DABCO-mediated isocyanide-based multicomponent reaction: Synthesis of highly substituted cyclopentenes

Article abstract of DOI:10.1039/c4ob00012a

Highly substituted cyclopentenes can be accessed rapidly from isocyanides, aldehydes and malononitrile or ethyl cyanoacetate (AB₂C₂) using DABCO as a catalyst under solvent-free conditions at 40 °C within 30 min. This journal is

Full text of DOI:10.1039/c4ob00012a

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Biomolecular Chemistry
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DABCO-mediated isocyanide-based
multicomponent reaction: synthesis of highly
substituted cyclopentenes†
Cite this: Org. Biomol. Chem., 2014,
12, 4628
Li-Rong Wen,*a Ming-Chao Lan,a Wen-Kui Yuana and Ming Li*a,b
Received 3rd January 2014,
Accepted 30th April 2014
Highly substituted cyclopentenes can be accessed rapidly from isocyanides, aldehydes and malononitrile
or ethyl cyanoacetate (AB2C2) using DABCO as a catalyst under solvent-free conditions at 40 °C within
30 min.
DOI: 10.1039/c4ob00012a
www.rsc.org/obc
Introduction
Five-membered carbocycles as structural frameworks are often
found in natural products and medicinally important agents,1
such as ent-isodehydrocostus lactone, (+)-podoandin,2 and
potential anti-HIV agents3 (Fig. 1). Additionally, some can be
used as potent potentiators of AMPA receptors,4 COX-2 inhibi-
tors,5 nucleoside-type antibodies6 and mannosidase inhibi-
tors.7 Due to the importance of cyclopentenes, remarkable
progress has been made which led to the development of
many innovative strategies, concepts and methodologies.8
Among the known synthetic methods, phosphine-catalyzed
[3 + 2] cycloaddition, developed by Lu in 1995,9 is considered
to be a powerful synthetic approach. Although a variety of
methodologies and protocols have been reported by a number
of organic or pharmaceutical chemists,10 it is of great impor-
tance to explore a novel and efficient synthetic method to meet
increasing scientific and practical demands.
Scheme 1
Zwitterionic species are known to arise from the addition of
nucleophiles such as triphenylphosphine,11 pyridine,12 quino-
line,13 as well as other nitrogen heterocycles14 to the activated
π systems. Isocyanides can also form zwitterions with suitable
electrophilic unsaturated systems and help develop novel pro-
tocols for the synthesis of heterocycles and carbocycles.15
Mironov discovered a new reaction of substituted arylidene-
malononitriles with three equiv. of isocyanides leading to the
cyclopentene derivatives.16 Based on our previous endeavors in
exploring novel multicomponent reactions,17 herein we report
a novel five-component reaction (AB2C2) of isocyanides, alde-
hydes, and malononitrile derivatives in a 1 : 2 : 2 ratio to yield
highly substituted cyclopentenes under solvent-free conditions
using DABCO as the catalyst (Scheme 1).
Fig. 1 Selected examples of cyclopentene-containing natural products
and medicinal agents.
aState Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and
Molecular Engineering, Qingdao University of Science and Technology, Qingdao
266042, P. R. China
Results and discussion
bState Key Laboratory of Institute of Elemento-organic Chemistry, Nankai University,
The reaction conditions were optimized using cyclohexyl iso-
cyanide 1a (0.5 mmol), benzaldehyde 2a (1.0 mmol) and malo-
nonitrile 3a (1.0 mmol) as model substrates with various
bases, solvents at different temperatures (Table 1). When the
model reaction was stirred in CH3CN at room temperature for
8 h or in refluxing THF for 4 h without the addition of any
Tianjin 300071, P. R. China. E-mail: liming928@qust.edu.cn,
wenlirong@qust.edu.cn
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, full spectroscopic data for all new compounds, and crystal data for 5e
and 6a (CIFs). CCDC 848387 and 906273. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4ob00012a
4628 | Org. Biomol. Chem., 2014, 12, 4628–4632
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Table 1 Optimization of the reaction conditions for 4aa
Table 2 Synthesis of 4/5 based on malononitrile 3aa
2/R2
4/5
Yieldb (%)
Entry Solvent Base (equiv.)
Temp. (°C) Time (h) Yieldb (%)
Entry
1
1
2
3
4
5
6
7
8
CH3CN
THF
CH3CN DABCO (1.0)
CH3CN Et3N (1.0)
CH3CN Ph3P (1.0)
CH3CN Pyridine (1.0) r.t.
THF
—
—
r.t
Reflux
r.t.
r.t.
r.t.
8
4
8
8
8
8
8
8
0
0
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1a
2a C6H5
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
5a
5b
5c
5d
5e
4 m
64
68
68
70
62
67
66
58
60
63
63
65
64
62
62
64
60
0
2b 4-ClC6H4
2c 4-BrC6H4
2d 4-FC6H4
2e 4-CH3C6H4
2f 3-ClC6H4
2g 3-BrC6H4
2h 3-CH3OC6H4
2i 3-CH3C6H4
2j 2-ClC6H4
2k 2-BrC6H4
2l 2-FC6H4
46
26
21
24
26
21
16
DABCO (1.0)
DABCO (1.0)
DABCO (1.0)
r.t.
r.t.
r.t.
Reflux
r.t.
40
60
(Et)2O
DCM
9
8
2
9
c
10
11
12
13
14
15
16
17
CH3CN DABCO (1.0)
—
42
64
62
63
64
35
64
10
11
12
13
14
15
16
17
18
d
—
—
—
—
—
—
—
DABCO (1.0)
DABCO (1.0)
DABCO (1.0)
DABCO (0.75) 40
DABCO (0.5) 40
DABCO (0.25) 40
DABCO (1.25) 40
10 min
10 min
10 min
10 min
10 min
10 min
10 min
d
d
d
d
d
d
2a C6H5
2b 4-ClC6H4
2c 4-BrC6H4
2d 4-FC6H4
2e 4-CH3C6H4
2m n-Bu
a Reaction conditions: 1a (0.5 mmol), 2a (1.0 mmol), 3a (1.0 mmol),
base (equiv. based on 1a), solvent (1 mL). b Isolated yields based on 1a.
c Complex mixture. d Solvent-free conditions.
a Reaction conditions: isocyanides
1 (0.5 mmol), aldehydes 2
(1.0 mmol), malononitrile 3a (1.0 mmol), DABCO (0.25 mmol), 40 °C,
10 min. b Isolated yields based on isocyanides.
base, only the Mironov product16 as a major product was
obtained and no target compound 4a was observed (Table 1, 4/5 in good yields. Unfortunately, however, aliphatic aldehydes
entries 1 and 2). However, when DABCO (1.0 equiv.) was such as butyraldehyde failed to afford the desired products.
added, a highly substituted cyclopentene 4a was obtained in a
It is worthwhile to note that when an aryl isocyanide such
moderate yield of 46% (Table 1, entry 3). Then other bases, as benzeneisocyanide was employed to react with benz-
such as Et3N, Ph3P, and pyridine, were examined, and the aldehyde 2a and malononitrile 3a under the optimized con-
results revealed that they all failed to effectively promote this ditions, the reaction system was complex and no target
reaction (Table 1, entries 4–6). Other solvents, such as THF, compound was observed.
(Et)2O, and DCM, were next tested in the presence of DABCO
The structures of the products 4 and 5 were identified by
(1.0 equiv.) at room or reflux temperature to improve the yield their IR, 1H NMR, 13C NMR, and HRMS spectral data, and
of 4a, but the results were all unsatisfactory (Table 1, entries unequivocally confirmed by X-ray diffraction analysis of single
7–10). After much careful experimentation, and considering crystals of 5e (Fig. S1 in ESI†).
that the solvent-free reaction is an important synthetic pro-
Encouraged by these results, we further pursued the scope
cedure from the viewpoint of green chemistry,18 solvent-free of the reactions with respect to the substrates 3. Surprisingly,
conditions were attempted. To our delight, a breakthrough when ethyl cyanoacetate 3b replaced 3a to react with 1a and 2a
result was achieved, and 4a was obtained in 64% isolated yield under the above conditions, the reaction did not give rise to
within only 10 min at 40 °C (Table 1, entry 12). Thus, the the tautomerization compounds like compounds 4/5 directly,
amount of DABCO was examined (Table 1, entries 14–17). The but a decarboxylation product 6a was obtained. This obser-
best result was obtained in the mole ratio 0.5 : 1.0 : 1.0 for vation was verified by the 1H NMR spectrum and the X-ray
1a/2a/3a using DABCO (0.5 equiv.) as the base at 40 °C single crystal diffraction analysis of 6a (Fig. S2 in ESI†).
under solvent-free conditions.
The feasibility of employing other aromatic aldehydes to
With the optimized conditions in hand, the scope of the react with ethyl cyanoacetate (3b) and isocyanides was also
five-component reaction was studied, and the results are illus- investigated, and the results demonstrated that all of the reac-
trated in Table 2.
tions proceeded smoothly (Table 3).
As can be seen from Table 2, various aromatic aldehydes 2
This observation motivated us to investigate diethyl malo-
bearing both electron-donating and electron-withdrawing sub- nate (3c) instead of 3a and 3b under the same conditions.
stituents at 2-, 3- or 4-position on the aromatic ring had Unfortunately, only a small amount of Passerini product 8 and
proven to be reliable substrates for this five-component reac- the product from the reaction of diethyl malonate (3c) with an
tion with 1a or 1b and 3a, providing the desired cyclopentenes aldehyde were observed (Scheme 2).
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Table 3 Synthesis of 6/7 based on ethyl cyanoacetate 3ba
Entry
1
2/R2
6/7
Yieldb (%)
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1b
1b
2a C6H5
6a
6b
6c
6d
6e
7a
7b
57
64
68
55
60
57
54
2b 4-ClC6H4
2d 4-FC6H4
2i 3-CH3C6H4
2l 2-FC6H4
2b 4-ClC6H
2e 4-CH3C6H4
Scheme 3 Exploitation of the mechanism.
a Reaction conditions: isocyanides
1 (0.5 mmol), aldehydes 2
(1.0 mmol), ethyl cyanoacetate 3b (1.0 mmol), DABCO (0.25 mmol),
40 °C, 30 min. b Isolated yields based on isocyanides.
Scheme 2 The study of the reaction of diethyl malonate 3c.
Next, we examined the possibility of applying this reaction
with two different aldehydes such as a bearing electron-with-
drawing substituent aldehyde, 2d, and a bearing electron-
donating substituent aldehyde, 2e. When the ratio of 2d and
2e was 1 : 1, 36% cross-coupling product (4de or 4ed) was
obtained, while 4d and 4e were also produced in 27 and 24%
yields, respectively (the yields were given by LC-MS based on
the peak area; see Fig. S3 in ESI†). As we can see from LC-MS,
it is very hard to isolate the cross-coupling products, and there-
fore their structures were not ascertained.
To justify the role of DABCO, we performed the reaction by
using directly 2-benzylidenemalononitrile (1 mmol) and cyclo-
hexyl isocyanide 1a (0.5 mmol) with 0.5 equiv. of DABCO
(0.25 mmol) in CH3CN (0.5 mL) at 40 °C for 6 h; the result
revealed that only the desired product 4a was obtained,
whereas just the Mironov product16 was obtained without
addition of DABCO (Scheme 3). Undoubtedly, in this reaction
process, the key step may be the formation of a zwitterion
derived from DABCO with one 2-arylidenemalononitrile
formed in situ from an aldehyde and a malononitrile, which
were attacked by another highly reactive zwitterion generated
in situ from an isocyanide and another 2-arylidenemalono-
nitrile. We think that in situ generation of both of the reactive
zwitterionic intermediates would be essential for the reaction.
Although we have not experimentally established the mecha-
nism of the reaction, a plausible mechanism is represented in
Scheme 4 based on the above experimental results. First, alde-
hydes 2 react with malononitrile 3a through Knoevenagel con-
Scheme 4 Proposed mechanism for products 4–7.
densation to give the intermediate arylidenemalononitriles A
using DABCO as the catalyst. Next, DABCO as a nucleophilic
trigger reacts with A to generate an intermediate B; meanwhile,
another A reacts with an isocyanide 1 to give a reactive zwitter-
ionic intermediate C, and then B and C undergo a [3 + 2] cycli-
zation to give the intermediate D with the elimination of
DABCO. The latter isomerizes to give the cyclopentenes 4 and
5, or provides products 6 and 7 through losing a molecule of
ethanol and decarboxylation followed by a [1,3]-H shift.
Conclusion
In conclusion, we have demonstrated a novel, rapid and direct
isocyanide-based five-component reaction for the synthesis of
fully substituted cyclopentenes via DABCO-catalyzed annula-
tions under solvent-free conditions within 30 min. The syn-
thetic procedures have the advantages of mild reaction
conditions, short reaction time, convenient handling as well as
a wide substrate scope, which make this method useful for the
4630 | Org. Biomol. Chem., 2014, 12, 4628–4632
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synthesis of potentially biologically active cyclopentene 7.40 Hz, 2H, ArH); 13C NMR (CDCl3, 125 MHz): δ = 13.2, 24.0,
derivatives.
24.3, 25.4, 33.3, 33.6, 52.5, 57.7, 59.2, 60.0, 62.1, 69.9, 118.8,
118.9, 128.4, 128.6, 129.1, 129.4, 130.7, 131.1, 134.3, 156.0,
162.9; IR (KBr) v: 3317, 2934, 2856, 2247, 2183, 1748, 1607,
1581, 1537, 1495, 1454, 1222, 752, 703 cm−1
; HRMS
Experimental
General remarks
(ESI-TOF+): m/z calcd for C28H30N3O2 [(M + H)+], 440.2338;
found, 440.2345.
All reagents and solvents were obtained from commercial sup-
Ethyl 3-(tert-butylamino)-2,5-bis(4-chlorophenyl)-1,4-dicyano-
pliers and used without further purification. All reagents were cyclopent-3-enecarboxylate (7a). White powder; yield: 57%;
1
weighed and handled in air at room temperature. Melting mp 154–156 °C; H NMR (CDCl3, 500 MHz): δ = 0.71–0.74 (t,
points were recorded on a RY-1 microscopic melting apparatus J = 7.18 Hz, 3H, CH3), 1.52 (s, 9H, CH3), 3.55–3.59 (m, 2H,
1
and uncorrected. H NMR spectra were recorded at 500 MHz CH2), 4.48 (s, 1H, NH), 4.61 (s, 1H, CH), 4.88 (s, 1H, CH),
and 13C NMR spectra were recorded at 125 MHz using a 7.34–7.42 (m, 8H, ArH); 13C NMR (CDCl3, 125 MHz): δ = 13.2,
Bruker Avance 500 spectrometer. Chemical shifts were 29.7, 30.2, 30.5, 53.6, 58.0, 58.4, 60.8, 62.5, 70.1, 118.3, 120.3,
reported in parts per million (δ) relative to tetramethylsilane 128.7, 129.4, 129.5, 130.5, 132.3, 132.8, 134.8, 136.0, 153.7,
(TMS). IR spectra were recorded on a Nicolet iS10 FT-IR 162.8; IR (KBr) v: 3362, 2979, 2936, 2874, 2251, 2187, 1741,
spectrometer and only major peaks are reported in cm−1. Mass 1613, 1575, 1534, 1492, 1461, 1398, 1219, 819 cm−1; HRMS
spectra were performed on an Ultima Global spectrometer (ESI-TOF+): m/z calcd for C26H26Cl2N3O2 [(M + H)+], 482.1402;
with an ESI source. The X-ray single-crystal diffraction was per- found, 482.1412.
formed on a Saturn 724+ instrument.
General procedure for the preparation of compounds
4–7. In a round bottom flask equipped with a magnetic bar, a
mixture of isocyanides 1 (0.5 mmol, 1 equiv.), aromatic alde-
Acknowledgements
hydes 2 (1.0 mmol, 2.0 equiv.), and 3 (1.0 mmol, 2.0 equiv.) This work was financially supported by the National Natural
was stirred at 40 °C with DABCO (0.25 mmol, 0.5 equiv.) as the Science Foundation of China (21372137 and 21072110) and
catalyst for an appropriate time. After completion of the reac- the Natural Science Foundation of Shandong Province
tion as indicated by TLC (petroleum ether–EtOAc, 10 : 1, v/v), (ZR2102BM003).
the crude product was purified by column chromatography
(petroleum ether–EtOAc, 10 : 1, v/v) and afforded the pure
products.
Notes and references
4-(Cyclohexylamino)-2,5-diphenylcyclopent-4-ene-1,1,3,3-tetra-
carbonitrile (4a). White powder; yield: 64%; mp 155–157 °C;
1H NMR (CDCl3, 500 MHz): δ = 0.99–1.93 (m, 10H, CH2),
3.06–3.08 (m, 1H, CH), 4.01 (d, J = 9.50 Hz, 1H, NH), 4.46 (s,
1H, CH), 7.47–7.78 (m, 10H, ArH); 13C NMR (CDCl3, 125 MHz):
δ = 24.6, 25.0, 33.7, 46.1, 48.1, 53.6, 58.6, 104.4, 110.7, 111.9,
112.1, 113.2, 128.6, 129.2, 129.4, 129.7, 129.8, 130.2, 131.4,
138.4; IR (KBr) v: 3365, 2925, 2852, 2250, 1663, 1517, 1501,
1457, 1098, 753, 702 cm−1; HRMS (ESI-TOF+): m/z calcd for
C27H24N5 [(M + H)+], 418.2032; found, 418.2039.
4-(tert-Butylamino)-2,5-diphenylcyclopent-4-ene-1,1,3,3-tetra-
carbonitrile (5a). White powder; yield: 64%; mp 113–114 °C;
1H NMR (CDCl3, 500 MHz): δ = 1.22 (s, 9H, CH3), 3.74 (s, 1H,
NH), 4.43 (s, 1H, CH), 7.51–7.58 (m, 8H, ArH), 7.78 (d, J = 6.55
Hz, 2H, ArH); 13C NMR (CDCl3, 125 MHz): δ = 30.6, 47.8, 48.2,
55.7, 58.8, 111.1, 111.7, 112.6, 112.9, 114.4, 128.6, 129.2, 129.6,
129.7, 129.9, 130.4, 130.7, 131.5, 139.7; IR (KBr) v: 3382, 3063,
2973, 2932, 2873, 2248, 1655, 1575, 1500, 1475, 1198, 1114,
754, 700 cm−1; HRMS (ESI-TOF+): m/z calcd for C25H21N5Na
[(M + Na)+], 414.1689; found, 414.1690.
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