Synthesis and evaluation of antitumor activities of novel chiral 1,2,4-triazole Schiff bases bearing γ-butenolide moiety

Background 1,2,4-Triazole derivatives have received much attention due to their versatile biological properties including antibacterial, antifungal, anticonvulsant, antiinflammatory, anticancer, and antiproliferative properties. 1,2,4-Triazole nucleus has been incorporated into a wide variety of therapeutically interesting molecules to transform them into better drugs. Schiff bases of 1,2,4-triazoles have also been found to possess extensive biological activities. On the other hand, γ-substituted butenolide moiety represents a biological important entity that is present in numerous biologically active natural products. Results We have described herein the synthesis of 12 hybrid 1,2,4-triazole Schiff bases bearing γ-substituted butenolide moiety. These compounds were synthesized by utilizing the tandem asymmetric Michael addition/elimination reaction as the key step. All the new compounds were evaluated for their in vitro anticancer activity. Conclusions Tandem asymmetric Michael addition/elimination approach has offered an easy access to new chiral 1,2,4-triazole compounds 7a-7l. All these chiral 1,2,4-triazole derivatives exhibited good anticancer activities towards Hela. Of all the tested compounds, the chiral compound 7l with an IC50 of 1.8 μM was found to be the most active.


Background
Cancer, a diverse group of diseases characterized by the proliferation and spread of abnormal cells, is a major worldwide problem. Therefore, the discovery and development of new potent and selective anticancer drugs are of high importance in modern cancer research.
Recently, we reported on the synthesis of a series of hybrid 1,3,4-thiadiazoles derivatives possessing γsubstituted butenolide moiety, which exhibited good anticancer activities against cervical cancer cells [25]. In continuation of our studies on the identification of potential active antitumor compounds, herein we report the synthesis and evaluation of a new series of hybrid 1,2,4-triazole Schiff bases bearing γ-substituted butenolide moiety as potential anticancer agents ( Figure 1). To the best of authors' knowledge, the synthesis and anticancer activities of this types of compounds have not been reported so far.
The structures of these new compounds 7a-l were characterized with IR, 1 H, 13 C NMR, and LC-MS spectra. In addition, the molecular structure of 7a was unambiguously confirmed through X-ray crystallography ( Figure 2). a All newly synthesized compounds 7a-l were initially evaluated for their in vitro anticancer activities against cervical cancer cell lines (HeLa) using the MTT assay, and the results were summarized in Table 1. All the compounds 7a-l displayed good inhibition activities on HeLa cell lines. Of all the studied compounds, the compound 7l exhibited the best inhibitory activity with an IC 50 of 1.8 μM.
Then, the growth inhibition rates of HeLa cell lines with compounds 7a-l at different concentrations (0.1-20 μM) were evaluated ( Table 2). After being treated with 20 μg/mL compound 7l for 24 h, the growth inhibition rate was the highest (90.0%).

Experimental
All the chemicals were used as-received without further purification unless otherwise stated. IR spectra were Figure 1 The general structure of target compounds. recorded on a FTIR-8400S spectrometer as KBr disks. 1 H NMR and 13 C NMR spectra were obtained with a Bruker Avance III 400 MHz spectrometer in chloroformd (CDCl 3 ) and tetramethylsilane was used as an internal standard. Diffraction measurement was made on a Bruker AXS SMART 1000 CCD diffractometer with graphitemonochromatized Mo Kα radiation (λ = 0.71073 Å). All the melting points were determined on a WRS-1B digital melting point apparatus and are uncorrected. Thin-layer chromatography (TLC) was carried out on silica GF254 plates (Qingdao Haiyang Chemical Co., Ltd., China).
General procedure for the synthesis of compounds 7 To an aqueous solution of dichloromethane was sequentially added the compounds 1 (1.0 mmol), potassium carbonate (1.0 mmol), 18-crown-6 (0.1 mmol), and the compounds 6 (1.1 mmol). The resulting mixture was stirred at room temperature, and the reaction was monitored by TLC. On completion of the reaction (10-20 h), the mixture was exacted and the organic layer was washed with saturated brine. Then the organic layer was dried over anhydrous MgSO 4 , filtered, and concentrated in vacuo The purification of the residue by silica gel column chromatography or crystallizations yielded the desired compounds 7a-l in 65-89% yields (For the characterization of compound 7a-7l, please see the Additional file 1: Supporting Information). Compound   Endnote a The molecular structure of the product 7a was determined by means of X-ray crystallographic studies. CCDC 829447 (7a) contains the supplementary crystallographic data for this article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Additional file
Additional file 1: Supporting Information Available. Experimental procedures, spectral data of new compounds.

Competing interests
The authors declare that they have no competing interests.
Authors' contributions ZS and XL carried out the design of the project, and drafted the manuscript. XL and HL synthesized target compounds. XZ evaluated in vitro anticancer activities against cervical cancer cell lines (HeLa). All authors read and approved the final manuscript.

Supporting information available
Experimental procedures, spectral data of new compounds.