New 3-(6-Bromo-2-oxo-1,3-benzoxazol-3(2H)-yl)propanoic Acid Derivatives: Synthesis and Biological Activity Against Bacterial Pathogens

Continuing our work in the field of synthesis and research of amino acids, their derivatives, and cyclization products, in this work, we synthesized various 3-(6-bromo-2-oxo-1,3-benzoxazol-3(2H)-yl)propanoic acid derivatives and investigated their antimicrobial activity. A total of eighteen synthesized chemical compounds (No. 1–18), including several structural analogues (e.g., 3a, 3b, 4a–4e, 8a–8m, 9a–9d), were evaluated for their antibacterial properties. The antibacterial activity was assessed using the Kirby–Bauer disk diffusion method, and inhibition zone diameters (mm) were measured against five representative bacterial strains: S. aureus, MRSA, B. subtilis, E. coli, and P. aeruginosa. The minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of the most active synthesized compounds were determined against representative Gram-positive and Gram-negative bacterial strains, including S. aureus, MRSA, B. subtilis, and E. coli. Overall, these results indicate that the tested compounds display selective antibacterial activity, mainly against Gram-positive bacteria, with compound 12 emerging as the most promising derivative in the series. The antibacterial activities of several synthesized compounds were systematically evaluated against S. aureus and MRSA over a 24 h incubation period, with optical density measured at ten time points. Bacterial growth was monitored spectrophotometrically at 600 nm (OD600) at 1, 2, 3, 4, 5, 6, 7, 8, 20, and 24 h, enabling a detailed assessment of growth kinetics and the temporal dynamics of inhibition. The effect of compound 11 on the growth kinetics of S. aureus was evaluated by quantifying viable bacterial counts (log10 CFU/mL) over a 6 h incubation period, and the results are presented in the time–kill curve. Compound 11 was selected for this experiment because it exhibited the most pronounced antibacterial activity against S. aureus in the disk diffusion assay. The cytotoxicity of compounds 9a, 11, 12, and 13 was evaluated at concentrations ranging from 125 to 1.95 µg/mL. The results showed a clear, concentration-dependent decrease in cytotoxicity for all tested compounds. The molecular structure of compound 3a was confirmed by a single-crystal X-ray diffraction.