Synthesis of functionalized cyclopentenes through allenic ketone-based multicomponent reactions

Article abstract of DOI:10.1039/c8ob02259c

A novel and efficient synthesis of diversely functionalized cyclopentene derivatives through the multicomponent reactions of 1,2-allenic ketones with 4-chloroacetoacetate and malononitrile/cyanoacetate under mild and metal-free conditions is presented. Me

Full text of DOI:10.1039/c8ob02259c

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Synthesis of functionalized cyclopentenes through
allenic ketone-based multicomponent reactions†
Cite this: Org. Biomol. Chem., 2018,
16, 8854
Qiang Wang, *a Tao Zhang,a Yunchang Fana and Xuesen Fan
*
b
A novel and efficient synthesis of diversely functionalized cyclopentene derivatives through the multicom-
ponent reactions of 1,2-allenic ketones with 4-chloroacetoacetate and malononitrile/cyanoacetate under
mild and metal-free conditions is presented. Mechanistically, the formation of title compounds involves a
cascade process including nucleophilic substitution, Michael addition and intramolecular aldol type reac-
tion. Interestingly, when 1-phenyl allenic ketones bearing electron-donating groups on the phenyl ring
were reacted with 4-chloroacetoacetate and cyanoacetate, methylenecyclo-pentanes, the regioisomer of
cyclopentenes, were formed with good selectivity and high efficiency.
Received 12th September 2018,
Accepted 2nd November 2018
DOI: 10.1039/c8ob02259c
rsc.li/obc
pentenes from easily accessible starting materials and accom-
plished under mild and metal-free conditions still remains as
Introduction
Five-membered carbocycles are ubiquitous in natural products, a hot topic in the synthetic community.
pharmaceuticals and functional materials.1 Among them,
In recent decades, allene derivatives have been developed
cyclopentene is a privileged scaffold in drug discovery as many as powerful and versatile building blocks in organic synthesis
compounds bearing this skeleton have been developed as due to their unique and rich reactivities.15 As a result, many
thrombin inhibitors,2 antifungal inhibitors,3 p38α MAP kinase types of carbocyclic, heterocyclic, or acyclic compounds could
inhibitors,4 or S-adenosylhomocysteine hydrolase inhibitors.5 be prepared in a more efficient and sustainable manner by
In addition, cyclopentene and its derivatives also serve as valu- using allene derivatives as the key substrates.16 Inspired by
able building blocks in synthetic chemistry.6 Owing to their these pioneering studies and as a continuation of our own
importance, a number of elegant methods for the preparation interest in allene chemistry,17,18 we would like to report herein
of cyclopentene derivatives were developed, which mainly a novel synthesis of highly functionalized cyclopentenes via
include aldol cyclocondensation,7 rearrangements of vinyl the three-component cascade reactions of easily obtainable
cyclopropanes,8 Nazarov cyclizations,9 phosphine-catalyzed 1,2-allenic ketones with commercially available 4-chloroaceto-
[3 + 2] cycloaddition reactions,10,11 domino ring-opening cycli- acetate and malononitrile/cyanoacetate.
zation of donor–acceptor cyclopropanes,12 cycloaddition of
cyclopropanes with alkenes or alkynes,13a bimetallic catalyzed
cycloaddition of cyclopropylcarboxamide with alkynes,13b
Results and discussion
silver-catalyzed oxidative intermolecular [3 + 2] annulation of
terminal alkynes with 4-vinyl acids,13c and some base-pro-
Initially, 4-chloroacetoacetate (1) was treated with malono-
nitrile (2a) in the presence of K2CO3 at rt for 20 min in
moted multicomponent reactions.14 While these literature pro-
tocols are generally reliable, the search for novel and efficient
methods for the preparation of diversely functionalized cyclo-
acetone. Then, 1-phenylbuta-2,3-dien-1-one (3a) was added,
and the resulting mixture was stirred at rt for 1.5 h. From this
reaction, ethyl 2-(2-benzoyl-4,4-dicyano-1-hydroxy-3-methyl-
cyclopent-2-en-1-yl)acetate (4a) was obtained in 60% yield
(Table 1, entry 1). To improve the efficiency, different solvents
were tried. It was thus found that polar solvents such as
acetone, CH3CN and DMF were more beneficial for the for-
mation of 4a than non-polar solvents such as THF, toluene
and dichloromethane (entries 1–3 vs. 4–6). Following studies
on the effect of different bases by using CH3CN as the solvent
showed that K2CO3 gave the best yield and its optimum
loading is 1.2 equiv. (entries 14, 16 and 17). To our delight, a
further study showed that raising the reaction temperature
aCollege of Chemistry and Chemical Engineering, Henan Key Laboratory of Coal
Green Conversion, Henan Polytechnic University, Jiaozuo, Henan 454000,
P. R. China. E-mail: wangqiang@hpu.edu.cn, incomer@163.com
bHenan Key Laboratory of Organic Functional Molecule and Drug Innovation,
Collaborative Innovation Centre of Henan Province for Green Manufacturing of Fine
Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of
Education, School of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang, Henan 453007, P. R. China. E-mail: xuesen.fan@htu.cn
†Electronic supplementary information (ESI) available: Experimental procedure,
characterization data and NMR spectra of all products, and the X-ray crystal
structures and data of 4n. See DOI: 10.1039/c8ob02259c
8854 | Org. Biomol. Chem., 2018, 16, 8854–8858
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Table 1 Optimization of the reaction conditions for 4aa
Table 2 Substrate scope for the synthesis of 4 and 5a,b
3/R1
3/R2
3/R3
Product/yield (%)
Entry
Base
Solvent
T/°C
Yieldb (%)
Entry
2
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
LiOH
KOH
TBAF
Et3N
K2CO3
K2CO3
K2CO3
K2CO3 (0.2 equiv.)
K2CO3 (1.0 equiv.)
Acetone
CH3CN
DMF
THF
Toluene
DCM
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
40
80
100
80
80
60
63
56
34
10
11
58
61
25
12
8
Trace
72
79
73
38
72
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2a
2a
2a
2a
C6H5
1-Naphthyl
3-ClC6H4
2,4-Di-ClC6H3
2-BrC6H4
3-BrC6H4
4-NCC6H4
2-Br-5-MeOC6H3
3-MeC6H4
4-MeC6H4
4-MeOC6H4
3,4-Di-MeOC6H3
C6H5
1-Naphthyl
2,4-Di-ClC6H3
2-BrC6H4
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
C6H5
C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Et
4a, 79
4b, 70
4c, 76
4d, 72
4e, 75
4f, 69
4g, 71
4h, 82
4i, 64
4j, 83
4k, 69
4l, 61
4m, 67
4n, 76
4o, 81
4p, 78
4q,71
4r, 60
4s, 77
4t, 78
5a, 69
5b, 71
5c, 73
5d, 68
—
9
9
10
11
12
13
14
15
16
17
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
3-BrC6H4
4-NCC6H4
2-Br-5-MeOC6H3
C6H5CH2CH2
3-MeC6H4
4-MeC6H4
4-MeOC6H4
3,4-Di-MeOC6H3
C6H5
a Reaction conditions: 1 (0.5 mmol), 2a (0.5 mmol), base (1.2 equiv.
based on 3), solvent (3 mL), air, rt, 20 min; then 3a (0.5 mmol), air,
1.5 h. b Isolated yield.
could improve the yield of 4a obviously, and the most suitable
temperature is 80 °C (entries 2 and 13–15). In summary of the
optimization study, treating 1 with 2a (1 : 1) in the presence of
K2CO3 (1.2 equiv.) at ambient temperature in CH3CN for
20 min, and then treating the resulting mixture with 3a
(1.0 equiv.) at 80 °C for 1.5 h afforded 4a in a yield of 79%
(Table 1, entry 14).
C6H5
Me
C6H5
—
—
—
a Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), K2CO3 (0.6 mmol),
solvent (3 mL), air, rt, 20 min; then 3 (0.5 mmol), air, 80 °C, 1.5 h.
b Isolated yield.
After having established the optimum reaction conditions,
the substrate scope of this three-component cascade reaction such as methyl and methoxy on the phenyl ring, a series of
was investigated, and the results are summarized in Table 2. It methylenecyclopentane derivatives (5a–5d), the regioisomer of
was firstly found that naphthyl derived allenic ketones reacted the corresponding cyclopentene derivatives, were obtained in
with 1 and 2a effectively to afford 4b in 70% yield (entry 2). 68–73% yield (entries 21–24). When allenic ketones bearing a
Next, 1-phenyl allenic ketones bearing either electron-donating substituent on the terminal or the internal position of the
groups such as Me and MeO or electron-withdrawing groups allenic unit were reacted with 1 and 2a, neither the cyclopen-
such as Cl, Br and CN on the phenyl ring participated in this tene nor the methylenecyclopentane-type product could be
reaction smoothly to afford the corresponding products 4c–4l obtained, most likely due to the steric effect (entries 25–28).
in moderate to good yields (entries 3–12). Meanwhile, it was Furthermore, when maletate, acetoacetate or acetylacetone was
also observed that various functional groups on the phenyl used to replace 2a or 2b to react with 1 and 3 under the
ring tolerated the reaction conditions very well without optimum reaction conditions as described above, only messy
showing obvious electronic and steric effects. Second, the mixtures were formed, and the formation of the desired five-
scope of substrate 2 was extended from malononitrile (2a) to membered carbocycle products was not observed. Finally, it is
cyanoacetate (2b). It turned out that when 1 and 2b were worth noting that the structure of 4n was confirmed by its
reacted with 1-naphthyl allenic ketones or 1-phenyl allenic spectroscopic data together with the X-ray diffraction analysis
ketones bearing electron-withdrawing groups such as chloro, (see the ESI† for details).
bromo and cyano on the phenyl ring, the desired cyclopentene
In further study, we were delighted to find that the treat-
products 4m–4s were obtained in moderate to good yields ment of 2-bromo-1-phenylethanone (6) with 2a and 3a under
(entries 13–19). Promisingly, the reaction was also compatible standard reaction conditions could afford 3-benzoyl-4-hydroxy-
with 1-alkyl allenic ketone to give 4t in 78% yield (Table 2, 2-methyl-4-phenylcyclopent-2-ene-1,1-dicarbonitrile (7) in
a
entry 20). Surprisingly, when 1 and 2b were reacted with yield of 56% (Scheme 1, (1)). On the other hand, the treatment
1-phenyl allenic ketones bearing electron-donating groups of ethyl 2-chloroacetate (8) with 2a and 3a under similar con-
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nated to give anion B, which then undergoes a conjugate
addition onto 3 to afford anion C. In the next stage, C under-
goes an intramolecular aldol type reaction to give anion D. A
rapid proton transfer occurs with D to afford E and F. Finally,
protonation of F gives product 4. On the other hand, with R as
an ester group and Ar as a phenyl unit bearing an electron-
donating group, anion E is relatively less stable and difficult to
be formed. Therefore, direct protonation of D occurs more
favourably to afford product 5.
Scheme 1 Reactions of 6/8 with 2a and 3a.
ditions afforded ethyl 3,3-dicyano-4-methyl-6-oxo-6-phenylhex-
4-enoate (9) in a yield of 62%, and the formation of the corres-
ponding cyclopentene product was not observed (Scheme 1, (2)).
To get some insight into the reaction mechanism, the fol-
lowing control experiments were carried out. First, 1 was
treated with 2a in the presence of K2CO3 in CH3CN at rt for
20 min. From this reaction, ethyl 5,5-dicyano-3-oxopentanoate
(A) was obtained in a yield of 82% (Scheme 2, (1)). Next, A was
let to react with 3a under similar conditions. From this reac-
tion, 4a was formed in 88% yield (Scheme 2, (2)). Third, 5a was
treated with NaOH in DMSO at 100 °C for 1 h, from which
cyclopentene derivative 4u was obtained in 15% yield
(Scheme 2, (3)).
Conclusions
In conclusion, we have developed an efficient and metal-free
method for the synthesis of diversely functionalized cyclopen-
tene derivatives via base-promoted cascade reactions of
4-chloroacetoacetate with malononitrile/cyanoacetate and 1,2-
allenic ketones. Notable features of this novel protocol include
readily available starting materials, broad substrate scope,
structurally diverse products, mild reaction conditions and
operational simplicity.
Experimental section
A typical procedure for the synthesis of 4a
Based on previous reports and the above-mentioned experi-
mental results, plausible mechanisms accounting for the for-
mation of 4 and 5 are shown in Scheme 3. Initially, intermedi-
ate A is formed through base-promoted nucleophilic substi-
tution of 1 with 2. Under the reaction conditions, A is deproto-
To a flask containing malononitrile (2a, 33.0 mg, 0.5 mmol)
and K2CO3 (82.8 mg, 0.6 mmol) in CH3CN (3 mL) was added
ethyl 4-chloroacetoacetate (1, 82.3 mg, 0.5 mmol). The mixture
was stirred at room temperature for 20 min. Then, 1-phenyl-
buta-2,3-dien-1-one (3a, 72.0 mg, 0.5 mmol) was added. The
resulting mixture was stirred at 80 °C for 1.5 h. When the reac-
tion was complete as determined by TLC analysis, it was
allowed to cool to room temperature and quenched with satu-
rated ammonium chloride, and the mixture was extracted with
ethyl acetate (3 × 15 mL). The combined organic layers were
dried over anhydrous Na2SO4 and concentrated under reduced
pressure. The residue was purified by flash chromatography
(SiO2) using EtOAc/petroleum ether (v/v = 1/5) as the eluent to
give 4a (133.5 mg, 79%). Other cyclopentene derivatives (4b–4t)
and methylenecyclopentanes (5a–5d) were obtained in a
similar manner.
Scheme 2 Control experiments.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful to the Natural Science Research Program from
the Education Department of Henan Province (No.
14A150048), the Henan Polytechnic University Foundation for
Doctor Teachers (B2010-65), the Foundation of Henan
Province (182102310728), and the Program for Innovative
Research Team of Henan Polytechnic University (T2018-3) for
Scheme 3 Proposed mechanisms for the formation of 4 and 5.
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financial support. The authors thank Dr D. Zhao, Dr Z.-Q. Xu
and Dr Y. Wang for their assistance with the collection and
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