Ruthenium(II)-Catalyzed Hydroarylation of Maleimides Using Carboxylic Acids as a Traceless Directing Group

Article abstract of DOI:10.1021/acs.orglett.7b01964

An efficient Ru(II)-catalyzed hydroarylation of maleimides with ready-stock aryl carboxylic acids has been developed based on weak carboxylate-directed ortho-C-H alkylation and concomitant decarboxylation processes, fabricating 3-aryl succinimides, a recurrent scaffold in drug molecules, in high yields (up to 97%). The protocol features operational simplicity, avoids the need for precious metal additives/oxidants, and offers broad substrate scope with formal meta- and para-selectivities. It represents the first example of Ru(II)-catalyzed direct arylation of maleimides with unbiased benzoic acids.

Full text of DOI:10.1021/acs.orglett.7b01964

Letter
pubs.acs.org/OrgLett
Ruthenium(II)-Catalyzed Hydroarylation of Maleimides Using
Carboxylic Acids as a Traceless Directing Group
Anup Mandal, Harekrishna Sahoo, Suman Dana, and Mahiuddin Baidya*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India
S
* Supporting Information
ABSTRACT: An efficient Ru(II)-catalyzed hydroarylation of
maleimides with ready-stock aryl carboxylic acids has been developed
based on weak carboxylate-directed ortho-C−H alkylation and
concomitant decarboxylation processes, fabricating 3-aryl succini-
mides, a recurrent scaffold in drug molecules, in high yields (up to
97%). The protocol features operational simplicity, avoids the need
for precious metal additives/oxidants, and offers broad substrate
scope with formal meta- and para-selectivities. It represents the first
example of Ru(II)-catalyzed direct arylation of maleimides with unbiased benzoic acids.
Scheme 1. Transition-Metal-Catalyzed Hydroarylation of
Maleimides
Cyclic imides, particularly succinimides embracing 3-aryl
substitution, are pivotal structural motifs frequently encoun-
tered in natural products and pharmaceuticals with diverse
biological activities (Figure 1).1 They have potential
applications in material science2 and also serve as useful
synthetic intermediates for the construction of important
heterocyclic frameworks such as pyrrolidines and γ-lactams.3
Consequently, synthetic endeavors toward this scaffold are in
high demand.
A straightforward route to access 3-aryl succinimides would
be the catalytic hydroarylation of maleimides through direct
cleavage of unactivated C−H bond of arenes (Scheme 1). In
this context, Prabhu et al. reported conjugate addition of aryl
ketones and amides to malemides under ruthenium catalysis.4
However, a stoichiometric amount of copper salt was essential
to promote the reaction, limiting its practicability. The Kim
group has used precious rhodium catalysts for such trans-
formations under acidic conditions.5 Recently, Ackermann and
Li et al. disclosed cobalt(III) catalysis for C−H alkylation of
arenes with maleimides, where substrates were modified
through the installation of a strongly coordinating pyrimidine
unit as a directing group.6 Apart from these selective
accomplishments, hydroarylation of maleimides through a C−
H bond activation strategy that maximizes step and atom
economy remains unsatisfied and urges immediate attention.7
In recent years, aromatic carboxylic acids have gained
considerable momentum in the arena of transition-metal-
catalyzed directed C−H bond activation/functionalization
processes.8 They are cheap and widely available in great
structural diversity, shelf-stable, and nontoxic, and the acid
group is easily modifiable to a variety of functional moieties.
Importantly, the carboxyl functionality could also be tracelessly
removed by protodecarboxylation and offers formal meta- or
para-functionalization of arenes depending on the existing
substitution pattern.9 However, progress of the traceless
directing group strategies using carboxylate substrates has
primarily been confined under Pd,10 Rh,11 and Ir12 catalysis
pioneered by Miura, Gooßen, Larossa, Yu, and others.
Application of Ru catalysis has been realized very recently
Figure 1. Examples of bioactive molecules featuring succinimide and
its derivatives.
Received: June 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01964
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of Hydroarylation Reaction
Scheme 2. Scope of Hydroarylation of Maleimides with
a
Respect to Benzoic Acids
b
entry
deviation from standard conditions
yield [%]
1
2
without Cy3PO
73
95
0
none
3
without Ru(II) catalyst
without NaHCO3
4
0
c
5
with (p-cymene)Ru(PPh3)Cl2 catalyst
Na2CO3 instead of NaHCO3
K2CO3 instead of NaHCO3
K3PO4 instead of NaHCO3
Li2CO3 instead of NaHCO3
KHCO3 instead of NaHCO3
NH4HCO3 instead of NaHCO3
toluene instead of DCE
1,4-dioxane instead of DCE
chlorobenzene
-
6
42
49
63
7
8
c
9
trace
10
11
12
13
14
15
16
17
18
19
20
46
c
trace
c
trace
12
9
c
DMF instead of DCE
DME instead of DCE
MeOH instead of DCE
at 120 °C
at 80 °C
Ph3P instead of Cy3PO
-
c
-
c
-
55
64
52
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), [Ru(p-
cymene)Cl2]2 (5 mol %), Cy3PO (10 mol %), base (1 equiv), solvent
b
c
(2 mL), 12 h. Isolated yields. TLC showed the presence of both
starting materials 1a and 2a.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), [Ru(p-cymene)-
Cl2]2 (5 mol %), Cy3PO (10 mol %), NaHCO3 (1 equiv), DCE (2
mL). Yields of isolated products are given.
when Hartwig and Zhao, Ackermann, and Gooßen independ-
ently disclosed hydroarylation of alkyne via a C−H
alkenylation−decarboxylation sequence and demonstrated
that, in contrast to other transition-metal catalysts, ruthenium
catalyst could facilitate the decarboxylation process under mild
conditions without the aid of copper or silver metal.13
Encouraged by these works and our recent interest in the
ruthenium catalysis,14a,b we posited that reactions of aromatic
carboxylic acids with maleimides under Ru(II) catalysis could
be an effective transformative alternative for the hydroarylation
of maleimides (Scheme 1b).15 Herein, we report the develop-
ment of this approach and present the first example of Ru(II)-
catalyzed, precious metallic additive free decarboxylative
hydroarylation of maleimides en route to synthesis of 3-aryl
succinimides under mild conditions. Notably, the reaction is
highly regiospecific and overrides competitive formal [4 + 2]
heterocyclization, Heck type addition, and ipso-substitution
processes (Scheme 1b).
Our investigation commenced by studying the model
reaction of p-toluic acid 1a with maleimide 2a based on
carboxylate-directed ortho C−H alkylation and a concomitant
decarboxylation process (Table 1, a detailed summary is
provided in the Supporting Information). The bench-stable
[Ru(p-cymene)Cl2]2 was considered as a preferred catalyst. To
our delight, when a mixture of 1a and 2a was exposed to
ruthenium(II) catalyst (5 mol %) in the presence of NaHCO3
base in DCE solvent at 100 °C, the hydroarylation reaction
proceeded efficiently, delivering aryl succinimide 3a in 73%
isolated yield with desired meta-selectivity (Table 1, entry 1).
Previously, we have found an improved reactivity of ruthenium-
(II) catalyst in the presence of phosphine oxide.14a,c Thus,
Cy3PO (10 mol %) was introduced into the reaction
conditions, and a remarkable enhancement in yield was
observed, rendering the desired product in 95% isolated yield
(entry 2). The reaction was unfruitful in the absence of either
[Ru(p-cymene)Cl2]2 or NaHCO3, signifying indispensable
roles of catalyst and base (entries 3 and 4). The reaction
completely shut down when [Ru(p-cymene)Cl2]2 catalyst was
replaced with (p-cymene)Ru(PPh3)Cl2 (entry 5). Screening of
other bases gave inferior results (entries 6−11). Except DCE,
other tested solvents did not affect the hydroarylation process
(entries 12−17). The output was also strongly governed by the
reaction temperature; the yield reduced significantly upon
lowering (80 °C) as well as increasing (120 °C) the reaction
temperature (entries 18 and 19). Examination of a phosphine
ligand such as Ph3P had a deleterious effect (entry 20).
Under the optimized reaction conditions, the scope of this
hydroarylation process was explored (Scheme 2). The reaction
is quite general. Parent benzoic acid and a broad range of
aromatic carboxylic acids bearing electron-donating as well as
electron-withdrawing groups at ortho-, meta-, and para-
positions reacted smoothly with maleimide 2a to afford 3-aryl
succinimides 3a−h in high yields (Scheme 2, 64−95% yields).
Di- and trisubstituted hindered benzoic acids also effectively
participated in this reaction delivering desired products 3i−l in
excellent yields. In general, the reaction proceeded faster with
B
DOI: 10.1021/acs.orglett.7b01964
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Scope of the Hydroarylation Process with Respect
to Maleimides
Scheme 5. Control Experiments
a
3m. Furthermore, heteroaryl carboxylic acids such as
thiophenecarboxylic acids (1n−o) and benzothiophene-2-
carboxylic acid (1p) were also suitable for this process and
offered 3-heteroarylated succinimides 3n−p in good yields.
The scope of this reaction with respect to maleimides was
also investigated (Scheme 3). The reactions of maleimides
having N-phenyl (4a,b), N-alkyl (4c,d), and sensitive N-allyl
(4e,f) functionalities readily furnished desired products in high
yields with envisioned meta-selectivity. Evaluation of electronic
effects on the aryl ring of N-aryl maleimides turned out
marginal; both electron-donating substituents such as methoxy
(4g,h) and methyl (4s−w) and electron-withdrawing groups,
for instance fluoro (4l−n) and trifluoromethyl (4r), effortlessly
led to the desired products. Compound 4s was crystallized, and
the X-ray analysis unambiguously established the observed
regioselectivity. The protocol tolerates different halogen
substituents (4i−q), which are useful synthetic handles for
further functionalization to complex molecules.
The synthetic utility of this process was further highlighted
through the oxidation of succinimide 3l to produce 3-arylated
maleimide 5 in 62% yield (Scheme 4). It is worth noting that 3-
aryl maleimides are an emerging scaffold for drug discovery and
diagnosis.16 Synthesis of pyrrole heterocycle 6 was also
accomplished by treating 3l with LiAlH4 in THF at room
temperature (Scheme 4).
To gain insight into the reaction mechanism, various
controlled experiments were performed (Scheme 5). The
presence of radical scavengers such as TEMPO, BHT, and 1,1-
diphenylethylene did not significantly affect the hydroarylation
process, and thus, the involvement of radical species is very
unlikely (Scheme 5a). The kinetic isotope effect studied
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), [Ru(p-cymene)-
Cl2]2 (5 mol %), Cy3PO (10 mol %), NaHCO3 (1 equiv), DCE (2
mL). Yields of isolated products are given.
through independent parallel experiment revealed kH/kD
=
1.51, indicating that the C−H bond breaking may be involved
in the rate-determining step (Scheme 5b). In-depth mechanistic
investigations are underway to fully elucidate the reaction
pathway.
Scheme 4. Synthesis of Maleimide and Pyrrole Heterocycle
In summary, we have disclosed direct hydroarylation of
maleimides through regioselective ortho-alkylation of benzoic
acids using carboxylic acid as a traceless directing group. The
reaction is catalyzed by readily available Ru(II) catalyst and
insensitive to air and moisture, avoids the use of metallic
oxidants or cocatalysts, and produced biologically relevant 3-
aryl succinimide in high yields under mild conditions. Notably,
this simple protocol, while circumventing other putative
organometallic processes, offers formal meta-functionalized
products from ortho- and para-substituted benzoic acids and
para-functionalized adducts from meta-substituted substrates.
We anticipate that this reaction will find application in the
synthesis of important pharmaceuticals. Furthermore, the
electron-rich aromatic carboxylic acids. The regioselectivity
pattern of this process is alluring. The reactions with ortho- (3a,
3c−e) and para-substituted benzoic acids (3a,c,d, and 3h)
provided meta-functionalized products. In the case of meta-
substituted benzoic acids, the hydroarylation occurred at the
less hindered site to provide para-functionalized products
exclusively (3f,g). Satisfyingly, both 1- and 2-naphthoic acid,
respectively, furnished 86% and 82% yields of the succinimide
C
DOI: 10.1021/acs.orglett.7b01964
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Organic Letters
Letter
4034. (e) Sharma, S.; Han, S. H.; Oh, Y.; Mishra, N. K.; Lee, S. H.; Oh,
J. S.; Kim, I. S. Org. Lett. 2016, 18, 2568. (f) Morita, T.; Akita, M.;
Satoh, T.; Kakiuchi, F.; Miura, M. Org. Lett. 2016, 18, 4598. (g) Also
see, ref 3c.
effectiveness of this strategy for various C−H bond
functionalizations is currently being explored in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
(8) For reviews, see: (a) De Sarkar, S.; Liu, W.; Kozhushkov, S. I.;
Ackermann, L. Adv. Synth. Catal. 2014, 356, 1461. (b) Drapeau, M. P.;
Gooßen, L. J. Chem. - Eur. J. 2016, 22, 18654. (c) Wei, Y.; Hu, P.;
Zhang, M.; Su, W. Chem. Rev. 2017, 117, 8864. (d) Arockiam, P. B.;
Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012, 112, 5879. (e) He, J.;
Wasa, M.; Chan, K. S. L.; Shao, Q.; Yu, J.-Q. Chem. Rev. 2017, 117,
8754. (f) Manikandan, R.; Jeganmohan, M. Org. Biomol. Chem. 2015,
13, 10420. For recent reports, see: (g) Engelman, K. L.; Feng, Y.;
Ison, E. A. Organometallics 2011, 30, 4572. (h) Giri, R.; Maugel, N.; Li,
J.; Wang, D.; Breazzano, S. P.; Saunders, L. B.; Yu, J. J. Am. Chem. Soc.
2007, 129, 3510. (i) Miao, J.; Ge, H. Org. Lett. 2013, 15, 2930.
(j) Chen, G.; Zhuang, Z.; Li, G.; Saint-denis, T. G.; Hsiao, Y.; Joe, C.
L.; Yu, J.-Q. Angew. Chem., Int. Ed. 2017, 56, 1506. (k) Shi, X.; Li, C.-J.
Adv. Synth. Catal. 2012, 354, 2933. (l) Ng, F.; Zhou, Z.; Yu, W. Chem. -
Eur. J. 2014, 20, 4474. (m) Chen, H.; Lin, C.; Xiong, C.; Zhang, Y.
Org. Chem. Front. 2017, 4, 455. (n) Yang, C.; He, X.; Zhang, L.; Han,
G.; Zuo, Y.; Shang, Y. J. Org. Chem. 2017, 82, 2081. (o) Zhu, Y.; Li, J.;
Han, T.; He, J.; Zhu, K. Eur. J. Org. Chem. 2017, 4, 806. (p) Biafora, A.;
Krause, T.; Hackenberger, D.; Belitz, F.; Gooßen, L. J. Angew. Chem.,
Int. Ed. 2016, 55, 14752. (q) Huang, L.; Weix, D. J. Org. Lett. 2016, 18,
5432. (r) Mei, R.; Zhu, C.; Ackermann, L. Chem. Commun. 2016, 52,
13171. (s) Simonetti, M.; Cannas, D. M.; Panigrahi, A.; Kujawa, S.;
Kryjewski, M.; Xie, P.; Larrosa, I. Chem. - Eur. J. 2017, 23, 549. (t) Mei,
R.; Zhang, S.; Ackermann, L. Org. Lett. 2017, 19, 3171.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01964.
Complete experimental details and characterization data
for the prepared compounds (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mbaidya@iitm.ac.in.
ORCID
Mahiuddin Baidya: 0000-0001-9415-7137
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge CSIR New Delhi for the financial
support (02(0212)/14/EMR-II). A.M. acknowledges UGC for
JRF, and H.S. and S.D. acknowledge IIT-Madras for HTRA.
We also thank the Department of Chemistry, IIT-Madras for
instrumental facilities.
(9) (a) Bhadra, S.; Dzik, W. I.; Gooßen, L. J. Angew. Chem., Int. Ed.
2013, 52, 2959. (b) Simonetti, M.; Larrosa, I. Nat. Chem. 2016, 8,
1086. (c) Font, M.; Quibell, J. M.; Perry, G. J. P.; Larrosa, I. Chem.
Commun. 2017, 53, 5584.
(10) (a) Maehara, A.; Tsurugi, H.; Satoh, T.; Miura, M. Org. Lett.
2008, 10, 1159. (b) Wang, C.; Rakshit, S.; Glorius, F. J. Am. Chem. Soc.
2010, 132, 14006. (c) Cornella, J.; Righi, M.; Larrosa, I. Angew. Chem.,
Int. Ed. 2011, 50, 9429. (d) Arroniz, C.; Ironmonger, A.; Rassias, G.;
Larrosa, I. Org. Lett. 2013, 15, 910. (e) Luo, J.; Preciado, S.; Larrosa, I.
J. Am. Chem. Soc. 2014, 136, 4109. (f) Luo, J.; Preciado, S.; Larrosa, I.
Chem. Commun. 2015, 51, 3127. (g) Tang, J.; Hackenberger, D.;
Gooßen, L. J. Angew. Chem., Int. Ed. 2016, 55, 11296.
(11) (a) Mochida, S.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett.
2010, 12, 5776. (b) Mochida, S.; Hirano, K.; Satoh, T.; Miura, M. J.
Org. Chem. 2011, 76, 3024. (c) Qin, X.; Sun, D.; You, Q.; Cheng, Y.;
Lan, J.; You, J. Org. Lett. 2015, 17, 1762. (d) Shi, X.; Liu, K.; Fan, J.;
Dong, X.; Wei, J.; Li, C. Chem. - Eur. J. 2015, 21, 1900. (e) Zhang, Y.;
Zhao, H.; Zhang, M.; Su, W. Angew. Chem., Int. Ed. 2015, 54, 3817.
(12) (a) Lee, D.; Chang, S. Chem. - Eur. J. 2015, 21, 5364. (b) Quan,
Y.; Xie, Z. J. Am. Chem. Soc. 2014, 136, 15513.
(13) (a) Zhang, J.; Shrestha, R.; Hartwig, J. F.; Zhao, P. Nat. Chem.
2016, 8, 1144. (b) Kumar, N. Y. P.; Bechtoldt, A.; Raghuvanshi, K.;
Ackermann, L. Angew. Chem., Int. Ed. 2016, 55, 6929. (c) Huang, L.;
Biafora, A.; Zhang, G.; Bragoni, V.; Gooßen, L. J. Angew. Chem., Int. Ed.
2016, 55, 6933. (d) Biafora, A.; Khan, B. A.; Bahri, J.; Hewer, J. M.;
Gooßen, L. J. Org. Lett. 2017, 19, 1232. For an amidation process, see:
(e) Shi, X.; Dong, X.; Fan, J.; Liu, K.; Wei, J.; Li, C. Sci. China: Chem.
2015, 58, 1286.
(14) (a) Mandal, A.; Dana, S.; Sahoo, H.; Grandhi, G. S.; Baidya, M.
Org. Lett. 2017, 19, 2430. (b) Dana, S.; Mandal, A.; Sahoo, H.; Baidya,
M. Org. Lett. 2017, 19, 1902. Also see: (c) Ackermann, L. Org. Lett.
2005, 7, 3123.
REFERENCES
■
(1) (a) Porter, R. J.; Penry, J. K.; Lacy, J. R.; Newmark, M. E.;
Kupferberg, H. J. Neurology 1979, 29, 1509. (b) Nakamura, N.;
Hirakawa, A.; Gao, J.; Kakuda, H.; Shiro, M.; Komatsu, Y.; Sheu, C.;
Hattori, M. J. Nat. Prod. 2004, 67, 46. (c) Geethangili, M.; Tzeng, Y.
eCAM. 2011, 2011, 212641. (d) Garad, D. N.; Tanpure, S. D.; Mhaske,
S. B. Beilstein J. Org. Chem. 2015, 11, 1008. (e) Han, Z.; Li, P.; Zhang,
Z.; Chen, C.; Wang, Q.; Dong, X.; Zhang, X. ACS Catal. 2016, 6, 6214.
(f) Kavitha, K.; Praveena, K. S. S.; Ramarao, E. V. V. S.; Murthy, N. Y.
S.; Pal, S. Curr. Org. Chem. 2016, 20, 1955. (g) Seiler, M. P.; Nozulak,
J. WO EP861 20010126, 2001.
(2) (a) Yan, J.; Zheng, B.; Pan, D.; Yang, R.; Xu, Y.; Wang, L.; Yang,
M. Polym. Chem. 2015, 6, 6133. (b) Yang, C. P.; Su, Y-Y.; Hsu, M. H.
Polym. J. 2006, 38, 132. (c) Braunecker, W. A.; Owczarczyk, Z. R.;
Garcia, A.; Kopidakis, N.; Larsen, R. E.; Hammond, S. R.; Ginley, D.
S.; Olson, D. C. Chem. Mater. 2012, 24, 1346.
(3) (a) Hubert, J. C.; Wunberg, J. B. P. A.; Speckamp, W. N.
Tetrahedron 1975, 31, 1437. (b) Wijnberg, J.B.P.A.; Schoemaker, H.E.;
Speckamp, W. N. Tetrahedron 1978, 34, 179. (c) Shintani, R.; Duan,
W.; Nagano, T.; Okada, A.; Hayashi, T. Angew. Chem., Int. Ed. 2005,
44, 4611.
(4) (a) Bettadapur, K. R.; Lanke, V.; Prabhu, K. R. Org. Lett. 2015,
17, 4658. (b) Lanke, V.; Bettadapur, K. R.; Prabhu, K. R. Org. Lett.
2015, 17, 4662. (c) Keshri, P.; Bettadapur, K. R.; Lanke, V.; Prabhu, K.
R. J. Org. Chem. 2016, 81, 6056.
(5) (a) Han, S. H.; Kim, S.; De, U.; Mishra, N. K.; Park, J.; Sharma,
S.; Kwak, J. H.; Han, S.; Kim, H. S.; Kim, I. S. J. Org. Chem. 2016, 81,
12416. (b) Han, S. H.; Mishra, K.; Jo, H.; Oh, Y.; Jeon, M.; et al. Adv.
Synth. Catal. 2017, 359, 2396.
(6) Zhang, Z.; Han, S.; Tang, M.; Ackermann, L.; Li, J. Org. Lett.
2017, 19, 3315.
(7) Other examples on C−H functionalization with maleimides, see:
(a) Shintani, R.; Duan, W.; Hayashi, T. J. Am. Chem. Soc. 2006, 128,
5628. (b) d’Herouville, F. L. B.; Millet, A.; Scalone, M.; VerMichelet,
E. J. Org. Chem. 2011, 76, 6925. (c) Thaler, T.; Guo, L.; Steib, A. K.;
Raducan, M.; Karaghiosoff, K.; Mayer, P.; Knochel, P. Org. Lett. 2011,
13, 3182. (d) Miura, W.; Hirano, K.; Miura, M. Org. Lett. 2015, 17,
(15) For a recent report on rhodium catalysis with AcOH additive,
see: Bettadapur, K. R.; Lanke, V.; Prabhu, K. R. Chem. Commun. 2017,
53, 6251. For examples on Ru-catalyzed hydroarylation of alkynes, see
(b) Reddy, M. C.; Jeganmohan, M. Chem. Commun. 2013, 49, 481.
(c) Manikandan, R.; Jeganmohan, M. Org. Lett. 2014, 16, 912.
(16) (a) Guan, Q.; Zuo, D.; Jiang, N.; Qi, H.; Zhai, Y.; Bai, Z.; Feng,
D.; Yang, L.; Jiang, M.; Bao, K.; Li, C.; Wu, Y.; Zhang, W. Bioorg. Med.
Chem. Lett. 2015, 25, 631. (b) Stewart, S. G.; Ho, L. A.; Polomska, M.
E.; Percival, A.; Yeoh, G. C. T. ChemMedChem 2009, 4, 1657.
D
DOI: 10.1021/acs.orglett.7b01964
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