Synthesis of 2-substituted 4,5-dihydro-4-oxo-3-furancarboxylates using acylative intramolecular cyclization of sulfonium salts

Article abstract of DOI:10.1016/j.tetlet.2017.11.035

A simple and efficient synthesis of 4,5-dihydro-4-oxo-3-furancarboxylates using an acylative intramolecular cyclization of sulfonium salts is described. The reaction involved the efficient formation of a mixed anhydride between a linear carboxylic acid and trifluoroacetic anhydride in the presence of N-methylimidazole, followed by the sequential conversion into a highly reactive acylammonium species in situ. This procedure is easily handled, uses readily available inexpensive reagents, and provides a variety of 2-substituted 4,5-dihydro-4-oxo-3-furancarboxylates.

Full text of DOI:10.1016/j.tetlet.2017.11.035

Accepted Manuscript
Synthesis of 2-Substituted 4,5-Dihydro-4-oxo-3-furancarboxylates Using Acy-
lative Intramolecular Cyclization of Sulfonium Salts
Sho Inagaki, Akari Sato, Haruka Sato, Satoru Tamura, Tomikazu Kawano
PII:
S0040-4039(17)31449-1
https://doi.org/10.1016/j.tetlet.2017.11.035
TETL 49478
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
11 October 2017
10 November 2017
16 November 2017
Please cite this article as: Inagaki, S., Sato, A., Sato, H., Tamura, S., Kawano, T., Synthesis of 2-Substituted 4,5-
Dihydro-4-oxo-3-furancarboxylates Using Acylative Intramolecular Cyclization of Sulfonium Salts, Tetrahedron
Letters (2017), doi: https://doi.org/10.1016/j.tetlet.2017.11.035
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Synthesis of 2-Substituted 4,5-Dihydro-4-oxo-3-furancarboxylates Using Acylative
Intramolecular Cyclization of Sulfonium Salts
Sho Inagaki, Akari Sato, Haruka Sato, Satoru Tamura and Tomikazu Kawano∗
Department of Medicinal and Organic Chemistry, School of Pharmacy, Iwate Medical University, Yahaba, Iwate 028-3694, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple and efficient synthesis of 4,5-dihydro-4-oxo-3-furancarboxylates using an acylative
intramolecular cyclization of sulfonium salts is described. The reaction involved the efficient
formation of a mixed anhydride between a linear carboxylic acid and trifluoroacetic anhydride in
the presence of N-methylimidazole, followed by the sequential conversion into a highly reactive
acylammonium species in situ. This procedure is easily handled, uses readily available
Available online
inexpensive reagents, and provides
furancarboxylates.
a
variety of 2-substituted 4,5-dihydro-4-oxo-3-
Keywords:
3
4
(2H)-Furanone
,5-Dihydro-4-oxo-3-furancarboxylate
2
009 Elsevier Ltd. All rights reserved.
Sulfonium salt
Acylative intramolecular cyclization
One-pot process
Introduction
mixture of N-halosuccinimide and 4-(dimethylamino)pyridine
DMAP), produces a variety of 4-substituted 3(2H)-furanones.
Regarding the reaction mechanism, we believe that using an elec-
trophilic acylating reagent will lead to 2-substituted 4,5-dihydro-
(
Substituted 3(2H)-furanones are important structures presen1t
in many natural products and biologically active compounds.
These derivatives are known to exhibit a variety of pharmaceuti-
4
-oxo-3-furancarboxylates 2 and the resulting carboxylate 2 will
2
cal activities. Therefore, many synthetic methodologies towards
5,6,7
be used as a versatile substance.
In this paper, we report the
3
,4
polysubstituted 3(2H)-furanones have been developed. Among
substituted 3(2H)-furanones, 4,5-dihydro-4-oxo-3-
furancarboxylates, which have an alkoxycarbonyl group at the 3-
position on the 3(2H)-furanone ring are also comprised in various
natural products, and are known to exhibit antiallergy, antima-
acylative intramolecular cyclization of sulfonium salts as a novel
procedure for the preparation of 2-substituted 4,5-dihydro-4-oxo-
3
-furancarboxylates. This procedure is simple and practical, and
use inexpensive commercially available reagents to give the tar-
get compounds via a one-pot process under mild conditions.
5
6
larial, and anticancer activities. In addition, these carboxylates
serve as versatile scaffolds to build 1H-pyrazole, oxazole, and
azepine skeletons. These attractive features of carboxylates have
Previous Work
O
7
Ph
S
O
O
BF4
Ph
motivated many organic chemists, which has resulted in the de-
velopment of various synthetic methodologies including the ac-
ylation/cyclization of chloroacetyl chloride to 1,3-dicarbonyl
OR
OR
Intramolecular Cyclization
O
1
OR
6
a,8
base
compounds,
the acylation/cyclization of ethyl α-
O
6c,8a,9
haloacetoacetates and isocyanates,
and the photo-oxidation-
Ph
O
O
Alkyl
OR
Alkyl-X, t-BuOK
10
intramolecular Michael addition of 2-furyl-1,3-diketones. To
date, an efficient and flexible approach for the introduction of
various functional groups at the 2-position in 4,5-dihydro-4-oxo-
S
Ph
Alkylative
Intramolecular Cyclization
O
A
O
3
-furancarboxylates has not been achieved, and therefore the
X
development of a novel synthetic methodology is required.
N-Halosuccinimide, DMAP
Halogenative
Intramolecular Cyclization
O
We have previously reported novel synthetic methodology for
OR
O
the construction of a substituted 3(2H)-furanone framework via
1
1
This Work
O
the intramolecular cyclization of a sulfonium salt (Scheme 1).
Thus, the reaction between enolate A, generated from sulfonium
salt 1 under basic condition, and electrophiles such as alkyl hal-
ides or coordination complexes, prepared from an equimolar
R'COX
OR
Acylative
Intramolecular Cyclization
O
R'
2
—
——
∗
Corresponding author. Tel.: +81-19-651-5111; fax: +81-19-698-1932; e-mail: author@tkawano@iwate-med.ac.jp

2
Tetrahedron
a
Scheme 1. Intramolecular cyclization of sulfonium salt 1
Table 1. Optimized conditions
Results and Discussion
We commenced the investigation of acylative intramolecular
cyclization with 3-ethoxycarbonyl-2-
oxopropyldiphenylsulfonium tetrafluoroborate (1a). In order to
find acylating agents suitable for this reaction, we screened a
variety of acetylating agents, such as acetyl chloride, acetic an-
hydride, and activated esters derived from acetic acid. Under
standard reaction conditions, 1.0 equiv of sulfonium salt 1a was
treated with 1.0 equiv of acetylating agent in the presence of 1.0
equiv of DMAP in acetonitrile (MeCN). Among the acetylating
agent used, the most reactive acetyl chloride gave 4-
chloroacetoacetate as a major product. This result suggested that
substitution reaction between sulfonium salt and eliminated chlo-
ride ion should be faster than acylation. Finally, only acetic an-
hydride underwent acylative intramolecular cyclization with
moderate yield (Table S1).
b
Entry
Reagent
Solvent
MeCN
MeCN
MeCN
MeCN
MeCN
Base
NMI
NMI
NMI
NMI
NMI
NMI
NMI
NMI
NMI
NMI
1
Yield/%
36
1
2
3
4
5
6
7
8
9
Ts
2
O
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
Tf
2
O
42
Piv
2
O
35
MNBA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
34
49
CH
2
Cl
2
37
Bearing these results in mind, we then used a mixed anhydride
generated in situ from readily available carboxylic acids and
electrophilic activating agents to simplify the experimental pro-
cedure. The examination of electrophilic activating agents for the
acylative intramolecular cyclization was carried out with butyric
acid in the presence of 3.0 equiv of N-methylimidazole (NMI) in
MeCN (Table 1). To our satisfaction, when sulfonic anhydrides
THF
40
DMF
DMA
NMP
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
56
54
1
0
52
d
11
Et
3
N
n.d.
such
as
p-toluenesulfonic
anhydride
O) were used, expected
-substituted product 2a was obtained in moderate yields (entries
and 2), but base-induced unsubstituted cyclized product, 5-
(Ts
2
O)
and
d
1
1
2
3
K
2
CO
3
n.d.
trifluoromethanesulfonic anhydride (Tf
2
1
2
d
pyridine
imidazole
4-MeOPy
DMAP
PPY
n.d.
14
7
11b
ethoxy-3(2H)-furanone, was detected as a byproduct. Among
1
5
10
55
50
59
72
48
other electrophilic activating agent tested, i.e. pivalic anhydride
(
Piv
2
O), 2-methyl-6-nitrobenzoic anhydride (MNBA), and
16
trifluoroacetic anhydride (TFAA), TFAA was the most efficient,
generating 2a in 49% yield (entry 5).
1
1
1
2
7
8
9
0
c
c
c
NMI
Next, we investigated the effect of the solvent on this reaction
NMI
(
entries 6–10). The results revealed that the use of polar aprotic
NMI
solvents facilitates the reaction and DMF was found to be the
most suitable solvent among those tested. With the best condi-
tions in hand, we then screened different bases. Poor nucleophilic
a
All reactions were performed using butyric acid 3a (0.2 mmol), reagent (1.0
equiv), base (3.0 equiv), and sulfonium salt 1 (1.0 equiv) in solvent (2.0 mL).
Ts O = p-toluenesulfonic anhydride, Tf O = trifluoromethanesulfonic anhy-
dride, Piv O = pivalic anhydride, MNBA = 2-methyl-6-nitrobenzoic anhy-
dride, TFAA = trifluoroacetic anhydride, MeCN = acetonitrile, DMF = N,N’-
dimethylformamide, DMA = N,N’-dimethylacetamide, DMP = N-
methylpyrrolidone, NMI = N-methylimidazole, 4-MeOPy = 4-
amines, such as triethylamine (Et
3
N), or inorganic bases prevent-
2
2
2
ed the acylative intramolecular reaction and only the
1
1b
intramolecular cyclization product was obtained (entries 11
and 12). Using aromatic nucleophilic amines, yields increased
corresponding to the basicity of the amine, but were lower than
that using NMI (entries 8 and 13–17). Lowering the temperature
of step 1 slightly increased the yield (entry 18). Modification of
the ester moiety of the sulfonium salt improved the yield signifi-
cantly. Thus, when the sulfonium salt with a 2,2,2-trifluoroethyl
group in the ester moiety was used, desired product 2b was ob-
tained in 72% yield (entry 19). However, the introduction of
b
methoxypyridine, PPY = 4-pyrrolidinopyridine. Isolated yield based on 1.
Step1) 0 °C, 0.5 h. n.d. = not detected.
c
d
With the optimized reaction condition in hand, we explored
the scope of various carboxylic acids (3) in the acylative
intramolecular cyclization (Table 2). The reaction using short-
chain carboxylic acid, such as butyric acid (3a), acetic acid (3b),
and propionic acid (3c) afforded desired products 2b, 2d, and 2e
in 72%, 70%, and 76% yields, respectively (entries 1–3).
Branched isobutyric acid (3d) remarkably diminished the product
yield, presumably due to its steric hindrance (entry 4). Similarly,
benzoic acid was difficult to be applied to the reaction (no entry).
When medium or long-chain linear carboxylic acid were used,
the reaction progressed successfully (entries 5 and 6). Further-
more, carboxylic acids 3g and 3h, bearing a carbon–carbon dou-
ble or triple bond produced 2i and 2j in 76% and 71% yield, re-
spectively (entries 7 and 8). Carboxylic acids containing a ben-
zene ring or a 2-, 3-, or 4-brominated benzene ring were tolerated
in the reaction (entries 9–13). Carboxylic acids containing a vari-
ety of other functional groups such as bromoalkyl (3n and 3o),
silyl ether (3p), Cbz-protected amine (3q), ketone (3r), acetal
1
,1,1,3,3,3-hexafluoropropan-2-yl group into the sulfonium salt
resulted in a diminished yield and this observation might be due
to steric hindrance (entry 20). It is noteworthy that any O-
acylating products and cyclized products bearing trifluoromethyl
group at 2-position were not detected under the reaction condi-
tions.
(
3s), and ester (3t), underwent acylative intramolecular cycliza-

tion to give corresponding 3(2H)-furanones 2p–2v in good yield
entries 14–20).
min of mixing, 0.1 M), revealed signals corresponding to a mix-
ture of mixed anhydride B and acylammonium species C in near-
ly 1:1.7 ratio (Figure S1). Among the signals, the chemical shift
of the α-position of the acylammonium species appeared at 3.63
(
a
Table 2. Reaction using various carboxylic acids
1
2
ppm, which is consistent with reported data. On the other hand,
the NMR spectrum in DMF-d revealed the presence of mixed
anhydride B and acylammonium species C in nearly 1:10 ratio
Figure S2). These results indicate that DMF promotes the effi-
7
(
cient formation of the acylammonium species in the reaction
system.
b
Entry
R
Carboxylic
Product 2
Yield/%
acid 3
Based on these results, a proposed reaction mechanism is de-
scribed in Scheme 2. Initially, mixed anhydride B is formed from
carboxylic acid 3 and TFAA in the presence of NMI, which is
immediately converted into highly reactive acylammonium salt C
in the presence of excess NMI. Enolate A, generated by the
deprotonation of the sulfonium salt 1b with the remaining NMI,
reacts with acylammonium C to produce acylated sulfonium D
and NMI. Enolate E, derived from the subsequent deprotonation
of D by regenerated NMI, undergoes ring closure reaction to give
1
2
3
4
Pr
Me
Et
3a
3b
3c
3d
2b
2d
2e
2f
72
70
76
20
iPr
5
6
3e
3f
2g
2h
71
61
2
-substituted 4,5-dihydro-4-oxo-3-furancarboxylate 2.
7
8
9
3g
3h
3i
2i
2j
76
71
77
2k
Ph
1
1
1
0
1
2
3j
3k
3l
2l
2m
2n
69
74
77
1
1
3
4
3m
3n
2o
2p
72
67
Scheme 2. Proposed Reaction Mechanism
1
1
1
5
6
7
3o
3p
3q
2q
2r
2s
70
70
70
Conclusion
We have developed a one-pot synthesis of 2-substituted 4,5-
dihydro-4-oxo-3-furancarboxylates via acylation followed by
intramolecular cyclization of sulfonium salts under mild condi-
tions. The procedure is suitable to linear chain carboxylic acids
containing a variety of functional groups. The use of DMF is
crucial for this reaction as it promotes the formation of a highly
reactive acylammonium species between the carboxylic acid and
TFAA in the presence of excess NMI. This procedure uses a
practical convenient operation and can be immediately applied
for the construction of related molecular frameworks. Further
development of the reaction using sulfonium salts is underway in
our laboratory.
1
8
3r
2t
64
1
2
9
0
3s
3t
2u
2v
65
62
a
All reactions were performed using carboxylic acid 3 (0.2 mmol), sulfonium
b
salt 1b (1.0 equiv), and NMI (3.0 equiv) in DMF (2.0 mL). Isolated yield
based on 1b.
Acknowledgments
We hypothesized that the mixed anhydride formed between
the carboxylic acid and TFAA in the presence of base reacts with
NMI to generate a highly reactive acylammonium species. A H
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
1
NMR monitoring study using an equimolar mixture of
phenylpropionic acid and TFAA in the presence of 3.0 equiv of
NMI in CDCl under the reproduced reaction conditions (after 30
3

4
Tetrahedron
Supplementary data
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Highlights
ꢀ
ꢀ
ꢀ
ꢀ
A flexible and efficient synthesis of 2-substituted 4,5-dihydro-4-oxo-3-furancarboxylates is reported.
A variety of functionalized linear carboxylic acids can be applied to the reaction.
The reactions are performed under mild conditions via one-pot process.
Oxopropyldiphenylsulfonium salts serve as a three-carbon unit for the construction of 5-membered ring.
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Synthesis of 2-Substituted 4,5-Dihydro-4-
oxo-3-furancarboxylates Using Acylative
Intramolecular Cyclization of Sulfonium
Salts
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Sho Inagaki, Akari Sato, Haruka Sato, Satoru Tamura, Tomikazu Kawano
Synthesis of 2-Substituted 4,5-Dihydro-4-
oxo-3-furancarboxylates Using Acylative
Intramolecular Cyclization of Sulfonium
Salts
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Sho Inagaki, Akari Sato, Haruka Sato, Satoru Tamura, Tomikazu Kawano