Green Microwave Chemistry Yields Promising New Antitubercular Compounds

Tuberculosis remains one of the deadliest infectious diseases on the planet, and a team of chemists in India now reports a greener, faster way to build molecules that could help fight it. In a study published in Discover Chemistry, researchers led by Santosh S. Pohare and Bhaskar S. Dawane of Swami Ramanand Teerth Marathwada University synthesized a family of fused pyrano-thiazole-2-one derivatives using an environmentally benign one-pot protocol, then screened the new compounds for activity against mycobacteria. Their results point to a thiophene-bearing molecule, designated compound 4b, as the most potent of the series, inhibiting mycobacterial growth at a concentration of just 6.19 micrograms per milliliter in a standard susceptibility assay.

The public health context behind the work is stark. According to the World Health Organization’s 2025 reporting cited by the authors, more than 10.7 million people are infected with tuberculosis and roughly 1.23 million die from it every year. In developing countries such as India, tuberculosis ranks among the ten leading causes of death, and the country’s Sample Registration System cause-of-death report for 2021 through 2023 records that 3.3 percent of fatalities in the 15 to 29 age group were attributable to the disease. Because the burden falls heavily on young adults, tuberculosis carries socioeconomic consequences that extend well beyond individual illness, striking at a vital segment of the workforce and underscoring the urgent need for new antitubercular drugs.

The chemical scaffolds at the heart of the study are hybrids of two biologically privileged heterocycles. Pyrans, six-membered oxygen-containing rings, appear in molecules with antitubercular, anticancer, antiviral, antibacterial, antimicrobial, and anti-inflammatory activity. Thiazoles, five-membered rings containing both nitrogen and sulfur, are similarly versatile, showing up in drugs for cancer, malaria, fungal infections, diabetes, inflammation, viral disease, and neurological disorders. Fusing the two motifs into a single pyrano[2,3-d]thiazole framework, the researchers reasoned, could produce compounds with meaningful antimycobacterial potential, and they set out to build a library of twelve such derivatives, labeled 4a through 4l, each differing in the aromatic group attached at the 7-position of the core.

Traditional routes to heterocyclic compounds of this kind suffer from well-known drawbacks: poor yields, long reaction times, expensive reagents, chemical waste, and difficult purification. To sidestep these problems, the team turned to multicomponent reactions, in which two or more reactants combine in a single vessel to deliver the product in one step, and supercharged the approach with microwave irradiation. Their recipe combines equimolar amounts of a substituted aromatic aldehyde, malononitrile, and thiazolidine-2,4-dione in polyethylene glycol 400 as solvent, with bleaching earth clay, a non-toxic and recyclable heterogeneous catalyst, added at 10 mole percent. One hour in a microwave oven at 210 watts delivered the fused products in yields of 86 to 92 percent.

The optimization experiments reveal why the chosen conditions matter. With no solvent or catalyst at all, the reaction simply did not proceed. Water, though attractive on green chemistry grounds, dissolved only two of the three reactants, leaving the aromatic aldehydes insoluble and the reaction incomplete. Conventional solvents such as ethanol, methanol, ethylene glycol, and glycerol gave product only in the presence of the clay catalyst, and even then required more than two hours. Polyethylene glycol 400 proved decisively superior. The authors attribute this to its high loss tangent, which couples efficiently with microwave radiation to produce rapid internal heating, and to its polyether chain, which creates a pseudo-crown-ether environment that stabilizes the transition states of the multicomponent reaction more effectively than simpler hydroxyl solvents. Its low vapor pressure also permits safe high-temperature microwave operation without significant pressure buildup, an advantage for scalability.

The proposed mechanism begins when the basic active sites on the clay surface deprotonate malononitrile, generating a resonance-stabilized dicyanomethanide anion. That anion attacks the substituted aromatic aldehyde to form a benzylidene malononitrile intermediate. The catalyst then activates thiazolidine-2,4-dione, converting it into a reactive anion that attacks the benzylidene intermediate nucleophilically. The resulting adduct undergoes intramolecular cyclization and tautomerization to build the fused pyrano-thiazole ring, and a final oxidative dehydrogenation step delivers the aromatic carbonitrile products. The products were purified by simple recrystallization from ethanol, and both the solvent and the catalyst could be recovered and reused, with the solvent sustaining four reaction cycles while yields slipped only from 94 to 90 percent.

Structural confirmation relied on infrared spectroscopy, proton and carbon-13 nuclear magnetic resonance, and mass spectrometry, with characteristic signals such as the amino stretch near 3376 inverse centimeters and the nitrile stretch near 2210 inverse centimeters matching the expected functional groups. With the library in hand, the team evaluated antitubercular activity against Mycobacterium smegmatis, a fast-growing, non-pathogenic risk group 2 organism used as a safe surrogate for Mycobacterium tuberculosis, which is classified as risk group 3. Using the Microplate Alamar Blue Assay, a resazurin-based viability test in which a blue color indicates no bacterial growth and a pink color indicates growth, the researchers tested concentrations from 6.25 to 100 micrograms per milliliter, with the first-line drug isoniazid serving as the positive control.

The screening results traced a clear structure-activity relationship. Compound 4b, carrying a thiophene ring at the 7-position, matched isoniazid’s sensitivity profile across nearly the whole concentration range and recorded the lowest minimum inhibitory concentration of the series at 6.19 micrograms per milliliter. The pyrazole analogue 4c, along with compounds 4g and 4h bearing methoxy and hydroxy substituents on the phenyl ring, showed sensitivity down to 12.5 micrograms per milliliter and MIC values of 11.01, 12.19, and 11.93 micrograms per milliliter respectively. Electron-donating groups at the meta and para positions of the phenyl ring consistently outperformed halogenated derivatives, while the electron-withdrawing nitro group fared worst. Compounds 4f and 4k were the weakest performers, with MIC values near 48 micrograms per milliliter.

To probe how the most active molecules might work at the molecular level, the team docked compounds 4b, 4c, 4g, and 4h into the crystal structure of enoyl-acyl carrier protein reductase, the enzyme known as InhA, using the Protein Data Bank entry 2H7M. InhA sits at the heart of the type II fatty acid synthesis pathway that builds mycolic acids, essential components of the mycobacterial cell wall, and it is the clinically validated target of isoniazid itself. Compound 4g posted the best docking score at minus 8.5 kilocalories per mole, forming pi-sulfur interactions with PHE149 and MET161 and alkyl contacts with MET199, LEU218, and ILE215. Compound 4b scored minus 7.5 kilocalories per mole with a similar interaction network, compound 4c scored minus 7.0, and compound 4h scored minus 4.0 while forming a conventional hydrogen bond with LYS165. These binding patterns support the idea that the compounds can occupy the InhA active site and potentially disrupt cell wall biosynthesis.

The authors present the protocol as a template for sustainable medicinal chemistry: a rapid, one-pot, microwave-assisted multicomponent reaction that cuts solvent waste, recycles both catalyst and medium, shortens reaction times from hours to sixty minutes, and still delivers high yields. Beyond the synthetic achievement, the biological findings sharpen the design rules for future analogues, favoring heteroaromatic substituents such as thiophene at the 7-position and electron-donating groups on pendant phenyl rings. While the activity was measured against a non-pathogenic surrogate strain rather than Mycobacterium tuberculosis itself, and docking evidence is computational rather than direct, the convergence of strong assay performance and favorable predicted binding to a validated drug target makes these fused pyrano-thiazole derivatives a promising starting point for the next round of antitubercular drug discovery.

Subject of Research: Green synthesis and antitubercular evaluation of fused pyrano-thiazole-2-one derivatives

Article Title: Antitubercular evaluation and synthesis of fused pyrano-thiazole-2-one derivatives via green and efficient protocol

Article References: Pohare, S. S., Mogle, P. P., Gaikwad, M. V., Zangade, S. B., & Dawane, B. S. (2026). Antitubercular evaluation and synthesis of fused pyrano-thiazole-2-one derivatives via green and efficient protocol. Discover Chemistry, 3(1), Article 460. https://doi.org/10.1007/s44371-026-00914-z

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00914-z

Keywords: tuberculosis, pyrano-thiazole, green chemistry, microwave-assisted synthesis, multicomponent reaction, PEG-400, bleaching earth clay, Mycobacterium smegmatis, molecular docking, InhA enzyme, heterocyclic compounds, drug discovery