The molecules at the heart of the study belong to a class known as [1]benzopyrano[b]dioxolo[g]quinolines, a mouthful of a name that describes a structure fusing three biologically privileged frameworks: a benzopyran (also called a chromene), a dioxole ring, and a quinoline. Benzopyrans are among the most ubiquitous scaffolds in medicinal chemistry. They appear in compounds with antibacterial, antiviral, antitumor and antiproliferative activities, and they show up in natural products such as Callistenone A, an antibacterial agent, and Urolithin M5, an antioxidant from the leaves of Canarium album. Because these frameworks keep proving their worth in drug discovery, chemists are constantly searching for faster and cleaner ways to build new derivatives.

The team’s strategy was a three-component condensation. They combined 4-hydroxycoumarin, a range of aromatic aldehydes, and 3,4-(methylenedioxy)aniline in water, then bombarded the mixture with ultrasonic irradiation at 80 watts while a trace amount of a Cu/ZnO@GO nanocomposite did the catalytic heavy lifting. The result: eight novel compounds, designated 4a through 4h, isolated in yields ranging from 93 to 97 percent. For comparison, running the same reaction in water without any catalyst, even under reflux for a grueling 300 minutes, produced only a 20 percent yield. Ultrasound alone, without the catalyst, managed just 48 percent in fifteen minutes. The synergy between the sound waves and the nanomaterial proved essential.

Ultrasound accelerates chemical reactions through a phenomenon called acoustic cavitation. As sound waves pass through a liquid, they generate microscopic bubbles that rapidly grow and collapse, producing intense local heating, high pressures and powerful shock waves. These transient hot spots dramatically enhance mass transfer and can create reactive surfaces on solid catalysts, all while the bulk solution remains at room temperature. In this study, the reaction proceeded at a comfortable 25 degrees Celsius, meaning the energy input is localized at the nanoscale rather than wasted heating an entire flask. That combination of ambient conditions and dramatically shortened reaction times is precisely what green chemistry advocates have been demanding for decades.

The catalyst itself is as much a story as the reaction it drives. Cu/ZnO@GO is a composite in which copper-doped zinc oxide nanoparticles are anchored to sheets of graphene oxide. Graphene-based supports are prized in catalysis for their enormous surface area, chemical and thermal stability, and low cost, providing an ideal platform on which to disperse catalytically active metal particles. The researchers prepared the Cu/ZnO nanoparticles using an aqueous extract of Petasites hybridus, commonly known as butterbur, whose rhizome was dried, powdered and steeped in boiling deionized water. The plant extract acted as both a reducing and a stabilizing agent, steering the formation of the nanoparticles without any synthetic capping chemicals. Zinc acetate and copper chloride were added to the extract, heated to 200 degrees Celsius, and the resulting nanoparticles were then dispersed with graphene oxide in a second round of plant-extract-assisted growth.

Characterization confirmed the composite was exactly what the team intended. Fourier-transform infrared spectroscopy revealed the characteristic Zn–O and Cu–O stretching vibrations, while X-ray diffraction showed the hexagonal wurtzite phase of ZnO alongside the expected copper reflections, with copper incorporation visibly damping the intensity of the primary ZnO planes. Scanning and transmission electron microscopy showed spherical nanoparticles effectively anchored to the graphene oxide support, and energy-dispersive X-ray analysis verified the elemental composition. It is a textbook example of a well-built heterogeneous catalyst: robust, well-defined, and ready to work in water.

Optimization experiments, centered on a model reaction producing the 4-bromophenyl derivative 4b, systematically varied the catalyst loading, solvent and energy source. Just 0.05 grams of Cu/ZnO@GO in 3 milliliters of water, under 80-watt sonication for fifteen minutes, delivered the maximum 97 percent yield. Conventional solvents such as ethanol, dichloromethane, acetonitrile and dimethylformamide all underperformed relative to water. Other conventional catalysts were tested and consistently fell short of the nanocomposite. Equally important, the catalyst proved durable: after each reaction it was recovered by centrifugation, washed with ethanol, dried, and reused, retaining its activity for at least four consecutive cycles without significant loss of performance. That recyclability directly reduces waste and cost, two of the biggest obstacles to sustainable industrial chemistry.

The proposed mechanism follows a well-established cascade. The acidic sites on the nanocatalyst activate the carbonyl group of the aromatic aldehyde, which then undergoes a Knoevenagel condensation with the methylenedioxyaniline to form an alkene intermediate. 4-Hydroxycoumarin adds to this alkene in a Michael-type addition, and the resulting intermediate cyclizes, dehydrates and aromatizes to deliver the fused quinoline product. The method worked smoothly across aldehydes bearing both electron-donating groups, such as methoxy, hydroxy and methyl substituents, and electron-withdrawing groups, including bromo, chloro, cyano and nitro variants. One limitation emerged: aliphatic aldehydes gave poor yields, apparently because their low boiling points cause them to vaporize under sonication.

What elevates the study beyond synthetic methodology is the biological evaluation. The team subjected four of the new compounds to the DPPH free radical scavenging assay, a standard test in which a compound’s ability to neutralize a stable purple radical is measured spectrophotometrically at 517 nanometers, with the industrial antioxidants butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ) as benchmarks. The activity ranking was TBHQ, followed by BHT roughly tied with compound 4e, then 4a, 4c and 4h. Compound 4e, which carries a hydroxyl group on its aryl ring, emerged as the standout performer, approaching the activity of the commercial standards at a concentration of 1000 parts per million. The finding matters because oxidative stress is implicated in diseases ranging from diabetes and cancer to atherosclerosis and Alzheimer’s, fueling the search for new antioxidant scaffolds.

The broader significance of the work lies in its demonstration that sustainable chemistry need not sacrifice speed or efficiency. Water replaces toxic organic solvents. Ultrasound replaces hours of energy-intensive reflux. A plant extract replaces synthetic reagents in catalyst preparation. A recyclable nanocomposite replaces stoichiometric activators. And the products themselves are not merely laboratory curiosities but candidates for antioxidant drug development. As the authors note, the approach represents a meaningful advance for green chemistry, and it offers a template that other laboratories can adapt: pair a well-designed nanocatalyst with acoustic energy and an aqueous medium, and even multi-component reactions that once demanded harsh conditions can be coaxed to completion in minutes, at room temperature, with yields that would have seemed implausible a generation ago.

Subject of Research: Ultrasound-assisted green synthesis of benzopyranoquinoline derivatives with antioxidant activity using a Cu/ZnO@GO nanocatalyst

Article Title: Ultrasound assisted aqua-mediated synthesis of novel [1]benzopyrano[b]dioxolo[g]quinolines with promising antioxidant activities using Cu/ZnO@GO nanocatalyst

Article References: Abdolmohammadi, S., Azadi, S., Imanov, H., Al Omari, R. H., Pal, A., & Vessally, E. (2026). Ultrasound assisted aqua-mediated synthesis of novel [1]benzopyrano[b]dioxolo[g]quinolines with promising antioxidant activities using Cu/ZnO@GO nanocatalyst. Journal of Saudi Chemical Society, 30(2), Article 16. https://doi.org/10.1007/s44442-026-00061-1

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00061-1

Keywords: green chemistry, ultrasound synthesis, nanocatalyst, Cu/ZnO@GO, graphene oxide, quinolines, benzopyran, antioxidant, DPPH assay, aqueous media, Petasites hybridus, heterogeneous catalysis

The catalyst itself is a layered particle with a magnetic iron oxide core wrapped in a silica shell, functionalized with an amino-silane compound called APTES and topped with sulfonic acid groups. This architecture gives the material two useful qualities at once: reactive acidic sites on the surface that can drive organic reactions, and a magnetic core that lets the particles be pulled out of a reaction mixture with a simple magnet rather than by filtration. The team built the material in four steps, starting with iron chloride dissolved in the butterbur extract, then layering on silica, APTES, and finally the sulfonic acid groups through a reaction with 1,4-butane sultone.

Once the catalyst was in hand, the researchers put it to work in a four-component reaction combining α,β-unsaturated compounds, guanidine, activated acetylenic compounds, and methyl iodide. The products are methyliminopyridopyrimidines, a class of fused nitrogen-containing ring systems that sit at the heart of many drug candidates. Pyridopyrimidine frameworks are known to display antibacterial, antiviral, antitumor, antimalarial, and anti-inflammatory properties, making them attractive targets for medicinal chemists. The reaction ran in plain water at room temperature and finished within roughly two to three hours, producing yields between 78 and 92 percent across the series of compounds the team made.

The choice of water as the solvent is more than a nod to environmental fashion. Water can actively accelerate organic reactions through hydrogen bonding at the interface between the aqueous phase and organic reactants, a phenomenon recognized since K. Barry Sharpless’s work on on-water chemistry in 2005. In this case, the reaction performed poorly without a catalyst, and heating the mixture to 100 degrees Celsius offered no improvement over room temperature. Water, it turned out, was the best solvent among those tested, and the catalyst at a loading of just 0.02 grams per reaction delivered the highest yields.

Characterization of the new catalyst was thorough. X-ray diffraction confirmed the cubic crystal structure of the magnetite core with no damage from the coating process, and the average crystallite size came out at about 32.5 nanometers. Electron microscopy showed spherical, uniform core-shell particles, while vibrating sample magnetometry revealed superparamagnetic behavior. The saturation magnetization dropped from 76.4 emu per gram for bare iron oxide to 35.5 emu per gram after coating, but that remained plenty strong for magnetic separation from solution. Energy-dispersive X-ray spectroscopy and elemental mapping confirmed the expected distribution of iron, oxygen, silicon, carbon, nitrogen, and sulfur throughout the material.

The catalyst also proved durable. Thermogravimetric analysis showed that the organic surface groups survive up to roughly 500 degrees Celsius, and hot filtration tests indicated that the catalyst does not leach into solution during reactions, meaning the material is genuinely heterogeneous rather than dissolving and acting from the liquid phase. After each run the particles were collected with a magnet, washed, dried, and reused, with considerable retention of catalytic activity across cycles.

The mechanism behind the catalysis involves cooperative action between Lewis acid and Lewis base sites on the particle surface. Silicon, iron, and the sulfonic acid protons bind to the oxygen atoms of carbonyl groups in the starting materials, increasing the electrophilicity of the carbonyl carbon and making it more vulnerable to nucleophilic attack. Meanwhile, oxygen and nitrogen sites on the surface act as bases, stripping protons from the reactants to form key intermediates. Guanidine first reacts with the unsaturated compound, then the acetylenic component joins in, and finally an intramolecular cyclization closes the ring to form the pyridopyrimidine product.

Having built the molecules, the researchers turned to biology. They tested four of the new compounds for antioxidant activity using the DPPH radical scavenging assay, a standard method that measures a compound’s ability to neutralize a stable free radical. Compound 5c stood out with an IC50 value of 12.56 micrograms per milliliter, meaning it took only that concentration to inhibit half of the DPPH radicals. That compares with 9.26 for the industrial antioxidant BHT and 8.75 for TBHQ, placing the synthetic compound in genuinely competitive territory. A second assay, in which the compound’s ability to reduce ferric iron was measured spectrophotometrically, produced a similar ranking, with TBHQ leading, followed by BHT, then compound 5c.

Antibacterial testing followed using the disk diffusion method against both Gram-positive and Gram-negative bacterial strains, with streptomycin and gentamicin as clinical benchmarks. Six of the compounds, designated 5b, 5c, 5e, 5f, 5i, and 5j, showed strong activity against both categories of bacteria. The largest zone of inhibition was observed against Escherichia coli, a Gram-negative organism whose outer membrane typically makes it harder to kill than Gram-positive species. That a synthetic heterocycle built in water at room temperature can match broad-spectrum antibacterial patterns is the kind of result that catches the attention of drug discovery teams.

The catalyst had one more trick to demonstrate. The team tested its ability to reduce 4-nitrophenol, a toxic water pollutant produced by the dye, pesticide, and pharmaceutical industries, into 4-aminophenol, a far less harmful compound and a useful industrial intermediate. In the presence of sodium borohydride, the catalyst drove the reduction rapidly, with the yellow color of the nitrophenol solution fading to colorless within minutes, confirmed by the disappearance of the 400-nanometer absorbance peak and the appearance of a new peak near 300 nanometers. Without the catalyst, the borohydride alone made no meaningful progress even after 15 hours. Given that global dye and pigment production runs at roughly 700,000 tons per year and that most of these compounds harm aquatic life, a cheap, magnetic, plant-built catalyst that clears such pollutants from water in seconds-to-minutes timescales represents a practical piece of green chemistry with potential well beyond the laboratory bench.

Subject of Research: Green synthesis of iminopyridopyrimidine compounds using a bio-derived magnetic nanocatalyst

Article Title: Green synthesis and evaluation of biological activity of novel iminopyridopyrimidine employing APTES@SiO2@Fe3O4-SO3H as nanocatalyst

Article References: Shirangi, H. S., Hojjati, M., Noorzaei, M., & Rostami-Charati, F. (2026). Green synthesis and evaluation of biological activity of novel iminopyridopyrimidine employing APTES@SiO2@Fe3O4-SO3H as nanocatalyst. Journal of Saudi Chemical Society, 30(4), Article 51. https://doi.org/10.1007/s44442-026-00079-5

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00079-5

Keywords: green chemistry, nanocatalyst, magnetite nanoparticles, pyridopyrimidine, Petasites hybridus, antioxidant, antibacterial, multicomponent reaction, 4-nitrophenol reduction, water as solvent, APTES, sulfonic acid