shown anti-cancer, anti-bacterial, anti-viral, anti-inflammatory and anti-oxidant activities, and strategic modification at various ring positions has tuned their pharmacokinetic and pharmacodynamic properties. It is this dual identity, drug lead and electronic material in one molecule, that makes the synthetic chemistry of carbazole worth cataloguing in exhaustive detail.

Ansari and Ali organize their review around named reactions, the eponymous transformations that serve as the shared vocabulary of synthetic organic chemistry. Just as doctors must know the names of organs, the authors argue, chemists must know their named reactions: the labels compress substrates, conditions and mechanisms into a single phrase that lets researchers think instantly about what a transformation can and cannot do. The review spans classical nineteenth-century chemistry, including the Bucherer, Graebe-Ullmann, Borsche-Drechsel and Fischer indole syntheses, alongside modern metal-catalyzed and photocatalytic methods such as the Ackermann reaction, Cadogan cyclization, Knölker synthesis and ring-closing metathesis.

The classical routes remain surprisingly relevant. The Borsche-Drechsel cyclization, first reported by Edmund Drechsel in 1888 and elaborated by Walther Borsche in 1908, condenses arylhydrazines with cyclohexanones to form hydrazones that undergo protonation, ene-amine tautomerization and a [3,3]-sigmatropic rearrangement, ultimately delivering tetrahydrocarbazoles that can be aromatized to the parent scaffold. This venerable sequence has carried chemists to an impressive roster of natural products, including murrayanine from the stem bark of Murraya koenigii, murrayacine, clausenol and the antifungal algal alkaloids Tjipanazole D and I. In a green-chemistry twist, Kotha’s team recently performed the Borsche-Drechsel cyclization in a deep eutectic solvent, a low-melting mixture of N,N’-dimethyl urea and L-(+)-tartaric acid, replacing harsh classical media with a sustainable alternative.

The Fischer indole synthesis, discovered by Emil Fischer in 1883, follows a closely related mechanistic logic: phenylhydrazines condense with aldehydes or ketones to form hydrazones, which tautomerize, rearrange sigmatropically and cyclize with loss of ammonia to deliver indole frameworks, including fused carbazoles. It was this strategy that Bonjouklian and co-workers used to reach Tjipanazole E and B, chlorinated carbazole alkaloids bearing N-glycosidic substituents that were first isolated from the blue-green alga Tolypothrix tjipanasensis. Meanwhile, the Bucherer carbazole synthesis, reported by Hans Theodor Bucherer in 1908, couples naphthols with arylhydrazines under acidic bisulfite conditions without any transition metal, proceeding through hydrogen sulfite addition, a [3,3]-sigmatropic rearrangement and final ammonia elimination to yield benzo-fused carbazoles.

On the modern side, palladium catalysis dominates. The Buchwald-Hartwig coupling, independently discovered by Stephen Buchwald and John Hartwig in 1994, forms carbon-nitrogen bonds between amines and aryl halides through the canonical catalytic cycle of oxidative addition, amine binding, deprotonation and reductive elimination. Zhou and Verkade showed that a double N-arylation of primary amines with 2,2′-dihalobiphenyls, mediated by a proazaphosphatrane ligand, delivers N-substituted carbazoles in a single operation. The method’s excellent functional-group tolerance made it the key step in the 2008 total synthesis of murrayazoline, an antiplatelet carbazole alkaloid from the genus Murraya. The related Ackermann reaction chains a Buchwald-Hartwig coupling with a direct arylation to build carbazoles from simple anilines and 1,2-dihaloarenes, a domino strategy that was used to synthesize murrayafoline A, a growth-inhibitory and fungicidal alkaloid isolated from Rutaceae plants.

Other modern methods trade metal catalysis for other tricks. The Cadogan cyclization, reported by John Cadogan in 1974, reductively cyclizes o-nitrobiphenyls with triethyl phosphite or triphenylphosphine, proceeding through nitroso intermediates and either a nitrene or a direct cyclization pathway; it underpinned the synthesis of siamenol, an anti-HIV carbazole alkaloid from Murraya siamensis. Microwave irradiation now compresses Cadogan reaction times from hours to minutes, and a visible-light variant using the organic photocatalyst 4CzIPN under blue LEDs delivers carbazoles in 40 to 87 percent yields. The Knölker synthesis, introduced by Hans-Joachim Knölker in 1999, takes a different approach entirely: a tricarbonyliron fragment protects a reactive cyclohexadiene, enforces anti-selective stereochemistry and enables hydride abstraction with triphenylmethyl tetrafluoroborate, a sequence that made possible the first total synthesis of the complete antiostatin A and B families. Ring-closing metathesis, the Nobel-recognized chemistry of Grubbs, Schrock and Chauvin, closes carbazole rings from pre-installed dienes under mild conditions and delivered murrayaquinone A, while the Wulff-Dötz benzannulation, in which a Fischer chromium carbene complex reacts with an alkyne through carbon monoxide insertion and 6π-electrocyclization, constructed the carbazole core of the Aspidospermidine alkaloids.

What emerges from the survey is not merely a catalogue but a decision framework. Each reaction carries trade-offs: the Graebe-Ullmann synthesis is reliable but demands temperatures above 200 degrees Celsius; the Cadogan cyclization is metal-free but generates stoichiometric phosphine oxide waste; the Buchwald-Hartwig coupling is modular but requires costly palladium and pre-halogenated substrates; the Knölker synthesis offers stereocontrol but demands specialized iron chemistry. The diverted Bischler-Napieralski cascade reported by Faltracco in 2021 illustrates how even well-worn reactions can surprise, delivering an unexpected carbazole instead of the anticipated dihydro-β-carboline through a spirointermediate pathway. By assembling these options side by side, with their mechanisms, substrate requirements and validated natural product targets, the review gives synthetic chemists a practical map for choosing the right tool, and gives students a masterclass in how the vocabulary of named reactions organizes more than a century of heterocyclic chemistry.

Subject of Research: Named reactions for the synthesis of carbazole and their mechanistic and applied significance

Article Title: Named reactions in Carbazole synthesis: Mechanistic insights and potential applications

Article References: Ansari, T., & Ali, R. (2026). Named reactions in Carbazole synthesis: Mechanistic insights and potential applications. Results in Chemistry, 31, Article 103913. https://doi.org/10.1016/j.rechem.2026.103913

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103913

Keywords: carbazole, named reactions, organic synthesis, heterocyclic chemistry, Buchwald-Hartwig coupling, Fischer indole synthesis, Knölker synthesis, ring-closing metathesis, natural products, OLEDs, medicinal chemistry, palladium catalysis

Polyhydroquinolines are not laboratory curiosities. These nitrogen-containing ring systems, close cousins of the dihydropyridine scaffolds found in calcium-channel blockers, have been linked to an impressive range of pharmacological activities, including hepatoprotective, antidiabetic, geroprotective, bronchodilator, vasodilator, antiatherosclerotic and antitumor effects. Related structures have also shown antiplasmodial, antibacterial and antimalarial activity. Because of this versatility, medicinal chemists have spent decades searching for efficient ways to assemble them, and the classic route traces all the way back to 1882, when Arthur Hantzsch first described a one-pot condensation of an aldehyde, a beta-ketoester and ammonia in acetic acid or ethanol.

The Hantzsch reaction is a prime example of a multicomponent reaction, a strategy in which three or more starting materials combine in a single step to form a product that incorporates most of the atoms of the reactants. Multicomponent reactions are prized for their atom economy and for slashing the number of synthetic steps required to reach complex heterocyclic scaffolds. Yet many modern versions of the polyhydroquinoline synthesis still carry baggage: metal salts that complicate separation, ionic liquids that are costly, catalysts that cannot be reused, or solvents that generate waste. The new study set out to strip away as much of that baggage as possible.

The researchers’ catalyst of choice, PFPAT, is a non-metallic organoammonium salt whose design is deceptively simple but chemically clever. The strongly electron-withdrawing pentafluorophenyl group, paired with a weakly coordinating triflate anion, makes the ammonium proton unusually acidic. That acidity allows PFPAT to activate carbonyl compounds through proton transfer and hydrogen-bonding interactions, the key first move in the multicomponent condensation. Because the catalyst is a metal-free organic salt, it avoids many of the separation and recyclability problems that plague soluble metal catalysts, particularly when reactions are scaled up.

The model reaction combined benzaldehyde, ethyl acetoacetate, dimedone and ammonium acetate, with reaction progress tracked by thin-layer chromatography. Without any catalyst, the mixture yielded only trace product even after prolonged reaction, and heating to 90 degrees Celsius alone brought only marginal improvement, a clear sign that thermal activation is not enough. When PFPAT was introduced, the picture changed dramatically. Raising the catalyst loading from 2.5 to 10 mole percent steadily improved both yield and reaction time, and 10 mole percent at 90 degrees Celsius emerged as the optimum. Pushing the loading further, to 12.5 or 15 mole percent, offered no meaningful benefit, indicating that the active catalytic sites were already sufficient for efficient conversion.

Solvent screening reinforced the green credentials of the protocol. The team tested acetonitrile, chloroform, dimethyl sulfoxide and dichloromethane, and even the best performer, DMSO, managed only a moderate 68 percent yield with longer reaction times. Nothing matched the solvent-free system. Under the optimized conditions, the model product was isolated in 86 percent yield in just three hours, with the crude solid obtained by simple filtration, washed with a minimal volume of ethyl acetate and petroleum ether, and purified by recrystallization from hot aqueous ethanol. The work-up is deliberately mundane, which is precisely the point for a procedure intended to be practical.

To benchmark PFPAT against the competition, the researchers ran the same model reaction with a lineup of commercially available Lewis and Brønsted acid catalysts under identical conditions. Metal salts such as zinc chloride, iron(III) chloride and iron(III) iodide delivered conversions of only 60 to 72 percent with isolated yields of 38 to 50 percent. The lanthanide triflate La(OTf)3 fared better at 85 percent conversion and 65 percent yield, while p-toluenesulfonic acid and heterogeneous titanium dioxide pushed past 90 percent conversion with yields above 70 percent over five hours. PFPAT outperformed them all, achieving complete conversion and an 86 percent yield in three hours. Calculated turnover numbers told the same story: PFPAT posted the highest TON of 8.6 and TOF of 2.87 per hour among the catalysts tested, reflecting its strong Brønsted acidity and its knack for accelerating the cyclocondensation under mild, solvent-free conditions.

The substrate scope revealed a clear and chemically satisfying pattern. Eleven substituted benzaldehydes were converted into their polyhydroquinoline products, spanning electron-donating groups such as methyl, methoxy and hydroxy as well as electron-withdrawing halogens and nitro groups. Electron-poor aldehydes reacted fastest and best: 4-nitrobenzaldehyde gave its product in 94 percent yield within just 1.5 hours, while electron-rich 4-methoxybenzaldehyde needed four hours and delivered 78 percent. The explanation lies in the first step of the mechanism, a Knoevenagel condensation in which the aldehyde carbonyl must be attacked. Electron-withdrawing substituents make the carbonyl carbon more electrophilic, speeding both the condensation and the subsequent cyclization, whereas electron-donating groups dampen electrophilicity and slow the cascade. The consistent trend across substrates suggests the catalytic system has a general affinity for electron-deficient aromatic aldehydes, a useful guide for anyone planning to apply the method.

Mechanistically, the reaction unfolds as a classic Hantzsch-type sequence. PFPAT first activates the aldehyde carbonyl through hydrogen bonding and acid interactions, priming it for nucleophilic attack. Ammonium acetate then reacts with ethyl acetoacetate to generate an enamine intermediate, while dimedone undergoes a PFPAT-catalyzed Knoevenagel condensation with the activated aldehyde to form a reactive alpha,beta-unsaturated intermediate. The enamine adds to this activated olefin in a Michael-type addition, followed by intramolecular cyclization and tautomerization to close the polyhydroquinoline ring. The catalyst’s ability to promote proton transfers and stabilize charged intermediates through each of these stages is what allows the whole cascade to proceed smoothly without solvent.

Recyclability, often the Achilles heel of homogeneous catalysts, proved to be a genuine strength here. After each run, PFPAT was recovered simply by extraction with ethyl acetate, dried under vacuum and reused without any further treatment. The catalyst retained its activity effectively through four consecutive cycles, with only a slight loss attributable to partial solubility in the aqueous phase. On the fifth cycle, performance dropped more sharply, with product yield falling to 70 percent and catalyst recovery to 63 percent, but the overall pattern still marks PFPAT as a viable, eco-friendly option for repeated use. Combined with yields of 72 to 94 percent across the substrate range, short reaction times, operational simplicity and the complete elimination of reaction solvents, the protocol offers a template for sustainable heterocyclic synthesis. The authors suggest that future work could expand the substrate scope, probe the biological activities of the products, and evaluate whether the process can be scaled for industrial application, a prospect that would bring a 140-year-old reaction firmly into the era of green chemistry.

Subject of Research: Green solvent-free Brønsted acid-catalyzed synthesis of polyhydroquinoline derivatives via Hantzsch multicomponent reaction

Article Title: An optimized and sustainable Brønsted acid-catalyzed green synthesis of polyhydroquinoline derivatives

Article References: Pande, P., Niralwad, K., & Patki, A. (2026). An optimized and sustainable Brønsted acid-catalyzed green synthesis of polyhydroquinoline derivatives. Discover Chemistry, 3(1), Article 551. https://doi.org/10.1007/s44371-026-00996-9

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00996-9

Keywords: polyhydroquinolines, PFPAT, green chemistry, Hantzsch reaction, Brønsted acid catalysis, multicomponent reactions, solvent-free synthesis, catalyst recyclability, heterocyclic chemistry, drug discovery, Knoevenagel condensation, sustainable synthesis