Synthesis and antimicrobial activities of some novel thiazole compounds Bazı Yeni Tiyazol Bileşiklerinin Sentezi ve Antimikrobiyal Aktiviteleri

Objective: The advent of resistant pathogenic microorganisms against current antimicrobial drugs prompted scien-tists to investigate novel molecules with new mechanisms. In this paper, some new 2-[2-[4-(ethyl/phenyl)cyclohex-ylidene]hydrazinyl]-4-(4-substitutedphenyl)thiazole ( 2a–2o ) derivatives were synthesized and studied for their antimicrobial activities. Materials and methods: The title compounds ( 2a–2o ) were obtained via the reaction of 4-(ethyl/phenyl)cyclohexane-1-one with appropriate phenacyl bromide in ethanol at room temperature. The chemical structures of the compounds were elucidated by FT-IR, 1 H-NMR, 13 C-NMR, HRMS and elemental analysis. Antimicrobial activity of the compounds was measured by using broth microdilution method. Chloramphenicol and ketoconazole were used as reference drugs. Results: Among the synthesized compounds, 2-[2-(4-phe-nylcyclohexylidene)hydrazinyl]-4-phenylthiazole ( 2h ) and 2-[2-(4-phenylcyclohexylidene)hydrazinyl]-4-(4-chlo-rophenyl)thiazole ( 2l ) have been found to exhibit potency almost four-fold better than ketoconazole against C. albicans with MIC 90 value of 1.95. Conclusion: The current study contributed to the knowl-edge of the antimicrobial activity of thiazole bearing compounds.


Introduction
One of the most significant current discussions in the world is the therapy of infectious diseases due to the increasing appearance of resistant pathogenic microorganisms against present antibacterial and antifungal drugs [1,2]. Recent research, thus, has tended to focus on developing new effective antimicrobial agents that act through different mechanisms than the conventional drugs, particularly for the treatment of the infections of hospitalized and immunosuppressed patients [3]. Therefore, the disclosure of novel and powerful antibacterial and antifungal drugs is very necessary.
Considering antimicrobial agents with innovative mode of actions, various heterocyclic rings have attracted a great interest over the years owing to their different biological activities. Among diverse heterocyclic compounds, thiazoles and their derivatives are crucial scaffolds in medicinal chemistry. In many pharmaceutically active compounds and natural products such as including thiamin and penicillin G, thiazole ring composes the scaffold of core molecular structure. Thiazole compounds are accompanied with improved lipophilicity and are metabolized via known biochemical reactions [4]. The enthusiasm for thiazoles is because of their potential natural action and magical physicochemical characteristics thus, some many potent drugs such as sulfathiazole (antimicrobial drug) and abafungin (antifungal drug) contain a thiazole ring. Thiazole and its derivatives are important pharmacophore and they have a broad range of biological activities including antimicrobial [5][6][7][8][9][10], antitumor [11,12], anti-inflammatory [13], anti-cancer [14], anti-tubercular [15,16], antiviral [17], antioxidant [18], anti HIV [19], antihypertensive [20], antischizophrenia [21], antiallergic [22] and analgesic activity [23]. Besides that, it was clear that thiazoles have gotten significant consideration because of their effective properties as antimicrobial agents.
Based on the above-mentioned findings to recognize new candidates that may be with a great value in designing new, potent and selective antimicrobial agents, we report in this paper the synthesis and antimicrobial activity of some new thiazole derivatives.

Antimicrobial activity assay
Antimicrobial activity studies were performed according to the following guides CLSI reference M07-A9 broth microdilution method [27] for bacterial strains and EUCAST definitive (EDef 7.1) method [28] for fungal strains.

Broth microdilution assay
Mueller-Hinton broth (Difco) was used to produce the bacterial strains. The strains were incubated at 37 °C for 24h. The yeasts were produced in RPMI after night long incubation at 37 °C. The inoculation of test microorganisms adjusted to match the turbidity of a Mac Farland 0.5 standard tube as determined with a spectrophotometer. For antibacterial and antifungal assays, the final inoculum size was 0.5-2.5 × 10 5 cfu/mL. For test, the two-fold serial dilutions technique utilized and test was carried out in Mueller-Hinton broth and RPMI at pH = 7. As controls, the last well on the microplates including only inoculated broth was held. In order to   Table 3. Structures of the obtained compounds were confirmed by FT-IR, 1 H-NMR, 13 C-NMR, HRMS and elemental analysis.

Results and discussion
In the IR spectra of the compounds (2a-2o), some significant specific bands were observed at 3381-3304 cm −1 and 1635-1435 cm −1 belong to N-H, C=N and C=C bonds.
In the 1 H NMR spectra of all final compounds, the protons of cyclohexyl ring were observed at 1.01-3.18 ppm. A singlet peak due to thiazole ring was resonated at 7.07-8.62 ppm. The other protons in aromatic region were observed at the range of 6.91-8.34 ppm. The signals correspond to N-H residue were observed at about 10.71-11.09 ppm. The other aromatic and aliphatic protons were observed at the expected regions.
In the 13 C NMR spectrum of the compounds, the signals belonging to -CH 2 -CH 3 group in compounds 2a-2g, were determined at 11.85-12.14 ppm. The carbon atoms belonging to cyclohexyl ring were determined at 21.26-43.03 ppm. The signals belonging to C 2 carbon of thiazole ring were resonated at 159. .58 ppm all the other aromatic and aliphatic carbons were observed at expected regions.

Antimicrobial activity
MICs were recorded as the minimum concentration of a compound that inhibits the growth of tested microorganisms. The results are summarized in Tables 1  and 2. The antibacterial assessment showed that the compounds possess no useful inhibitory action. On the other hand, some of the compounds tested illustrated remarkable antifungal activity when compared with reference drug, ketoconazole. In the antifungal activity, compound 2h with nonsubstituted phenyl ring, compound 2l with para-chloro substituent on phenyl ring and compound 2m with para-fluoro substituent on phenyl ring exhibited significant antifungal activity against tested fungi species. According to results, it is clear that besides nonsubstituted derivative, halogen substituted derivatives possessed enhanced antimicrobial activity.
In comparing their MIC values with that of chloramphenicol, compound 2h and 2l, showed potency approximately four-fold better than a reference drug against C. albicans with MIC 90 values of 1.95. In addition, compound 2h, 2l and 2m exhibited equal level of activity with ketoconazole against C. krusei and C. parapsilosis. Also compound 2h, 2l and 2m showed comparable activities against C. glabrata and the other compounds were found less active than the reference agent used.
Since, the addition of halogen substituents (in particular F, Cl, Br, I) will increase the lipophilicity of molecules and as consequence increase penetration to the bacterial cell [29], it appears that nonsubstitution or substitution on para position with chloro or fluoro on phenyl ring attached to the thiazole moiety contribute to the outstanding antifungal activity.

Conclusion
In the present study, we synthesized a new series of 4-(4-substitutedphenyl)-2-[2-[4-(ethyl/phenyl)cyclohexylidene] hydrazinyl]thiazole (2a-2o) derivatives and screen for their antibacterial and antifungal activity. According to the results, it was observed that some of the compounds exhibited remarkable effects. Among the them, compound 2h with nonsubstituted phenyl ring and compound 2l with chloro substituent at para position on phenyl ring were found to be the most promising antifungal agents with MIC values of 1.95 that is approximately four-fold better than the reference drug chloramphenicol against C. albicans.

Conflict of interest statement:
The authors declare no conflicts of interest.