Khellinoflavanone, a Semisynthetic Derivative of Khellin, Overcomes Benzo[a]pyrene Toxicity in Human Normal and Cancer Cells That Express CYP1A1.

Cytochrome P450 family 1 (CYP1) enzymes catalyze the metabolic activation of environmental procarcinogens such as benzo[a]pyrene, B[a]P, into carcinogens, which initiates the process of carcinogenesis. Thus, stopping the metabolic activation of procarcinogens can possibly prevent the onset of cancer. Several natural products have been reported to show unique ability in inhibiting CYP1 enzymes. We found that khellin, a naturally occurring furanochromone from Ammi visnaga, inhibits CYP1A1 enzyme with an IC50 value of 4.02 μM in CYP1A1-overexpressing human HEK293 suspension cells. To further explore this natural product for discovery of more potent and selective CYP1A1 inhibitors, two sets of semisynthetic derivatives were prepared. Treatment of khellin with alkali results in opening of a pyrone ring, yielding khellinone (2). Claisen-Schmidt condensation of khellinone (2) with various aldehydes in presence of potassium hydroxide, at room temperature, provides a series of furanochalcones 3a-v (khellinochalcones). Treatment of khellinone (2) with aryl aldehydes in the presence of piperidine, under reflux, affords the flavanone series of compounds 4a-p (khellinoflavanones). The khellinoflavanone 4l potently inhibited CYP1A1 with an IC50 value of 140 nM in live cells, with 170-fold selectivity over CYP1B1 (IC50 for CYP1B1 = 23.8 μM). Compound 4l at 3× IC50 concentration for inhibition of CYP1A1 completely protected HEK293 cells from CYP1A1-mediated B[a]P toxicity. Lung cancer cells, A549 (p53+) and Calu-1 (p53-null), blocked in growth at the S-phase by B[a]P were restored into the cell cycle by compound 4l. The results presented herein strongly indicate the potential of these khellin derivatives for further development as cancer chemopreventive agents.

Introduction

Ammi visnaga (commonly known as “Khella”; family: Apiaceae) is a widespread flowering plant native to Europe, Asia, and North Africa and has many therapeutic properties.[1] The khella seeds are being used in traditional medicine for respiratory conditions including bronchitis, asthma, and cough.[2] The seeds of this plant contain a γ-pyrone, furanochromone khellin as a major constituent, which is a vasodialator and antispasmodic.[1a,3] Inspired from the vasodialatory and antispasmodic property of khellin, Roger Altounyan, an Anglo-Armenian physician discovered cromoyln sodium, a chromone class of antiasthma drug.[4] Khellin has also been extensively studied for its cardioactive properties and is being used for angina pectoris therapy.[5] On the basis of the cardioactive property of khellin, the Labaz company from Belgium has discovered an antiarrhythmic drug amiodarone[6] via medicinal chemistry on the benzofuran scaffold. Other known pharmacological activities of khellin and its derivatives include melanin inhibition[1d,7] and anti-inflammatory,[8] anticancer,[9] and cancer chemopreventive activity.[10]

Many natural compounds and their derivatives modulate the expression and activities of cytochrome P450 family 1 (CYP1) enzymes which are known to bioactivate procarcinogens.[11] The cytochrome P450 proteins comprise a large family of enzymes that can catalyze the oxidative metabolism of a variety of xenobiotic chemicals including toxic compounds, drugs, and chemical carcinogens as well as endogenous compounds. As a part of our continuing efforts to discover CYP1A1 inhibitors as cancer chemopreventive agents,[12] the furanochromone khellin (1) was identified as a selective CYP1A1 inhibitor (CYP1A1 IC50 = 4.02 μM, CYP1B1 IC50 = 34.6 μM) via screening of institutional natural products repository (Figure ).[10]

Figure 1

Structure of khellin (1), its CYP1A1 and CYP1B1 inhibitory activity, and interactions with the CYP1A1 active site. The H-bonding is shown by the dark blue dotted lines, whereas π–π interactions are shown by the light blue dotted line.

To further explore this natural product for fine-tuning its potency and selectivity profile, semisynthetic modifications were carried out. Literature reports indicated that semisynthetic modifications of khellin have been performed earlier,[13] however, never been explored as cancer chemopreventive agents. Herein, we report synthesis of two series of khellin derivatives (khellinochalcones and khellinoflavanones) and their biological evaluation as CYP1A1 inhibitors and possible cancer chemopreventive agents.

The in vivo studies[14] have demonstrated that CYP1A1 is involved in benzo[a]pyrene (B[a]P)-induced carcinogenesis in mice. These studies have also shown that polycyclic aromatic hydrocarbon (PAH)-treated mice overexpress CYP1A1 in the lungs and liver. Therefore, herein, we also studied the ability of khellin derivatives to protect lung cancer cells and normal cells from B[a]P-induced toxicity.

Results and Discussion

Synthesis of Khellin Derivatives

Khellin was identified as one of the “hit” (active) from the natural products repository screening,[10] in the search of new leads for cancer chemoprevention. However, the level of activity was in a micromolar range (IC50, 4.02 μM); therefore, it was necessary to modify the structure of khellin to modulate its ability to inhibit CYP1A1. On the basis of the structural features of khellin, it was decided to convert khellin to khellinone followed by its condensation with aryl aldehydes. The presence of the acetyl group in khellinone provides an easiest handle for chemical modifications.

Khellin (1) was treated with potassium hydroxide in ethanol, resulting in the formation of khellinone (2).[15] Two series of compounds were prepared from khellinone (2) as follows:[16] (a) furanochalcones by the Claisen–Schmidt condensation of khellinone with different substituted aryl aldehydes in presence of KOH and (b) furanoflavanones by treatment of khellinone with aryl aldehydes in the presence of piperidine. Khellinone (2) was treated with aryl aldehydes under an alkaline condition (KOH), at room temperature, to obtain khellinochalcones 3a–v (Figure , path b). Interestingly, with slight modifications in the reaction conditions (base and reaction temperature), another series of products were formed. Khellinone (2), when it was reacted with aryl aldehydes in ethanol with catalytic amounts of piperidine (instead of KOH), under reflux conditions, yielded furanoflavanones 4a–p (khellinoflavanones) (Figure path c). Mechanistically, the flavanones were formed via cyclization of chalcones. All synthesized compounds were characterized based on their spectroscopic properties. The coupling constant of the newly formed bond (C8–C9) in chalcones was found to be 16 Hz, which confirmed the trans orientation of the double bond. The 1H NMR spectrum of flavanone 4b showed the signals of H-7 at δ 5.45 ppm and H-6 at δ 3.04 and 2.94 ppm, confirming the formation of the flavanone skeleton. Further, the 13C NMR and DEPT135 spectrum confirmed the appearance of extra −CH at δ 45.0 (C-6) and −CH at δ 78.4 (C-7). Interestingly, when khellinone (2) was treated with thiophene-2-carboxaldehyde, it underwent double aldehyde coupling, producing product 4p. The 1H NMR spectrum of 4p did not show the presence of signal for CH2; however, the extra proton appeared at δ 7.13 ppm, indicating the presence of an additional olefinic proton.

Figure 2

Synthesis of khellin derivatives 3a–v and 4a–p. Reagents and conditions: (a) KOH (1 M), ethanol, 80 °C, 12 h, 61%; (b) RCHO (R = aryl or heteroaryl), KOH (1 M), methanol, 0 °C to rt, 12–14 h, 40–85%; and (c) RCHO (R = aryl or heteroaryl), piperidine, ethanol, reflux at 70 °C, 12 h, 51–68%. The “R” groups of different derivatives 3a–v and 4a–p are shown in Tables and 3.

Table 1

In Vitro CYP1A1 Inhibition by Furanochalcones in Sacchrosomes and HEK293 Cells, Overexpressing CYP1A1, at 10 μM Concentration

α-Naphthoflavone (ANF) was used as a positive control.

The percentage inhibition values represent mean and standard deviations (±SD) of results obtained from two independent experiments.

Table 3

In Vitro CYP1A1 Inhibition by Furanoflavanones in Sacchrosomes and HEK293 Suspension Cells, Overexpressing CYP1A1, at 10 μM Concentrationsa

The IC50 values represent average (±SD) of results obtained from three independent experiments.

Pyridyl Furanochalcone Derivative 3g Is a Pan Inhibitor of CYP1 Family Enzymes

A series of furanochalcones 3a–v were tested for inhibition of CYP1A1 in yeast microsomes (Sacchrosomes) and human live HEK293 cells, overexpressing CYP1A1, at 10 μM (Table ).[10,12] The parent natural product khellin (1) showed 88 and 81% inhibition of CYP1A1, at 10 μM, in the two assays, respectively. Several chalcone derivatives showed higher inhibition of CYP1A1 in both assays. The furanochalcone 3a, bearing a simple phenyl ring (as R group) showed 92 and 97% inhibition of CYP1A1 in the microsomal and live cells assays, respectively. This result was expected, as chalcones have been reported to possess potent CYP1A1 inhibition activity.[12a] The most potent derivative among the series of chalcones was the compound 3g, which bears a pyridin-3yl moiety as the R group. This compound showed 97 and 99% inhibition of CYP1A1 in Sacchrosomes and in live human cells, respectively. The results of in vitro CYP1A1 inhibition by furanochalcones in Sacchrosomes and HEK293 cells, at 10 μM concentration, are shown in Table .

The IC50 values were then determined for selected furanochalcones in a panel of enzymes belonging to cytochrome P450 subfamilies 1, 2, and 3. The results are shown in Table . Chalcones 3e (bearing 4-fluoro-phenyl as the R group) and 3p (bearing 2,4,5-tri-OMe phenyl as the R group) displayed highly selective inhibition of CYP1A1 with IC50 values of 0.32 and 0.30 μM, respectively. These two compounds do not inhibit the other two enzymes from the CYP1 subfamily, that is, CYP1A2 or CYP1B1 (IC50 > 10 μM). Further, they do not inhibit the CYP2 and CYP3 subfamily of enzymes commonly involved in drug metabolism (Table ). The pyridyl furanochalcone 3g showed inhibition of Sacchrosomes containing CYP1A1, CYP1A2, and CYP1B1 with IC50 values of 0.34, 0.17, and 0.66 μM, respectively, whereas no inhibition of the CYP2 and CYP3 subfamily of enzymes was observed (IC50 > 20 μM). The compound 3g showed IC50 values of 0.47 and 0.27 μM for CYP1A1 and CYP1B1 inhibition in live human cell assays, respectively.

Table 2

IC50 Values of Selected Furanochalcones in a Panel of CYP Enzymes in Sacchrosomes

	IC50 in μM (Sacchrosomes)a
entry	1A1	1A2	1B1	2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4
3e	0.32 ± 0.04	>10	>10	>20
3g	0.34 ± 0.03	0.17 ± 0.02	0.66 ± 0.04	>20
3p	0.30 ± 0.05	>20	>20	>20
3r	0.28 ± 0.03	4.2 ± 0.8	0.78 ± 0.04	>20

The IC50 values represent average (±SD) of results obtained from three independent experiments.

The in silico docking studies have shown that the chalcone 3g displays H-bonding and π–π interactions with the active site residues of CYP1A1. The carbonyl oxygen of the chalcone 3g showed three-way H-bonding interactions with Asp 320, Thr 497, and Asn 222 residues, whereas the pyridine ring displays π–π stacking with Phe 224 and Phe 258 residues. The furan ring of the chalcone 3g has been oriented toward the heme of the CYP1A1 enzyme (Figure ), as it was observed in the case of khellin (Figure ).

Figure 3

Interactions of chalcone 3g with the CYP1A1 (PDB ID: 4I8V) active site. The H-bonding is shown by the dark blue dotted lines, whereas π–π interactions are shown by the light blue dotted lines.

Furanoflavanone Derivative 4l Is a Potent and Selective Inhibitor of the CYP1A1 Enzyme

A series of furanoflavanones 4a–p were tested for the inhibition of the CYP1A1 enzyme borne on yeast microsomes (Sacchrosomes) and CYP1A1-overexpressing HEK293 cells, at 10 μM concentration (Table ). Several flavanone derivatives showed >85% inhibition of the CYP1A1 enzyme in Sacchrosomes as well as in live human cells overexpressing CYP1A1. The compound 4a, bearing a simple phenyl as the R group, has shown 97 and 87% inhibition of CYP1A1 in Sacchrosomes and in live human cells, respectively. The 3-pyridyl-substituted flavanone (compound 4e) also showed potent inhibition of CYP1A1 enzyme in both assay systems.

Next, the IC50 values were determined for three best flavanones, in live human cell assays expressing specific CYP enzymes. Compound 4a was found to be a dual inhibitor of CYP1A1 and CYP1B1 with slight selectivity toward CYP1B1. However, the compounds 4e and 4l were selective CYP1A1 inhibitors, showing 9.2- and 170-fold selectivity over CYP1B1 (Table ), respectively. These compounds do not inhibit CYP1A2, CYP2D6, and CYP3A4 (IC50 > 10 μM). The excellent selectivity profile of flavanone 4l against CYP2D6, CYP3A4, and also within the CYP family 1 enzymes, indicated its potential for further exploration.

Table 4

IC50 Values of Selected Furanochalcones for CYP1A1, CYP1A2, CYP1B1, CYP2D6, and CYP3A4 Inhibition in Live Human HEK293 Cells

	IC50 in μM (live cells)a
entry	CYP1A1	CYP1A2	CYP1B1	CYP2D6	CYP3A4
4a	1.84 ± 0.14	>10	0.566 ± 0.08	>10	>10
4e	0.45 ± 0.06	>10	4.16 ± 0.25	>10	>10
4l	0.14 ± 0.04	>10	23.8 ± 1.2	>10	>10

The IC50 values represent average (±SD) of results obtained from three independent experiments.

The in silico docking studies of flavanone 4l with CYP1A1 have shown that it displays interactions similar to chalcone 3g; however, the molecule was found to be flipped by 180° inside the active site. The furan ring of chalcone 3g was oriented toward the heme, whereas it is away from heme in the case of flavanone 4l. However, despite this flip, the three-way H-bonding interaction of carbonyl oxygen with Asp 320, Thr 497, and Asn 222 residues was common in both the inhibitors. The flavanone 4l also showed two π–π stacking interactions with Phe 224 and Phe 258 residues. The interactions of 4l with CYP1A1 are shown in Figure .

Figure 4

Interactions of flavanone 4l with the CYP1A1 (PDB ID: 4I8V) active site.

On comparison of the CYP1A1 and CYP1B1 inhibition data of naphthyl pyridochalcones published earlier by us,[17] with the data of khellinochalcones and khellinoflavanones, an interesting structure–activity relationship was observed. The 3-pyridyl naphthyl chalcone[17] (structure shown in Figure ) has been reported as potent and selective CYP1B1 inhibitor with an IC50 value of 4 nM, showing selectivity over CYP1A1. The replacement of the naphthyl ring with 4,7-dimethoxybenzofuran has resulted in improved CYP1A1 inhibitory activity (Figure ). The 4,7-dimethoxybenzofuran ring bearing 3-pyridyl chalcone 3g showed CYP1A1 inhibition with an IC50 value of 470 nM. Compound 3g inhibited CYP1B1 in live human cell assay with an IC50 value of 270 nM, indicating slight selectivity toward CYP1B1. Furthermore, when the chalcone skeleton was cyclized in the form of a flavanone (3g vs 4e), there was a huge gain in CYP1A1 selectivity. This indicated that flavanones have better selectivity toward CYP1A1, compared with chalcones. Interestingly, when the pyridyl ring of flavanone 4e was replaced with substituted phenyl, the potent and selective CYP1A1 inhibitor 4l was identified. The compound 4l bearing a 3-bromo-4-fluorophenyl ring instead of 3-pyridyl (of 4e) inhibited CYP1A1 with an IC50 value of 140 nM and 170-fold selectivity over CYP1B1 (IC50 for CYP1B1 = 23.8 μM). The CYP1A1 and CYP1B1 activities of chalcones and flavanones, indicating selectivity within these two enzymes, are depicted in Figure .

Figure 5

Structure–activity relationship between chalcones and flavanones.

Furanochalcone 3g and Furanoflavanone 4l Protect CYP1A1-Overexpressing Normal Human Cells from CYP1A1-Mediated B[a]P Toxicity

CYP1A1 is an aryl hydroxylase which provides metabolic activation of PAHs and polyhalogenated aromatic hydrocarbons. It is mainly expressed in the extrahepatic tissue such as the lung, placenta, skin, and so forth.[18] The CYP1A1 enzyme is known to metabolically activate B[a]P to its carcinogenic form.[18,19] Therefore, to investigate the potential of identified CYP1A1 inhibitors to stop the process of metabolic activation, thereby protecting normal cells from carcinogenesis, the B[a]P toxicity in normal HEK293 cells and HEK293 cells,[20] overexpressing CYP1A1, was studied.[10,12] EC50 of B[a]P in normal HEK293 cells was found to be 14 μM, whereas in CYP1A1-overexpressing cells, it was highly toxic (EC50 1.4 μM), indicating that CYP1A1 metabolizes B[a]P to its toxic form. The cotreatment of these CYP1A1-overexpressing cells with CYP1A1 inhibitors 3g or 4l, at their 3× IC50 concentrations, resulted in nullifying of B[a]P toxicity. The cells were completely protected from B[a]P toxicity as indicated by restoration of the B[a]P EC50 to normal (i.e., 14 μM) (Figure ).

Figure 6

Protection of CYP1A1-overexpressing HEK293 cells from B[a]P toxicity. B[a]P was used in the concentration range of 0.05–100 μM in the presence of 3× IC50 values of the compounds 3g or 4l (as determined in the human cell assay where cells expressed CYP1A1 when they were grown in suspension). The cell viability was determined by 3-4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assay. The cell viability determination using sulforhodamine B assay[21] provided results similar to the MTT assay. EC50 values are presented in μM concentrations and represent the average of three independent experiments (***P < 0.001).

Furanoflavanone 4l Prevents B[a]P-Mediated S-Phase Block in Human Lung Cancer Cells, A549 and Calu-1

The human lung cancer cell lines A549[22] and Calu-1[23] are known to express CYP1A1 enzyme. Furthermore, the in vivo studies in mice model[14] have shown that PAH-treated mice overexpress CYP1A1 in the lung tissues; therefore, lung cancer cell lines were chosen for this study. The effect of CYP1A1 inhibitors on the cell cycle, in the presence of B[a]P, was investigated by first pretreating A549 (ATCC CCL-185) and Calu-1 (ATCC HTB-54) lung cancer cells (which express CYP1A1; Figure A) with the most potent CYP1A1 inhibitor, compound 4l, and then exposing cells to B[a]P. The endogenous CYP1A1 is expected to metabolize B[a]P to its diol-epoxide and corresponding O-quinone, products which are known to intercalate DNA. DNA intercalation should block cells primarily at the S-phase of the cell division cycle. It has been suggested that DNA adduct formation causes DNA mutations that lead to the onset of cancer.[24]

Figure 7

Western blot and FACS analyses of A549 and Calu-1 lung cancer cells. (A) Expression of CYP1A1, in 5 μg of total protein, isolated from A549 (lane 1), Calu-1cells (lane 3), normal HEK293 cells (lane 2; as negative control), and normal WI-38 cells (lane 4; as negative control) was confirmed by western blotting; CYP1A1-bearing Sacchrosomes (1 pmol) were taken as the positive control (lane 5). The CYP1A1-specific antibody, used for confirming CYP1A1 expression in the upper panel, was from ProteinTech (#13241-1-A); β-actin was used as a loading control and the blot (lower panel) was probed with an antibody from Sigma (# A2228). (B,E) FACS analyses of untreated A549 (B) and Calu-1 cells (E) growing asynchronously. FACS analyses of A549 cells, treated with B[a]P, in the absence of compound 4l (C) and in the presence of compound 4l (D). FACS analyses of Calu-1 cells, treated with B[a]P, in the absence of compound 4l (F) and in the presence of compound 4l (G). Percentage of cells in G0/G1, S, and G2/M phases of the cell division cycle is shown in the upper right-hand corner of each of the FACS analyses.

Upon exposure of asynchronous A549 and Calu-1 lung cancer cells (Figure B,E) to a 15 μM concentration of B[a]P for 3 h and then releasing cells into fresh medium for 57 h, the cells did face a block at the S-phase of the cell cycle (Figure C,F). This would be expected if the CYP1A1 protein (Figure A), expressed within A549 and Calu-1cells, were to be a functional enzyme. However, when cells were first preincubated for 20 h with the CYP1A1-specific inhibitor, compound 4l, at its 3× IC50 concentrations, and then treated with a 15 μM concentration of B[a]P for 3 h, followed by release into fresh medium, the cells were preponderantly released from the block (Figure D,G). This would imply that CYP1A1-mediated metabolism of B[a]P in A549 and Calu-1 cells has been prevented by compound 4l, demonstrating the efficacy of the compound in preventing B[a]P-mediated toxicity in lung cancer cells.

Conclusions

In summary, we have identified that the semisynthetic modifications on the natural product khellin have resulted in significant improvement in its CYP1A1 inhibitory activity. The furanochalcone derivative 3g and furanoflavanone derivative 4l displayed potent inhibition of CYP1A1 with IC50 values of 470 and 140 nM, respectively. These inhibitors completely protected normal human cells from the toxicity of procarcinogen B[a]P via inhibition of its metabolic activation. Moreover, compound 4l also prevented manifestation of B[a]P-mediated S-phase block (i.e. DNA intercalation that occurs at S-phase upon B[a]P treatment) in lung cancer cells. Thus, the leads identified herein have potential for further investigation as cancer chemopreventive agents.

Experimental Section

General

The commercially available khellin was purchased from Sigma-Aldrich (CAS: 82-02-0, Lot≠ BCBJ3836U). NMR spectra were recorded on Bruker-AVANCE DPX FT-NMR 500 and 400 MHz instruments. Chemical shift values for protons/carbons are reported in parts per million downfield from tetramethylsilane. Electrospray ionization mass spectrometry (ESIMS) and high-resolution mass spectrometry (HRMS) spectra were recorded on Agilent 1100 LC-Q-TOF and HRMS-6540-UHD machines. IR spectra were recorded on a PerkinElmer IR spectrophotometer. The melting points were recorded on a digital melting point apparatus.

The purity of all derivatives was determined using high-performance liquid chromatography (HPLC) analysis. The sample solution of compounds of concentration 0.5 mg/mL was prepared in acetonitrile and filtered through a 0.45 μ membrane filter injected (15 μL) in the HPLC system (Waters) using a Chromolith C-18e column (Merck, 50 × 4.6 mm). The binary mobile phase consisting of A (water) and B (acetonitrile) was used. The isocratic elution was used as follows: 80 min run time with 90% acetonitrile and 10% water. The flow rate was 1 mL/min and the results were analyzed using a UV-PDA detector.

Synthesis of Khellinone (2)

The mixture of khellin (900 mg, 3.46 mmol) and potassium hydroxide (1 M) in ethanol was refluxed at 90 °C over a period of 12–14 h. After completion of the reaction, ethanol was evaporated over a vacuo rotavapor and the obtained residue was partitioned in between dichloromethane and water. The dichloromethane layer was washed with brine solution and was dried over anhydrous sodium sulfate. The solvent was evaporated over the vacuo rotavapor and the obtained crude product was purified using silica gel column chromatography (5–10% ethyl acetate in hexane) to get khellinone (2) as a yellow powder. The purchased khellin along with khellinone were characterized using spectral analysis.

Khellin (1): white crystals; HPLC: tR = 4.6 min (99% purity); mp 155–156 °C; IR (CHCl3) νmax: 3436, 3160, 3137, 2989, 2931, 2960, 2830, 1619, 1586, 1471, 1444, 1424, 1364, 1380, 1300, 1265 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.51 (d, J = 2.2 Hz, 1H, CH), 6.91 (d, J = 4.0 Hz, 1H, CH), 4.15 (s, 3H, OMe), 4.05 (s, 3H, OMe), 2.73 (s, 3H, Me); 13C NMR (100 MHz, CDCl3): δ (ppm) 206.2 (C=O), 153.5 (C-7a), 152.3 (C-6), 151.6 (C-3), 143.8 (OCH=CH), 128.8 (C-7), 110.8 (C-3a), 110.5 (C-5), 106.7 (OCH=CH), 61.0 (OMe), 60.9 (OMe), 33.2 (Me); HR-ESIMS m/z: 237.0759 [M + H]+ calcd for C12H12O5 + H+ (237.0757).[25]

1-(6-Hydroxy-4,7-dimethoxy-benzofuran-5-yl) (2)

Yellow crystals; HPLC: tR = 4.6 min (99% purity); yield: 61%; mp 169–170 °C; IR (CHCl3) νmax: 3436, 3160, 3137, 2989, 2931, 2960, 2830, 1619, 1586, 1471, 1444, 1424, 1364, 1380, 1300, 1265 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.51 (d, J = 2.2 Hz, 1H, CH), 6.91 (d, J = 4.0 Hz, 1H, CH), 4.15 (s, 3H, OMe), 4.05 (s, 3H, OMe), 2.73 (s, 3H, Me); 13C NMR (100 MHz, CDCl3): δ (ppm) 206.2 (C=O), 153.5 (C-7a), 152.3(C-6), 151.6 (C-3), 143.8 (OCH=CH), 128.8 (C-7), 110.8 (C-3a), 110.5 (C-5), 106.7 (OCH=CH), 61.0 (OMe), 60.9 (OMe), 33.2 (Me); HR-ESIMS m/z: 237.0759 [M + H]+ calcd for C12H12O5 + H+ (237.0757).[15]

Synthesis of Furanochalcones 3a–v

Khellinone (80 mg, 0.338 mmol) was reacted with different aromatic aldehydes (1.2 equiv) in the presence of catalytic amount of 1 M KOH in 50 mL of methanol at a temperature of 0–1 °C over a period of 12–14 h. After completion of the reaction, methanol was evaporated over the vacuo rotavapor and the obtained residue was partitioned in between dichloromethane and water. The dichloromethane layer was washed with brine solution and was dried over anhydrous sodium sulfate. The solvent was evaporated over the vacuo rotavapor and the obtained crude product was purified using silica gel column chromatography (5–25%) to get furanochalcones 3a–v in 40–85% yield.

(1-(6-Hydroxy-4,7-dimethoxy-benzofuran-5-yl)-3-phenyl-propenone) (3a)

Yellow-orange solid; HPLC: tR = 4.7 min (99% purity); yield: 41%; mp 123–126 °C; IR (CHCl3) νmax: 3860, 3791, 3697, 3436, 3060, 2930, 2850, 1630, 1606, 1559, 1494, 1446, 1360 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.88 (d, J = 2.2 Hz, 2H, CH), 7.64–7.67 (m, 2H), 7.53 (d, J = 2.2 Hz, 1H, OCH=CH), 7.42–7.45 (m, 3H), 6.89 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.05 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.7 (C=O), 153.2 (C-9a), 151.9 (C-6), 150.7 (C-4), 144.1(CH=CH), 143.5 (OCH=CH), 135.1, 130.4, 129.6, 129.0, 128.5, 127.0, 112.8, 111.9, 105.2 (OCH=CH), 62.0, 61.0; HR-ESIMS m/z: 325.1078 [M + H]+ calcd for C19H16O5 + H+ (325.1075).[16]

3-(4-Bromo-phenyl)-1-(6-hydroxy-4,7-dimethoxy-benzofuran-5-yl)-propenone) (3b)

White solid; HPLC: tR = 4.5 min (100% purity); yield: 52%; mp 135–137 °C; IR (CHCl3) νmax: 3400, 2919, 2850, 1682, 1613, 1544, 1414, 1389, 1435, 1349, 1279, 112, 1012 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.82 (d, J = 12.0 Hz, 2H, CH), 7.58 (d, J = 8.0 Hz, 1H, CH), 7.53 (d, J = 4.0 Hz, 1H, OCH=CH), 7.40 (d, J = 12.0 Hz, 2H, CH), 6.89 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.04 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.4 (C=O), 153.1 (C-6), 152.1 (C-9a), 150.7 (C-4), 144.3 (OCH=CH), 141.4, 137.3, 133.1, 130.9, 130.5, 127.2, 123.1, 112.7, 111.7, 106.2 (OCH=CH), 62.0, 61.0; HR-ESIMS m/z: 403.0175 [M + H]+ calcd for C19H15BrO5 + H+ (403.0175).

3-(4-Chlorophenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3c)

Orange crystals; HPLC: tR = 49.6 min (90% purity); yield: 48%; mp 152–154 °C; IR (CHCl3) νmax: 3448, 3053, 2928, 2868, 2304, 1730, 1656, 1619, 1585, 1462, 1386, 1327, 1313, 12679, 1262, 1210, 1149, 1039 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.98 (d, J = 4.0 Hz, 2H, CH), 7.52 (d, J = 4.0 Hz, 1H, OCH=CH), 7.40 (d, J = 8.0 Hz, 2H, CH), 7.19 (m, 2H, CH), 6.88 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.05 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.5 (C=O), 153.2 (C-6), 152.0 (C-9a), 150.7 (C-4), 144.2 (OCH=CH), 141.9, 136.3, 133.6, 129.6, 129.3, 127.5, 127.4, 112.7, 111.8, 106.2 (OCH=CH), 62.0, 61.0; HR-ESIMS m/z: 359.0677 [M + H]+ calcd for C19H15 ClO5 + H+ (359.0608).[26]

3-(3-Bromo-4-fluorophenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one) (3d)

White solid; HPLC: tR = 5.0 min (97% purity); yield: 44%; mp 186–188 °C; IR (CHCl3) νmax: 3400, 2921, 2850, 1630, 1557, 1494, 1463, 1442, 1417, 1382, 1359, 1332, 1299, 1269, 1151, 1091 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.85 (m, 1H, CH), 7.76 (d, J = 8.0 Hz, 2H, CH), 7.54 (t, J = 4.0 Hz, 2H, CH), 7.12 (t, J = 12.0 Hz, 1H, CH), 6.89 (d, J = 2.2 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.05 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.5 (C=O), 165.3 (d, 1JCF = 251 Hz), 162.8, 153.1, 151.9, 150.6, 144.2 (OCH=CH), 142.2, 131.4, 130.4 (d, 3JCF = 8.0 Hz), 130.3, 129.6, 126.7, 116.2 (2JCF = 22.0 Hz), 112.8, 111.9, 106.1 (OCH=CH), 62.0, 61.0; HR-ESIMS m/z: 423.0060 [M + H]+ calcd for C19H14 BrFO5 + H+ (423.0060).

3-(4-Fluro-phenyl)-1-(6-hydroxy-4,7-dimethoxy-benzofuran-5-yl)-propenone (3e)

Cream colored oil; HPLC: tR = 48.4 min (98% purity); yield: 40%; mp 129–130 °C; IR (CHCl3) νmax: 3400, 2922, 2851, 1628, 1601, 1556, 1544, 1510, 1461, 1443, 1413, 1360, 1297, 1299, 1266, 1151, 1091 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.82 (s, 2H, CH), 7.64 (dd, J = 8.0, 5.5 Hz, 2H, CH), 7.53 (d, J = 2.2 Hz, 1H, OCH=CH), 7.12 (t, J = 8.6 Hz, 2H, CH), 6.88 (d, J = 2.3 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.05 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.5 (C=O), 165.0 (d, 1JCF = 251.2 Hz), 163.0, 153.1, 151.9, 150.7, 144.2 (OCH=CH), 142.2, 131.4, 130.4 (d, 3JCF = 8.75 Hz), 126.7 (d, 4JCF = 3.0 Hz), 116.2 (d, 2JCF = 22.5 Hz), 112.8, 111.8, 106.2 (OCH=CH), 62.0, 61.0; HR-ESIMS m/z: 365.0838 [M + Na]+ calcd for C19H15FNaO5 (365.0801).[27]

3-(2,4-Difluro-phenyl)-1-(6-hydroxy-4,7-dimethoxy-benzofuran-5-yl)-propenone) (3f)

White solid; HPLC: tR = 42.4 min (98% purity); yield: 40%; mp 154–156 °C; IR (CHCl3) νmax: 3400, 2919, 2850, 1633, 1618, 1588, 1562, 1542, 1464, 1438, 1412, 1377, 1357, 1286, 1211, 1153, 1119 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.83 (d, J = 16.0 Hz, 1H, CH), 7.72 (d, J = 12.0 Hz, 1H, CH), 7.54 (d, J = 2.2 Hz, 1H, OCH=CH), 7.14 (d, J = 4.0 Hz, 2H, CH), 6.89 (d, J = 4.0 Hz, 1H, CH), 6.86 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.07 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.2 (C=O), 164.8, 162.0, 153.2, 152.2, 150.8, 144.3 (OCH=CH), 140.2, 138.5, 129.5, 112.5, 111.6, 111.0, 110.9, 110.7, 105.4, 105.2 (OCH=CH), 61.9, 61.0; HR-ESIMS m/z: 361.0912 [M + H]+ calcd for C19H14F2O5 + H+ (361.0882).

1-(6-Hydroxy-4,7-dimethoxybenzofuran-5-yl)-3-(pyridin-3-yl)prop-2-en-1-one (3g)

Yellow-orange crystals; HPLC: tR = 49.6 min (95%); yield: 85%; mp 112–114 °C; IR (CHCl3): 3400, 2919, 2850, 1633, 1618, 1588, 1562, 1542, 1464, 1438, 1412, 1377, 1357, 1286, 1211, 1153, 1119 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.88 (s, 1H, CH), 8.63 (d, J = 4.0 Hz, 1H, CH), 7.94 (m, 2H, CH), 7.81 (d, J = 16.0 Hz, 1H, CH), 7.54 (d, J = 2.3 Hz, 1H, OCH=CH), 7.37 (dd, J = 8.0, 4.9 Hz, 1H, CH), 6.89 (d, J = 2.3 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.06 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.2 (C=O), 153.2, 152.2, 150.9, 149.8, 144.3, 139.3, 134.8, 131.0, 128.9, 123.8, 112.5, 112.5, 111.6, 111.5, 106.3, 61.8, 61.1; HR-ESIMS m/z: 326.1033 [M + H]+ calcd for C18H15NO5 + H+ (326.1023).

1-(6-Hydroxy-4,7-dimethoxybenzofuran-5-yl)-3-(4-methoxyphenyl)prop-2-en-1-one (3h)

Yellow crystals; HPLC: tR = 5.3 min (100% purity); yield: 52%; mp 148–149 °C IR (CHCl3) νmax: 3435, 2923, 2851, 1630, 1606, 1564, 1543, 1456, 1438, 1422, 1404, 1358, 1306, 1293, 1250, 1153, 1119 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.84 (d, J = 16.0 Hz, 2H, CH), 7.62 (d, J = 4.0 Hz, 2H, CH), 7.53 (d, J = 2.3 Hz, 1H, OCH=CH), 7.30 (d, J = 12.0 Hz, 1H, CH), 6.95 (d, J = 4.0 Hz, 2H, CH), 6.89 (d, J = 12.0 Hz, 1H, CH), 6.88 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.04 (s, 3H, OMe), 3.87 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.6 (C=O), 161.7, 153.2, 151.7, 150.6, 144.1 (OCH=CH), 143.8, 130.3, 127.8, 124.5, 114.5, 113.4, 112.9, 112.0, 105.1 (OCH=CH), 62.0, 61.0, 55.4; HR-ESIMS m/z: 355.1171 [M + H]+ calcd for C20H18O6 + H+ (354.1103).[27]

3-(2,6-Dichlorophenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3i)

Orange-yellow crystals; HPLC: tR = 5.3 min (100% purity); yield: 57%; mp 298–299 °C; IR (CHCl3) νmax: 3399, 3161, 3090, 2951, 2921, 2851, 1640, 1613, 1577, 1472, 1441, 1427, 1378, 1357, 1328, 1301, 1275, 1242, 1213, 1185, 1145 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.98 (d, J = 8.0 Hz, 2H, CH), 7.52 (d, J = 2.3 Hz, 1H, OCH=CH), 7.40 (d, J = 8.0 Hz, 2H, CH), 7.23 (d, J = 8.0 Hz, 1H, CH), 6.88 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.07 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.4 (C=O), 153.4, 152.2, 151.1, 143.9 (OCH=CH), 141.7, 136.3, 135.3, 134.7, 132.6, 129.9, 129.3, 128.8, 112.3, 111.5, 105.3 (OCH=CH), 61.9 (OMe), 61.1 (OMe); HR-ESIMS m/z: 393.0287 [M + H]+ calcd for C19H14Cl2O5 + H+ (393.0291).

3-(5-Bromo-2-ethoxyphenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3j)

Orange powder; HPLC: tR = 5.1 min (100% purity); yield: 55%; mp 260–261 °C; IR (CHCl3) νmax: 3399, 3161, 3090, 2951, 2921, 2851, 1640, 1613, 1577, 1472, 1441, 1427, 1378, 1357, 1328 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.12 (d, J = 16.0 Hz, 1H, CH), 7.90 (d, J = 16.0 Hz, 1H, CH), 7.73 (d, J = 4.0 Hz, 1H, CH), 7.53 (d, J = 2.2 Hz, 1H, OCH=CH), 7.43 (d, J = 4.0 Hz, 1H, CH), 6.88 (d, J = 2.2 Hz, 1H, OCH=CH), 6.82 (d, J = 8.0 Hz, 1H, CH), 4.13 (m, 2H, CH2), 4.09 (s, 3H, OMe), 4.04 (s, 3H, OMe), 1.50 (t, J = 7.0 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.7 (C=O), 157.1, 157.0, 153.2, 151.9, 150.8, 144.2 (OCH=CH), 137.2, 134.0, 130.9, 128.1, 126.1, 113.9, 112.8, 112.8, 111.9, 106.2 (OCH=CH), 64.4, 62.0, 61.0, 14.7; HR-ESIMS m/z: 447.0433 [M + H]+ calcd for C21H19BrO6 + H+ (447.0437).

3-(2-Chlorophenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3k)

Yellow powder; HPLC: tR = 5.1 min (100% purity); yield: 55%; mp 150–151 °C; IR (CHCl3) νmax: 3435, 2922, 2851, 2650, 2342, 1693, 1628, 1591, 1571, 1469, 1439, 1408, 1363, 1316 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.23 (d, J = 16.0 Hz, 1H, CH), 7.86 (d, J = 8.0 Hz, 1H, CH), 7.74 (d, J = 2.2 Hz, 1H, CH), 7.53 (d, J = 2.3 Hz, 1H, OCH=CH), 7.33 (dd, J = 2.3, 4.0 Hz, 2H, CH), 6.88 (d, J = 2.3 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.05 (s, 3H, OMe);13C NMR (100 MHz, CDCl3): δ (ppm) 194.4 (C=O), 153.1, 152.1, 150.7, 144.3 (OCH=CH), 141.1, 137.3, 133.1, 130.9, 130.5, 129.5, 128.3, 127.2, 123.1, 112.7, 111.7, 105.2, 62.0, 61.0; HR-ESIMS m/z: 359.0680 [M + H]+ calcd for C19H15ClO5 + H+ (359.0680).[26]

3-(4,7-Dimethoxy-6-hydroxybenzofuran-5-yl)-1-(2-3-dimethoxyphenyl)-3-oxopropene (3l)

HPLC: tR = 4.7 min (100% purity); yield: 57%; mp 260–262 °C; IR (CHCl3) νmax: 3400, 2923, 2851, 1627, 1561, 1511, 1463, 1439, 1383, 1360, 1301, 1264, 1064, 1022 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.83 (d, J = 16.0 Hz, 2H, CH), 7.53 (d, J = 4.0 Hz, 1H, OCH=CH), 7.25 (d, J = 2.2 Hz, 2H, CH), 7.16 (d, J = 4.0 Hz, 1H, CH), 6.91 (d, J = 12.0 Hz, 1H, CH), 6.87 (d, J = 4.0 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.03 (s, 3H, OMe), 3.95 (s, 3H, OMe), 3.93 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 195.6 (C=O), 163.8, 161.1, 154.0, 152.2, 151.3, 144.7, 140.3, 131.4, 130.2, 125.3, 125.2, 118.0, 113.6, 112.9, 106.2, 105.8, 62.7, 61.7, 56.2, 56.2; HR-ESIMS m/z: 385.1278 [M + H]+ calcd for C21H20O7 + H+ (385.1281).

3-(4,7-Dimethoxy-6-hydroxybenzofuran-5-yl)-1-(2-4-dimethoxyphenyl)-3-oxopropene (3m)

Colorless oil; HPLC: tR = 4.7 min (100% purity); yield: 55%; mp 264–266 °C; IR (CHCl3) νmax: 3400, 2923, 2851, 1627, 1561, 1511, 1463, 1439, 1383, 1360, 1301, 1264, 1064, 1022 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.21 (d, J = 16.0 Hz, 1H, CH), 7.93 (d, J = 16.0 Hz, 1H, CH), 7.61 (d, J = 8.0 Hz, 1H, CH), 7.51 (d, J = 2.2 Hz, 1H, OCH=CH), 6.87 (s, 1H, CH), 6.53 (d, J = 4.0 Hz, 1H, OCH=CH), 6.48 (s, 1H, CH), 4.09 (s, 3H, OMe), 3.91 (s, 3H, OMe), 3.87 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.9 (C=O), 163.2, 160.5, 153.3, 151.5, 150.6, 144.0, 139.6, 130.7, 129.4, 124.6, 117.3, 112.9, 112.2, 105.5, 105.2, 98.4, 62.0, 61.0, 55.5, 55.5; HR-ESIMS m/z: 385.1276 [M + H]+ calcd for C21H20O7 + H+ (385.1281).[26]

1-(6-Hydroxy-4,7-dimethoxybenzofuran-5-yl)-3-(perfluorophenyl)prop-2-en-1-one (3n)

White solid; HPLC: tR = 5.0 min (92% purity); yield: 50%; mp 112–114 °C; IR (CHCl3) νmax: 3400, 2924, 2853, 1726, 1656, 1500, 1462, 1385, 1280, 1209, 1151, 1053, 1021 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.12 (d, J = 16.0 Hz, 1H, CH), 7.81 (d, J = 16.0 Hz, 1H, CH), 7.53 (d, J = 4.0 Hz, 1H, OCH=CH), 6.91 (d, J = 4.0 Hz, 1H, OCH=CH), 4.17 (s, 3H, OMe), 4.11 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.3 (C=O), 153.5, 152.2, 151.0, 144.9, 144.1 (OCH=CH), 133.0, 132.9, 129.3, 127.2, 112.1, 111.3, 108.6, 105.1 (OCH=CH), 61.3, 61.0; HR-ESIMS m/z: 415.0603 [M + H]+ calcd for C19H11F5O5 + H+ (415.0599).

1-(6-Hydroxy-4,7-dimethoxybenzofuran-5-yl)-3-(2,4,5-trimethoxyphenyl)prop-2-en-1-one (3o)

Reddish orange crystals; HPLC: tR = 8.7 min (100% purity); yield: 50%; mp 131–133 °C; IR (CHCl3) νmax: 3434, 2930, 2867, 1724, 1656, 1517, 1463, 1385, 1263, 1210, 1159, 1084, 1024 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.29 (d, J = 16.0 Hz, 1H, CH), 7.92 (d, J = 16.0 Hz, 1H, CH), 7.59 (s, 1H, OCH=CH), 7.22 (m, 1H, CH), 6.94 (s, OCH=CH, 1H), 6.60 (s, 1H, CH), 4.16–3.97 (m, 15H, OMe); 13C NMR: (100 MHz, CDCl3): δ (ppm) 194.7, 154.8, 153.2, 152.7, 151.5, 150.5, 144.0, 143.3, 139.3, 129.6, 124.4, 115.7, 113.0, 112.2, 111.2, 105.1, 96.7, 62.0 (OMe), 61.0 (OMe), 56.5 (OMe), 56.3 (OMe), 56.1 (OMe); HR-ESIMS m/z: 415.1390 [M + H]+ calcd for C22H21O8 + H+ (415.1390).

3-(3-Bromo-4-methoxyphenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3p)

Orange crystals; HPLC: tR = 5.4 min (100% purity); yield: 52%; mp 261–263 °C; IR (CHCl3) νmax: 3454, 2927, 2866, 1730, 1654, 1590, 1464, 1386, 1365, 1326, 1312, 1279, 1102, 1084, 1048 cm –1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.11 (d, J = 16.0 Hz, 1H, CH), 7.86 (d, J = 16.0 Hz, 1H, CH), 7.73 (s, 1H, CH), 7.52 (s, 1H, CH), 6.88 (s, 1H, CH), 6.84 (d, J = 8.0 Hz, 1H, CH), 4.09 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.91 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.8 (C=O), 157.7, 153.2, 151.9, 150.7, 144.2, 137.1, 134.0, 134.0, 130.9, 128.3, 128.3, 126.1, 113.0, 112.7, 105.2, 62.0 (OMe), 61.0 (OMe), 55.8; HR-ESIMS m/z: 432.0300 [M + H]+ calcd for C20H17BrO6 + H+ (432.0281).

3-(Benzo[d][1,3]dioxol-5-yl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3q)

Orange crystals; HPLC: tR = 5.6 min (92% purity); yield: 53%; mp 248–251 °C; IR (CHCl3) νmax: 3743, 3385, 3130, 2850, 1729, 1627, 1565, 1542, 1489, 1470, 1446, 1353, 1300, 1255 cm –1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.78 (d, J = 16.0 Hz, 2H, CH), 7.53 (d, J = 4.0 Hz, 1H, OCH=CH), 7.15 (m, 2H, CH), 6.88 (d, J = 2.2 Hz, 1H, OCH=CH), 6.87 (d, J = 8.0 Hz, 1H, CH), 6.04 (s, 2H, CH2), 4.09 (s, 3H, OMe), 4.04 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.4 (C=O), 153.2, 151.7, 150.6, 149.9, 148.4, 144.1, 143.8, 143.6, 129.6, 125.4, 124.9, 112.8, 111.9, 108.7, 106.6, 105.2, 101.6, 62.0 (OMe), 61.0 (OMe); HR-ESIMS m/z: 369.0968 [M + H]+ calcd for C20H17O7 + H+ (369.0954).[26]

1-(6-Hydroxy-4,7-dimethoxybenzofuran-5-yl)-3-(thiophen-3-yl)prop-2-en-1-one (3r)

Orange crystals; HPLC: tR = 3.9 min (85% purity); yield: 42%; mp 139–141 °C; IR (CHCl3) νmax: 3584, 3136, 2922, 2850, 1626, 1586, 1561, 1543, 1470, 1442, 1364, 1416, 1297, 1131 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.89 (d, J = 16.0 Hz, 1H, CH), 7.73 (d, J = 16.0 Hz, 1H, CH), 7.62 (s, CH, 1H), 7.52 (s, 1H, CH), 7.40 (d, J = 8.0 Hz, 2H, CH), 6.87 (s, CH, 1H), 4.09 (s, 3H, OMe), 4.03 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.8 (C=O), 153.1, 151.8, 150.6, 144.1, 138.5, 137.2, 129.6, 129.1, 127.0, 126.6, 125.3, 112.8, 111.9, 105.1, 62.0 (OMe), 61.0 (OMe); HR-ESIMS m/z: 331.0619 [M + H]+ calcd for C17H15O5S + H+ (331.0634).

3-(2,4-Dichlorophenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-on (3s)

Orange crystals; HPLC: tR = 5.0 min (95% purity); yield: 42%; mp 148–149 °C; IR (CHCl3) νmax: 3399, 3161, 3090, 2951, 2921, 2851, 1640, 1613, 1577, 1472, 1441, 1427, 1378, 1357, 1328, 1301, 1275, 1242, 1213, 1185, 1145 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.14 (d, J = 16.0 Hz, 1H, CH), 7.83 (d, J = 16.0 Hz, 1H, CH), 7.67 (d, J = 12.0 Hz, 1H, CH), 7.53 (d, J = 4.0 Hz, 1H, OCH=CH), 7.48 (d, J = 3.0 Hz, 1H, CH), 7.31 (m, 1H, CH), 6.88 (d, J = 2.2 Hz, 1H, OCH=CH), 4.09 (s, 3H, OMe), 4.03 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.2 (C=O), 153.2, 152.1, 150.7, 144.3, 137.5, 136.3, 136.1, 132.0, 130.1, 129.8, 129.5, 128.4, 127.6, 112.6, 111.7, 105.2, 61.9, 61.0; HR-ESIMS m/z: 393.0275 [M + H]+ calcd for C19H14Cl2O5 + H+ (393.0291).

1-(6-Hydroxy-4,7-dimethoxybenzofuran-5-yl)-3-(naphthalen-2-yl)prop-2-en-1-one (3t)

Orange crystals; HPLC: tR = 7.1 min (99% purity); yield: 42%; mp 158–159 °C; IR (CHCl3) νmax: 3584, 3136, 2922, 2850, 1626, 1586, 1561, 1543, 1470, 1442, 1379, 1297, 1149 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.03 (m, 3H, CH), 7.86 (m, 4H, CH), 7.54 (d, J = 2.2 Hz, 1H, OCH=CH), 7.53 (m, 3H, CH), 6.90 (d, J = 2.2 Hz, 1H, OCH=CH), 4.10 (s, 3H, OMe), 4.07 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.6 (C=O), 153.5, 153.2, 150.7, 144.2, 143.7, 134.3, 133.4, 132.7, 130.7, 128.7, 128.6, 127.8, 127.4, 127.1, 126.8, 123.7, 112.9, 112.0, 110.6, 105.2, 62.1, 60.5; HR-ESIMS m/z: 375.1194 [M + H]+ calcd for C23H18O5 + H+ (375.1127).[26]

3-(2-Fluorophenyl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3u)

Orange crystals; HPLC: tR = 5.0 min (95% purity); yield: 52%; mp 139–141 °C; IR (CHCl3) νmax: 3400, 2922, 2851, 1628, 1601, 1556, 1544, 1510, 1461, 1443, 1413, 1360, 1297, 1299, 1266, 1151 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.96 (q, J = 15.8 Hz, 2H, CH), 7.64 (t, J = 7.1 Hz, 1H, CH), 7.53 (d, J = 2.2 Hz, 1H, OCH=CH), 7.38 (m, 1H, CH), 7.17 (m, 2H, CH), 6.89 (d, J = 4.0 Hz, 1H, OCH=CH); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.8 (C=O), 162.8 (d, 1JCF = 246 Hz), 160.7, 153.3, 152.1, 150.9, 144.1, 136.0, 131.8 (d, 3JCF = 8.75 Hz), 129.8 (d, 4JCF = 3.7 Hz), 129.6 (d, 3JCF = 6.0 Hz), 124.5 (d, 4JCF = 3.7 Hz), 123.3, 116.4 (d, 2JCF = 21.2 Hz), 112.5, 111.7, 105.3, 61.8, 61.0; HR-ESIMS m/z: 432.0300 [M + H]+ calcd for C20H17BrO6 + H+ (432.0281).[26]

3-(Anthracen-2-yl)-1-(6-hydroxy-4,7-dimethoxybenzofuran-5-yl)prop-2-en-1-one (3v)

Orange crystals; HPLC: tR = 7.4 min (100% purity); yield: 52%; mp 156–158 °C; IR (CHCl3) νmax: 3419, 2920, 2850, 2103, 1632, 1569, 1442, 1408, 1359, 1215, 1146, 1065 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.82 (d, J = 15.8 Hz, 1H, CH), 8.49 (s, 1H, CH), 8.40 (d, J = 8.4 Hz, 2H, CH), 8.05 (d, J = 7.4 Hz, 2H, CH), 7.76 (d, J = 15.8 Hz, 1H, CH), 7.53 (m, 5H, CH), 6.87 (d, J = 2.2 Hz, 1H, OCH=CH), 4.11 (s, 3H, OMe), 4.07 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 194.3 (C=O), 153.3, 152.3, 150.9, 144.0, 140.2, 135.9, 131.3, 130.3, 129.7, 129.2, 128.9, 128.9, 128.4, 128.4, 128.3, 126.3, 126.3, 125.4, 126.4, 111.7, 111.5, 105.5, 61.4, 61.1; HR-ESIMS m/z: 425.1374 [M + H]+ calcd for C27H20O5 + H+ (425.1383).

Synthesis of Furanoflavonones 4a–p

Khellinone (100 mg, 0.423 mmol) was treated with different aldehydes (1.2 equiv) in the presence of catalytic amount of piperidine in ethanol (2 mL) at 70 °C for 12 h. The solvent was evaporated on the rotary evaporator and the remaining mixture was diluted with water and dichloromethane. The organic layer was separated, dried over anhydrous sodium sulfate, and concentrated to dryness on the rotary evaporator. The obtained crude product was purified using silica gel column chromatography (5–40%) to yield furanoflavonones 4a–p in 51–68% yield.

4,9-Dimethoxy-7-phenyl-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4a)

Yellow powder; HPLC: tR = 4.1 min (100% purity); yield: 65%; mp 148–150 °C; IR (CHCl3) νmax: 3444, 3021, 2932, 2851, 2347, 2072, 1682, 1614, 1542, 1477, 1435, 1351, 1278, 1163, 1066, 1029 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.52 (d, J = 2.2 Hz, 1H, OCH=CH), 7.50 (m, 2H, CH), 6.93 (d, J = 2.2 Hz, 1H, OCH=CH), 5.49 (dd, J = 12.3, 3.0 Hz, 1H, CH), 4.08 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.09 (dd, J = 16.7, 12.4 Hz, 1H), 2.94 (dd, J = 16.7, 3.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.7 (C=O), 152.0 (C-9a), 151.4 (C-4), 150.0 (C-8a), 144.4 (OCH=CH), 138.8 (C-1′), 130.0 (C-9), 128.8 (Ar-3′, Ar-5′), 128.5 (Ar-4), 126.0 (Ar-2′, Ar-6′), 115.6 (C-4a), 111.4 (C-3a), 105.6 (OCH=CH), 79.3 (C-7), 61.5 (OMe), 61.4 (OMe), 46.1 (C-6); HR-ESIMS m/z: 325.1072 [M + H]+ calcd for C19H16O5 + H+ (325.1070).

7-(2-Bromophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4b)

Yellow powder; HPLC: tR = 4.3 min (100% purity); yield: 63%; mp 146–148 °C; IR (CHCl3) νmax: 3708, 3418, 2922, 2850, 2400, 2095, 1659, 1644, 1477, 1435, 1349, 1302 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.68 (d, J = 1.4 Hz, 1H, CH), 7.53 (d, J = 2.3 Hz, 1H, OCH=CH), 7.51 (m, 1H, CH), 7.42 (d, J = 7.8 Hz, 1H, CH), 7.29 (d, J = 8.0 Hz, 1H, CH), 6.94 (d, J = 2.3 Hz, 1H, OCH=CH), 5.45 (dd, J = 12.2, 3.2 Hz, 1H, CH), 4.08 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.04 (dd, J = 12.2, 1.3 Hz, 1H), 2.94 (dd, J = 6.7, 3.3 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.7 (C=O), 152.0 (C-9a), 151.0 (C-4), 150.0 (C-8a), 144.5 (OCH=CH), 141.0 (C-1′), 131.7 (C-3′), 130.4 (C-4′), 130.0 (C-9), 129.1 (C-5′), 124.6 (C-6′), 122.9 (C-2′), 115.7 (C-4a), 111.2 (C-3a), 106.7 (OCH=CH), 78.4 (C-7), 61.4 (OMe), 61.4 (OMe), 46.0 (C-6); HR-ESIMS m/z: 403.0170 [M + H]+ calcd for C19H15BrO5 + H+ (403.1750).

7-(Benzo[d][1,3]dioxol-5-yl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4c)

Yellow powder; HPLC: tR = 3.5 min (100% purity); yield: 60%; mp 150–154 °C; IR (CHCl3) νmax: 3743, 3584, 3415, 2922, 2850, 1681, 1614, 1542, 1504, 1491, 1477, 1445, 1390, 1349, 1300, 1277 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.52 (d, J = 2.2 Hz, 1H, OCH=CH), 7.02 (d, J = 1.4 Hz, 1H, CH), 6.94 (d, J = 2.3 Hz, 1H, OCH=CH), 5.99 (s, 2H, CH2), 5.39 (dd, J = 12.2, 3.2 Hz, 1H, CH), 4.07 (s, 3H, OMe), 4.04 (s, 3H, OMe), 3.06 (dd, J = 16.7, 12.3 Hz, 1H), 2.89 (dd, J = 16.7, 3.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.7 (C=O), 152.0 (C-9a), 151.3 (C-4), 150.0 (C-8a), 148.0 (C-3′), 147.8 (C-4′), 144.4 (OCH=CH), 132.6 (C-9), 129.9 (C-1′), 119.9 (C-6′), 115.5 (C-4a), 111.2 (C-3a), 108.3 (C-5′), 106 (C-2′), 105.6 (OCH=CH), 101.3 (CH2), 79.2 (C-7), 61.5 (OMe), 61.4 (OMe), 46.1 (C-6); HR-ESIMS m/z: 369.0968 [M + H]+ calcd for C20H16O7 + H+ (369.0968).

7-(5-Bromo-2-methoxyphenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4d)

Yellow powder; HPLC: tR = 4.4 min (100% purity); yield: 66%; mp 178–180 °C; IR (CHCl3) νmax: 3743, 3584, 3415, 2922, 2850, 1681, 1614, 1542, 1504, 1491, 1477, 1445, 1390, 1349, 1300, 1277 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.79 (d, J = 2.0 Hz, 1H, CH), 7.53 (d, J = 2.3 Hz, 1H, OCH=CH), 7.43 (dd, J = 8.7, 2.5 Hz, 1H, CH), 6.94 (d, J = 2.3 Hz, 1H, OCH=CH), 6.79 (d, J = 8.8 Hz, 1H, CH), 5.73 (dd, J = 12.9, 2.7 Hz, 1H, CH), 4.09 (s, 3H, OMe), 4.07 (s, 3H, OMe), 3.82 (OMe), 2.99 (dd, J = 16.8, 2.9 Hz, 1H), 2.83 (dd, J = 16.8, 12.9 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.8 (C=O), 154.8 (C-2′), 151.9 (C-9a), 151.6 (C-4), 150.1 (C-8a), 144.4 (OCH=CH), 131.8 (C-4′), 130.1 (C-9), 129.8 (C-1′), 129.6 (C-6′), 115.8 (C-4a), 113.3 (C-3a), 112.2 (C-3′), 111.4 (C-5′), 105.6 (OCH=CH), 74.2 (C-7), 61.5 (OMe), 61.4 (OMe), 45.1 (C-6); HR-ESIMS m/z: 433.0274 [M + H]+ calcd for C20H17O7 + H+ (433.0281).

4,9-Dimethoxy-7-(pyridin-3-yl)-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4e)

Yellow powder; HPLC: tR = 3.3 min (100% purity); yield: 65%; mp 149–151 °C; IR (CHCl3) νmax: 3584, 3380, 3124, 3074, 2937, 2850, 1668, 1599, 1479, 1547, 1437, 1344, 1392, 1285, 1078 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.78 (s, 1H, CH), 8.65 (d, J = 4.6 Hz, 1H, CH), 7.86 (d, J = 7.9 Hz, 1H, CH), 7.54 (d, J = 2.1 Hz, 1H, OCH=CH), 7.38 (dd, J = 7.8, 4.8 Hz, 1H, CH), 6.95 (d, J = 2.1 Hz, 1H, OCH=CH), 5.54 (dd, J = 12.3, 3.1 Hz, 1H, CH), 4.09 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.10 (dd, J = 16.6, 12.4 Hz, 1H), 2.97 (dd, J = 16.7, 3.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.8 (C=O), 152.0 (C-9a), 150.9 (C-4), 150.1 (C-8a), 150.0 (C-6′), 147.8 (C-4′), 144.5 (OCH=CH), 134.3 (C-9), 133.6 (C-2′), 130.0 (C-1′), 123.6 (C-3′), 115.8 (C-4a), 111.2 (C-3a), 106.6 (OCH=CH), 77.3 (C-7), 61.4 (OMe), 61.3 (OMe), 45.6 (C-6); HR-ESIMS m/z: 326.1021 [M + H]+ calcd for C18H16NO5 + H+ (325.1023).

4,9-Dimethoxy-7-(thiophen-3-yl)-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4f)

Yellow powder; HPLC: tR = 5.3 min (89% purity); yield: 66%; mp 159–161 °C; IR (CHCl3) νmax: 3428, 1927, 2851, 2079, 1673, 1651, 1620, 1477, 1542, 1435, 1348, 1300, 1216 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.52 (d, J = 2.2 Hz, 1H, OCH=CH), 7.37 (m, 2H, CH), 7.21 (dd, J = 4.9, 1.3 Hz, 1H, CH), 6.92 (d, J = 2.3 Hz, 1H, OCH=CH), 5.60 (dd, J = 10.3, 4.1 Hz, 1H, CH), 4.07 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.12 (dd, J = 16.7, 10.3 Hz, 1H), 3.04 (dd, J = 16.7, 4.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.5 (C=O), 152.0 (C-9a), 151.0 (C-4), 150.0 (C-8a), 144.4 (OCH=CH), 139.8 (C-1′), 128.8 (C-9), 126.7 (C-3′), 125.8 (C-2′), 122.6 (C-5′), 115.5 (C-4a), 111.4 (C-3a), 106.7 (OCH=CH), 75.4 (C-7), 61.4 (OMe), 61.4 (OMe), 45.1 (C-6); HR-ESIMS m/z: 331.0643 [M + H]+ calcd for C17H14 O5S+ H+ (331.0634).

4,9-Dimethoxy-7-(4-nitrophenyl)-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4g)

Yellow powder; HPLC: tR = 3.6 min (100% purity); yield: 61%; mp 180–181 °C; IR (CHCl3) νmax: 3745, 3383, 2850, 2918, 1734, 1702, 1604, 1519, 1503, 1467, 1345, 1273, 1213, 1149, 1066 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.10 (d, J = 8.7 Hz, 2H, CH), 7.56–7.49 (dd, J = 7.6, 5.5 Hz, 2H, CH), 6.91 (d, J = 2.3 Hz, 1H, OCH=CH), 4.88 (dd, J = 6.5, 4.4 Hz, 1H, CH), 4.12 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.47 (dd, J = 16.2, 4.0 Hz, 1H), 3.04 (dd, J = 14.6, 6.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 196.5 (C=O), 159.6 (C-9a), 154.7 (C-4), 147.1 (C-1′), 146.9 (C-4′), 144.5 (OCH=CH), 143.1 (C-8a), 130.6 (C-2′, C-6′), 127.7 (C-9), 123.5 (C-3′, C-5′), 113.8 (C-4a), 108.3 (C-3a), 106.1 (OCH=CH), 85.4 (C-7), 61.2 (OMe), 60.8 (OMe), 37.1 (C-6); HR-ESIMS m/z: 370.0933 [M + H]+ calcd for C19H15NO7+ H+ (370.0931).

7-(2-Chlorophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4h)

Yellow powder; HPLC: tR = 4.4 min (100% purity); yield: 65%; mp 142–143 °C; IR (CHCl3) νmax: 3625, 3584, 3441, 2934, 2849, 2079, 1682, 1651, 1542, 1435, 1391, 1303, 1278 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.67 (t, J = 7.3 Hz, 1H, CH), 7.54 (d, J = 2.1 Hz, 1H, OCH=CH), 7.40–7.34 (m, 1H, CH), 7.25 (dd, J = 12.2, 4.4 Hz, 1H, CH), 7.15–7.08 (m, 1H, CH), 6.95 (d, J = 2.1 Hz, 1H, OCH=CH), 5.77 (dd, J = 12.5, 3.0 Hz, 1H), 4.12 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.07 (dd, J = 16.7, 12.6 Hz, 1H), 2.96 (dd, J = 16.8, 3.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.3 (C=O), 152.0 (C-9a), 151.4 (C-4), 150.2 (C-8a), 144.5 (OCH=CH), 136.7 (C-1′), 131.7 (C-2′), 130.0 (C-9), 129.7 (C-3′), 129.5 (C-4′), 127.4 (C-6′), 127.1 (C-5′), 115.7 (C-4a), 111.3 (C-3a), 105.7 (OCH=CH), 77.5 (C-7), 61.5 (OMe), 61.5 (OMe), 44.9 (C-6); HR-ESIMS m/z: 359.0670 [M + H]+ calcd for C19H15ClO5 + H+ (359.0675).

(2,4-Dichlorophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4i)

Yellow powder; HPLC: tR = 4.5 min (100% purity); yield: 64%; mp 123–125 °C; IR (CHCl3) νmax: 3584, 3354, 3019, 2930, 2850, 1683, 1614, 1543, 1479, 1485, 1348, 1278, 1165, 1130, 1102, 1018 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.79 (d, J = 8.4 Hz, 1H, CH), 7.55 (d, J = 2.1 Hz, 1H, OCH=CH), 7.44 (m, 1H, CH), 7.38 (dd, J = 8.4, 2.1 Hz, 1H, CH), 6.96 (d, J = 2.3 Hz, 1H, OCH=CH), 5.78 (dd, J = 13.0, 2.8 Hz, 1H, CH), 4.11 (s, 3H, OMe), 4.05 (s, 3H, OMe), 3.03 (dd, J = 16.8, 2.9 Hz, 1H), 2.86 (dd, J = 16.8, 13.0 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 189.9 (C=O), 152.0 (C-9a), 151.1 (C-4), 150.2 (C-8a), 144.6 (OCH=CH), 135.3 (C-9), 134.8 (C-1′), 132.3 (C-4′), 130.1 (C-2′), 129.5 (C-3′), 128.1 (C-6′), 127.8 (C-5′), 115.8 (C-4a), 111.1 (C-3a), 105.7 (OCH=CH), 76.0 (C-7), 61.5 (OMe), 61.5 (OMe), 44.8 (C-6); HR-ESIMS m/z: 393.0282 [M + H]+ calcd for C19H15 Cl2O5+ H+ (393.0291).

4,9-Dimethoxy-7-(naphthalen-2-yl)-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4j)

Yellow powder; HPLC: tR = 5.2 min (100% purity); yield: 60%; mp 143–145 °C; IR (CHCl3) νmax: 3418, 2925, 2852, 2085, 1680, 1616, 1545, 1497, 1436, 1390, 1302, 1249, 1126, 1018 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.79 (m, 4H, CH), 7.63 (d, J = 8.5 Hz. 1H, OCH=CH), 7.52 (m, 3H, CH), 6.94 (d, J = 2.2 Hz, 1H, OCH=CH), 5.66 (dd, J = 16.7, 12.2 Hz, 1H, CH), 4.09 (s, 3H, OMe), 4.07 (s, 3H, OMe), 3.20 (dd, J = 16.7, 12.2 Hz, 1H), 3.04 (dd, J = 16.7, 3.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.7 (C=O), 152.1 (C-9a), 151.4 (C-4), 150.1 (C-8a), 144.4 (OCH=CH), 136.1 (C-9), 133.8 (C-8′), 133.1 (C-3′), 130.0 (C-1′), 128.7 (C-9′), 128.2 (C-10′), 127.7 (C-4′, C-7′), 115.6 (C-3a), 111.4 (C-4a), 106.7 (OCH=CH), 79.4 (C-7), 61.5 (OMe), 61.4 (OMe), 46.0 (C-6), HR-ESIMS m/z: 375.1230 [M + H]+ calcd for C23H18O5 + H+ (375.1227).

7-(2,6-Dichlorophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4k)

Yellow powder; HPLC: tR = 4.4 min (100% purity); yield: 65%; mp 143–145 °C; IR (CHCl3) νmax: 3444, 3011, 2936, 2848, 2111, 1682, 1614, 1581, 1564, 1477, 1434, 1376, 1348, 1277, 1247, 1211 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.53 (d, J = 2.2 Hz, 1H, OCH=CH), 7.39 (d, J = 8.0 Hz, 2H, CH), 7.28 (m, 1H, CH), 6.95 (d, J = 2.2 Hz, 1H, OCH=CH), 6.25 (dd, J = 14.4, 3.1 Hz, 1H, CH), 4.11 (s, 3H, OMe), 4.03 (s, 3H, OMe), 3.77 (dd, J = 16.8, 14.5 Hz, 1H), 2.69 (dd, J = 16.8, 3.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ (ppm) 190.1 (C=O), 152.3 (C-9a), 151.5 (C-4), 150.3 (C-8a), 144.5 (OCH=CH), 135.1 (C-9), 134.8 (C-1′), 132.5 (C-2′, C-6′), 130.2 (C-3′), 129.7 (C-4′), 128.9 (C-5′), 115.4 (C-3a), 111.0 (C-4a), 105.7 (OCH=CH), 75.9 (C-7), 61.9 (OMe), 61.5 (OMe), 41.0 (C-6); HR-ESIMS m/z: 393.0331 [M + H]+ calcd for C19H14Cl2O5 + H+ (393.0291).

7-(3-Bromo-4-fluorophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4l)

Yellow powder; HPLC: tR = 4.1 min (95% purity); yield: 68%; mp 140–143 °C; IR (CHCl3) νmax: 3429, 2933, 2850, 2091, 1681, 1614, 1544, 1498, 1478, 1435, 1417, 1391, 1349, 1278, 1249, 1067 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.73 (dd, J = 6.4, 2.0 Hz, 1H, CH), 7.54 (d, J = 2.2 Hz, 1H, OCH=CH), 7.44–7.39 (m, 1H, CH), 7.18 (t, J = 8.4 Hz, 1H, CH), 6.95 (d, J = 2.2 Hz, 1H, OCH=CH), 5.43 (dd, J = 12.1, 3.1 Hz, 1H, CH), 4.08 (s, 3H, OMe), 4.06 (s, 3H, OMe), 3.03 (dd, J = 16.7, 12.1 Hz, 1H), 2.92 (dd, J = 16.7, 3.3 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ (ppm) 189.9 (C=O), 160.0 (d, 1JCF = 246 Hz, C-4′), 158.0 (C-9a), 152.4 (C-4), 150.8 (C-8a), 150.1 (C-9) 144.6 (OCH=CH), 136.2 (d, 4JCF = 3.7 Hz, CH-1′), 131.4 (C-2′), 129.9 (C-3a), 126.8 (d, 3JCF = 7.2 Hz, CH-6′), 116.9 (CH-5′), 111.1 (C-4a), 109.6 (d, 2JCF = 21.2 Hz, C-3′), 106.7 (OCH=CH), 77.9 (C-7), 61.4 (OMe), 61.4 (OMe), 45.9 (C-6); HR-ESIMS m/z: 423.0060 [M + H]+ calcd for C19H14BrFO5 + H+ (423.0060).

7-(2-Fluorophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4m)

Yellow powder; HPLC: tR = 3.7 min (93% purity); yield: 64%; mp 142–145 °C; IR (CHCl3) νmax: 3584, 3360, 3126, 3070, 2938, 2843, 1687, 1682, 1615, 1543, 1505, 1478, 1459, 1436, 1349, 1302, 1278, 1252, 1233, 1165, 1130, 1067 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.67 (t, J = 7.3 Hz, 1H, CH), 7.54 (d, J = 2.1 Hz, 1H, OCH=CH), 7.37 (dd, J = 13.3, 6.0 Hz, 1H, CH), 7.25 (dd, 1H, J = 12.2, 4.4 Hz, CH), 7.15–7.08 (m, 1H, CH), 6.95 (d, J = 2.1 Hz, 1H, OCH=CH), 5.77 (dd, J = 12.5, 3.0 Hz, 1H, CH), 4.10 (s, 3H, OMe), 4.05 (s, 3H, OMe), 3.07 (dd, J = 16.7, 12.6 Hz, 1H), 2.96 (dd, J = 16.8, 3.2 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ (ppm) 190.4 (C=O), 160.7 (d, 1JCF = 246 Hz, C-2′), 158.7 (C-9a), 152.1 (C-4), 151.4 (C-8a), 150.2 (C-9), 144.5 (OCH=CH), 130.2 (d, 3JCF = 8.7 Hz, CH-4′), 130.0 (C-3a), 127.4 (CH-6′), 126.2 (CH-1′), 124.6 (d, 4JCF = 3.7 Hz CH-5′), 115.8 (d, 2JCF = 21.2 Hz, CH-3′), 111.1 (C-4a), 105.7 (OCH=CH), 73.8 (CH-7), 61.5 (OMe), 61.5 (OMe), 45.1 (C-6); HR-ESIMS m/z: 343.0990 [M + H]+ calcd for C19H15FO5+ H+ (343.0976).

7-(4-Fluorophenyl)-4,9-dimethoxy-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4n)

Yellow powder; HPLC: tR = 4.4 min (100% purity); yield: 61%; mp 142–145 °C; IR (CHCl3) νmax: 3744, 3397, 2920, 2351, 1683, 1575, 1512, 1477, 1414, 1348, 1301, 1226, 1126, 1065 cm–1; 1HNMR (400 MHz, CDCl3): δ (ppm) 7.53 (d, J = 2.2 Hz, 1H, OCH=CH), 7.49 (dd, J = 8.5, 5.4 Hz, 2H, CH), 7.12 (t, J = 8.6 Hz, 1H, CH), 6.94 (d, J = 2.2 Hz, 1H, OCH=CH), 5.47 (dd, J = 12.3, 2.9 Hz, 1H, CH), 4.09 (s, 3H, OMe), 4.05 (s, 3H, OMe), 3.07 (dd, J = 16.7, 12.3 Hz, 1H), 2.93 (dd, J = 16.7, 3.2 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ (ppm) 190.5 (C=O), 163.7 (d, 1JCF = 246 Hz, C-4′), 161.7 (C-9a), 152.0 (C-4), 151.2 (C-8a), 150.0 (C-9), 144.5 (OCH=CH), 134.6 (CH-1′), 127.9 (d, 3JCF = 8.7 Hz, C-2′, C-6′), 115.8 (d, 2JCF = 20.0 Hz, C-3′, C-5′), 115.6 (C-3a), 111.2 (C-4a), 105.6 (OCH=CH), 78.6 (C-7), 61.4 (OMe), 61.4 (OMe), 45.9 (C-6); HR-ESIMS m/z: 343.0990 [M + H]+ calcd for C19H15FO5 + H+ (343.0976).

(E)-6-Benzylidene-4,9-dimethoxy-7-phenyl-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4o)

Yellow powder; HPLC: tR = 3.7 min (87% purity); yield: 51%; mp 159–161 °C; IR (CHCl3) νmax: 3585, 3415, 2922, 2850, 2094, 1641, 1619, 1475, 1437, 1348, 1277, 1126, cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 8.09 (s, 1H, CH), 7.98 (d, J = 2.1 Hz, 1H, CH), 7.50 (m, 5H, CH), 7.44 (t, J = 6.9 Hz, 4H), 7.38 (d, J = 7.1 Hz, 1H, CH), 7.18 (d, J = 2.1 Hz, 1H, CH), 6.76 (s, 1H, CH), 4.00 (s, 3H, OMe), 3.94 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 180.0, 151.2, 149.8, 147.6, 145.4, 138.0, 137.1, 133.3, 132.5, 130.1, 129.9, 129.8, 129.0, 128.9, 127.2, 115.3, 111.7, 105.7, 77.2, 61.1, 60.8; HR-ESIMS m/z: 413.1383 [M + H]+ calcd for C26H20O5 + H+ (413.1383).

(Z)-4,9-Dimethoxy-6-(thiophen-2-ylmethylene)-7-(thiophen-3-yl)-6,7-dihydro-5H-furo[3,2-g]chromen-5-one (4p)

Yellow powder; HPLC: tR = 4.6 min (95% purity); yield: 55%; mp 148–150 °C; IR (CHCl3) νmax: 3429, 2933, 2850, 2091, 1681, 1614, 1544, 1498, 1478, 1435, 1417, 1391, 1349, 1278, 1249, 1067 cm–1; 1H NMR (400 MHz, CDCl3): δ (ppm) 7.94–7.91 (m, 2H, CH), 7.86 (d, J = 1.9 Hz, 1H, CH), 7.67 (dd, J = 4.9, 2.9 Hz, 1H, CH), 7.53 (d, J = 2.9 Hz, 1H, OCH=CH), 7.38–7.34 (m, 1H, CH), 7.24 (d, J = 5.0 Hz, 1H, CH), 7.18 (d, J = 5.0 Hz, 1H, CH), 7.13 (d, J = 2.2 Hz, 1H, 1H), 6.83 (s, 1H, CH), 3.96 (s, 3H, OMe), 3.86 (s, 3H, OMe); 13C NMR (100 MHz, CDCl3): δ (ppm) 180.8 (C=O), 151.6, 150.2, 148.4, 144.1, 139.1, 135.8, 132.1, 131.0, 130.8, 129.1, 128.6, 127.0, 126.8, 126.7, 124.7, 116.5, 112.7, 105.5, 75.1, 61.5, 61.4; HR-ESIMS m/z: 425.0495 [M + H]+ calcd for C22H16O5S2 + H+ (425.0511).

CYP Screening and Protection from B[a]P Toxicity

The activity of test compounds for inhibition of CYP enzymes was investigated using Sacchrosomes and recombinant human HEK293 cells using protocols as described in our earlier publications.[10,12] The protection from B[a]P toxicity in CYP1A1-expressing normal adherent HEK293 human cells was studied as per protocols described in our earlier publications.[10,12]

For IC50 determination for CYP inhibition, the enzyme inhibition was plotted using a sigmoidal curve (four-parameter variable slope equation) and half-inhibitory concentration (IC50) values were analyzed statistically using GraphPad Prism Software (Version 6.0). The IC50 values of CYP inhibitors were determined by fitting a nonlinear curve of uninhibited fraction with incubation concentration of the inhibitors, using the Hill equation. The EC50 values for B[a]P toxicity were determined by plotting the dose–response curve of cell viability versus log concentration of B[a]P or B[a]P + test compound. EC50 values were obtained using GraphPad Prism Software (Version 6.0), indicating the concentration of B[a]P required for 50% reduction in cell viability.

Profiling of Khellinochalcones on a Panel of 12 Humanized CYP450 Enzymes

IC50 values of furanochalcone 3g were obtained by kinetic microplate assay using 15 min reaction time. Recombinant human P450s from a Saccharomyces cerevisiae microsome expression system supplied from CYP Design were used. Each 100 μL reaction was performed in 100 mM potassium phosphate buffer, pH 7.4, and with a reduced nicotinamide adenine dinucleotide phosphate (NADPH) regenerating system (final well concentrations: 0.04 units of glucose-6-phosphate dehydrogenase, 1.3 mM NADP+, 3.3 mM glucose-6-phosphate, 3.3 mM MgCl2, and 50 μM tribasic sodium citrate). The test samples were serially diluted in 10% dimethyl sulfoxide (DMSO) in decreasing final well concentrations from 20 μM downward/0.5% DMSO. CYP2E1 is sensitive to solvents, and so test samples were diluted in 1% DMSO in decreasing final well concentrations from 10 μM downward/0.5% DMSO. The assay was initiated by adding the substrate and the NADPH regenerating system. P450 content and substrate concentrations were preoptimized.[12]

In Vitro Cell Proliferation Assays and Flow Cytometric Analyses

In vitro cell proliferation assays and flow cytometric analyses were performed as described earlier.[28] Control cells (i.e., in the absence of any compound) and cells (i.e., treated with compound) were harvested after treatment with trypsin. The cells were fixed in chilled (−20 °C) 70% ethanol for an hour, after washing once with phosphate-buffered saline (PBS). The fixed cells were centrifuged at room temperature. The pellets were resuspended in PBS, in the presence of DNase-free ribonuclease (0.5 mg/mL; Sigma-Aldrich # R-5503) before staining with a propidium iodide solution (50 μg/mL; Sigma-Aldrich, # P-4170) for an hour (in the dark) at 4 °C. Cell cycle analysis was performed on the Beckman Coulter (Epics Altra) fluorescence-activated cell sorter (Beckman Coulter UK Ltd). To exclude cell doublets or cell clumps, all events that represent single cells were gated. Cytograms of the fluorescence peak of propidium iodide were plotted against the integrated fluorescence/linear signal. Data points on a straight line, within a single gate, were isolated and the gated data were employed for plotting a histogram that represents a complete cell cycle. The total number of events did not exceed 200 events per second. Data acquisition was stopped after collection of around 10 000 events.

In Silico Docking with CYP1A1

The docking of identified CYP1A1 inhibitors with the crystal structure of CYP1A1 (PDB ID: 4I8V)[18] was performed using the protocol as described earlier.[10,12]