Palladium-Acid Co-Catalyzed Cleavage of Alkynoates to Construct Dibenzo[c,h]xanthene Derivatives

Article abstract of DOI:10.1002/adsc.201800770

A novel palladium-acid co-catalyzed C(sp)?C(sp²) cleavage of alkynoates for the construction of dibenzo[c,h]xanthene derivatives is reported. A catalytic amount of triflic acid (TfOH) afforded an efficient transformation into the final product. Furthermore, a reduction process of alkynoates was observed during the formation of dibenzo[c,h]xanthene, preliminary mechanistic studies indicated that the protonation of the new-formed methyl group in dibenzo[c,h]xanthene mainly came from water. (Figure presented.).

Full text of DOI:10.1002/adsc.201800770

Title: Palladium-acid co-catalyzed cleavage of alkynoates to construct
dibenzo[c,h]xanthene derivatives
Authors: Bocheng Tang, Miao Wang, Jintian Ma, Zixuan Wang,
Yandong Wu, and Anxin Wu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800770
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10.1002/adsc.201800770
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium-acid co-catalyzed cleavage of alkynoates to construct
dibenzo[c,h]xanthene derivatives
Bo-Cheng Tang, Miao Wang, Jin-Tian Ma, Zi-Xuan Wang, Yan-Dong Wu and An-Xin
Wu*
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal
University, Wuhan 430079, P. R. China.
E-mail: chwuax@mail.ccnu.edu.cn
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
made some progresses.10 Among these, “2-3”, “1-4”,
Abstract. A novel palladium-acid co-catalyzed C(sp)-
C(sp2) cleavage of alkynoates for the construction of and “4-5” cleavage of alkynoates are described.
However, different from decarboxylation11 of alkynyl
dibenzo[c,h]xanthene derivatives is reported. A catalytic
amount of triflic acid (TfOH) afforded an efficient
transformation into the final product. Furthermore, a
reduction process of alkynoates was observed during the
carboxylic acids, there are few reactions involve
direct “1-2” cleavage of alkynoates because of the
inherent differences between carboxyl and ester group.
Alkynyl carboxylic acids as the precursor of
alkynoates often participate in decarboxylative
reactions, which have been used as an efficient tool to
construct the alkyne-contained skeletons. For example,
Lee reported a palladium catalyzed decarboxylative
C-H alkynylation of benzoxazoles and α,β-ynoic
acids,12 Li described silver catalyzed oxidative
decarboxylative coupling of carboxylic acid and
anhydrides to produce 1,2-diketones.13 These
decarboxylative reactions are similar with the
previous “1-2” cleavage as well as the structure
between alkynyl carboxylic acids and alkynoates.
Based on the decarboxylative reactions and our consi-
formation
of
dibenzo[c,h]xanthene,
preliminary
mechanistic studies indicated that the protonation of the
new-formed methyl group in dibenzo[c,h]xanthene mainly
came from water.
Keywords: Palladium-acid co-catalyzed; C(sp)-C(sp2)
cleavage of alkynoates; Reduction of alkynoates.
Alkynoates are among the most common alkyne
derivatives, presenting a relatively complicated
molecular structure with an extra ester group in
comparison with alkynes,1 that may increase the
diversity of the relevant reactions. The classic
reaction modes of alkynoates are participated as a
whole structure in multicomponent reaction and
coupling reaction, such as C2 cyclization reaction and
Sonogashira, Fujiwara-type reaction.2-4 Up to date,
transition-metal-catalyzed reactions of alkynoates
have seen much research progress. The ester group
can serve as another nucleophilic or electrophilic site
to participate in reactions, resulting in relatively
diverse reaction modes such as the cleavage of
alkynoates, mainly including “2-3”, or “1-4”
cleavage.5 In particular, Jiao has developed a “2-3”
cleavage of alkynoates as a C1 unit for cyclization via
dioxygen activation (Scheme 1a).6 Zhang and Xu
described a copper catalyzed approach for the “1-4"
cleavage of alkynoates to construct 4-quinolone
derivatives (Scheme 1b).7,8 In addition, Jiang reported
a novel synthesis of methylene-γ-lactone skeletons
based on a palladium/oxygen catalytic system via “4-
5” cleavage (Scheme 1c).9 This elegant work is
complementary to the palladium/alkyne reaction
system. Moreover, the use of alkynoates for the
cyclization reactions to construct heterocycles have
Scheme 1. Representative cleavage modes of alkynoates
1
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10.1002/adsc.201800770
Advanced Synthesis & Catalysis
stent interests in alkynoate derivatives, we envisaged only trace amount of 3aa was observed when the
that if the “1-2” cleavage of alkynoates can be reaction was conducted without H2O2 (entry 11).
realized in one-pot under mild conditions. Herein, we Interestingly, the reaction ran smoothly when H2O2
report the first example of palladium-acid co- was replaced by H2O (4 equiv.), although gave
catalyzed C(sp)-C(sp2) cleavage and annulation of relatively low yield (entry 12). Nevertheless, using
alkynoates (Scheme 1d), and this work might be a common Lewis acids such as Cu(OAc)2, Sc(OTf)3
complementary to the cleavage modes of alkynoates.
and FeCl3 in place of the palladium catalyst produced
Initially, the reaction of naphthalen-1-ol 1a and a complex mixture. Other palladium sources were
methyl but-2-ynoate 2a was screened in toluene at also investigated but did not improve the reaction
100 ℃ in the presence of 10% Pd(OAc)2, and outcome. (see the Supporting Information (SI)).
TfOH (1 equiv.). Surprisingly, product 3aa was
Interestingly, product 4aa was obtained in 17%
obtained in 20% yield (Table1, entry1). Therefore, yield when 10% AcOH was selected as the additive
this investigation was extended to increase the yield (entry 10 versus 14,15). A series of ligands were
of 3aa. We first investigated the amount of TfOH, to screened to further improve the yield of 4aa. To our
our delight, 5% TfOH was tested to be appropriate for satisfaction, bathophen as the optimal ligand well
the formation of 3aa (entries 1-5). Preliminary control promoted the reaction, affording 4aa with satisfactory
experiments were conducted to demonstrate that yields and selectivities (entries 16-21). Different from
TfOH was necessary, while the single catalyst system Trost’s work14, the addition of NaOAc and HCOOH
did not complete the catalytic cycle. Hence a instead of PhMe resulted in a complex mixture.
palladium-acid co-catalyzed pathway might be Besides, the catalytic amounts of AcOH was
involved (entry 6). Specificity tests further compatible in our system, which indicated a different
demonstrated the necessity of TfOH, as no reactions pathway for the 4aa formation compared with Trost’s
occurred when TsOH, TFA, AcOH or PivOH were work. Finally, a range of different temperatures were
used instead of TfOH (entries 7 - 10). Importantly,
subsequently tested, with the optimal temperatures
found to be 130 °C for 3aa and 100 °C for 4aa. (see
the Supporting Information (SI)).
Table 1. Optimization of the Reaction Conditionsab.
yieldb (%)
entry
additive
acid
(3aa/4aa)
1
2
3
4
5
TfOH (100%)
TfOH (50%)
TfOH (20%)
TfOH (10%)
TfOH (5%)
-
H2O2
H2O2
H2O2
H2O2
H2O2
20/0
41/0
53/trace
60/8
64/5
0/0
6
H2O2
7
8
9
PivOH (10%)
TFA (10%)
TsOH (10%)
AcOH (10%)
TfOH (5%)
TfOH (5%)
TfOH (5%)
AcOH (5%)
AcOH (50%)
AcOH (10%)
AcOH (10%)
AcOH (10%)
AcOH (10%)
AcOH (10%)
AcOH (10%)
-
H2O2
H2O2
H2O2
H2O2
0/trace
0/10
0/0
0/17
trace/6
46/5
51/5
0/7
0/trace
0/0
0/0
0/0
0/0
10
11
12
13
14
15
16
17
18
19
20
21
22
-
H2O
H2O+O2
-
-
dppp
Xantphos
DPEphos
2-2’-bpy
1,10-phen
bathophen
bathophen
0/55
0/65
0/54
Scheme 2. Scope of Substrates to form 3 and 4ab.
a) Reaction conditions: (condition A) 1a (1.0 mmol), 2a (0.5
mmol), TfOH (5%), Pd(OAc)2 (10%), H2O2 (2mmol),
PhMe (3.0 mL) at 130 °C for the formation of 3aa.
(condition B) 1a (0.5mmol), 2a (0.5mmol), bathophen
(22%), Pd(OAc)2 (10%), HOAc (10%), AgSbF6 (20%),
PhMe (3.0mL) at 100 °C for the formation of 4aa.
Isolated yields based on 2a. Reactions were conducted in a
pressure vessel.
a) Reaction conditions: (condition A) 1a (1.0 mmol), 2a (0.5
mmol), TfOH (5%), Pd(OAc)2 (10%), H2O2 (2mmol),
PhMe (3.0 mL) at 130 °C for the formation of 3aa.
(condition B) 1a (0.5mmol), 2a (0.5mmol), bathophen
(22%), Pd(OAc)2 (10%), HOAc (10%), AgSbF6 (20%),
PhMe (3.0mL) at 100 °C for the formation of 4aa.
Isolated yields based on 2a. Reactions were conducted in a
pressure vessel.
b)
b)
2
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10.1002/adsc.201800770
Advanced Synthesis & Catalysis
The substrate scope of this cascade reaction was thalen-1-ol and 4-(trifluoromethyl)phenol all showed poor
subsequently evaluated. As shown in Scheme 2, the reactivity, none of the desired products were observed
scope of the 3aa series was first investigated. We under the standard conditions, probably because the
found that alkynoate derivatives with a series of alkyl nucleaophilicity at the ortho-position of the phenol
substituents were tolerated (3aa-3ag). As predicted, derivatives declined sharply. Gratifyingly, the conditions
the alkynoate with a phenyl substituent gave a much were mild enough to be compatible with halogen
lower yield owing to strong steric effects (3ah), substituted naphthalen-1-ol, which provided possibilities
which were also responsible for no reaction occurring for further functionalization (4ca-4da). The exact
with alkynoates bearing substituted aryl group (3ai- structures of 3aa, and 4ab were further unambiguously
3aj). Confusingly, various naphthols failed to afford elucidated by single-crystal X-ray diffraction analysis.15
the desired products. We also investigated the
To gain some insights into the reaction mechanism,
several control experiments were conducted (Scheme
3). Firstly, the reaction was conducted under a
nitrogen atmosphere, which resulted in the same
yields of 3aa and 4aa (Scheme 3a). Since the air is
substrate scope of the 4aa series, compared with
Trost’s work14, the halogen and electron-donating-
group substituted naphthalen-1-ol and various aryl-
substituted alkynoates were good substrates in our
system, which provided an effective supplement for
this strategy. Moreover, alkynoate derivatives bearing
alkyl substituents (4aa-4af) and heterocyclic
substituent (4aj) reacted smoothly to afford the
corresponding products in moderate to good yields.
Besides, the aryl substituents bearing electron-
donating and halogen groups at the para position
were also tolerated (4ah-4ai), while these substituents
at ortho position resulted in 100% recovery of the
starting materials, probably due to steric effects.
Furthermore, electron-donating-group substituted
naphthalen-1-ol, naphthalen-2-ol, and phenol were
good substrates for this transformation, furnishing the
desired products in moderate yields (4ea-4kh).
However, electron-withdrawing-group substituted
phenol derivatives such as 4-nitrophenol, 4-nitronaph-
the only oxidant, suggesting that
a
possible
protonation process existed and was more favorable
than a reductive elimination process.7 To further
verify the possibility of protonation during the
formation of 4aa, several deuterium labeling
experiments were performed. When the reaction was
conducted in d8-PhMe, no [D]-4aa was observed,
which suggested that the proton did not come from
PhMe (Scheme 3b). Next, after 1equiv of DOAc was
added, [D]-4aa was obtained in 26% yield, along with
25% D (Scheme 3c). Unsurprisingly, 2.5equiv of
DOAc gave the 50% D of the [D]-4aa but only in 8%
yield (Scheme 3d). Considering the 4aa could be
obtained in 54% yields without the HOAc as the
additive, which indicated that protodemetalation
could be caused by active protons mainly coming
from the hydroxyl group of naphthalen-1-ol,
generating in-situ the active HOAc to participate in
the catalytic cycle. Finally, varied H2O2 to D2O gave
75%D of the [D]-3ab under condition A (Scheme 3e).
This phenomenon clearly indicated that the proton of
the methyl mainly came from water. Hence a TfOH-
assisted hydrolysis of ester process might be
involved.
Based on the results of the current study and
previous reports,16-21 a tentative mechanism has been
proposed using 1a and 2a as example (Scheme 4). For
3aa, initially the Pd(OAc)2 acts as a Lewis acid to
activate the hydroxyl group.17,18 An intermolecular
nucleophilic substitution might then occur, caused by
another naphthalen-1-ol to form intermediate B,
followed by migratory insertion of the alkynoate to
produce
intermediate
C.
Subsequently
protodemetalation would give intermediate D. Then a
hydrolysis of ester group assisted by TfOH might
occur with the release of MeOH and CO2 to form
intermediate E. Finally, intramolecular nucleophilic
cyclization and protonation affords desired product
3aa. For the formation of 4aa, the palladium center of
intermediate A’ might coordinate with the alkynoates
and lead to intramolecular nucleopalladation to form
intermediate C’.19-21 Protodemetalation, mainly
caused by HOAc generated in-situ, might then occur
Scheme 3. Control Experiments.
3
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10.1002/adsc.201800770
Advanced Synthesis & Catalysis
to deliver intermediate D’, completing the catalytic
cycle through tautomerism and nucleophilic attack,
and affording the final product.
switched by using bathophen and TfOH, respectively.
Investigations of the potential applications of these
two transformations are underway in our laboratory.
To demonstrate the synthetic potential of this
method, the gram scale experiments of naphthalen-1-
ol and methyl hex-2-ynoate under condition A and B
were carried out. Gratifyingly, the reactions
Experimental Section
A mixture of naphthalen-1-ol 1a (1.0 mmol), 2a (0.5
proceeded well at the scale of 10 mmol and the mmol), TfOH (5 %), H2O2 (4 eq) and palladium acetate
(10 %) in PhMe (3 mL), the mixture was stirred at 130 °C,
till almost completed conversion of the substrates by TLC
analysis, the mixture was quenched with water (25 mL),
extracted with EtOAc (3 × 50 mL). The combined organic
layers were washed with brine, dried over anhydrous
Na2SO4 and concentrated under reduced pressure. The
crude product was purified by column chromatography on
silica gel (eluent: petroleum ether) to afford the product
3aa. as a white solid.
desired product 3ac and 4ac was isolated in 41% and
53% yield respectively.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21472056 and 21602070) and the
Fundamental Research Funds for the Central Universities
(CCNU15ZX002 and CCNU16A05002) for financial support.
This work was also supported by the 111 Project B17019.
References
[1] For selected reviews please see: a) R. Chinchilla, C.
Najera, Chem. Rev. 2014, 114, 1783; b) Y. Yamamoto,
Chem. Soc. Rev. 2014, 43, 1575; c) E. Negishi, L.
Anastasia, Chem. Rev. 2003, 103, 1979; d) B. M. Trost,
J. T. Masters, Chem. Soc. Rev. 2016, 45, 2212; e) X. F.
Wu, H. Neumann, M. Beller, Chem. Rev. 2013, 113, 1.
[2] a) Y. Fujiwara, Science. 2000, 287, 1992; b) C. G. Jia,
T. Kitamura, Y. Fujiwara, Acc. Chem. Res. 2001, 34,
633; c) R. Chinchilla, C. Najera, Chem. Soc. Rev. 2011,
40, 5084; d) Q. D. Jiang, H. Y. Li, W. P. Su, Org. Lett.
2018, 20, 2424.
[3] a) G. Choudhary, R. K. Peddinti, Green. Chem. 2011,
13, 3290; b) G. Kumaraswamy, D. Rambabu, N.
Jayaprakash, Eur. J. Org. Chem. 2009, 2009, 4158; c)
K. L. Obydennov, E. L. Klimareva, M. F. Kosterina,
Tetrahedron Letters. 2013, 54, 4876; d) A. A. Santilli,
A. C. Scotese, J. Heterocyclic. Chem. 1979, 16, 213; e)
I. Ambrogio, S. Cacchi, G. Fabrizi, Tetrahedron
Letters. 2007, 48, 7721; f) M. Chinnapattu, K. L.
Sathiyanarayanan, P. S. Lyer, RSC Adv. 2015, 5, 37716;
g) T. Murata, Y. Mizobe, S. Motomura, J. Am. Chem.
Soc. 1994, 116, 3389; h) H. Kuniyasu, T. Yoshizawa, N.
Kambe, Tetrahedron Lett. 2010, 51, 6818; i) S. A. Kazi,
G. F. Kelso, M. Hearn, Tetrahedron. 2010, 66, 9461; j)
D. Ramanathan, K. Pitchumani, Eur. J. Org. Chem.
2015, 2015, 463; k) Z. Z. Shi, C. Zhang, N. Jiao, Angew.
Chem. Int. Ed. 2009, 48, 4572.
Scheme 4. Proposed Mechanisms.
In conclusion, we have developed a novel
palladium-acid co-catalyzed C(sp)-C(sp2) cleavage of
alkynoates
for
the
construction
of
dibenzo[c,h]xanthene derivatives, TfOH was found to
be crucial as a specific acid cocatalyst for the
synthesis of dibenzo[c,h]xanthenes. While the
synthesis of benzo[h]coumarin derivatives provided
an effective supplement for Trost’s work because of
the broad substrate scope and the different mechanism.
Furthermore, these two transformations could be
[4] a) X. Zhang, X. F. Xu, W. Yi, Org. Lett. 2016, 18,
4864; b) A. E. Diaz-Alvarez, P. Crochet, V. Cadierno,
Adv. Synth. Catal. 2010, 352, 2427; c) J. Karunakaran,
M. Nandakumar, A. K. Mohanakrishnan, Org. Biomol.
4
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800770
Advanced Synthesis & Catalysis
Chem. 2016, 14, 4247; d) W. B. Liu, C. Chen, H. L. Liu, [17] M. R. Kuram, M. Bhanuchandra, A. K. Sahoo, Angew.
Adv. Synth. Catal. 2015, 357, 4050; e) C. Ma, H. F.
Ding, M. Bian, Angew. Chem. Int. Ed. 2006, 45, 7793;
f) V. Nair, R. S. Menon, S. Bindu, Org. Lett. 2004, 6,
767; g) V. Nair, S. Bindu, Org. Lett. 2003, 5, 665; h) S.
Safaei, I. Mohammadpoor-Baltork, A. R. Khosropour,
New. J. Chem. 2013, 37, 2037; i) J. W. Sun, H. Y. Hu,
F. Y. Wang, Y. Liu, RSC. Adv. 2014, 4, 36498; j) J.
Zheng, Z. Li, H. F. Jiang, Org. Lett. 2016, 18, 3514; k)
H. B. Zhong, D. Yang, S. Q. Wang, J. H. Huang, Chem.
Commun. 2012, 48, 3236.
Chem. Int. Ed. 2013, 52, 4607.
[18] a) Y. Wang, J. Y. Gu, Z. J. Shi, Org. Lett. 2017, 19,
1326; b) S. Luo, F. X. Luo, X. S. Zhang, Z. J. Shi,
Angew. Chem. Int. Ed. 2013, 52, 1.
[19] A. Minatti, K. Muniz, Chem. Soc. Rev. 2007, 36, 1142.
[20] N. Asao, T. Nogami, K. Takahashi, Y. Yamamoto, J.
Am. Chem. Soc. 2002, 124, 764.
[21] Y. J. Xiao, J. L. Zhang, Angew. Chem. Int. Ed. 2008,
47, 1903.
[5] a) D. K. Rayabarapu, S. Thota, C. H. Cheng, Angew.
Chem. Int. Ed. 2001, 40, 1286; b) Y. B. Huang, D. H.
Yan, H. F. Jiang, Chem. Commun. 2018, 54, 1742; c) A.
Z. Wang, H. F. Jiang, J. Am. Chem. Soc. 2008, 130,
5030; d) Z. Long, Y. D. Yang, J. S. You, Org. Lett.
2017, 19, 2781.
[6] a) T. Shen, Y. Q. Zhang, N. Jiao, J. Am. Chem. Soc.
2016, 138, 13147; b) T. Sonehara, S. Murakami, S.
Yamazaki, M. Kawatsura, Org. Lett. 2017, 19, 4299.
[7] X. F. Xu, X. Zhang, Org. Lett. 2017, 19, 4984.
[8] X. F. Xu, R. Z. Sun, X. Zhang, W. Yi, Org. Lett. 2018,
20, 1893.
[9] L. B. Huang, Q. Wang, H. F. Jiang, Angew. Chem. Int.
Ed. 2012, 51, 5696.
[10] a) S. Y. Ni, Jia. C, Y. Pan, Green. Chem. 2016, 18,
3935; b) D. Q. Zheng, J. Y. Yu, J. Wu, Angew. Chem.
Int. Ed. 2016, 55, 11925; c) W. Wei, J. W. Wei, H.
Wang, Chem. Commun. 2015, 51, 768; d) K. L. Yan, D.
S. Yang, H. Wang, J. Org. Chem. 2015, 80, 1550.
[11] a) J. D. Weaver, A. J. Grenning, J. A. Tunge, Chem.
Rev. 2011, 111, 1846; b) Y. Wei, P. Hu, W. P. Su,
Chem. Rev. 2017, 117, 8864; c) D. Zhao, C. Gao, J. S.
You, Chem. Commun. 2010, 46, 9049; d) C. Feng, T. P.
Loh, Chem. Commun. 2010, 46, 4779; e) T. Li, P. Sun,
J. Mao, Tetrahedron. 2012, 68, 6413; f) A. Park, K.
Park, S. Lee, Org. Lett. 2011, 13, 944; g) X. Li, F.
Yang, Y. Wu, J. Org. Chem. 2013, 78, 4543; h) J.
Hwang, K. Park, S. Lee, J. Org. Chem. 2014, 79, 3267.
[12] J. S. Kim, D. J. Kang, P. H. Lee, Eur. J. Org. Chem.
2013, 2013, 7902.
[13] a) H. X. Zou, Y. Li, J. H. Li, J. N. Xiang, Adv. Synth.
Catal. 2018, 360, 1439; b) Y. Li, M. Hu, J. H. Li, ACS
Catal. 2017, 7, 6757; c) J. W. Qiu, B. L. Hu, J. H. Li,
Org. Biomol. Chem. 2015, 13, 3122; d) W. W. Zhang,
X. G. Zhang, J. H. Li, J. Org. Chem. 2010, 75, 5259.
[14] B. M. Trost, F. D. Toste, K. Greenman, J. Am. Chem.
Soc. 2003, 125, 4518.
[15] CCDC-1843759 and CCDC-1843764 contain the
supplementary crystallographic data for compounds 3aa
and 4ab, respectively. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif (see
also the Supporting Information (SI)).
[16] D. Pan, Y. Wei, M. Shi, Org. Lett. 2016, 18, 3930.
5
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10.1002/adsc.201800770
Advanced Synthesis & Catalysis
UPDATE
Palladium-acid co-catalyzed cleavage of alkynoates
to construct dibenzo[c,h]xanthene derivatives
Adv. Synth. Catal. Year, Volume, Page – Page
Bo-Cheng Tang, Miao Wang, Jin-Tian Ma, Zi-
Xuan Wang, Yan-Dong Wu and An-Xin Wu*
6
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