Synthesis, characterization, antimicrobial and biofilm inhibitory activities of new N-oxide esters

Abstract

Novel N-oxide esters were synthesized from tertiary amine esters, which in turn were obtained from the reaction of 11-bromoundecanoic acid and different alkyl amines. The synthesized N-oxide esters were characterized by fourier transform infrared, proton nuclear magnetic resonance, carbon-13 nuclear magnetic resonance, and mass spectral analysis. The synthetic route involved three steps; initially, 11-bromoundecanoic acid was converted into methyl 11-bromoundecanoate, which was further converted into tertiary amine (mono and di) esters by reacting with different aliphatic amines (hexyl, dodecyl, octadecyl, dioctyl, and dicyclohexyl amine). Finally, the obtained amine esters were converted into N-oxide esters by treating with m-chloroperbenzoic acid. The synthesized N-oxide esters (4a–e) were studied for their antimicrobial and anti-biofilm activities. Among all the synthesized N-oxide esters, compounds 4a and e showed good antimicrobial activity especially towards pathogenic gram-positive bacterial strains with minimum inhibitory concentration values in the range of 7.8–31.2 μg mL−1. The compounds which exhibited antimicrobial activity were also effective in anti-biofilm activity and it was found that the compound 4e exhibited promising biofilm inhibitory activity with IC50 value of 6.4 μg mL−1 against Bacillus subtilis MTCC 121. Further, compound 4e showed increased levels of intracellular reactive oxygen species accumulation, which may be contributing to the bactericidal activity in B. subtilis MTCC 121.