Synthesis of Some New Thiazole Derivatives and Their Biological Activity Evaluation

New 2-(4-arylpiperazine-1-yl)-N-[4-(2-(4-substituted phenyl)thiazol-4-yl)phenyl]acetamide derivatives were synthesized and evaluated for their antimicrobial and anticholinesterase activities. Acetylcholinesterase inhibitory activities of the compounds were foundweak contrary to expectations. It is unlikely that antifungal activity of the compoundswas found significant, especially against Candida parapsilosis.


Introduction
The treatment of infectious diseases still remains a challenging task because of combination of factors including increasing number of multidrug resistant, microbial pathogens and emerging newer infectious diseases [1].The therapeutic problem is a crucial part of hospitalized patients, immunosuppressed patients with AIDS, and those undergoing anticancer therapy or organ transplants.In spite of a large number of antibiotics and chemotherapeutics available for medical use, the emerging resistance to old and antibiotics has created a substantial need for novel classes of antimicrobial agents [2].So, it is necessary to design new and effective molecules as antimicrobial agents.
Small-ring heterocycles including nitrogen and sulfur have been under investigation for a long time on account of their synthetic diversity and therapeutic relevance.Among the wide range of heterocycles explored to privileged candidates in drug discovery, thiazoles have been identified to play a necessary role in medical chemistry [3].
Thiazole ring is a structural fragment of natural compounds such as thiamine (vitamin B1), thiamine pyrophosphate (TPP, a coenzyme important in respiration in the Krebs cycle), epothilones, carboxylase, and the large family of macrocyclic thiopeptide antibiotics, thiostrepton and micrococcin P1 [4,5].Thiazole derivatives are associated with a broad spectrum of biological properties, including anticonvulsant, antimicrobial, antituberculous, bacteriostatic activities, antiviral, antimalarial, anticancer, hypertension, inflammation, schizophrenia, HIV infections, hypnotics and more recently for the treatment of pain, as fibrinojen receptor antagonists with antithrombotic activity, as new inhibitors of bacterial DNA gyrase B [6][7][8][9][10].Thiazole derivatives are also found in application in the drug development for the treatment of allergies [11].
Many currently notable drugs contain piperazine ring and amide moiety as part of their molecular structure [12].Piperazines are also among the most important building blocks in today's drug discovery and are found in biologically active compounds across a number of different therapeutic areas such as antifungal, antibacterial, antimalarial, and antipsychotic activities [13].
In our previous study, cholinesterase activity of some thiazole-piperazine derivatives was reported [14].Prompted from the data we have obtained before and the above literature research, we synthesized a new series of thiazolepiperazine derivatives and investigated their antimicrobial and anticholinesterase activities so as to obtain new biologically active compounds.

Experimental
2.1.Material.Melting points ( ∘ C, uncorrected) of the synthesized compounds were determined by using electrothermal melting point apparatus (Electrothermal, Essex, UK). 1 H NMR spectra were recorded with a Bruker 500 MHz digital FT-NMR spectrometer (Bruker Bioscience, Billerica, MA, USA) and 13 C NMR spectra were recorded with a Bruker 125 MHz digital FT-NMR spectrometer in DMSOd 6 solvent including TMS as an internal standard.Coupling constants () are reported as Hertz.IR spectra were obtained in KBr pressed pellets via IR, Shimadzu 8400S spectrophotometer (Shimadzu, Tokyo, Japan).Mass spectra of fast atom bombardment (FAB) were obtained on a VG Quattro mass spectrometer VG Quattro Mass spectrometer (Fisons Instruments Vertriebs GmbH, Mainz, Germany).The purities of compounds were checked by TLC on silica gel 60 F 254 (Merck KGaA, Darmstadt, Germany).Starting materials were either commercially available or synthesized according to the methods given in the literature.(1.5 mmol), appropriate piperazine derivative (1.8 mmol) and K 2 CO 3 (1.5 mmol) in acetone, was refluxed for 2 h.After cooling, the solvent was evaporated until dryness.The residue was treated with 25 mL of water.Solidified product was filtered, washed with water, and recrystallized from ethanol to give the compounds 6a-6p., s, N-H).For C 29 H 30 N 4 O 2 S calculated: (%) C 69.85, H 6.06, N 11.24; found: (%) C 69.92, H 6.14, N 11.31.MS: / 499 (M + 1).

Anticholinesterase Activity.
The inhibition of acetylcholinesterase enzyme, AChE (E.C.3.1.1.7 from Electric Eel, 500 units), was determined spectrophotometrically using acetylthiocholine (ATC) as substrate according to the modified method of Ellman's test [16].Enzyme solutions were prepared in gelatin solution (1%), at a concentration of 2.5 units/mL.AChE and compound solution (50 L) which is prepared in 2% dimethyl sulfoxide at 0.1 and 1 mM concentrations were added to 3.0 mL phosphate buffer (pH 8 ± 0.1) and incubated at 25 ∘ C for 5 min.The reaction was started by adding 5,5-dithio-bis(2-nitrobenzoic)acid, DTNB (50 L), and ATC (10 L) to the enzyme-inhibitor mixture.Spectrophotometric measurements were performed on a 1700 Shimadzu UV-1700 UV-Vis spectrophotometer at 412 nm.Donepezil hydrochloride was used as a positive control and all processes were assayed in triplicate [17].
Whereupon considering many studies on thiazole derivatives biological profile, we tend to screen antimicrobial activity of the synthesized 2,4-disubstituted thiazole compounds.Sixteen new compounds were tested for their antibacterial and antifungal activities.The antifungal activity of the compounds was found to be higher than their antimicrobial activities (Table 1).Compounds 6c and 6d showed higher activity than the other compounds against E. faecalis which is a Gram-positive bacterium.Among the tested Gram-negative bacteria, compounds 6c-6f, 6i and 6j exhibited the highest activity against K. pneumonia with a MIC value of 100 g/mL which is two-fold standard drug's MIC value.Compound 6l showed equipotent activity to standard drug againstanother Gram-negative bacterium P. aeruginosa.Against two tested E. coli species (ATCC 35218 and ATCC 25922), only compounds 6d and 6m displayed moderate antibacterial activity compared with standard drug chloramphenicol.As for the antifungal activity, compounds showed higher activities against four tested Candida species.Compounds 6a, 6b, 6i, and 6m exhibited same efficiency with ketoconazole against C. albicans and also compound 6f towards C. glabrata.Against C. krusei, the MIC value was found as 50 g/mL for compound 6a which was very high according to ketoconazole's MIC value (3.13 g/mL).Among all Candida species, the most sensitive Candida was established as C. parapsilosis.Compounds 6b, 6c, and 6h were found twice as active as standard drug, whereas compounds 6a, 6d, 6e, 6f, 6m, and 6n showed equipotent activity.All the other compounds displayed half the potency of ketoconazole against the same microorganism.
The structures of the tested compounds were based on a thiazole ring which has phenyl moiety at second and fourth positions of it.The substitutions of the phenyl ring at second position are methoxy, chloro and fluoro groups.The difference at the fourth position of the thiazole ring is pyridyl, pyrimidinyl benzyl, and furoyl piperazine moieties bonded to phenyl ring with an amide linker.2-Furoyl piperazine substitution has attracted attention for its antibacterial activity.In antifungal activity evaluation, nonsubstituted phenyl group and different substituted piperazine substitutions were observed with significant activity according to Candida species.
NDactivity of the compounds was found promising.Compounds 6b, 6c, and 6h exhibited two-fold anticandidal activity compared with ketoconazole against C. parapsilosis.