Regiospecific synthesis of polysubstituted furans with mono- to tricarboxylates from various sulfonium acylmethylides and acetylenic esters

Article abstract of DOI:10.1039/c9ra03563j

Polysubstituted furans were prepared in moderate to good yields from various sulfur ylides and alkyl acetylenic carboxylates. The direct reactions of dimethylsulfonium acylmethylides with dialkyl acetylenedicarboxylates afforded dialkyl furan-3,4-dicarboxylates through a tandem sequence of Michael addition, intramolecular nucleophilic addition, 4π ring opening, intramolecular Michael addition, and elimination. The method was extended to synthesize furan-3-carboxylate, -2,4-dicarboxylates, and -2,3,4-tricarboxylates as well. The current method provides a direct and simple strategy in the synthesis of structurally diverse polysubstituted furans with mono to tricarboxylate groups from safe and readily available dimethylsulfonium acylmethylides and different alkyl acetylenic carboxylates.

Full text of DOI:10.1039/c9ra03563j

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Regiospecific synthesis of polysubstituted furans
with mono- to tricarboxylates from various
sulfonium acylmethylides and acetylenic esters†
Cite this: RSC Adv., 2019, 9, 25034
*
Jun Dong, Hongguang Du
and Jiaxi Xu
Polysubstituted furans were prepared in moderate to good yields from various sulfur ylides and alkyl
acetylenic carboxylates. The direct reactions of dimethylsulfonium acylmethylides with dialkyl
acetylenedicarboxylates afforded dialkyl furan-3,4-dicarboxylates through a tandem sequence of Michael
addition, intramolecular nucleophilic addition, 4p ring opening, intramolecular Michael addition, and
elimination. The method was extended to synthesize furan-3-carboxylate, -2,4-dicarboxylates, and
-2,3,4-tricarboxylates as well. The current method provides a direct and simple strategy in the synthesis
of structurally diverse polysubstituted furans with mono to tricarboxylate groups from safe and readily
available dimethylsulfonium acylmethylides and different alkyl acetylenic carboxylates.
Received 12th May 2019
Accepted 2nd August 2019
DOI: 10.1039/c9ra03563j
rsc.li/rsc-advances
alkynoates with carbonyl compounds in the presence of metal
leads to polysubstituted furans as well.6
Introduction
Sulfur ylides (sulfonium and sulfoxonium methylides) have
been used as universal synthetic precursors for various chem-
ical transformations.7 The reactions of sulfur ylides and alkynes
were also applied in the synthesis of furan derivatives. One
interesting and powerful method to construct the furan motif
was designed through the precious metal gold-catalyzed addi-
tion of sulfur ylides to terminal alkynes in an inter- or intra-
molecular reaction. The reactions were the electrophilic
addition of metal gold-carbenes to electron-rich alkynes
(Scheme 1(I)).8 Another method for the synthesis of furan
derivatives was the reaction of sulfonium ylides with dialkyl
acetylenedicarboxylates. The reactive intermediate sulfonium
acylmethylides were trapped by acetylenic esters to yield
substituted furans in an inter- or intramolecular reaction.9
Mostly, furan-2,3-dicarboxylic acid derivatives were obtained
with this method10 and only two examples were reported on the
synthesis of furan-3,4-dicarboxylic acid derivatives (Scheme
1(II)).11 We, herein, report the regioselective synthesis of
Furan derivatives are not only important building blocks in
organic chemistry, but also natural products found in various
natural sources, most in plants, algae, and microorganisms,
and structural motifs in biologically drug molecules (Fig. 1).1
Polysubstituted furans represent important cores and moieties
of some biological compounds as well as acting as useful
intermediates in organic synthesis.2 The development of effi-
cient methods for their preparation has been an important
research area in organic chemistry.
Historically, the classical approaches such as Paal–Knorr
and Feist–Benary syntheses of furan derivatives have been
widely applied. The Paal–Knorr method relies on an acid-
catalyzed intramolecular cyclization of 1,4-dicarbonyl
compounds,3 while the Feist–Benary method provides a strategy
for the efficient preparation of polysubstituted furan derivatives
via intermolecular annulation of b-dicarbonyl compounds and
a-haloketones.4 In the past few decades, many efforts for the
synthesis of polysubstituted furans have been made to explore
the annulation of alkynes and other unsaturated compounds.
Such as, oxazoles, 1,3,4-oxadiazoles, and furobenzopyran
undergo inter- or intramolecular Diels–Alder/retro-Diels–Alder
reactions with electron-poor alkynes to generate poly-
substituted furans.5 The addition/oxidative cyclization of alkyl
State Key Laboratory of Chemical Resource Engineering, Department of Organic
Chemistry, College of Chemistry, Beijing University of Chemical Technology, Beijing
100029, People's Republic of China. E-mail: jxxu@mail.buct.edu.cn
† Electronic supplementary information (ESI) available: Analytic data and copies
of 1H and 13C NMR spectra of furan products 3, and copies of HR-MS spectra of
unknown furan products 3. See DOI: 10.1039/c9ra03563j
Fig. 1 Furan-derived natural products and drugs.
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product 3aa was 80% when the reaction was conducted at
ꢀ
110 C (Table 1, entry 10).
With the optimized reaction conditions, the reaction scope
was then evaluated (Table 2). Various sulfonium ylides 2 were
examined in the reaction with dimethyl acetylenedicarboxylate
(DMAD) (1a). We were pleased to nd that the application of
various sulfonium ylides 2 led to the corresponding furan-3,4-
dicarboxylates 3 in moderate to good yields. Various func-
tional groups such as methyl, uoro, chloro, bromo, tri-
uoromethyl, and cyano on the aryl group of the sulfur ylides 2
were well tolerated under optimized reaction conditions (Table
2,
3ab–3ah).
For
electron-decient
2-(dimethyl-l4-
sulfanylidene)-1-(4-nitrophenyl)ethan-1-one (2i), the desired
furan-3,4-dicarboxylate 3ai was isolated in 45% yield under the
optimized reaction conditions. In comparison with dime-
thylsulfonium monosubstituted benzoylmethylides 2a–2i, the
sulfur ylide with more substituted on the aryl group was tested
as well. 1-(3,4-Dichlorophenyl)-2-(dimethyl-l4-sulfanylidene)
ethan-1-one (1j) gave rise to the corresponding furan-3,4-
dicarboxylate 3aj in 74% yield. Aer replacing the phenyl
group with naphthyl, the corresponding products 3ak and 3al
were obtained in 85% and 75% yields, respectively. Aer
investigating the reaction of different dimethylsulfonium 2-aryl-
2-oxoethylides 2 with dimethyl acetylenedicarboxylate, diethyl
acetylenedicarboxylate (1b) was evaluated with three represen-
tative sulfur ylides 2a, 2b, 2f, affording the desired products 3ba,
3bb, and 3bf in 56%, 53%, and 68% yields, respectively, under
the optimized reaction conditions.
Scheme 1 Synthesis of polysubstituted furans from sulfur ylides and
alkynes.
structurally diverse polysubstituted furans with mono to tri-
carboxylates from various acetylenic esters and dimethylsulfo-
nium acylmethylides and the mechanistic rationale on the
selective generation of dialkyl furan-3,4-dicarboxylates.
To extend the application of the synthetic method, several
alkyl propynoates were attempted to prepare alkyl 2-substituted
furan-3-carboxlyates (Table 2). Gratifyingly, ethyl 4,4,4-
Results and discussion
Table 1 Screening of reaction conditionsa
To understand the mechanism and to explore the substrate
scope of the reaction of dialkyl acetylenedicarboxylates and
dimethylsulfonium acylmethylides, dimethyl acetylenedi-
carboxylate (DMAD) (1a) and 2-(dimethyl-l4-sulfanylidene)-1-
phenylethan-1-one (dimethylsulfonium benzoylmethylide) (2a)
were employed as the model substrates to optimize the reaction
conditions in 1,2-dichloroethane (DCE) under nitrogen atmo-
sphere. We rst optimized the reactant ratio (Table 1, entries 1–
3). When the ratio of 1a : 2a was increased from 1 : 2 to 1 : 2.4,
the yield of product dimethyl 2-phenylfuran-3,4-dicarboxylate
(3aa) decreased from 44% to 34% (Table 1, entries 1 and 2).
The yield dropped sharply to 15% when the ratio of 1a : 2a was
2 : 1 (Table 1, entry 3). Solvents were also screened. Product 3aa
was obtained in 57% in polar DMF as solvent (Table 1, entry 4).
The reactions with MeCN and DMSO as solvents gave product
3aa in 50% and 81% yields, respectively (Table 1, entries 5 and
6). The results indicated that DMSO was the best choice. The
yield decreased to 70% when the reaction was conducted
without nitrogen protection (Table 1, entry 7). It was found that
Entry
1a : 2a
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
1 : 2
1 : 2.4
2 : 1
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
DCE
DCE
DCE
DMF
MeCN
DMSO
DMSO
DMSO
DMSO
DMSO
4
4
4
4
4
4
4
6
2
4
44
34
15
57
50
81(79)c
70d
80
9
73
80
10e
a
All reactions were conducted on a 0.125 mmol scale of 1a in 1 mL of
b
the extension of the reaction time had no signicant effect on solvent at 80 ꢀC. NMR yield of the crude product using 1,3,5-
c
trimethoxybenzene as an internal standard. Yield of the isolated
the yield of product 3aa (Table 1, entry 8). When the reaction
time was shortened from 4 h to 2 h, the yield was slightly
d
e
product. Without nitrogen protection. The reaction conducted at
110 ꢀC.
dropped to 73% (Table 1, entry 6 vs. 9). At last, the yield of
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Table 2 Synthesis of polysubstituted furans from various alkyl acety-
yield of 15%, while diacyl stabilized sulfonium ylide 2n reacted
with ethyl propynoate (1e), giving the desired furan-2,4-
dicarboxylate 3en in a moderate yield of 48%. However, in
a previous report,10a the reaction of diacyl stabilized sulfonium
ylide 2n and ethyl propynoate (1e) produced 2-ethyl 4-methyl 5-
methylfuran-2,4-dicarboxylate (4en) only in 73% yield in 1,4-
lenic esters and sulfur ylidesa
ꢀ
dioxane at 160 C, showing a different reaction regioselectivity
from our results. To verify the different selectivity, we conducted
the same reaction in 1,4-dioxane at 160 ꢀC under microwave
irradiation, affording
a mixture of 4-ethyl 2-methyl 5-
methylfuran-2,4-dicarboxylate (3en) and 2-ethyl 4-methyl 5-
methylfuran-2,4-dicarboxylate (4en) in a ratio of 1 : 1.81 in 53%
total yield. However, under our optimal conditions, only 4-ethyl
2-methyl 5-methylfuran-2,4-dicarboxylate (3en) was obtained
regiospecically in DMSO as solvent. The results revealed that
solvent and temperature played important effects on the reac-
tion pathways, resulting in regioselective formation of different
furan derivatives. The exact reason for the selective control is
not clear now possible it is attributed the stability of interme-
diates in different solvents during reaction.
The reactions of both sulfur ylides 2a and 2m with methyl
propynoate (1d) gave rise to the corresponding products 3da
and 3dm in low yields because methyl propynoate (1d) was not
electron-decient enough compared with other acetylenic esters
and then it was difficult for sulfonium ylides 2a or 2m to
undergo Michael addition to methyl propynoate (1d).
Aer obtaining the above information, we proposed the
following reaction mechanism (Scheme 2). Dimethyl acetyle-
nedicarboxylate (1a) and 2-(dimethyl-l4-sulfanylidene)-1-
phenylethan-1-one (2a) are used to illustrate the proposed
mechanism. First, 2-(dimethyl-l4-sulfanylidene)-1-phenylethan-
1-one (2a) undergoes Michael addition to dimethyl acetylene-
dicarboxylate (1a) to generate intermediate A, which further
takes place an intramolecular nucleophilic addition to the
carbonyl group of the benzoyl group to produce zwitterionic
intermediate B. The intermediate B undergoes a 4p ring
opening to form intermediate enolate C. The enolate in inter-
mediate C takes place an intramolecular Michael addition fol-
lowed by the elimination of dimethyl sulde to give furan 3aa.
In the previous report,11 the conversion of intermediates A to C
was assumed as the benzoyl group shi followed by tautome-
rization. In our viewpoint, it is an intramolecular nucleophilic
addition followed by a 4p ring opening process. This is a more
reasonable conversion pathway.
a
Reaction conditions A: acetylenic ester 1 (0.125 mmol) and sulfur ylide
2 (0.250 mmol) in DMSO (1 mL) were stirred under nitrogen at 80 ꢀC for
4 h. b Reaction conditions B: acetylenic ester 1 (0.125 mmol) and sulfur
ylide 2 (0.250 mmol) in DMSO (1 mL) were stirred under nitrogen at
160 ꢀC for 15 min under microwave irradiation.
triuorobut-2-ynoate (1c) was also successful for the synthesis
of the corresponding triuoromethylated furan-3-carboxylate
derivatives. The desired 4-triuoromethylfuran-3-carboxylates
3ca–3cf were obtained in moderate yields of 44–57%.
However, methyl propynoate (1d) only produced the corre-
sponding furan-3-carboxylate 3da in a low yield of 15% when it
was reacted with sulphur ylide 2a.
Further extension of the synthetic strategy in the preparation
of trialkyl furan-2,3,4-tricarboxylates was performed (Table 2). It
was worth noting that no desired products were observed for
diacyl stabilized sulfonium ylides 2m and 2n at 80 ꢀC when they
were applied in the reaction. To our delight, the desired trialkyl
furan-2,3,4-tricarboxylates 3am, 3bm, 3an, and 3bn were ob-
tained in moderate to good yields of 48% to 75% when the
reaction temperature was increased from 80 ꢀC to 160 ꢀC under
microwave irradiation due to the microwave assistance.11,12
Finally, the synthetic strategy was tested in the preparation
of dialkyl furan-2,4-dicarboxylates as well (Table 2). The reaction
of diacyl stabilized sulfonium ylide 2m and methyl propynoate
(1d) afforded the desired furan-2,4-dicarboxylate 3dm in a low
Scheme 2 Proposed reaction mechanism.
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broad singlet (brs), doublet (d), triplet (t), quartet (q), and
multiplet (m). The IR spectra (KBr pellets, v [cmÀ1]) were
recorded on
a Nicolet 5700 FTIR spectrometer. HRMS
measurements were carried out on an Agilent LC/MSD TOF
mass spectrometer. LRMS measurements were carried out on
a Thermo Trace 1300/ISQ QD system. Melting points were ob-
tained on a Yanaco MP-500 melting point apparatus and are
uncorrected. All the microwave reactions were conducted in
a CEM Discover SP microwave system equipped with an infrared
temperature detector.
General procedure for the synthesis of sulfur ylides 2a–2l
Scheme 3 Selected reported application.
To a solution of halomethyl carbonyl compound 4 (10 mmol) in
acetone (15 mL) was added dimethyl sulde (620 mg, 10 mmol).
Aer the mixture was stirred for 12 h, the residue was ltered
and washed with acetone. The solid product was used as
sulfonium halide without further purication. The corre-
sponding sulfonium halide was added to a solution of NaOH
Selected applications of polysubstituted furans, dimethyl 2-
phenylfuran-3,4-dicarboxylates, from the literature are listed in
Scheme 3,13 One for synthesis of diaryldihydrofuranones,13a the
other for furanmethanol.13b
ꢀ
(400 mg, 10 mmol) in water (10 mL) at 0 C. The solution was
stirred for 30 min and then extracted several times with
dichloromethane. The combined organic layers were washed
with water and brine sequentially, dried over Na2SO4, ltered,
and concentrated. The sulfur ylide 2 was obtained and can be
used directly without further purication.
Conclusions
Polysubstituted furans with carboxylate(s) were prepared in
moderate to good yields from dimethylsulfonium acylmethy-
lides with alkyl acetylenecarboxylates through
a tandem
sequence of Michael addition, intramolecular nucleophilic
addition, 4p ring opening, intramolecular Michael addition,
and elimination. The current method provides a direct and
simple strategy in the efficient preparation of dialkyl 2-
substituted furan-3,4-dicarboxylates from safe and readily
available dimethylsulfonium acylmethylides and dialkyl acety-
lenedicarboxylates. In addition, the current method can also be
applied for the synthesis of alkyl 2-arylfuran-3-carboxylates,
dialkyl 5-substituted furan-2,4-dicarboxylates, and trialkyl 5-
General procedure for the synthesis of sulfur ylides 2m and
2n14
To a suspension of NCS (3.472 g, 26 mmol) in anhydrous
dichloromethane (1ꢀ10 mL) was added dimethyl sulde (2.7 mL,
36.9 mmol) at À78 C under nitrogen. The mixture was stirred
for 1 h at the same temperature. A solution of an active meth-
ylene compound (20 mmol) was added at the same temperature.
Aer addition of triethylamine (4.2 mL, 30.3 mmol), the
resulting mixture was stirred for another 1 h. The reaction
mixture was treated with cold brine (60 mL) and extracted with
ether (180 mL). The organic layer was washed with brine (60 mL)
three times, dried over Na2SO4, ltered, and concentrated. The
residue was puried by column chromatography with chloro-
form and methanol (20 : 1, v/v) as eluent to afford the desired
yilde 2m or 2n.
substituted
furan-2,3,4-tricarboxylates,
even
alkyl
4-
triuoromethylfuran-3-carboxylates, showing versatile applica-
tion. A new reasonable reaction mechanism is proposed as well.
Experimental
Unless otherwise noted, all materials were purchased from
commercial suppliers. Flash column chromatography was per-
formed using silica gel (normal phase, 200–300 mesh) from
Branch of Qingdao Haiyang Chemical Industry. The petroleum
General procedure for the synthesis of furan derivatives 3
Reaction conditions A. To a stirred suspension of sulfur ylide
ether (PE) used for column chromatography is the 60–90 ꢀC 2 (0.25 mmol) in DMSO (1 mL) was added acetylenic ester 1
fraction, and the removal of residue solvent was accomplished (0.125 mmol). The mixture was further stirred at 80 ꢀC for 4
under rotovap. The reactions were monitored by thin-layer hours under nitrogen. Aer cooling and addition of water (10
chromatography (TLC) on silica gel GF254 coated 0.2 mm mL), the mixture was extracted with DCM, and combined
plates from Institute of Yantai Chemical Industry. The plates organic layer was washed with brine, dried over anhydrous
were visualized under UV light, as well as the other TLC stains sodium sulfate, and concentrated in vacuo. The resulting
(10% phosphomolybdic acid in ethanol; 1% potassium residue was puried by silica gel column chromatography with
permanganate in water; 10 g of iodine absorbed on 30 g of silica petroleum ether and ethyl acetate (10 : 1, v/v) as eluent to afford
1
gel). H and 13C NMR spectra were recorded on a Bruker 400 furan 3.
MHz spectrometer in CDCl3 with TMS as an internal standard,
Reaction conditions B. To a stirred suspension of sulfur ylide
and the chemical shis (d) are reported in parts per million 2 (0.25 mmol) in DMSO (1 mL) was added acetylenic ester 1
(ppm). All coupling constants (J) in 1H NMR spectra are absolute (0.125 mmol). The mixture was further stirred at 160 ꢀC for
values given in hertz (Hz) with peaks labelled as singlet (s), 15 min under microwave irradiation with N2 protection. Aer
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cooling and addition of water (10 mL), the mixture was extrac-
ted with DCM. The combined organic layer was washed with
brine, dried over anhydrous sodium sulfate, and concentrated
in vacuo. The resulting residue was puried by silica gel column
chromatography with petroleum ether and ethyl acetate (10 : 1,
v/v) as eluent to afford furan 3.
R. Yan, J. Huang, J. Luo and P. Wen, Synlett, 2010, 1071–
1074.
7 (a) L. Q. Lu, T. R. Li, Q. Wang and W. J. Xiao, Chem. Soc. Rev.,
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Conflicts of interest
8 (a) S. Kramer and T. Skrydstrup, Angew. Chem., Int. Ed., 2012,
51, 4681–4684; (b) X. Huang, B. Peng, M. Luparia,
L. F. Gomes, L. F. Veiros and N. Maulide, Angew. Chem.,
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There are no conicts to declare.
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