Design, Synthesis, and Biological Evaluation of Novel Benzofuran Derivatives Bearing N-Aryl Piperazine Moiety

A series of novel hybrid compounds between benzofuran and N-aryl piperazine have been synthesized and screened in vitro for anti-inflammatory activity in lipopolysaccharide (LPS)-stimulated RAW-264.7 macrophages and for anticancer activity against three human tumor cell lines. The results demonstrated that derivative 16 not only had inhibitory effect on the generation of NO (IC50 = 5.28 μM), but also showed satisfactory and selective cytotoxic activity against human lung cancer line (A549) and gastric cancer cell (SGC7901) (IC50 = 0.12 μM and 2.75 μM, respectively), which was identified as the most potent anti-inflammatory and anti-tumor agent in this study.

Nitrogen heterocycles are an important class of compounds having versatile biological activities, which are used in drug design and synthesis, generally as active units [16,17]. Piperazine-one of the most biological active moieties-represents a series of important organic compounds that make up the core structures in medicines and have been widely used in the development of drug molecular design [18,19].
In previous work, we reported that benzofuran compounds containing N-heterocyclic moieties displayed potent cytotoxic activities [20], and the hybrids between chalcone and N-aryl piperazine bearing acetophenone showed potent anti-inflammatory and anticancer activities [21,22]. In addition, the acetophenone substituent was vital for modulating cytotoxic activities.

Scheme 1. Structures of biological benzofuran agents.
Based on these results, in the present research, we have designed and synthesized novel hybrids towards the recombination of benzofuran and N-aryl piperazine (Scheme 2). In order to study the structure-activity relationship (SAR) of hybrid compounds, various R-X were selected, including α-bromoacetophenone, benzyl bromide, and alkyl bromide. The potent in vitro anti-inflammatory activity in lipopolysaccharide (LPS)-stimulated RAW-264.7 macrophages and anticancer activities of compounds against a panel of human cancer cell lines (A549, Hela, and SGC7901) were evaluated, respectively. Scheme 2. Designed strategy of benzofuran hybrids.

Chemistry
The general synthetic route used to synthesize hybrid compounds is outlined in Scheme 3. Treatment of commercial 5-diethylaminosalicylaldehyde (1) with 2-bromo-4′-fluoro acetophenone (2) gave the 2-phenzoylbenzofuran compound (3) in the presence of K2CO3 in refluxing acetone. Then, the key benzofuran-piperazine intermediate (4) was prepared by substitution with piperazine from compound (3) in the presence of K2CO3 at 110 °C in DMF. With the desired intermediate (4) in hand, we began the synthesis of amines by treatment of compound (4) with several commercially available R-X. To get further insight into the structure-activity relationship, the tertiary amines  were prepared with excellent yields by the reaction of intermediate (4) with various R-X. To compare the biological activities of substituents of the NH group of piperazine ring, we prepared the title compounds by substitution of α-bromoacetophenone, benzyl bromide, alkyl bromide, and hetero aromatic bromide. Comparative data for novel hybrid compounds with respect to structures and yield are provided in Table 1. All of the synthesized compounds were characterized by 1 H-NMR and 13 C-NMR, and some representative compounds were characterized by high-resolution mass spectrometry (HRMS) analysis. The spectra of title compounds were in Supplementary Materials. Scheme 2. Designed strategy of benzofuran hybrids.

Chemistry
The general synthetic route used to synthesize hybrid compounds is outlined in Scheme 3. Treatment of commercial 5-diethylaminosalicylaldehyde (1) with 2-bromo-4 -fluoro acetophenone (2) gave the 2-phenzoylbenzofuran compound (3) in the presence of K 2 CO 3 in refluxing acetone. Then, the key benzofuran-piperazine intermediate (4) was prepared by substitution with piperazine from compound (3) in the presence of K 2 CO 3 at 110 • C in DMF. With the desired intermediate (4) in hand, we began the synthesis of amines by treatment of compound (4) with several commercially available R-X. To get further insight into the structure-activity relationship, the tertiary amines (5-25) were prepared with excellent yields by the reaction of intermediate (4) with various R-X. To compare the biological activities of substituents of the NH group of piperazine ring, we prepared the title compounds by substitution of α-bromoacetophenone, benzyl bromide, alkyl bromide, and hetero aromatic bromide. Comparative data for novel hybrid compounds with respect to structures and yield are provided in Table 1. All of the synthesized compounds were characterized by 1 H-NMR and 13 C-NMR, and some representative compounds were characterized by high-resolution mass spectrometry (HRMS) analysis. The spectra of title compounds were in Supplementary Materials.              As shown in Table 2, the substituents of the NH group of the piperazine ring have an obvious influence on anti-inflammatory activities. To our delight, benzofuran-piperazine compounds 16, 18, and 22 displayed good anti-inflammatory activity on the generation of NO (IC50 < 10 μM). Especially, compound 16 was found to be the most potent anti-inflammatory agent (IC50 = 5.28 μM). In addition, compounds 5, 9, and 24 showed potent anti-inflammatory activity (IC50 < 20 μM). However, hetero aromatic compound 25 displayed no activity compared to others. Notably, except for the mentioned hybrids, most of the derivatives had little or no inhibitory effects on the release of NO (IC50 > 20 μM). On the other hand, we could find that the substituents of the NH group of the piperazine ring played an important role. Overall, keto-substituents contributed to better activity, alkyl and aryl substituents led to weaker activity, and the pyridyl substituent had no effect on anti-inflammatory activity (keto-> alkyl ≈ aryl > pyridyl). So, in future research, we will focus on the keto-substituents.

Anti-Inflammatory Activity
RAW 264.7 cells are widely used to establish inflammatory models in vitro. In this work, we investigated the anti-inflammatory activity of synthetic compounds in LPS-induced RAW 264.7 on the generation of NO. The results of the title hybrids are summarized in Table 2. As shown in Table 2, the substituents of the NH group of the piperazine ring have an obvious influence on anti-inflammatory activities. To our delight, benzofuran-piperazine compounds 16, 18, and 22 displayed good anti-inflammatory activity on the generation of NO (IC 50 < 10 µM). Especially, compound 16 was found to be the most potent anti-inflammatory agent (IC 50 = 5.28 µM). In addition, compounds 5, 9, and 24 showed potent anti-inflammatory activity (IC 50 < 20 µM). However, hetero aromatic compound 25 displayed no activity compared to others. Notably, except for the mentioned hybrids, most of the derivatives had little or no inhibitory effects on the release of NO (IC 50 > 20 µM). On the other hand, we could find that the substituents of the NH group of the piperazine ring played an important role. Overall, keto-substituents contributed to better activity, alkyl and aryl substituents led to weaker activity, and the pyridyl substituent had no effect on anti-inflammatory activity (keto-> alkyl ≈ aryl > pyridyl). So, in future research, we will focus on the keto-substituents.

Anticancer Activity
The anticancer activities of novel synthesized derivatives were evaluated against human lung cancer cell line (A549), human cervical carcinoma (Hela), and human gastric carcinoma (SGC7901) by MTT [3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2-H-tetrazolium bromide] assay, using cisplatin (DDP) as the reference drug. The anti-tumor results for the hybrids are summarized in Table 3. As shown in Table 3, the structures of the hybrid compounds have an obvious influence on cytotoxic activities. There were three series of substituents of the piperazine ring, including keto-, alkyl, and aryl. In general, derivatives bearing keto-substituent (16-24) were most active, displaying similar or better cytotoxic activity in vitro compared to cisplatin (DDP). However, hetero aromatic compound 25 had weak cytotoxic activity against Hela, and alkyl-substituted compounds showed no activity, except for hybrid 9 (IC 50 = 19.27 µM against A549). Furthermore, the electron withdrawing group or halide substituent at position 4 of the benzene ring of the acetophenone moiety could be more sensitive to cytotoxic activity, such as 4-F (19), 4-Cl (20), and 4-CN (23). Compound 16 showed the most potent cytotoxic activity and pronounced selectivity against A549 (IC 50 = 0.12 µM) and SGC7901 (IC 50 = 2.75 µM). In addition, some aryl-substituted compounds displayed good inhibitory activity. For example, hybrids 11 and 12 had selective anti-tumor activity against cancer cells (IC 50 = 8.57-16.27 µM).
The biological results suggested that the existence of a keto-substituent played an important role in the anti-inflammatory and anticancer activity of compounds. In all synthesized derivatives, hybrid 16 had better inhibitory effect on the generation of NO and showed more potent cytotoxic activity against A549 and SGC7901, and could be identified as the most potent anti-inflammatory and anti-tumor agent among those studied. The structure-activity relationship (SAR) results are summarized in Scheme 4.
Overall, although only a few compounds were found to exhibit excellent anti-inflammatory and antitumor activities, we could study the tendency of benzofuran-piperazine compounds and conduct further medicine chemistry research following the results.

General Information
Starting materials were commercially available and analytically pure. Melting points were measured on a YANACO microscopic melting point meter and were uncorrected. 1 H-NMR and 13 C-NMR spectra were recorded on Bruker AV 300 and Bruker AV 400 spectrometers (Bruker, Karlsruhe, Germany) using TMS as internal standard and CDCl3 as solvent, respectively. Thin layer chromatographic (TLC) analysis was carried out on silica gel plates GF254. High-resolution mass spectra were performed on an ESI Q-TOF MS spectrometer (Agilent, Santa Clara, CA, USA).

General Procedure
General procedure for the synthesis of compound 3: To a solution of acetone (50 mL), 5-diethylamino salicylaldehyde (1.93 g, 10 mmol) and 2-bromo-4′-fluoroacetophenone (2.17 g, 10 mmol), was added K2CO3 (2.76 g, 20 mmol) and left to react for 4 h in reflux. The reaction was poured into 100 mL cold water. After stirring for 10 min, the mixture was filtered, and the filtrate was concentrated in vacuo and dried to afford a yellow solid.
General procedure for the preparation of compound 4: To a stirred solution of compound 3 (1.56 g, 5 mmol) and K2CO3 (1.38 g, 10 mmol) in dried DMF (20 mL), piperazine (0.86 g, 10 mmol) was added, and the reaction mixture was stirred for 12 h at 110 °C. After completion of the reaction as indicated by TLC, the reaction was quenched by the addition of CHCl3 (50 mL) and washed with water (3 × 20 mL). The organic layer was dried by anhydrous sodium sulfate, concentrated in vacuo, and purified by column chromatography to afford a brown solid.
General procedure for the preparation of hybrid derivatives 5-7: To a stirred solution of compound 4 (0.5 mmol) in dried DMF (5 mL), NaH (0.06 g, 60% in oil) was added, and the mixture was stirred at 0 °C for 1 h. Then, R-X (1 mmol) was added, and after completion of the reaction as indicated by TLC, the reaction was quenched by the addition of H2O (30 mL) and was extracted with DCM (3 × 10 mL). The organic layer was dried by anhydrous sodium sulfate, concentrated in vacuo, and purified by column chromatography (1% MeOH/DCM) to afford products.
General procedure for the preparation of derivatives 8-14, 25: To a stirred solution of compound 4 (0.5 mmol) and Cs2CO3 (0.5 g) in dried DCM (15 mL), R-X (0.6 mmol) was added, and the reaction mixture was stirred for 12 h at room temperature (r.t.). After completion of the reaction as indicated by TLC, the mixture was filtered off. Then, the organic layer was concentrated in vacuo and purified by column chromatography (1% MeOH/DCM) to afford products.

General Information
Starting materials were commercially available and analytically pure. Melting points were measured on a YANACO microscopic melting point meter and were uncorrected. 1 H-NMR and 13 C-NMR spectra were recorded on Bruker AV 300 and Bruker AV 400 spectrometers (Bruker, Karlsruhe, Germany) using TMS as internal standard and CDCl 3 as solvent, respectively. Thin layer chromatographic (TLC) analysis was carried out on silica gel plates GF 254 . High-resolution mass spectra were performed on an ESI Q-TOF MS spectrometer (Agilent, Santa Clara, CA, USA).

General Procedure
General procedure for the synthesis of compound 3: To a solution of acetone (50 mL), 5-diethylamino salicylaldehyde (1.93 g, 10 mmol) and 2-bromo-4 -fluoroacetophenone (2.17 g, 10 mmol), was added K 2 CO 3 (2.76 g, 20 mmol) and left to react for 4 h in reflux. The reaction was poured into 100 mL cold water. After stirring for 10 min, the mixture was filtered, and the filtrate was concentrated in vacuo and dried to afford a yellow solid.
General procedure for the preparation of compound 4: To a stirred solution of compound 3 (1.56 g, 5 mmol) and K 2 CO 3 (1.38 g, 10 mmol) in dried DMF (20 mL), piperazine (0.86 g, 10 mmol) was added, and the reaction mixture was stirred for 12 h at 110 • C. After completion of the reaction as indicated by TLC, the reaction was quenched by the addition of CHCl 3 (50 mL) and washed with water (3 × 20 mL). The organic layer was dried by anhydrous sodium sulfate, concentrated in vacuo, and purified by column chromatography to afford a brown solid.
General procedure for the preparation of hybrid derivatives 5-7: To a stirred solution of compound 4 (0.5 mmol) in dried DMF (5 mL), NaH (0.06 g, 60% in oil) was added, and the mixture was stirred at 0 • C for 1 h. Then, R-X (1 mmol) was added, and after completion of the reaction as indicated by TLC, the reaction was quenched by the addition of H 2 O (30 mL) and was extracted with DCM (3 × 10 mL). The organic layer was dried by anhydrous sodium sulfate, concentrated in vacuo, and purified by column chromatography (1% MeOH/DCM) to afford products.
General procedure for the preparation of derivatives 8-14, 25: To a stirred solution of compound 4 (0.5 mmol) and Cs 2 CO 3 (0.5 g) in dried DCM (15 mL), R-X (0.6 mmol) was added, and the reaction mixture was stirred for 12 h at room temperature (r.t.). After completion of the reaction as indicated by TLC, the mixture was filtered off. Then, the organic layer was concentrated in vacuo and purified by column chromatography (1% MeOH/DCM) to afford products.
General procedure for the preparation of derivatives 15-24: To a stirred solution of compound 4 (0.5 mmol) and K 2 CO 3 (0.2 g) in dried DCM (10 mL), R-X (0.6 mmol) was added, and the reaction mixture was stirred for 12 h at r.t. After completion of the reaction as indicated by TLC, the reaction was quenched by the addition of 5% NaOH (20 mL) and was extracted with DCM (3 × 10 mL). The organic layer was dried using anhydrous sodium sulfate, concentrated in vacuo, and purified by column chromatography (1% MeOH/DCM) to afford products.  90 (s, 1H), 1.23 (t, J = 6.9 Hz, 6H); 13       (100 µL) was mixed with an equal volume of Griess reagent at r.t. for 10 min. Absorbance was measured at 540 nm in a microplate reader.

Antitumor Activity
The assay was carried out using the method described previously. About 1 × 10 4 cell/well were seeded into 96-well microtiter plates. At twenty-four hours post-seeding, cells were treated with vehicle control or various concentrations of samples for 48 h. Twenty microliters of MTT solution (5 mg/mL) was added to each well, and the tumor cells were incubated at 37 • C in a humidified atmosphere of 5% CO 2 air for 4 h. Upon removal of MTT/medium, 150 µL of DMSO was added to each well, and the plate was agitated at oscillator for 5 min to dissolve the MTT-formazan. The assay plate was read at a wavelength of 570 nm using a microplate reader.

Conclusions
In summary, a series of novel hybrid compounds between benzofuran and N-aryl piperazine have been synthesized and screened in vitro for anti-inflammatory and anticancer activity. The results demonstrated that derivative 16 not only had an inhibitory effect on the generation of NO (IC 50 = 5.28 µM), but also displayed good cytotoxic activity against A549 and SGC7901 (IC 50 = 0.12 µM and 2.75 µM, respectively), which was considered to be the most potent anti-inflammatory and anti-tumor agent in this study. Further research is currently underway, and the results will be reported in due course.