Halogenated Azabicyclo Hydrazones Show Strong Antimicrobial Activity in Lab Tests

A new family of fluorinated azabicyclo–benzohydrazone compounds has been synthesized and screened against a panel of pathogenic bacteria and fungi, with the halogen-bearing members of the series outperforming standard drugs in several laboratory assays. The study, which combined experimental chemistry with computational modeling, produced five hybrid molecules built on a rigid 3-azabicyclo[3.3.1]nonan-9-one core linked through a hydrazone bond to a 4-fluorobenzohydrazide pharmacophore. The work offers a preliminary but systematic look at how aryl substituents tune the biological activity of this scaffold, which the authors describe as a privileged framework in medicinal chemistry because of its occurrence in biologically significant natural alkaloids such as hetisine-type diterpenoids.

The synthetic route was deliberately short. In the first step, a one-pot Mannich-type cyclocondensation of cyclohexanone, substituted benzaldehydes, and methylammonium formate in ethanol, gently heated overnight, delivered the bicyclic ketone intermediates 5(a–e), which were purified by filtration, washing with an ethanol–ether mixture, and recrystallization from ethanol and chloroform. In the second step, these ketone intermediates were condensed with 4-fluorobenzohydrazide in a 1:1 methanol–chloroform mixture under catalytic acetic acid at reflux for four hours, forming the defining hydrazone C=N bond.

The five products were isolated in yields ranging from 52 to 68 percent after simple recrystallization, with no chromatographic purification required. The series was designed so that the only variable was the aryl substituent: hydrogen, para-methoxy, para-methyl, meta-bromo, and para-fluoro groups were placed on the rings at the 2- and 4-positions of the bicyclic core. Structural confirmation relied on standard spectroscopic tools.

Infrared spectra showed N–H stretching bands between 3202 and 3334 cm⁻¹, aliphatic and aromatic C–H stretching between 2845 and 3067 cm⁻¹, and hydrazone C=N absorptions between 1604 and 1655 cm⁻¹, confirming hydrazone formation. Proton NMR spectra displayed amide NH singlets between δ 7.96 and 11.99 ppm and aromatic multiplets between δ 7.22 and 7.79 ppm, while carbon NMR data placed the imine carbonyl between 165.8 and 168.1 ppm and the amide carbonyl between 162.6 and 163.4 ppm. Elemental analysis matched the calculated values for each formula, and melting points between 200 and 214 °C supported the purity of the isolated white crystalline solids.

The authors note that the route tolerated both electron-donating and electron-withdrawing substituents without significant side-product formation. Antimicrobial testing used a broth microdilution format against five bacterial strains—two Gram-positive, Bacillus subtilis and Staphylococcus aureus, and three Gram-negative, Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa—and four fungal isolates comprising Aspergillus flavus, Aspergillus niger, Candida albicans, and a Candida species. The halogenated derivatives 6d and 6e, bearing meta-bromo and para-fluoro groups respectively, delivered the strongest results.

Against the fungal panel, 6d and 6e reached MIC values of 6.25 to 12.5 µg/mL across the Aspergillus and Candida isolates, a two- to four-fold improvement compared with the fluconazole reference. The unsubstituted and electron-rich analogues were markedly weaker, with MIC values frequently in the 100 to 200 µg/mL range. The activity pattern points to a clear substituent effect.

The authors attribute the halogen advantage to improved lipophilicity, better penetration through microbial membranes, and stronger dipole or hydrogen-bond-like interactions with target enzymes, consistent with known trends for halogenated heterocyclic antimicrobials. Electron-donating methoxy and methyl groups, by contrast, appear to reduce electrophilicity at the pharmacophore and may partially obstruct binding. Because the MIC values come from a two-fold dilution series spanning 200 to 3.12 µg/mL and a small compound set, the authors frame these comparisons as fold-change observations relative to the standard drugs rather than statistically validated rankings, noting that MIC is an ordinal endpoint conventionally compared descriptively.

Antioxidant behavior followed the opposite electronic trend. In DPPH radical-scavenging, metal-chelation, and lipid-peroxidation assays, the electron-rich methoxy and methyl derivatives 6b and 6c were the most active, with DPPH IC₅₀ values of roughly 2.05 µg/mL, close to the 1.97 µg/mL measured for the α-tocopherol control. The bromo and fluoro analogues were the weakest scavengers, an outcome the authors link to the electron-withdrawing character of halogens, which depletes aromatic electron density and hinders the proton donation needed to stabilize radicals.

All five compounds showed concentration-dependent activity across the tested range of 30 to 120 µg/mL, and the assays were run in triplicate with one-way analysis of variance followed by Duncan's multiple range test confirming significant differences at p ≤ 0.05. On the computational side, density functional theory calculations on the unsubstituted reference compound 6a, performed at the B3LYP level with the 6-311G basis set in the gas phase, mapped the electronic landscape of the scaffold. The highest occupied molecular orbital sits mainly on the piperidine ring and its adjacent substituents while the lowest unoccupied orbital localizes on the fluorine-substituted benzene ring, and the orbital gap of 4.5334 eV indicates a relatively hard, electronically stable molecule, with computed chemical hardness of 2.2667 eV and softness of 0.4412 eV.

Electrostatic potential maps highlighted the carbonyl oxygen and hydrazide N–H region as the most electron-rich sites, marking them as the likely points of electrophilic attack or contact with biological targets. Mulliken population analysis confirmed strong negative charges on the three nitrogen atoms, the carbonyl oxygen, and the fluorine substituent, with the piperidine ring nitrogen 4N carrying the most electron-rich value of −0.6090 atomic units. Molecular docking against enoyl-acyl carrier protein reductase from Mycobacterium tuberculosis, a validated enzyme in the fatty acid synthase-II pathway responsible for mycolic acid biosynthesis, suggested that the compounds can form stable contacts within the enzyme active site, with the reference compound recording a GLIDE-XP docking score of −6.38, van der Waals interaction energy of −41.34, and total binding energy of −47.42, contacting SER20 and PHE97.

The authors are careful to note that this target was chosen for its mechanistic relevance and that the experimental antimicrobial assays used non-tubercular organisms, so the docking results indicate a possible additional anti-tubercular application that remains to be experimentally verified rather than an explanation of the measured activity. Predicted pharmacokinetic profiles were broadly acceptable: most members satisfied Lipinski's criteria with at most one violation, showed high predicted gastrointestinal absorption, topological polar surface areas of 53.49 to 71.95 Ų, and no flagged toxicity risks in the Osiris panel, although the bulky bromo analogue 6d fell short of two drug-likeness thresholds and scored lowest on predicted overall developability with a bioavailability score of 0.17. The study's own conclusions emphasize that the series is small and the structure–activity analysis preliminary.

The strongest antimicrobial candidates are the halogenated derivatives 6d and 6e, while the antioxidant and predicted developability profiles favor the methoxy and methyl analogues 6b and 6c, a divergence the authors suggest could guide separate optimization paths. They identify in vivo efficacy and toxicity testing, enzyme inhibition and binding assays, and expansion of the substituent set to enable more robust quantitative structure–activity analysis as the necessary next steps before any of these hybrids can be considered genuine lead compounds. For now, the work stands as an early map of a scaffold that combines a rigid bicyclic core, a fluorinated hydrazide arm, and tunable aryl electronics—a combination the authors argue is worth pursuing further in the search for new antimicrobial and antioxidant agents.

Subject of Research: Synthesis and biological evaluation of fluorinated azabicyclo–benzohydrazone hybrid compounds

Article Title: Design, synthesis, antioxidant and antimicrobial activities of novel fluorinated azabicyclo–benzohydrazone hybrids supported by DFT, molecular docking, ADME, and PETRA-Osiris analyses

Article References: Dar, T. A., Balasankar, T., Fahelelbom, K. M., Dar, M. A., Gundeboiyna, S., Ahmed, S., Bhat, A. R., Tebeje, B. A., & Tataringa, G. (2026). Design, synthesis, antioxidant and antimicrobial activities of novel fluorinated azabicyclo–benzohydrazone hybrids supported by DFT, molecular docking, ADME, and PETRA-Osiris analyses. Results in Chemistry, 31, Article 103964. https://doi.org/10.1016/j.rechem.2026.103964

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103964

Keywords: medicinal chemistry, azabicyclononane, hydrazone, antimicrobial, antioxidant, halogenation, DFT, molecular docking, ADME, drug discovery, structure–activity relationship, fluorine