The research team, led by Yogesh Bhaskar Singh Tanwer, Suman Sourabh, and Sabyasachi Bhunia of the Central University of Jharkhand, together with Sanchari Pal and Debjit Das of Triveni Devi Bhalotia College, combined three simple ingredients in a single flask: a substituted salicylaldehyde, either 4-hydroxycoumarin or 1,3-dimethylbarbituric acid, and an electron-rich arene such as 1,3,5-trimethoxybenzene. Heating this mixture at 90 degrees Celsius in a mixture of lemon juice and ethanol produced the desired fused chromene products in yields ranging from 74 to 87 percent for the chromeno[4,3-b]chromene series and good to excellent yields for the chromeno[2,3-d]pyrimidine analogues. No metals, no exotic reagents, no chromatographic purification—just filtration and a wash with aqueous ethanol in most cases.

The appeal of multicomponent reactions lies in their efficiency. Rather than assembling a target molecule step by step, isolating and purifying intermediates at each stage, a multicomponent reaction stitches three or more building blocks together in one operation. This convergence saves time, energy, solvent, and manpower, which is why such reactions are prized in pharmaceutical and combinatorial chemistry. But traditional versions of these reactions often rely on volatile organic solvents and toxic or costly metal catalysts, undermining their green credentials. The Indian team’s approach sidesteps both problems by using a food-derived catalyst in a largely benign medium.

Lemon juice has been attracting growing attention as a biocatalyst in recent years. It is cheap, widely available, water-soluble, biodegradable, and non-toxic, and its natural acidity—derived primarily from citric acid—allows it to substitute for harmful metallic and non-metallic acid catalysts in a range of organic transformations. In the new study, the researchers found that lemon juice alone in water failed to deliver any product even after 24 hours of reflux, largely because the organic starting materials dissolve poorly in the aqueous juice. Adding ethanol as a co-solvent solved the solubility problem dramatically: with 2.0 milliliters of lemon juice and 1.5 milliliters of ethanol at 90 degrees Celsius, the model reaction between salicylaldehyde, 1,3,5-trimethoxybenzene, and 4-hydroxycoumarin delivered the product 4a in 89 percent yield.

With the optimized conditions in hand, the team explored how broadly the method could be applied. A variety of salicylaldehyde derivatives reacted smoothly with 1,3,5-trimethoxybenzene and 4-hydroxycoumarin, furnishing nine different chromeno[4,3-b]chromene products in 74 to 87 percent yields. Switching the coumarin component for 1,3-dimethylbarbituric acid redirected the chemistry toward chromeno[2,3-d]pyrimidines, seven derivatives of which were obtained in good to excellent yields. The researchers also demonstrated that dibenzylaniline, another strongly electron-rich arene, could serve as the arene component, giving the corresponding products in good yields, including a chromeno[2,3-d]pyrimidine in 78 percent yield.

The substrate scope does have a clear boundary. The reaction tolerates a range of substituents on the salicylaldehyde ring, but it works only with strongly electron-rich arenes. Weaker candidates such as mesitylene, dimethoxybenzene, and methoxynaphthalene showed little to no activity, a limitation that reflects the electrophilic character of the key intermediate in the reaction pathway. Even so, the products obtained represent a class of compounds that had not previously been accessible through a general protocol using direct arene functionalization at the 7 and 5 positions, making the method a genuine addition to the synthetic toolbox.

To understand how the reaction actually works, the team ran a series of control experiments. When salicylaldehyde and 1,3,5-trimethoxybenzene were heated together without the coumarin component, no adduct formed, ruling out a direct coupling pathway. When salicylaldehyde and 4-hydroxycoumarin were combined under the optimized conditions, a condensed product—labeled intermediate B—formed in 66 percent yield within six hours. The researchers isolated this intermediate, confirmed its structure by nuclear magnetic resonance spectroscopy, and then showed that reacting it with 1,3,5-trimethoxybenzene gave the final chromeno[4,3-b]chromene in 91 percent yield. This established that the three-component reaction proceeds through intermediate B rather than through a different sequence.

Based on these observations, the authors proposed a plausible mechanism. The acidity of the lemon juice first promotes a Knoevenagel condensation between the salicylaldehyde and the 4-hydroxycoumarin, generating the electron-poor Michael acceptor B. The electron-rich arene then adds to this acceptor in a Michael-type addition, producing an intermediate containing a tertiary carbon center. Finally, an acid-driven intramolecular ring closure followed by dehydration delivers the fused chromene product. In other words, the citric acid in the juice orchestrates every key step of the cascade, from initial condensation to final cyclization, without any metal assistance.

Practical considerations further strengthen the case for the method. The structures of two representative products were unambiguously confirmed by single-crystal X-ray crystallographic analysis, and all compounds were characterized by proton and carbon NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Crucially, the team demonstrated that the reaction can be scaled up: a 10-millimole-scale run of the model compound proceeded with effectiveness and yields comparable to the small-scale experiments, suggesting genuine potential for commercial and industrial application. Because nearly all products precipitate from the reaction mixture, purification requires only filtration and washing, eliminating extraction steps, column chromatography, and the large volumes of harmful organic solvents they consume.

The significance of the work extends beyond the specific molecules made. Chromene derivatives are distributed widely in natural products, drugs, and edible plants, and their fused hybrids with coumarin or barbituric acid units display a striking range of pharmacological activities, including antitumor, anti-inflammatory, cytotoxic, antioxidant, antithrombotic, and antiplatelet effects, along with useful photophysical properties. A hybrid framework combining chromene with other bioactive analogues may incorporate the characteristics of both and enhance biological activity, which is precisely why synthetic chemists keep seeking better ways to build them. By replacing toxic metals and hazardous solvents with a kitchen staple, the Indian team has shown that the principles of green chemistry—waste prevention, safer solvents, catalysis, and energy efficiency—can be honored without sacrificing yield, scope, or scalability. The study was funded by the Science and Engineering Research Board and the University Grants Commission of India, and it stands as a vivid reminder that sometimes the most sustainable catalyst is sitting in the refrigerator.

Subject of Research: Lemon juice-mediated green synthesis of chromeno[4,3-b]chromene and chromeno[2,3-d]pyrimidine derivatives via one-pot three-component reactions

Article Title: Lemon juice mediated environmentally benign synthesis of functionalized chromeno[4,3-b]chromene/chromeno[2,3‐d]pyrimidine derivatives via one-pot three component reaction

Article References: Lemon juice mediated environmentally benign synthesis of functionalized chromeno[4,3-b]chromene/chromeno[2,3‐d]pyrimidine derivatives via one-pot three component reaction. (n.d.). https://doi.org/10.1007/s44509-026-00001-7

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00001-7

Keywords: lemon juice, green chemistry, multicomponent reaction, chromene, chromenopyrimidine, organocatalysis, salicylaldehyde, 4-hydroxycoumarin, barbituric acid, Knoevenagel condensation, sustainable synthesis, metal-free catalysis

The significance of the work lies in solving a long-standing awkwardness with deep eutectic solvents, or DESs. These mixtures, typically formed by combining a hydrogen bond donor and acceptor such as choline chloride and urea, have surged in popularity because they are inexpensive, low in toxicity, thermally stable, and essentially non-volatile. They offer a tunable, environmentally friendlier alternative to conventional ionic liquids and organic solvents. But as anyone who has tried to stir one into a reaction flask knows, their viscous, homogeneous nature makes separating them from products and recycling them a genuine headache. That limitation has held DESs back from routine use as catalysts at scale.

Immobilizing a DES on a solid support converts a sticky liquid into a handleable solid, which is precisely the strategy Sharifirad and Khodaei pursued. Earlier efforts in this direction have attached DESs to silica frameworks, magnetic particles, and metal-organic frameworks, with the researchers’ own group previously reporting DES hybrids with UiO-66 and magnetic pectin supports. The new work pushes the concept further by choosing a porous organic polymer as the substrate, specifically a triazine-containing porous polyamide. The polymer’s amide groups act as hydrogen-bond acceptors that capture choline chloride in situ, forming the deep eutectic phase directly on the polymer surface rather than coating it on afterward.

The choice of support is not arbitrary. Porous organic polymers, or POPs, are a class of amorphous or semi-crystalline frameworks distinguished by permanent porosity, high surface areas, and remarkable chemical and thermal robustness. Triazine-based variants are particularly attractive because the electron-poor triazine rings serve as Lewis basic sites that can activate substrates, while the rigid aromatic skeleton resists degradation. The authors emphasize that such frameworks facilitate effective mass transfer, letting reactants diffuse deep into the pores where the catalytic sites reside. In ChCl@TPA, the immobilized choline chloride hydrogen-bond network and the triazine moieties work in concert, creating a bifunctional catalyst in which each component compensates for the other’s limitations.

The researchers characterized the material with an extensive battery of techniques. Fourier transform infrared spectroscopy confirmed the hydrogen-bonding interaction between choline chloride and the amide groups, while field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy mapped the morphology and elemental distribution. Powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis probed the framework’s structure and thermal resilience, and Brunauer-Emmett-Teller plus Barrett-Joyner-Halenda measurements quantified the porosity that underpins the catalyst’s accessibility. Together these analyses established that the DES had genuinely integrated into the polymer rather than simply decorating it, and that the composite retained its structural integrity through repeated catalytic runs.

The first reaction target was the Biginelli reaction, a classic three-component condensation of an aldehyde, a beta-ketoester, and urea that yields 3,4-dihydropyrimidinones, heterocycles whose skeletons appear in calcium channel blockers and a range of drug leads. Traditional Biginelli protocols often lean on Lewis acids such as indium(III) bromide or iron(III) tosylate, which raise cost, toxicity, and waste concerns. ChCl@TPA promoted the reaction efficiently under mild, green conditions, and the authors attribute its performance to a synergistic push-pull mechanism: the hydrogen-bond network polarizes the carbonyl partners while the triazine basic sites assist in deprotonation steps, accelerating the sequence of iminium formation, Knoevenagel-type condensation, and cyclization.

The second target showcased the catalyst’s range. Tetrazoles, five-membered rings containing four nitrogens, are celebrated as bioisosteres of carboxylic acids and appear in blockbuster drugs as well as in energetic materials. The team used ChCl@TPA to prepare 5-substituted-1H-tetrazoles, building on the researchers’ earlier experience with tetrazole synthesis using other recoverable solid catalysts. Because azide chemistry can be hazardous and metal catalysts can complicate purification, a metal-free heterogeneous system that operates under gentle conditions is a meaningful addition to the toolbox. The hydrogen-bond-rich microenvironment inside the polymer pores appears to facilitate the 1,3-dipolar cycloaddition between nitriles and azide that forges the tetrazole ring.

Recyclability, often the Achilles heel of immobilized catalysts, emerged as a highlight. The solid could be recovered and reused over multiple cycles with excellent retention of activity, and post-reaction characterization confirmed that both the hydrogen-bonded DES and the polyamide framework survived intact. This durability matters practically: in an industrial setting, a catalyst that loses performance after a handful of runs erases the environmental gains it promised. The metal-free composition also means no risk of metal contamination in pharmaceutical intermediates, a compliance headache that plagues many heterogeneous alternatives.

The work fits into a broader movement to replace metal catalysts with engineered organic frameworks. Reviews of POP-based organocatalysis note a proliferation of nitrogen-rich, sulfonated, and ionic-liquid-modified polymers for applications spanning carbon dioxide conversion, biomass transformation, and fine chemical synthesis. What distinguishes ChCl@TPA is the combination of two green chemistry motifs, a DES phase and a porous polyamide, into a single synergistic material, effectively giving a liquid solvent the recoverability of a solid without altering its fundamental chemistry.

Funded and supported by Razi University, the study points toward a future where catalysts are designed less around precious metals and more around engineered non-covalent interactions. If immobilized DESs can match or outperform their metallic counterparts across a widening set of reactions, the economics of green synthesis could shift substantially. For now, Sharifirad and Khodaei’s hydrogen-bonded hybrid stands as a demonstration that the gentlest force in chemistry, the hydrogen bond, can be harnessed to hold a catalyst together, keep it working, and let chemists simply wash and reuse it.

Subject of Research: A metal-free heterogeneous catalyst formed by immobilizing a choline chloride deep eutectic solvent on a triazine-based porous polyamide for green synthesis of dihydropyrimidinones and tetrazoles

Article Title: Engineering of Porous Polyamide-Supported Deep Eutectic Solvent as a Metal-Free Heterogeneous Catalyst for the Synthesis of 3,4-Dihydropyrimidinones and 5-Substituted-1H-Tetrazoles

Article References: Sharifirad, F., & Khodaei, M. M. (2026). Engineering of Porous Polyamide-Supported Deep Eutectic Solvent as a Metal-Free Heterogeneous Catalyst for the Synthesis of 3,4-Dihydropyrimidinones and 5-Substituted-1H-Tetrazoles. Catalysis Letters, 156(10), Article 284. https://doi.org/10.1007/s10562-026-05529-8

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05529-8

Keywords: deep eutectic solvents, porous organic polymers, triazine-based polyamide, choline chloride, heterogeneous catalysis, metal-free catalyst, Biginelli reaction, dihydropyrimidinones, tetrazoles, green chemistry, catalyst recyclability, hydrogen bonding