INTRODUCTION:

German chemist Friedlieb Ferdinand Runge first isolated quinoline from coal tar in 1834; he dubbed it leukol ("white oil" in Greek). The main source of commercial quinoline is still coal tar. Although this heterocycle based on nitrogen is not particularly helpful by itself, it is readily changed with simple to complicated functions to produce a variety of compounds that are widely used in the domains of industrial and medical chemistry^1-2. It is a heterocyclic aromatic chemical that contains nitrogen. Its molecular weight is 129.16g/mol and its chemical formula is C₉H₇N³. The log P value is 2.04 while the acidic pKb and basic pKa values are 4.85 and 9.5, respectively. It is an odorous, colourless, hygroscopic liquid.

Samples that are old, especially if they are exposed to light, turn yellow and then brown. In comparison to most organic solvents and hot water, quinoline is very weakly soluble in cold water. Quinoline alone only has a few uses, but several of its derivatives have a wide range of uses. Quinine, an alkaloid found in plants, is a notable example. There are about 200 quinoline and quinazoline alkaloids that are physiologically active^4-5. A weak tertiary base is quinoline. With acids, it can produce salt and exhibits reactions akin to those of pyridine and benzene. Both nucleophilic and electrophilic substitution reactions are demonstrated. When consumed orally and inhaled, it is safe for people^6-7. The quinoline scaffold has emerged as a key building block to produce novel pharmaceuticals among heterocyclic compounds.

Numerous biological activities can be found in quinoline and its derivatives^8-9. Many medications being developed for the treatment of various disorders have a quinoline-containing system as a structural component. Quinoline is also present in a number of therapeutically useful drugs¹⁰. Some quinoline-based derivatives which shows different activity are Cinoxacin shows antibacterial activity¹¹^-12, antimicrobial activity¹³, oxolinic acid show antimicrobial activity¹⁴. Ciprofloxacin show antibacterial activity¹⁵. Gatifloxacin shows antibacterial activity. Some quinoline-based derivatives which shows different activity are Cinoxacin shows antibacterial activity^16-18, oxolinic acid show antimicrobial activity^19-20, Ciprofloxacin show antibacterial activity²¹.

MATERIAL AND METHODS:

All the chemicals and solvents used in this work are of synthetic grade and purchased from Bio-chemika Reagents, SD fine chemical limited and moly chem Mumbai. Thin layer chromatography (TLC) was used to monitor the reaction's development using silica gel plates. Visualization of TLC plate was done by TLC chamber. After the completion of synthesis. Alcohols were evaporated when the mixture was cooled to room temperature. To obtain the intended product, the residue was purified using column chromatography. Utilizing melting point apparatus, the melting points of compounds that were produced were determined. All synthetic compounds have their Fourier Transform infrared (FTIR) spectra recorded. The infrared spectra were captured using the KBr pellet technique on a Bruker FTIR spectrophotometer. Using KBr Disc, ¹H-NMR spectra were acquired from a Bruker-DRX 600MHz spectrophotometer by using dimethyl sulfoxide (DMSO-d6) as solvent. Chemical shifts are expressed as ppm. Docking studies was done using Molegro Virtual Docker.

Detailed procedure for the synthesis of diethyl 2-(chloromethyl)-6-ethylquinoline-3,4-dicarboxylate (VD-01):

Isatin (1.0g, 6.8mmol), ethyl acetoacetate (1ml), concentrated H₂SO₄ (1.0mL, 8.4mmol), and ethanol (10 mL) were combined in a 25mL round-bottom flask and refluxed at 80⁰C for 1.5hours while being watched by thin-layer chromatography (TLC) until the material began to exhibit complete consumption. Alcohols were evaporated once the mixture was cooled to room temperature. Water was then put to it. Ethyl acetate was used to extract the mixture. The organic phase was concentrated after being rinsed with brine and dried with sodium sulphate (Na₂SO₄). To obtain the intended product, the residue was purified using column chromatography on silica gel. Light yellow colour oil is obtained²².

Spectral Data of diethyl 2-(chloromethyl)-6-ethylquinoline-3,4-dicarboxylate (VD-01):

IR (KBr, cm^-1): O-H group shows stretching vibration on wavelength 3679.08 cm^-1. Aromatic CH stretching shows 3209.52 cm^-1wavelength. CH group shows vibration at 2856.96 cm^-1, 1736.59cm^-1. C=N ring shows stretching vibration at 1419.04cm^-1. C(=O)–O group shows stretching vibration at 1159.45cm^-1. C–Cl group shows stretching at 1096.77cm^-1. C–C stretching was shown at 1035.58cm^-1 and 815.07cm^-1. C–H out of plane bending shown at 768.55cm^-1.¹H-NMR (600MHz, DMSOD6), (q, 1.057, 2H), (s, 2.466), (q, 3.742, 3H), (s, 4.048). 4.048 (R-CH₂-Cl), 3.742 (RO-CH₃), 2.466 (R-CO-CH₃), 1.057 (R-CH2-R).

Detailed procedure for the synthesis of diethyl 6-ethyl-2-propylquinoline-3,4-dicarboxylate (VD-02):

Isatin (1.0 g, 6.8 mmol), ethyl butyryl acetate (2.15 gm), concentrated H₂SO₄ (1.0 mL, 8.4 mmol), and ethanol (10 mL) were combined in a 25 mL round-bottom flask and refluxed with continuous stirring at 80 to 100⁰C for two hours while being watched by thin-layer chromatography (TLC) until the material began to show complete consumption. Alcohols were evaporated once the mixture was cooled to room temperature. Water was then put to it. Ethyl acetate was used to extract the mixture. The organic phase was concentrated after being rinsed with brine and dried with sodium sulphate (Na₂SO₄)²².

Spectral Data of diethyl 6-ethyl-2-propylquinoline-3,4-dicarboxylate (VD-02):

IR (KBr, cm^-1): O-H group shows stretching vibration on wavelength 3679.08 cm^-1. Aromatic C-H stretching shows 3209.52 cm^-1wavelength. C-H group shows vibration at 2856.96 cm^-1. C=N ring shows stretching vibration at 1439.04cm^-1. C(=O)–O group shows stretching vibration at 1159.45cm^-1. C–C stretching was shown at 1035.58cm^-1 and 815.07cm^-1. C–H out of plane bending shown at 788.55cm^-1.¹H-NMR (600MHz, DMSO-d6), (q, 1.057, 2H), (s, 2.466), (s, 4.048), 3.742 (RO-CH₃), 2.466 (R-CO-CH₃), 1.057 (R-CH2-R).

Detailed procedure for the synthesis of 3-acetyl-2-methylquinoline-4-carboxylic acid (VD-03):

250 mg of potassium hydroxide and 147 mg of isatin were combined with 5 ml of water and agitated at room temperature for 15 to 30 minutes. After adding 0.40ml of strong hydrochloric acid to the mixture to bring the pH level down to 2-3, 0.30ml of ethyl aceto-acetate, and 25 mg of CuSO₄.5H₂O were added. When the mixture was agitated, precipitate appeared. TLC (Rf value = 0.38; CHCl₃) was used to keep track of the reaction's development. The precipitate was filtered, washed with water, and recrystallized once the initial material had disappeared to produce the pure product²³.

Spectral Data of 3-acetyl-2-methylquinoline-4-carboxylic acid (VD-03):

IR (KBr, cm^-1): C-H group (methyl group) shows stretching vibration at 2988.10cm^-1. C=O group shows stretching vibration at 1642.94cm^-1. O-H group shows bending vibration at 1413.81cm^-1. N-C group shows stretching vibration at 1387.27cm^-1. C-C group shows stretching vibration at 1162.16cm^-1. C=C shows stretching vibration at 1097.02cm^-1. CH group shows stretching vibration at 716.01cm^-1. ¹H-NMR (600MHz, DMSO-d6), (s, 2.546, 3H), (s, 6.352, H), (s, 11.057, H), (t, 7. 593, 7.510, 7.499; q, 7.150, 7.073, 6.928, 6.918, quinoline). 11.057 (R-COOH), 2.546 (Ar-COCH₃).

Detailed procedure for the synthesis of 1,2,3,4-tetrahydroacridine-9-carboxylic acid (VD-04):

250 mg of potassium hydroxide and 147 mg of isatin were combined with 5 ml of water and agitated at room temperature for 15 to 30 minutes. Following the addition of 2 ml of ketone (ethyl aceto-acetate) and 25 mg of CuSO₄.5H₂O, the liquid was acidified to a pH of 2-3 using 0.40 ml of strong hydrochloric acid. When the mixture was agitated, precipitate appeared. TLC (Rf value= 0.32; CHCl₃) was used to keep track of the reaction's development. The precipitate was filtered, washed with water, and recrystallized once the initial material had disappeared to produce the pure product²³^.

Spectral Data of synthesis of 1,2,3,4-tetrahydroacridine-9-carboxylic acid (VD-04):

IR (KBr, cm^-1): C=O group shows stretching vibration at 1642.94cm^-1. O-H group shows bending vibration at 1413.81cm^-1. N-C group shows stretching vibration at 1387.27cm^-1. C-C group shows stretching vibration at 1162.16cm^-1. C=C group shows stretching vibration at 1097.02cm^-1. C-H group shows stretching vibration at 716.01cm^-1.¹H-NMR (600MHz, DMSO-d6), (s, 2.546, 3H), (s, 6.352, H), (s, 11.057, H), (t, 7. 593, 7.510, 7.499; q, 7.150, 7.073, 6.928, 6.918, quinoline). 11.057 (R-COOH).

In silico studies:

Molecular docking studies:

Molecular docking studies of forty-three compounds were done by using Molegro virtual docker (MVD) software. Molecules are designed using chemdraw professional 15.0 and 2D str. converted into energy minimized 3D str. and the str. are saved as .Mole file. Out of forty-three molecules four molecules show very good hydrogen bonding with amino acid in the active pocket of bacterial protein (PDB ID-3ACX). Moldock score and hydrogen bonding of the test compounds were compared with Chloramphenicol as standard drug. Mol dock score are mentioned in table1.

Name of compound	Moldock Score	Rerank score	H-Bonding
VD-01	-78.3595	-64.5277	-2.68018
VD-02	-81.3343	-66.5526	-2.2235
VD-03	-104.854	-82.3475	-2.17157
VD-04	-127.897	-86.8756	-4.30042
Chloramphenicol	-99.384	-80.3699	-5.00341

Evaluation of antibacterial activity:

Using the cup-plate method, in-vitro antibacterial activity was tested against gram positive and gram-negative bacteria cultures that had been grown for 24 hours. The antibacterial activity of the recently synthesised compounds has been tested against E. coli, Staphylococcus aureus, S. typhi, and B. subtilis. Chloramphenicol was used as standard for measuring antibacterial activity. After a 24hour incubation period at 25°C, the zone of inhibition was compared to a reference medication Chloramphenicol. The outcomes are listed below in table 2:

Table 2: Antibacterial activity

Compounds	Antibacterial activity (Zone of Inhibition) mm
Compounds	E. coli	S. aureus	S. typhi	B. subtilis
Chloramphenicol	24	26	24	23
VD-01	25	22	25	20
VD-02	23	27	23	24
VD-03	21	20	24	25
VD-04	27	26	25	23

RESULTS:

The compounds were synthesised as per literature. The docking studies of compounds show very good binding with bacterial protein PDB ID-3ACX with mol. dock score between 78.3595 to -124.166. We used Chloramphenicol as reference compound. Chloramphenicol have Moldock score -99.4262. Compound 1, 2, 3, 4-tetrahydroacridine-9-carboxylic acid (VD-04) having higher Moldock score of -127.897 and show good antibacterial activity. The docking poses of standard drug and synthesized compounds are shown below in figure 1 to 7.

Figure 1: Structure of 3ACX.

Figure 2: 3D structure of protein i.e., 3ACX.

Figure 3: Chloramphenicol docked into binding site of 3ACX.

Figure 4: diethyl 2-(chloromethyl)-6-ethylquinoline-3,4-dicarboxylate (VD-01) docked into binding site of 3ACX.

Figure 5: diethyl 6-ethyl-2-propylquinoline-3,4-dicarboxylate (VD-02) docked into binding site of 3ACX.

Figure 6: 3-acetyl-2-methylquinoline-4-carboxylic acid (VD-03) docked into binding site of 3ACX.

Figure 7: 1,2,3,4-tetrahydroacridine-9-carboxylic acid (VD-04) docked into binding site of 3ACX.

Figure 8: Comparison of Antibacterial activity (Zone of Inhibition) mm with Standard drug.

CONCLUSION:

Four derivatives of quinoline were synthesized by reacting isatin with beta keto esters, and the confirmation was done by FTIR and NMR. The synthesized compounds show good antibacterial activity against gram positive and gram-negative bacteria. The result of molecular docking revealed that compounds exhibit higher docking score as compare with standard ligand Chloramphenicol. Compound VD-04 exhibit higher Moldock score -127.897.

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Received on 19.03.2024 Revised on 10.09.2024

Accepted on 14.01.2025 Published on 27.02.2025

Available online from March 05, 2025

Asian J. Pharm. Tech. 2025; 15(1):1-5.

DOI: 10.52711/2231-5713.2025.00001

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