Pd/Cu-catalyzed cascade Sonogashira coupling/cyclization reactions to highly substituted 3-formyl furans

Article abstract of DOI:10.1039/c0ob00985g

An efficient palladium/copper-catalyzed approach to the synthesis of highly substituted 3-formyl furans from the reactions of readily available α-bromoenaminones with terminal alkynes has been developed. This methodology was realized by the cascade reactions of Sonogashira coupling and the subsequent intramolecular cyclization. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0ob00985g

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Pd/Cu-catalyzed cascade Sonogashira coupling/cyclization reactions to
highly substituted 3-formyl furans†
Jingyu Yang, Chengyu Wang, Xin Xie, Hongfeng Li, Ende Li and Yanzhong Li*
Received 5th November 2010, Accepted 17th December 2010
DOI: 10.1039/c0ob00985g
An efficient palladium/copper-catalyzed approach to the
synthesis of highly substituted 3-formyl furans from the reac-
tions of readily available a-bromoenaminones with terminal
alkynes has been developed. This methodology was realized
by the cascade reactions of Sonogashira coupling and the
subsequent intramolecular cyclization.
from the Feist–Benary synthesis.17 This has some drawbacks such
as the protection and deprotection of other functional groups on
the furan ring. Therefore, the development of an efficient and
direct approach for 3-formyl furan formation is highly desired.
During the course of our ongoing study on the development of
heterocycle forming protocols,18 we found that functionalized 1,4-
dihydropyridines could be efficiently prepared from enaminones.19
Enaminones are versatile reagents that combine the ambident
nucleophilicity of enamines with the ambident electrophilicity
of enones. We envisioned that enaminones incorporating an
alkyne moiety at the a-position might undergo transition metal-
catalyzed cyclization to substituted furans with a C-3 formyl group
after hydrolysis, because the inherent behavior of the push-pull-
substituted enaminones may further facilitate the nucleophilic
attack of the carbonyl oxygen to the triple bond (Scheme 1).
Transition metal-catalyzed cascade reactions have been proved
as powerful tools for the synthesis of a variety of organic
compounds in a one-pot manner.1 These reactions allow the
rapid construction of elaborate or highly functionalized organic
molecules in one single operation. Polysubstituted furans are
very important as core structures of many natural products
and pharmaceuticals.2 They are also useful building blocks in
synthetic organic chemistry.3 As a result, a variety of synthetic
procedures have been disclosed for furan ring formation.4 Among
the many approaches to substituted furans, transition metal-
catalyzed cyclization reactions are the most effective. The acyclic
precursors involve allenyl ketones,5 alkynyl ketones,6 2-(1-alkynyl)-
2-alken-1-ones,7 enynols or phenols,8 1,3-enynyl ketones9 and
others.10 Although these methods are efficient for substituted
furan formation, procedures for the preparation of formylated
furans are rare,11 and the metal mediated direct synthesis of
trisubstituted furans with a C-3 or C-4 formyl group has not
been reported,12 to the best of our knowledge. 3-Formyl furan
derivatives are versatile synthetic intermediates for the preparation
of core structures of bioactive natural products that have shown
a wide range of activities such as anticancer, antifungal and
antifouling agents,13 for example, for the preparation of bioactive
butenolides and (-)-nakadomarin A which showed cytotoxic
activity against murine lymphoma L1210 cells.14 Generally, 3-
formyl furans could be synthesized in two ways. First is the
metalation of furan rings followed by formylation.15 This could be
problematic because metalation usually occurs preferentially at the
C-2 and C-5 positions. Second is the reduction of carboxylic acids
or carboxylic esters16 on the furan rings obtained, for example,
Scheme 1
Herein, we would like to report the effective synthesis of 3-
formyl trisubstituted furans through cascade Sonogashira cou-
pling/cyclization reactions (Scheme 2).
The requisite a-bromoenaminones were synthesized via bromi-
nation of N-substituted enaminones which were readily pre-
pared through conjugate addition of anilines or aliphatic
amines with terminal alkynones or ethyl propiolate.19 The re-
action of (Z)-2-bromo-3-(butylamino)-1-phenylprop-2-en-1-one
(1a: Scheme 2, R2 = Bu, R3 = H) bearing a secondary amine
group with phenylacetylene (2a) was first carried out using
Pd/Cu as the catalysts. However, no Sonogashira coupling
product was obtained after screening a variety of reaction
conditions. We thought that protected amine might be necessary
for the coupling reaction. Then a p-toluenesulfonyl protected
a-bromoenaminone of (Z)-N-(2-bromo-3-oxo-3-phenylprop-1-
enyl)-N-butyl-4-methylbenzenesulfonamide (1b) was prepared in-
stead to react with 2a (Table 1). When the reaction was carried
out using Pd(PPh3)2Cl2 (5 mol%) and CuI (10 mol%) as catalysts,
Et3N as the base in THF, only the Sonogashira coupling prod-
uct, namely (E)-N-(2-benzoyl-4-phenylbut-1-en-3-ynyl)-N-butyl-
4-methylbenzenesulfonamide (4a), was obtained in 80% yield
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Institute of Medicinal Chemistry, Department of Chemistry, East China
Normal University, 3663 North Zhongshan Road, Shanghai, 200062,
People’s Republic of China. E-mail: yzli@chem.ecnu.edu.cn; Fax: (+86)
021-62233969
† Electronic supplementary information (ESI) available: Experimental
details and spectroscopic characterization of all new compounds. See DOI:
10.1039/c0ob00985g
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Table 1 Optimization studies for the formation of 3-formyl furan 3a
entry
catalyst (5 mol%)
ligand (10 mol%)
H2O
solvent
time (h)
yield(%)a
1b
2b
3b
4
5
6
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(OAc)2
—
—
—
—
—
—
—
—
—
—
—
—
THF
12
12
12
14
24
24
16
16
17
20
4a(80)c
trace
—
63
67
64
62
81
82
DMF
CH3CN
iPr2NH
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
—
1.0
10.0
5.0
5.0
5.0
7
8
9d
10
—
Ph3P
76
a Isolated yields. b 3.0 equiv of Et3N was added as the base. c The reaction was carried out at 50 ◦C. d 2.0 equiv of 2a was used.
Scheme 2
(Table 1, entry 1). Changing the solvent to DMF gave a trace
amount of 3-formyl furan 3a (Table 1, entry 2). Other solvents
such as CH3CN gave no desired products (Table 1, entry 3).
bromoenaminones to produce the desired 3-formyl trisubstituted
furans in good to high yields. a-Bromoenaminone 1c with a 4-
methoxylphenyl substituent located on the carbonyl moiety led
to the formation of 3h in 80% yield (Table 2, entry 8). The
substituents on carbonyl carbon could also be 4-chlorophenyl,
2-furanyl or 1-naphthyl, and the corresponding 3-formyl furans
were obtained in 60–83% yields (Table 2, entries 9–11). When a-
bromoenaminone with a cyclohexyl group on carbonyl carbon
was employed, the reaction with phenylacetylene was completed
in a much longer reaction time to give the desired product 3l
in moderate yield (Table 2, entry 12). The cyclization has also
been successfully extended to the N,N-diisopropyl substituted a-
bromoenaminone 1h, and 62% yield of 3a was obtained through
the reaction with phenylacetylene (Table 2, entry 13). When
an acetyl protected a-bromoenaminone 1i, namely N-(2-bromo-
3-oxo-3-phenylprop-1-enyl)-N-(4-methoxyphenyl)acetamide, was
reacted with phenyacetylene under the standard reaction con-
ditions, 3a was isolated in 84% yield along with N-(4-
methoxyphenyl)acetamide in 93% yield. However, when N, N-
diphenyl substituted a-bromoenaminone such as (Z)-2-bromo-3-
(diphenylamino)-1-phenylprop-2-en-1-one was used to react with
2a, only trace amount of the desired furan was detected with most
of the a-bromoenaminone remaining.
i
Interestingly, when Pr2NH was used as the solvent, 3a could be
obtained in 63% yield (Table 1, entry 4). To our delight, Et3N gave
the desired 3a in 67% yield after 24 h (entry 5). It is worthy to note
that the addition of H2O (5 equiv) can remarkably shorten the
reaction time and give a much higher yield (81%) of the expected
product (Table 1, entry 8). 2 equiv of phenyacetylene gave a similar
result as that of 1.5 equiv (Table 1, entry 9). Other Pd catalysts
such as Pd(OAc)2 in the presence of PPh3 gave a lower yield of
the desired product even with a longer reaction time (Table 1,
entry 10). It was clear that the optimized reaction condition was
to use 5 mol% of Pd(PPh3)2Cl2 and 10 mol% of CuI as the catalysts,
5 equiv of H2O and Et3N as the solvent at 80 ◦C.
With the optimized reaction conditions in hand, we next exam-
ined the substrate scope of this catalytic method for the synthesis
of 3-formyl furans using a variety of a-bromoenaminones 1 and
terminal alkynes 2 (Table 2). Firstly, we investigated the electronic
effects of the aromatic substituents on terminal alkynes. It was
found that an electron-withdrawing (-Cl) aryl group afforded
the corresponding product 3b in 76% yield (Table 2, entry
2). An electron-donating (-OMe) aryl group resulted in 83%
yield with dramatically less reaction time (Table 2, entry 3). 1-
Ethynylnaphthalene (2d) gave good result (Table 2, entry 4). Alkyl
substituted alkynes such as oct-1-yne (2f) and propargylic ether
2g were also compatible under the reaction conditions, furnishing
3f, 3g in 63% and 35% yields, respectively (Table 2, entries 6, 7).
The reaction proceeded smoothly with various N-Ts substituted a-
To understand the reaction mechanism, we stopped the reaction
of 1b with phenylacetylene after 2 h. The coupling product 4a
was isolated in 85% yield [Scheme 3, eqn (1)]. The isolated 4a was
subjected to the standard conditions to afford 3a in 91% yield after
18 h [Scheme 3, eqn (2)]. Without the addition of Pd(PPh3)Cl2,
10 mol% of CuI also furnished 3a in 92% in 17 h [Scheme 3,
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Table 2 Synthesis of various of trisubstituted 3-formyl furans
Entry
1
Bromoenaminone
Alkyne
Time (h)
16
Product
Yield (%)a
81
2
3
4
5
1b
1b
1b
1b
21
11
35
11
76
83
73b
69
6
7
8
1b
1b
21
50
20
63
35
80
2a
2a
2a
9
12
14
60
83
10
11
12
13
2a
2a
2a
14
42
12
78
41c
62
3a
a Isolated yields. Unless noted, all the reactions were carried out at 80 ◦C with Pd(PPh3)2Cl2 (5 mol%) and Cul (10 mol%) as catalysts, 1.5 equiv of alkyne
and 5.0 equiv of H2O in Et3N. b Naph is 1-naphthyl. c Hex is cyclohexyl.
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Scheme 3
Scheme 4 A proposed reaction pathway.
eqn (3)]. Surprisingly, without any metal catalysts, 3a could also
Experimental section
be produced in 54% yield, however, a longer reaction time of 72 h
was required. Moreover, the reaction of 1b with phenylacetylene
under the optimal reaction conditions without the addition of CuI
could also afford the desired 3a in a much lower yield of 54% after
15 h. These results indicated that copper salts could accelerate the
intramolecular cyclization process.
On the basis of the above observations, a possible reaction
mechanism is proposed in Scheme 4. First, Sonogashira coupling
of a-bromoenaminone 1 and terminal alkyne took place to give
the alkynylated enaminone 4. Then the subsequent coordination of
the alkynyl moiety to the Cu(I) salt enhances the electrophilicity of
the triple bond,20 which facilitates an intramolecular cyclization of
the carbonyl oxygen onto the alkyne to afford the iminium cation
6. Hydrolysis of 6 and protonation with regeneration of the Cu(I)
catalyst led to the desired 3-formyl furan 3.
In conclusion, we have successfully developed an efficient
method for the synthesis of trisubstituted furans with a formyl
group at the C-3 position that are difficult to access by other meth-
ods, through Pd/Cu-catalyzed Sonogashira coupling/cyclization
cascade reaction. Further applications of this novel Pd/Cu-
catalyzed trisubstituted furan formation procedure to extend the
scope of synthetic utility of the reaction are under progress in our
group.
A typical procedure for the Pd-catalyzed one-pot synthesis of
2,5-diphenylfuran-3-carbaldehyde (3a)
To a solution of (Z)-N-(2-bromo-3-oxo-3-phenylprop-1-enyl)-N-
butyl-4-methyl benzenesulfonamide (1b) (218 mg, 0.5 mmol) in
Et3N was added H2O (0.045 mL, 2.5 mmol), phenylacetylene
2a (0.082 mL, 0.75 mmol), Pd(PPh3)2Cl2 (18 mg, 5 mmol%)
and CuI (10 mg, 10 mmol%) in this sequence. The resulting
solution was stirred at 80 ◦C for 16 h, the solvent was evapo-
rated under reduced pressure and the residue was diluted with
EtOAc and washed with satd. aq NH4Cl. The organic layer was
separated, and the aqueous layer was extracted with EtOAc. The
combined organic layers were dried over Na2SO4. The solvent
was evaporated under reduced pressure, and the product was
isolated by chromatography on a silica gel column to give the
yellow solid product 2◦,5-diphenylfuran-3-carbaldehyde (3a); 101
1
mg (81%); mp 84–85 C. H NMR (400 MHz, CDCl3, Me4Si)
d 7.04 (s, 1H), 7.28–7.32 (m, 1H), 7.37–7.51 (m, 5H), 7.69–7.77
(m, 4H), 10.10 (s, 1H); 13C NMR (100 MHz, CDCl3, Me4Si) d
103.85, 124.06, 124.63, 127.80, 128.33, 128.69, 128.73, 128.89,
129.18, 130.00, 153.79, 160.23, 185.42; HRMS calcd for C17H12O2
248.0837, found 248.0834.
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (No. 20572025 and 20872037) for financial support. We
also thank the Laboratory of Organic Functional Molecules, the
Sino-French Institute of ECNU for support.
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