Abstract
2-Oxo-2H-chromene-3-carbohydrazide derivatives 2a,b react with 2-{[4-(substituted thiazol-2-yl)iminoethyl)-phenyl]hydrazono}-3-oxo-butyric acid ethyl esters 4a–c to give 3-methyl-1-[(2-oxo-2H-chromen-3-yl) carbonyl]-4-{[4-(substituted thiazol-2-yl)iminoethyl)-phenyl]hydrazono}]-2-pyrazolin-5-one derivatives 5a–f. A considerable increase in the reaction rate had been observed with better yield using microwave irradiation for the synthesis of compounds 2a, b, 3a–c, and 5a–f. The synthesized products were tested against B. subtilis, S. aureus, and E. coli as well as C. albicans compared with tetracycline and nystatin as reference drugs.
1. Introduction
Pyrazolones are an increasingly popular functionality with wide-range applications. They are used as antibacterial [1], antifungal [2], anti-inflammatory [3], cytotoxic [4], analgesic [5], antiviral [6], and SARS-corona virus 3C-like protease inhibitors [7] agents. Furthermore, 4-(1,3-benzothiazol-2-yl)hydraziny lidene-2,4-dihydro-3H-pyrazol-3-one derivatives have shown good antioxidant activity [8]. On the other hand, the iminothiazoline derivatives have been reported to exhibit antibacterial and antifungal activities [9, 10]. Coumarin derivatives possess various biological activities namely antibacterial [11, 12], antifungal [13], antitumor [14], anticoagulant [15], and anti-inflammatory agents [16]. These compounds are used as additives in food and cosmetics [17], dispersed fluorescent brightening agents, and as dyes for tuning lasers [18]. Microwave irradiation is well known to promote the synthesis of a variety of organic compounds [19–21] where chemical reactions are accelerated because of selective absorption of microwave by polar molecules. In view of these observations we report herein the synthesis of some pyrazolone derivatives, using conventional and microwave irradiated methods, with the hope to get better antimicrobial agents.
2. Experimental
Melting points were estimated using a Stuart apparatus in open capillaries. Purity of compounds was checked by TLC. IR spectra were recorded as KBr pellets on a Jasco FTIR 460 plus spectrophotometer. 1H-NMR spectra were recorded using a Bruker AV 500 MHz spectrometer using DMSO-d6 as solvent and TMS as an internal standard. The mass spectral data were obtained with a Micro Spectrometer model 7070 at 70 eV and a 90°C inlet temperature. Elemental analyses were performed on a Perkin-Elmer 240 microanalyser in the Faculty of Science, Cairo University. Microwave irradiations (MWIs) were carried out in a SANYO EM-700T domestic oven (700 W).
3. Synthesis of 2-Oxo-2H-Chromene-3-carbohydrazides (2a,b)
3.1. Method I
A mixture of ethyl-2-oxo-2H-chromene-3-carboxylates (1a,b) (0.01 mole) and hydrazine hydrate 98% (0.5 mole) was refluxed for 2 h. The reaction mixture was poured into water and then the separated solid was filtered off and recrystallized from ethanol to give 2a, b (Tables 1 and 2).
3.2. Method II
Hydrazine hydrate (0.01 mole), 1a,b (0.01 mole), and absolute ethanol (2 mL) were irradiated in an Erlenmeyer flask under MWI for 3 min. The reaction mixture was cooled, and the separated solid was filtered off and washed with water to yield 2a,b (Table 3).
4. Synthesis of 1-(4-Aminophenyl)-1-(substituted thiazol-2-yl)iminoethanes 3a–c
4.1. Method I
A mixture of 4-aminoacetophenone (0.01 mole) and the appropriate amine, namely, (2-aminothiazole, 2-amino-5-methylthiazol and 2-aminobenzothiazole) (0.01 mole for each) in DMF (30 mL) was heated under reflux for 4 h. The precipitated solid formed after cooling was collected by filtration. Then, it was washed with water, dried, and crystallized to afford 3a–c (Tables 1 and 2).
4.2. Method II
A solution of 4-aminoacetophenone (0.01 mole) in methanol (30 mL) and 2-aminothiazoles (0.01 mole) was put in round-bottomed flask placed in a microwave oven and irradiated for 2.0–3.5 min, and then the solvent was removed by vacuum distillation. The solid product was filtered, dried, and recrystallized from ethanol to give 3a–c (Table 3).
5. Synthesis of 2-{[4-(Substituted thiazol-2-yl)iminoethyl)-phenyl]Hydrazono}-3-oxo-Butyric Acid Ethyl Ester 4a–c
A solution of sodium nitrite (0.01 mole) in water (10 mL) was added to an ice-cooled mixture of 3a–c (0.01 mole) in concentrated HCl (10 mL) and water (10 mL). The diazotized compound was dropped while cooling with stirring over a cold mixture of ethyl acetoacetate (0.01 mole) and sodium acetate (2 g in 10 mL water) in ethanol (20 mL). The reaction mixture was stirred at room temperature for 8 h. The precipitated solid was collected by filtration. Then it was washed with water, dried, and recrystallized to afford 4a–c (Tables 1 and 2).
6. Synthesis of 3-Methyl-1-[(2-oxo-2H-chromen-3-yl) carbonyl]-4-{[4-(substituted thiazol-2-yl)iminoethyl)-phenyl]hydrazono}]-2-pyrazolin-5-one Derivatives (5a–f)
6.1. Method I
A mixture of 4a–c (0.002 mole for each) and (2a,b) (0.002 mole for each) in acetic acid (15 mL) was refluxed for 8 h. The reaction mixture was then allowed to stand overnight. After conclusion of the reaction (TLC), the reaction mixture was poured onto crushed ice to give sticky product which was extracted from ether for three times. The organic layer was washed with water. Then, it was dried with MgSO4 and filtered. After that, the solvent was removed under vacuum to give 5a–f (Tables 1 and 2).
6.2. Method II
A mixture of 4a–c (0.01 mole for each), 2a,b (0.01 mole for each), and glacial acetic acid (2 mL) in an Erlenmeyer flask was exposed to pulsed microwave irradiation using microwave oven for 3 min. The reaction mixture was poured onto crushed ice; the separated solid was filtered, then washed with water, and dried to give the pyrazolin-5-ones 5a–f (Table 3).
7. Results and Discussion
Treatment of ethyl-2-oxo-2H-chromene-3-carboxylates (1a,b) [22] with hydrazine hydrate under heating reflux afforded 2-oxo-2H-chromene-3-carbohydrazide derivatives 2a,b. Again, compounds 2a,b were obtained in a 94–98% yield by irradiation of (1a,b) with hydrazine hydrate in ethanol under MWI for 3 min (Figure 1).
The IR (KBr) of compounds 2a,b displayed absorption bands at 3321(NH2), 3078(NH), 1730(lactone C=O), and 1682 cm−1 for amidic CO. The structure of 2b was also confirmed by its mass spectrum that shows molecular ion peaks (M+) at m/z 283(61.06%, 79Br) and m/z 284(32.67%, 81Br).
It has been found that the 4-aminoacetophenone reacted with 2-amino-5-methylthiazole or 2-aminobenzothiazole in DMF to give the imino derivatives (3b,c), similar to compound (3a) [23]. It was found that the higher yields of compounds 3a–c were obtained at 500 watt for 2-3 min of microwave irradiation (Figure 2).
IR spectrum (KBr) of 3b, as an example, showed bands at 3400-3388 (NH2), and 1640 cm−1 for C=N. The 1H-NMR spectrum (DMSO-d6) of 3b showed signals at δ 2.01 (s. 3H, CH3), 2.72 (s. 3H, CH3), 6.88 (s, 2H, NH2, D2O-exchangeable), and 7.22–7.82 ppm (m, 5H, ArH). Diazotization of 3a–c followed by coupling with ethyl acetoacetate in presence of sodium acetate gave 2-{[4-(substituted thiazol-2-yl)iminoethyl)-phenyl]hydrazono}-3-oxo-butyric acid ethyl esters 4a–c (Figure 3).
IR spectra of 4a–c revealed no absorption band in amino group region; furthermore, it displayed absorption bands at 1760 and 1560 cm−1 for ester group and (N=N), respectively. The 1H-NMR spectrum (DMSO-d6) of compound 4c, as an example, exhibited signals at δ 1.27 (t, 3H, OCH2CH3), 2.57 (s, 3H, CH3), 2.80 (s, 3H, CH3), 3.22 (s, 1H, CH), 4.27–4.29 (q, 2H, OCH2CH3) and 6.66–7.98 ppm (m, 8H, ArH).
Refluxing compounds 2a,b and 4a–c in acetic acid for 8 hrs lead to 3-methyl-1-[(2-oxo-2H-chromen-3-yl) carbonyl]-4-{[4-(substituted thiazol-2-yl)iminoethyl)-phenyl]hydrazono}]-2-pyrazolin-5-one derivatives 5a–f. These pyrazolin-5-one derivatives were obtained in good yield through the irradiation of a mixture of 2a,b and 4a–c using microwave oven for 3-4 min (Figure 4).
The 1H-NMR spectrum (DMSO-d6) of compounds 5a, as an example, showed signals at δ 2.03 (s, 3H, CH3), 2.92 (s, 3H, CH3), 4.58 (s, 1H, CH), 7.31–7.52 (m, 10H, ArH), 9.72 (s, 1H, CH-4), and 11.40 ppm (s, 1H, NH, disappeared after D2O exchange). The infrared spectrum of compounds 5a–f revealed absorption bands characteristic for NH, lactone CO, CO, and C=N. The mass spectra of compounds containing bromine atoms (5d,5e, and 5f) showed fragments corresponding to the typical bromine isotope (79Br and 81Br) patterns. Thus, the mass spectrum of 5d shows its M+1 and M+ peaks at m/z 578 (22.52%) and 577 (78.93%), respectively (Table 3).
8. Antimicrobial Activity
The antimicrobial activity of new compounds was investigated against a variety of microorganisms, including the gram-positive bacteria Bacillus subtilis and Staphylococcus aureus and the gram-negative bacteria Escherichia coli as well as the unicellular fungi Candida albicans. The agar plate disc-diffusion method [24] was used to assess the activity of the compounds. Sterilized filter paper discs (5 mm in diameter) were wetted with 10 μL each of a solution of the tested compound (10 mg/mL of the compound in DMF). The discs were then allowed to dry and were placed on the surface of agar plates seeded with the test organism. Nutrient agar was used for bacterial plating and sabourauds dextrose agar for fungi. Each plate contained 15 mL of the agar medium, previously seeded with 0.2 mL of an 18 h-old broth culture of each organism. The inoculated plates were incubated at 37°C for 48 h with the test discs in place, and the inhibition zones were measured in mm. Controls including the use of the solvent DMF without test compounds that showed no antimicrobial activity for this solvent. The antibacterial reference tetracycline discs and the antifungal reference nystatin discs were tested concurrently as standards.
Concerning the data of antimicrobial activity in Table 4, some of the synthesized compounds showed antibacterial activity comparable to that of tetracycline (the reference drug used). Concerning the activity against gram-positive bacteria (Bacillus subtilis), 3-methyl-1-[(2-oxo-2H-chromen-3-yl)carbonyl]-4-[(4-thiazol-2-yl)iminoethyl)phenyl) hydrazon]-2-pyrazolin-5-one derivative (5a) showed excellent activity, and compounds 5b and 5d exhibited good activity, whereas compounds 4c,5e, and 5f showed moderate activity. All synthesized compounds exhibited moderate activity against Staphylococcus aureus. Compounds 5a and 5f showed excellent activity against gram-negative bacteria Escherichia coli. On the other hand, the unicellular fungi Candida albicans showed high responses to 5d.
9. Conclusion
This paper describes the synthesis, spectral characterization, and screening of antimicrobial activity of some 3-methylpyrazolin-5-one derivatives bearing side chains, imino-thiazole-phenylhydrazone derivatives, and coumarin moiety. Microwave-assisted synthesis was also used to improve the yield and reaction time. The synthesized compounds showed a wide range of potentially promising antimicrobial activities.
Acknowledgment
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project no. RGP-VPP-133.