Synthesis of substituted furans through domino aldol/homo-Michael reactions of formylcyclopropane 1,1-diesters with 1,3-dicarbonyls

Article abstract of DOI:10.1002/ejoc.201200162

A Lewis acid catalyzed domino aldol/homo-Michael reaction of formylcyclopropane 1,1-diesters with dicarbonyls has been successfully developed. This method provides efficient and direct construction of highly functionalized furans from easily available sta

Full text of DOI:10.1002/ejoc.201200162

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201200162
Synthesis of Substituted Furans through Domino Aldol/Homo-Michael
Reactions of Formylcyclopropane 1,1-Diesters with 1,3-Dicarbonyls
Jilai Bao,[a] Jun Ren,[a] and Zhongwen Wang*[a]
Keywords: Small ring systems / Domino reactions / Oxygen heterocycles / Michael addition / Aldol reactions
A Lewis acid catalyzed domino aldol/homo-Michael reaction
of formylcyclopropane 1,1-diesters with dicarbonyls has been
successfully developed. This method provides efficient and
direct construction of highly functionalized furans from easily
available starting materials.
clohexane-1,3-dione (2a), by which substituted hydrobenzo-
furan 3a was afforded efficiently (Scheme 1). Several suit-
ably substituted examples (Figure 1) could be selected as
potential targets for synthesis. Herein, we report our recent
results.
Introduction
Furan is a common and important nucleus broadly exi-
sting in natural products (e.g., pseudopteranes, furanocem-
branes, and 4-oxybenzufurans; Figure 1),[1,2] pharmaceuti-
cals, and agrochemicals. Additionally, furans can also be
employed as useful intermediates in organic synthesis.[3] Al-
though many synthetic methods have been developed for
the synthesis of furans,[4,5] the development of a novel
method for the synthesis of highly functionalized furans un-
der mild conditions and from readily available starting ma-
terials remains an important goal.
Figure 1. Representative furanoterpenoid and 4-oxybenzofuran
natural products.
Scheme 1.
The domino reaction represents one of the most efficient
transformations for cyclic skeletons.[6] We have recently de-
veloped a [3+2] IMCC (intramolecular [3+2] cross-cycload-
dition) strategy on functionalized cyclopropanes for the
construction of bridged [n.2.1] skeletons.[7] During the fur-
ther development of the [3+2] IMCC-based domino process
aimed at providing a more efficient method for the con-
struction of polycyclic skeletons (e.g., cortistatin A[8]), we
observed an unexpected domino process from readily avail-
able formylcyclopropane (FCP) 1,1-diester 1a[9,10] and cy-
Results and Discussion
The first experiment was carried out between FCP 1,1-
diester 1a and cyclohexane-1,3-dione (2a) in DCE (1,2-
dichloroethane) at 40 °C under the catalysis of Sc(OTf)3
(20 mol-%). After 40 h, instead of the bridged product, we
obtained compound 3a, which was supposed to be an inter-
rupted [3+2] IMCC product (Table 1, Entry 10). Several
other Lewis acids (LAs) were tested, among which
Sc(OTf)3 proved to be suitable. Solvent screening showed
that DCE was the best choice. It was also discovered that
increasing the amount of 1a afforded a better result
(Table 1, Entries 10, 14, and 20). Reaction at 25 °C (Table 1,
Entry 22) gave a better yield than that at 40 °C (Table 1,
Entry 21).
[a] State Key Laboratory of Elemento-Organic Chemistry,
Institute of Elemento-Organic Chemistry, Nankai University,
Tianjin 300071, P. R. China
E-mail: wzwrj@nankai.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200162.
Eur. J. Org. Chem. 2012, 2511–2515
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2511

J. Bao, J. Ren, Z. Wang
Table 2. Investigation of the reaction scope.[a]
SHORT COMMUNICATION
Table 1. Optimization of the reaction conditions.
Entry
T
[°C]
Catalyst
Solvent
t
[h]
Yield[d]
[%]
1[a]
15
40
40
40
40
40
40
40
40
40
80
40
40
40
40
40
40
––
CuOTf
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
PhMe
THF
DCM
CHCl3
CCl4
EtOH
DCE
DCE
DCE
DCE
DCE
DCE
24
40
40
40
40
40
40
40
40
40
20
40
40
40
40
40
40
40
24
40
72
40
40
0
38
35
20
11
24
29
–
2[a]
Entry
R1
2
R2
–(CH2)3–
–(CH2)3–
–(CH2)3–
R3
t
[h]
Product
Yield[b]
[%]
3[a]
Cu(OTf)2
Sn(OTf)2
AgOTf
Bi(OTf)3
Yb(OTf)3
ZnCl2
Zn(OTf)2
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
TiCl4
4[a]
5[a]
1
2
3
4
5
6
7
8
Et
Me
iPr
Et
Et
Et
Et
Et
Et
Et
2a
2a
2a
2b
2c
2d
2e
2f
40
40
40
18
40
40
40
40
40
40
40
12
2.5
18
14
14
14
3a
3b
3c
3d
3e
3f/4f
3g/4g
3h/4h
3i/4i
3j/4j
3k
3l
3m
3n
84
67
61
90
56
32/41
26/49
38/42
42/28
25/34
100
99
6[a]
7[a]
8[a]
Me
Me
9[a]
–
C6H5
C6H5
C6H5
C6H5
C6H5
Me
10[a]
11[a]
12[b]
13[b]
14[b]
15[b]
16[b]
17[b]
18[b]
19[b,f]
20[c]
21[c]
22[c]
23[c]
43
18
28
28
27
28
21
9
50
20
69[e]
64[e]
84[e]
67[e]
iBu
Et
Me
Me
9
2g p-MeC6H4
2h p-ClC6H4
2i
2j
10
11
12
13
14
15
16
17
Et
Et
Me
C6H5
C6H5
Me
Me
Me
OEt
OEt
OEt
OMe
OMe
OEt
OEt
Me
Me
Et
Me
Et
2j
98
86
62
50
2k
2k
2i
40
40
40
40
3o
3p
3q
2l CH2CO2Et
28
25
[a] Reaction conditions: 1 (2.0 equiv.), 2 (1.0 equiv.), Sc(OTf)3
(20 mol-%), 4 Å MS (50 mg), and DCE (5 mL, 0.1 m) under a N2
atmosphere at room temperature. [b] Isolated yield based on 2.
–78 to 25
[a] Reaction conditions: 1a (1.0 equiv.), 2a (1.5 equiv.), catalyst
(20 mol-%), 4 Å MS (50 mg), and the solvent (5 mL, 0.1 m) under
a N2 atmosphere. The crude product was purified by column
chromatography. [b] 1a (1.0 equiv.), 2a (2.0 equiv.). [c] 1a
(2.0 equiv.), 2a (1.0 equiv.). [d] Isolated yield based on 1a. [e] Iso-
lated yield based on 2a. [f] Anhydrous MgSO4 (250 mg) was added
instead of 4 Å MS.
Scheme 2. Unusual interruption of furan generation.
With the optimal reaction condition in hand, the scope
of both FCP 1,1-diesters 1 and 1,3-dicarbonyls 2 was inves-
tigated. The results of these experiments are summarized in
Table 2. Reactions between FCP 1,1-diester 1 with symmet-
rical 1,3-diketones 2 afforded 3. Both aryl and alkyl dike-
tones worked well. Whereas pentane-2,4-dione (2b) gave
product 3d in excellent yield (Table 2, Entry 4), 1,3-di-
phenylpropane-1,3-dione (2c) gave product 3e in moderate
yield (Table 2, Entry 5). For unsymmetrical 1,3-diketones,
two regioselective isomers were obtained (Table 2, En-
tries 6–10). We also delightfully found that β-keto esters
also worked well (Table 2, Entries 11–17). In some examples
(Table 2, Entries 9, 10, and 17), together with the main
products, decarbonylative products were obtained as by-
products.[11] Both phenyl- and alkyl-substituted β-keto es-
ters gave domino products 3 in moderate to excellent yields.
The reaction of FCP 1,1-diester 1a with 1H-indene-
1,3(2H)-dione (from Aldrich) only gave aldol/dehydration
product 3r (Scheme 2). Because of the noted and unusual
interruption of furan generation, and to further explore the
possible mechanism, several other experiments were carried
out (Scheme 3). First, we examined the process in Table 1
isted but were not captured at a low temperature (Table 1,
Entry 23). Then, a mono-ketone was introduced to take the
place of the 1,3-diketones under standard aldol conditions,
to find out whether similar aldol/dehydration products ex- Scheme 3. Protocol used to explore the possible mechanism.
2512 Eur. J. Org. Chem. 2012, 2511–2515
www.eurjoc.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Substituted Furans through Domino Aldol/Homo-Michael reactions
Scheme 4. Possible mechanism.
but unfortunately, the reaction of FCP 1,1-diester 1a with
1-(p-tolyl)ethanone (6) failed. However, Mukaiyama aldol
product 8 was successfully obtained from 1a and tert-butyl-
dimethyl [(1-phenylvinyl)oxy]silane (7) under TiCl4 catalysis
(100 mmol-%), which was subsequently converted into
furan product 3s when the temperature was elevated from
–78 °C to higher than 12 °C (Scheme 3). Also, a one-pot
experiment gave the same result. At last, compound 5 was
synthesized independently (see the Supporting Infor-
mation), but no reaction occurred when compound 5 was
treated under the TiCl4 catalytic conditions. These results
indicated that the final furan product came from intermedi-
ate 8 instead of dehydration intermediate 5. Only if dehy-
dration gave a more stable product would furan generation
be terminated (i.e., 3r).
On the basis of these results, a possible mechanism was
proposed (Scheme 4), which is similar to that of the Feist–
Benary reaction[12] of formyl epoxide with 2-haloketone: re-
action between 1 and 2 gives aldol product 9, which can
subsequently undergo an intramolecular SN2-like homo-
Michael reaction[7c,13] to afford ring-opened intermediate
10. Instead of [3+2] IMCC product 11, intermediate 10
undergoes a dehydration/deprotonation process to afford
furan product 3 or 4 through intermediate 13. Depending
on the experimental results, the pathway to 12 by dehy-
dration of 9 has been proved to be impossible.
Conclusions
We have developed a new LA-catalyzed domino aldol/
homo-Michael reaction of FCP 1,1-diesters with dicarbon-
yls. Features of this reaction include mild reaction condi-
tions, readily available and structurally diverse starting ma-
terials, and good yields. This supplies a convenient method
for the synthesis of substituted furans.
Experimental Section
General Procedure for Lewis Acid Catalyzed Domino Reactions of
FCP 1,1-Diesters 1 and 1,3-Dicarbonyls 2: To an oven-dried, 50-
mL, three-necked flask was charged with 4 Å molecular sieves
(50 mg) and Sc(OTf)3 (50 mg, 0.1 mmol). 1,3-Dicarbonyl
2
(0.5 mmol), FCP 1,1-diester 1 (1.0 mmol), and dry DCE (5 mL)
were then added sequentially under a positive pressure of nitrogen.
The reaction mixture was stirred at room temperature for the given
period of time. After completion of the reaction (as monitored by
TLC), the solvent was evaporated in vacuo, and the residue was
purified by flash chromatography to afford products 3.
Supporting Information (see footnote on the first page of this arti-
cle): General methods, complete experimental details, and charac-
terization data for all compounds.
Acknowledgments
These furan products also have potential utilization. 4-
Oxybenzofuran is a common substructure existing in natu-
ral and synthetic compounds (Figure 1) and synthetic bio-
logically active compounds. Oxidation of 3a easily afforded
compound 14 in excellent yield (Scheme 5).[14] This supplied
an efficient method to prepare suitably substituted benzo-
furan products and their analogues.
We thank the National Natural Science Foundation of China,
National Key Project of Scientific and Technical Supporting Pro-
grams (973 Program) (2010CB126106), National Key Technologies
R&D Program (2011BAE06B05), and “111” Project of Ministry of
Education of China (B06005) for financial support.
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Eur. J. Org. Chem. 2012, 2511–2515
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2513

J. Bao, J. Ren, Z. Wang
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Received: February 10, 2012
Published Online: March 27, 2012
Eur. J. Org. Chem. 2012, 2511–2515
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