Synthesis and Antibacterial Evaluation of Some Novel 1,3,4-oxadiazol Derivatives Incorporated with Quinoline Moiety

Quinoline moiety를 가지고 있는 1,3,4-oxadiazol 유도체의 합성 및 항균활성

Abstract

5-(3,4,5-Triethoxyphenyl)-1,3,4-oxadiazole-2-thiol 6 on treatment with substituted 3-(bromomethyl)-2-chloroquinoline or 2-(p-tolyloxy)-3-(bromomethyl)quinoline 4a-j afforded the corresponding 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-chloroquinoline or 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-(p-tolyloxy)quinoline 7a-j, in the presence of $K_2CO_3$ and DMF under stirring at ambient temperature. All the synthesized compounds were further screened for their antibacterial activities. Some of our compounds showed excellent antibacterial activities against test organisms and reference standard.

5-(3,4,5-Triethoxyphenyl)-1,3,4-oxadiazole-2-thiol 6을 3-(bromomethyl)-2-chloroquinoline or 2-(p-tolyloxy)-3-(bromomethyl) quinoline 4a-j 화합물과 반응시켜서 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-chloroquinoline 또는 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-(p-tolyloxy)quinoline 7a-j를 합성하였다. 합성한 화합물들에 대한 항균활성을 측정하였으며, 화합물 7d, 7i 및 7j 은 우수한 활성을 나타내었다.

INTRODUCTION

Quinoline scaffold is prevalent in a variety of pharmacologically active compounds as well as in naturally occurring products.1 Quinoline-containing drugs are widely used in the treatment of malaria,2 HIV-1 replication inhibitors,3 antimicrobial and anti-tuberculosis drugs4 and antihelmintic properties.5 Beside this quinolines have also occupied a unique position in the design and synthesis of novel biologically active compounds since they are often used as anti-inflammatory, antiasthmatic, antibacterial, antihypertensive, antitumor,6,7 antiproliferative,8 anticancer9 and antiparasitic agents.10

In addition, gallic acid and its related compounds are widely distributed in fruits & plants.11,12 It has been reported to have anticarcinogenic, antioxidative, antimutagenic, antiallergic and anti-inflammatory activities.13 Gallic acid has been a building block of choice for different pharmaceutical leads due to the presence of this moiety in several bioactive natural products.14 Hence, numerous derivatisations have been done and are reported as anticancer,15 HIV-1 Integrase16 and HIV-1RT inhibitors,17 antioxidants,18 antimalarials agents,19 etc.

Literature review also revealed that 1,3,4-Oxadiazoles are an important class of heterocyclic compounds with a wide range of pharmaceutical and biological activities. Their synthesis and transformations have been of interest from a long time. They have revealed anti-inflammatory, anticonvulsant and analgesic activities.20,21 They have also shown antibacterial,22 antifungal23 and muscle relaxant24 properties.

Prompted by the above-mentioned biological properties of quinoline and oxadiazole it was contemplated to synthesize a novel series of quinoline incorporated oxadiazole and its derivatives (Scheme 1, 2 & 3, Table 1). Antibacterial activities of the newly synthesized compounds are discussed in this paper.

Scheme 1

Scheme 2

Scheme 3

Table 1Characterization data for compounds 7(a-j)

 

RESULT AND DISCUSSION

Chemistry

In continuation on our research work to synthesize potent bioactive heterocycles.25 Herein, we report the synthesis of a series of oxadiazole derivatives synthesized via 3-(bromomethyl)-2-chloroquinolines or 2-(p-tolyloxy)-(bromomethyl) quinolines 4a-j which were synthesized by reduction of substituted 2-chloroquinoline-3-carbaldehyde or 2-(p-tolyloxy)quinoline-3-carbaldehyde 2a-j in presence of catalytic amount of NaBH4 and methanol giving (2-chloroquinolin-3yl)methanol or (2-(p-tolyloxy)quinolin- 3yl)methanol 3a-j, which was further brominated with PBr3 in presence of DCM under ice cold condition to afford 4a-j (Scheme 1).

On the other hand, 3,4,5-triethoxybenzohydrazide 5 was further reacted with carbon disulfide in presence of potassium hydroxide under reflux condition to afford 5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazole-2-thiol 6 (Scheme 2). Finally 5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazole-2-thiol 6 was alkylated with substituted 3-(bromomethyl)-2-chloroquinoline or 2-(p-tolyloxy)-(bromomethyl)quinoline 4a-j to afford 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-chloroquinoline or 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-(p-tolyloxy) quinoline 7a-j in presence of K2CO3 and DMF at ambient temperature stirring for 15-20 min.

Spectral analysis

The structures of the synthesized compounds were confirmed by spectral analysis (IR, 1H NMR and Mass). The IR spectrum of compound 2a showed a peak at 1722 cm-1 due to C=O stretch. In 1H NMR spectrum it exhibited two singlets, one at δ 2.41 due to CH3 proton, second at δ 10.65 due to CHO proton. Mass spectrum was consisted with assigned structure.

As estimated the 1H NMR spectrum of compound 3a showed singlet at δ 3.71 due to the presence of OH proton, while another singlet at δ 10.65 due to CHO proton get disappeared due to reduction of aldehyde. Similarly, in the 1H NMR spectrum of compound 4a the broad peak of OH disappeared due to bromination.

1H NMR spectrum of compound 6 showed a highly deshielded singlet at δ 11.23 attributed to SH proton, which get disappeared in compound 7a due to the alkylation of compound 6 with compound 4a-j. The IR and mass spectral data of compound 4, 6 and 7a were consisted with the assigned structure.

The antibacterial screening results revealed that most of the newly synthesized compounds exhibited promising antibacterial activities. Generally, the test compounds showed better activity against the Gram Negative bacteria (Table 2). Out of the compounds tested, compounds 7d, 7i and 7j exhibited excellent antibacterial activity against the Gram Negative bacteria i.e. Salmonella typhi and Pseudomonus aeroginosa and moderate activity against gram positive bacteria i.e. Bacillus subtilis and Staphylococcus aureus as compared with the broad spectrum antibiotic Streptomycine and Ampicilline.

Table 2Minimal inhibitory concentrations (MIC μg/mL) of tested compounds 7(a-j)

Conclusion

In conclusion, we have synthesized some novel 1,3,4-oxadiazol derivatives incorporated with quinoline moiety and evaluate their in-vitro antibacterial activity. Out of the compounds tested, compounds 7d, 7i and 7j exhibited excellent antibacterial activity against the Gram Negative bacteria i.e. Salmonella typhi and Pseudomonas aeroginosa and moderate activity against gram positive bacteria i.e. Bacillus subtilis and Staphylococcus as compared with commercially available drug.

 

EXPERIMENTAL SECTION

All chemicals and solvents were purchased from Merck, Spectrochem and S.D. Fine-chem. (India). Melting points were determined in open capillaries on Kumar’s melting point apparatus (India) and are uncorrected. IR spectra were recorded on JASCO FT-IR 4100, Japan using KBr discs. 1H-NMR spectra were recorded on a Varian as 400 MHz spectrometer in CDCl3/DMSO-d6, chemical shifts (δ) are in ppm relative to TMS, and coupling constants (J) are expressed in hertz (Hz). Mass spectra were recorded on Single-Quadrupole Mass Detector 3100, Waters. Elemental analyses were performed on CHNS analyzer Flash 1112, Thermo Finnigan. The progress of the reactions was monitored by TLC on Merck silica plates. Multiplicities are shown as the abbreviations: s (singlet), brs (broad singlet), d (doublet), t (triplet), m (multiplet). Solvents were commercially available materials of reagent grade.

Synthesis of 2-chloroquinoline-3-carbaldehyde (1a): The compound 1a was prepared as per procedure reported in the literature.2 m.p: 148 ℃ IR-(KBr): 2739, 1710, 1605 and 755 (cm-1); 1H NMR-(DMSO-d6): δ 10.36 (s, 1H), 8.57 (s, 1H), 8.06 (d, 1H), 7.92 (m, 2H), 7.75 (dd, 1H); MS: m/z 192.3 (M+); Anal. Calcd for C10H6ClNO: C, 62.68; H, 3.16; N, 7.31; O, 8.35; found C, 62.74; H, 3.21; N, 7.20; O, 8.31.

Synthesis of 2-(p-tolyloxy)quinoline-3-carbaldehyde (2a): To a mixture of p-cresol (0.031 mmol, 3.38 gms) and K2CO3 (0.068 mmol, 9.51 gms) in DMF, compound 1a (0.031 mmol, 6 gms) was added and the reaction mixture was stirred at 85-90 ℃ for 5 hrs. The completion of the reaction was monitored by TLC. After completion, water (50 ml) was poured in the reaction mixture & the solid thus obtained was filtered off & recrystallized from ethyl acetate. 2a m.p: 129℃, IR-(KBr): 2945, 2750, 1720, 1600 and 1225 (cm-1); 1H NMR-(DMSO-d6): δ 10.65 (s, 1H), 8.71 (s, 1H), 7.88 (d, 1H), 7.74 (d, 1H), 7.71 (m, 1H), 7.45 (m, 1H), 7.39 (d, 2H), 7.19 (d, 2H), 2.41 (s, 3H); MS: m/z 264.1 (M+); Anal. Calcd for C17H13NO2: C, 77.55; H, 4.98; N, 5.32; O, 12.15; found C, 77.63; H, 5.01; N, 5.21; O, 12.09.

Synthesis of (2-chloroquinolin-3-yl)methanol (3a): To the mixture of compound 2a in methanol, sodium borohydride was added portion wise, & the mixture was stirred at room temperature for 15-20 min. The completion of the reaction was monitored by TLC & reaction mass was concentrated under vacuum. The reaction mass was poured into ice cold water and the solid thus obtained was filtered & recrystallized from ethyl acetate.

Compounds 3a: m.p: 134 ℃ IR-(KBr): 2945, 2750, 1722, 1600 and 1125 (cm-1); 1H NMR-(DMSO-d6): δ 8.21 (s, 1H), 8.18 (dd, 1H), 7.91 (m, 1H), 7.82 (dd, 1H), 7.51 (dd, 1H), 4.96 (s, 2H), 3.75 (s, 1H); MS: m/z 193.9 (M+); Anal. Calcd for C10H8ClNO: C, 62.03; H, 4.16; N, 7.23; O, 8.26; found C, 62.21; H, 4.19; N, 7.18; O, 8.21.

Compounds 3f: IR-(KBr): 3427, 2920, 1720, 1520 and 1225 (cm-1); 1H NMR-(DMSO-d6): δ 8.05 (s, 1H), 7.62 (d, 1H), 7.48 (d, 2H), 7.41 (d, 1H), 7.25 (dd, 2H), 7.23 (d, 2H), 4.74 (s, 2H), 4.01 (s, 1H), 3.51 (s, 1H), 2.46 (s, 3H); MS: m/z 266.1 (M+); Anal. Calcd for C17H15NO2: C, 76.96; H, 5.70; N, 5.28; O, 12.06; found C, 77.13; H, 5.75; N, 5.17; O, 12.01.

Synthesis of 3-(bromomethyl)-2-chloroquinoline (4a): Compound 3a was dissolved in DCM at 5 ℃, after 10-15 min. of stirring calculated amount of PBr3 was added drop wise and the mixture was stirred at room temperature for 1 hr. The completion of the reaction was monitored by TLC. The DCM was removed under vacuum and the reaction mass was poured on ice cold water & the solution was neutralized by adding saturated solution of NaHCO3. The solid thus obtained was filtered and recrystallized from ethyl acetate.

Compound 4a: m.p: 179 ℃ IR-(KBr): 1670, 1630, 750 and 710 (cm-1); 1H NMR-(CDCl3): δ 8.25 (s, 1H), 8.16 (dd, 1H), 7.68 (dd, 1H), 7.40 (m, 1H), 7.52 (m, 1H), 4.47 (s, 2H); MS: m/z 257.4 (M+); Anal. Calcd for C10H7BrClN: C, 46.82; H, 2.75; N, 5.46; found C, 46.93; H, 2.81; N, 5.34.

Compound 4i: IR-(KBr): 2930, 1624, 1560, 1150 and 735 (cm-1); 1H NMR-(CDCl3): δ 8.05 (s, 1H), 7.61 (dd, 1H), 7.48 (s, 1H), 7.40 (dd, 1H), 7.23 (m, 2H), 7.16 (d, 2H), 4.74 (s, 2H), 2.46 (s, 3H), 2.38 (s, 3H); MS: m/z 343.1 (M+); Anal. Calcd for C18H16BrNO: C, 63.17; H, 4.71; N, 4.09; O, 4.68; found C, 63.31; H, 4.83; N, 3.96; O, 4.52.

Synthesis of 5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazole-2-thiol (6): To the compound 5 (0.01 mole) in ethanol 50 ml was added a solution of KOH (0.015 mole) in ethanol 20 ml, followed by the addition of CS2 (20 ml). The reaction mixture was heated under reflux for 8hrs. Then it was concentrated, acidified with dilute hydrochloric acid & the resulting solid was collected, washed with water & recrystallized with ethyl acetate to afford the desired product. m.p: 203 ℃, IR-(KBr): 2870, 2580 and 1570 (cm-1); 1H NMR-(CDCl3): δ 11.23 (s, 1H), 7.13 (s, 2H), 4.02 (m, 6H), 1.54 (t, 9H); MS: m/z 310.9 (M+); Anal. Calcd for C14H18N2O4S: C, 54.18; H, 5.85; N, 9.03; O, 20.62; found C, 54.31; H, 5.92; N, 8.91; O, 20.51.

Synthesis of 3-((5-(3,4,5-triethoxyphenyl)-1,3,4-oxadiazol-2-ylthio)methyl)-2-(p-tolyloxy) quinoline) (7a): To the mixture of 6 (1 eq.) and K2CO3 (1.2 eq.) in DMF, 4a (1.2 eq.) was added and the reaction mixture was stirred at room temperature for 15-20 min. The completion of the reaction was monitored by TLC. After completion ice cold water was added to the reaction mass and the solid thus obtained was filtered off and recrystallized from ethyl acetate.

Compound 7a: IR-(KBr): 2890, 1680, 1180 and 1090 (cm-1); 1H NMR-(CDCl3): δ 8.27 (dd, 1H), 8.13 (s, 1H), 7.92 (dd,1H), 7.86 (dd, 1H), 7.61 (m, 1H), 6.69 (s, 2H), 4.35 (s, 2H), 3.95 (m, 6H), 1.54 (t, 9H); MS: m/z 486.8 (M+); Anal. Calcd for C24H24ClN3O4S: C, 59.31; H, 4.98; N, 8.65; S, 6.60; found C, 59.52; H, 5.03; N, 8.38; S, 6.54.

Compound 7c: IR-(KBr): 2922, 1660, 1310 and 1210 (cm-1); 1H NMR-(CDCl3): δ 8.21 (d, 1H), 8.10 (s, 1H), 7.87 (d, 1H), 7.45 (s, 1H), 6.71 (s, 2H), 4.32 (s, 2H) 4.01 (m, 6H), 3.95 (s, 3H), 1.57 (t, 9H); MS: m/z 515.9 (M+); Anal. Calcd for C25H26ClN3O5S: C, 58.19; H, 5.08; N, 8.14; S, 6.21; found C, 58.34; H, 5.16; N, 8.01; S, 6.13.

Compound 7f: IR-(KBr): 2910, 1670, 1315 and 1275 (cm-1); 1H NMR-(CDCl3): δ 8.02 (dd, 1H), 7.92 (s, 1H), 7.81 (dd, 2H), 7.56 (m, 1H), 7.32 (d, 2H), 7.21 (d, 2H), 7.05 (s, 2H), 4.45 (s, 2H), 3.94 (m, 6H), 2.81 (s, 3H), 1.55 (t, 9H); MS: m/z 558.2 (M+); Anal. Calcd for C31H31N3O5S: C, 66.77; H, 5.60; N, 7.54; S, 5.75; found C, 66.86; H, 5.67; N, 7.43; S, 5.69.

Compound 7i: IR-(KBr): 2950, 1645, 1290 and 1150 (cm-1); 1H NMR-(CDCl3): δ 8.05 (d, 1H), 7.89 (s, 1H), 7.68 (d, 2H), 7.27 (d, 2H), 7.02 (d, 2H), 6.63 (s, 2H), 4.37 (s, 2H), 3.94 (m, 6H), 2.86 (s, 3H), 1.57 (t, 9H); MS: m/z 572.1 (M+); Anal. Calcd for C32H33N3O5S: C, 67.23; H, 5.82; N, 7.35; S, 5.61; found C, 67.41; H, 5.91; N, 7.25; S, 5.53.

Antibacterial activity

The MICs of the chemical compounds assays were carried out as described by well-diffusion method.26 Two Gram-positive (Staphylococcus aureus ATCC 25923 and Bacillus subtilis ATCC 6633) and two Gram-negative (Salmonella typhimurium (ATCC No, 23564) and Pseudomonas aeruginosa ATCC 27853) bacteria were used as quality control strains. Streptomycin and Ampicilline were used as standard antibacterial agent. The bacterial liquid cultures were prepared in fusion broth for their activity tests. The compounds were dissolved in DMSO at concentration of 1 mg/ml. Antibacterial activity of DMSO against the test organisms was investigated, and was found to be nil. Molten nutrient agar (15 cm3), kept at 45 ℃, was then poured into the Petri dishes and allowed to solidify. Ten millimeter diameter holes were then punched carefully using a sterile cork borer and completely filled with the test solutions. The plates were incubated for 24 h at 37 ℃. After 24 h, the inhibition zone that appeared around the holes in each plate was measured. Antibacterial activity was determined by examining the minimal inhibitory concentration (MICs, μg/mL) of the tested compounds, which are recorded in (Table 2).