Synthesis and Structure–Activity Relationship Studies of Novel Schiff Base Derivatives and Their Biological Applications
Keywords:
Schiff base; 4-aminoantipyrine; azomethine linkage; structure–activity relationship; antimicrobial activity; substituent effectsAbstract
A series of twelve novel Schiff base derivatives (SBD-1 to SBD-12) was designed, synthesised and evaluated in order to establish a systematic relationship between aryl substituent character and biological potency. The derivatives were prepared by acid-catalysed condensation of 4-aminoantipyrine with twelve structurally and electronically distinct aromatic aldehydes spanning strongly electron-donating, moderately electron-donating, unsubstituted, halogenated and strongly electron-withdrawing classes. All twelve compounds were obtained in synthetically useful isolated yields ranging from 74.3% to 92.1%, with a series mean of 84.9%, and their structures were unambiguously confirmed by ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, proton and carbon-13 nuclear magnetic resonance spectroscopy, mass spectrometry and elemental microanalysis. Diagnostic azomethine ν(C=N) absorption appeared consistently in the 1602–1622 cm⁻¹ region with concurrent disappearance of the aldehydic ν(C=O) and amine ν(N–H) bands, while the azomethine proton resonated as a sharp singlet between δ 8.28 and δ 8.92 ppm. The derivatives were screened in vitro against four bacterial strains (Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa) and two fungal strains (Candida albicans and Aspergillus niger). The 2,4-dichloro-substituted derivative SBD-10 emerged as the most potent member of the series, producing a 23 mm zone of inhibition and a minimum inhibitory concentration of 7.81 µg/mL against S. aureus, approaching the potency of ciprofloxacin. Activity declined monotonically with increasing electron-donating character, the dimethylamino derivative SBD-4 being the least active. Structure–activity analysis indicated that potency is governed jointly by substituent-induced lipophilicity and by the electrophilicity of the azomethine carbon, with the ortho-hydroxyl derivative SBD-5 behaving anomalously due to intramolecular hydrogen bonding. The findings identify SBD-10 and SBD-5 as promising lead scaffolds for further optimisation.
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