Sequential Suzuki-Miyaura Coupling/Lewis Acid-Catalyzed Cyclization: An Entry to Functionalized Cycloalkane-Fused Naphthalenes

Article abstract of DOI:10.1021/acs.orglett.0c02020

Functionalized angular cycloalkane-fused naphthalenes were prepared using a two-step process involving a Pd-catalyzed Suzuki-Miyaura coupling of aryl pinacol boronates and vinyl triflates followed by a boron trifluoride etherate-catalyzed cycloaromatization.

Full text of DOI:10.1021/acs.orglett.0c02020

pubs.acs.org/OrgLett
Letter
Sequential Suzuki−Miyaura Coupling/Lewis Acid-Catalyzed
Cyclization: An Entry to Functionalized Cycloalkane-Fused
Naphthalenes
́
́
́
Camilo Mahecha-Mahecha, Frederic Lecornue,* Sumita Akinari, Thomas Charote,
́
́
Diego Gamba-Sanchez, Tomohiko Ohwada, and Sebastien Thibaudeau*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c02020
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Functionalized angular cycloalkane-fused naphtha-
lenes were prepared using a two-step process involving a Pd-
catalyzed Suzuki−Miyaura coupling of aryl pinacol boronates and
vinyl triflates followed by a boron trifluoride etherate-catalyzed
cycloaromatization.
olysubstituted naphthalenes and their functionalized
employed.10 Cascade reactions11 and isomerization12 allowed
the generation of original fused derivatives as well as the metal-
catalyzed annulation and coupling reactions.13 The dehydro-
genation of 1,4-dihydronaphthalenes,14 aldol-type condensa-
tion,15 and acid-promoted cyclization of naphthyl butanol
derivatives16 complete this synthetic arsenal. Another strategy
relies on the intramolecular acid-catalyzed Friedel−Crafts
reaction that is favored by the aromatization process.17
Aromaticity is known to be an important thermodynamic
driving force18 for various synthetic and enzymatic trans-
formations.19 It has been used to generate phenanthrene
derivatives through acid-promoted electrophilic aromatic
substitution processes20 but rarely exploited to generate
functionalized naphthalenes21 or cyclic fused analogues.22
Here we disclose a concise entry to functionalized angular
naphthalene derivatives,23 through a sequential Suzuki−
Miyaura coupling/cycloaromatization of ketal (or acetal)-
containing pinacol boronic esters and vinyl triflates (Figure 2).
We envisioned that boronic esters could ideally act as
nucleophiles in the Pd-catalyzed coupling reaction in the
presence of a strong electrophilic vinyl triflate to form the first
C−C bond. The masked electrophilic carbonyl group could
then be activated to react with the nucleophilic styrene
derivative. Building upon the thermodynamically favored
cycloaromatization, the second C−C bond being formed, we
P
derivatives are most common structures for a broad
range of applications (Figure 1). Important fluorophores,1 they
Figure 1. Representative naphthalene-containing high-value synthetic
and natural molecules.
are valuable intermediates in the synthesis of complex
molecules (azo dyes) for preparing materials (naphthalene
diimides).2 Fused naphthalenes also constitute the essential
structural core of numerous natural products and active
pharmaceutical ingredients (Naproxen, Naftopidil, and Sur-
amin)3 and are also actively exploited as ligands.4,5 Therefore,
the development of straightforward synthetic strategies to
access functionalized naphthalenes is desirable, and recent
ongoing efforts in this direction have been made.
The prevalent method is the use of a Diels−Alder reaction
followed by oxidative aromatization.6,7 The selective hydro-
genation of aromatic and polyaromatic compounds is also a
standard reaction,8 but mainly limited to non-functionalized
systems, needing alternative developments.9 Ene-allene isomer-
ization and electrocyclization have also been successfully
Received: June 18, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02020
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
(diphenylphosphino)ferrocene] dichloropalladium(II) catalyst
(5 mol %), a substoichiometric amount of copper chloride as
an additive (0.75 equiv), and an excess of sodium hydroxide (4
equiv). After reaction for 2 h at 90 °C in 1,4-dioxane, the
desired aryl-substituted functionalized olefin 5a was synthe-
sized in 76% yield. (Figure 3B).
Our next goal was to validate the feasibility of exploiting the
acid-promoted cycloaromatization of substrates 5 to target the
functionalized naphthalenes 6. Cyclohexene 5a was selected as
a model substrate to screen various acidic conditions (Table
1). First, the use of triflic acid in excess was explored without
Figure 2. General strategy toward functionalized cycloalkane-fused
naphthalenes.
found the dehydration would end up being formation of the
desired naphthalene.
To test this hypothesis, the functionalized brominated
aromatic ketals 2 were synthesized (Figure 3A). Starting
Table 1. Optimization of the Acid-Catalyzed Cyclization of
Olefin 5a to Naphthalene 6a
a
b
entry
1
2
acid
TfOH
TfOH
equiv
reaction time (h)
yield (%)
c
10
7
5
1
−
d
c
−
3
4
5
6
MgBr2
AlCl3
7
7
10
1
0.1
0.1
0.1
0.1
1
24
7
69
17
e
BF3·OEt2
BF3·OEt2
BF3·OEt2
CeCl3·7H2O
Ce(OTf)3
Bi(OTf)3
CSA
24
24
1.5
17
4
4
3
16
94
90
86
14
35
87
79
68
f
7
8
f g
9 ,
10 ,
f g
f
11
12
f
CSA
0.15
a
b
Equivalents of acid. Yield after purification over column
c
d
chromatography. Complex mixture. Reaction performed at 0 °C.
e
f
g
Side product formation. Reflux. EtOH used as the solvent.
success. Under these conditions, only a complex mixture of
compounds was obtained, highlighting the sensibility of the
process in the presence of an excess of strong Brønsted acids
(Table 1, entries 1 and 2). We thus moved to explore Lewis
acid activation by testing magnesium bromide in excess.
Gratifyingly, after reaction for 24 h at room temperature, the
targeted naphthalene product 6a could be synthesized in 69%
yield, confirming the ability to generate functionalized
naphthalene with this sequential protocol. When aluminum
chloride was used, the desired product was also formed, but
among side products. Conversely, the use of an excess of boron
trifluoride diethyl etherate as an acid promoter was very
efficient, favoring the clean formation of the desired product in
94% yield (Table 1, entry 5). Using a stoichiometric amount of
the acid was found not to be detrimental to the reaction (Table
1, entry 6). With one additional step in the optimization of this
reaction, it was found that the use of a catalytic amount of the
acid (15 mol %) was sufficient to favor the reaction, the
desired naphthalene derivative 6a being formed after reaction
for 1.5 h in a reasonable 86% yield. The cyclization reaction
could also be catalyzed by other Lewis acids (Table 1, entries
8−10), but the need to use a higher temperature could be seen
as a limitation when using sensitive substrates. Given that the
aromatization of the system liberates acidic protons, we
hypothesized that the process could also be promoted by a
Brønsted acid. This was confirmed by testing the reaction in
the presence of camphorsulfonic acid (Table 1, entries 11 and
Figure 3. (A) General synthetic procedure for generating the
functionalized pinacol boronic esters 3 and vinyl triflates 4. (B)
Suzuki−Miyaura coupling between boronic esters 3a and vinyl triflate
4b under optimized conditions.
from commercially available aldehydes (or ketones) 1, a
straightforward one-pot Wittig reaction/acetal formation
allows for the synthesis of the desired products in overall
good yields. Alternatively, the ketals can also be generated
directly from methyl- and phenyl-substituted ketones, obtained
from the commercially available 2-bromophenylacetic acid.
The Miyaura borylation of protected derivatives led to the
formation of the desired arylboronic acid pinacol esters 3 in
reasonable yields.24 Cycloalkenyl triflates 4 were synthesized
from their ketone precursors after reaction with trifluorome-
thanesulfonic anhydride or N-phenyltrifluoromethanesulfoni-
mide. With the key substrates 3 and 4 in hand, the earliest
objective of the proposed approach was to form the first C−C
bond following a Suzuki−Miyaura coupling reaction.
Our first attempt with substrates 3a and 4b led to the
desired formation of aryl-substituted cyclohexene derivative 5a.
However, the modest obtained yield after purification
motivated us to screen other reaction conditions (see the
Supporting Information).25 Typically, the optimized coupling
reaction was performed between vinyl triflate 4b and aryl
pinacol boronate 3a in the presence of the 1,1′-[bis-
B
https://dx.doi.org/10.1021/acs.orglett.0c02020
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Figure 4. Suzuki−Miyaura coupling between boronic esters 3a−3i and vinyl triflates 4a−4f under optimized conditions.
12). Tentative efforts to perform this sequence in a single pot
or sequentially without purification of intermediate 5a failed in
our hands.
were then applied to the synthesis of cyclopentene and
cycloheptene derivatives 5h and 5i with success. These
compounds further cyclized to generate the five- and seven-
membered fused ring systems in good yields (formation of
products 6h and 6i). The reaction between vinyl triflate
derived from acetophenone and substrate 3a allowed us to
generate targeted olefin 5j in a modest yield. This alkene was
found to be a good candidate for the synthesis of aryl-
substituted naphthalene 6j. Exploiting an alternative synthetic
strategy based on Ullmann-type coupling between iodocyclo-
hexene and indole derivative, product 5k could be generated in
an excellent 93% yield (see the Supporting Information).
Unfortunately, the cyclization did not occur to furnish the
desired indolo-fused naphthalene derivative, and thus whatever
the cyclization conditions used. Interestingly, a slightly
modified protocol allowed the efficient generation of the
targeted nitrogen-containing analogue 5l. The subsequent
challenging formation of the piperidine-fused dihydrobenzoi-
soquinoline 6l in excellent yield confirmed that the method
could also be considered as an efficient tool to access nitrogen-
containing polycyclic derivatives of biological interest.28 To
further explore the potential of the strategy, alkyl- and aryl-
substituted products 5m−5o were also synthesized in good
yields. Gratifyingly, their efficient cyclization highlighted the
With these optimized conditions in hand, the scope of this
tandem process was next explored by submitting a series of
boronic esters 3 and triflates 4 to the Suzuki−Miyaura
coupling reaction, followed by the cyclization reaction of the
generated alkenes 5 under optimized cyclization conditions in
the presence of BF3·OEt2 (0.15 equiv) in dichloromethane for
1.5 h at reflux (Figure 4). Naphthyl-substituted olefin 5b was
generated in 60% yield. The reaction was found to be more
demanding with boronic ester exhibiting withdrawing groups
or with heteroaromatic boronic ester (formation of products
5d and 5e) than with the electronically enriched substrate
(formation of 5c). Under the optimized cyclization conditions,
cyclohexane-fused naphthalene derivatives 6b−6d could then
be efficiently generated in good to very good yields.
Interestingly, the method also allowed the direct access to
heteroaromatic derivatives, as exemplified with the formation
of benzothiophene 6e in 81% yield. Dihydronaphthyl
derivatives 5f and 5g were synthesized following the same
procedure and then acting as good substrates to generate
original angular naphthalene derivatives of interest,26,27 6f and
6g, in very good yields. The optimized reaction conditions
C
https://dx.doi.org/10.1021/acs.orglett.0c02020
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Notes
ability to generate efficiently elaborated and regioselectively
substituted cyclohexane-fused naphthalene derivatives 6m−6o.
Taking into account these data, we found the cyclization step
must follow a Friedel−Crafts-type mechanism. Any temporary
formation of aldehyde after acetal partial acid hydrolysis was
not observed when following the reaction by in situ NMR
spectroscopy. Starting from the vinyl methyl ether analogue of
substrate 5a, we could obtain product 6a in only 47%
yield.17,21 An analogous aldehyde was also tested under the
same conditions, generating a crude mixture containing only
traces of the desired product. This suggests a concerted
mechanism implicating acetal activation and alkene trapping
followed by subsequent eliminations to afford the desired
aromatic.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the CNRS and the University of Poitiers.
The authors also acknowledge financial support from the
́
European Union (ERDF) and “Region Nouvelle Aquitaine”
(SUPERDIV project-HABISAN program). The authors also
thank the French fluorine network (GIS-FLUOR). The
authors are also thankful for a JSPS visiting Fellows grant to
S.A. (16J08260). C.M.-M. acknowledges the Faculty of Science
of Universidad de los Andes for financial support during his
internship.
In conclusion, using a straightforward sequential two-step
Pd- and acid-catalyzed procedure from easily accessible
substrates, functionalized cycloalkane-fused naphthalenes can
be efficiently prepared. This strategy that can be applied to a
variety of different substrates can be seen as a general entry to
fused ring compounds by formation of two consecutive
selective C−C bonds.29
REFERENCES
■
(1) (a) Benedetti, E.; Kocsis, L. S.; Brummond, K. M. J. Am. Chem.
Soc. 2012, 134, 12418−12421. (b) Takaishi, K.; Iwachido, K.;
Takehana, R.; Uchiyama, M.; Ema, T. J. Am. Chem. Soc. 2019, 141,
6185−6190. (c) Berezin, A. A.; Sciutto, A.; Demitri, N.; Bonifazi, D.
Org. Lett. 2015, 17, 1870−1873.
(2) (a) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M.
A.; Wasielewski, M. R.; Marder, S. R. Adv. Mater. 2011, 23, 268−284.
(b) Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li,
W.; Lin, Y. Y.; Dodabalapur, A. Nature 2000, 404, 478−481. (c) Sim,
J.; Yim, H.; Ko, N.; Choi, S. B.; Oh, Y.; Park, H. J.; Park, S. Y.; Kim, J.
Dalton Trans. 2014, 43, 18017−18024.
(3) (a) Wang, Y.; Zhang, X.; Zhao, J.; Xie, S.; Wang, C. J. Med.
Chem. 2012, 55, 3502−3512. (b) Noorduin, W. L.; Kaptein, B.;
Meekes, H.; van Enckevort, W. J. P.; Kellogg, R. M.; Vlieg, E. Angew.
Chem., Int. Ed. 2009, 48, 4581−4583. (c) Dai, J.; Liu, Y.; Zhou, Y. D.;
Nagle, D. G. J. Nat. Prod. 2007, 70, 1824−1826. (d) Marchais-
Oberwinkler, S.; Wetzel, M.; Ziegler, E.; Kruchten, P.; Werth, R.;
Henn, C.; Hartmann, R. W.; Frotscher, M. J. Med. Chem. 2011, 54,
534−547. (f) Ji, K.; Yang, F.; Gao, S.; Tang, J.; Gao, J. Chem. - Eur. J.
2016, 22, 10225−10229.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02020.
■
sı
1
Experimental procedures, characterization data, and H,
13C, and 19F NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
́
́
Sebastien Thibaudeau − Universite de Poitiers, UMR-CNRS
̀
7285, IC2MP, Equipe Synthese Organique-Groupe Superacide,
86073 Poitiers, France; orcid.org/0000-0002-6246-5829;
Email: sebastien.thibaudeau@univ-poitiers.fr
(4) Shibasaki, M.; Matsunaga, S. In Privileged Chiral Ligands and
Catalysts; Zhou, Q. L., Ed.; Wiley: New York, 2011; pp 295−332 and
references cited therein.
(5) For a recent example, see: Bai, H.-Y.; Tan, F.-X.; Liu, T.-Q.; Zhu,
G.-D.; Tian, J.-M.; Ding, T.-M.; Chen, Z.-M.; Zhang, S.-Y. Nat.
Commun. 2019, 10, 3063.
́
́
́
́
Frederic Lecornue − Universite de Poitiers, UMR-CNRS 7285,
̀
IC2MP, Equipe Synthese Organique-Groupe Superacide, 86073
Poitiers, France; Email: frederic.lecornue@univ-poitiers.fr
(6) (a) Hilt, G.; Janikowski, J.; Hess, W. Angew. Chem., Int. Ed. 2006,
Authors
̈
45, 5204−5206. (b) Mockel, R.; Hilt, G. Org. Lett. 2015, 17, 1644−
́
Camilo Mahecha-Mahecha − Universite de Poitiers, UMR-
CNRS 7285, IC2MP, Equipe Synthese Organique-Groupe
Superacide, 86073 Poitiers, France; Laboratory of Organic
Synthesis, Bio and Organocatalysis, Chemistry Department,
Universidad de los Andes, Bogota, Colombia; orcid.org/
0000-0002-0034-1169
Sumita Akinari − Universite de Poitiers, UMR-CNRS 7285,
IC2MP, Equipe Synthese Organique-Groupe Superacide, 86073
Poitiers, France; Graduate School of Pharmaceutical Sciences,
The University of Tokyo, Tokyo 113-0033, Japan
Thomas Charote − Universite de Poitiers, UMR-CNRS 7285,
IC2MP, Equipe Synthese Organique-Groupe Superacide, 86073
1647. (c) Kramer, N. J.; Hoang, T. T.; Dudley, G. D. Org. Lett. 2017,
19, 4636−4639.
̀
(7) For recently developed electrochemically and photocatalytically
activated versions, see: (a) Ma, Y.; Lv, J.; Liu, C.; Yao, X.; Yan, G.; Yu,
W.; Ye, J. Angew. Chem., Int. Ed. 2019, 58, 6756−6760. (b) Chatterjee,
T.; Lee, D. S.; Cho, E. J. J. Org. Chem. 2017, 82, 4369−4378.
(8) For a recent example, see: Luo, W.; Shi, H.; Schachtl, E.;
́
́
́
Gutierrez, O. Y.; Lercher, J. A. Angew. Chem., Int. Ed. 2018, 57,
̀
14555−14559.
(9) Fu, W.; Zhao, W.; Zhang, L.; Zhang, T.; Tang, T.; Chen, Q. Ind.
Eng. Chem. Res. 2019, 58, 17289−17299.
(10) Chen, D.; Xing, G.; Yao, J.; Zhou, H. RSC Adv. 2016, 6,
103320−103323.
́
̀
Poitiers, France
(11) Feng, T.; He, Y.; Zhang, X.; Fan, X. Adv. Synth. Catal. 2019,
361, 1271−1276.
́
Diego Gamba-Sanchez − Laboratory of Organic Synthesis, Bio
and Organocatalysis, Chemistry Department, Universidad de los
Andes, Bogota, Colombia; orcid.org/0000-0001-5432-5108
Tomohiko Ohwada − Graduate School of Pharmaceutical
Sciences, The University of Tokyo, Tokyo 113-0033, Japan;
orcid.org/0000-0001-5390-0203
(12) (a) Zhu, Z.-B.; Wei, Y.; Shi, M. Chem. - Eur. J. 2009, 15, 7543−
7548. (b) Furstner, A.; Mamane, V. J. Org. Chem. 2002, 67, 6264−
̈
́
6267. (c) Yu, J.; Yan, H.; Zhu, C. Angew. Chem., Int. Ed. 2016, 55,
1143−1146.
(13) (a) Dudnik, A. S.; Schwier, T.; Gevorgyan, V. Org. Lett. 2008,
10, 1465−1468. (b) Gogoi, P.; Bezboruah, P.; Boruah, R. C. Eur. J.
Org. Chem. 2013, 2013, 5032−5035. (c) Fukutani, T.; Hirano, K.;
Satoh, T.; Miura, M. Org. Lett. 2009, 11, 5198−5201. (d) Liu, Z.;
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02020
D
https://dx.doi.org/10.1021/acs.orglett.0c02020
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Larock, R. C. J. Org. Chem. 2007, 72, 223−232. (e) Mamane, V.;
Hannen, P.; Fu
̈
rstner, A. Chem. - Eur. J. 2004, 10, 4556−4575.
(14) Thummel, R. P.; Cravey, W. E.; Cantu, D. B. J. Org. Chem.
1980, 45, 1633−1637.
(15) Di Raddo, P.; Harvey, R. G. Tetrahedron Lett. 1988, 29, 3885−
3886.
(16) Jackson, A. H.; Shannon, P. V. R.; Taylor, P. W. J. Chem. Soc.,
Perkin Trans. 2 1981, 2, 286−297.
(17) (a) Ichikawa, J.; Yokota, M.; Kudo, T.; Umezaki, S. Angew.
Chem., Int. Ed. 2008, 47, 4870−4873. (b) Fuchibe, K.; Jyono, H.;
Fujiwara, M.; Kudo, T.; Yokota, M.; Ichikawa, J. Chem. - Eur. J. 2011,
17, 12175−12185. (c) Ponra, S.; Vitale, M. R.; Michelet, V.;
Ratovelomanana-Vidal, V. Arkivoc 2016, ii, 62−81.
(18) Schleyer, P. V. R.; Puhlhofer, F. Org. Lett. 2002, 4, 2873−2876.
̈
(19) For a recent example of the promotion of C−C bond
activation, see: Xu, Y.; Qi, X.; Zheng, P.; Berti, C.; Liu, P.; Dong, G.
Nature 2019, 567, 373−378.
(20) (a) Li, Y.; Yagi, A.; Itami, K. Chem. Sci. 2019, 10, 5470−5475.
(b) Takahashi, I.; Hayashi, M.; Fujita, T.; Ichikawa, J. Chem. Lett.
2017, 46, 392−394.
(21) (a) Fujita, T.; Takahashi, I.; Hayashi, M.; Wang, J.; Fuchibe, K.;
Ichikawa, J. Eur. J. Org. Chem. 2017, 2017, 262−265. (b) Murai, M.;
Hosokawa, N.; Roy, D.; Takai, K. Org. Lett. 2014, 16, 4134−4137.
(22) Melidonie, J.; Dmitrieva, E.; Zhang, K.; Fu, Y.; Popov, A. A.;
Pisula, W.; Berger, R.; Liu, J.; Feng, X. J. Org. Chem. 2020, 85, 215−
223.
(23) Chen, S.-C.; Zhang, Q.; Zheng, Q.; Tang, C.; Lu, C.-Z. Chem.
Commun. 2012, 48, 1254−1256.
(24) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508−7510.
(25) (a) Nguyen, Q.; Sun, K.; Driver, T. G. J. Am. Chem. Soc. 2012,
134, 7262−7265. (b) Deng, J. Z.; Paone, D. V.; Ginnetti, A. T.;
Kurihara, H.; Dreher, S. D.; Weissman, S. A.; Stauffer, S. R.; Burgey,
C. S. Org. Lett. 2009, 11, 345−347.
(26) Shen, Y.; Chen, C.-F. Chem. Rev. 2012, 112, 1463−1535.
(27) Oxidative dehydrogenation would generate corresponding
polycyclic aromatic compounds. See ref 15 and: (a) Neumann, R.;
Lissel, M. J. Org. Chem. 1989, 54, 4607−4610. (b) Kamata, K.; Kasai,
J.; Yamaguchi, K.; Mizuno, N. Org. Lett. 2004, 6, 3577−3580.
(c) Zhang, W.; Ma, H.; Zhou, L.; Sun, Z.; Du, Z.; Miao, H.; Xu, J.
Molecules 2008, 13, 3236−3245.
(28) Tan, Q.; Yang, Z.; Jiang, D.; Cheng, Y.; Yang, J.; Xi, S.; Zhang,
M. Angew. Chem., Int. Ed. 2019, 58, 6420−6424.
(29) For recent examples of sequential two-step fused ring
formation, see: (a) Wang, M.; Kong, L.; Wang, Y.; Song, B.; Sun,
Y.; Tang, R.; Li, Y. Org. Lett. 2018, 20, 6130−6134. (b) Ambler, B. R.;
Turnbull, B. W. H.; Suravarapu, S. R.; Uteuliyev, M. M.; Huynh, N.
O.; Krische, M. J. J. Am. Chem. Soc. 2018, 140, 9091−9094. (c) Deng,
L.; Jin, L.; Dong, G. Angew. Chem., Int. Ed. 2018, 57, 2702−2706.
E
https://dx.doi.org/10.1021/acs.orglett.0c02020
Org. Lett. XXXX, XXX, XXX−XXX