Synthesis and Biological Evaluation of Novel γ-Alkylidene Butenolides

Three new series of 4-substituted-5-alkylidene-2,5-dihydrofuran-2-ones were synthesized. The in vitro activity test results showed that some of them exhibited good antibacterial and cytotoxic activities. Among them compound 5c showed the most potent antibacterial activity against Escherichia coli with the MIC value of 20.00 μg/mL. Compound 9c showed good cytotoxic activity against Ec9706 cells with IC 50 value of 19.39 μM, better than that of the reference compound fluorouracil (IC 50 = 37.74 μM).


Results and Discussion
Based on the previous researches and our previous work on the synthesis and bioactivity of -alkylidene buteno-lides [38][39][40][41], series of new ,-unsaturated--lactones bearing a nitrogen-containing unit such as piperazine and piperidine are designed and synthesized in the present work.The antibacterial and cytotoxic activities of the synthetic compounds were also evaluated.
The synthesis of compound 3 was reported in early studies [42][43][44].The butenolide intermediates 4, 6, and 8 were prepared via introducing nitrogen-containing moiety such as piperazine or piperidine to the C-4 position of the lactone 3 in good yields (Scheme 1).In this reaction, secondary amines were logical choice as nucleophiles because of the base-sensitive nature of the butenolide 3.In addition, lower alkalinity catalysts such as Na 2 CO 3 were used in the nucleophilic substitution reaction.The corresponding amines piperidine, methyl benzylamine, and benzoyl piperazine are all secondary amines, so the catalyst must be relatively weak alkaline in order to avoid the occurrence of adverse  reaction.Compounds 4, 6, and 8 were reacted with various aromatic aldehydes to produce a series of novel -alkylidene butenolides 5, 7, and 9 except for compound 5d (compound 4 was reacted with acetone to produce 5d).The structures of compounds 5, 7, and 9, including the stereochemistry of the exocyclic carbon-carbon double bond, were established by 1D and 2D NMR spectroscopy.In the NMR spectrum of 5c and 9c (Figure 2), single peaks at H 6.41 and H 6.40, respectively, could be observed, suggesting the products were single isomers.In order to determine the geometry at the double bonds (Δ 5,11 ) of 5c and 9c (Figure 2), NOESY experiment was investigated.The correlation between H-11 at H 6.41 of 5c and H-6 proved that the geometry of the double bond (Δ 5,11 ) in the structure of 5c is Z configuration.The stereochemistry of 9c was similar to that of 5c.Similarly, most of the other synthetic compounds were confirmed to be single isomers, in which the geometry of the double bond (Δ 5,11 ) is Z configuration.
The antibacterial activity 1 in vitro of compounds 5, 7, and 9 was evaluated against Escherichia coli and Staphylococcus aureus as described in Table 1.Among them is compound 5c with potent antibacterial activity against Escherichia coli (MIC = 20 g/mL), which was better than antibacterial drug chloramphenicol (MIC 90 = 110 g/mL).The results indicated that the introduction of the nitrogen heterocyclic and alkylidene to the lactone was favorable for the antibacterial activity in some cases.
The cytotoxic activity 2 in vitro of synthetic -alkylidene butenolides was also investigated against Ec9706 cells, Hela cells, SPCA1 cells, and HepG-2 cells.In cytotoxic activity test, first, each compound was tested at 100 M concentration if the compounds show no inhibitory activity for each selected cell, their IC 50 values would not be tested.The results were described in Table 2. Some of them showed good cytotoxic activity.Among them compound 9c showed excellent cytotoxic activity against Ec9706 cells with IC 50 value of 19 indicated that the introduction of the nitrogen heterocyclic and -alkylidene to the lactone was efficient for the cytotoxic activity in some case.

Experimental
3.1.Chemistry.All reagents and solvents were obtained from commercial suppliers.All the reactions were monitored by TLC.Melting points were determined on a Beijing Keyi XT5 apparatus, and the temperature was not corrected.IR spectra were recorded as KBr pellets on a Thermo Nicolet (IR200) Spectrometer. 1 H and 13 C NMR spectra were recorded on a Bruker DPX-400 spectrometer at 400 and 100 MHz with TMS as internal standard.Mass spectra were taken by Waters Q-Tof micromass spectrometer.

4-(Piperidin-1-ylmethyl)furan-2(5H)-one (4).
To an CH 3 CN (50 mL) solvent of compound 3 (0.1 mol) and anhydrous sodium carbonate (0.06 mol) 1.2 eq.piperidine (0.12 mol) was added.The mixture was stirred at room temperature for 3 h, and the reaction was monitored by TLC until completion, the solid salt was filtered, and the filtrate was evaporated under reduced pressure to steam out of most of the acetonitrile, the reaction mixture was washed with water and ethyl acetate, dried and purified by column chromatography (elution with chloroform ether-ethylacetate,

4-((Benzyl(methyl)amino)methyl)furan-2(5H)-one (6).
To a CH 3 CN (50 mL) solvent of compound 3 (0.1 mol) 1.2 eq methyl benzylamine (0.12 mol) and catalytic amount of anhydrous sodium carbonate were added.The mixture was stirred at room temperature for 3 h and the reaction was monitored by TLC until completion.The reaction solution was evaporated under reduced pressure to steam out most of the CH 3 CN solvent, the reaction mixture was washed with water and ethyl acetate, dried and purified by column chromatography (elution with chloroform ether-ethylacetate, 4 : 1) which afforded 19.5 g compound 6.Yellow oil.Yield: 90%.

General Procedure for the Preparation of (7a-d).
Compound 5 (0.01 mol) and anhydrous sodium carbonate (0.01 mol) were dissolved in methanol solvent, and then the respective aromatic aldehyde (0.012 mol) was added.The mixture was stirred at room temperature for 3-12 h and the reaction was monitored by TLC until completion.The reaction solution was evaporated under reduced pressure to steam out of most of the methanol solvent.The reaction mixture was washed with water and ethyl acetate, dried and purified by column chromatography.

4-((4-Benzoylpiperazin-1-yl)methyl)furan-2(5H)-one (8).
To a CH 3 CN (50 mL) solvent of compound 3 (0.1 mol) 1.1 eq.benzoyl piperazine 12 (0.11 mol) and catalytic amount of anhydrous sodium carbonate were added.The mixture was stirred at room temperature for 2 h, and the reaction was monitored by TLC until completion.The reaction solution was evaporated under reduced pressure to steam out of most of the CH 3 CN solvent, the reaction mixture was washed with water and ethyl acetate, dried and purified by ethyl acetate crystallize to afforded 27.2 g compound 8. White crystal.Yield: 95%, mp: 112.5-113.0∘ C. IR (KBr, cm 3.1.6.General Procedure for the Preparation of (9a-f).Compound 8 (0.01 mol) and anhydrous sodium carbonate (0.01 mol) was dissolved in methanol solvent, then the respective aromatic aldehyde (0.012 mol) was added.The mixture was stirred at room temperature for 3-12 h and the reaction was monitored by TLC to complete.The reaction solution was vaporated under reduced pressure to steam out of most of the methanol solvent, the reaction mixture was washed with water and ethyl acetate, dried and purified by column chromatography.

Conclusions
In summary, a series of novel -alkylidene butenolides bearing nitrogen unit have been synthesized by a simple and general method in good yields.Their antibacterial activity and cytotoxic activity were evaluated.The results revealed that the introduction of nitrogen heterocyclic and -alkylidene to the lactone was efficient for the bioactivities.

Endnotes
1.In vitro antibacterial activity was studied with tube double dilution method.In order to study the bacteriostasis to staphylococcus aureus and Escherichia coli.
The minimum inhibitory concentration (MIC) was measured.The sample was dissolved to 1.0 mg/mL high concentration solution, geometric diluted to less than 200 g/mL for several different concentration.The bacterias have been incubated 16-18 h and diluted 200 times, in the above-mentioned several different concentrations of liquid by adding 0.1 mL solution of bacteria.With a sample solution without bacteria as a test group, to observe whether the liquid was contaminated, a solution of diluted bacteria without sample was used as a control group to observe whether the bacteria can grow and another tube with only 1 mL medium was used as a blank control group to observe whether the medium was contaminated.Samples for each of the groups were activated in a shaker with 225 r/min and 37

Table 1 :
.39 M, which was better than that of the standard drug Fluorouracil (IC 50 = 37.74 M).Meanwhile, it exhibited good cytotoxic activity against HepG-2 and SPCA1 with IC 50 values of 49.96 M and 50.15 M, respectively.The results MIC a,b of butenolide derivatives against Escherichia coli and Staphylococcus aureus.
a MIC (g/mL): minimum inhibitory concentration.b Defined as 100% inhibition of the growth of control.c Escherichia coli; d Staphylococcus aureus.e No detection.f MIC90 (g/mL).
∘ C. IR (KBr, cm −1 ): 1758, 1627, 1434, 1280, 994, 697.1 ∘ C for 16-18 h to check the results.With naked eye to observe the clear level of each group's medium and sequential observation of drug control tubes, control tubes, and control tubes of bacteria, control tubes needed to be clarified; otherwise it would mean that samples or medium has been contaminated.The control tube of bacteria that has become muddy proved to have normal bacterial growth.Comparison of the test tube, among the test tubes with clarified liquid medium, with the minimum sample concentration poses the drug concentration as the minimum inhibitory concentration of drugs.All the experiment was carried out in triplicate.2.In vitrocytotoxicity study: the cells were purchased from the Type Culture Collection of the Chinese Academy of Sciences, Shanghai, China.The cytotoxicity of compounds at various concentrations or fluorouracil (10 g/mL) was measured by MTT(3-[4, 5]-dimethylthiazol-2-yl-2,5-di-phenyltetra-zolium bromide) method.HepG-2.2.15 cells (3.0 × 10 4 cells/mL) were seeded in 96-wellplates for 24 h.After attachment to plates, the supernatants in each well were replaced very carefully with 200L of fresh RMPI1640 containing different concentrations of the compounds or fluorouracil.Untreated cells were used as the control.Remove the medium after HepG-2.2.15 cells were treated for 3 days as explained previously.Cells were cultured at 37 ∘ C for 4 h after 150 L of DMEM medium containing MTT (5 mg/mL) was added into each well.Remove the medium and add 200 L of DMSO into each well.The absorption (A) at 490 nm was measured after a ten-minute shaking.The IC 50 value was the concentration that achieved 50% cytotoxicity to HepG-2.2.15.All the experiment was carried out in triplicate.Data were analyzed statistically by the SPSS11.5 program software package, and a  value of 0.05 was regarded as significant.