Synthesis & Biological Evaluation of Oxime Based Derivatives as Antibacterial Agents

Bacterial infections remain a major concern for global public health, even with notable progress in antimicrobial therapy and infection-control measures. Disease-causing bacteria are responsible for numerous clinical conditions, ranging from respiratory and gastrointestinal infections to skin, soft-tissue, and urinary tract diseases, as well as severe systemic illnesses such as sepsis and tuberculosis. The excessive and improper use of antibiotics has accelerated the development of antimicrobial resistance (AMR), reducing the effectiveness of many traditional antibacterial drugs. As a result, multidrug-resistant strains of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Mycobacterium tuberculosis have become increasingly common, leading to poorer therapeutic outcomes, higher death rates, and rising healthcare expenditures across the world. Antimicrobial resistance develops through several biological processes, including the breakdown of drugs by bacterial enzymes, modification of drug-binding targets, decreased uptake through cell membranes, and the activation of efflux systems that expel antibiotics from the cell. These resistance traits can emerge through spontaneous genetic mutations or be transferred between bacteria via horizontal gene exchange, allowing resistance to spread rapidly. At the same time, the discovery of new classes of antibacterial agents has slowed, while existing therapies continue to lose effectiveness. This situation highlights the urgent need for new antibacterial compounds that offer greater activity, broader coverage, and novel modes of action. Consequently, modern medicinal chemistry increasingly emphasizes the redesign of established pharmacophores and the introduction of new functional groups to overcome resistance.

MATERIAL AND METHODS

All chemicals employed in the synthesis were procured from Merck (Mumbai), Sigma, Loba Chemie (Mumbai), Rankem (Haryana), and Avera Laboratories (Hyderabad). All solvents, reagents, and catalysts were of analytical grade and were used without further purification. The progress and purity of the synthesized compounds were monitored by thin-layer chromatography (TLC) using silica gel–coated glass plates as the stationary phase, with dichloromethane: methanol (10:1) as the mobile phase. The crude products were purified by recrystallization using suitable solvents. Further purification of the final compounds was achieved by column chromatography employing silica gel (230–400 mesh) packed in a sintered glass column. Melting points were determined by the open capillary method using an Analab scientific melting point apparatus and are reported as uncorrected values. Infrared (IR) spectra were recorded using the KBr pellet technique on an FT-IR 8400S spectrophotometer (Shimadzu, Japan). ^1H-NMR and ^13C-NMR spectra of the synthesized compounds were recorded on a BRUKER AVANCE II 400 spectrometer operating at 400 and 100 MHz, respectively. Mass spectra were obtained using a WATERS Q-TOF MICROMASS (LC-MS) instrument at the Sophisticated Analytical Instrument Facility (SAIF), Panjab University, Chandigarh. Chemical shift values are expressed in δ (ppm). In vitro antimicrobial studies were carried out in the Department of Biotechnology, Maharaja Ranjeet Singh College, Indore, India.

Scheme

Physical data of title Compound IV(a-f)

General procedure for the preparation of II (a-f)

General Procedure for Synthesis of 2 (a–f). Add (0.1 mol) of 80% KOH to a suspension of (0.1 mol) of 2-amino-5- mercapto-1,3,4-thiadiazole, in 15 mL of water. Solution was clarified with activated charcoal and diluted with 32 mL of ethanol, 0.1 mol of 1 (a–f) was added rapidly with stirring. Thick reaction mixture was formed, stirred vigorously at room temperature, and then diluted with 200 mL of cold water. The solid was obtained by filtration, washed with water and ether. 2 (a–f) were obtained. General procedure for the preparation of III (a-f) A mixture of compound II (a-f) (1 mmol), hydroxylamine hydrochloride (2 mmol) and sodium bi-carbonate (2 mmol) were taken in 10mL sealed; add equal volume of absolute methanol and dichloromethan into the sealed vial. The reaction was carried out at room temperature for 24 hrs to give the crude compound III (a-f). Recrystallized the crude III (a-f) by the mixture of ethanol and dichloromethane to gave the pure brownish amorphous compound IV (a-f). m.p. (°C) IV(a)159-161, IV(b)170-172, IV(c)140-144, IV(d)168-170, IV(e)164-166, IV(f)181 182; Yield (%) IV(a) 65, IV(b) 61, IV(c) 69, IV(d) 55, IV(e) 63, IV(f)68.

Antibacterial Evaluation

In-vitro Antibacterial Activity of Title Compounds III (a–f) All synthesized compounds were screened for their in-vitro antibacterial activity against selected Gram-positive and Gram-negative bacterial strains. Standard antibacterial agents, namely ciprofloxacin, gatifloxacin, and streptomycin, were employed as reference drugs. The Gram-positive organisms used in the study were Staphylococcus aureus (NCIM 2079) and Bacillus subtilis (NCIM 2250), while the Gram-negative organisms included Escherichia coli (NCIM 2109) and Pseudomonas aeruginosa (NCIM 2036). The antibacterial evaluation was carried out at the Department of Biotechnology, Maharaja Ranjeet Singh College, Indore, India.

Composition of Nutrient Agar

• Peptone:10.0g

• Sodium chloride:5.0g

• Beef extract: 10.5g

• Distilled water: upto 1000mL

• pH: 7.4±0.2

Nutrient agar (Hi-Media) was used as the microbiological medium for culturing the test organisms. The antibacterial potential of the test compounds was assessed against S. aureus and B. subtilis (Gram-positive) as well as E. coli and P. aeruginosa (Gram-negative). Antibacterial activity is generally defined by the ability of a compound to inhibit bacterial growth in a nutrient medium, which was determined using the disk diffusion method. This method is widely employed for evaluating antibacterial efficacy. Culture Medium Nutrient broth was used for the preparation of bacterial inocula, while nutrient agar served as the medium for antibacterial screening.

Fig. No.1 Zone of Inhibition