Gold-catalyzed cascade cyclization of (azido)ynamides: An efficient strategy for the construction of indoloquinolines

Article abstract of DOI:10.1021/ol5012604

(Azido)ynamides were efficiently converted into indoloquinolines by the use of a gold catalyst. While ynamides bearing an allylsilane gave terminal alkenes, ynamides bearing a simple alkene gave cyclopropanes. This reaction proceeds through the formation

Full text of DOI:10.1021/ol5012604

Letter
pubs.acs.org/OrgLett
Gold-Catalyzed Cascade Cyclization of (Azido)ynamides: An Efficient
Strategy for the Construction of Indoloquinolines
Yusuke Tokimizu, Shinya Oishi, Nobutaka Fujii,* and Hiroaki Ohno*
Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
S
* Supporting Information
ABSTRACT: (Azido)ynamides were efficiently converted
into indoloquinolines by the use of a gold catalyst. While
ynamides bearing an allylsilane gave terminal alkenes,
ynamides bearing a simple alkene gave cyclopropanes. This
reaction proceeds through the formation of an α-amidino gold
carbenoid.
uring the past decade, gold has attracted considerable
attention as an effective π-acid for the activation of alkynes
and antitumor activities.10 It was envisaged that the gold-
catalyzed reaction of (azido)ynamide A would lead to the
formation of an α-amidino gold-carbenoid B, which could be
used to produce various indoloquinolines C−E through an
intramolecular trapping reaction with an alkene or arene
(Scheme 1). For example, the reaction of the carbenoid B with
D
and allenes,1 and the gold-catalyzed reactions of alkynes and
alkenes have been used extensively to synthesize a broad range of
natural products and complex molecules in an efficient and
chemoselective manner.2 In addition to its ability to activate π-
bonds, gold ions bearing alkenyl or alkynyl ligands are electron-
donating and have the ability to form gold-carbenoid species,
which have been reported as intermediates in the reactions of
enynes,3 diynes,4 and alkynes with N-oxides,5 as well as the
rearrangement reactions of 1,2-propargylic esters.5a,6 In 2005,
Toste et al.7a reported the use of azide as an effective nitrene
equivalent for the generation of a gold-carbenoid species in their
synthesis of pyrroles. Following on from this pioneering work,
Gagosz7b and Zhang7c independently reported the development
of a novel method for the synthesis of indoles from alkynyl azides
by the nucleophilic addition of alcohols or arenes to gold
carbenoids. In contrast to these studies, there have been no
reports in the literature to date pertaining to the reactivity of
ynamides with azides, despite the potential utility of this reaction
in the synthesis of α-carboline-type fused indoles.
Scheme 1. Concept of This Work
an allylsilane or simple alkene moiety would give terminal alkene
C or cyclopropane D, respectively. It was also envisaged that the
reaction of carbenoid B with an arene would be effective for the
formation of fused indole E. Herein, we report the development
of a novel method for the construction of indoloquinoline
compounds by the gold-catalyzed cascade cyclization of
(azido)ynamides.
The (azido)ynamide 1a was prepared by the reaction of
tosylamides with a Waser-type alkynylating reagent (Scheme
2).11 It is noteworthy that bromoalkyne, which has been widely
used as an alkynylating reagent for the preparation of ynamides,12
was ineffective in our hands for the preparation of (azido)-
ynamides. Ynamides 1b−g were also prepared in a similar
manner (see Supporting Information).
As a part of our ongoing research toward the development of
gold-catalyzed reactions for the construction of the core
structures of pharmaceutically important compounds,8 we
became interested in the direct synthesis of the α-carbolines
and indoloquinolines. These heterocyclic frameworks can be
found in numerous bioactive natural products (Figure 1),9 as well
as a wide range of synthetic derivatives exhibiting antimalarial
For our initial experiments, we investigated the optimization of
the reaction conditions for the synthesis of indoloquinoline 2a
Figure 1. Natural products containing an α-carboline/indoloquinoline
framework.
Received: May 2, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol5012604 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
decreased loading of the catalyst (1 mol %) (entries 10−12). To
the best of our knowledge, this reaction represents the first
example of the use of allylsilane as a nucleophilic trapping agent
for the capture of a gold-carbenoid species.13
Scheme 2. Preparation of (Azido)ynamide 1a
With the optimal reaction conditions in hand (Table 1, entry
12), we proceeded to evaluate the substituent effect of the
(azido)ynamide 1 (Table 2). Ynamide 1b bearing an electron-
Table 2. Evaluation of the Substituent Effect
Table 1. Optimization of Reaction Conditions
a
entry
substrate (E/Z)
R1
R2
product (%)
1
2
3
4
5
6
1b (Z)
1c (Z)
1d (Z)
1e (Z)
1f (Z)
OMe
CF3
H
H
H
2b (91)
2c (60)
2d (97)
Me
yield
a
b
H
CO2Me
Cl
2e (90)
entry
catalyst
solvent
conditions
(%)
b
H
2f (86)
1
IPrAuCl/AgNTf2
PPh3AuCl/AgNTf2
JohnPhosAuCl/AgNTf2
BrettPhosAuCl/AgNTf2
XPhosAuCl/AgNTf2
L1AuCl/AgNTf2
L2AuCl/AgNTf2
PtCl4
ClCH2CH2Cl 50 °C, 15 h
ClCH2CH2Cl rt, 10 min
ClCH2CH2Cl 50 °C, 1.5 h
ClCH2CH2Cl 50 °C, 10 h
ClCH2CH2Cl 50 °C, 10 h
ClCH2CH2Cl 50 °C, 4 h
ClCH2CH2Cl rt, 10 min
ClCH2CH2Cl 40 °C, 12 h
ClCH2CH2Cl 70 °C, 2 h
ClCH2CH2Cl rt, 10 min
11
64
74
43
47
37
78
63
trace
90
94
94
1g (E)
b
H
H
2a (95)
2
a
Isolated yield. 2 mol % of the catalyst was used.
3
4
donating methoxy group as the R1 substituent reacted smoothly
under the optimized conditions to give the desired product in
91% yield (entry 1). In contrast, ynamide 1c bearing an electron-
withdrawing trifluoromethyl group at the same position reacted
much less efficiently and gave the corresponding product in only
60% yield (entry 2). Electron-donating and -withdrawing groups
(i.e., methyl, and methoxycarbonyl and chloro groups,
respectively) positioned para (R2) to the alkyne were well
tolerated (entries 3−5), although the electron-deficient
ynamides 1e and 1f required an increase in the loading of the
catalyst to 2 mol % (entries 4 and 5). The application of the
optimized reaction conditions to (E)-allylsilane 1g resulted in the
formation of 2a in 95% yield (entry 6), which was the same
product as that obtained from the corresponding (Z)-isomer 1a
(Table 1). These reaction conditions were also successfully
applied to the allylamine-type ynamide 3, where they gave
exomethylene derivative 4 in 66% yield (Scheme 3).14
5
6
7
8
9
PtCl2
b
10
11
12
[L2AuCl/AgOTf]
b
[L2AuCl/AgOTf]
CH3NO2
CH3NO2
rt, 10 min
rt, 30 min
bc
,
[L2AuCl/AgOTf]
a
b
Isolated yield. The catalyst was used following filtration through a
c
pad of Celite. 1 mol % of the catalyst was used.
Scheme 3. Reaction of Allylamine-Type Ynamides 3
using ynamide 1a as a model substrate (Table 1). The reaction of
1a with 5 mol % of IPrAuCl/AgNTf2 in 1,2-dichloroethane at 50
°C gave the desired product 2a, albeit in a low yield of 11% (entry
1). The use of PPh3AuCl/AgNTf2 as the catalyst led to a
significant improvement in the yield, with 2a being isolated in
64% yield within 10 min at rt (entry 2). Several other phosphine
ligands were also screened in the reaction (entries 3−7) with the
use of the triarylphosphine L2 providing 2a in 78% yield after 10
min at rt (entry 7). PtCl4 was also found to catalyze the reaction,
whereas PtCl2 showed barely any activity (entries 8 and 9).
Following an extensive period of screening, we found that a
combination of L2AuCl/AgOTf in nitromethane provided the
highest level of activity with 2a being formed in 94% yield with a
We then moved on to investigate the reaction of (azido)-
ynamides with simple alkenes with the aim of synthesizing
cyclopropane-fused indoloquinolines (Table 3). As expected,
ynamides 5a−d, which had a phenyl or n-butyl group as one of
their alkene substituents, provided the corresponding cyclo-
propane compounds 6a−d in good to excellent yields (entries
1−4).15 PPh3AuCl/AgNTf2 was used as the catalyst for the
reaction of ynamide 5b because the reaction did not proceed to
completion with L2AuCl/AgOTf (entry 2). These reactions
were found to be stereospecific and provided different products
depending on the geometry of the alkene (compare entry 1 vs 3
or entry 2 vs 4). In contrast, the reaction of ynamide 5e bearing a
B
dx.doi.org/10.1021/ol5012604 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 3. Reaction Scope for the Synthesis of Cyclopropanes
Scheme 5. Construction of Medium-Sized Rings through
Arylation
b
entry
substrate
R1
Ph
R2
product (%)
1
2
3
4
5
5a
5b
5c
5d
5e
H
6a (91)
c
n-Bu
H
H
6b (60)
Ph
n-Bu
H
6c (97)
6d (90)
H
d
H
6e
a
Reactions were conducted with 5 mol % of [L2AuCl/AgOTf] in 1,2-
b
c
dichloroethane. Isolated yield. 5 mol % of PPh3AuCl/AgNTf2 was
used as the catalyst. Produced as an inseparable mixture.
d
Scheme 6. Plausible Mechanisms for the Reactions of
(Azido)ynamides
terminal alkene moiety afforded cyclopropane 6e as an
inseparable mixture of products, presumably because of its
instability (entry 5).
We also investigated the ring-opening reactions of cyclo-
propane 6c to evaluate the synthetic utility of these cyclo-
propane-fused indoloquinolines 6 (Scheme 4). The use of
Scheme 4. Ring-Opening Reaction of Cyclopropane
ethanol, NaN3, and TMSCl as nucleophiles for the ring-opening
reaction of 6c led to the efficient diastereoselective cleavage of
the cyclopropane ring to give the corresponding indoloquinoline
derivatives 7a−c in excellent yields.15 These results demon-
strated that oxygen, nitrogen, and halogen functional groups
could be introduced stereoselectively to the α-position of the side
chain.
To complete this research, we examined the reaction of
(azido)ynamides 8 bearing an aryl group as the trapping
functional group (Scheme 5). As expected, the (azido)ynamides
8 underwent a cascade cyclization reaction via the arylation of the
corresponding gold carbenoids with a benzene or indole ring to
give the azocine- or azepine-fused pentacyclic indoles 9a and 9b,
respectively, in moderate yields.
Plausible reaction mechanisms for the formation of 2a, 6, and 9
by the intramolecular reactions of (azido)ynamides are shown in
Scheme 6. Coordination of the gold catalyst to the alkyne as
shown in 10 would induce the nucleophilic attack of the azide
nitrogen to form 11. The gold-carbenoids 12a−c would then be
formed by the release of molecular nitrogen. Ynamide 1a bearing
an allylsilane moiety would undergo an electrophilic cyclization
reaction through the gold carbenoid 12a to give a carbocation,
and subsequent elimination of the TMS group followed by
deauration with concomitant aromatization would give indolo-
quinoline 2a. The carbenoid 12b without a silyl group would
undergo cyclopropanation of the corresponding gold carbenoid
to give 6.16 Compound 2a could also be formed from the
cyclopropane intermediate 6 (R1 = H, R2 = CH2TMS) by the
desilylative cleavage of the cyclopropane ring. In the case of
ynamides bearing an aryl group, arylation of the carbenoid
moiety would take place from 12c to give 9 via a nucleophilic
addition or C−H insertion pathway.17
In summary, we have developed a novel method for the
synthesis of indoloquinoline compounds via the gold-catalyzed
cascade cyclization of (azido)ynamides. We have shown that a
variety of trapping functional groups can be used for this reaction,
including allylsilane, simple alkenes, and arenes. This reaction
provides a new approach for the synthesis of biologically
interesting α-carboline and indoloquinoline-type compounds.
Further studies focused on the application of this reaction to the
total synthesis of indoloquinoline alkaloids are currently
underway in our laboratory.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and characterization data for all new
compounds. This material is available freely via the Internet at
http://pubs.acs.org.
C
dx.doi.org/10.1021/ol5012604 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(8) (a) Watanabe, T.; Oishi, S.; Fujii, N.; Ohno, H. Org. Lett. 2007, 9,
4821. (b) Hirano, K.; Inaba, Y.; Watanabe, T.; Oishi, S.; Fujii, N.; Ohno,
H. Adv. Synth. Catal. 2010, 352, 368. (c) Hirano, K.; Inaba, Y.;
Takahashi, N.; Shimano, M.; Oishi, S.; Fujii, N.; Ohno, H. J. Org. Chem.
2011, 76, 1212. (d) Hirano, K.; Inaba, Y.; Takasu, K.; Oishi, S.;
Takemoto, Y.; Fujii, N.; Ohno, H. J. Org. Chem. 2011, 76, 9068.
(e) Suzuki, Y.; Naoe, S.; Oishi, S.; Fujii, N.; Ohno, H. Org. Lett. 2012, 14,
326. (f) Naoe, S.; Suzuki, Y.; Hirano, K.; Inaba, Y.; Oishi, S.; Fujii, N.;
Ohno, H. J. Org. Chem. 2012, 77, 4907. (g) Chiba, H.; Oishi, S.; Fujii, N.;
Ohno, H. Angew. Chem., Int. Ed. 2012, 51, 9169. (h) Chiba, H.; Sakai, Y.;
Oishi, S.; Fujii, N.; Ohno, H. Tetrahedron Lett. 2012, 53, 6273. (i) Chiba,
H.; Sakai, Y.; Ohara, A.; Oishi, S.; Fujii, N.; Ohno, H. Chem.Eur. J.
2013, 19, 8875.
(9) For isolation of communesins, see: (a) Jadulco, R.; Edrada, R. A.;
Ebel, R.; Berg, A.; Schaumann, K.; Wray, V.; Steube, K.; Proksch, P. J.
Nat. Prod. 2004, 67, 78. For isolation of perophoramidine, see:
(b) Verbitski, S. M.; Mayne, C. L.; Davis, A. R.; Concepcion, G. P.;
Ireland, C. M. J. Org. Chem. 2002, 67, 7124. For isolation of oxaline, see:
(c) Naqel, D. W.; Pachler, K. G. R.; Steyn, P. S.; Wessels, P. L.; Gafner,
G.; Kruger, G. J. J. Chem. Soc., Chem. Commun. 1974, 1021.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: nfujii@pharm.kyoto-u.ac.jp.
*E-mail: hohno@pharm.kyoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas for “Integrated Organic Synthesis
based on Reaction Integration: Development of New Methods
and Creation of New Substances” (No. 2105) and Platform for
Drug Design, Discovery, and Development from the MEXT,
Japan. Y.T. is grateful for Research Fellowships from the Japan
Society for the Promotion of Science (JSPS) for Young
Scientists.
(10) (a) Cimanga, K.; Bruyne, T. D.; Pieters, L.; Vlietinck, A. J. J. Nat.
Prod. 1997, 60, 688. (b) Sayed, I. E.; Veken, P. V.; Steert, K.; Dhooghe,
REFERENCES
■
̀
L.; Hostyn, S.; Baelen, G. V.; Lemiere, G.; Maes, B. U. W.; Cos, P.; Maes,
(1) For selected reviews, see: (a) Hashmi, A. S. K. Chem. Rev. 2007,
107, 3180. (b) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239.
(c) Arcadi, A. Chem. Rev. 2008, 108, 3266. (d) Patil, N. T.; Yamamoto,
Y. Chem. Rev. 2008, 108, 3395. (e) Widenhoefer, R. A. Chem.Eur. J.
L.; Joossens, J.; Haemers, A.; Pieters, L.; Augustyns, K. J. Med. Chem.
́
2009, 52, 2979. (c) Wang, L.; Switalska, M.; Mei, Z.-W.; Lu, W.-J.;
Takahara, Y.; Feng, X.-W.; El-Sayed, I. E.-T.; Wietrzyk, J.; Inokuchi, T.
Bioorg. Med. Chem. 2012, 20, 4820. (d) Mei, Z.-W.; Wang, L.; Lu, W.-J.;
Pang, C.-Q.; Maeda, T.; Peng, W.; Kaiser, M.; Sayed, I. E.; Inokuchi, T. J.
Med. Chem. 2013, 56, 1431.
(11) For pioneering works, see: (a) Brand, J. P.; Charpentier, J.; Waser,
J. Angew. Chem., Int. Ed. 2009, 48, 9346. (b) Brand, J. P.; Waser, J. Angew.
Chem., Int. Ed. 2010, 49, 7304. (c) Brand, J. P.; Chevalley, C.; Scopelliti,
R.; Waser, J. Chem.Eur. J. 2012, 18, 5655. A nitrobenzene-type
alkynylating reagent was reported by our group; see: (d) Ohta, Y.;
Tokimizu, Y.; Oishi, S.; Fujii, N.; Ohno, H. Org. Lett. 2010, 12, 3963.
(12) Zhang, X.; Zhang, Y.; Huang, J.; Hsung, R. P.; Kurtz, K. C. M.;
Oppenheimer, J.; Petersen, M. E.; Sagamanova, I. K.; Shen, L.; Tracey,
M. R. J. Org. Chem. 2006, 71, 4170.
(13) For a related gold-catalyzed reaction of propargylsilane-derived
propargyl esters (1,3-acyl migration), see: Wang, A.; Zhang, L. Org. Lett.
2006, 8, 4585.
(14) When CH3NO2 was used as a solvent, 4 was given in 36% yield.
(15) The unambiguous structure assignments for 6c and 7b were
confirmed by X-ray analysis (see Supporting Information).
(16) For representative examples of cyclopropanation of the gold
2008, 14, 5382. (f) Furstner, A. Chem. Soc. Rev. 2009, 38, 3208.
̈
(g) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49, 5232. (h) Bandini,
M. Chem. Soc. Rev. 2011, 40, 1358. (i) Corma, A.; Leyva-Perez, A.;
́
Sabater, M. J. Chem. Rev. 2011, 111, 1657. (j) Krause, N.; Winter, C.
Chem. Rev. 2011, 111, 1994. (k) Abbiati, G.; Marinelli, F.; Rossi, E.;
Arcadi, A. Isr. J. Chem. 2013, 53, 856. (l) Ohno, H. Isr. J. Chem. 2013, 53,
869.
(2) For selected reviews, see: (a) Hashmi, A. S. K.; Rudolph, M. Chem.
Soc. Rev. 2008, 37, 1766. (b) Rudolph, M.; Hashmi, A. S. K. Chem. Soc.
Rev. 2012, 41, 2448.
(3) For selected reviews, see: (a) Nieto-Oberhuber, C.; Lop
Jimen
ez-Nunez, E.; Echavarren, A. M. Chem.Eur. J. 2006, 12, 5916.
(b) Jimen
́
ez, S.;
́
́
̃
́
ez-Nunez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326.
́
̃
(c) dos Santos Comprido, L. N.; Hashmi, A. S. K. Isr. J. Chem. 2013, 53,
883. (d) Obradors, C.; Echavarren, A. M. Chem. Commun. 2014, 50, 16.
(4) (a) Ye, L.; Wang, Y.; Aue, D. H.; Zhang, L. J. Am. Chem. Soc. 2012,
134, 31. (b) Hashmi, A. S. K.; Braun, I.; Nosel, P.; Schadlich, J.; Wieteck,
̈
̈
M.; Rudolph, M.; Rominger, F. Angew. Chem., Int. Ed. 2012, 51, 4456.
(c) Hansmann, M. M.; Rudolph, M.; Rominger, F.; Hashmi, A. S. K.
Angew. Chem., Int. Ed. 2013, 52, 2593. (d) Wang, Y.; Yepremyan, A.;
Ghorai, S.; Todd, R.; Aue, D. H.; Zhang, L. Angew. Chem., Int. Ed. 2013,
carbenoid, see: (a) Lop
Nieto-Oberhuber, C.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45,
6029. (b) Gorin, D. J.; Dube, P.; Toste, F. D. J. Am. Chem. Soc. 2006, 128,
14480. (c) Jimenez-Nunez, E.; Raducan, M.; Lauterbach, T.; Molawi, K.;
́ ́ ́ ́
ez, S.; Herrero-Gomez, E.; Perez-Galan, P.;
́
52, 7795. (e) Nosel, P.; Lauterbach, T.; Rudolph, M.; Rominger, F.;
̈
́
́
̃
Hashmi, A. S. K. Chem.Eur. J. 2013, 19, 8634. (f) Nosel, P.; dos Santos
̈
Solorio, C. R.; Echavarren, A. M. Angew. Chem., Int. Ed. 2009, 48, 6152.
(d) Qiana, D.; Zhang, J. Chem. Commun. 2011, 47, 11152. (e) Iqbal, N.;
Sperger, C. A.; Fiksdahl, A. Eur. J. Org. Chem. 2013, 907. (f) Oh, C. H.;
Piao, L.; Kim, J. H. Synthesis 2013, 174.
Comprido, L. N.; Lauterbach, T.; Rudolph, M.; Rominger, F.; Hashmi,
A. S. K. J. Am. Chem. Soc. 2013, 135, 15662.
(5) For a review, see: (a) Xiao, J.; Li, X. Angew. Chem., Int. Ed. 2011, 50,
7226. For pioneering works, see: (b) Ye, L.; Cui, L.; Zhang, G.; Zhang,
L. J. Am. Chem. Soc. 2010, 132, 3258. (c) Ye, L.; He, W.; Zhang, L. J. Am.
Chem. Soc. 2010, 132, 8550. For selected publications, see: (d) Davies,
P. W.; Cremonesi, A.; Martin, N. Chem. Commun. 2011, 47, 379.
(e) Mukherjee, A.; Dateer, R. B.; Chaudhuri, R.; Bhunia, S.; Karad, S. N.;
Liu, R.-S. J. Am. Chem. Soc. 2011, 133, 15372. (f) Bhunia, S.; Ghorpade,
S.; Huple, D. B.; Liu, R.-S. Angew. Chem., Int. Ed. 2012, 51, 2939.
(6) For reviews, see: (a) Marion, N.; Nolan, S. P. Angew. Chem., Int. Ed.
(17) For representative examples of arylation of the gold carbenoid,
see: (a) Hashmi, A. S. K.; Braun, I.; Rudolph, M.; Rominger, F.
Organometallics 2012, 31, 644. (b) Hashmi, A. S. K.; Wieteck, M.; Braun,
I.; Nosel, P.; Jongbloed, L.; Rudolph, M.; Rominger, F. Adv. Synth. Catal.
̈
2012, 354, 555. (c) Wang, Y.; Ji, K.; Lan, S.; Zhang, L. Angew. Chem., Int.
Ed. 2012, 51, 1915. (d) Yang, L.-Q.; Wang, K.-B.; Li, C.-Y. Eur. J. Org.
Chem. 2013, 2775.
́
2007, 46, 2750. For a mechanistic study, see: (b) Garayalde, D.; Gomez-
Bengoa, E.; Huang, X.; Goeke, A.; Necado, C. J. Am. Chem. Soc. 2010,
132, 4720.
(7) (a) Gorin, D. J.; Davis, N. R.; Toste, F. D. J. Am. Chem. Soc. 2005,
127, 11260. (b) Wetzel, A.; Gagosz, F. Angew. Chem., Int. Ed. 2011, 50,
7354. (c) Lu, B.; Luo, Y.; Liu, L.; Ye, L.; Wang, Y.; Zhang, L. Angew.
Chem., Int. Ed. 2011, 50, 8358. (d) Yan, Z.-Y.; Xiao, Y.; Zhang, L. Angew.
Chem., Int. Ed. 2012, 51, 8624. (e) Gronnier, C.; Boissonnat, G.; Gagosz,
F. Org. Lett. 2013, 15, 4234.
D
dx.doi.org/10.1021/ol5012604 | Org. Lett. XXXX, XXX, XXX−XXX