Synthesis and antimicrobial activity of some new hydrazone-bridged thiazole-pyrrole derivatives

Abstract

In this work, we synthesized fourteen different compounds which contain hydrazone bridged thiazole and pyrrole rings. For this purpose, pyrrole-2-carboxaldehydes were reacted directly with thiosemicarbazide in ethanol and then obtained thiosemicarbazones were condensed with α-bromoacetophenone derivatives (Hantzsch reaction) to give 1-substituted pyrrole-2-carboxaldehyde [4-(4-substituted phenyl)-1,3-thiazol-2-yl] hydrazones. The structures of the obtained compounds were elucidated by using IR, ¹H-NMR and FAB⁺-MS spectral data and elemental analyses results. All of the compounds were screened for their antibacterial and antifungal activities against twelve different microorganisms by using microbroth dilution method. Ketoconazole and chloramphenicol were used as standard drugs. All of the compounds showed good activity against Staphylococcus aureus and Enterococcus faecalis.

Keywords::

• Antimicrobial activity
• thiazole
• hydrazone
• pyrrole

Introduction

Throughout history, there has been a continual battle between humans and the multitude of microorganisms that causes infections and diseases¹. The high incidence of microbial disorders due to the difficulties in antimicrobial prophylaxis and treatment, the rapid emergence of new infections, the threat of multidrug-resistant microorganisms against current antibiotics, and many induced side effects due to widespread use of antimicrobial agents are major problems to overcome the common pathogens. Thus, there is a need to search for new and efficacious antimicrobial agents for the treatment of resistant infections^2–7.

Among various heterocycles that have been explored for developing pharmaceutically important molecules, thiazoles, fused thiazoles and thiazoles linked to various heterocylic rings through different linkages have recently attracted great attention⁸. Thiazoles and their derivatives have concerned continuing interest over the years because of their varied biological activities such as antiallergic, antihypertensive, antiinflammatory, antischizophrenic, antibacterial, anti-HIV, and FabH inhibitors⁹. It has also been reported in the literature that phenyl-thiazole analogues possess efficient antifungal activity¹⁰. Moreover, thiazolylhydrazines¹¹ and (4-aryl-thiazol-2-yl)hydrazines¹² have been shown to exhibit significant antimicrobial activity. These studies confirmed that thiazole ring is a good pharmacophore group for the design of bioactive molecules¹³ which is also act as a bioisoster of the imidazole ring^14,15.

Hydrazone is a versatile moiety^16,17 that has an obvious role especially in antimicrobial activity¹⁸. Additionally, the pyrrole containing heterocyclic compounds have attracted attention particularly as antimicrobial agents^19,20.

For these reasons, in this paper we report synthesis and biological evaluation of a new series of 2,4-disubstituted-1,3-thiazoles bearing pyrrole ring on double bond C=N and a 4′-substituted phenyl in position C₄ of thiazole nucleus, respectively.

Experimental

Chemistry

All chemicals were purchased from Sigma-Aldrich Chemical Co. All melting points (m.p.) were determined by Electrothermal 9100 digital melting point apparatus and were uncorrected. Spectroscopic data were recorded with the following instruments: ¹H-NMR, Bruker 400 MHz spectrometer; MS-FAB, VG Quattro Mass spectrometer and Elemental analyses were performed on a Perkin Elmer EAL 240 elemental analyser (Perkin-Elmer, Norwalk, CT, USA).

Preparation of 1-substituted pyrrol-2-carbaldehyde thiosemicarbazones (1a, 1b)

A mixture of thiosemicarbazide (0.27 g, 3 mmol) and 2-formylpyrrole or N-methyl-2-formylpyrrole (3 mmol) in ethanol (20 mL) was refluxed for 2 h. The progress of the reaction was monitored by TLC. The resulting mixture was cooled, poured into ice water, filtered and then recrystallized from ethanol to afford thiosemicarbazones (1a, 1b)²¹.

Preparation of 1-substituted pyrrole-2-carboxaldehyde [4-(4-substituted phenyl)-1,3-thiazol-2-yl]) hydrazones (2a–2n)

An anhydrous ethanolic solution (10 mL) of 1a or 1b (0.80 mmol) and α-bromoacetophenone (0.16 g, 0.80 mmol) was stirred at room temperature, until all the thiosemicarbazone disappeared in TLC. Then the deposit was filtered and recrystallized from ethanol to give target compounds (2a–2n).

Pyrrole-2-carboxaldehyde (4- phenyl-1,3-thiazol-2-yl) hydrazone (2a):

Yield: 90%. M.p. 125°C. IR (KBr) ν_max(cm−¹): 3380-3122 (N-H), 3056 (aromatic C-H), 1619-1483 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 6.16 (brs, 1H, pyrrole-H), 6.50 (brs, 1H, pyrrole-H), 6.97 (brs, 1H, pyrrole-H), 7.33 (s, 1H, C₅-H thiazole), 7.35–7.47 (m, 3H, Ar-H), 7.81–7.83 (m, 2H, Ar-H), 8.07 (s, 1H, -CH=N-N), 11.35 (s, 1H, pyrrole N-H), 11.80 (brs, 1H, C=N-NH, D₂O exch.).For C₁₄H₁₂N₄S calculated: 62.66 % C, 4.51 % H, 20.88 % N; found: 62.67 % C, 4.53 % H, 20.86 % N. MS (FAB) [M+1]⁺: m/z 267.

Pyrrole-2-carboxaldehyde [4-(4-methylphenyl)-1,3-thiazol-2-yl] hydrazone (2b):

Yield: 82%. M.p. 156°C. IR (KBr) ν_max(cm⁻¹): 3385-3150 (N-H), 3026 (aromatic C-H), 1610-1477 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 2.32 (s, 3H, C-CH₃), 6.12 (brs, 1H, pyrrole-H), 6.40 (brs, 1H, pyrrole-H), 6.89 (brs, 1H, pyrrole-H), 7.17 (s, 1H, C₅-H thiazole), 7.21 (d, 2H, Ar-H, J = 8.4 Hz), 7.72 (d, 2H, Ar-H, J = 8 Hz), 7.91 (s, 1H, -CH=N-N), 11.21 (s, 1H, pyrrole N-H), 11.79 (brs, 1H, C=N-NH, D₂O exch.). For C₁₅H₁₄N₄S calculated: 63.80 % C, 5.00 % H, 19.84 % N; found: 63.82 % C, 5.02 % H, 19.81 % N. MS (FAB) [M+1]⁺: m/z 283.

Pyrrole-2-carboxaldehyde [4-(4-methoxyphenyl)-1,3-thiazol-2-yl] hydrazone (2c):

Yield: 85%. M.p. 155 ^°C. IR (KBr) ν_max(cm⁻¹): 3369-3135 (N-H), 3021 (aromatic C-H), 1598-1487 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.78 (s, 3H, -OCH₃), 6.13 (brs, 1H, pyrrole-H), 6.41 (brs, 1H, pyrrole-H), 6.90 (brs, 1H, pyrrole-H), 6.97 (d, 2H, Ar-H, J = 8.5 Hz), 7.08 (s, 1H, C₅-H thiazole), 7.77 (d, 2H, Ar-H, J = 8 Hz), 7.93 (s, 1H, -CH=N-N), 11.22 (s, 1H, pyrrole N-H), 11.75 (brs, 1H, C=N-NH, D₂O exch.). For C₁₅H₁₄N₄OS calculated: 60.38 % C, 4.73 % H, 18.78 % N; found: 60.34 % C, 4.70 % H, 18.75 % N. MS (FAB) [M+1]⁺: m/z 299.

Pyrrole-2-carboxaldehyde [4-(4-bromophenyl)-1,3-thiazol-2-yl) hydrazone (2d):

Yield: 80%. M.p. 170°C. IR (KBr) ν_max(cm⁻¹): 3349-3126 (N-H), 3016 (aromatic C-H), 1592-1479 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 6.14 (brs, 1H, pyrrole-H), 6.46 (brs, 1H, pyrrole-H), 6.94 (brs, 1H, pyrrole-H), 7.38 (s, 1H, C₅-H thiazole), 7.62 (d, 2H, Ar-H, J = 8.8 Hz), 7.78 (d, 2H, Ar-H, J = 8.8 Hz), 8.01 (s, 1H, -CH=N-N), 11.31 (s, 1H, pyrrole N-H), 11.82 (brs, 1H, C=N-NH, D₂O exch.). For C₁₄H₁₁BrN₄S calculated: 48.43 % C, 3.19 % H, 16.14 % N; found: 48.41 % C, 3.22 % H, 16.15 % N. MS (FAB) [M+1]⁺: m/z 348.

Pyrrole-2-carboxaldehyde [4-(4-chlorophenyl)-1,3-thiazol-2-yl] hydrazone (2e):

Yield: 81%. M.p. 200°C. IR (KBr) ν_max(cm⁻¹): 3386-3220 (N-H), 3027 (aromatic C-H), 1609-1475 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 6.15 (brs, 1H, pyrrole-H), 6.49 (brs, 1H, pyrrole-H), 6.97 (brs, 1H, pyrrole-H), 7.38 (s, 1H, C₅-H thiazole), 7.51 (d, 2H, Ar-H, J = 8.8 Hz), 7.85 (d, 2H, Ar-H, J = 8.4 Hz), 8.08 (s, 1H, -CH=N-N), 11.36 (s, 1H, pyrrole N-H), 11.72 (brs, 1H, C=N-NH, D₂O exch.). For C₁₄H₁₁ClN₄S calculated: 55.53 % C, 3.66 % H, 18.50 % N; found: 55.52 % C, 3.65 % H, 18.52 % N. MS (FAB) [M+1]⁺: m/z 303.5.

Pyrrole-2-carboxaldehyde [4-(4-fluorophenyl)-1,3-thiazol-2-yl] hydrazone (2f):

Yield: 88%. M.p. 166°C. IR (KBr) ν_max(cm⁻¹): 3376-3145 (N-H), 3015 (aromatic C-H), 1611-1483 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 6.12 (brs, 1H, pyrrole-H), 6.41 (brs, 1H, pyrrole-H), 6.90 (brs, 1H, pyrrole-H), 7.21–7.25 (m, 3H, Ar-H and C₅-H thiazole), 7.87 (d, 2H, Ar-H, J = 8.1 Hz), 7.92 (s, 1H, -CH=N-N), 11.21 (s, 1H, pyrrole N-H), 11.79 (brs, 1H, C=N-NH, D₂O exch.).For C₁₄H₁₁FN₄S calculated: 58.73 % C, 3.87 % H, 19.57 % N; found: 58.72 % C, 3.85 % H, 19.55 % N. MS (FAB) [M+1]⁺: m/z 287.

Pyrrole-2-carboxaldehyde [4-(4-nitrophenyl)-1,3-thiazol-2-yl] hydrazone (2g):

Yield: 86%. M.p. 200°C. IR (KBr) ν_max(cm⁻¹): 3368-3190 (N-H), 3023 (aromatic C-H), 1612-1449 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 6.15 (brs, 1H, pyrrole-H), 6.44 (brs, 1H, pyrrole-H), 6.93 (brs, 1H, pyrrole-H), 7.67 (s, 1H, C₅-H thiazole), 7.98 (s, 1H, -CH=N-N), 8.30 (d, 2H, Ar-H, J = 8.4 Hz), 8.33 (d, 2H, Ar-H, J = 9.2 Hz), 11.28 (s, 1H, pyrrole N-H), 11.82 (brs, 1H, C=N-NH, D₂O exch.). For C₁₄H₁₁N₅O₂S calculated: 53.66 % C, 3.54 % H, 22.35 % N; found: 53.62 % C, 3.55 % H, 22.37 % N. MS (FAB) [M+1]⁺: m/z 314.

1-Methylpyrrole-2-carboxaldehyde (4- phenyl-1,3-thiazol-2-yl) hydrazone (2h):

Yield: 76%. M.p. 224°C. IR (KBr) ν_max(cm⁻¹): 3368-3156 (N-H), 3013 (aromatic C-H), 2969 (aliphatic C-H), 1589-1457 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.87 (s, 3H, N-CH₃), 6.17 (brs, 1H, pyrrole-H), 6.51 (brs, 1H, pyrrole-H), 7.01 (brs, 1H, pyrrole-H), 7.33 (s, 1H, C₅-H thiazole), 7.36–7.47 (m, 3H, Ar-H), 7.82 (d, 2H, Ar-H, J = 8 Hz), 8.18 (s, 1H, -CH=N-N), 11.91 (brs, 1H, C=N-NH, D₂O exch.).For C₁₅H₁₄N₄S calculated: 63.80 % C, 5.00 % H, 19.84 % N; found: 63.82 % C, 5.04 % H, 19.83 % N. MS (FAB) [M+1]⁺: m/z 283.

1-Methylpyrrole-2-carboxaldehyde [4-(4-methylphenyl)-1,3-thiazol-2-yl] hydrazone (2i):

Yield: 81%. M.p. 182°C. IR (KBr) ν_max(cm⁻¹): 3389-3179 (N-H), 3024 (aromatic C-H), 2987 (aliphatic C-H), 1585-1446 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 2.33 (s, 3H, C-CH₃), 3.87 (s, 3H, N-CH₃), 6.10 (brs, 1H, pyrrole-H), 6.48 (brs, 1H, pyrrole-H), 6.98 (brs, 1H, pyrrole-H), 7.23 (s, 1H, C₅-H thiazole), 7.24 (d, 2H, Ar-H, J = 8 Hz), 7.71 (d, 2H, Ar-H, J = 8 Hz), 8.12 (s, 1H, -CH=N-N), 11.92 (brs, 1H, C=N-NH, D₂O exch.). For C₁₆H₁₆N₄S calculated: 64.84 % C, 5.44 % H, 18.90 % N; found: 64.82 % C, 5.41 % H, 18.93 % N. MS (FAB) [M+1]⁺: m/z 297.

1-Methylpyrrole-2-carboxaldehyde [4-(4-methoxyphenyl)-1,3-thiazol-2-yl] hydrazone (2j):

Yield: 82%. M.p. 209°C. IR (KBr) ν_max(cm⁻¹): 3388-3169 (N-H), 3009 (aromatic C-H), 2974 (aliphatic C-H), 1579-1477 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.80 (s, 3H, O-CH₃), 3.87 (s, 3H, N-CH₃), 6.12 (brs, 1H, pyrrole-H), 6.52 (brs, 1H, pyrrole-H), 7.00-7.02 (m, 3H, Ar-H and pyrrole-H), 7.16 (s, 1H, C₅-H thiazole), 7.75 (d, 2H, Ar-H, J = 8 Hz), 8.18 (s, 1H, -CH=N-N), 11.89 (brs, 1H, C=N-NH, D₂O exch.). For C₁₆H₁₆N₄OS calculated: 61.52 % C, 5.16 % H, 17.93 % N; found: 61.53 % C, 5.18 % H, 17.96 % N. MS (FAB) [M+1]⁺: m/z 313.

1-Methylpyrrole-2-carboxaldehyde [4-(4-bromophenyl)-1,3-thiazol-2-yl] hydrazone (2k):

Yield: 76%. M.p. 214°C. IR (KBr) ν_max(cm⁻¹): 3392-3189 (N-H), 3015 (aromatic C-H), 2986 (aliphatic C-H), 1579-1454 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.87 (s, 3H, N-CH₃), 6.09 (brs, 1H, pyrrole-H), 6.45 (brs, 1H, pyrrole-H), 6.97 (brs, 1H, pyrrole-H), 7.36 (s, 1H, C₅-H thiazole), 7.61 (d, 2H, Ar-H, J = 8.4 Hz), 7.78 (d, 2H, Ar-H, J = 8.8 Hz), 8.08 (s, 1H, -CH=N-N), 11.88 (brs, 1H, C=N-NH, D₂O exch.). For C₁₅H₁₃BrN₄S calculated: 49.87 % C, 3.63 % H, 15.51 % N; found: 49.86 % C, 3.65 % H, 15.53 % N. MS (FAB) [M+1]⁺: m/z 362.

1-Methylpyrrole-2-carboxaldehyde [4-(4-chlorophenyl)-1,3-thiazol-2-yl] hydrazone (2l):

Yield: 85%. M.p. 202°C. IR (KBr) ν_max(cm⁻¹): 3369-3156 (N-H), 3033 (aromatic C-H), 2967 (aliphatic C-H), 1589-1457 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.87 (s, 3H, N-CH₃), 6.09 (brs, 1H, pyrrole-H), 6.45 (brs, 1H, pyrrole-H), 6.96 (brs, 1H, pyrrole-H), 7.35 (s, 1H, C₅-H thiazole), 7.47 (d, 2H, Ar-H, J = 8.4 Hz), 7.86 (d, 2H, Ar-H, J=8.8 Hz), 8.11 (s, 1H, -CH=N-N), 11.89 (brs, 1H, C=N-NH, D₂O exch.). For C₁₅H₁₃ClN₄S calculated: 56.87 % C, 4.14 % H, 17.68 % N; found: 56.89 % C, 4.15 % H, 17.69 % N. MS (FAB) [M+1]⁺: m/z 317.5.

1-Methylpyrrole-2-carboxaldehyde [4-(4-fluorophenyl)-1,3-thiazol-2-yl] hydrazone (2m):

Yield: 73%. M.p. 222°C. IR (KBr) ν_max(cm⁻¹): 3318-3196 (N-H), 3024 (aromatic C-H), 2979 (aliphatic C-H), 1593-1446 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.87 (s, 3H, N-CH₃), 6.11 (brs, 1H, pyrrole-H), 6.50 (brs, 1H, pyrrole-H), 7.00 (brs, 1H, pyrrole-H), 7.26-7.30 (m, 3H, Ar-H and C₅-H thiazole), 7.88 (d, 2H, Ar-H, J = 8 Hz), 8.16 (s, 1H, -CH=N-N), 11.82 (brs, 1H, C=N-NH, D₂O exch.). For C₁₅H₁₃FN₄S calculated: 59.98 % C, 4.36 % H, 18.65 % N; found: 59.97 % C, 4.38 % H, 18.69 % N. MS (FAB) [M+1]⁺: m/z 301.

1-Methylpyrrole-2-carboxaldehyde [4-(4-nitrophenyl)-1,3-thiazol-2-yl] hydrazone (2n):

Yield: 72%. M.p. 199°C. IR (KBr) ν_max(cm⁻¹): 3368-3186 (N-H), 3028 (aromatic C-H), 2989 (aliphatic C-H), 1610-1446 (C=C and C=N).¹H NMR (DMSO-d₆) δ ppm: 3.87 (s, 3H, N-CH₃), 6.09 (brs, 1H, pyrrole-H), 6.43 (brs, 1H, pyrrole-H), 6.95 (brs, 1H, pyrrole-H), 7.64 (s, 1H, C₅-H thiazole), 8.08 (s, 1H, -CH=N-N), 8.09 (d, 2H, Ar-H, J = 8.8 Hz), 8.26 (d, 2H, Ar-H, J = 8.8 Hz), 11.92 (brs, 1H, C=N-NH, D₂O exch.). For C₁₅H₁₃N₅O₂S calculated: 55.03 % C, 4.00 % H, 21.39 % N; found: 55.02 % C, 3.95 % H, 21.35 % N. MS (FAB) [M+1]⁺: m/z 328.

Microbiology

The study was designed to compare MICs obtained by the CLSI reference M7–A7 broth microdilution method^22,23. MIC readings were performed twice for each chemical agent. Final products were tested for their in-vitro growth inhibitory activity against human pathogenic as Gram-positive bacteria; Staphylococcus aureus (ATCC 25923), Enterococcus faecalis (ATCC 29212) and Listeria monocytogenes (obtained from Faculty of Pharmacy Anadolu University, Eskisehir, Turkey), as Gram-negative bacteria; Pseudomonas aeruginosa (ATCC 27853), Klebsiella pneumoniae (ATCC 13883), Escherichia coli (ATCC 35218), Salmonella typhimurium (NRRL B- 4420) and Yersinia entercolitica (ATCC 35669) and yeast as Candida albicans, Candida glabrata (ATCC 36583), Candida krusei (obtained from Faculty of Medicine Osmangazi University, Eskisehir, Turkey) and Candida parapsilosis (ATCC 22019). Chloramphenicol and ketoconazole were used as control drugs.

In order to ensure that the solvent per se had no effect on bacteria or yeast growth, a control test was also performed containing inoculated broth supplemented with only DMSO at the same dilutions used in our experiments and found inactive in culture medium.

Antimicrobial assay

The cultures were obtained from Mueller–Hinton broth (Difco) for the bacterial strains after overnight incubation at 35 ± 1°C. The yeasts were maintained in Sabouroud dextrose broth (Difco) after overnight incubation 35 ± 1°C. The inocula of test microorganisms adjusted to match the turbidity of a Mac Farland 0.5 standard tube as determined with a spectrophotometer and the final inoculum size was 0.5–2.5 × 10⁵ CFU/mL for antibacterial and antifungal assays. Testing was carried out in Mueller–Hinton broth and Sabouroud dextrose broth (Difco) at pH 7 and the two-fold serial dilutions technique was applied. The last well on the microplates containing only inoculated broth was kept as controls and the last well with no growth of microorganism was recorded to represent the MIC expressed in μg/mL. For both the antibacterial and antifungal assays the compounds were dissolved in DMSO. Further dilutions of the compounds and standard drugs in test medium were prepared at the required quantities of 800, 400, 200, 100, 50, 25, 12.5, 6.25, 3.125, 1.5625 μg/mL concentrations with Mueller–Hinton broth and Sabouroud dextrose broth. Each experiment in the antimicrobial assays was replicated twice in order to define the MIC values. Chloramphenicol and ketoconazole were used as control drugs.

Results and discussions

Chemistry

In this work, we synthesized fourteen different compounds, which contain hydrazone bridged thiazole and pyrrole rings. Pyrrole-2-carboxaldehyde and N-methylpyrrole-2-carboxaldehyde were reacted directly with thiosemicarbazide in ethanol by refluxing and the obtained thiosemicarbazones (1a, 1b) subsequently were condensed with α-bromoacetophenone derivatives (Hantzsch reaction) to give 1-substituted pyrrole-2-carboxaldehyde [4-(4-substituted phenyl)-1,3-thiazol-2-yl] hydrazones (2a-n) as shown in Scheme 1. All synthesized compounds were fully characterized by analytical and spectral data.

The FT-IR spectra of the final products showed characteristic absorption bands at 3120–3330 cm⁻¹ for –NH– and at 1590–1670 cm⁻¹ for azomethine group (–CH=N–). The ¹H-NMR spectra of compounds showed signals at δ 7.91–8.18 and δ 11.72–11.92 corresponding to azomethine (–CH=N–) proton and hydrazide (NH) proton, respectively. The broad singlet peak seen at δ 11.21–11.36 indicated the pyrrole N-H proton for the compounds 2a-g. The C₅-H proton of the thiazole was observed as a singlet at δ 7.08–7.67. The appearance of a pair of doublets and/or multiplets at δ 6.97–8.33 was due to the aromatic protons of phenyl ring. The C-H protons of the pyrrole ring were resonated at δ 6.09–7.01 region as broad singlets as expected. M+1 peaks in FAB-MS spectra were in agreement with the calculated molecular weight of the target compounds (2a–2n). Elemental analysis results for C, H, and N elements were satisfactory with calculated values of the compounds.

Scheme 1.  Synthesis of the compounds (2a-n). Reagents: (i) thiosemicarbazide, ethanol, reflux; (ii) α-bromo-4-substituted acetophenone, ethanol, rt.

Antimicrobial activity

Antimicrobial activity was investigated by finding minimum inhibitory concentration (MIC) of the synthesized compounds and reference agents against S. aureus, L. monocytogenes, E. coli, P. aeruginosa, Y. enterocolitica, E. faecalis, K. pneumoniae S. typhimurium, C. albicans, C. glabrata, C. krusei, C. parapsilosis. Resuts are summarized in Table 1.

Table 1.  Antimicrobial activities of the compounds (2a-n) (µg/mL).

Compounds	A	B	C	D	E	F	G	H	I	J	K	L
2a	200	200	400	200	6.25	50	100	200	50	50	200	6.25
2b	200	400	400	200	25	50	200	400	100	200	50	50
2c	200	400	400	200	25	50	200	400	200	200	50	25
2d	200	200	400	200	6.25	12.5	200	200	100	200	200	100
2e	200	100	200	200	6.25	12.5	200	200	100	200	200	200
2f	200	100	200	200	12.5	12.5	400	400	100	200	50	100
2g	200	100	200	200	12.5	12.5	400	200	100	200	50	200
2h	200	200	200	200	12.5	12.5	200	200	100	100	100	100
2i	200	200	200	200	6.25	12.5	200	200	100	100	100	100
2j	200	200	200	400	12.5	12.5	200	200	100	100	100	100
2k	200	200	200	400	6.25	12.5	200	200	100	100	100	100
2l	200	200	200	400	12.5	50	400	400	100	200	50	100
2m	100	100	400	100	6.25	50	50	200	50	50	100	12.5
2n	200	200	200	200	25	50	200	200	200	100	100	100
Ref. ^1	50	6.25	6.25	100	12.5	12.5	100	12.5	–	–	–	–
Ref. ^2	–	–	–	–	–	–	–	–	12.5	50	1.56	1.56

Reference 1: Chloramphenicol, Reference 2: Ketoconazole.

A: E. coli, B: L. monocytogenes, C: Y. enterocolitica, D: P. aeruginosa, E: E. faecalis, F: S. aureus, G: K. pneumoniae, H: S. typhimurium, I: C. albicans, J: C. glabrata, K: C. krusei, L: C. parapsilosis.

In general evaluating, it was observed that all of the compounds had higher antimicrobial activity against gram-positive bacteria than gram negative bacteria and fungi.

Most of the tested compounds revealed higher selectivity toward E. faecalis and S. aureus, whereas all of the compounds lacked antibacterial activity against L. monocytogenes among gram positives. Comparing with chloramphenicol, compounds 2d-l had similar MIC value of 12.5 µg/mL against S. aureus. Among all of the strains, E. faecalis was the most susceptible species. While compounds 2a, 2d, 2e, 2i, 2k and 2m showed higher antimicrobial activity, compounds 2f, 2g, 2h, 2j and 2l showed equipotent activity to standard drug against E. faecalis.

With regard to the activity against K. pneumoniae, potential activity was displayed by compound 2m (MIC 50 µg/mL), which were twice as active as chloramphenicol (MIC 100 µg /mL). The compounds 2b, 2c, 2d, 2e, 2h, 2i, 2j, 2k and 2n (MIC 200 µg/mL) displayed half the potency of chloramphenicol, meanwhile compound 2a (MIC 100 µg/mL) showed equal potency to chloramphenicole (MIC 100 µg/mL) against the same organism.

Compound 2m was determined as the most active compound against P. aeruginosa and E. coli. Among other gram negative bacterial strains, Y. enterocolitica and S. typhimurium exhibited resistance all of the tested compounds.

The antifungal activity of the compounds was studied against four Candida species, the most sensitive Candida was established as C. glabrata toward 2a and 2m compounds, which had the same MIC value (50 µg/mL) with ketoconazole. However, there was not any obvious inhibitory effect against other Candida strains.

An insight into the structures of the active compounds revealed that the tested compounds belong two main structure series; pyrrole-2-carboxaldehyde [4-(4-substituted phenyl)-1,3-thiazol-2-yl] hydrazone (2a-g) and 1-methylpyrrole-2-carboxaldehyde [4-(4- substituted phenyl)-1,3-thiazol-2-yl] hydrazone (2h-n). These series differ from each other due to methyl substituent at N-H position of the pyrrole ring. It was observed that methyl substitution (N-CH₃) on pyrrole ring influenced the activity essentially. It seemed that most of the compounds bearing methyl substituent on pyrrole moiety had higher activity according to the compounds, which do not contain methyl substitution.

On the other hand, it was determined that substitution of the phenyl moiety at para position also caused changes in activity. In antibacterial activity evaluating, the most active compound was 2e, which includes fluoro substituent on phenyl ring and non-substituted pyrrole. Among the N-methylpyrrole moiety containing compounds (2h-n), the compound 2m, which carries fluoro substituent on phenyl ring displayed notable improvement in the spectrum of both antibacterial and antifungal activity. Furthermore, compound 2m seemed to be much more active than chloramphenicol against E. faecalis and K. pneumoniae bacterial strains.

In antifungal activity evaluating, it was observed that 2a and 2m were the most active compounds. In compounds 2g and 2n, nitro substitution on phenyl moiety at para position caused a reduction in both antibacterial and antifungal activity. Thus, it can be claimed that substitution of phenyl moiety at para position with an electron withdrawing group reduces the antimicrobial activity. Among all of the compounds, there were not any clear inhibitory activity against L. monocytogenes, Y. enterocolitica, S. typhimurium and C. krusei.

Declaration of interest

The authors report no declarations of interest.