Synthesis, characterization, crystal structure and cytotoxicities of 2-aroyl-3-aryl-5 H -furo[3,2-g ]chromene derivatives

Five 2-aroyl-3-aryl-5 H -furo[3,2-g ]chromene derivatives have been synthesized, and their structures were characterized by IR, NMR, ESI-MS and elemental analysis. The crystal structure for 2-benzoyl-3-(4-methoxyphenyl)-6,7-dihydro-5 H -furo[3,2-g ]chromene has been determined by single-crystal X-ray diffraction. X-ray analysis reveals that the pyran ring adopts a half-chair conformation, while the fused furo[3,2-g ]chromene ring is approximately coplanar with a slight distortion. The preliminary pharmacological test showed all target compounds exhibit cytotoxicities against the U2OS-EGFP-4F12G cell line.


Introduction
Selective estrogen receptor modulators (SERMs) are of great value in the treatment of various estrogen dependent diseases such as breast cancer, osteoporosis, Alzheimer's disease, and coronary heart disease.Tamoxifen is being used frequently as a drug for the prevention and treatment of breast cancer. 1 Raloxifene, another SERM, is commonly used in the prevention and treatment of osteoporosis. 2However, the clinical usefulness of marketed SERMs is often accompanied by adverse effects such as increased incidence of endometrial cancer and hot flashes, 3,4 therefore, it is necessary and urgent to find more effective SERMs.

Raloxifene
As known, the furochromene ring is an interesting structure for its various biological activities including anticancer, 5 antitumor, 6 and antimycobacterial activity. 72-Aroylbenzofuran derivatives exhibit selective cytotoxicity against a tumorigenic cell line. 8By use of a panel of 24 different human tumor cell lines, mean IC50 values of the most potent 2-aryolbenzofurans with N-hydroxyacrylamide is 0.75 M. 9 According to the structure-activity relationship of raloxifene [10][11][12] and the principle of bioisosterism and hybridization, based on replacing the central core of the benzothiophene in raloxifene structure with a furo[3,2-g]chromene moiety, a series of 2-aroyl-3-aryl-5H-furo[3,2-g]chromene derivatives were designed and synthesized as the target compounds.
The target compounds were characterized by IR, NMR, ESI-MS and elemental analysis.Fortunately, a single crystal of 5a was grown, and the structure was characterized by X-ray diffraction analysis.The preliminary pharmacological test showed all target compounds exhibit cytotoxicities against U2OS-EGFP-4F12G cell line.
Compounds 4 were synthesized by Fries rearrangement with stannic chloride as the solvent and catalyst.The two single peaks at δ = 6.43 and 7.30 ppm in the proton NMR indicated the rearrangement may occur at the C6 position of the chroman ring, because the two protons do not couple.When the rearrangement occurred at the C8 position of the chroman ring, the two protons must couple.So we speculated that the product of the rearrangement reaction should be 6-aroyl-7-hydroxychroman derivatives.In order to confirm the structures of the products, a single crystal of 5a was grown for X-ray diffraction analysis.X-ray analysis reveals that our speculation was correct.The molecular structure of the target compound 5a is shown in Figure 1, and the crystal packing of the target compound 5a in Figure 2.   24).The result confirms the screw structure (Figure 3) constructed by the furo[3,2-g]chromene ring and the different benzene rings.However, the crystal belongs to the triclinic crystal system and space group of P-1 (Figure 2).
Crystallographic data for structure 5a has been deposited at the Cambridge Crystallographic Data Center, CCDC No. 775960 for compound 5a.Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44 1223 336033; email: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk ).

Cytotoxicity
The target compounds were evaluated for cytotoxicity in vitro by MTT assay against the U2OS-EGFP-4F12G (human osteosarcoma) cell line.The screening result indicates that all target compounds exhibit cytotoxicities (Table 1), and the 5H-furo[3,2-g]chromene scaffold becomes a novel moiety with cytotoxicity.Compound 5a was superior to compound 5b, which indicated that the methoxy group at the para position on the phenyl group at C3 position of furo[3,2-g]chromene scaffold was good for cytotoxicity.
Among all target compounds, the cytotoxicity of compound 5a without substitutents was more potent than those of the others, while the compound 5c with methyl group, compound 5d with methoxy group and compound 5e with chloro group led to a huge decrease in biological activity, which indicated that the steric effect played an important role in cytotoxicity.

Conclusions
The target compounds were synthesized and the corresponding molecular structures were experimentally characterized by IR, NMR, ESI-MS and elemental analysis.The crystal structure for compound 5a has been determined by single-crystal X-ray diffraction with a triclinic space group P-1.X-ray analysis reveals that the pyran ring adopts a half-chair conformation.The fused furo[3,2-g]chromene ring is approximately coplanar except atom C(5), while the whole molecular forms the screw structure.All target compounds showed cytotoxicities against U2OS-EGFP-4A12G cells.

Experimental Section
General.The melting point was determined on open capillary tubes and the thermometer was uncorrected.IR spectra were recorded in the range of 4000-400 cm -1 using KBr pellets on a Bruker AFS55 spectrometer.Proton NMR spectra was measured on Bruker spectrometers operating at 300 MHz or 600 MHz with CDCl3 as the solvent and TMS as the internal standard.13 C-NMR spectra was measured on Bruker spectrometers operating at 100 MHz with DMSO-d6 as the solvent.Mass spectra were recorded on a electrospray (ESI) ion source in a Waters spectrometer at 3.5 kV spray voltage.The single-crystal structure was determined on a Bruker SMART 1000 CCD diffractometer.Elemental analysis was carried out on a Perkin Elmer 2400 elemental analyzer.All the reagents were of analytical reagent grade.

General procedure for preparing of 7-aroyloxychromans (3)
To a solution of 2 (40 mmol) and triethylamine (44 mmol) in 80 mL dichloromethane, was added aroyl chloride (44 mmol).The mixture was stirred at room temperature for 3h and monitored by TLC.The mixture was washed with diluted hydrochloric acid (1 mol/L) and water (20 mL × 3), and the combined organic layer was dried over MgSO4.After distillation under reduced pressure, the residue was recrystallized from ethanol.

General procedure for preparing of 6-aroyl-7-hydroxychromans (4)
A mixture of 3 (30 mmol), stannic chloride (150 mmol) was refluxed for 8 h.The solution was poured to ice water and appeared the precipitation, the solid was washed with cold water and recrystallized from ethanol.

Figure 1 .
Figure 1.Structure of the target compound 5a.

Figure 2 .
Figure 2. Crystal packing of the target compound 5a viewed along the C axis.

Figure 3 .
Figure 3.The screw structure (right) in the target compound 5a.