A facile access and evaluation of some novel thiazole and 1,3,4-thiadiazole derivatives incorporating thiazole moiety as potent anticancer agents

Background Many heterocyclic compounds containing thiazole or 1,3,4-thiadiazole ring in their skeletons have been reported to possess various pharmacological activities especially anticancer activities. Results 4-Methyl-2-phenylthiazole-5-carbohydrazide (2) was used as a synthon to prepare 2-(4-methyl-2-phenylthiazole-5-carbonyl)-N-phenylhydrazinecarbothioamide (3) and 2-(2-(4-methyl-2-phenylthiazole-5-carbonyl)hydrazono)-N′-phenylpropane hydrazonoyl chlorides 5a–c. In addition, thioamide 3 was used as starting material for preparation of a new series of thiadiazole derivatives via its reaction with hydrazonoyl chlorides 5a–c in dioxane using triethylamines as catalyst. In addition, a series of thiazole derivatives was synthesized by reaction of thioamide 3 with a number of α-halo compounds, namely, 3-chloropentane-2,4-dione (8) or 2-chloro-3-oxo-N-phenyl butanamide (10) phenacyl bromide 12 ethyl chloroacetate (14) in EtOH in the presence of triethylamine. The structures assigned for all the new products were elucidated based on both elemental analyses and spectral data and the mechanisms of their formation was also discussed. Moreover, the new products was evaluated in vitro by MTT assays for their anticancer activity against cell lines of Hepatocellular carcinoma cell line (HepG-2). The best result observed for compounds 7b (IC50 = 1.61 ± 1.92 (μg/mL)) and 11 (IC50 = 1.98 ± 1.22 (μg/mL)). The structure–activity relationships have been suggested based on their anticancer results. Conclusions A novel series of new pharmacophores containing thiazole moiety have been synthesized using a facile and convenient methods and evaluated as potent anticancer agents.

The reaction of compound 2 with the appropriate hydrazonoyl chlorides 4a-c [32] in refluxing ethanol yielded the corresponding condensation product 5 (Scheme 2). The IR spectra of the latter products revealed a carbonyl and two NH absorption bands (see "Experimental" part). Their 1 HNMR showed two D 2 O exchangeable signals of two NH protons in the regions δ 10.03-10.06 and δ 10.57-10.59 ppm. Also, their mass spectra confirmed the assigned structure 5 (Scheme 2). Treatment of thioamide derivative 3 with the appropriate hydrazonoyl halides of type 5a-c in refluxing EtOH   containing TEA gave the corresponding thiadiazole derivatives 7a-c (Scheme 2). Their structures were elucidated on the basis of their spectral data and elemental analysis (see "Experimental").

Anticancer activity
The synthesized compounds were tested as anticancer agents against human Hepatocellular carcinoma cell line (HepG-2) using colorimetric MTT assay. We also included the well-known anticancer standard drug (Cisplatin) in the same assay to compare the potency of the synthesized compounds. The IC 50 (the concentration of test compounds required to kill 50% of cell population) was determined (Table 1, Fig. 3).
From the data of Table 1, we concluded the following structure-activity relationships (SARs): • The thiazole ring is essential for the activity.
• Less number of thiazole ring as in compounds 5a-c lead to drastic drop in activity. • 1,3,4-Thiadiazole ring is crucial for the cytotoxic activity. • Presence of methyl group (electron donating group) at position 4 of the phenyl ring in compound 7b increase its activity more than compound 7a. • The presence of the N-phenylcarboxamide group in compound 11 leads to increasing of its cytotoxic activity.

Chemistry General
Melting points were measured on an Electrothermal IA 9000 series digital melting point apparatus (Bibby Sci.

Anticancer activity
The cytotoxic evaluation of the synthesized compounds was carried out at the Regional Center for Mycology and Biotechnology at Al-Azhar University, Cairo, Egypt according to the reported method [33].

Conclusions
We successfully synthesized a series of novel heterocycles containing thiazole and 1,3,4-thiadiazole rings by a facile and convenient method. The structure of the newly prepared compounds was established based on both elemental analysis and spectroscopic data. The anticancer activity of the synthesized compounds was measured and showed promising activity.