Synthesis and In Vitro Antibacterial Evaluation of Schiff Bases Derived FROM 2-Chloro-3-Quinolinecarboxaldehyde

Schiff bases are organic compounds that contain imine or azomethine (C=NR) group and are often produced from condensation of aldehydes or ketones with primary aliphatic/aromatic amines. Quinoline Schiff bases are formed if either or both of the reaction participants possess quinoline ring. Quinoline nucleus is an essential part of the chemical structure of natural products, and pharmaceutical and biologically active compounds (Figure 1). Quinoline Yellow WS is a water-soluble greenish yellow food additive which is derived from the dye Quinoline Yellow SS (1). Bosutinib as a tyrosine kinase agonist is prescribed to treat Philadelphia chromosome-positive leukemia (2). Furthermore, Apomorphine as a morphine derivative, dopamine D2 inhibitor, and emetic agent has been used in the treatment of acute poisoning and parkinsonism (3). Mefloquine is an effective antimalaria drug against Plasmodium falciparum parasite (4). Imiquimod acts as an immune response modifier and helps to treat genital and anal warts, actinic keratoses, and superficial basal cell carcinoma (5). Cinchocaine or dibucaine is a surface anesthetic with high toxicity that has been restricted to spinal anesthesia (6). Schiff bases are suitable multidentate ligands for metal complexation. The therapeutic potential of both coordinated and uncoordinated forms has been proven against pathogenic bacteria (7), fungi (8), viruses (9), protozoa (10), and helminths (11). In addition, quinolones are highly bioavailable antibiotics with a diverse range of activities and functions that are used to treat respiratory and urinary tract infections. The antibiotic examples of current quinolones and fluoroquinolones are as follows: moxifloxacin, lomefloxacin, gatifloxacin, norfloxacin, ofloxacin, nalidixic acid, rosoxacin, and ciprofloxacin. Mallandur et al synthesized some quinolineand benzimidazole-based Schiff bases; their antibacterial effects were assessed against Escherichia coli, Staphylococcus aureus, and Salmonella typhi via disc diffusion and Avicenna Journal of Medical Biochemistry

Other methods were proposed to synthesize imines besides the reaction of aldehydes/ketones and amines. The acid-catalyzed interaction of hydrazoic acid with tertiary alcohols to afford imines is referred to as the Schmidt reaction (16). The in situ oxidation of primary alcohols to corresponding aldehydes catalyzed by N-heterocyclic carbene (NHC)-silver (I) complexes (17), manganese dioxide (18), palladium (19), ortho-naphthoquinone (20), and ferric nitrate (21) has produced imines in good yields. The retro-aza-Henry-type reaction of amines with nitrostyrenes (22), intermolecular alkyne hydroamination (23), Pd-catalyzed reaction of aryl halides, and bulky arylamines (24) were also developed for this purpose. In order to expand potential antimicrobial agents, the inhibitory activity of some synthesized Schiff bases of 2-chloro-3-quinolinecarboxaldehyde was evaluated against the pathogenic genera Rhodococcus, Streptococcus, Staphylococcus, Enterococcus, Bacillus, Shigella, and Proteus.

Chemicals
All reagents were prepared from reputable chemical companies. The uncorrected melting points were determined by Kruss KSP1N melting point apparatus. Aluminum TLC plates (20×20 cm) containing silica gel coated with fluorescent indicator F254 were used to monitor the progress of reactions. Bruker Tensor-27 FT-IR spectrometer was applied to record the FT-IR spectra of compounds. 1 H and 13 C NMR spectra were registered using a Bruker 400 MHz-NMR spectrometer.
Synthesis of 2-chloroquinoline-3-carbaldehyde (2) A total of 120 mmol phosphoryl chloride (18.36 g) was gradually added to 10 mmol acetanilide (1, 1.34 g) and 30 mmol dry DMF (2.30 mL) at 0-5°C (21). The solution was warmed to 90°C, and stirred under these conditions for 16 hours. The contents of reaction were cooled to the room temperature, and poured into 100 g crashed ice. The precipitate was filtered and washed with water. The solid was recrystallized from acetonitrile to achieve a yield of 90% (1.97 g) of pure white compound 2.
General Procedure for the Synthesis of Quinoline Schiff Bases 4a-g A solution of 1 mmol aniline derivatives 3a-g in 5 mL ethanol was added dropwise to a solution containing 1 mmol 2-chloroquinoline-3-carbaldehyde (2) at room temperature. The mixture was refluxed for 3-7 hours. The progress of the reaction was checked by TLC, including a mixture of ethyl acetate-hexane (2: 1). The precipitate was then filtered off, washed with ethanol, and recrystallized from appropriate solvent to give Schiff bases 4a-g as yellow solids.
(E)-1-  M02-A11 guideline (26). In this respect, 3 colonies of initial suspensions were inoculated on MHA plates (100 mm) by a swab, and 5 sterile blank discs were included on inoculated media. Then, 10 μL of compounds (10240 μg mL -1 ) and/or antibiotic (17.6 μg mL -1 ) were moved onto the dedicated discs. They were incubated at 37°C for 18 hours. The IZD values were measured in millimeter using caliper.
Determining the Minimum Inhibitory Concentration The broth microdilution method was adopted for the determination of minimum inhibitory concentration (MIC) values according to the following CLSI M07-A9 guideline (26): 20 μL of derivatives were dissolved in Determining the Minimum Bactericidal Concentration The time-kill test was performed for the determination of minimum bactericidal concentration (MBC) values according to the following CLSI M26-A guideline (26): samples of all microwells without turbidity in the previous experiment were cultured using a swab in plates containing MHA medium. They were incubated for 24 hours at 37°C. The lowest concentration of derivatives or antibiotic in which bacteria could not survive was considered as the MBC value.

Results
As shown in Scheme 1, quinoline Schiff bases 4a-g were synthesized via condensation of 2-chloroquinoline-3carbaldehyde (2) and aniline derivatives 3a-g in the absence of any catalyst. The data are reported in Table 1.
The reaction progress between 1 mmol of both quinoline 2 and aniline 3a was checked in 5 mL of different solvents in thermal heating under reflux conditions. As shown in Table 2, the greatest amount of products were obtained in ethanolic solutions. Moreover, the inhibitory activity of synthesized  heterocycles 2 and 4a-g, as well as antibiotic ceftriaxone were evaluated against a variety of pathogenic bacteria. The results are recorded as IZD, MIC, and MBC values in Table 3.

Discussion
In vitro antibacterial potentials of all synthesized heterocyclic derivatives were studied and compared with those of ceftriaxone. All of our synthesized heterocycles could inhibit the growth of B. subtilis subsp. spizizenii. The blocking effects against E. faecalis and S. aureus were observed with only quinoline Schiff base 4c containing 2-hydroxyaniline substituent. The quinoline 4g bearing 4-hydroxyaniline substituent was the only effective heterocycle and the only Schiff base derivative on gramnegative Proteus vulgaris and gram-positive S. pneumoniae, respectively. Moreover, the heterocyclic compounds 4c and 4g affected a wider range of tested bacterial strains. Schiff bases prepared by the condensation of 2-chloro quinoline-3-carbaldehyde derivatives and a substituted 5-benzimidazolecarboxylic hydrazide could inhibit E. coli strains with MICs in the range of 25 to 50 μg mL -1 (12). Cu (II) complexes of 2-sulfanyl or 2-hydroxyquinoline-3-carbaldehyde Schiff bases exhibited better antibacterial effects than their corresponding ligands (14). Furthermore, moderate antibacterial activities were observed with two synthesized Schiff bases based on 2-chloroquinoline-3carbaldehyde (15). It has been found that quinolone and fluoroquinolone antibiotics block the growth of bacteria via enzyme inhibition (31). DNA gyrase and topoisomerase IV as DNA topology controllers are two essential enzymes produced by the most bacteria whose function is impaired in the presence of quinolones and fluoroquinolones. They also play a key role in the repair, deactivation, replication, and transcription of DNA. In addition, molecular docking studies predicted effective interactions of 6-chloro-2hydroxyquinoline-3-carbaldehyde Schiff bases introduced into the active site of target protein (13).

Conclusion
To conclude, synthesized 2-chloroquinoline-3carbaldehyde Schiff bases showed moderate inhibitory properties against some important pathogenic gramnegative and gram-positive strains. Changes in the structure and position of substituents on quinoline ring, complexation, and the use of new condensing primary amines and their equivalents may improve antimicrobial effects.