Design, Synthesis and Anticancer Screening of Novel Benzothiazole-Piperazine-1,2,3-Triazole Hybrids.

A library of novel regioselective 1,4-di and 1,4,5-trisubstituted-1,2,3-triazole based benzothiazole-piperazine conjugates were designed and synthesized using the click synthesis approach in the presence and absence of the Cu(I) catalyst. Some of these 1,2,3-triazole hybrids possess in their structures different heterocyclic scaffold including 1,2,4-triazole, benzothiazole, isatin and/or benzimidazole. The newly designed 1,2,3-triazole hybrids were assessed for their antiproliferative inhibition potency against four selected human cancer cell lines (MCF7, T47D, HCT116 and Caco2). The majority of the synthesized compounds demonstrated moderate to potent activity against all the cancer cell lines examined. Further, we have established a structure activity relationship with respect to the in silico analysis of ADME (adsorption, distribution, metabolism and excretion) analysis and found good agreement with in vitro activity.

Recently, a combination of benzothiazole and 1,2,3-triazole moieties has received great interest in improving the effectiveness of bioactive molecules with fascinating anticancer activity [26,27].
In continuation of our research on the synthesis and biological evaluation of benzothiazoles and 1,2,3-triazoles [28][29][30][31][32], we here report the click synthesis and antiproliferative evaluation of new series of benzothiazole-piperazine-1,2,3-triazole hybrids incorporating different functionalities and/or heterocyclic moieties on the 1,2,3-triazole ring. The newly designed hybrids have been evaluated for their anticancer activity against breast and colon cancer cell lines in order to investigate if the combination of these heterocyclic units in one scaffold could generate active pharmaceutical ingredients (API) dotted with relevant chemotherapeutic activities comparable to the clinically approved standard drugs. To support in vitro biological assay, in silico techniques have been widely used. In this research study, we briefly describe in silico ADME prediction, with an emphasis on structure pattern recognition.
The 2-(1-piperazinyl)benzothiazole [33] was prepared from 2-chlorobenzothiazole by nucleophilic substitution of the chlorine atom by piperazine in the presence of sodium bicarbonate in an aqueous solution of 2-propanol, followed by purification with column chromatography. The structure of the newly synthesized compound 2 is in agreement with IR, 1 H-NMR, 13 C-NMR and mass spectra. Its IR spectrum showed clearly the absence of the piperazine amino group and the presence of a strong absorption band at 1685 cm −1 attributed to the acetyl carbonyl (C=O) group. Moreover, the 1 H-NMR confirmed the absence of NH proton, which was clear evidence of the success of the acylation of the piperazine nucleus. The spectrum exhibited characteristic singlet at δH 3.33 ppm assigned to the methylene acetyl protons (CH2CO). The aromatic and aliphatic protons resonated at their expected chemical shifts. Structural assignment of the acetyl side chain in the structure of compound 2 has been also supported by 13 C-NMR analysis through the appearance of two characteristic signals at δC 59.8 and δC 169.7 ppm belonging to the CH2 and C=O carbons, respectively. The formation of the azido derivative 3 was also established on the basis of its spectroscopic data. In the IR spectrum, the presence of the ki azido group in its structure was confirmed by the appearance of an absorption band at 2170 cm −1 . In contrast, no change has been observed in the protons and carbons assignment to the azido derivative 3 compared to its corresponding precursor 2, except a downfield shift of the CH2CO protons from δH 3.33 ppm for compound 2 to δH 4.22 ppm for compound 3 (See experimental section). The structure of the newly synthesized compound 2 is in agreement with IR, 1 H-NMR, 13 C-NMR and mass spectra. Its IR spectrum showed clearly the absence of the piperazine amino group and the presence of a strong absorption band at 1685 cm −1 attributed to the acetyl carbonyl (C=O) group. Moreover, the 1 H-NMR confirmed the absence of NH proton, which was clear evidence of the success of the acylation of the piperazine nucleus. The spectrum exhibited characteristic singlet at δ H 3.33 ppm assigned to the methylene acetyl protons (CH 2 CO). The aromatic and aliphatic protons resonated at their expected chemical shifts. Structural assignment of the acetyl side chain in the structure of compound 2 has been also supported by 13 C-NMR analysis through the appearance of two characteristic signals at δ C 59.8 and δ C 169.7 ppm belonging to the CH 2 and C=O carbons, respectively. The formation of the azido derivative 3 was also established on the basis of its spectroscopic data. In the IR spectrum, the presence of the ki azido group in its structure was confirmed by the appearance of an absorption band at 2170 cm −1 . In contrast, no change has been observed in the protons and carbons assignment to the azido derivative 3 compared to its corresponding precursor 2, except a downfield shift of the CH 2 CO protons from δ H 3.33 ppm for compound 2 to δ H 4.22 ppm for compound 3 (See experimental section).
The success of the click synthesis of the 1,4-disubstituted-1,2,3-triazole hybrids 5a-e has been clearly evidenced by 1 H-NMR analysis through the appearance of one distinct singlet in the aromatic region at δH 7.76-8.66 ppm attributable the 1,2,3-triazolyl proton. The OH proton of the hydroxylated alkyl residue linked to the 1,2,3-triazole ring resonated as a triplet at δH 5.22 ppm for compound 5a and as a broad singlet at δH 4.44-6.02 ppm for compounds 5b-d. Moreover, the ester functionality characterizing the 1,2,3-triazole 5e appeared as a triplet at δH 1.32 ppm and a quartet at δH 4.30-4.35 ppm assigned to the ester methyl and methylene protons, respectively. In the 13 C-NMR spectra, the absence of signals on the Sp-carbon regions and appearance of new signals in the aliphatic area assigned to the alkyl side chain carbons confirmed the success of the cycloaddition reaction.
Under the same optimized copper (I) catalyzed click synthesis, a new library of benzothiazole-piperazine-1,2,3-triazole hybrids 5f-l tagged different heterocyclic scaffolds, including 1,2,4-triazole, benzothiazole, benzimidazole and/or isatin, has been successfully designed and synthesized in 87-90% yield through the ligation of compound 3 with the appropriate heterocyclic alkynes 4f-l. It should be noted that the propargylated heterocyclic precursors 4f-l needed for the construction of compounds 5f-l were synthesized via the alkylation of 4,5-disubstituted-1,2,4-triazole-3-thiones, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole and/or isatin with propargyl bromide, in the presence of potassium carbonate in DMF at room temperature. The 1 H-NMR spectra of compounds 5f-l were fully characterized by the appearance of two new diagnostic singlets at δH 4.44-5.00 and δH 7.90-8.09 ppm assigned to the methylene SCH2/NCH2 protons and the triazolic H-5 proton, respectively. The spectra also revealed the presence of 4-10 extra aromatic protons in the aromatic region related to the aromatic heterocyclic alkyne building blocks, which confirm their incorporation through 1,3-dipolar cycloaddition. The 13 C-NMR analysis also confirmed the success of the click synthesis through; first the absence of the Sp-carbon signals from their usual chemical shift region, and second the presence of new signal in the upfield region at 25.5-35.5 ppm assignable to the SCH2/NCH2 carbon. Scheme 2. Click synthesis of 1,4-disubstituted-1,2,3-triazoles 5a-e carrying functionalized alkyl side chains.
The success of the click synthesis of the 1,4-disubstituted-1,2,3-triazole hybrids 5a-e has been clearly evidenced by 1 H-NMR analysis through the appearance of one distinct singlet in the aromatic region at δ H 7.76-8.66 ppm attributable the 1,2,3-triazolyl proton. The OH proton of the hydroxylated alkyl residue linked to the 1,2,3-triazole ring resonated as a triplet at δ H 5.22 ppm for compound 5a and as a broad singlet at δ H 4.44-6.02 ppm for compounds 5b-d. Moreover, the ester functionality characterizing the 1,2,3-triazole 5e appeared as a triplet at δ H 1.32 ppm and a quartet at δ H 4.30-4.35 ppm assigned to the ester methyl and methylene protons, respectively. In the 13 C-NMR spectra, the absence of signals on the Sp-carbon regions and appearance of new signals in the aliphatic area assigned to the alkyl side chain carbons confirmed the success of the cycloaddition reaction.
Under the same optimized copper (I) catalyzed click synthesis, a new library of benzothiazole-piperazine-1,2,3-triazole hybrids 5f-l tagged different heterocyclic scaffolds, including 1,2,4-triazole, benzothiazole, benzimidazole and/or isatin, has been successfully designed and synthesized in 87-90% yield through the ligation of compound 3 with the appropriate heterocyclic alkynes 4f-l. It should be noted that the propargylated heterocyclic precursors 4f-l needed for the construction of compounds 5f-l were synthesized via the alkylation of 4,5-disubstituted-1,2,4-triazole-3-thiones, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole and/or isatin with propargyl bromide, in the presence of potassium carbonate in DMF at room temperature. The 1 H-NMR spectra of compounds 5f-l were fully characterized by the appearance of two new diagnostic singlets at δ H 4.44-5.00 and δ H 7.90-8.09 ppm assigned to the methylene SCH 2 /NCH 2 protons and the triazolic H-5 proton, respectively. The spectra also revealed the presence of 4-10 extra aromatic protons in the aromatic region related to the aromatic heterocyclic alkyne building blocks, which confirm their incorporation through 1,3-dipolar cycloaddition. The 13 C-NMR analysis also confirmed the success of the click synthesis through; first the absence of the Sp-carbon signals from their usual chemical shift region, and second the presence of new signal in the upfield region at 25.5-35.5 ppm assignable to the SCH 2 /NCH 2 carbon.
The newly synthesized 4,5-disubstituted 1,2,3-triazoles 6a,b are in agreement with the IR, 1 H-NMR, 13 C-NMR and HRMS spectral data. In the 1 H-NMR spectrum of compound 6a, the two non-equivalents methyl ester protons were recorded as two characteristic singlets at δH 3.97 and δH 4.00 ppm. The ethyl ester protons attributable to compound 6b also appeared as two sets of multiplets at δH 1.24-1.33 and δH 4.29-4.39 ppm, respectively. The methoxy and ethoxy carbons characteristic of the two non-equivalent ester functionalities resonated at their expected chemical shift in the aliphatic region at δC 14.1-14.4 and δC 52.7-62.8 ppm. In addition, the carbonyl ester carbons appeared in the downfield region at δC 168.2-168.5 ppm. Scheme 3. Click synthesis of 1,4-disubstituted-1,2,3-triazoles 5f-l carrying heterocyclic moieties.

Antiproliferative Activity
Compounds 2, 3, 5a-l, and 6a,b were tested for their in vitro antiproliferation activities against two human breast cancer cell lines; MCF7 and T47D, and two human colon cancer cell lines; HCT116 and Caco2 according to the protocol described in the ISO 10993-5 guide [37]. As shown in Table 1, relative to the parent compounds 2 and 3, some of the hybrid compounds exhibited good antiproliferative activity against both breast and colon carcinoma cell lines. Among the synthesized compounds, the 3-hydroxypropyl hybrid compound 5b has demonstrated the most potent antiproliferative activity with IC 50 of 38 µM and 33 µM against the breast carcinoma cell lines of T47D and MCF7, respectively, and IC 50 of 48 µM and 42 µM against the colon carcinoma cell lines of HCT116 and Caco2, respectively.
In addition, title compounds 5c, 5d, 5h, and 5g tethering 1,4-disubstituted-1,2,3-triazoles have exhibited appreciable activity against all studied cell lines. From the trend activity, cell proliferation assay demonstrates that hybrid molecules tethering ethyl ester substitution on the triazole moiety is less favored for anticancer activity (title compound 5e). On the other hand, aryl and alkyl substituted triazole is favored for anticancer activity. The thiomethyl-triazole substituent exhibited variable effects on the anticancer activity profile that appeared to be dependent on the substituent's size and position on the terminal triazole ring. For example, the 5-methyl-4-phenyl thiomethyl-triazole substitution (title compound 5f) had atrocious effect on activity, while the 4-methyl-5-phenyl thiomethyl-triazole substitution (title compound 5g) exhibited more potent antiproliferative activity against all examined cancer cell lines. Interestingly, the hybrid molecules with 1,2,3-triazole tri-substitutions (title compounds 6a and 6b) have shown poor antiproliferative profile against all cancer cell lines in this study.

In Silico ADME and LogP Analysis
In silico ADME (adsorption, distribution, metabolism, and excretion) was performed to confirm the reliability in vitro biological activity. Evaluation of in silico ADME is a reliable technique to confirm the potential of a drug candidate. Before going in clinical trial, preliminary agreement is needed for ADME and can be easily provided [38]. For good absorption and permeability, cLogP should be less than 5. The hydrophilicity and cLogP values are correlated because hydrophilicity depends on, and is expressed in terms of, the cLogP value. Any drug to be active should not have more than one violation [39]. To qualify the preliminary requirement, logP and ADME analysis have preformed for synthesized benzothiazole-piperazine conjugates (2, 3, 5a-5l, and 6a,b).
Violations of Lipinski's rule and predicted ADME parameters (molecular weight (MW), logP, topological polar surface are (tPSA), number of hydrogen donors (nON), and acceptors (nOHNH) and volume) are presented in Table 2. From Table 2, screening data for ADME and logP revealed that all compounds are safe. According to these data, compounds comply Lipinski's rule of five and number of violation except compound 5i. The ADME parameters are in good agreement and may have good pharmacokinetic profile with good lipophilicity.

General
All melting points were measured on a Stuart Scientific SMP1 apparatus (Red Hill, UK) and are uncorrected. The IR spectra were measured in a KBr matrix with a Perkin-Elmer 1430 series FTIR spectrometer (PerkinElmer, Santa Clara, California, USA). The NMR spectra were recorded with an Advance Bruker spectrophometer (Fällanden, Switzerland) at 400 MHz for the 1 H-NMR analysis and at 100 MHz for the 13 C-NMR analysis, using Tetramethylsilane (TMS) (0.00 ppm) as the internal. A Finnigan MAT 95XL spectrometer (Darmstadt, Germany) was used for the determination of the EI mass spectra. Elemental analyses were performed using a GmbH-Vario EL III Element Analyzer (Hanau, Germany). In silicon ADME and logP value has been calculated using Molinspiration Cheminformatics software (Nova ulica 61, SK-900 26 Slovensky Grob, Slovak Republic) on http://www.molinspiration.com.

Synthesis and Characterization of 2-Azido-1-(4-(benzo[d]thiazol-2-yl)piperazin-1-yl)ethanone (3)
A mixture of compound 2 (10 mmol) and sodium azide (12 mmol) in a mixture of acetone:water (4:1) (100 mL) was stirred for 24 h at room temperature. The excess of solvent was evaporated under vacuum. Potassium carbonate (11 mmol) was added to a stirred solution of compound 1 (10 mmol) dissolved in DMF (25 mL) and stirring was continued for 2 h. Then, propargyl bromide (11 mmol) was added and the reaction mixture was stirred at room temperature overnight. The mixture was poured onto crushed ice water and the precipitate thus formed was collected by filtration and recrystallized from ethanol to afford the desired alkynes 4f-l. A solution of copper sulfate (0.8 mmol) and sodium ascorbate (1.1 mmol) in water (10 mL) was added with stirring to a mixture of the appropriate alkyne 4a-l (1 mmol) and benzothiazoleazide 3 (1 mmol) in DMSO (10 mL). Then, the reaction mixture was heated at 80 • C for 8 h, until the consumption of the starting material as indicated by TLC. The reaction mixture was quenched with ice water and the solid thus formed was collected by filtration, washed with saturated solution of ammonium chloride and recrystallized from ethanol to give the desired 1,2,3-triazoles 5a-l.

Conclusions
In the present work, we have designed and synthesized a series of novel potential anticancer agents based benzothiazole-piperazine-1,4-disubstituted-1,2,3-triazole molecular hybrids utilizing the Cu(I)-catalyzed 1,3-dipolar cycloaddition coupling between the appropriate 2-azido-1-(4-(benzo[d]thiazol-2-yl)piperazin-1-yl)ethanone with different functionalized and/or heterocyclic terminal alkynes. On the other hand, novel 4,5-diester-1,2,3-triazoles were synthesized using an efficient and quick green free solvent click synthesis in the absence of the copper catalyst. The synthesized compounds were evaluated against four different human cancer cell lines representing breast and colon cancers. Majority of the hybrid molecules displayed substantial antiproliferative activity. Among them, compound 5b exhibited the most potent antiproliferative activity against all examined cancer lines. Preliminary in vitro screening showed that all compounds exerted good biological profile, which was further confirmed by clogP and ADME analysis. ADME and clogP analysis were in good agreement and follow Lipinski rule of five and violation rule. R. carried out the experimental work, collected data, interpreted the results and prepared the manuscript. S.K.B. performed the biological assays, wrote the paper and edited English language. P.K.S. Performed the in silico ADME study, discussed the results and commented on the manuscript.