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

Article abstract of DOI:10.1039/c5ra14273c

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 is

Full text of DOI:10.1039/c5ra14273c

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Selective synthesis of 2,5-disubstituted furan-3-
carboxylates and the isomeric 2,4-disubstituted
furan-3-carboxylates†
Cite this: RSC Adv., 2015, 5, 79906
a
a
b
a
Panpan Chen, Yinggao Meng, Qinghua Yang, Jie Wu, Yuanyuan Xiao,a
*
*
c
a
a
*
Dhilli Rao Gorja, Chuanjun Song and Junbiao Chang
Received 20th July 2015
Accepted 15th September 2015
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.
DOI: 10.1039/c5ra14273c
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Introduction
This article can be cited before page numbers have been issued, to do this please use: P. chen, Y. Meng,
Furans are a fundamental class of ve-membered heterocycles.
Q. Yang, J. Wu, Y. Xiao, D. R. Gorja, C. Song and J. Chang, RSC Adv., 2015, DOI: 10.1039/C5RA14273C.
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 anti-
Fig. 1 Representative furan natural products.
allergic,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
Scheme 1 Proposed silver-mediated cyclization to form 2,5-disub-
Transition metals, PhI(O2CCF3)2, PhSeSePh, NBS and even
iodine could be used as catalysts for these transformations to
obtain highly substituted furans.
stituted furan-3-carboxylates.
Over the last decades, silver-mediated cyclization for the
construction of heterocyclic compounds has received consid-
erable attention.11 Recently, Lei and co-worker have developed
useful silver-mediated oxidative cross-coupling/cyclization
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-prop-
anoates,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 rst
silver-mediated cyclization of 3-substituted 2-(2-bromoallyl)-3-
oxo-1-carboxylates to poly-substituted furans.
strategies
between
2-aminopyridines,12
1,3-dicarbonyl
compounds,13 b-enamino esters14 and terminal alkynes to create
polysubstituted imidazole[1,2-a]pyridines, furans, and pyrroles,
aCollege of Chemistry and Molecular Engineering, Zhengzhou University, 100 Science
Avenue, Zhengzhou 450001, P. R. China. E-mail: changjunbiao@zzu.edu.cn; chjsong@
zzu.edu.cn; wujie@zzu.edu.cn
bSchool of Pharmaceutical Sciences, Zhengzhou University, 100 Science Avenue,
Zhengzhou 450001, P. R. China
cChina-US (Henan) Hormel Cancer Institute, 127 Dongming Road, Zhengzhou 450003,
P. R. China
Results and discussion
† Electronic supplementary information (ESI) available: CCDC 1413437. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra14273c
Initial study was carried out with methyl 2-benzoyl-4-
bromopent-4-enoate 1a, which was treated with 2 equivalent
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of DBU and a catalytic amount of Ag2CO3 (0.05 eq.) at 60 ꢀC shorter time and 3a was isolated in good yield (entry 12). Even
(entry 1, Table 1). When the reaction was complete (6 h), two better result was obtained when Cs2CO3 was used (entry 13).
main spots appeared on the TLC plate. Aer being separated by With Cs2CO3 as our optimized base, we next investigated the
column chromatography, the two products were found to be the effect of the reaction temperature. The reaction proceeded
desired furan 2a and the isomerized product 3a, respectively. slowly at temperatures below 60 ꢀC while the yield of 3a was
Inspired by the results, we then optimized the reaction condi- moderate (entries 14 and 15). By contrast, at temperatures
tions in order to obtain either regioisomer selectively.
above 60 ꢀC, the reaction proceeded much faster and the
In the presence of 2 equivalent of DBU, the ratio of 2a : 3a product yield was higher (entries 16–18). When the reaction was
ꢀ
improved with increased loading of Ag2CO3 (entries 1 and 3–6, conducted at 80 C, 3a was isolated in the highest yield (92%)
Table 1). When 0.5 equivalent of Ag2CO3 was used, 2a was iso- (entry 17). Slightly less satisfactory result was obtained when the
lated as the sole product in 83% yield (entry 6). The ratio of reaction temperature was raised to 90 ꢀC (entry 18). Finally, we
2a : 3a decreased either changing the base from DBU to Cs2CO3 inspected the effect of the quantities of Cs2CO3 to the reaction
(entry 7 vs. 6), or changing the silver salt from Ag2CO3 to (entries 17 and 19–21) and found that the reaction could work
AgNO3/silica gel (entry 2 vs. 1). In the absence of a base, no equally well when as low as 0.75 equivalent of the base was
reaction occurred at all (entry 8). When silver salt was removed applied (entry 20). Further reduction of the amount of Cs2CO3
from the reaction system, isomer 3a was the only product resulted in reduced yield of the product (entry 21).
obtained although the reaction proceeded slowly and only
Having the optimized reaction conditions in hand, we then
moderate yield was obtained (entry 9). Inspired by the ndings tested the generality and scope of the present 2,5-disubstituted
and in order to obtain 3a selectively, we then explored other furan-3-carboxylates and 2,4-disubstituted furan-3-carboxylates,
bases. Similar to DBU, when conducted in the presence of and the results were collected in Table 2. In the presence of 0.5
K3PO4 or NaHCO3, the reaction was sluggish and 3a was iso- equivalent of Ag2CO3 and 2 equivalent of DBU, the reactions of
lated in a moderate yield (entries 10 and 11). To our delight, 3-aryl substituted substrates 1 bearing either an electron-
when K2CO3 was applied, the reaction was complete in much donating group (entries 2 and 3) or electron-withdrawing
Table 1 Optimization of the reaction conditions for the selective formation of 2a or 3a, respectivelya
Entry
Base
Silver salt
Temperature (ꢀC)
Reaction time (h)
Ratio of 2a : 3a
Yield (%)
1
2
3
4
5
6
7
8
DBU (2)b
Ag2CO3 (0.05)b
60
60
60
60
60
60
60
60
60
60
60
60
60
40
50
70
80
90
80
80
80
6
6
5.5
4
3.5
1
1
20
12
25
22
4
3
9
8
1
0.7
0.6
0.7
0.7
0.7
1 : 3c
DBU (2)b
AgNO3/silica gel (0.05)b
1 : 8c
DBU (2)b
Ag2CO3 (0.25)b
1 : 0.15c
1 : 0.03c
1 : 0.02c
1 : 0
DBU (2)b
Ag2CO3 (0.35)b
DBU (2)b
Ag2CO3 (0.4)b
DBU (2)b
Ag2CO3 (0.5)b
83e
Cs2CO3 (2)b
—
Ag2CO3 (0.5)b
1 : 0.14c
Ag2CO3 (0.5)b
—
d
9
DBU (2)b
—
—
—
—
—
—
—
—
—
—
—
—
—
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
0 : 1
45e
60e
40e
74e
78e
46e
70e
85e
92e
89e
92e
92e
70e
10
11
12
13
14
15
16
17
18
19
20
21
K3PO4 (2)b
NaHCO3 (2)b
K2CO3 (2)b
Cs2CO3 (2)b
Cs2CO3 (2)b
Cs2CO3 (2)b
Cs2CO3 (2)b
Cs2CO3 (2)b
Cs2CO3 (2)b
Cs2CO3 (1)b
Cs2CO3 (0.75)b
Cs2CO3 (0.5)b
Anhydrous DMF was found to be the most appropriate solvent through screening. b Numbers in parenthesis indicated the equivalents of reagents
a
used. c Ratio determined on the basis of H NMR integration of the crude reaction mixture. d No reaction. e Isolated yields.
1
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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
1
Substrate
2,5-Disubstituted furan-3-carboxylates
2,4-Disubstituted furan-3-carboxylates
2
3
4
5
6
7
8
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Table 2 (Contd.)
Entry
9
Substrate
2,5-Disubstituted furan-3-carboxylates
2,4-Disubstituted furan-3-carboxylates
10
11
12
13
14
15
a
75% of the unreacted 1n was recovered.
group (entries 4–6) at the phenyl ring proceeded smoothly to ring gave 2g in poor yield (entry 7). Under the reaction condi-
give 2,5-disubstituted furan-3-carboxylates 2b–f in good yields. tions, both substrates bearing naphthalyl and protected pyrrolyl
However, reaction of substrate 1g having a 2,4-dimethoxyphenyl group (1h and 1i) could cyclize to afford good yields of the
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gave methylenecyclopropane III.20 Activation of the double bond
in III with silver7b,21 (path a) followed by concomitant cyclopro-
pane ring opening and cyclization would generate V.22 b-Elimi-
nation 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 ring20,23 (path b) would generate VII, which
tautomerized to produce 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.
Fig. 2 X-ray single crystal structure of 2i.
expected products 2h and 2i, respectively (entries 8 and 9). The
structure of 2i was unambiguously conrmed by X-ray single
crystal diffraction (Fig. 2).19 Aliphatic b-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 con-
verted 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. Aer 25 h, the desired product 2n was isolated in 11%
yield together with substantial amount (75%) of the unreacted
starting material recovered.
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
Based on the above results and literature reports, plausible
mechanisms for the regioselective formation of 2,5-disubsti-
tuted 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. Intra-
molecular attack of the allene function with the enolate carbon
ꢀ
mixture was stirred at 60 C until the substrate was consumed
(monitored by TLC). Aer being cooled to ambient temperature,
the mixture was treated with 1 M HCl (5 mL) and ltered. The
ltrate was extracted with ethyl acetate (10 mL ꢁ 3). The
combined organic extracts were washed with brine (10 mL),
then dried over anhydrous sodium sulphate, ltered and
Scheme 2 Proposed mechanism for the regioselective formation of 2,5-disubstituted furan-3-carboxylates 2 and 2,4-disubstituted furan-3-
carboxylates 3.
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1
concentrated in vacuo. The residue was puried by column colorless oil; yield 85%; H NMR (400 MHz, CDCl3): d 7.57 (m,
chromatography on silica gel to afford 2,3,5-trisubstituted 1H), 7.37 (m, 1H), 7.29 (m, 1H), 7.20 (m, 1H), 6.36 (q, J ¼ 1.0 Hz,
furans 2a–o.
1H), 3.62 (s, 3H), 2.28 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz,
Methyl 5-methyl-2-phenylfuran-3-carboxylate (2a).6a The CDCl3): d 163.8, 155.0, 152.3, 132.9, 132.3, 132.1, 130.8, 126.9,
crude product was puried by column chromatography on silica 124.0, 116.7, 107.2, 51.6, 13.6; IR (neat): nmax/cmꢂ1 1723, 1570,
gel (1% ethyl acetate in petroleum ether) to give 2a as a colorless 1467, 1438, 1383, 1275, 1209, 1102, 1058; HRMS (ESI): m/z calcd
oil; yield 83%; 1H NMR (400 MHz, CDCl3): d 7.97–7.95 (m, 2H), for C13H1279BrO3: 294.9964 [M + H]+; found 294.9963.
7.44–7.35 (m, 3H), 6.43 (s, 1H), 3.81 (s, 3H), 2.35 (s, 3H); 13C
Methyl 2-(20,40-dimethoxyphenyl)-5-methylfuran-3-carboxylate
NMR (100 MHz, CDCl3): d 164.4, 156.2, 151.3, 130.1, 129.1, (2g). The crude product was puried by column chromatography
128.1, 114.2, 108.8, 51.6, 51.4, 13.5; IR (neat): nmax/cmꢂ1 1721, on silica gel (7% ethyl acetate in petroleum ether) to give 2g as a
1559, 1492, 1435, 1378, 1274, 1212, 1097, 1016; HRMS (ESI): m/z colorless oil; yield 38%; 1H NMR (400 MHz, CDCl3): d 7.36 (d, J ¼
calcd for C13H13O3: 217.0859 [M + H]+; found 217.0859.
8.4 Hz, 1H), 6.56–6.49 (m, 2H), 6.37 (q, J ¼ 1.0 Hz, 1H), 3.83 (s, 3H),
Methyl 5-methyl-2-(40-methylphenyl)furan-3-carboxylate (2b).6a 3.76 (s, 3H), 3.70 (s, 3H), 2.32 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100
The crude product was puried by column chromatography on MHz, CDCl3): d 164.4, 161.9, 158.5, 153.5, 151.1, 138.8, 131.9, 115.5,
silica gel (0.5% ethyl acetate in petroleum ether) to give 2b as an 107.2, 104.3, 98.7, 55.6, 55.4, 51.2, 13.4; IR (neat): nmax/cmꢂ1 1719,
orange oil; yield 70%; 1H NMR (400 MHz, CDCl3): d 7.86 (d, J ¼ 8.0 1623, 1582, 1504, 1438, 1280, 1209, 1090, 1032; HRMS (ESI): m/z
Hz, 2H), 7.23 (d, J ¼ 8.0 Hz, 2H), 6.41 (s, 1H), 3.80 (s, 3H), 2.39 (s, calcd for C15H17O5: 277.1071 [M + H]+; found 277.1073.
3H), 2.34 (s, 3H); 13C NMR (100 MHz, CDCl3): d 164.4, 156.6, 150.9,
Methyl 2-(60-methoxynaphthalen-20-yl)-5-methylfuran-3-
139.2, 128.9, 128.1, 127.3, 113.6, 108.7, 51.6, 21.5, 13.5; IR (neat): carboxylate (2h). The crude product was puried by column
nmax/cmꢂ1 1719, 1587, 1480, 1381, 1223, 1086; HRMS (ESI): m/z chromatography on silica gel (10% ethyl acetate in petroleum
calcd for C14H15O3: 231.1016 [M + H]+; found 231.1016.
ether) to give 2h as a yellow solid; yield 65%; mp 77 ꢀC; 1H NMR
Methyl 2-(40-methoxyphenyl)-5-methylfuran-3-carboxylate (2c). (400 MHz, CDCl3): d 8.46 (m, 1H), 8.03 (m, 2H), 7.81 (m, 1H),
The crude product was puried by column chromatography on 7.76 (m, 1H), 7.17–7.13 (m, 2H), 6.45 (q, J ¼ 1.0 Hz, 1H), 3.94 (s,
silica gel (5% ethyl acetate in petroleum ether) to give 2c as an 3H), 3.83 (s, 3H), 2.38 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz,
orange oil; yield 75%; 1H NMR (400 MHz, CDCl3): d 7.98–7.94 (m, CDCl3): d 164.4, 158.4, 156.4, 151.0, 134.7, 130.3, 128.4, 127.6,
2H), 6.99–6.95 (m, 2H), 6.41 (q, J ¼ 1.0 Hz, 1H), 3.87 (s, 3H), 3.83 126.4, 125.9, 119.1, 113.8, 108.8, 105.6, 55.3, 51.5, 29.7, 13.4; IR
(s, 3H), 2.35 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d (KBr): nmax/cmꢂ1 1714, 1626, 1602, 1548, 1496, 1392, 1278, 1259,
164.4, 160.2, 156.4, 150.5, 129.6, 122.7, 113.5, 112.8, 108.5, 55.3, 1202, 1164, 1092, 1031; HRMS (ESI): m/z calcd for C18H16NaO4:
51.4, 13.3; IR (neat): nmax/cmꢂ1 1717, 1609, 1580, 1504, 1440, 1378, 319.0941 [M + Na]+; found 319.0943.
1303, 1210; HRMS (ESI): m/z calcd for C14H15O4: 247.0965 [M +
Methyl 5-methyl-2-(N-tosyl-1H-pyrrol-30-yl)furan-3-carboxylate
(2i). The crude product was puried by column chromato-
H]+; found 247.0962.
Ethyl 5-methyl-2-(40-nitrophenyl)furan-3-carboxylate (2d). graphy on silica gel (10% ethyl acetate in petroleum ether) to give
The crude product was puried by column chromatography on 2i as a colorless solid; yield 68%; mp 111 ꢀC; 1H NMR (400 MHz,
silica gel (3% ethyl acetate in petroleum ether) to give 2d as a CDCl3): d 8.14 (m, 1H), 7.84–7.82 (m, 2H), 7.31 (m, 2H), 7.18 (m,
yellow solid; yield 74%; mp 72 ꢀC; 1H NMR (400 MHz, CDCl3): d 1H), 6.94 (m, 1H), 6.33 (q, J ¼ 1.2 Hz, 1H), 3.85 (s, 3H), 2.41 (s,
8.25–8.20 (m, 4H), 6.50 (q, J ¼ 1.0 Hz, 1H), 4.31 (q, J ¼ 7.2 Hz, 3H), 2.31 (d, J ¼ 1.2 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 164.2,
2H), 2.38 (d, J ¼ 1.0 Hz, 1H), 1.35 (t, J ¼ 7.2 Hz, 3H); 13C NMR 151.6, 150.4, 145.3, 135.9, 130.2, 127.2, 120.8, 119.1, 113.0, 112.8,
(100 MHz, CDCl3): d 163.4, 153.0, 152.8, 147.3, 135.8, 128.4, 108.1, 51.5, 21.8, 13.4; IR (KBr): nmax/cmꢂ1 1713, 1649, 1620, 1594,
123.5, 117.7, 110.0, 61.0, 14.3, 13.5; IR (KBr): nmax/cmꢂ1 1724, 1439, 1366, 1293, 1266, 1258, 1189, 1173, 1086; HRMS (ESI): m/z
1598, 1569, 1516, 1354, 1313, 1300, 1281, 1260, 1215, 1095, calcd for C18H17NNaO5S: 382.0720 [M + Na]+; found 382.0729.
1028; HRMS (ESI): m/z calcd for C14H14NO5: 276.0866 [M + H]+;
Diethyl 5,500-dimethyl-[2,20:50,200-terfuran]-3,300-dicarboxylate
found 276.0865.
(2j). The crude product was puried by column chromatography
Methyl 5-methyl-2-(30-triuoromethylphenyl)furan-3-carboxylate on silica gel (2% ethyl acetate in petroleum ether) to give 2j as a
(2e). The crude product was puried by column chromatography yellow solid; yield 64%; mp 80 ꢀC; 1H NMR (400 MHz, CDCl3): d
on silica gel (0.5% ethyl acetate in petroleum ether) to give 2e as an 7.60 (s, 2H), 6.43 (q, J ¼ 1.2 Hz, 2H), 4.33 (q, J ¼ 7.1 Hz, 4H), 2.40
1
orange oil; yield 69%; H NMR (400 MHz, CDCl3): d 8.27 (s, 1H), (d, J ¼ 1.2 Hz, 6H), 1.37 (t, J ¼ 7.1 Hz, 6H); 13C NMR (100 MHz,
8.22 (d, J ¼ 7.8 Hz, 1H), 7.61 (d, J ¼ 7.8 Hz, 1H), 7.53 (t, J ¼ 7.8 Hz, CDCl3): d 163.2, 151.8, 147.0, 144.8, 144.7, 114.6, 108.7, 60.6,
1H), 6.46 (s, 1H), 3.83 (s, 3H), 2.37 (s, 3H); 13C NMR (100 MHz, 14.5, 13.7; IR (KBr): nmax/cmꢂ1 1713, 1607, 1538, 1378, 1261,
CDCl3): d 163.9, 154.1, 151.9, 131.1 (q, JF–C ¼ 1.2 Hz), 130.6, 130.5 (q, 1215, 1100, 1052, 1024; HRMS (ESI): m/z calcd for C20H20NaO7:
JF–C ¼ 32.3 Hz), 128.5, 125.3 (q, JF–C ¼ 3.7 Hz), 124.8 (q, JF–C ¼ 4.0 395.1101 [M + Na]+; found 395.1100.
Hz), 124.0 (q, JF–C ¼ 270.8 Hz), 115.3, 109.1, 51.7, 13.4; IR (neat):
Ethyl 2,5-dimethylfuran-3-carboxylate (2k). The crude
nmax/cmꢂ1 1722, 1558, 1439, 1380, 1330, 1268, 1215, 1168, 1127, product was puried by column chromatography on silica gel
1102, 1030; HRMS (ESI): m/z calcd for C14H12F3O3: 285.0733 [M + (petroleum ether) to give 2k as a yellow oil; yield 88%; 1H NMR
H]+; found 285.0723.
(400 MHz, CDCl3): d 6.20 (q, J ¼ 0.8 Hz, 1H), 4.25 (q, J ¼ 7.1 Hz,
Methyl 2-(20-bromophenyl)-5-methylfuran-3-carboxylate (2f). 2H), 2.51 (s, 3H), 2.23 (d, J ¼ 0.8 Hz, 3H), 1.32 (t, J ¼ 7.1 Hz, 3H);
The crude product was puried by column chromatography on 13C NMR (100 MHz, CDCl3): d 164.5, 157.7, 150.0, 114.1, 106.3,
silica gel (2% ethyl acetate in petroleum ether) to give 2f as a 60.0, 14.5, 13.8, 13.3; IR (neat): nmax/cmꢂ1 1716, 1624, 1590,
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1406, 1283, 1231, 1206, 1083; HRMS (ESI): m/z calcd for chromatography on silica gel to afford 2,3,4-trisubstituted
C9H13O3: 169.0859 [M + H]+; found 169.0858.
furans 3a–o.
Ethyl 2-isopropyl-5-methylfuran-3-carboxylate (2l). The crude
Methyl 4-methyl-2-phenylfuran-3-carboxylate (3a).24 The
product was puried by column chromatography on silica gel crude product was puried by column chromatography on silica
(petroleum ether) to give 2k as a colorless oil; yield 55%; 1H gel (0.5% ethyl acetate in petroleum ether) to give 3a as a
NMR (400 MHz, CDCl3): d 6.19 (q, J ¼ 1.0 Hz, 1H), 4.25 (q, J ¼ 7.1 colorless oil; yield 92%; 1H NMR (400 MHz, CDCl3): d 7.78–7.76
Hz, 2H), 3.71 (heptet, J ¼ 7.0 Hz, 1H), 2.24 (d, J ¼ 1.0 Hz, 3H), (m, 2H), 7.43–7.37 (m, 3H), 7.24 (s, 1H), 3.80 (s, 3H), 2.19 (s, 3H);
1.32 (t, J ¼ 7.1 Hz, 3H), 1.24 (d, J ¼ 7.0 Hz, 6H); 13C NMR (100 13C NMR (100 MHz, CDCl3): d 165.1, 158.1, 139.3, 130.5, 129.2,
MHz, CDCl3): d 165.9, 164.4, 149.8, 112.1, 106.2, 60.0, 27.2, 21.0, 128.5, 128.1, 122.6, 114.1, 51.4, 10.3; IR (neat): nmax/cmꢂ1 1717,
14.5, 13.4; IR (neat): nmax/cmꢂ1 1724, 1469, 1250, 1214, 1069, 1490, 1436, 1290, 1066; HRMS (ESI): m/z calcd for C13H13O3:
1025; HRMS (ESI): m/z calcd for C11H17O3: 197.1172 [M + H]+; 217.0859 [M + H]+; found 217.0859.
found 197.1173.
Methyl 4-methyl-2-(40-methylphenyl)furan-3-carboxylate (3b).24
Methyl 5-ethyl-2-(40-methylphenyl)furan-3-carboxylate (2m). The crude product was puried by column chromatography on
The crude product was puried by column chromatography on silica gel (0.5% ethyl acetate in petroleum ether) to give 3b as an
silica gel (petroleum ether) to give 2m as a yellow oil; yield 80%; orange oil; yield 90%; 1H NMR (400 MHz, CDCl3): d 7.68–7.66 (m,
1H NMR (400 MHz, CDCl3): d 7.85 (d, J ¼ 8.3 Hz, 2H), 7.23 (d, J ¼ 2H), 7.24–7.22 (m, 3H), 3.81 (s, 3H), 2.39 (s, 3H), 2.20 (s, 3H); 13
C
8.3 Hz, 2H), 6.41 (t, J ¼ 0.9 Hz, 1H), 3.81 (s, 3H), 2.69 (qd, J ¼ 7.6, NMR (100 MHz, CDCl3): d 165.1, 158.4, 139.2, 138.9, 128.8, 128.3,
0.9 Hz, 2H), 2.39 (s, 3H), 1.28 (t, J ¼ 7.6 Hz, 3H); 13C NMR (100 127.6, 122.5, 113.5, 51.3, 21.5, 10.3; IR (neat): nmax/cmꢂ1 1722,
MHz, CDCl3): d 164.5, 156.5, 156.4, 139.2, 128.9, 128.2, 127.4, 1590, 1480, 1377, 1223, 1090; HRMS (ESI): m/z calcd for C14H15O3:
113.5, 107.2, 51.6, 21.6, 21.3, 12.0; IR (neat): nmax/cmꢂ1 1721, 231.1016 [M + H]+; found 231.1018.
1504, 1290, 1233, 1099; HRMS (ESI): m/z calcd for C15H17O3:
Methyl 2-(40-methoxyphenyl)-4-methylfuran-3-carboxylate (3c).
The crude product was puried by column chromatography on
245.1172 [M + H]+; found 245.1172.
Methyl 2-(40-methylphenyl)-4,5,6,7-tetrahydrobenzofuran-3- silica gel (1% ethyl acetate in petroleum ether) to give 3c as a
1
ꢀ
carboxylate (2n). The crude product was puried by column yellow solid; yield 85%; mp 70 C; H NMR (400 MHz, CDCl3): d
chromatography on silica gel (petroleum ether) to give 2n as a 7.76–7.73 (m, 2H), 6.41 (q, J ¼ 1.0 Hz, 1H), 6.96–6.92 (m, 2H), 3.84
colorless oil; yield 52%; 1H NMR (400 MHz, CDCl3): d 7.72 (d, J ¼ (s, 3H), 3.80 (s, 3H), 2.18 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz,
8.2 Hz, 2H), 7.21 (d, J ¼ 8.2 Hz, 2H), 3.79 (s, 3H), 2.66–2.61 (m, CDCl3): d 165.1, 160.3, 158.5, 138.7, 130.0, 123.1, 122.5, 113.6,
4H), 2.38 (s, 3H), 1.88–1.73 (m, 4H); 13C NMR (100 MHz, CDCl3): 113.0, 55.4, 51.3, 10.4; IR (KBr): nmax/cmꢂ1 1717, 1608, 1501, 1438,
d 165.2, 156.3, 150.7, 138.9, 128.8, 128.3, 127.9, 119.1, 112.8, 1254, 1177, 1074, 1028; HRMS (ESI): m/z calcd for C14H14NaO4:
51.3, 23.2, 23.0, 22.8, 22.7, 21.6; IR (neat): nmax/cmꢂ1 1720, 1557, 269.0784 [M + Na]+; found 269.0784.
1501, 1438, 1283, 1217, 1095, 1037; HRMS (ESI): m/z calcd for
17H19O3: 271.1329 [M + H]+; found 271.1328.
Ethyl 4-methyl-2-(40-nitrophenyl)furan-3-carboxylate (3d).
C
The crude product was puried by column chromatography on
Ethyl 5-benzyl-2-isopropylfuran-3-carboxylate (2o). The silica gel (3% ethyl acetate in petroleum ether) to give 3d as a
crude product was puried by column chromatography on silica yellow solid; yield 58%; mp 83 ꢀC; 1H NMR (400 MHz, CD3OD): d
gel (0.3% ethyl acetate in petroleum ether) to give 2o as a 8.28 (m, 2H), 8.04 (m, 2H), 7.53 (s, 1H), 4.33 (q, J ¼ 6.8 Hz, 2H),
colorless oil; yield 89%; 1H NMR (400 MHz, CDCl3): d 7.33–7.22 2.22 (s, 3H), 1.33 (t, J ¼ 6.8 Hz, 3H); 13C NMR (100 MHz,
(m, 5H), 6.18 (s, 1H), 4.24 (q, J ¼ 7.2 Hz, 2H), 3.91 (s, 1H), 3.72 CD3OD): d 165.3, 155.9, 149.0, 142.4, 137.4, 130.0, 124.5, 124.3,
(heptet, J ¼ 7.0 Hz, 1H), 1.30 (t, J ¼ 7.2 Hz, 3H), 1.25 (d, J ¼ 7.0 117.9, 61.9, 14.4, 10.1; IR (neat): nmax/cmꢂ1 1712, 1616, 1591,
Hz, 6H); 13C NMR (100 MHz, CDCl3): d 166.4, 164.2, 152.3, 137.7, 1545, 1520, 1488, 1474, 1382, 1348, 1297, 1280, 1100, 1072,
128.8, 128.6, 126.7, 112.2, 107.0, 60.0, 34.3, 27.3, 20.9, 14.5; IR 1024; HRMS (ESI): m/z calcd for C14H13NNaO5: 298.0686 [M +
(neat): nmax/cmꢂ1 1715, 1576, 1496, 1468, 1455, 1380, 1208, Na]+; found 298.0688.
1059; HRMS (ESI): m/z calcd for C17H21O3: 273.1485 [M + H]+;
found 273.1484.
Methyl 4-methyl-2-(30-triuoromethylphenyl)furan-3-carboxylate
(3e). The crude product was puried 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): d 8.08 (s, 1H), 8.00
(d, J ¼ 7.9 Hz, 1H), 7.63 (d, J ¼ 7.9 Hz, 1H), 7.53 (t, J ¼ 7.9 Hz, 1H),
7.29 (s, 1H), 3.82 (s, 3H), 2.21 (s, 3H); 13C NMR (100 MHz, CDCl3): d
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 164.7, 156.2, 140.0, 131.6 (q, JF–C ¼ 1.2 Hz), 131.2, 130.6 (q, JF–C
¼
of 3-substituted 2-(2-bromoallyl)-3-oxo-1-carboxylates (1a–o) 32.3 Hz), 128.6, 125.6 (q, JF–C ¼ 3.8 Hz), 125.4 (q, JF–C ¼ 4.0 Hz),
(1.0 mmol) in anhydrous DMF (15 mL). The resulting mixture 124.1 (q, JF–C ¼ 271.0 Hz), 123.0, 115.1, 51.5, 10.2; IR (neat):
was stirred at 80 C until the substrate was consumed (moni- nmax/cmꢂ1 1720, 1604, 1547, 1479, 1439, 1381, 1331, 1283, 1213,
ꢀ
tored by TLC). Aer being cooled to ambient temperature, the 1168, 1128, 1073, 1001; HRMS (ESI): m/z calcd for C14H11F3NaO3:
mixture was treated with 1 M HCl (5 mL) and ltered. The 307.0553 [M + Na]+; found 307.0558.
ltrate was extracted with ethyl acetate (10 mL ꢁ 3). The
Methyl 2-(20-bromophenyl)-4-methylfuran-3-carboxylate (3f).
combined organic extracts were washed with brine (10 mL), The crude product was puried by column chromatography on
then dried over anhydrous sodium sulfate, ltered and silica gel (0.5% ethyl acetate in petroleum ether) to give 3f as a
1
concentrated in vacuo. The residue was puried by column colorless oil; yield 80%; H NMR (400 MHz, CDCl3): d 7.65 (m,
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1H), 7.45 (m, 1H), 7.37 (m, 1H), 7.28 (m, 1H), 6.44 (q, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 165.0, 160.2, 137.7, 121.3,
1H), 3.70 (s, 3H), 2.36 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz, 113.6, 59.9, 14.5, 14.5, 10.2; IR (neat): nmax/cmꢂ1 1716, 1611,
CDCl3): d 163.8, 155.0, 152.3, 133.0, 132.3, 132.1, 130.8, 126.9, 1563, 1418, 1385, 1298, 1272, 1099; HRMS (ESI): m/z calcd for
124.1, 116.8, 107.2, 51.6, 13.6; IR (neat): nmax/cmꢂ1 1722, 1570, C9H13O3: 169.0859 [M + H]+; found 169.0858.
1467, 1438, 1383, 1276, 1209, 1102, 1058, 1012; HRMS (ESI): m/z
Ethyl 2-isopropyl-4-methylfuran-3-carboxylate (3l). The crude
product was puried by column chromatography on silica gel
calcd for C13H1279BrO3: 294.9964 [M + H]+; found 294.9965.
Methyl 2-(20,40-dimethoxyphenyl)-4-methylfuran-3-carboxylate (petroleum ether) to give 3l as a colorless oil; yield 76%; 1H NMR
(3g). The crude product was puried by column chromato- (400 MHz, CDCl3): d 7.03 (q, J ¼ 1.2 Hz, 1H), 4.28 (q, J ¼ 7.1 Hz,
graphy on silica gel (2% ethyl acetate in petroleum ether) to give 2H), 3.71 (heptet, J ¼ 7.0 Hz, 1H), 2.12 (d, J ¼ 1.2 Hz, 3H), 1.34 (t,
3g as an orange oil; yield 82%; 1H NMR (400 MHz, CDCl3): d 7.36 J ¼ 7.1 Hz, 3H), 1.23 (d, J ¼ 7.0 Hz, 6H); 13C NMR (100 MHz,
(d, J ¼ 8.4 Hz, 1H), 7.22 (q, J ¼ 1.0 Hz, 1H), 6.54 (dd, J ¼ 8.4, 2.3 CDCl3): d 168.0, 164.9, 137.7, 121.0, 111.8, 59.9, 27.7, 20.8, 14.4,
Hz, 1H), 6.49 (d, J ¼ 2.3 Hz, 1H), 3.82 (s, 3H), 3.75 (s, 3H), 3.69 (s, 10.2; IR (neat): nmax/cmꢂ1 1718, 1560, 1282, 1210, 1065; HRMS
3H), 2.18 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 165.2, (ESI): m/z calcd for C11H17O3: 197.1172 [M + H]+; found
161.9, 158.2, 154.9, 138.9, 131.4, 121.6, 115.4, 113.0, 104.4, 98.6, 197.1173.
55.5, 55.4, 51.1, 9.7; IR (neat): nmax/cmꢂ1 1717, 1618, 1504, 1438,
1292, 1211, 1161, 1090, 1032; HRMS (ESI): m/z calcd for C15H17O5: The crude product was puried by column chromatography on
277.1071 [M + H]+; found 277.1071.
silica gel (0.3% ethyl acetate in petroleum ether) to give 3m as a
Methyl 4-ethyl-2-(40-methylphenyl)furan-3-carboxylate (3m).
1
Methyl 2-(60-methoxynaphthalen-20-yl)-4-methylfuran-3- yellow oil; yield 60%; H NMR (400 MHz, CDCl3): d 7.65 (d, J ¼
carboxylate (3h). The crude product was puried by column 8.2 Hz, 2H), 7.24–7.22 (m, 3H), 3.80 (s, 3H), 2.67 (qd, J ¼ 7.4, 1.0
chromatography on silica gel (1% ethyl acetate in petroleum Hz, 2H), 2.39 (s, 3H), 1.23 (t, J ¼ 7.4 Hz, 3H); 13C NMR (100 MHz,
1
ꢀ
ether) to give 3h as a colorless solid; yield 80%; mp 114 C; H CDCl3): d 165.1, 158.5, 139.2, 138.3, 129.3, 128.9, 128.4, 127.8,
NMR (400 MHz, CDCl3): d 8.24 (s, 1H), 7.84–7.73 (m, 3H), 7.28 112.9, 51.4, 21.5, 18.5, 13.8; IR (neat): nmax/cmꢂ1 1716, 1500,
(q, J ¼ 1.0 Hz, 1H), 7.17–7.13 (m, 2H), 3.93 (s, 3H), 3.82 (s, 3H), 1437, 1293, 1208, 1077; HRMS (ESI): m/z calcd for C15H17O3:
2.22 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 165.2, 245.1172 [M + H]+; found 245.1170.
158.6, 158.5, 139.2, 134.9, 130.0, 128.4, 128.0, 126.4, 126.3,
Methyl 2-(40-methylphenyl)-4,5,6,7-tetrahydrobenzofuran-3-
125.7, 122.7, 119.3, 113.8, 105.7, 55.5, 51.4, 10.4; IR (neat): carboxylate (2n). The crude product was puried by column
nmax/cmꢂ1 1713, 1627, 1592, 1537, 1504, 1479, 1458, 1392, 1378, chromatography on silica gel (petroleum ether) to give 2n as a
1302, 1287, 1273, 1258, 1202, 1164, 1119, 1093, 1069, 1029; colorless oil; yield 11%; 1H NMR (400 MHz, CDCl3): d 7.72 (d, J ¼
HRMS (ESI): m/z calcd for C18H16NaO4: 319.0946 [M + Na]+; 8.2 Hz, 2H), 7.21 (d, J ¼ 8.2 Hz, 2H), 3.79 (s, 3H), 2.66–2.61 (m,
found 319.0943.
4H), 2.38 (s, 3H), 1.88–1.73 (m, 4H); 13C NMR (100 MHz, CDCl3):
Methyl 4-methyl-2-(N-tosyl-1H-pyrrol-30-yl)furan-3-carboxylate d 165.2, 156.3, 150.7, 138.9, 128.8, 128.3, 127.9, 119.1, 112.8,
(3i). The crude product was puried by column chromato- 51.3, 23.2, 23.0, 22.8, 22.7, 21.6; IR (neat): nmax/cmꢂ1 1720, 1557,
graphy on silica gel (7% ethyl acetate in petroleum ether) to give 1501, 1438, 1283, 1217, 1095, 1037; HRMS (ESI): m/z calcd for
3i as an orange oil; yield 78%; 1H NMR (400 MHz, CDCl3): d 8.06
(m, 1H), 7.80 (d, J ¼ 8.4 Hz, 2H), 7.30 (d, J ¼ 8.4 Hz, 2H), 7.16 (m,
C
17H19O3: 271.1329 [M + H]+; found 271.1328.
Ethyl 4-benzyl-2-isopropylfuran-3-carboxylate (3o). The
1H), 7.11 (q, J ¼ 1.0 Hz, 1H), 6.84 (m, 1H), 3.85 (s, 3H), 2.40 (s, crude product was puried by column chromatography on silica
3H), 2.15 (d, J ¼ 1.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 164.9, gel (0.5% ethyl acetate in petroleum ether) to give 3o as a
153.7, 145.4, 138.3, 135.9, 130.3, 127.2, 122.3, 121.1, 120.7, 119.3, colorless oil; yield 71%; 1H NMR (400 MHz, CDCl3): d 7.32–7.21
113.1, 112.9, 51.4, 21.8, 10.6; IR (neat): nmax/cmꢂ1 1713, 1606, (m, 5H), 6.18 (s, 1H), 4.23 (q, J ¼ 7.1 Hz, 2H), 3.91 (s, 1H), 3.73
1538, 1494, 1474, 1454, 1438, 1373, 1303, 1231, 1214, 1174, 1117, (heptet, J ¼ 7.0 Hz, 1H), 1.30 (t, J ¼ 7.1 Hz, 3H), 1.25 (d, J ¼ 7.0
1018; HRMS (ESI): m/z calcd for C18H17NNaO5S: 382.0720 [M + Hz, 6H); 13C NMR (100 MHz, CDCl3): d 166.4, 164.2, 152.3, 137.6,
Na]+; found 382.0723.
128.8, 128.6, 126.7, 112.2, 107.0, 60.0, 34.3, 27.3, 20.9, 14.4; IR
Diethyl 4,400-dimethyl-[2,20:50,200-terfuran]-3,300-dicarboxylate (neat): nmax/cmꢂ1 1715, 1576, 1497, 1468, 1455, 1381, 1208,
(3j). The crude product was puried by column chromatography 1059; HRMS (ESI): m/z calcd for C17H21O3: 273.1485 [M + H]+;
on silica gel (5% ethyl acetate in petroleum ether) to give 3j as a found 273.1483.
yellow solid; yield 60%; mp 175 ꢀC; 1H NMR (400 MHz, CDCl3): d
7.54 (s, 2H), 7.27 (q, J ¼ 1.2 Hz, 2H), 4.37 (q, J ¼ 3.6 Hz, 4H), 2.21
(d, J ¼ 1.2 Hz, 6H), 1.39 (t, J ¼ 3.6 Hz, 6H); 13C NMR (100 MHz,
CDCl3): d 163.8, 149.1, 145.1, 139.7, 122.7, 114.8, 114.2, 60.6,
14.4, 10.5; IR (KBr): nmax/cmꢂ1 1701, 1600, 1523, 1307, 1231,
1122, 1101, 1024; HRMS (ESI): m/z calcd for C20H20NaO7:
395.1101 [M + Na]+; found 395.1098.
Acknowledgements
We are grateful to Zhengzhou University (#1421316040) and the
NSFC (#81330075; #21172202) for nancial support.
Ethyl 2,4-dimethylfuran-3-carboxylate (3k). The crude
product was puried by column chromatography on silica gel
(petroleum ether) to give 3k as a colorless oil; yield 85%; 1H
NMR (400 MHz, CDCl3): d 7.01 (q, J ¼ 1.1 Hz, 1H), 4.27 (q, J ¼ 7.2
Hz, 2H), 2.52 (s, 3H), 2.12 (d, J ¼ 1.1 Hz, 3H), 1.34 (t, J ¼ 7.2 Hz,
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DOI: 10.1039/C5RA14273C
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RSC Advances
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