Stereoselective synthesis, X-ray analysis, computational studies and biological evaluation of new thiazole derivatives as potential anticancer agents

Background The synthesis of new thiazole derivatives is very important because of their diverse biological activities. Also , many drugs containing thiazole ring in their skeletons are available in the market such as Abafungin, Acotiamide, Alagebrium, Amiphenazole, Brecanavir, Carumonam, Cefepime, and Cefmatilen. Results Ethyl cyanoacetate reacted with phenylisothiocyanate, chloroacetone, in two different basic mediums to afford the thiazole derivative 6, which reacted with dimethylformamide- dimethyl acetal in the presence of DMF to afford the unexpected thiazole derivative 11. The structures of the thiazoles 6 and 11 were optimized using B3LYP/6-31G(d,p) method. The experimentally and theoretically geometric parameters agreed very well. Also, the natural charges at the different atomic sites were predicted. HOMO and LUMO demands were discussed. The anticancer activity of the prepared compounds was evaluated and showed moderate activity. Conclusions Synthesis of novel thiazole derivatives was done. The structure was established using X-ray and spectral analysis. Optimized molecular structures at the B3LYP/6-31G(d,p) level were investigated. Thiazole derivative 11 has more electropositive S-atom than thiazole 6. The HOMO–LUMO energy gap is lower in the former compared to the latter. The synthesized compounds showed moderate anticancer activity. Electronic supplementary material The online version of this article (10.1186/s13065-018-0420-7) contains supplementary material, which is available to authorized users.


Introduction
Currently marketed anticancer medications have increasing problems of various toxic side effects and development of resistance to their action. So, there is an urgent clinical need for the synthesis of novel anticancer agents that are potentially more effective and have higher safety profile. The synthesis of different thiazole derivatives has attracted great attention due to their diverse biological activities that include anticonvulsant [1,2], antimicrobial [3,4], anti-inflammatory [5,6], anticancer [7], antidiabetic [8], anti-HIV [9], anti-Alzheimer [10], antihypertensive [11], and antioxidant activities [12]. The reaction between active methylene compounds with phenylisothiocyanate and α-haloketones in DMF in the presence of potassium hydroxide is the simple and convenient method for the synthesis of many thiazole derivatives [13][14][15]. In continuation of our interest in the synthesis of new biologically active heterocyclic rings [16][17][18][19][20][21][22] and motivated by these information, it was thought worthwhile to synthesize some novel thiazole derivatives and to test their antitumor activity in order to discover new potentially biologically active drugs of synthetic origin.

Chemistry
The thiazole derivative 6 was previously obtained by the reaction of ethyl cyanoacetate with phenylisothiocyanate and propargyl bromide in DMF-NaH [23]. The presence of many functional groups attached to this bioactive thiazole ring motivated us to prepare it again to use it as a precursor for some new heterocycles bearing the bioactive thiazole ring. In this research, we used, instead of propargyl bromide, other reagents, such as chloroacetone, and we studied the configuration of the isolated products.
Next, fusion of thiazole 6 with DMF-DMA in presence of DMF afforded the unexpected thiazole derivative 11 (Scheme 2). The structure of the isolated product was elucidated based on its elemental and spectral analysis (IR, NMR, MS and X-ray) (see "Experimental section") (Figs. 3,4).
In many reports dimethylformamide were used as a formylating agent for indole [25], thiophene [26], and substituted benzene [27]. Based on these information, we suggested that the reaction was started via formylation of thiazole derivative 6 by DMF to afford the formyl derivative 7, which involved a reversible opening of the thiazole ring to give intermediate 8. The subsequent cyclization of 8 afforded 9, which underwent dehydration to give the methyl ketone 10. Reaction of intermediate 10 with dimethylformamide-dimethylacetal For more details see (Additional file 1: Tables S1-S6) (these files are available in the ESI section).

Geometry optimization
The optimized molecular geometries of the thiazole derivatives 6 and 11 are shown in Fig. 5 and the results of the calculated bond distances and angles are given in Additional file 1: Table S7. Good correlations were obtained between the calculated and experimental bond distances with correlation coefficients ranging from 0.991 to 0.996 (Fig. 6). The maximum differences between the calculations and experiments not exceed 0.03 Å for both compounds indicating the well prediction of the molecular geometries.

Charge population analysis
The natural population analysis is performed to predict the natural charges (NC) at the different atomic sites (Additional file 1: Table S8). The ring sulphur atom has natural charge of 0.5079 and 0.5499e for thiazole 6 and thiazole 11, respectively. In both cases, the S-atoms have electropositive nature where higher positive charge is found in thiazole 11 probably due to the presence of carbonyl group as electron withdrawing group directly attached to the ring while in thiazole 6, there is one methyl as electron releasing group via inductive effect attached to the ring. The negative sites are related to the nitrogen and oxygen sites as also further confirmed from the molecular electrostatic potential (MEP) maps shown in Fig. 7.

Frontier molecular orbitals
The HOMO and LUMO levels of the thiazole derivatives Since the HOMO and LUMO levels are mainly located over the π-system of the studied compound so the HOMO-LUMO intramolecular charge transfer is mainly a π-π* transition.

Cytotoxic activity
The anti-cancer activity of the thiazole derivatives 6 and 11 was determined against the Human Colon Carcinoma (HCT-116) cell line in comparison with the anticancer drug vinblastine, using MTT assay [28,29]. The cytotoxic activity was expressed as the mean IC 50 (the concentration of the test compounds required to kill half of the cell population) of three independent experiments ( Table 1). The results revealed that thiazole 11 has moderate anticancer activity against colon carcinoma (HCT-116), while thiazole 6 has less activity.

Chemistry General
All the melting points were measured on a Gallen Kamp apparatus in open glass capillaries and are uncorrected. The IR Spectra were recorded using Nicolet 6700 FT-IR

Method A
To a stirred solution of ethyl cyanoacetate (1.13 g, 1.07 mL, 10 mmol), in dimethylformamide (10 mL) was added potassium carbonate (1.38 g, 10 mmol). Stirring was continued at room temperature for 30 min, then phenylisothiocyanate (1.35 g, 1.2 mL, 10 mmol) was added dropwise to this mixture and stirring was continued for another 1 h. To this reaction mixture, chloroacetone (0.92 g, 0.8 mL, 10 mmol) was added and the mixture was stirred for additional 3 h at room temperature. Finally, the content was poured on cold water (50 mL

Method B
A mixture of ethyl cyanoacetate (1.13 g, 1.07 mL, 10 mmol) in sodium ethoxide (0.23 g Sodium in 10 ml of absolute ethanol) was stirred for 10 min. To this mixture, phenyl isothiocyanate (1.35 g, 10 mmol) was added dropwise and the mixture was stirred for another 1 h. Chloroacetone (0.92 g, 0.8 mL, 10 mmol) was added to the reaction mixture and stirring was continued for 3 h. Finally, it was poured on cold water and the solid precipitate that formed was filtered and recrystallized from DMF to afford the same product which obtained from method A, yield 65%.

X-Ray analysis
The thiazoles of 6 and 11 were obtained as single crystals by slow evaporation from DMF solution of the pure compound at room temperature. Data were collected on a BrukerAPEX-II D8 Venture area diffractometer, equipped with graphite monochromatic Mo Kα radiation, λ = 0.71073 Å at 100 (2) K. Cell refinement and data reduction were carried out by Bruker SAINT. SHELXT [30,31] was used to solve structure. The final refinement was carried out by full-matrix least-squares techniques with anisotropic thermal data for nonhydrogen atoms on F. CCDC 1504892 and 1505279 contain the supplementary crystallographic data for this compound can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_reque st/cif.

Computational details
The X-ray structure coordinates of the studied thiazoles were used for geometry optimization followed by frequency calculations. For this task, we used Gaussian  03 software [32] and B3LYP/6-31G(d,p) method. All obtained frequencies are positive, and no imaginary modes were detected. GaussView4.1 [33] and Chemcraft [34] programs have been used to extract the calculation results and to visualize the optimized structures.

Cytotoxic activity
The cytotoxic activity of the synthesized compounds was determined against Human Colon Carcinoma (HCT-116) by the standard MTT assay [28,29].