A new efficient domino approach for the synthesis of coumarin-pyrazolines as antimicrobial agents targeting bacterial d -alanine- d -alanine ligase

The inhibition of d -alanine- d -alanine ligase (Ddl) prevents bacterial growth, which makes this enzyme an attractive and viable target in the urgent search for novel effective antimicrobial drugs. In this work, a series of novel coumarin-linked pyrazoline inhibitors of d -alanine- d -alanine ligase were synthesized and evaluated as inhibitors of Escherichia coli DdlB ligase in order to target resistant strains of bacteria using environmentally benevolent β-cyclodextrin as a supramolecular catalyst via one-pot four component synthesis in water as a green reaction media. All the newly synthesized compounds have been characterized by elemental analysis and various spectroscopic methods. The new procedure has noteworthy advantages including easy work-up, short reaction times, high yields of products and column-free synthesis. The synthesized compounds were evaluated in vitro for their antimicrobial activity. Among the synthesized compounds, namely 3-(5-(4-hydroxy-3-methoxyphenyl)-4,5-dihydro-1 H -pyrazol-3-yl)-2 H -chromen-2-one ( 5f ) was found to be the most potent d -alanine- d -alanine ligase enzyme inhibitor, with an IC 50 value 106 μM, and the compound 3-(5-( p -tolyl)-4,5-dihydro-1 H -pyrazol-3-yl)-2 H -chromen-2-one ( 5g ) was found to be the second-most potent inhibitor of the DdlB enzyme, with an IC 50 value 111 μM against the standard d -cycloserine. In addition, SAR study provided evidence that the –OH, –CH 3 and –OCH 3 groups at the 4- and 3-position of the coumarins linked to the pyrazolines scaffold increased enzymatic inhibition, while the molecular docking study of most active compounds 5a , 5g , and 5j against DdlB enzyme of E. coli exhibited good binding properties. This work thus highlights the coumarin-linked pyrazoline motif as a very promising tool for the development of novel antimicrobial compounds acting through an interesting bactericidal mechanism of action.