Direct Assembly of 4-Substituted Quinolines with Vinyl Azides as a Dual Synthon via C=C and C-N Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.8b01718

An unprecedented Zn-promoted selective cleavage of vinyl azides for the synthesis of 4-substituted quinolines is developed. In this conversion, vinyl azides function as a dual synthon via C=C and C-N bond cleavage with two C=C bonds and one C=N bond formation in a one-step manner. The reaction is appreciated for its readily accessible substrates, high step economy, mild conditions, and use of air as the sole oxidant.

Full text of DOI:10.1021/acs.orglett.8b01718

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Direct Assembly of 4‑Substituted Quinolines with Vinyl Azides as a
Dual Synthon via CC and C−N Bond Cleavage
Jinghe Cen, Jianxiao Li, Yu Zhang, Zhongzhi Zhu, Shaorong Yang,* and Huanfeng Jiang*
Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640, P. R. China
S
* Supporting Information
ABSTRACT: An unprecedented Zn-promoted selective cleavage of vinyl azides for the synthesis of 4-substituted quinolines is
developed. In this conversion, vinyl azides function as a dual synthon via CC and C−N bond cleavage with two CC bonds
and one CN bond formation in a one-step manner. The reaction is appreciated for its readily accessible substrates, high step
economy, mild conditions, and use of air as the sole oxidant.
Scheme 1. Vinyl Azides Serve as a Multipurpose Precursor
and Its Application
ulticomponent reactions (MCRs) have expanded
Mrapidly in recent decades, since they allow the synthesis
of complex molecules, especially natural-product-like molec-
ular frameworks with simplicity and synthetic efficiency.1
Instead of serendipitous discovery, recently, chemists have
been focusing on creating logic-based MCRs through rational
design or combinatorial tactics.2,3 As one of the innovative
strategies to further explore MCRs, the dual synthon approach,
which employs one reactant to generate multiple fragments,
has garnered increasing attention.4 Such a strategy can
effectively minimize competing reactions by reducing the
number of starting materials. However, the major challenge
that remains in the dual synthon approach is how to achieve
the selective cleavage of one reactant and subsequent
incorporation into the target molecule. Thus, finding a reaction
precursor that can serve as a multiplepurpose fragment may be
the key point in addressing this issue. Recently, with the
emergence of many molecules that can serve as multipurpose
building blocks such as DMF,5 DMSO,6 and MeOH,7
opportunities arose for rational design of MCRs via a dual
synthon strategy.
According to the reported literature, vinyl azides are
energetic and versatile building blocks exhibiting distinct and
unprecedented chemical reactivity for the synthesis of a great
number of nitrogen heterocycles8 such as pyrroles,9a−c
pyrazoles,9d imidazoles,9e,f thiazoles,9g triazoles,9h pyridines,9i
quinolines,9j isoquinolines,9k and imidazo[1,2-a] pyridines.9l
Among them, the vinyl azides serve as a multipurpose
precursor for various units depending on the types of bond
(N−N, C−N, CC, and C−C) cleavage (Scheme 1). For
example, vinyl azides were employed as a pivotal three-atom
synthon to construct N-containing heterocycles through N−N
cleavage with the release of N2.10 Chiba and co-workers11
developed a [4 + 2] annulation by using vinyl azides as an
olefin fragment via C−N bond cleavage. Subsequently, Liu’s
group12 demonstrated the gold catalyzed CC cleavage of
vinyl azides with the methylene (CH2) adding to the
terminal alkynyl carbons of propargyl esters. Moreover, a series
of elegant studies were also reported by Chiba and other
groups13 who adopted vinyl azides as enamine-type nucleo-
Received: June 1, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01718
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Organic Letters
Letter
philes toward carbon electrophiles to synthesize amide-
containing molecules by N−N bond and C−C bond cleavages.
All these diverse cleavage methods permit vinyl azides to
function as an excellent dual synthon candidate. Despite the
elegant development of various cleavages of vinyl azides,
selective cleavage of vinyl azides and assembly in one molecule
have rarely been investigated.14 Inspired by previous work, we
envision a unique reaction system wherein two cleavage
methods of vinyl azides can be successfully carried out and
assembled into one molecule via MCRs. As part of our
continuing interest in developing new MCRs to synthesize
heterocycles,15 herein, we wish to disclose a Zn-promoted
MCR of anilines and vinyl azides to give 4-substituded
quinolones, which are not easily obtained in the traditional
Povarov reaction.16 Notably, no ligand and additive were
required in this procedure.
reaction was carried out under a N2 atmosphere (Table 1,
entry 12), which indicated that O2 as an oxidant may play a
crucial role in this transformation.
Having identified the optimized conditions, we evaluated the
substrate scope of the anilines. As shown in Scheme 2, the
a
Scheme 2. Substrate Scope of Anilines
To validate our hypothesis, we chose readily accessible p-
toluidine 1a and (1-azidovinyl)benzene 2a as model substrates
to optimize the reaction conditions. In the presence of
Zn(OTf)2 (0.5 equiv) and using MeCN as solvent, the
reaction gained the corresponding quinoline 3a in 43% yield
(Table 1, entry 1). A series of Lewis acids, such as ZnCl2,
a
Table 1. Optimization of Reaction Conditions
b
entry
Lewis acid
equiv
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
Zn(OTf)2
ZnCl2
0.5
0.5
0.5
0.5
0.5
0.5
1.0
1.5
2.0
1.5
1.5
1.5
−
80
80
80
80
80
80
80
80
80
90
100
90
90
43
28
trace
35
33
n.d.
56
67
66
82 (80)
74
Zn(OAc)2
Mg(OTf)2
Eu(OTf)3
Cu(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
−
a
Reaction conditions: a mixture of 1 (0.2 mmol), 2a (0.5 mmol), and
Zn(OTf)2 (1.5 equiv) in 2.5 mL MeCN was stirred at 90 °C for 2 h;
b
c
isolated yields based on 1 were given. Reaction time was 5 h. 1
mmol scale.
c
electronic effect played a significant role in this reaction.
Electron-neutral and electron-rich anilines converted more
efficiently than their electron-poor analogues. The anilines with
electron-donating groups on the aryl ring at different positions
worked well, furnishing 3a, 3c−3f, 3i, 3j, 3l−3o in 75−84%
yields. However, yields slightly declined when the substrates
contained electron-withdrawing groups 3g, 3h, 3k; they also
required a longer reaction time. In addition, more-activated
anilines 1p, 1q, 1r gave the desired products in good yields.
When 1 mmol of 1r was used as the substrate, the qiunoline
product 3r was obtained in 70% yield (Scheme 2). It was
noteworthy that when naphthalen-2amine 1s was used in this
transformation, the ring closing happened at the more
congested α-position instead of the β-position 3s, which
reflected the electronic factor, rather than steric factor
significantly affecting this reaction. The structure of 3s was
further confirmed by single-crystal X-ray analysis (CCDC:
1814447). Naphthalen-1-amine also reacted smoothly and
gave 3t in 77% yield.
d
n.d.
n.d.
a
Reaction conditions: a mixture of 1a (0.2 mmol) and 2a (0.5 mmol)
b
in 2.5 mL of MeCN was stirred in air for 2 h. Yields were determined
c
by 1HNMR using bromochloromethane as internal standard. Isolated
d
yield. The reaction was carried out under N2.
Zn(OAc)2, Mg(OTf)2, Eu(OTf)3, and Cu(OTf)2 were also
screened, but did not give better results (Table 1, entries 2−6).
It should be noted that the product yield was remarkably
improved when the Zn(OTf)2 loading was increased from 0.5
to 1.5 equiv (Table 1, entries 7 and 8). Further increasing the
amount of Zn(OTf)2 did not result in a higher yield of 3a
(Table 1, entry 9). In the following optimization, we were
pleased to find that the product yield further improved to 82%
by increasing the reaction temperature to 90 °C (Table 1,
entry 10). Further increasing the reaction temperature
decreased the yield (Table 1, entry 11). In the absence of
Zn(OTf)2, no desired product was observed, and the p-
toluidine 1a was recovered in quantitative yield. Additionaly,
vinyl azide 2a was almost converted into 2H-azirine (Table 1,
entry 13). However, no desired product was detected when the
Encouraged by these results, we next examined the general
applicability of diverse vinyl azides (Scheme 3). It was found
that electron-deficient substituents, including F, Cl, and Br, at
the different aryl positions proved to be well-tolerated and
generated the corresponding halo-substituted products, which
B
DOI: 10.1021/acs.orglett.8b01718
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Organic Letters
Letter
a
furnishing the corresponding product 5f in lower yield (30%).
These results not only confirm that the 2-position carbon atom
of the quinoline product originates from vinyl azides but also
indicate that the β-substituted vinyl azides tend to provide the
carbon atom in the 2-position of quinoline, while the terminal
vinyl azides contribute the olefin moiety. The lower yields in
the crossover reaction might be attributed to the side reactions
and the difficulties of isolation while increasing the number of
the starting materials.
Scheme 3. Substrate Scope of Vinyl Azides
To gain insight into the possible mechanism of this
transformation, a series of control experiments were inves-
tigated. The reaction of 1a proceeded well in the presence of a
radical scavenger such as 2,2,6,6-tetramethylpiperidinyloxy
(TEMPO) or BHT (2,6-di-tert-4-methylphenol) and gener-
ated the quinoline product 3a in excellent yields. Therefore, a
radical process could be excluded for this transformation
(Scheme 5, eq 1). 2H-Azirine was proposed as an intermediate
Scheme 5. Mechanistic Studies and Control Experiments
a
Reaction conditions: 1b (0.2 mmol), 2 (0.5 mmol), and Zn(OTf)2
(1.5 equiv) in 2.5 mL of MeCN were stirred at 90 °C in air for 2 h;
isolated yields based on 1b were reported.
could further undergo coupling reactions (Scheme 3, 4a−4c
and 4j−4l). To our delight, vinyl azides containing a strong
electron-withdrawing group (CF3, CN, CH3COO) were also
viable substrates. Notably, vinyl azides bearing pyridine were
also effective and gave the corresponding product 4p in 51%
yield. Unfortunately, α-alkyl vinyl azides 4n failed to convert
into the desired product, probably due to its low activity.
We wonder what results will be given in a crossover reaction
between two different vinyl azides with aniline (Scheme 4).
a
Scheme 4. Further Exploration of the Present Strategy
of the transformation. However, when p-toluidine and 2H-
azirine were employed under the standard conditions, the
reaction gave the undesired dimeric product 6a with p-
toluidine with 84% recovery (Scheme 5, eq 2). This result
might exclude the involvement of 2H-azirine in our reaction
system.
To further probe the source of the nitrogen atom of the
quinoline products, 15N isotope experiments were carried out.
When 15N-labeled aniline and vinyl azides were utilized in the
reaction, the 15N-labeled product was obtained in 73% yield
(Scheme 5, eq 3), detected by HRMS (for details, see the
Supporting Information (SI)). This result demonstrates that
the nitrogen atom of the quinoline originated from aniline.
However, the starting meterials decomposed and no desired
product was observed when only β-methyl vinyl azides 2q were
employed in the reaction, which demonstrated that terminal
vinyl azides were indispensable in this transformation (Scheme
5, eq 4). Furthermore, when aldimine 10a was employed as a
substrate under the standard conditions, the desired product
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), 2′ (0.3 mmol),
and Zn(OTf)2 (1.5 equiv) in 2.5 mL of MeCN were stirred at 90 °C
in air for 5 h. Isolated yields based on 1 were reported.
When one β-substituted vinyl azide and one terminal vinyl
azide were chosen to conduct this reaction, to our surprise,
only 2,4-disubsituted quinolines were obtained as main
products with high regioselectivity. Regarding the terminal
vinyl azide partners, it was proven that vinyl azides with an
electron-donating group proceeded well with moderate yields
(5a−5e). Terminal vinyl azides bearing a halogen atom at the
p-position of the aromatic ring exhibited lower reactivity,
C
DOI: 10.1021/acs.orglett.8b01718
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Organic Letters
Letter
11a was given in 60% yield (Scheme 5, eq 5). These results
suggest that aldimine might be the intermediate involved in
this transformation.
On the basis of the above preliminary results and the
reported literature, a plausible reaction pathway is illustrated
(Scheme 6). Initially, intermediate A was formed by the
*E-mail: jianghf@scut.edu.cn.
ORCID
Huanfeng Jiang: 0000-0002-4355-0294
Notes
The authors declare no competing financial interest.
Scheme 6. Possible Reaction Mechanism
ACKNOWLEDGMENTS
■
The authors thank the National Key Research and Develop-
ment Program of China (2016YFA0602900), the National
Natural Science Foundation of China (21420102003,
21502055, and 21642005), and the Fundamental Research
Funds for the Central Universities (2015ZY001).
REFERENCES
■
(1) (a) Wang, Q.; Wang, D.-X.; Wang, M.-X.; Zhu, J. Acc. Chem. Res.
2018, 51, 1290. (b) Deibl, N.; Ament, K.; Kempe, R. J. Am. Chem. Soc.
2015, 137, 12804. (c) Deibl, N.; Kempe, R. Angew. Chem., Int. Ed.
2017, 56, 1663. (d) Li, Y.; Zhu, F.; Wang, Z.; Wu, X.-F. Chem.
Commun. 2018, 54, 1984. (e) Yang, D.; Yu, Y.; Wu, Y.; Feng, H.; Li,
X.; Cao, H. Org. Lett. 2018, 20, 2477.
coordination of zinc to the azide group, increasing the
electrophilicity of the olefin.17 Next, nucleophilic attack by
aniline generates intermediate B, with the loss of nitrogen.
Then, intermediate B produces the imine intermediate C
through C−C bond cleavage18 giving benzonitrile as a
byproduct, which was detected by GC-MS (for details, please
see the SI). Subsequently, intermediate C undergoes an
intramolecular cyclization [4 + 2]-annulation with 2a to
produce intermediate E, which further generates intermediate
F with the elimination of HN3.11 Finally, aromatization of F by
O2 in air furnishes the desired quinoline product.
In conclusion, we have discovered an unprecedented
transformation of vinyl azides, in which two types of cleavage
of vinyl azides were involved in a one-step procedure to
assemble 4-substituted quinolines. In this conversion, vinyl
azides function as a dual synthon to provide a logic-based
strategy to further explore MCRs. The reaction features high
step economy, mild conditions, easy operation, and utilization
of environmentally friendly air as the sole oxidant. Further
studies to clearly understand the reaction mechanism and the
synthetic applications are ongoing in our group.
(2) For reviews, see: (a) Ganem, B. Acc. Chem. Res. 2009, 42, 463.
(b) Godineau, E.; Landais, Y. Chem. - Eur. J. 2009, 15, 3044.
(3) For selected examples see: (a) Huang, B.; Zeng, L.; Shen, Y.;
Cui, S. Angew. Chem., Int. Ed. 2017, 56, 4565. (b) Liu, Z.; Liu, Z.-Q.
Org. Lett. 2017, 19, 5649. (c) Kanazawa, J.; Maeda, K.; Uchiyama, M.
J. Am. Chem. Soc. 2017, 139, 17791.
(4) (a) Guo, W.; Liao, J.; Liu, D.; Li, J.; Ji, F.; Wu, W.; Jiang, H.
Angew. Chem., Int. Ed. 2017, 56, 1289. (b) Xu, C.; Jiang, S.-F.; Wen,
X.-H.; Zhang, Q.; Zhou, Z.-W.; Wu, Y.; Jia, F.-C.; Wu, A. Adv. Synth.
Catal. 2018, 360, 2267. (c) Das, U. K.; Shimon, L. J. W.; Milstein, D.
Chem. Commun. 2017, 53, 13133.
(5) Ding, S.; Jiao, N. Angew. Chem., Int. Ed. 2012, 51, 9226.
(6) For selected examples, see: (a) Shen, T.; Huang, X.; Liang, Y.-F.;
Jiao, N. Org. Lett. 2015, 17, 6186. (b) Tomita, R.; Yasu, Y.; Koike, T.;
Akita, M. Angew. Chem., Int. Ed. 2014, 53, 7144. (c) Ashikari, Y.;
Nokami, T.; Yoshida, J. J. Am. Chem. Soc. 2011, 133, 11840. (d) Xu,
R.; Wan, J.-P.; Mao, H.; Pan, Y. J. Am. Chem. Soc. 2010, 132, 15531.
(e) Ren, X.; Chen, J.; Chen, F.; Cheng, J. Chem. Commun. 2011, 47,
6725. (f) Gao, Q.; Wu, X.; Li, Y.; Liu, S.; Meng, X.; Wu, A. Adv. Synth.
Catal. 2014, 356, 2924. (g) Jiang, Y.; Loh, T.-P. Chem. Sci. 2014, 5,
4939. (h) Jia, T.; Bellomo, A.; Baina, K. EL; Dreher, S. D.; Walsh, P. J.
J. Am. Chem. Soc. 2013, 135, 3740. (i) Jia, T.; Bellomo, A.; Montel, S.;
Zhang, M.; El Baina, K.; Zheng, B.; Walsh, P. J. Angew. Chem., Int. Ed.
2014, 53, 260.
ASSOCIATED CONTENT
* Supporting Information
■
(7) (a) For a review, see: Natte, K.; Neumann, H.; Beller, M.;
Jagadeesh, R. V. Angew. Chem., Int. Ed. 2017, 56, 6384. (b) For an
example, see: Chakrabarti, K.; Maji, M.; Panja, D.; Paul, B.; Shee, S.;
Das, G. K.; Kundu, S. Org. Lett. 2017, 19, 4750.
(8) For reviews, see: (a) Jung, N.; Brase, S. Angew. Chem., Int. Ed.
2012, 51, 12169. (b) Hu, B.; DiMagno, S. G. Org. Biomol. Chem.
2015, 13, 3844. (c) Hayashi, H.; Kaga, A.; Chiba, S. J. Org. Chem.
2017, 82, 11981. (d) Fu, J.; Zanoni, G.; Anderson, E. A.; Bi, X. Chem.
Soc. Rev. 2017, 46, 7208.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01718.
Experimental procedures, condition screening table,
characterization data, and copies of NMR spectra for
all products (PDF)
(9) (a) Chiba, S.; Wang, Y.-F.; Lapointe, G.; Narasaka, K. Org. Lett.
2008, 10, 313. (b) Chen, F.; Shen, T.; Cui, Y.; Jiao, N. Org. Lett.
2012, 14, 4926. (c) Wu, Y.; Zhu, L.; Yu, Y.; Luo, X.; Huang, X. J. Org.
Chem. 2015, 80, 11407. (d) Zhang, G.; Ni, H.; Chen, W.; Shao, J.;
Liu, H.; Chen, B.; Yu, Y. Org. Lett. 2013, 15, 5967. (e) Luo, J.; Chen,
W.; Shao, J.; Liu, X.; Shu, K.; Tang, P.; Yu, Y. RSC Adv. 2015, 5,
55808. (f) Shao, J.; Liu, X.; Shu, K.; Tang, P.; Luo, J.; Chen, W.; Yu,
Y. Org. Lett. 2015, 17, 4502. (g) Chen, B.; Guo, S.; Guo, X.; Zhang,
G.; Yu, Y. Org. Lett. 2015, 17, 4698. (h) Liu, Z.; Ji, H.; Gao, W.; Zhu,
G.; Tong, L.; Lei, F.; Tang, B. Chem. Commun. 2017, 53, 6259.
(i) Wang, Y.-F.; Chiba, S. J. Am. Chem. Soc. 2009, 131, 12570.
(j) Wang, Q.; Huang, J.; Zhou, L. Adv. Synth. Catal. 2015, 357, 2479.
(k) Wang, Y.-F.; Toh, K. K.; Lee, J.-Y.; Chiba, S. Angew. Chem., Int.
Accession Codes
CCDC 1814447 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: lisryang@scut.edu.cn.
D
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Organic Letters
Letter
Ed. 2011, 50, 5927. (l) Donthiri, R. R.; Pappula, V.; Reddy, N. N. K.;
Bairagi, D.; Adimurthy, S. J. Org. Chem. 2014, 79, 11277.
(10) (a) Stokes, B. J.; Dong, H.; Leslie, B. E.; Pumphrey, A. L.;
Driver, T. G. J. Am. Chem. Soc. 2007, 129, 7500. (b) Wang, Y.-F.;
Chiba, S. J. Am. Chem. Soc. 2009, 131, 12570. (c) Wang, Y.-F.; Toh,
K. K.; Ng, E. P. J.; Chiba, S. J. Am. Chem. Soc. 2011, 133, 6411.
(d) Chen, F.; Shen, T.; Cui, Y.; Jiao, N. Org. Lett. 2012, 14, 4926.
(e) Chen, B.; Guo, S.; Guo, X.; Zhang, G.; Yu, Y. Org. Lett. 2015, 17,
4698. (f) Zhu, Z.; Tang, X.; Li, J.; Li, X.; Wu, W.; Deng, G.; Jiang, H.
Org. Lett. 2017, 19, 1370.
(11) Zhu, X.; Wang, Y.-F.; Zhang, F.-L.; Chiba, S. Chem. - Asian J.
2014, 9, 2458.
(12) Wagh, S. B.; Liu, R.-S. Chem. Commun. 2015, 51, 15462.
(13) (a) Zhang, F.-L.; Wang, Y.-F.; Lonca, G. H.; Zhu, X.; Chiba, S.
Angew. Chem., Int. Ed. 2014, 53, 4390. (b) Zhang, Z.; Kumar, R. K.;
Li, G.; Wu, D.; Bi, X. Org. Lett. 2015, 17, 6190. (c) Lin, C.; Shen, Y.;
Huang, B.; Liu, Y.; Cui, S. J. Org. Chem. 2017, 82, 3950.
(14) Kanchupalli, V.; Katukojvala, S. Angew. Chem., Int. Ed. 2018, 57,
5433.
(15) For selected examples, see: (a) Peng, J.; Gao, Y.; Hu, W.; Gao,
Y.; Hu, M.; Wu, W.; Ren, Y.; Jiang, H. Org. Lett. 2016, 18, 5924.
(b) Peng, J.; Gao, Y.; Zhu, C.; Liu, B.; Gao, Y.; Hu, M.; Wu, W.; Jiang,
H. J. Org. Chem. 2017, 82, 3581. (c) Liu, W.; Jiang, H.; Huang, L. Org.
Lett. 2010, 12, 312. (d) Zhang, M.; Jiang, H.; Liu, H.; Zhu, Q. Org.
Lett. 2007, 9, 4111. (e) Cao, H.; Wang, X.; Jiang, H.; Zhu, Q.; Zhang,
M.; Liu, H. Chem. - Eur. J. 2008, 14, 11623. (f) Jiang, H.; Yang, J.;
Tang, X.; Li, J.; Wu, W. J. Org. Chem. 2015, 80, 8763. (g) Liu, D.;
Guo, W.; Wu, W.; Jiang, H. J. Org. Chem. 2017, 82, 13609. For
reviews, see: (h) Wu, W.; Jiang, H. Acc. Chem. Res. 2014, 47, 2483.
(i) Huang, H.; Ji, X.; Wu, W.; Jiang, H. Chem. Soc. Rev. 2015, 44,
1155. (j) Li, J.; Yang, S.; Wu, W.; Jiang, H. Eur. J. Org. Chem. 2018,
2018, 1284.
(16) Ji, X.; Huang, H.; Li, Y.; Chen, H.; Jiang, H. Angew. Chem., Int.
Ed. 2012, 51, 7292.
(17) Huang, W.; Liu, S.; Chen, B.; Guo, X.; Yu, Y. RSC Adv. 2015, 5,
32740.
(18) Chiba, S.; Zhang, L.; Ang, G. Y.; Hui, B. W.-Q. Org. Lett. 2010,
12, 2052.
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