Synthesis of benzofurans from the cyclodehydration of α-phenoxy ketones mediated by Eaton’s reagent

Abstract

Cyclodehydration of α-phenoxy ketones promoted by Eaton’s reagent (phosphorus pentoxide–methanesulfonic acid) is used to prepare 3-substituted or 2,3-disubstituted benzofurans with moderate to excellent yields under mild conditions. The method provides a facile access to benzofurans from readily available starting materials such as phenols and α-bromo ketones. The reaction is highly efficient, which is attributed to the good reactivity and fluidity of Eaton’s reagent. The reaction can be applied to prepare naphthofurans, furanocoumarins, benzothiophenes, and benzopyrans.

Keywords

benzofurans cyclodehydration Eaton’s reagent electrophilic aromatic substitution naphthofurans

Introduction

The benzofuran moiety is found in many natural products with medically useful bioactivities such as anti-inflammatory, anti-oxidant, antitumor, anti-HIV, antiplatelet, analgesic, and so on (Figure 1).^1,2 Besides pharmaceutical applications, they also serve as excellent organic electronic materials due to their prominent electrochemical and photochemical properties.^3–6

Figure 1.

Selected benzofuran compounds with bioactivities.

Due to their remarkable properties, benzofurans have attracted much interest from synthetic chemists in order to develop efficient synthetic methods to access them. Synthetic approaches toward this advantageous scaffold have been developed from various starting materials.^7–9 Among them, transition-metal-catalyzed cyclization is most popular (Scheme 1, Path 1).^10–20 Many of these strategies rely on the cyclo-aromatization of alkynes or alkenes with phenol derivatives via inter- or intramolecular furan annulation catalyzed by Pd, Rh, Ag, Au, Co, Cu, and so on. In these pathways, expensive transition-metal catalysts are a prerequisite and the substrates are often synthesized via multi-step sequences. Recently, a few methods based on “green” chemistry starting from easily accessible materials have been developed (Scheme 1, Path 2), which involve microwave-aided synthesis,^21–24 electrochemical catalysis,^25,26 and nanoparticle catalysis.^27,28

Scheme 1.

Methods for the synthesis of benzofurans and the procedure described in this work.

As easily accessible starting materials, α-phenoxy ketones 1 have often been used to undertake cyclodehydration reactions to give benzofurans 2. α-Phenoxy ketones 1 can be conveniently synthesized from α-bromo ketones 3 and phenols 4 in the presence of K₂CO₃ (Scheme 2).

Scheme 2.

Synthesis of α-phenoxy ketones 1 from α-bromo ketones 3 and phenols 4.

As a typical example, the synthesis of 3-phenylbenzofuran 2a from α-phenoxy acetophenone 1a is illustrated in Scheme 3. It can be accessed by refluxing 1a in ethanol in the presence of a base (Path 1) such as KOH with subsequent acidification^29,30 or in the presence of acid (Path 2), for example, Lewis acids such as TiCl₄,³¹ Bi(OTf)₃,³² BCl₃,^33,34 and Al₂O₃³⁵ or Brønsted acids such as triflic acid,³⁶ methanesulfonic acid (MeSO₃H),^37–40 and p-toluenesulfonic acid.^41,42 In addition, a few transition-metal catalysts (path 3) such as Ir or Pd have been used to facilitate cyclodehydration reaction.^43–45 These protocols usually require long reaction times even in refluxing solvents, which encouraged us to find an alternative method to prepare benzofurans under moderate conditions. According to the literature, MeSO₃H and P₂O₅⁴⁶ are both reported as reagents for cyclodehydration reactions; however, their combined use to synthesize benzofurans is seldom reported. To our knowledge, there is only one example of this synthesis reported by Vedejs.⁴⁷ The P₂O₅–MeSO₃H combination is known as Eaton’s reagent,⁴⁸ which is an alternative reagent to polyphosphoric acid (PPA) for the promotion of (cyclo)dehydration reactions.^49–54 Herein, we have applied Eaton’s reagent to mediate the synthesis of 3-substituted or 2,3-disubstituted benzofurans efficiently under mild conditions.

Scheme 3.

Synthesis of 3-phenylbenzofuran 2a from 2-phenoxy acetophenone 1a.

Results and discussion

Initially, the cyclodehydration of 2-phenoxy acetophenone 1a to benzofurans 2a/2a’ was promoted by PPA, MeSO₃H or Eaton’s reagent and the results are shown in Table 1. Both PPA and Eaton’s reagent are supposed to be used in large excess to remove water generated during the dehydration procedure. When 1a was reacted in PPA, the reaction temperature must be maintained below 55°C to yield 2a as the only product (Table 1, Entry 1). Otherwise, both 2a and 2-phenylbenzofuran 2a’ are obtained when reacted between 55°C and 132°C, and the ratio of 2a’ in the mixed products increased when the temperature was increased until 2a’ was produced as the only isomer at 132°C (Table 1, Entries 2 and 3). Therefore, very careful control of the temperature is required to obtain 2a as the only isomer when using PPA, which is quite difficult because of the high viscosity. On the contrary, when the reaction is conducted in Eaton’s reagent, benzofuran 2a was obtained as the only product, and the reaction can be easily manipulated due to its good fluidity even at lower temperatures. When the reaction temperature was increased from 30°C to 45°C (Table 1, Entries 4–6), the reaction time was dramatically shortened from 1.5 h to 10 min and the yield increased from 71% to 89%. The yield decreased due to decomposition at higher reaction temperatures, but none of the product 2a’ was formed. When 1a was reacted in MeSO₃H at 45°C, it took 31 h to complete the transformation into 2a with a 56% yield (Table 1, Entry 7). Obviously, Eaton’s reagent exhibits the advantage of much higher reactivity than either PPA or MeSO₃H without any of the isomerization product being formed.

Table 1.

Cyclodehydration of 1a to phenylbenzofurans 2a/2a’.^a,b

Entry	Reagent	Temp (°C)	Time	Products and yield (%)^b	Ratio of 2a:2a’
1	PPA	55	2 h	2a, 78	100:0
2	PPA	55–132	2 h	mixture of 2a and 2a’^c	–^d
3	PPA	132	2 h	2a’, 73	0:100
4	Eaton’s	30	1.5 h	2a, 71	100:0
5	Eaton’s	40	0.5 h	2a, 81	100:0
6	Eaton’s	45	10 min	2a, 89	100:0
7	MeSO₃H	45	31 h	2a, 56	100:0

PPA: polyphosphoric acid.

Isolated yield.

Reaction conditions: 1a (1 mmol) in 3.0 mL of PPA or Eaton’s reagent was reacted at the indicated temperature until thin-layer chromatography monitoring showed the reaction was completed.

HPLC analysis.

The ratio of 2a:2a’ depends on the reaction temperature.

To investigate the correlation between 2a and 2a’, pure 2a was heated in PPA at 132°C and was completely transformed into 2a’ in 80% yield after 2 h, which is the same reaction time needed for 1a to be transformed into 2a’ under these conditions. This indicated the 2a’ is presumably formed by isomerization from 2a. This transformation may proceed by a mechanism as shown in Scheme 4. Protonation of 2a under the strongly acidic conditions results a stable α-oxy carbocation A.^55–58 A 1,2-phenyl shift in the oxonium cation A leads to the benzylic cation B, which on deprotonation affords 2-phenylbenzofuran 2a’.

Scheme 4.

The plausible mechanism for the isomerization of 2a to 2a’.

Next, we investigated the reaction scope of α-phenoxy-acetophenones 1, which were prepared from α-bromo acetophenone 3a with various substituted phenols 4a–4l (Table 2). In addition, α-naphthol 4m, β-naphthol 4n, 4-hydroxycoumarin 4o, 7-hydroxycoumarin 4p, and thiophenol 4q were also used as phenols. Generally, the cyclodehydration gave the expected benzofurans 2a–2q in satisfactory yields. However, the yields depend greatly on the electronic properties of the substituents. Electron-donating substituents favor the cyclo-aromatization and the corresponding phenoxy-acetophenones 1a–1h afforded benzofurans 2a–2h in goods yields (59%–97%). Phenoxy-acetophenones 1i–1k, prepared from phenols with electron-withdrawing substituents, gave benzofurans 2i–2k in lower yields (50%–56%). For phenoxy-acetophenones with strong electron-withdrawing substituents such as acetyl, carboxylic acid, or nitro groups, none of the expected products 2r–2v were obtained. The cyclodehydration reaction had to be carried out at a higher temperature (75°C) and proceeded with a very slow reaction rate, and only traces of products were evident due to decomposition. Benzodifuran 2l, naphthofurans (2m and 2n) and furanocoumarins (2o and 2p) were also prepared from the corresponding phenoxy-acetophenones 1l–1p in 50%–98% yield. Furanocoumarins 2o and 2p were obtained at 90°C because of the electron-deficient character of coumarin. Benzothiophene 2q was prepared from α-phenylthio acetophenone 1q in 59% yield at −10°C to avoid C–S bond cleavage leading to diphenyl disulfide (PhS–SPh).^59,60 Notably, the cyclodehydration of 1d from 3-methylphenol 4d and α-bromo acetophenone 3a afforded the regio-isomers of the benzofurans of 2d:2d” = 51:49 when reacted at 45°C. However, 2d was obtained as the predominant product (2d:2d” = 97:3) when the reaction temperature was increased to 150°C because 2d is more thermodynamically stable than 2d’’. (For details, please see Figure S1 in the Supporting Information).

Table 2.

Scope of the phenol precursors 4.^a,b

Isolated yield.

Reagents and conditions, unless otherwise specified: 1 mmol of 1 in 3.0 mL of freshly prepared Eaton’s reagent, reacted at 45 °C with thin-layer chromatography monitoring.

Reacted at 90°C.

Reacted at −10°C.

We then investigated the reaction scope of phenoxy ketones 1, which were prepared from phenol 4a or naphthols (4m, 4n) with a series of α-bromo ketones 3a–3r. The results for cyclodehydration reactions are listed in Table 3. This reaction exhibited good functional group tolerance for α-bromo ketones. The method was applicable for the preparation of 3-aryl (2a, 2ab–2ai, 2mf, and 2nf), 3-alkyl (2am and 2an) benzofurans and 2,3-disubstituted benzofurans (2aj–2al and 2ao–2ar) from α-substituted phenoxy ketones in good yields (53%–94%). For α-phenoxy aliphatic ketones carrying no aryl groups (1am–1ap), the reaction temperature should be kept at 10°C to obtain good yields (53%–89%). Notably, benzopyran (5) can also be synthesized from β-phenoxy propiophenone in 39% yield at 0°C to avoid decomposition.

Table 3.

Scope of the α-bromo ketone precursors 3.^a,b

Isolated yield.

Reagents and conditions: 1 mmol of phenoxy ketones 1 in 3.0 mL of Eaton’s reagent, reaction temperature is 45 °C unless otherwise specified.

Reacted at 10°C.

Reacted at 0°C.

This cyclodehydration reaction presumably proceeds through a mechanism involving intramolecular electrophilic aromatic substitution (EAS) followed by dehydration to afford the benzofurans. Eaton’s reagent is quite effective because its strong Brønsted acidity can promote the cyclization, and the powerful phosphoric carboxylic mixed anhydride⁶¹ can drive the dehydration. In the EAS step, electron-donating substituents on the phenoxy ring can increase electron density, which leads to a much more effective electrophilic attack by the protonated carbonyl. Therefore, a significant substituent effect has been shown on the phenoxy ring. On the contrary, protonation of the carbonyl group by MeSO₃H can greatly enhance its electrophilicity and decreases the electron demand, leading to good functional group tolerance of the ketone precursors.

Conclusion

In conclusion, we have developed a facile synthesis of 3-substituted or 2,3-disubstituted benzofurans from α-phenoxy ketones with moderate to excellent yields (50%–98%) promoted by Eaton’s reagent under mild conditions. This reaction can also be applied to the syntheses of naphthofurans, furanocoumarins, benzothiophene, and a benzopyran. Due to the use of readily available starting materials and the high efficiency of this approach, we believe it will be applied to other cyclodehydration reactions.

Experimental

General methods

All commercially available chemicals are used without purification. Thin-layer chromatography (TLC) was performed on Liangchen Guiyuan silica gel GF254 plates. Column chromatography was performed on Liangchen Guiyuan silica gel (300–400 mesh) with petroleum ether as the eluent. Melting points were measured on a RY-1G instrument without correction. ¹H Nuclear magnetic resonance (NMR) and ¹³C NMR spectra were recorded on a Bruker Avance III HD 600 spectrometer (Bruker, Switzerland) at 600 and 151 MHz, respectively. The chemical shifts are reported relative to CHCl₃ (δ = 7.26 for ¹H NMR and δ = 77.16 for ¹³C NMR). The infrared spectra were recorded on a Nicolet iS50 FTIR spectrometer (ThermoFisher Scientific, USA). Mass spectra were obtained on an Agilent Ultimate 3000/Thermo LCQ Fleet Trap LC-MS system for electrospray ionization mass spectrometry (ESI-MS) (ThermoFisher Scientific, USA). High-resolution mass spectrometry (HRMS) spectra (ESI) were acquired using an Agilent 6210 TOF LC/MS System (Agilent, USA) or on a Thermo Q Exactive Plus instrument (ThermoFisher Scientific, USA). The α-phenoxy ketones were prepared by heating phenols with α-bromoketones in K₂CO₃/acetone. Eaton’s reagent was prepared by stirring 8 g of P₂O₅ with 80 g of MeSO₃H. The reaction details are given in the Supporting Information.

Typical procedure for the synthesis of benzofurans 2a–2q, 2ab–2ar, 2mf, 2nf, and benzopyran 5

To a 10-mL round-bottom flask containing 3 mL Eaton’s reagent in an oil or ethanol bath which was kept in the indicated reaction temperature, the α-phenoxy ketone (1 mmol) was added in one portion after being pretreated to the same temperature. The mixture was stirred at this temperature with TLC monitoring. After the starting material had been consumed, the resulting dark green mixture was poured into 100 mL of ice water and extracted with diethyl ether (3 × 15 mL). The combined organic layer was washed with a saturated solution of NaHCO₃ (10 mL) and water (10 mL), then dried over anhydrous Na₂SO₄ and concentrated. The crude product was purified by flash chromatography to give benzofurans 2 or benzopyran 5. The reaction details are summarized in Table S1 of Supplemental Material.

3-Phenylbenzofuran (2a): Colorless oil. The NMR spectra are in good agreement with the literature.^{62
¹}H NMR (600 MHz, CDCl₃): δ = 7.85 (dd, J = 7.7, 0.5 Hz, 1H), 7.80 (s, 1H), 7.68 − 7.63 (m, 2H), 7.56 (d, J = 8.1 Hz, 1H), 7.50 − 7.46 (m, 2H), 7.41 − 7.37 (m, 1H), 7.37 − 7.34 (m, 1H), 7.31 (td, J = 7.7, 1.0 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 156.0, 141.5, 132.2, 129.1, 127.7, 127.6, 126.6, 124.7, 123.2, 122.4, 120.5, 111.9. IR(KBr): υ_max = 3033, 1605, 1574, 1489, 1451, 1217, 1107, 1092, 1012, 962, 856, 771, 763, 744, and 694 cm⁻¹. MS (ESI): m/z = 195.07, (100) (M + H)⁺.

5-Methyl-3-phenylbenzofuran (2b): Pale yellow oil. The NMR spectra are in good agreement with the literature.^{63
¹}H NMR (600 MHz, CDCl₃): δ = 7.73 (s, 1H), 7.64 − 7.60 (m, 3H), 7.48 − 7.43 (m, 2H), 7.41 (d, J = 8.4 Hz, 1H), 7.37 − 7.33 (m, 1H), 7.14 (dd, J = 8.4, 1.4 Hz, 1H), 2.46 (s, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.4, 141.6, 132.6, 132.4, 129.1 127.6, 127.5, 126.7, 125.9, 122.2, 120.3, 111.4, 21.6. IR(KBr): υ_max = 2921, 2360, 2341, 1467, 1230, 1188, 1107, 1090, 964, 903, 795, 762, 744, and 695 cm⁻¹. MS (ESI): m/z = 209.10, (100) (M + H)⁺.

7-Methyl-3-phenylbenzofuran (2c): Colorless oil. The NMR spectra are in good agreement with the literature.^{64
¹}H NMR (600 MHz, CDCl₃): δ = 7.85 (s, 1H), 7.74 (d, J = 7.7 Hz, 1H), 7.73 − 7.69 (m, 2H), 7.55 − 7.50 (m, 2H), 7.45 − 7.40 (m, 1H), 7.28 (t, J = 7.5 Hz, 1H), 7.22 (d, J = 7.2 Hz, 1H), 2.63 (s, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.0, 141.2, 132.5, 129.1, 127.7, 127.5, 126.1, 125.6, 123.2, 122.6, 122.2, 118.0, 15.2. IR(KBr): υ_max = 3054, 2923, 1608, 1496, 1446, 1416, 1379, 1350, 1223, 1177, 1147, 1116, 1074, 1022, 907, 843, 779, 759, 745, 696, and 549 cm⁻¹. MS (ESI): m/z = 209.06, (100) (M + H)⁺.

6-Methyl-3-phenylbenzofuran (2d): Yellow solid, m.p. 39°C. ¹H NMR (500 MHz, CDCl₃): δ = 7.75 (s, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.69 − 7.65 (m, 2H), 7.49 (t, J = 7.7 Hz, 2H), 7.41 − 7.36 (m, 2H), 7.16 (d, J = 8.0 Hz, 1H), 2.52 (s, 3H). ¹³C NMR (151 MHz, CDCl3): δ = 156.4, 140.9, 135.0, 132.4, 129.1, 127.53, 127.48, 124.6, 124.1, 122.2, 120.0, 112.1, 21.8. IR(KBr): υ_max = 3057, 3032, 2920, 2859, 1628, 1606, 1567, 1492, 1447, 1425, 1349, 1326, 1235, 1169, 1142, 1142, 1093, 1072, 1030, 965, 920, 912, 857, 798, 761, 745, 696, 673, and 612 cm⁻¹. MS (ESI): m/z = 247.02, (100) [M + K]⁺. HRMS (ESI): m/z (M + H)⁺ calcd for C₁₅H₁₃O⁺: 209.09609; found: 209.09608.

4-Methyl-3-phenylbenzofuran (2d”): Colorless oil. ¹H NMR (500 MHz, CDCl₃): δ = 7.53 (s, 1H), 7.46 − 7.37 (m, 6H), 7.21 (dd, J = 8.8, 6.9 Hz, 1H), 6.99 (dd, J = 7.3, 0.8 Hz, 1H), 2.24 (s, 3H). ¹³C NMR (151 MHz, CDCl3): δ = 155.5, 142.1, 133.1, 132.2, 130.3, 128.2, 127.7, 126.0, 124.55, 124.51, 123.5, 109.4, 19.9. IR(KBr): υ_max = 3054, 3028, 2955, 2923, 1605, 1568, 1494, 1444, 1413, 1295, 1223, 1164, 1141, 1106, 1071, 1025, 962, 913, 827, 778, 759, 747, and 700 cm⁻¹. MS (ESI): m/z = 247.09, (100) [M + K]⁺. HRMS (ESI): m/z (M + H)⁺ calcd for C₁₅H₁₃O⁺: 209.09609; found: 209.09569.

5-Methoxy-3-phenylbenzofuran (2e): Colorless oil. The NMR spectra are in good agreement with the literature.^{36,65 ¹}H NMR (600 MHz, CDCl₃): δ = 7.79 (s, 1H), 7.67 (d, J = 7.3 Hz, 2H), 7.53 (t, J = 7.7 Hz, 2H), 7.49 (d, J = 8.9 Hz, 1H), 7.42 (t, J = 7.4 Hz, 1H), 7.33 (d, J = 2.6 Hz, 1H), 7.02 (dd, J = 8.9, 2.6 Hz, 1H), 3.90 (s, 3H). ¹³C NMR (126 MHz, CDCl₃): δ = 156.3, 150.8, 142.3, 132.2, 129.1, 127.6, 127.1, 122.5, 113.4, 112.4, 102.8, 56.1. IR(KBr): υ_max = 2929, 2831, 1607, 1560, 1492, 1471, 1451, 1440, 1272, 1259, 1195, 1173, 1136, 1107, 1026, 968, 905, 834, 797, 761, 753, 740, 696, 680, and 646 cm⁻¹. MS (ESI): m/z = 225.03, (100) [M + H]⁺.

5-(tert-Butyl)-3-phenylbenzofuran (2f): Colorless oil. The NMR spectra are in good agreement with the literature.^{64
¹}H NMR (600 MHz, CDCl₃): δ = 7.87 (d, J = 1.9 Hz, 1H), 7.79 (s, 1H), 7.71 − 7.68 (m, 2H), 7.55 − 7.52 (m, 3H), 7.47 (dd, J = 8.7, 1.9 Hz, 1H), 7.45 − 7.41 (m, 1H), 1.45 (s, 9H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.2, 146.3, 141.7, 132.5, 129.1, 127.7, 127.5, 126.2, 122.7, 122.6, 116.4, 111.2, 35.0, 32.1. IR(KBr): υ_max = 3059, 2961, 2905, 2868, 1606, 1476, 1364, 1258, 1218, 1111, 1075, 970, 895, 879, 828, 804, 789, 763, 748, 724, 696, 664, and 508 cm⁻¹. MS (ESI): m/z = 251.16, (100) (M + H)⁺.

3,5-Diphenylbenzofuran (2g): Colorless oil. ¹H NMR (600 MHz, CDCl₃): δ = 8.01 (d, J = 1.4 Hz, 1H), 7.83 (s, 1H), 7.70 − 7.67 (m, 2H), 7.65 − 7.63 (m, 2H), 7.62 (d, J = 8.5 Hz, 1H), 7.59 (dd, J = 8.5, 1.7 Hz, 1H), 7.50 (t, J = 7.7 Hz, 2H), 7.46 (t, J = 7.7 Hz, 2H), 7.40 (t, J = 7.4 Hz, 1H), 7.36 (t, J = 7.4 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.6, 142.1, 141.9, 137.0, 132.1, 129.2, 128.9, 127.8, 127.71, 127.70, 127.2, 127.1, 124.5, 122.7, 119.1, 112.0. IR(KBr): υ_max = 3058, 3031, 2923, 1600, 1462, 1454, 1426, 1341, 1260, 1211, 1107, 1093, 1029, 965, 878, 807, 789, 754, 694, 656, 624, and 537 cm^–1. MS (ESI): m/z = 271.27, (100) [M + H]⁺. HRMS (ESI): m/z (M + H)⁺ calcd for C₂₀H₁₅O⁺: 271.1117; found: 271.1119.

3,7-Diphenylbenzofuran (2h): White solid, m.p. 92°C–94°C (Lit⁶⁶ 96°C–97°C) (PE/Acetone, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{66
¹}H NMR (600 MHz, CDCl₃): δ = 7.90 − 7.87 (m, 2H), 7.85 (s, 1H), 7.82 (dd, J = 7.8, 1.0 Hz, 1H), 7.70 − 7.66 (m, 2H), 7.55 − 7.48 (m, 5H), 7.44 − 7.38 (m, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 153.2, 141.6, 136.6, 132.2, 129.1, 128.9, 128.8, 127.9, 127.8, 127.7, 127.4, 126.1, 124.4, 123.7, 122.6, 119.7. IR(KBr): υ_max = 3052, 2923, 1605, 1572, 1491, 1444, 1408, 1344, 1218, 1205, 1156, 1122, 1085, 1022, 1073, 969, 877, 834, 798, 760, 746, 698, 644, 586, and 555 cm⁻¹. MS (ESI): m/z = 271.15, (100) (M + H)⁺.

5-Fluoro-3-phenylbenzofuran (2i): Colorless oil. The NMR spectra are in good agreement with the literature.^{36
¹}H NMR (600 MHz, CDCl₃): δ = 7.82 (s, 1H), 7.60 (dt, J = 7.9, 1.5 Hz, 2H), 7.52 − 7.45 (m, 4H), 7.42 − 7.37 (m, 1H), 7.08 (td, J = 9.0, 2.6 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 159.6 (d, J = 238.5 Hz), 152.1, 143.1, 131.7, 129.2, 127.8, 127.5, 127.4, 122.7 (d, J = 4.0 Hz), 112.6, 112.5 (d, J = 15.8 Hz), 106.2 (d, J = 25.5 Hz). ¹⁹F NMR (565 MHz, CDCl₃): δ = −120.52. IR(KBr): υ_max = 3058, 2925, 1746, 1625, 1595, 1493, 1466, 1450, 1338, 1270, 1250, 1164, 1127, 1104, 1087, 974, 916, 857, 800, 766, 756, 740, 696, 682, 639, 611, and 557 cm⁻¹. MS (ESI): m/z = 212.88, (100) (M + H)⁺.

5-Chloro-3-phenylbenzofuran (2j): Colorless oil. The NMR spectra are in good agreement with the literature.^{64
¹}H NMR (600 MHz, CDCl₃): δ = 7.80 (d, J = 2.5 Hz, 1H), 7.79 (s, 1H), 7.61 (t, J = 1.6 Hz, 1H), 7.60 (t, J = 1.4 Hz, 1H), 7.51 − 7.46 (m, 3H), 7.42 − 7.38 (m, 1H), 7.32 (dd, J = 8.7, 2.2 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.3, 142.7, 131.5, 129.2, 128.9, 128.1, 127.9, 127.6, 125.0, 122.3, 120.3, 112.9. IR(KBr): υ_max = 3064, 2923, 2853, 1608, 1484, 1451, 1440, 1368, 1319, 1257, 1219, 1163, 1142, 1102, 1065, 1029, 1010, 968, 912, 866, 826, 801, 760, 742, 722, 690, 660, 644, and 547 cm⁻¹. MS (ESI): m/z = 229.01, (100) (M + H)⁺.

5-Bromo-3-phenylbenzofuran (2k): Colorless oil. The NMR spectra are in good agreement with the literature.^{64
¹}H NMR (600 MHz, CDCl₃): δ = 7.95 (d, J = 1.7 Hz, 1H), 7.78 (s, 1H), 7.60 (t, J = 1.5 Hz, 1H), 7.59 (d, J = 1.0 Hz, 1H), 7.51 − 7.47 (m, 2H), 7.45 (dd, J = 8.7, 1.8 Hz, 1H), 7.44 − 7.41 (m, 1H), 7.41 − 7.37 (m, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.7, 142.5, 131.4, 129.2, 128.7, 128.0, 127.7, 127.6, 123.3, 122.1, 116.4, 113.4. IR(KBr): υ_max = 3059, 2925, 1560, 1491, 1447, 1327, 1292, 1268, 1249, 1217, 1143, 1108, 1098, 1052, 1028, 965, 866, 798, 783, 759, 741, 712, 695, 648, and 539 cm⁻¹. MS (ESI): m/z = 273.11, (100) (M + H)⁺.

3,7-Diphenylbenzo[1,2-b:4,5-b']difuran (2l): White solid, m.p. 170°C–172°C (PE/Acetone, 4:1, v/v). ¹H NMR (600 MHz, CDCl₃): δ = 7.64 (s, 2H), 7.56 (s, 2H), 7.10 − 7.05 (m, 4H), 6.99 − 6.95 (m, 2H), 6.87 (t, J = 7.6 Hz, 4H). ¹³C NMR (151 MHz, CDCl₃): δ = 153.1, 142.8, 132.9, 128.5, 127.7, 127.1, 124.0, 119.7, 108.8. IR(KBr): υ_max = 3128, 3027, 1605, 1557, 1488, 1446, 1416, 1358, 1261, 1228, 1204, 1151, 1127, 1109, 1072, 1033, 1016, 929, 843, 807, 787, 753, 725, 696, 678, 601, 592, 569, and 493 cm⁻¹. MS (ESI): m/z = 311.21, (100) [M + H]⁺. HRMS (ESI): m/z (M + H)⁺ calcd for C₂₂H₁₅O₂⁺: 311.10666; found: 311.10660.

3-Phenylnaphtho[1,2-b]furan (2m): White solid, m.p. 108°C–110°C (Lit⁶⁷ 111°C–113°C) (PE/Acetone, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{67
¹}H NMR (600 MHz, CDCl₃): δ = 8.38 (d, J = 8.2 Hz, 1H), 7.98 (d, J = 8.2 Hz, 1H), 7.95 (s, 1H), 7.91 (d, J = 8.6 Hz, 1H), 7.76 − 7.70 (m, 3H), 7.66 − 7.62 (m, 1H), 7.54 (m, 3H), 7.42 (t, J = 7.4 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 151.6, 140.7, 132.4, 131.7, 129.2, 128.4, 127.8, 127.6, 126.6, 125.5, 123.8, 123.7, 122.1, 121.8, 120.3, 118.9. IR(KBr): υ_max = 3053, 1603, 1561, 1518, 1444, 1386, 1246, 1218, 1127, 1068, 1026, 918, 867, 810, 790, 757, 742, 695, 631, and 565 cm⁻¹. MS (ESI): m/z = 245.08, (100) (M + H)⁺.

1-Phenylnaphtho[2,1-b]furan (2n): Colorless oil. The NMR spectra are in good agreement with the literature.^{68
¹}H NMR (600 MHz, CDCl₃): δ = 8.07 (d, J = 8.4 Hz, 1H), 7.99 (d, J = 8.1 Hz, 1H), 7.81 (d, J = 9.0 Hz, 1H), 7.77 − 7.72 (m, 2H), 7.66 (dt, J = 3.3, 1.9 Hz, 2H), 7.59 − 7.54 (m, 2H), 7.54 − 7.50 (m, 1H), 7.48 (ddd, J = 8.1, 7.0, 1.2 Hz, 1H), 7.41 (ddd, J = 8.2, 6.9, 1.3 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 153.3, 141.8, 133.3, 131.0, 130.0, 129.1, 128.7, 128.5, 128.0, 126.1, 126.1, 124.6, 124.5, 123.5, 120.9, 112.8. IR(KBr): υ_max = 3051, 1524, 1488, 1443, 1384, 1294, 1251, 1224, 1110, 993, 947, 858, 803, 754, 700, 634, 611, 559, and 517 cm⁻¹. MS (ESI): m/z = 245.07, (100) (M + H)⁺.

3-Phenyl-4H-furo[3,2-c]chromen-4-one (2o): White crystal, m.p. 162°C–164°C (Lit⁶⁹ 177°C) (PE/EA, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{69
¹}H NMR (600 MHz, CDCl₃): δ = 7.92 (dd, J = 7.8, 1.5 Hz, 1H), 7.78 (s, 1H), 7.76 (d, J = 1.6 Hz, 2H), 7.57 − 7.51 (m, 1H), 7.46 (t, J = 7.6 Hz, 3H), 7.42 − 7.34 (m, 2H). ¹³C NMR (151 MHz, CDCl₃): δ = 159.0, 158.0, 152.8, 141.4, 131.1, 129.2, 128.8, 128.7, 128.5, 126.9, 124.6, 121.1, 117.3, 113.0, 108.7. IR(KBr): υ_max = 3150, 3052, 1740, 1629, 1594, 1543, 1497, 1487, 1449, 1414, 1366, 1317, 1286, 1249, 1213, 1155, 1101, 1072, 1045, 966, 927, 888, 793, 753, 722, 695, 632, 612, and 523 cm⁻¹. MS (ESI): m/z = 263.06, (100) (M + H)⁺.

9-Phenyl-2H-furo[2,3-h]chromen-2-one (2p): Pale yellow needles, m.p. 112°C–114°C (PE/Acetone, 4:1, v/v). ¹H NMR (500 MHz, CDCl₃): δ = 7.80 (d, J = 9.6 Hz, 1H), 7.76 − 7.73 (m, 2H), 7.72 (d, J = 1.1 Hz, 1H), 7.53 − 7.48 (m, 2H), 7.47 (d, J = 8.5 Hz, 1H), 7.45 − 7.39 (m, 2H), 6.37 (d, J = 9.5 Hz, 1H). ¹³C NMR (126 MHz, CDCl₃): δ = 160.3, 158.4, 149.4, 144.6, 142.9, 130.5, 129.3, 128.6, 128.3, 124.5, 123.1, 115.3, 114.4, 114.0, 109.2. IR(KBr): υ_max = 3150, 3052, 1740, 1629, 1594, 1543, 1497, 1487, 1449, 1414, 1366, 1317, 1286, 1249, 1213, 1155, 1101, 1072, 1045, 999, 966, 927, 888, 793, 753, 722, 695, 632, 612, and 523 cm⁻¹. MS (ESI): m/z = 285.04, (100) [M + Na]⁺. HRMS (ESI): m/z (M + H)⁺ calcd for C₁₇H₁₃O₃⁺: 265.07918; found: 265.07675.

3-Phenylbenzo[b]thiophene (2q): Colorless oil. The NMR spectra are in good agreement with the literature.^{70
¹}H NMR (600 MHz, CDCl₃): δ = 7.98 − 7.93 (m, 2H), 7.63 (t, J = 1.6 Hz, 1H), 7.61 (t, J = 1.5 Hz, 1H), 7.54 − 7.50 (m, 2H), 7.46 − 7.40 (m, 4H). ¹³C NMR (151 MHz, CDCl₃): δ = 140.8, 138.2, 138.1, 136.2, 128.9(2C), 127.7, 124.6, 124.5, 123.5, 123.1(2C). IR(KBr): υ_max = 3057, 3027, 1483, 1425, 1347, 833, 762, 731, 697, 636, 573, and 511 cm⁻¹. MS (ESI): m/z = 211.06, (100) (M + H)⁺.

3-(o-Tolyl)benzofuran (2ab): Pale yellow liquid. The NMR spectra are in good agreement with the literature.^{14
¹}H NMR (500 MHz, CDCl₃): δ = 7.63 (s, 1H), 7.56 (d, J = 8.2 Hz, 1H), 7.47 (d, J = 7.8 Hz, 1H), 7.40 (dd, J = 7.2, 1.5 Hz, 1H), 7.36 − 7.31 (m, 3H), 7.31 − 7.27 (m, 2H), 2.33 (s, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.2, 142.3, 137.0, 131.0, 130.6, 130.5, 128.0, 126.0, 124.5, 122.9, 121.5, 120.8, 111.7, 20.6. IR(KBr): υ_max = 3061, 2924, 2853, 1606, 1488, 1452, 1379, 1332, 1219, 1103, 1086, 1008, 962, 857, 774, 747, and 723 cm⁻¹. MS (ESI): m/z = 209.14, (100) (M + H)⁺.

3-(m-Tolyl)benzofuran (2ac): Colorless oil. The NMR spectra are in good agreement with the literature.^{64
¹}H NMR (500 MHz, CDCl₃): δ = 7.85 (d, J = 7.3 Hz, 1H), 7.78 (s, 1H), 7.55 (d, J = 8.0 Hz, 1H), 7.46 (d, J = 7.1 Hz, 2H), 7.36 (dt, J = 9.2, 4.6 Hz, 2H), 7.31 (td, J = 7.6, 0.9 Hz, 1H), 7.20 (d, J = 7.5 Hz, 1H), 2.44 (s, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.9, 141.4, 138.8, 132.1, 129.0, 128.39, 128.35, 126.7, 124.8, 124.6, 123.1, 122.5, 120.6, 111.9, 21.7. IR(KBr): υ_max = 3034, 2920, 1609, 1452, 1336, 1234, 1188, 1108, 1012, 857, 833, 783, 768, 744, 699, and 614 cm⁻¹. MS (ESI): m/z = 209.11, (100) (M + H)⁺.

3-(p-Tolyl)benzofuran (2ad): Yellow oil. The NMR spectra are in good agreement with the literature.^{14
¹}H NMR (600 MHz, CDCl₃): δ = 7.84 (dd, J = 7.7, 0.6 Hz, 1H), 7.77 (s, 1H), 7.58 − 7.53 (m, 3H), 7.38 − 7.33 (m, 1H), 7.32 − 7.29 (m, 3H), 2.43 (s, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.9, 141.2, 137.4, 129.8, 129.2, 127.5, 126.8, 124.6, 123.0, 122.3, 120.6, 111.9, 21.4. IR(KBr): υ_max = 3026, 2920, 1578, 1508, 1452, 1342, 1307, 1220, 1209, 1108, 1092, 1012, 964, 857, 824, 796, 769, 745, 565, 516, and 421 cm⁻¹. MS (ESI): m/z = 209.32, (100) (M + H)⁺.

3-(4-Fluorophenyl)benzofuran (2ae): Yellow oil. The NMR spectra are in good agreement with the literature.^{14
¹}H NMR (600 MHz, CDCl₃): δ = 7.81 − 7.77 (m, 1H), 7.76 (s, 1H), 7.63 − 7.58 (m, 2H), 7.57 − 7.55 (m, 1H), 7.37 (m, 1H), 7.34 − 7.30 (m, 1H), 7.20 − 7.15 (m, 2H). ¹³C NMR (151 MHz, CDCl₃): δ = 162.4 (d, J = 246.6 Hz), 155.9, 141.3, 129.2 (d, J = 8.0 Hz), 128.2 (d, J = 3.3 Hz), 126.5, 124.8, 123.2, 121.5, 120.3, 116.1 (d, J = 21.5 Hz), 112.0. IR(KBr): υ_max = 3056, 1817, 1597, 1570, 1505, 1452, 1341, 1298, 1218, 1159, 1109, 1090, 1013, 964, 932, 904, 857, 839, 813, 770, 745, 657, 578, 563, and 521 cm⁻¹. ¹⁹F NMR (565 MHz, CDCl₃): δ = −115.10. MS (ESI): m/z = 251.06, (100) (M + H)⁺.

3-(4-Nitrophenyl)benzofuran (2af): Yellow solid, m.p. 142° C–144° C (Lit⁷¹ 143° C) (CHCl₃/PE, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{71
¹}H NMR (500 MHz, CDCl₃): δ = 8.35 (d, J = 8.9 Hz, 2H), 7.94 (s, 1H), 7.86 − 7.80 (m, 3H), 7.62 − 7.58 (m, 1H), 7.44 − 7.40 (m, 1H), 7.37 (td, J = 7.5, 1.2 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 156.1, 147.1, 143.0, 139.2, 127.9, 125.6, 125.4, 124.5, 123.8, 120.8, 120.2, 112.3. IR(KBr): υ_max = 2959, 2924, 1599, 1565, 1508, 1449, 1343, 1316, 1259, 1215, 1185, 1137, 1111, 1091, 1013, 964, 852, 804, 761, 746, and 693 cm⁻¹. HRMS (ESI): m/z (M + Na)⁺ calcd for C₁₄H₉NNaO₃⁺: 262.0475; found: 262.0478.

3-((1,1'-Biphenyl) (2ag): White needles, m.p. 138° C–140° C (PE/EA, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{72
¹}H NMR (500 MHz, CDCl₃): δ = 7.92 − 7.88 (m, 1H), 7.85 (s, 1H), 7.76 − 7.70 (m, 4H), 7.69 − 7.64 (m, 2H), 7.58 (d, J = 7.7 Hz, 1H), 7.51 − 7.45 (m, 2H), 7.38 (td, J = 7.8, 1.4 Hz, 2H), 7.34 (td, J = 7.5, 1.2 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 156.0, 141.5, 140.8, 140.5, 131.2, 129.0, 128.0, 127.8, 127.6, 127.2, 126.6, 124.8, 123.2, 122.0, 120.6, 112.0. IR(KBr): υ_max = 3033, 1606, 1569, 1487, 1451, 1347, 1253, 1218, 1167, 1146, 1121, 1095, 960, 929, 857, 838, 805, 767, 734, and 691 cm⁻¹. MS (ESI): m/z = 271.15, (100) (M + H)⁺.

3-(Naphthalen-1-yl)benzofuran (2ah): Colorless liquid. The NMR spectra are in good agreement with the literature.^{73
¹}H NMR (500 MHz, CDCl₃): δ = 7.98 (d, J = 8.4 Hz, 1H), 7.93 (t, J = 8.0 Hz, 2H), 7.82 (s, 1H), 7.63 − 7.56 (m, 3H), 7.55 − 7.50 (m, 1H), 7.47 − 7.41 (m, 2H), 7.38 (ddd, J = 8.4, 7.3, 1.2 Hz, 1H), 7.23 (dd, J = 7.2, 0.9 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.4, 142.9, 134.0, 132.3, 129.4, 128.6, 128.49, 128.48, 127.8, 126.3, 126.2, 126.1, 125.7, 124.7, 123.0, 121.1, 120.9, 111.8. IR(KBr): υ_max = 3046, 2924, 2853, 1594, 1507, 1451, 1396, 1347, 1322, 1291, 1255, 1226, 1183, 1147, 1100, 1017, 936, 856, 798, 777, 767, 746, 670, 650, 615, and 583 cm⁻¹. MS (ESI): m/z = 267.23, (100) (M + Na)⁺.

3-(Naphthalen-2-yl)benzofuran (2ai): White solid, m.p. 76° C–78° C (PE/EA, 4:1, v/v). ¹H NMR (500 MHz, CDCl₃): δ = 8.16 (d, J = 0.6 Hz, 1H), 8.02 − 7.95 (m, 2H), 7.95 − 7.88 (m, 3H), 7.77 (dd, J = 8.4, 1.7 Hz, 1H), 7.62 (m, 1H), 7.54 (pd, J = 6.9, 1.5 Hz, 2H), 7.41 (dd, J = 7.9, 1.5 Hz, 1H), 7.39 (dd, J = 7.5, 1.3 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 156.0, 141.8, 133.9, 132.8, 129.7, 128.8, 128.1, 127.9, 126.7, 126.6, 126.12, 126.07, 125.9, 124.8, 123.2, 122.4, 120.6, 112.0. IR(KBr): υ_max = 3141, 3052, 1629, 1601, 1507, 1450, 1324, 1274, 1208, 1198, 1141, 1116, 1094, 1017, 981, 947, 932, 855, 836, 824, 804, 765, 745, 623, and 564 cm⁻¹. MS (ESI): m/z = 267.19, (100) [M + Na]⁺. HRMS (ESI): m/z (M + H)⁺ calcd for C₁₈H₁₃O⁺: 245.09601; found: 245.09599.

2-Methyl-3-phenylbenzofuran (2aj): Colorless oil. The NMR spectra are in good agreement with the literature.^{74
¹}H NMR (600 MHz, CDCl₃): δ = 7.58 (d, J = 7.9 Hz, 1H), 7.52 − 7.49 (m, 2H), 7.48 (d, J = 7.9 Hz, 1H), 7.46 (d, J = 7.9 Hz, 1H), 7.36 (s, 1H), 7.25 (d, J = 6.8 Hz, 1H), 7.24 − 7.20 (m, 1H), 2.54 (s, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.2, 151. 5, 133.0, 129.1, 128.9, 127.1, 123.7, 122.7, 119.5, 117.0, 110.9, 13.0. IR(KBr): υ_max = 3055, 2952, 2919, 2851, 1624, 1610, 1598, 1497, 1473, 1455, 1389, 1361, 1283, 1251, 1207, 1175, 1105, 1075, 1018, 1008, 963, 922, 904, 849, 769, 749, 700, 666, 653, 611, and 561 cm⁻¹. MS (ESI): m/z = 209.09, (100) (M + H)⁺.

2-Ethyl-3-phenylbenzofuran (2ak): Colorless oil. The NMR spectra are in good agreement with the literature.^{75
¹}H NMR (600 MHz, CDCl₃): δ = 7.63 (dd, J = 7.7, 0.6 Hz, 1H), 7.58 − 7.51 (m, 5H), 7.45 − 7.40 (m, 1H), 7.35 − 7.30 (m, 1H), 7.30 − 7.26 (m, 1H), 2.95 (q, J = 7.5 Hz, 2H), 1.42 (t, J = 7.6 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 156.4, 154.2, 133.0, 129.2, 129.0, 128.9, 127.1, 123.7, 122.7, 119.6, 116.3, 111.0, 20.4, 13.1. IR(KBr): υ_max = 3055, 2974, 2935, 2876, 1610, 1455, 1311, 1281, 1240, 1195, 1174, 1106, 1057, 1012, 980, 963, 898, 847, 769, 749, 700, and 611 cm⁻¹. MS (ESI): m/z = 223.15, (100) (M + H)⁺.

2,3-Diphenylbenzofuran (2al): White solid, m.p. 120° C–122° C (Lit⁷⁶ 121° C–123° C; Acetone/PE, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{76
¹}H NMR (600 MHz, CDCl₃): δ = 7.72 − 7.67 (m, 2H), 7.59 (d, 1H), 7.56 − 7.52 (m, 3H), 7.50 (td, J = 7.6, 1.7 Hz, 2H), 7.46 − 7.41 (m, 1H), 7.38 − 7.29 (m, 4H), 7.29 − 7.26 (m, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.1, 150.7, 133.0, 130.8, 130.4, 129.9, 129.1, 128.6, 128.5, 127.8, 127.2, 124.8, 123.1, 120.2, 117.7, 111.3. IR(KBr): υ_max = 3061, 1601, 1497, 1455, 1440, 1368, 1254, 1203, 1110, 1062, 1027, 1008, 960, 891, 827, 764, 748, 693, 621, and 608 cm⁻¹. MS (ESI): m/z = 271.12, (100) (M + H)⁺.

3-Methylbenzofuran (2am): Colorless oil. The NMR spectra are in good agreement with the literature.^{77
¹}H NMR (500 MHz, CDCl₃): δ = 7.55 − 7.51 (m, 1H), 7.48 − 7.44 (m, 1H), 7.41 (d, J = 1.3 Hz, 1H), 7.31 − 7.27 (m, 1H), 7.26 − 7.21 (m, 1H), 2.25 (d, J = 1.3 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.4, 141.5, 129.1, 124.2, 122.3, 119.5, 115.8, 111.4, 8.1. IR(KBr): υ_max = 2959, 2929, 2871, 1453, 1185, 1088, and 743 cm⁻¹. MS (ESI): m/z = 133.05, (100) (M + H)⁺.

3-Propylbenzofuran (2an): Colorless oil. ¹H NMR (600 MHz, CDCl₃): δ = 7.55 (d, J = 7.5 Hz, 1H), 7.46 (d, J = 8.1 Hz, 1H), 7.40 (s, 1H), 7.30 − 7.26 (m, 1H), 7.24 − 7.20 (m, 1H), 2.65 (t, J = 7.2 Hz, 2H), 1.74 (dq, J = 14.8, 7.3 Hz, 2H), 1.00 (t, J = 7.3 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.5, 141.2, 128.5, 124.1, 122.3, 120.6, 119.8, 111.5, 25.8, 22.4, 14.2. IR(KBr): υ_max = 2959, 2929, 2871, 1453, 1219, 1185, 1089, 856, 772, 743 cm⁻¹. MS (ESI) calcd for C₁₁H₁₃O⁺(M + H)⁺ 161.10; found: 161.05. MS (ESI): m/z = 161.05, (100) [M + H]⁺. HRMS (ESI): m/z (M + Na)⁺ calcd for C₁₁H₁₂NaO₃⁺: 183.07817; found: 183.07820.

2-Ethyl-3-methylbenzofuran (2ao): Colorless oil. The NMR spectra are in good agreement with the literature.^{78
¹}H NMR (500 MHz, CDCl₃): δ = 7.42 − 7.39 (m, 1H), 7.38 − 7.36 (m, 1H), 7.21 − 7.19 (m, 1H), 7.19 − 7.16 (m, 1H), 2.74 (q, J = 7.6 Hz, 2H), 2.15 (s, 3H), 1.28 (t, J = 7.6 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 155.6, 153.9, 130.7, 123.1, 122.1, 118.8, 110.6, 108.9, 19.9, 12.9, 7.9. IR(KBr): υ_max = 3036, 2973, 2937, 2876, 1631, 1474, 1455, 1315, 1276, 1238, 1179, 1133, 1107, 875, 828, and 743 cm⁻¹. MS (ESI): m/z = 161.06, (100) (M + H)⁺.

3-Ethyl-2-methylbenzofuran (2ap): Colorless oil. The NMR spectra are in good agreement with the literature.^{31
¹}H NMR (500 MHz, CDCl₃): δ = 7.52 − 7.48 (m, 1H), 7.42 (m, 1H), 7.25 − 7.20 (m, 2H), 2.67 (q, J = 7.6 Hz, 2H), 2.43 (s, 3H), 1.28 (t, J = 7.6 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.1, 150.1, 129.7, 123.0, 122.0, 118.9, 116.1, 110.7, 17.1, 14.6, 12.0. IR(KBr): υ_max = 3036, 2967, 2930, 2873, 1633, 1474, 1455, 1390, 1278, 1249, 1180, 1150, 1136, 1110, 1063, 1010, 952, 895, 848, and 743 cm⁻¹. MS (ESI): m/z = 161.10, (100) (M + H)⁺.

1,2,3,4-Tetrahydrodibenzo[b, d]furan (2aq): Pale red oil. The NMR spectra are in good agreement with the literature.^{79
¹}H NMR (500 MHz, CDCl₃): δ = 7.43 − 7.37 (m, 2H), 7.22 − 7.16 (m, 2H), 2.75 (tt, J = 6.3, 1.9 Hz, 2H), 2.63 (tt, J = 6.0, 1.9 Hz, 2H), 1.97 − 1.91 (m, 2H), 1.85 (tdd, J = 6.3, 4.6, 1.3 Hz, 2H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.4, 154.2, 129.0, 123.1, 122.2, 118.5, 113.0, 110.9, 23.6, 23.1, 22.8, 20.6. IR(KBr): υ_max = 3054, 2932, 2845, 1639, 1476, 1453, 1362, 1297, 1274, 1257, 1222, 1189, 1121, 1008, 951, 875, 852, 820, and 742 cm⁻¹. MS (ESI): m/z = 173.13, (100) (M + H)⁺.

7,8,9,10-Tetrahydro-6H-cyclohepta[b]benzofuran (2ar): Colorless oil. The NMR spectra are in good agreement with the literature.^{80
¹}H NMR (600 MHz, CDCl₃): δ = 7.43 − 7.39 (m, 1H), 7.39 − 7.35 (m, 1H), 7.22 − 7.17 (m, 2H), 2.93 − 2.90 (m, 2H), 2.73 − 2.68 (m, 2H), 1.87 − 1.86 (m, 2H), 1.84 − 1.77 (m, 4H). ¹³C NMR (151 MHz, CDCl₃): δ = 156.5, 153.4, 130.6, 122.9, 122.0, 118.4, 116.0, 110.6, 30.7 29.2, 28.4, 26.5, 23.3. IR(KBr): υ_max = 3035, 2923, 2849, 1474, 1455, 1308, 1277, 1239, 1227, 1209, 1170, 1095, 1069, 1008, 814, 742 cm⁻¹. MS (ESI): m/z = 187.11, (100) (M + H)⁺.

1-(4-Nitrophenyl)naphtho[2,1-b]furan (2mf): Yellow solid, m.p. 138° C–140° C (Lit⁸¹ 139° C–140° C) (CHCl₃/PE, 4:1, v/v). The NMR spectra are in good agreement with the literature.^{81
¹}H NMR (600 MHz, CDCl₃): δ = 8.42 − 8.35 (m, 2H), 7.98 (d, J = 8.1 Hz, 1H), 7.88 (d, J = 8.3 Hz, 1H), 7.82 (d, J = 9.0 Hz, 1H), 7.81 − 7.79 (m, 2H), 7.76 (s, 1H), 7.72 (d, J = 9.0 Hz, 1H), 7.52 − 7.45 (m, 1H), 7.41 (ddd, J = 8.2, 7.0, 1.2 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 153.7, 147.7, 142.4, 140.5, 131.1, 130.7, 129.4, 128.0, 126.9, 126.6, 124.9, 124.1, 123.1, 122.9, 120.0, 112.8. IR(KBr): υ_max = 3071, 2925, 1600, 1515, 1380, 1342, 1250, 1226, 1115, 995, 948, 863, 850, 811, 795, 753, and 697 cm⁻¹. HRMS (ESI): m/z (M + Na)⁺ calcd for C₁₈H₁₁NNaO₃⁺: 312.0631; found 312.0637.

3-(4-Nitrophenyl)naphtho[1,2-b]furan (2nf): Yellow solid, m.p. 218° C–220° C (CHCl₃/PE, 4:1, v/v). ¹H NMR (600 MHz, CDCl₃): δ = 8.37 (d, J = 8.7 Hz, 2H), 8.36 (d, J = 7.7 Hz, 1H),8.06 (s, 1H), 7.99 (d, J = 8.2 Hz, 1H), 7.87 (d, J = 8.7 Hz, 2H), 7.86 (d, J = 8.6 Hz, 1H), 7.79 (d, J = 8.6 Hz, 1H), 7.66 (t, J = 7.6 Hz, 1H), 7.57 (t, J = 7.5 Hz, 1H). ¹³C NMR (151 MHz, CDCl₃): δ = 152.0, 147.1, 142.0, 139.4, 131.9, 128.5, 128.1, 127.0, 126.0, 124.61, 124.58, 122.1, 121. 7, 121.1, 120.3, 118.1. IR(KBr): υ_max = 3133, 3062, 1597, 1571, 1508, 1453, 1379, 1340, 1312, 1294, 1278, 1251, 1221, 1154, 1120, 1106, 1065, 1026, 1009, 963, 919, 859, 851, 821, 811, 766, 755, 699, 687, 637, and 614 cm⁻¹. HRMS (ESI): m/z (M + H)⁺ calcd for C₁₈H₁₂NO₃⁺: 290.0812; found 290.0814.

4-Phenyl-2H-chromene (5): Pale yellow oil. The NMR spectra are in good agreement with the literature.^{82
¹}H NMR (500 MHz, CDCl₃): δ = 7.43 − 7.32 (m, 5H), 7.18 − 7.13 (m, 1H), 7.01 (dd, J = 7.7, 1.6 Hz, 1H), 6.90 (dd, J = 8.1, 1.1 Hz, 1H), 6.85 (td, J = 7.5, 1.2 Hz, 1H), 5.80 (t, J = 3.9 Hz, 1H), 4.86 (d, J = 3.9 Hz, 2H). ¹³C NMR (151 MHz, CDCl₃): δ = 154.9, 138.4, 137.3, 129.4, 128.8, 128.5, 127.9, 126.0, 123.8, 121.3, 120.1, 116.3, 65.4. IR(KBr): υ_max = 3059, 2933, 1739, 1603, 1484, 1448, 1347, 1260, 1223, 1148, 1115, 1065, 1031, 1011, 928, 800, 758, and 700 cm⁻¹. MS (ESI): m/z = 209.13, (100) (M + H)⁺.

Supplemental Material

SI20191104 – Supplemental material for Synthesis of benzofurans from the cyclodehydration of α-phenoxy ketones mediated by Eaton’s reagent

Supplemental material, SI20191104 for Synthesis of benzofurans from the cyclodehydration of α-phenoxy ketones mediated by Eaton’s reagent by Zhanwei Ma, Min Zhou, Lin Ma and Min Zhang in Journal of Chemical Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (grant nos. 21861002 and 21462003), the Scientific Research Foundation for Returned Scholars of Ministry of Education ((2013)1792), the Key Scientific Research Projects of Guangxi Department of Education (grant no. 2011ZD003), and the Scientific Research Foundation of Guangxi University (grant no. XGZ111491).

ORCID iD

Min Zhang

Supplemental material

Supplemental material for this article is available online.

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