Synthesis of new coumarin derivatives and assessment of their antimicrobial efficacy.

Aim: Developing new antimicrobial agents in response to the urgent challenge of antimicrobial resistance.

Methods: Synthesis of the targeted coumarins, elucidation of their structures using spectroscopic tools, and investigation of their antimicrobial activity.

Results: Coumarin-pyrazole 11 with CF₃ in the 3-position of the pyrazole ring displayed the lowest minimum inhibitory concentrations (MICs) and the minimum bactericidal concentrations (MBCs) with values of 1.95 and 15.6 µg/ml, respectively, against Bacillus pumilis. In addition, it exhibited the best inhibitory activity against Saccharomyces cerevisiae (MIC = 3.91 µg/ml) compared to the rest of the derivatives (7.81-62.5 µg/ml). Surprisingly, coumarin 14 with the S-CH₃ group had higher ability to inhibit the Staphylococcus faecalis strain with an MIC value of 1.95 µg/ml, which is twice that of penicillin G (MIC = 3.91 µg/ml). At the same time, compounds 6, 8, 11, 16, and penicillin G showed similar activity with an MIC value of 3.91 µg/ml against Staphylococcus faecalis. Also, the lowest MIC value (3.91 µg/ml) was obtained for S-CH₃ derivative 14 against Enterobacter cloacae. Coumarins 14 and 1,3,4-thiadiazine derivative 6 recorded the lowest MBC (15.6 µg/ml) against Escherichia coli.

Conclusion: Finally, it can be concluded that some designed coumarins have a high potential to act as potent antimicrobial agents. Some of them displayed higher efficacy than or equal to the reference drug.