Dual Directing-Groups-Assisted Redox-Neutral Annulation and Ring Opening of N-Aryloxyacetamides with 1-Alkynylcyclobutanols via Rhodium(III)-Catalyzed C-H/C-C Activations

Article abstract of DOI:10.1021/acs.orglett.9b00812

A cascade [3 + 2] annulation and ring opening of N-aryloxyacetamides with 1-alkynylcyclobutanols via Rh(III)-catalyzed redox-neutral C-H/C-C activations using internal oxidative O-NHAc and -OH as the dual directing groups has been achieved. This reaction provided an efficient and regioselective approach to benzofuran derivatives with good functional group compatibility and high yields.

Full text of DOI:10.1021/acs.orglett.9b00812

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Dual Directing-Groups-Assisted Redox-Neutral Annulation and Ring
Opening of N‑Aryloxyacetamides with 1‑Alkynylcyclobutanols via
Rhodium(III)-Catalyzed C−H/C−C Activations
Jin-Long Pan,† Chang Liu,† Chao Chen,† Tuan-Qing Liu,† Man Wang,† Zhenliang Sun,*,†,‡
and Shu-Yu Zhang*,†
†Shanghai Jiao Tong University Affiliated Sixth People’s Hospital South Campus & Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.
R. China
‡Shanghai University of Medicine & Health Sciences Affiliated Sixth People’s Hospital South Campus, Shanghai 201499, China
S
* Supporting Information
ABSTRACT: A cascade [3 + 2] annulation and ring opening
of N-aryloxyacetamides with 1-alkynylcyclobutanols via Rh-
(III)-catalyzed redox-neutral C−H/C−C activations using
internal oxidative O−NHAc and −OH as the dual directing
groups has been achieved. This reaction provided an efficient
and regioselective approach to benzofuran derivatives with
good functional group compatibility and high yields.
Scheme 1. Transition-Metal-Catalyzed C−H
Functionalization with Alkynyl Alcohols
ransition-metal-catalyzed C−H bond activation and
Tfunctionalization has been emerging as an area of great
interest to chemists.1 The past decade has witnessed the
development of transition-metal-catalyzed C−H functionaliza-
tion enabled by oxidizing directing groups (ODGs), which
avoid the use of stoichiometric amounts of external oxidants.2
In 2013, Liu and Lu pioneered an efficient synthesis of
benzofuran and enamide derivatives via a Cp*Rh(III)-
catalyzed coupling between N-aryloxyacetamides and alkynes
with tunable selectivity under mild conditions.3 Afterward,
increasing attention has been devoted to transition-metal-
catalyzed C−H functionalization using N-aryloxyacetamides as
the privileged substrates for the redox-neutral coupling with
different coupling partners, such as alkynes,4 alkenes,5 diazos,6
and others.7
Among the different functionalized alkynes, alkynyl alcohols
are widely used as important building blocks in synthetic
chemistry.4e−h,8 In 2018, Yi revealed an efficient and mild
Ir(III)-catalyzed C−H annulation of N-aryloxyacetamides with
tertiary propargyl alcohols to deliver benzofurans (Scheme 1,
eq 1).4f In the same year, Yi developed the Rh(III)-catalyzed
and solvent-controlled C−H functionalization of N-arylox-
yacetamides with secondary or primary propargyl alcohols for
the divergent synthesis of chalcones and benzofurans (Scheme
1, eqs 2 and 3).4g Almost simultaneously, Deng reported a
similar Rh(III)-catalyzed coupling of N-aryloxyacetamides with
secondary propargyl alcohols to construct chalcones under
redox-neutral conditions.4h In 2017, Zeng described a Rh(III)-
catalyzed C−H/N−H annulation of 1-alkynylcyclobutanols
with arylamines to afford indole skeletons through a sequential
C−H/C−C cleavage (Scheme 1, eq 4).9
Enlightened by the above elegant work and as part of our
ongoing interest in transition-metal-catalyzed C−H functiona-
lization,4i,10 herein, we present our new development of
Rh(III)-catalyzed cascade [3 + 2] annulation and ring opening
Received: March 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00812
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Letter
of N-aryloxyacetamides with 1-alkynylcyclobutanols. In this
reaction, O−NHAc served as both the directing group and the
internal oxidant. Moreover, O−NHAc and −OH, which
avoided extra steps for removal of undesired directing groups,
were traceless in the products. Notably, this methodology
provided an efficient and regioselective approach to 1-
(benzofuran-2-yl)butan-1-ones with good functional group
tolerance and high yields under mild conditions (Scheme 1, eq
5).
N-Phenoxyacetamide 1a and 1-(phenylethynyl)cyclobutanol
2a were chosen as the model substrates for optimization of the
reaction parameters, and the results were summarized in Table
1. Initially, the substrates was treated with [Cp*RhCl2]2 (2.5
Cp*M catalysts (M = Co, Ir, or Ru) did not show any catalytic
activity. Under the optimal reaction conditions, N-phenox-
ypivalamide did not afford 3aa, and neither 1-(phenylethynyl)-
cyclopropanol nor 1-(phenylethynyl)cyclopentanol gave the
corresponding products.
With the optimized reaction conditions in hand, we first
examined the scope and limitations of N-aryloxyacetamides in
the present Cp*Rh(III)-catalyzed redox-neutral cascade [3 +
2] annulation and ring-opening reaction. As shown in Scheme
2, para-substituted N-aryloxyacetamides bearing alkyl, phenyl,
a b
,
Scheme 2. Substrate Scope
a b
,
Table 1. Optimization of the Reaction Conditions
entry
solvent
CH2Cl2
additive
yield (%)
1
2
3
4
5
6
7
8
CsOAc
KOAc
19
25
36
58
83
83
81
31
79
81
85
0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
ClCH2CH2Cl
THF
NaOAc
NaHCO3
KHCO3
Na2CO3
K2CO3
Cs2CO3
Na3PO4
K3PO4
K2HPO4
KH2PO4
−
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
9
10
11
12
13
14
15
16
17
0
85
78
39
0
MeCN
MeOH
CH2Cl2
c
18
93
a
Reaction conditions: 1a (0.20 mmol), 2a (0.20 mmol), [Cp*RhCl2]2
(2.5 mol %), and additive (0.25 equiv) in solvent (2.0 mL) at rt for 24
b
c
h. Isolated yield calculated based on 2a. 1.2 equiv of 1a was used.
mol %) and CsOAc (0.25 equiv) in dichloromethane (0.1 M)
at room temperature for 24 h, and the desired product 3aa was
obtained in 19% yield (entry 1). Intrigued by the isolation of
3aa, we then attempted to investigate different additives,
including basic acetates, bicarbonates, carbonates, phosphates,
and hydrogen phosphates. The results suggested that K2HPO4
was more practical and provided a better yield. No target
product was generated when KH2PO4 or no additive was used
(entries 2−13). Next, a survey of solvents indicated that 1,2-
dichloroethane gave an identical yield and that the reaction
was sluggish when polar solvents were employed. Especially,
no desired product was observed in MeOH (entries 14−17).
To further improve the conversion of 2a, the amount of 1a was
increased to 1.2 equiv and finally we were pleased to find that
the desired product 3aa could be isolated in 93% yield (entry
18). Notably, the reaction was not sensitive to the air
atmosphere and moisture. Control experiments showed that
Cp*Rh(III) was crucial toward the success of this reaction, as
its omission led to no formation of 3aa. Other representative
a
Reaction conditions: 1 (0.24 mmol), 2 (0.20 mmol), [Cp*RhCl2]2
(2.5 mol %), and K2HPO4 (0.25 equiv) in CH2Cl2 (2.0 mL) at rt for
24 h. Isolated yield calculated based on 2.
b
trifluoromethoxy, and halogens led to the formation of the
desired products in good yields (3ba−3fa, 3ia−3ma).
However, electron-deficient substrates were less reactive,
affording the target products in 33% and 50% yield,
respectively (3ga and 3ha). For meta- substituted substrates,
the reaction proceeded smoothly at the less sterically hindered
position with excellent regioselectivity (3na−3pa). Substrate
containing methyl at the ortho-position gave 3qa in moderate
yield. Additionally, 3,4-disubstituted and 2-naphthalenyl
substrates were well-tolerated (3ra and 3sa), and the reaction
could be successfully applied to late-stage modification of a
B
DOI: 10.1021/acs.orglett.9b00812
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Letter
bioactive dopamine derivative, as exemplified by the
construction of 3ta with good yield and functional group
compatibility. The structure of 3ha was unambiguously
confirmed by X-ray diffraction analysis.
and 2a with a 45% deuterated hydroxyl group. In contrast, the
reaction between 1a and 2a was performed smoothly in the
presence of D2O, and 16% deuterium was incorporated. These
results indicated that one hydrogen in the terminal methyl
group of 3aa did not directly come from either aromatic ring of
1a or the hydroxyl group of 2a, and that metal protonation was
likely to be involved in the reaction.9 (Scheme 4a). The KIE
Subsequently, we explored a wide range of 1-alkynylcyclo-
butanols under standard conditions. To our delight, various
functionalities on the para-, meta-, or ortho-positions of the
aromatic rings were compatible, giving the corresponding
products in good to excellent yields (3ab−3ak). 2-
Naphthalenyl, 2- or 3-thiophenyl substrates were well-
tolerated, while 2- or 3- pyridinyl substrates gave unsatisfactory
yields, probably due to the coordination of nitrogen with the
catalyst (3al−3ap). Moreover, the reaction could be extended
to alkenyl or alkanyl enthynylcyclobutanols (3aq−3as).
Substrates bearing substituents, such as phenyl, methoxycar-
bonyl, and benzyloxy, at the 3-position of cyclobutanols
showed different reaction activities, generating the correspond-
ing products in yields ranging from 29% to 84% (3at−3av).
To demonstrate the practicality of this method in organic
synthesis, a scaled-up reaction of 1a (6 mmol) and 2a (5
mmol) was performed under the optimal reaction conditions
and the product 3aa was isolated in 86% yield (Scheme 3a).
Scheme 4. Experimental Mechanistic Investigations
Scheme 3. Synthetic Applications
value were determined to be 1.03, which suggested that C−H
bond cleavage might not be involved in the rate-determining
step (Scheme 4b). In the presence of equivalent 2,2,6,6-
tetramethylpiperidine-N-oxyl (TEMPO), or 2,6-di-tert-butyl-4-
methylphenol (BHT) as a radical scavenger, 3aa could still be
isolated in 93% and 87% yields, respectively. These experi-
ments implied that the reaction might not proceed in a radical
pathway (Scheme 4c). A control experiment showed that 2a
could not converted to ketone 7 under the standard
conditions, and substrate 2a was recovered in 90% yield.
This experiment illustrated that [3 + 2] annulation of 1a and 7
was not involved in this methodology.
Based on the aforementioned experimental results, together
with precedent literature,4f,g,12 we have proposed a plausible
mechanism for the Cp*Rh(III)-catalyzed cascade [3 + 2]
annulation and ring opening, as depicted in Scheme 5. Initially,
the reaction was triggered by the reversible N−H deprotona-
tion and C−H activation processes of 1a with the active
Cp*Rh(III) species to give a five-membered rhodacycle A via a
concerted metalation−deprotonation (CMD) process. The
Further transformations of the products were subsequently
conducted to prove their versatility. Reduction of 3aa was
readily available using NaBH4 as a reductant, affording alcohol
4 in 94% yield. Epoxidation of 3aa with m-CPBA smoothly
gave 5 in 70% yield (Scheme 3b). Intramolecular cyclization of
3ak was possible via a palladium-catalyzed cascade arylation−
aromatization, delivering 5-β-naphthol derivative 6 in 87%
yield (Scheme 3c).11
To get a better understanding of the reaction mechanism, a
series of experiments were then carried out. First, 1a was
treated with D2O under the optimal reaction conditions, and
47% deuterium incorporation was observed at the ortho
position of the directing group O−NHAc in the recycled
substrate. This experiment demonstrated that C−H bond
activation was largely reversible. Next, no evident deuterium
incorporation was found in the terminal methyl group of 3aa
either in the reaction of [D5]-1a and 2a or in the reaction of 1a
C
DOI: 10.1021/acs.orglett.9b00812
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Letter
*E-mail: zhangsy16@sjtu.edu.cn.
Scheme 5. Proposed Catalytic Pathway
ORCID
Jin-Long Pan: 0000-0002-1085-4566
Shu-Yu Zhang: 0000-0002-1811-4159
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the “Thousand Youth Talents
Plan”, NSFC (81872418, 21672145, and 51733007), the
Shanghai Natural Science Foundation (18ZR1431700,
17JC1403700), the Innovation Fund (IFPM016A002) from
Joint Research Center for Precision Medicine of Shanghai Jiao
Tong University and the Municipal Human Resources
Development Program for Outstanding Young Talents in
Medical and Health Sciences in Shanghai (2017YQ048). We
gratefully thank Prof. Yong-Qiang Tu (Shanghai Jiao Tong
University) for helpful suggestions and comments on this
manuscript.
regioselective migratory insertion of 2a into the C−Rh bond
stemmed from the formation of hydrogen bonding between
the dual directing groups to afford a seven-membered
rhodacycle B. The oxidative addition of Rh(III) into the N−
O bond generated the Cp*Rh(V) nitrenoid C, which afforded
D through the reductive elimination and rhodium insertion
into the C−C bond. The following metal protonation led to
the formation of 3aa and the regeneration of the active
Cp*Rh(III) species for the next catalytic cycle.
In summary, we have developed efficient and regioselective
[3 + 2] annulation/ring opening cascade reactions of readily
available N-aryloxyacetamides and 1-alkynylcyclobutanols via
rhodium(III)-catalyzed redox-neutral C−H/C−C activations
using the internal oxidative O−NHAc and −OH as the dual
directing groups, which are traceless in the final benzofuran
derivatives. This methodology features good functional group
compatibility and high yields to provide substituted benzofuran
derivatives. Further studies on the synthetic applications of this
method and developments of new transition-metal-catalyzed
C−H functionalizations with other coupling partners are
underway in our laboratory.
REFERENCES
■
(1) For selected reviews, see: (a) Gulías, M.; Mascaren
̃
as, J. L.
Angew. Chem., Int. Ed. 2016, 55, 11000. (b) Gensch, T.; Hopkinson,
M. N.; Glorius, F.; Wencel-Delord, J. Chem. Soc. Rev. 2016, 45, 2900.
(c) Gensch, T.; James, M. J.; Dalton, T.; Glorius, F. Angew. Chem., Int.
Ed. 2018, 57, 2296. (d) Sambiagio, C.; Schonbauer, D.; Blieck, R.;
Dao-Huy, T.; Pototschnig, G.; Schaaf, P.; Wiesinger, T.; Zia, M. F.;
Wencel-Delord, J.; Besset, T.; Maes, B. U. W.; Schnu
Soc. Rev. 2018, 47, 6603.
(2) For selected reviews, see: (a) Huang, H.; Ji, X.; Wu, W.; Jiang, H.
Chem. Soc. Rev. 2015, 44, 1155. (b) Zhu, R.-Y.; Farmer, M. E.; Chen,
Y.-Q.; Yu, J.-Q. Angew. Chem., Int. Ed. 2016, 55, 10578. (c) Zhu, C.;
Wang, C.-Q.; Feng, C. Tetrahedron Lett. 2018, 59, 430. (d) Wang, Z.;
Xie, P.; Xia, Y. Chin. Chem. Lett. 2018, 29, 47.
̈
̈
rch, M. Chem.
(3) Liu, G.; Shen, Y.; Zhou, Z.; Lu, X. Angew. Chem., Int. Ed. 2013,
52, 6033.
(4) For selected examples, see: (a) Chen, Y.; Wang, D.; Duan, P.;
Ben, R.; Dai, L.; Shao, X.; Hong, M.; Zhao, J.; Huang, Y. Nat.
Commun. 2014, 5, 4610. (b) Zhou, J.; Shi, J.; Qi, Z.; Li, X.; Xu, H. E.;
Yi, W. ACS Catal. 2015, 5, 6999. (c) Hu, S.; Lu, L.; Zhu, T.; Wu, Q.;
Chen, Y.; Li, J. J.; Zhao, J. Org. Biomol. Chem. 2018, 16, 43. (d) Li, M.;
Wang, J.-H.; Li, W.; Wen, L.-R. Org. Lett. 2018, 20, 7694. (e) Zhou,
Z.; Liu, G.; Chen, Y.; Lu, X. Org. Lett. 2015, 17, 5874. (f) Chen, W.;
Liu, F.-X.; Gong, W.; Zhou, Z.; Gao, H.; Shi, J.; Wu, B.; Yi, W. Adv.
Synth. Catal. 2018, 360, 2470. (g) Yi, W.; Chen, W.; Liu, F.-X.;
Zhong, Y.; Wu, D.; Zhou, Z.; Gao, H. ACS Catal. 2018, 8, 9508.
(h) Zhou, W.; Mei, Y.-L.; Li, B.; Guan, Z.-Y.; Deng, Q.-H. Org. Lett.
2018, 20, 5808. (i) Pan, J.-L.; Xie, P.; Chen, C.; Hao, Y.; Liu, C.; Bai,
H.-Y.; Ding, J.; Wang, L.-R.; Xia, Y.; Zhang, S.-Y. Org. Lett. 2018, 20,
7131. (j) Yuan, W.-K.; Zhu, M.-H.; Geng, R.-S.; Ren, G.-Y.; Zhang,
L.-B.; Wen, L.-R.; Li, M. Org. Lett. 2019, 21, 1654. (k) Li, Y.; Shi, D.;
Tang, Y.; He, X.; Xu, S. J. Org. Chem. 2018, 83, 9464. (l) Li, Y.; Shi,
D.; He, X.; Wang, Y.; Tang, Y.; Zhang, J.; Xu, S. J. Org. Chem. 2019,
84, 1588.
(5) For selected examples, see: (a) Shen, Y.; Liu, G.; Zhou, Z.; Lu,
X. Org. Lett. 2013, 15, 3366. (b) Lerchen, A.; Knecht, T.; Daniliuc, C.
G.; Glorius, F. Angew. Chem., Int. Ed. 2016, 55, 15166. (c) Wang, Y.;
Chen, Y.; Yang, Y.; Zhou, B. Org. Chem. Front. 2018, 5, 1844. (d) Li,
Y.; Tang, Y.; He, X.; Shi, D.; Wu, J.; Xu, S. Chem. - Eur. J. 2017, 23,
7453. (e) Duan, P.; Lan, X.; Chen, Y.; Qian, S.-S.; Li, J. J.; Lu, L.; Lu,
Y.; Chen, B.; Hong, M.; Zhao, J. Chem. Commun. 2014, 50, 12135.
(f) Zhang, H.; Wang, K.; Wang, B.; Yi, H.; Hu, F.; Li, C.; Zhang, Y.;
Wang, J. Angew. Chem., Int. Ed. 2014, 53, 13234. (g) Wang, X.;
Lerchen, A.; Daniliuc, C. G.; Glorius, F. Angew. Chem., Int. Ed. 2018,
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00812.
Experimental procedures, characterization data, and
copies of NMR spectra (PDF)
Accession Codes
CCDC 1884405 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: zhenliang6@126.com.
D
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57, 1712. (h) Wang, X.; Lerchen, A.; Gensch, T.; Knecht, T.;
Daniliuc, C. G.; Glorius, F. Angew. Chem., Int. Ed. 2017, 56, 1381.
(6) For selected examples, see: (a) Hu, F.; Xia, Y.; Ye, F.; Liu, Z.;
Ma, C.; Zhang, Y.; Wang, J. Angew. Chem., Int. Ed. 2014, 53, 1364.
(b) Zhou, J.; Shi, J.; Liu, X.; Jia, J.; Song, H.; Xu, H. E.; Yi, W. Chem.
Commun. 2015, 51, 5868. (c) Hu, Z.; Liu, G. Adv. Synth. Catal. 2017,
359, 1643. (d) Wu, Y.; Chen, Z.; Yang, Y.; Zhu, W.; Zhou, B. J. Am.
Chem. Soc. 2018, 140, 42.
(7) For selected examples, see: (a) Duan, P.; Yang, Y.; Ben, R.; Yan,
Y.; Dai, L.; Hong, M.; Wu, Y.-D.; Wang, D.; Zhang, X.; Zhao, J. Chem.
Sci. 2014, 5, 1574. (b) Li, B.; Lan, J.; Wu, D.; You, J. Angew. Chem.,
Int. Ed. 2015, 54, 14008. (c) Wu, Q.; Chen, Y.; Yan, D.; Zhang, M.;
Lu, Y.; Sun, W.-Y.; Zhao, J. Chem. Sci. 2017, 8, 169. (d) Wang, X.;
Gensch, T.; Lerchen, A.; Daniliuc, C. G.; Glorius, F. J. Am. Chem. Soc.
2017, 139, 6506. (e) Wu, Q.; Yan, D.; Chen, Y.; Wang, T.; Xiong, F.;
Wei, W.; Lu, Y.; Sun, W.-Y.; Li, J. J.; Zhao, J. Nat. Commun. 2017, 8,
14227. (f) Xiong, F.; Lu, L.; Sun, T.-Y.; Wu, Q.; Yan, D.; Chen, Y.;
Zhang, X.; Wei, W.; Lu, Y.; Sun, W.-Y.; Li, J. J.; Zhao, J. Nat. Commun.
2017, 8, 15912. (g) Yan, D.; Wang, G.; Xiong, F.; Sun, W.-Y.; Shi, Z.;
Lu, Y.; Li, S.; Zhao, J. Nat. Commun. 2018, 9, 4293.
(8) For selected examples, see: (a) Iwasawa, N.; Matsuo, T.;
Iwamoto, M.; Ikeno, T. J. Am. Chem. Soc. 1998, 120, 3903.
(b) Markham, J. P.; Staben, S. T.; Toste, F. D. J. Am. Chem. Soc.
2005, 127, 9708. (c) Yeom, H.-S.; Lee, Y.; Jeong, J.; So, E.; Hwang,
S.; Lee, J.-E.; Lee, S. S.; Shin, S. Angew. Chem., Int. Ed. 2010, 49, 1611.
(d) Jones, C.; Nguyen, Q.; Driver, T. G. Angew. Chem., Int. Ed. 2014,
53, 785. (e) Tian, Q.; Chen, B.; Zhang, G. Green Chem. 2016, 18,
6236.
(9) Hu, X.; Chen, X.; Zhu, Y.; Deng, Y.; Zeng, H.; Jiang, H.; Zeng,
W. Org. Lett. 2017, 19, 3474.
(10) For recent selected examples, see: (a) Pan, J.-L.; Li, Q.-Z.;
Zhang, T.-Y.; Hou, S.-H.; Kang, J.-C.; Zhang, S.-Y. Chem. Commun.
2016, 52, 13151. (b) Pan, J.-L.; Chen, C.; Ma, Z.-G.; Zhou, J.; Wang,
L.-R.; Zhang, S.-Y. Org. Lett. 2017, 19, 5216. (c) Bai, H.-Y.; Ma, Z.-G.;
Yi, M.; Lin, J.-B.; Zhang, S.-Y. ACS Catal. 2017, 7, 2042. (d) Zhang,
T.-Y.; Lin, J.-B.; Li, Q.-Z.; Kang, J.-C.; Pan, J.-L.; Hou, S.-H.; Chen,
C.; Zhang, S.-Y. Org. Lett. 2017, 19, 1764. (e) Ding, J.; Jiang, W.; Bai,
H.-Y.; Ding, T.-M.; Gao, D.; Bao, X.; Zhang, S.-Y. Chem. Commun.
2018, 54, 8889. (f) Fu, X.; Bai, H.-Y.; Zhu, G.-D.; Huang, Y.; Zhang,
S.-Y. Org. Lett. 2018, 20, 3469.
(11) Zhu, C.; Wang, D.; Zhao, Y.; Sun, W.-Y.; Shi, Z. J. Am. Chem.
Soc. 2017, 139, 16486.
(12) Yang, Y.-F.; Houk, K. N.; Wu, Y.-D. J. Am. Chem. Soc. 2016,
138, 6861.
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