Selective synthesis of 2,5-disubstituted furan-3-carboxylates and the isomeric 2,4-disubstituted furan-3-carboxylates

Abstract

An unprecedented Ag₂CO₃ and DBU mediated cyclization of 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates leading to the formation of 2,5-disubstituted furan-3-carboxylates has been reported. In the absence of a silver salt, the isomeric 2,4-disubstituted furan-3-carboxylates are obtained.

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Introduction

Furans are a fundamental class of five-membered heterocycles. Especially, functionalized furans play an important role as structural elements of many natural products (e.g. pochonin G I,¹ sarcofuranocembrenolide B II,² and the sesquiterpenoid III³ isolated from the mycelia of the edible mushroom Pleurotus cornucopiae fermented on rice, Fig. 1) which have shown diverse biological properties, such as hair-growth stimulation,¹ antiallergic,² antiapoptotic,² and anticancer activities.³ Moreover, they are important building blocks in organic synthesis.⁴ Therefore, the synthesis of furans has attracted a great deal of attention from synthetic organic chemists. Generally, there are two strategies to construct the furan ring: (i) Paal–Knorr synthesis,⁵ i.e. cyclization of 1,4-dicarbonyl compounds; (ii) cyclization of propargyl/allyl dicarbonyl compounds,⁶ allenyl ketones,⁷ enynols,⁸ alkynyl epoxides⁹ or propargyl ethers.¹⁰ Transition metals, PhI(O₂CCF₃)₂, PhSeSePh, NBS and even iodine could be used as catalysts for these transformations to obtain highly substituted furans.

Fig. 1 Representative furan natural products.

Over the last decades, silver-mediated cyclization for the construction of heterocyclic compounds has received considerable attention.¹¹ Recently, Lei and co-worker have developed useful silver-mediated oxidative cross-coupling/cyclization strategies between 2-aminopyridines,¹² 1,3-dicarbonyl compounds,¹³ β-enamino esters¹⁴ and terminal alkynes to create polysubstituted imidazole[1,2-a]pyridines, furans, and pyrroles, respectively. Silver-mediated intramolecular addition of heteroatoms to acetylene,¹⁵ alkene,¹⁶ and allenic intermediates¹⁷ has also been widely applied in the assembly of bioactive heterocycles. Given that 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates 1 could be easily accessed through nucleophilic substitution of 2,3-dibromo-1-propene with 3-oxo-1-propanoates,^6j,18 we wonder a silver-mediated cyclization would lead to the formation of furan-3-carboxylates 2 (Scheme 1). To the best of our knowledge, this demonstration represents the first silver-mediated cyclization of 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates to poly-substituted furans.

Scheme 1 Proposed silver-mediated cyclization to form 2,5-disubstituted furan-3-carboxylates.

Results and discussion

Initial study was carried out with methyl 2-benzoyl-4-bromopent-4-enoate 1a, which was treated with 2 equivalent of DBU and a catalytic amount of Ag₂CO₃ (0.05 eq.) at 60 °C (entry 1, Table 1). When the reaction was complete (6 h), two main spots appeared on the TLC plate. After being separated by column chromatography, the two products were found to be the desired furan 2a and the isomerized product 3a, respectively. Inspired by the results, we then optimized the reaction conditions in order to obtain either regioisomer selectively.

Table 1 Optimization of the reaction conditions for the selective formation of 2a or 3a, respectively^a

Entry	Base	Silver salt	Temperature (°C)	Reaction time (h)	Ratio of 2a : 3a	Yield (%)
a Anhydrous DMF was found to be the most appropriate solvent through screening.b Numbers in parenthesis indicated the equivalents of reagents used.c Ratio determined on the basis of ^1H NMR integration of the crude reaction mixture.d No reaction.e Isolated yields.
1	DBU (2)^b	Ag2CO3 (0.05)^b	60	6	1 : 3^c
2	DBU (2)^b	AgNO3/silica gel (0.05)^b	60	6	1 : 8^c
3	DBU (2)^b	Ag2CO3 (0.25)^b	60	5.5	1 : 0.15^c
4	DBU (2)^b	Ag2CO3 (0.35)^b	60	4	1 : 0.03^c
5	DBU (2)^b	Ag2CO3 (0.4)^b	60	3.5	1 : 0.02^c
6	DBU (2)^b	Ag2CO3 (0.5)^b	60	1	1 : 0	83^e
7	Cs2CO3 (2)^b	Ag2CO3 (0.5)^b	60	1	1 : 0.14^c
8	—	Ag2CO3 (0.5)^b	60	20	—^d
9	DBU (2)^b	—	60	12	0 : 1	45^e
10	K3PO4 (2)^b	—	60	25	0 : 1	60^e
11	NaHCO3 (2)^b	—	60	22	0 : 1	40^e
12	K2CO3 (2)^b	—	60	4	0 : 1	74^e
13	Cs2CO3 (2)^b	—	60	3	0 : 1	78^e
14	Cs2CO3 (2)^b	—	40	9	0 : 1	46^e
15	Cs2CO3 (2)^b	—	50	8	0 : 1	70^e
16	Cs2CO3 (2)^b	—	70	1	0 : 1	85^e
17	Cs2CO3 (2)^b	—	80	0.7	0 : 1	92^e
18	Cs2CO3 (2)^b	—	90	0.6	0 : 1	89^e
19	Cs2CO3 (1)^b	—	80	0.7	0 : 1	92^e
20	Cs2CO3 (0.75)^b	—	80	0.7	0 : 1	92^e
21	Cs2CO3 (0.5)^b	—	80	0.7	0 : 1	70^e

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In the presence of 2 equivalent of DBU, the ratio of 2a : 3a improved with increased loading of Ag₂CO₃ (entries 1 and 3–6, Table 1). When 0.5 equivalent of Ag₂CO₃ was used, 2a was isolated as the sole product in 83% yield (entry 6). The ratio of 2a : 3a decreased either changing the base from DBU to Cs₂CO₃ (entry 7 vs. 6), or changing the silver salt from Ag₂CO₃ to AgNO₃/silica gel (entry 2 vs. 1). In the absence of a base, no reaction occurred at all (entry 8). When silver salt was removed from the reaction system, isomer 3a was the only product obtained although the reaction proceeded slowly and only moderate yield was obtained (entry 9). Inspired by the findings and in order to obtain 3a selectively, we then explored other bases. Similar to DBU, when conducted in the presence of K₃PO₄ or NaHCO₃, the reaction was sluggish and 3a was isolated in a moderate yield (entries 10 and 11). To our delight, when K₂CO₃ was applied, the reaction was complete in much shorter time and 3a was isolated in good yield (entry 12). Even better result was obtained when Cs₂CO₃ was used (entry 13). With Cs₂CO₃ as our optimized base, we next investigated the effect of the reaction temperature. The reaction proceeded slowly at temperatures below 60 °C while the yield of 3a was moderate (entries 14 and 15). By contrast, at temperatures above 60 °C, the reaction proceeded much faster and the product yield was higher (entries 16–18). When the reaction was conducted at 80 °C, 3a was isolated in the highest yield (92%) (entry 17). Slightly less satisfactory result was obtained when the reaction temperature was raised to 90 °C (entry 18). Finally, we inspected the effect of the quantities of Cs₂CO₃ to the reaction (entries 17 and 19–21) and found that the reaction could work equally well when as low as 0.75 equivalent of the base was applied (entry 20). Further reduction of the amount of Cs₂CO₃ resulted in reduced yield of the product (entry 21).

Having the optimized reaction conditions in hand, we then tested the generality and scope of the present 2,5-disubstituted furan-3-carboxylates and 2,4-disubstituted furan-3-carboxylates, and the results were collected in Table 2. In the presence of 0.5 equivalent of Ag₂CO₃ and 2 equivalent of DBU, the reactions of 3-aryl substituted substrates 1 bearing either an electron-donating group (entries 2 and 3) or electron-withdrawing group (entries 4–6) at the phenyl ring proceeded smoothly to give 2,5-disubstituted furan-3-carboxylates 2b–f in good yields. However, reaction of substrate 1g having a 2,4-dimethoxyphenyl ring gave 2g in poor yield (entry 7). Under the reaction conditions, both substrates bearing naphthalyl and protected pyrrolyl group (1h and 1i) could cyclize to afford good yields of the expected products 2h and 2i, respectively (entries 8 and 9). The structure of 2i was unambiguously confirmed by X-ray single crystal diffraction (Fig. 2).¹⁹ Aliphatic β-keto esters 1k and 1l (entries 11 and 12), as well as substrates with a substituted 2-bromoallyl moiety 1m–o (entries 13–15) could also be converted into the corresponding furan-3-carboxylates 2k–o in good to excellent isolated yield. To our delight, trifuran 2j was obtained in good yield starting from 1j (entry 10). On the other hand, in the presence of 0.75 equivalent of Cs₂CO₃, most substrates reacted smoothly to give 2,4-disubstituted furan-3-carboxylates in good to excellent isolated yield (entries 1–13 and 15). In general, substrates bearing electron-donating groups at the phenyl ring (entries 2, 3 and 7) gave higher yields than those bearing electron-withdrawing groups (entries 4 and 5). Quite unexpectedly, reaction of substrate 1n was sluggish. After 25 h, the desired product 2n was isolated in 11% yield together with substantial amount (75%) of the unreacted starting material recovered.

Table 2 Substrate scope for the selective formation of 2,5-disubstituted furan-3-carboxylates 2 or 2,4-disubstituted furan-3-carboxylates 3, respectively

Entry	Substrate	2,5-Disubstituted furan-3-carboxylates	2,4-Disubstituted furan-3-carboxylates
a 75% of the unreacted 1n was recovered.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

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Fig. 2 X-ray single crystal structure of 2i.

Based on the above results and literature reports, plausible mechanisms for the regioselective formation of 2,5-disubstituted furan-3-carboxylates 2 and 2,4-disubstituted furan-3-carboxylates 3 were proposed in Scheme 2. Base promoted elimination of 1 generated the allenic intermediate I. Intramolecular attack of the allene function with the enolate carbon gave methylenecyclopropane III.²⁰ Activation of the double bond in III with silver^7b,21 (path a) followed by concomitant cyclopropane ring opening and cyclization would generate V.²² β-Elimination of V gave VI, which aromatized to produce furan 2 by tautomerization. On the other hand, in the absence of silver salt, cyclopropane ring opening and subsequent intramolecular nucleophilic attack at the less sterically hindered carbon atom in the cyclopropane ring^20,23 (path b) would generate VII, which tautomerized to produce 2,4-disubstituted furan-3-carboxylates 3.

Scheme 2 Proposed mechanism for the regioselective formation of 2,5-disubstituted furan-3-carboxylates 2 and 2,4-disubstituted furan-3-carboxylates 3.

Conclusion

In summary, we have developed a valuable approach towards the synthesis of 2,5-disubstituted furan-3-carboxylates and the isomeric 2,4-disubstituted furan-3-carboxylates. The former was accessed by treatment of 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates with Ag₂CO₃/DBU, while the latter could be easily obtained by treatment of the same substrates with Cs₂CO₃.

Experimental section

General information

Melting points were determined on a XT4A hot-stage apparatus and are uncorrected. IR spectra were obtained using an IFS25 FT-IR spectrometer. ¹H and ¹³C NMR spectra were obtained on a Agilent AV400 instrument. High-resolution mass spectra were recorded on a Micromass Q-TOF mass spectrometer. Flash column chromatography was performed over silica gel 200–300 mesh.

General procedure for the synthesis of 2,5-disubstituted furan-3-carboxylates (2a–o)

DBU (2.0 mmol) and silver carbonate (0.5 mmol) were added to a solution of 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates (1a–o) (1.0 mmol) in anhydrous DMF (15 mL). The resulting mixture was stirred at 60 °C until the substrate was consumed (monitored by TLC). After being cooled to ambient temperature, the mixture was treated with 1 M HCl (5 mL) and filtered. The filtrate was extracted with ethyl acetate (10 mL × 3). The combined organic extracts were washed with brine (10 mL), then dried over anhydrous sodium sulphate, filtered and concentrated in vacuo. The residue was purified by column chromatography on silica gel to afford 2,3,5-trisubstituted furans 2a–o.

Methyl 5-methyl-2-phenylfuran-3-carboxylate (2a)^6a. The crude product was purified by column chromatography on silica gel (1% ethyl acetate in petroleum ether) to give 2a as a colorless oil; yield 83%; ¹H NMR (400 MHz, CDCl₃): δ 7.97–7.95 (m, 2H), 7.44–7.35 (m, 3H), 6.43 (s, 1H), 3.81 (s, 3H), 2.35 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.4, 156.2, 151.3, 130.1, 129.1, 128.1, 114.2, 108.8, 51.6, 51.4, 13.5; IR (neat): ν_max/cm⁻¹ 1721, 1559, 1492, 1435, 1378, 1274, 1212, 1097, 1016; HRMS (ESI): m/z calcd for C₁₃H₁₃O₃: 217.0859 [M + H]⁺; found 217.0859.

Methyl 5-methyl-2-(4′-methylphenyl)furan-3-carboxylate (2b)^6a. The crude product was purified by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 2b as an orange oil; yield 70%; ¹H NMR (400 MHz, CDCl₃): δ 7.86 (d, J = 8.0 Hz, 2H), 7.23 (d, J = 8.0 Hz, 2H), 6.41 (s, 1H), 3.80 (s, 3H), 2.39 (s, 3H), 2.34 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.4, 156.6, 150.9, 139.2, 128.9, 128.1, 127.3, 113.6, 108.7, 51.6, 21.5, 13.5; IR (neat): ν_max/cm⁻¹ 1719, 1587, 1480, 1381, 1223, 1086; HRMS (ESI): m/z calcd for C₁₄H₁₅O₃: 231.1016 [M + H]⁺; found 231.1016.

Methyl 2-(4′-methoxyphenyl)-5-methylfuran-3-carboxylate (2c). The crude product was purified by column chromatography on silica gel (5% ethyl acetate in petroleum ether) to give 2c as an orange oil; yield 75%; ¹H NMR (400 MHz, CDCl₃): δ 7.98–7.94 (m, 2H), 6.99–6.95 (m, 2H), 6.41 (q, J = 1.0 Hz, 1H), 3.87 (s, 3H), 3.83 (s, 3H), 2.35 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.4, 160.2, 156.4, 150.5, 129.6, 122.7, 113.5, 112.8, 108.5, 55.3, 51.4, 13.3; IR (neat): ν_max/cm⁻¹ 1717, 1609, 1580, 1504, 1440, 1378, 1303, 1210; HRMS (ESI): m/z calcd for C₁₄H₁₅O₄: 247.0965 [M + H]⁺; found 247.0962.

Ethyl 5-methyl-2-(4′-nitrophenyl)furan-3-carboxylate (2d). The crude product was purified by column chromatography on silica gel (3% ethyl acetate in petroleum ether) to give 2d as a yellow solid; yield 74%; mp 72 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.25–8.20 (m, 4H), 6.50 (q, J = 1.0 Hz, 1H), 4.31 (q, J = 7.2 Hz, 2H), 2.38 (d, J = 1.0 Hz, 1H), 1.35 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 163.4, 153.0, 152.8, 147.3, 135.8, 128.4, 123.5, 117.7, 110.0, 61.0, 14.3, 13.5; IR (KBr): ν_max/cm⁻¹ 1724, 1598, 1569, 1516, 1354, 1313, 1300, 1281, 1260, 1215, 1095, 1028; HRMS (ESI): m/z calcd for C₁₄H₁₄NO₅: 276.0866 [M + H]⁺; found 276.0865.

Methyl 5-methyl-2-(3′-trifluoromethylphenyl)furan-3-carboxylate (2e). The crude product was purified by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 2e as an orange oil; yield 69%; ¹H NMR (400 MHz, CDCl₃): δ 8.27 (s, 1H), 8.22 (d, J = 7.8 Hz, 1H), 7.61 (d, J = 7.8 Hz, 1H), 7.53 (t, J = 7.8 Hz, 1H), 6.46 (s, 1H), 3.83 (s, 3H), 2.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 163.9, 154.1, 151.9, 131.1 (q, J_F–C = 1.2 Hz), 130.6, 130.5 (q, J_F–C = 32.3 Hz), 128.5, 125.3 (q, J_F–C = 3.7 Hz), 124.8 (q, J_F–C = 4.0 Hz), 124.0 (q, J_F–C = 270.8 Hz), 115.3, 109.1, 51.7, 13.4; IR (neat): ν_max/cm⁻¹ 1722, 1558, 1439, 1380, 1330, 1268, 1215, 1168, 1127, 1102, 1030; HRMS (ESI): m/z calcd for C₁₄H₁₂F₃O₃: 285.0733 [M + H]⁺; found 285.0723.

Methyl 2-(2′-bromophenyl)-5-methylfuran-3-carboxylate (2f). The crude product was purified by column chromatography on silica gel (2% ethyl acetate in petroleum ether) to give 2f as a colorless oil; yield 85%; ¹H NMR (400 MHz, CDCl₃): δ 7.57 (m, 1H), 7.37 (m, 1H), 7.29 (m, 1H), 7.20 (m, 1H), 6.36 (q, J = 1.0 Hz, 1H), 3.62 (s, 3H), 2.28 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 163.8, 155.0, 152.3, 132.9, 132.3, 132.1, 130.8, 126.9, 124.0, 116.7, 107.2, 51.6, 13.6; IR (neat): ν_max/cm⁻¹ 1723, 1570, 1467, 1438, 1383, 1275, 1209, 1102, 1058; HRMS (ESI): m/z calcd for C₁₃H₁₂⁷⁹BrO₃: 294.9964 [M + H]⁺; found 294.9963.

Methyl 2-(2′,4′-dimethoxyphenyl)-5-methylfuran-3-carboxylate (2g). The crude product was purified by column chromatography on silica gel (7% ethyl acetate in petroleum ether) to give 2g as a colorless oil; yield 38%; ¹H NMR (400 MHz, CDCl₃): δ 7.36 (d, J = 8.4 Hz, 1H), 6.56–6.49 (m, 2H), 6.37 (q, J = 1.0 Hz, 1H), 3.83 (s, 3H), 3.76 (s, 3H), 3.70 (s, 3H), 2.32 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.4, 161.9, 158.5, 153.5, 151.1, 138.8, 131.9, 115.5, 107.2, 104.3, 98.7, 55.6, 55.4, 51.2, 13.4; IR (neat): ν_max/cm⁻¹ 1719, 1623, 1582, 1504, 1438, 1280, 1209, 1090, 1032; HRMS (ESI): m/z calcd for C₁₅H₁₇O₅: 277.1071 [M + H]⁺; found 277.1073.

Methyl 2-(6′-methoxynaphthalen-2′-yl)-5-methylfuran-3-carboxylate (2h). The crude product was purified by column chromatography on silica gel (10% ethyl acetate in petroleum ether) to give 2h as a yellow solid; yield 65%; mp 77 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.46 (m, 1H), 8.03 (m, 2H), 7.81 (m, 1H), 7.76 (m, 1H), 7.17–7.13 (m, 2H), 6.45 (q, J = 1.0 Hz, 1H), 3.94 (s, 3H), 3.83 (s, 3H), 2.38 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.4, 158.4, 156.4, 151.0, 134.7, 130.3, 128.4, 127.6, 126.4, 125.9, 119.1, 113.8, 108.8, 105.6, 55.3, 51.5, 29.7, 13.4; IR (KBr): ν_max/cm⁻¹ 1714, 1626, 1602, 1548, 1496, 1392, 1278, 1259, 1202, 1164, 1092, 1031; HRMS (ESI): m/z calcd for C₁₈H₁₆NaO₄: 319.0941 [M + Na]⁺; found 319.0943.

Methyl 5-methyl-2-(N-tosyl-1H-pyrrol-3′-yl)furan-3-carboxylate (2i). The crude product was purified by column chromatography on silica gel (10% ethyl acetate in petroleum ether) to give 2i as a colorless solid; yield 68%; mp 111 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.14 (m, 1H), 7.84–7.82 (m, 2H), 7.31 (m, 2H), 7.18 (m, 1H), 6.94 (m, 1H), 6.33 (q, J = 1.2 Hz, 1H), 3.85 (s, 3H), 2.41 (s, 3H), 2.31 (d, J = 1.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.2, 151.6, 150.4, 145.3, 135.9, 130.2, 127.2, 120.8, 119.1, 113.0, 112.8, 108.1, 51.5, 21.8, 13.4; IR (KBr): ν_max/cm⁻¹ 1713, 1649, 1620, 1594, 1439, 1366, 1293, 1266, 1258, 1189, 1173, 1086; HRMS (ESI): m/z calcd for C₁₈H₁₇NNaO₅S: 382.0720 [M + Na]⁺; found 382.0729.

Diethyl 5,5′′-dimethyl-[2,2′:5′,2′′-terfuran]-3,3′′-dicarboxylate (2j). The crude product was purified by column chromatography on silica gel (2% ethyl acetate in petroleum ether) to give 2j as a yellow solid; yield 64%; mp 80 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.60 (s, 2H), 6.43 (q, J = 1.2 Hz, 2H), 4.33 (q, J = 7.1 Hz, 4H), 2.40 (d, J = 1.2 Hz, 6H), 1.37 (t, J = 7.1 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 163.2, 151.8, 147.0, 144.8, 144.7, 114.6, 108.7, 60.6, 14.5, 13.7; IR (KBr): ν_max/cm⁻¹ 1713, 1607, 1538, 1378, 1261, 1215, 1100, 1052, 1024; HRMS (ESI): m/z calcd for C₂₀H₂₀NaO₇: 395.1101 [M + Na]⁺; found 395.1100.

Ethyl 2,5-dimethylfuran-3-carboxylate (2k). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 2k as a yellow oil; yield 88%; ¹H NMR (400 MHz, CDCl₃): δ 6.20 (q, J = 0.8 Hz, 1H), 4.25 (q, J = 7.1 Hz, 2H), 2.51 (s, 3H), 2.23 (d, J = 0.8 Hz, 3H), 1.32 (t, J = 7.1 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.5, 157.7, 150.0, 114.1, 106.3, 60.0, 14.5, 13.8, 13.3; IR (neat): ν_max/cm⁻¹ 1716, 1624, 1590, 1406, 1283, 1231, 1206, 1083; HRMS (ESI): m/z calcd for C₉H₁₃O₃: 169.0859 [M + H]⁺; found 169.0858.

Ethyl 2-isopropyl-5-methylfuran-3-carboxylate (2l). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 2k as a colorless oil; yield 55%; ¹H NMR (400 MHz, CDCl₃): δ 6.19 (q, J = 1.0 Hz, 1H), 4.25 (q, J = 7.1 Hz, 2H), 3.71 (heptet, J = 7.0 Hz, 1H), 2.24 (d, J = 1.0 Hz, 3H), 1.32 (t, J = 7.1 Hz, 3H), 1.24 (d, J = 7.0 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 165.9, 164.4, 149.8, 112.1, 106.2, 60.0, 27.2, 21.0, 14.5, 13.4; IR (neat): ν_max/cm⁻¹ 1724, 1469, 1250, 1214, 1069, 1025; HRMS (ESI): m/z calcd for C₁₁H₁₇O₃: 197.1172 [M + H]⁺; found 197.1173.

Methyl 5-ethyl-2-(4′-methylphenyl)furan-3-carboxylate (2m). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 2m as a yellow oil; yield 80%; ¹H NMR (400 MHz, CDCl₃): δ 7.85 (d, J = 8.3 Hz, 2H), 7.23 (d, J = 8.3 Hz, 2H), 6.41 (t, J = 0.9 Hz, 1H), 3.81 (s, 3H), 2.69 (qd, J = 7.6, 0.9 Hz, 2H), 2.39 (s, 3H), 1.28 (t, J = 7.6 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.5, 156.5, 156.4, 139.2, 128.9, 128.2, 127.4, 113.5, 107.2, 51.6, 21.6, 21.3, 12.0; IR (neat): ν_max/cm⁻¹ 1721, 1504, 1290, 1233, 1099; HRMS (ESI): m/z calcd for C₁₅H₁₇O₃: 245.1172 [M + H]⁺; found 245.1172.

Methyl 2-(4′-methylphenyl)-4,5,6,7-tetrahydrobenzofuran-3-carboxylate (2n). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 2n as a colorless oil; yield 52%; ¹H NMR (400 MHz, CDCl₃): δ 7.72 (d, J = 8.2 Hz, 2H), 7.21 (d, J = 8.2 Hz, 2H), 3.79 (s, 3H), 2.66–2.61 (m, 4H), 2.38 (s, 3H), 1.88–1.73 (m, 4H); ¹³C NMR (100 MHz, CDCl₃): δ 165.2, 156.3, 150.7, 138.9, 128.8, 128.3, 127.9, 119.1, 112.8, 51.3, 23.2, 23.0, 22.8, 22.7, 21.6; IR (neat): ν_max/cm⁻¹ 1720, 1557, 1501, 1438, 1283, 1217, 1095, 1037; HRMS (ESI): m/z calcd for C₁₇H₁₉O₃: 271.1329 [M + H]⁺; found 271.1328.

Ethyl 5-benzyl-2-isopropylfuran-3-carboxylate (2o). The crude product was purified by column chromatography on silica gel (0.3% ethyl acetate in petroleum ether) to give 2o as a colorless oil; yield 89%; ¹H NMR (400 MHz, CDCl₃): δ 7.33–7.22 (m, 5H), 6.18 (s, 1H), 4.24 (q, J = 7.2 Hz, 2H), 3.91 (s, 1H), 3.72 (heptet, J = 7.0 Hz, 1H), 1.30 (t, J = 7.2 Hz, 3H), 1.25 (d, J = 7.0 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 166.4, 164.2, 152.3, 137.7, 128.8, 128.6, 126.7, 112.2, 107.0, 60.0, 34.3, 27.3, 20.9, 14.5; IR (neat): ν_max/cm⁻¹ 1715, 1576, 1496, 1468, 1455, 1380, 1208, 1059; HRMS (ESI): m/z calcd for C₁₇H₂₁O₃: 273.1485 [M + H]⁺; found 273.1484.

General procedure for the synthesis of 2,4-disubstituted furan-3-carboxylates (3a–o)

Cesium carbonate (0.75 mmol, 244 mg) was added to a solution of 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates (1a–o) (1.0 mmol) in anhydrous DMF (15 mL). The resulting mixture was stirred at 80 °C until the substrate was consumed (monitored by TLC). After being cooled to ambient temperature, the mixture was treated with 1 M HCl (5 mL) and filtered. The filtrate was extracted with ethyl acetate (10 mL × 3). The combined organic extracts were washed with brine (10 mL), then dried over anhydrous sodium sulfate, filtered and concentrated in vacuo. The residue was purified by column chromatography on silica gel to afford 2,3,4-trisubstituted furans 3a–o.

Methyl 4-methyl-2-phenylfuran-3-carboxylate (3a)²⁴. The crude product was purified by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 3a as a colorless oil; yield 92%; ¹H NMR (400 MHz, CDCl₃): δ 7.78–7.76 (m, 2H), 7.43–7.37 (m, 3H), 7.24 (s, 1H), 3.80 (s, 3H), 2.19 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.1, 158.1, 139.3, 130.5, 129.2, 128.5, 128.1, 122.6, 114.1, 51.4, 10.3; IR (neat): ν_max/cm⁻¹ 1717, 1490, 1436, 1290, 1066; HRMS (ESI): m/z calcd for C₁₃H₁₃O₃: 217.0859 [M + H]⁺; found 217.0859.

Methyl 4-methyl-2-(4′-methylphenyl)furan-3-carboxylate (3b)²⁴. The crude product was purified by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 3b as an orange oil; yield 90%; ¹H NMR (400 MHz, CDCl₃): δ 7.68–7.66 (m, 2H), 7.24–7.22 (m, 3H), 3.81 (s, 3H), 2.39 (s, 3H), 2.20 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.1, 158.4, 139.2, 138.9, 128.8, 128.3, 127.6, 122.5, 113.5, 51.3, 21.5, 10.3; IR (neat): ν_max/cm⁻¹ 1722, 1590, 1480, 1377, 1223, 1090; HRMS (ESI): m/z calcd for C₁₄H₁₅O₃: 231.1016 [M + H]⁺; found 231.1018.

Methyl 2-(4′-methoxyphenyl)-4-methylfuran-3-carboxylate (3c). The crude product was purified by column chromatography on silica gel (1% ethyl acetate in petroleum ether) to give 3c as a yellow solid; yield 85%; mp 70 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.76–7.73 (m, 2H), 6.41 (q, J = 1.0 Hz, 1H), 6.96–6.92 (m, 2H), 3.84 (s, 3H), 3.80 (s, 3H), 2.18 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.1, 160.3, 158.5, 138.7, 130.0, 123.1, 122.5, 113.6, 113.0, 55.4, 51.3, 10.4; IR (KBr): ν_max/cm⁻¹ 1717, 1608, 1501, 1438, 1254, 1177, 1074, 1028; HRMS (ESI): m/z calcd for C₁₄H₁₄NaO₄: 269.0784 [M + Na]⁺; found 269.0784.

Ethyl 4-methyl-2-(4′-nitrophenyl)furan-3-carboxylate (3d). The crude product was purified by column chromatography on silica gel (3% ethyl acetate in petroleum ether) to give 3d as a yellow solid; yield 58%; mp 83 °C; ¹H NMR (400 MHz, CD₃OD): δ 8.28 (m, 2H), 8.04 (m, 2H), 7.53 (s, 1H), 4.33 (q, J = 6.8 Hz, 2H), 2.22 (s, 3H), 1.33 (t, J = 6.8 Hz, 3H); ¹³C NMR (100 MHz, CD₃OD): δ 165.3, 155.9, 149.0, 142.4, 137.4, 130.0, 124.5, 124.3, 117.9, 61.9, 14.4, 10.1; IR (neat): ν_max/cm⁻¹ 1712, 1616, 1591, 1545, 1520, 1488, 1474, 1382, 1348, 1297, 1280, 1100, 1072, 1024; HRMS (ESI): m/z calcd for C₁₄H₁₃NNaO₅: 298.0686 [M + Na]⁺; found 298.0688.

Methyl 4-methyl-2-(3′-trifluoromethylphenyl)furan-3-carboxylate (3e). The crude product was purified by column chromatography on silica gel (1% ethyl acetate in petroleum ether) to give 3e as an orange oil; yield 65%; ¹H NMR (400 MHz, CDCl₃): δ 8.08 (s, 1H), 8.00 (d, J = 7.9 Hz, 1H), 7.63 (d, J = 7.9 Hz, 1H), 7.53 (t, J = 7.9 Hz, 1H), 7.29 (s, 1H), 3.82 (s, 3H), 2.21 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.7, 156.2, 140.0, 131.6 (q, J_F–C = 1.2 Hz), 131.2, 130.6 (q, J_F–C = 32.3 Hz), 128.6, 125.6 (q, J_F–C = 3.8 Hz), 125.4 (q, J_F–C = 4.0 Hz), 124.1 (q, J_F–C = 271.0 Hz), 123.0, 115.1, 51.5, 10.2; IR (neat): ν_max/cm⁻¹ 1720, 1604, 1547, 1479, 1439, 1381, 1331, 1283, 1213, 1168, 1128, 1073, 1001; HRMS (ESI): m/z calcd for C₁₄H₁₁F₃NaO₃: 307.0553 [M + Na]⁺; found 307.0558.

Methyl 2-(2′-bromophenyl)-4-methylfuran-3-carboxylate (3f). The crude product was purified by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 3f as a colorless oil; yield 80%; ¹H NMR (400 MHz, CDCl₃): δ 7.65 (m, 1H), 7.45 (m, 1H), 7.37 (m, 1H), 7.28 (m, 1H), 6.44 (q, J = 1.0 Hz, 1H), 3.70 (s, 3H), 2.36 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 163.8, 155.0, 152.3, 133.0, 132.3, 132.1, 130.8, 126.9, 124.1, 116.8, 107.2, 51.6, 13.6; IR (neat): ν_max/cm⁻¹ 1722, 1570, 1467, 1438, 1383, 1276, 1209, 1102, 1058, 1012; HRMS (ESI): m/z calcd for C₁₃H₁₂⁷⁹BrO₃: 294.9964 [M + H]⁺; found 294.9965.

Methyl 2-(2′,4′-dimethoxyphenyl)-4-methylfuran-3-carboxylate (3g). The crude product was purified by column chromatography on silica gel (2% ethyl acetate in petroleum ether) to give 3g as an orange oil; yield 82%; ¹H NMR (400 MHz, CDCl₃): δ 7.36 (d, J = 8.4 Hz, 1H), 7.22 (q, J = 1.0 Hz, 1H), 6.54 (dd, J = 8.4, 2.3 Hz, 1H), 6.49 (d, J = 2.3 Hz, 1H), 3.82 (s, 3H), 3.75 (s, 3H), 3.69 (s, 3H), 2.18 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.2, 161.9, 158.2, 154.9, 138.9, 131.4, 121.6, 115.4, 113.0, 104.4, 98.6, 55.5, 55.4, 51.1, 9.7; IR (neat): ν_max/cm⁻¹ 1717, 1618, 1504, 1438, 1292, 1211, 1161, 1090, 1032; HRMS (ESI): m/z calcd for C₁₅H₁₇O₅: 277.1071 [M + H]⁺; found 277.1071.

Methyl 2-(6′-methoxynaphthalen-2′-yl)-4-methylfuran-3-carboxylate (3h). The crude product was purified by column chromatography on silica gel (1% ethyl acetate in petroleum ether) to give 3h as a colorless solid; yield 80%; mp 114 °C; ¹H NMR (400 MHz, CDCl₃): δ 8.24 (s, 1H), 7.84–7.73 (m, 3H), 7.28 (q, J = 1.0 Hz, 1H), 7.17–7.13 (m, 2H), 3.93 (s, 3H), 3.82 (s, 3H), 2.22 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.2, 158.6, 158.5, 139.2, 134.9, 130.0, 128.4, 128.0, 126.4, 126.3, 125.7, 122.7, 119.3, 113.8, 105.7, 55.5, 51.4, 10.4; IR (neat): ν_max/cm⁻¹ 1713, 1627, 1592, 1537, 1504, 1479, 1458, 1392, 1378, 1302, 1287, 1273, 1258, 1202, 1164, 1119, 1093, 1069, 1029; HRMS (ESI): m/z calcd for C₁₈H₁₆NaO₄: 319.0946 [M + Na]⁺; found 319.0943.

Methyl 4-methyl-2-(N-tosyl-1H-pyrrol-3′-yl)furan-3-carboxylate (3i). The crude product was purified by column chromatography on silica gel (7% ethyl acetate in petroleum ether) to give 3i as an orange oil; yield 78%; ¹H NMR (400 MHz, CDCl₃): δ 8.06 (m, 1H), 7.80 (d, J = 8.4 Hz, 2H), 7.30 (d, J = 8.4 Hz, 2H), 7.16 (m, 1H), 7.11 (q, J = 1.0 Hz, 1H), 6.84 (m, 1H), 3.85 (s, 3H), 2.40 (s, 3H), 2.15 (d, J = 1.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 164.9, 153.7, 145.4, 138.3, 135.9, 130.3, 127.2, 122.3, 121.1, 120.7, 119.3, 113.1, 112.9, 51.4, 21.8, 10.6; IR (neat): ν_max/cm⁻¹ 1713, 1606, 1538, 1494, 1474, 1454, 1438, 1373, 1303, 1231, 1214, 1174, 1117, 1018; HRMS (ESI): m/z calcd for C₁₈H₁₇NNaO₅S: 382.0720 [M + Na]⁺; found 382.0723.

Diethyl 4,4′′-dimethyl-[2,2′:5′,2′′-terfuran]-3,3′′-dicarboxylate (3j). The crude product was purified by column chromatography on silica gel (5% ethyl acetate in petroleum ether) to give 3j as a yellow solid; yield 60%; mp 175 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.54 (s, 2H), 7.27 (q, J = 1.2 Hz, 2H), 4.37 (q, J = 3.6 Hz, 4H), 2.21 (d, J = 1.2 Hz, 6H), 1.39 (t, J = 3.6 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 163.8, 149.1, 145.1, 139.7, 122.7, 114.8, 114.2, 60.6, 14.4, 10.5; IR (KBr): ν_max/cm⁻¹ 1701, 1600, 1523, 1307, 1231, 1122, 1101, 1024; HRMS (ESI): m/z calcd for C₂₀H₂₀NaO₇: 395.1101 [M + Na]⁺; found 395.1098.

Ethyl 2,4-dimethylfuran-3-carboxylate (3k). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 3k as a colorless oil; yield 85%; ¹H NMR (400 MHz, CDCl₃): δ 7.01 (q, J = 1.1 Hz, 1H), 4.27 (q, J = 7.2 Hz, 2H), 2.52 (s, 3H), 2.12 (d, J = 1.1 Hz, 3H), 1.34 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.0, 160.2, 137.7, 121.3, 113.6, 59.9, 14.5, 14.5, 10.2; IR (neat): ν_max/cm⁻¹ 1716, 1611, 1563, 1418, 1385, 1298, 1272, 1099; HRMS (ESI): m/z calcd for C₉H₁₃O₃: 169.0859 [M + H]⁺; found 169.0858.

Ethyl 2-isopropyl-4-methylfuran-3-carboxylate (3l). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 3l as a colorless oil; yield 76%; ¹H NMR (400 MHz, CDCl₃): δ 7.03 (q, J = 1.2 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 3.71 (heptet, J = 7.0 Hz, 1H), 2.12 (d, J = 1.2 Hz, 3H), 1.34 (t, J = 7.1 Hz, 3H), 1.23 (d, J = 7.0 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 168.0, 164.9, 137.7, 121.0, 111.8, 59.9, 27.7, 20.8, 14.4, 10.2; IR (neat): ν_max/cm⁻¹ 1718, 1560, 1282, 1210, 1065; HRMS (ESI): m/z calcd for C₁₁H₁₇O₃: 197.1172 [M + H]⁺; found 197.1173.

Methyl 4-ethyl-2-(4′-methylphenyl)furan-3-carboxylate (3m). The crude product was purified by column chromatography on silica gel (0.3% ethyl acetate in petroleum ether) to give 3m as a yellow oil; yield 60%; ¹H NMR (400 MHz, CDCl₃): δ 7.65 (d, J = 8.2 Hz, 2H), 7.24–7.22 (m, 3H), 3.80 (s, 3H), 2.67 (qd, J = 7.4, 1.0 Hz, 2H), 2.39 (s, 3H), 1.23 (t, J = 7.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 165.1, 158.5, 139.2, 138.3, 129.3, 128.9, 128.4, 127.8, 112.9, 51.4, 21.5, 18.5, 13.8; IR (neat): ν_max/cm⁻¹ 1716, 1500, 1437, 1293, 1208, 1077; HRMS (ESI): m/z calcd for C₁₅H₁₇O₃: 245.1172 [M + H]⁺; found 245.1170.

Methyl 2-(4′-methylphenyl)-4,5,6,7-tetrahydrobenzofuran-3-carboxylate (2n). The crude product was purified by column chromatography on silica gel (petroleum ether) to give 2n as a colorless oil; yield 11%; ¹H NMR (400 MHz, CDCl₃): δ 7.72 (d, J = 8.2 Hz, 2H), 7.21 (d, J = 8.2 Hz, 2H), 3.79 (s, 3H), 2.66–2.61 (m, 4H), 2.38 (s, 3H), 1.88–1.73 (m, 4H); ¹³C NMR (100 MHz, CDCl₃): δ 165.2, 156.3, 150.7, 138.9, 128.8, 128.3, 127.9, 119.1, 112.8, 51.3, 23.2, 23.0, 22.8, 22.7, 21.6; IR (neat): ν_max/cm⁻¹ 1720, 1557, 1501, 1438, 1283, 1217, 1095, 1037; HRMS (ESI): m/z calcd for C₁₇H₁₉O₃: 271.1329 [M + H]⁺; found 271.1328.

Ethyl 4-benzyl-2-isopropylfuran-3-carboxylate (3o). The crude product was purified by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 3o as a colorless oil; yield 71%; ¹H NMR (400 MHz, CDCl₃): δ 7.32–7.21 (m, 5H), 6.18 (s, 1H), 4.23 (q, J = 7.1 Hz, 2H), 3.91 (s, 1H), 3.73 (heptet, J = 7.0 Hz, 1H), 1.30 (t, J = 7.1 Hz, 3H), 1.25 (d, J = 7.0 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 166.4, 164.2, 152.3, 137.6, 128.8, 128.6, 126.7, 112.2, 107.0, 60.0, 34.3, 27.3, 20.9, 14.4; IR (neat): ν_max/cm⁻¹ 1715, 1576, 1497, 1468, 1455, 1381, 1208, 1059; HRMS (ESI): m/z calcd for C₁₇H₂₁O₃: 273.1485 [M + H]⁺; found 273.1483.

Acknowledgements

We are grateful to Zhengzhou University (#1421316040) and the NSFC (#81330075; #21172202) for financial support.

References and notes

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Footnote

† Electronic supplementary information (ESI) available: CCDC 1413437. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra14273c

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