Rhodium-catalyzed annulative coupling of: N-aryl-2-aminopyridine and propargylic amine via selective C-C and C-H bond activation

Article abstract of DOI:10.1039/c9cc09777e

A Rh(iii)-catalyzed/Cu(ii)-mediated cascade reaction between N-aryl-2-aminopyridine and propargylic amine has been developed. Selective C(sp²)-H bond activation and C(sp)-C(sp³) cleavage occurred during the reaction, which was followed by a cyclization reaction to provide an unprecedented synthetic route to form 1,2-disubstituted indoles in yields up to 85%.

Full text of DOI:10.1039/c9cc09777e

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DOI: 10.1039/C9CC09777E
Journal Name
COMMUNICATION
Rhodium-catalyzed annulative coupling of N-aryl-2-aminopyridine
and propargylic amine via selective C−C and C-H bond activation
Shiyu He, Xufei Yan, Yanxi Lei, Haifeng Xiang and Xiangge Zhou*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
(a) Substitution reactions of propargylic amines with alkynes by Nakamura
R3
NR1
R
R3
NR12
R2
2
0 mol% CuCl, base
2
THF, 100 °C
R2
R
Abstract: A Rh(III)-catalyzed/Cu(II)-mediated cascade reaction
between N-aryl-2-aminopyridine and propargylic amine has been
(
b) Lu[N(SiMe3)2]3-catalyzed alkynyl exchange reaction by Zhou
R5
R
R5
2
3
R6
R4
developed. Selective C(sp )-H bond activation and C(sp)-C(sp )
cleavage occurred during reaction, which was followed by
cyclization reaction to provide an unprecedented synthetic route to
form 1,2-disubstituted indoles in yields up to 85%.
Lu[N(SiMe3)2]3 10 mol%
R6
N
N
R4
H
H
toluene, 130 °C
R
(c) Pd-catalyzed coupling reaction between two molecules of o-bromobenzylamine by Lautens
Ar
Ar
Ar
PdBrLn
Ts
NR8
R7
N
NHR8
Pd(PPh3)4 (5 mol %)
R7
R7
Arylated indoles are frequently encountered as core
structural motifs in bioactive products and pharmaceutical
K3PO4 (2 equiv), toluene
Br
120 C, 24 h
NHTs
R7
R7
Ar
1
molecules. For example, 2-arylindoles could serve as significant
(d) This work
(
Cp*RhCl2)2 5 mol%
scaffolds as marketed drugs such as cytotoxic inhibitor and liver
H
N
N
Cu(OAc)2 2.5 equiv.
N
2
R9
R9
X receptors. On the other hand, the cleavage of non-strained
N
NBn2
Na2C2O4 4 equiv.
THF, 120°C, air, 24 h
R10
R10
C-C bond has attracted increasing attentions as it can directly
reorganize the molecular skeletons and thereby synthesize new
compounds conveniently. Various strategies have been
developed to realize C-C single bonds activation in spite of the
intrinsic challenges including thermodynamic stability and
Scheme 1 Transition metal-catalyzed β-carbon elimination
of amines.
3
Compared with the C-C bond activation of alcohol via β-
carbon elimination, transition metal-catalyzed β-carbon
elimination of the amines is rarely reported probably due to the
constrained directionality of σ-orbital. For example, the
cleavage of C-C bonds involving β-carbon elimination via M-O-
C-C intermediate generated from the secondary/tertiary
4
8
4
formation of a strong M-N bond. In 2010, Nakamura and co-
workers described the first example of the transition metal-
catalyzed β-carbon elimination of amines, in which the
substitution reactions of propargylic amines with alkynes
proceeded in the presence of copper(I) catalyst (Scheme 1a).
alcohols has been well explored. Among them, the tertiary
propargylic alcohols are proved to be good precursors of
terminal alkynes with ketone extrusion through β-carbon
elimination, which can participate in dozens of cross coupling
5
reactions. For example, Wen group described the reaction
Zhou et al. reported a similar reaction with Lu[N(SiMe
3
)
2
]
3
as
between 1-(pyridin-2-yl)-1H-indoles and tert-propargyl alcohols
by using Rh/Cu catalyst system to offer a novel synthetic route
of pyrido[2,1-a]indoles, which involved three C-C bonds
9
catalyst (Scheme 1b). Lautens and co-workers realized the
generation of 5,6-dihydro-phenanthridine skeletons by Pd(0)-
catalyzed coupling reaction between two molecules of N-
sulfonyl substituted racemic o-bromobenzylamine (Scheme
6
cleavage in one pot. In our previous study, a Rh-catalyzed Cu-
mediated reaction of 4-methylphenyl-2-animopyridine with γ-
substituted tert-propargyl alcohols was also investigated,
1
0
1
c). Despite these accomplishments, applications of β-carbon
7
elimination of amines are largely underdeveloped. In
consideration of that propargylic amines are potentially
important building blocks in organic synthesis, developing
divergent methodologies to access valuable scaffolds with
general substrate availability by C-C bond activation of
propargylic amines is in great demand. On the basis of our
affording 2-arylindoles successfully.
S. He, X. Yan, Y. Lei, Prof. Dr. H. Xiang, Prof. Dr. X. Zhou*
College of Chemistry, Sichuan University, Wangjiang Road 29, Chengdu 610064,
China. Email: zhouxiangge@scu.edu.cn
†
Electronic Supplementary Information (ESI) available: Experimental procedures,
characterization data, and copies of NMR spectra. CCDC 1967129. For ESI and
crystallographic data in CIF or other electronic format See DOI:
11
previous work on C-C activation and synthesis of heterocyclic
1
0.1039/x0xx00000x.
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] or other copper
compounds,12 herein we developed an annulative coupling RhCl
reaction between propargylic amines with arylamines to form salts such as CuBr
, Pd(OAc)
, Ni(OAc)
, Cu(TFA)
, RuCl
, [Cp*Co(CO)I
3
2
2
3
2
View Article Online
DOI: 10.1039/C9CC09777E
2
2
3 2 2 2
, Cu(NO ) and CuF ·H O resulted in
2
-arylated indoles (Scheme 1d). This reaction involves the inferior results (Table 1, entries 2-10). Then, the solvent effects
selective cleavage of the carbon-carbon bond of the propargylic were studied, and THF exhibited to be better than others
amines, functionalization of carbon-hydrogen bond of including MeCN, toluene, DCM, and MeOH in yield of 56%
arylamines and cyclization procedure, which contributes the (Table 1, entries 11-15). Furthermore, it was reported that
8
first example of employing propargylic amines as coupling addition of base would be beneficial to the C-C bond cleavage.
partners to obtain indole derivatives.
Therefore, a series of bases were screened, and Na
2
C
2
O
4
was
and
found to be better than others including NaHCO , K C O
3
2 2 4
Table 1 Optimization of reaction conditionsa
Na CO , affording the desired product 3a in 68% yield (Table 1,
2
3
Catalyst
Additive, Base
entries 16-22). Moreover, inert atmosphere seemed to be
unnecessary for the reaction (Table 1, entry 23). At last, after
screening the amount of base and 2a, as well as reaction
temperature (Table 1, entries 24-29), the best yield 82% was
obtained. The control experiments further proved the necessity
of the Rh/Cu catalyst system, and no product was obtained in
the absence either of them (Table 1, entries 30-31). These
experiments concluded the optimized reaction condition
H
N
N
N
N
Solvent
20°C, air, 24 h
NBn2
1
1
a
2a
3a
b
Entry
Catalyst
Additive
Solvent
Base
Yield (%)
1
[Cp*RhCl
RhCl
Pd(OAc)
Ni(OAc)
RuCl
[Cp*Co(CO)I
2
]
2
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
2
2
2
2
2
2
dioxane
dioxane
dioxane
dioxane
Dioxane
Dioxane
dioxane
dioxane
dioxane
dioxane
THF
MeCN
toluene
DCM
MeOH
THF
THF
THF
THF
THF
THF
THF
THF
THF
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Et
48
0
0
0
0
6
0
0
0
14
56
38
trace
18
12
52
60
43
68
66
54
trace
60
53
70
82
63
77
45
0
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
2
2
consists of 1a (0.2 mmol), 2a (3 equiv), [Cp*RhCl
Cu(OAc) (2.5 equiv.) and Na (4.0 equiv) in THF (1.0 mL) at
20 °C for 24 h in the air.
2 2
] (5 mol%),
3
2
2 2 4
C O
2
]
1
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
[Cp*RhCl
-
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
]
2 2
CuBr
Cu(TFA)
Cu(NO
CuF ·H
2
2
Scheme 2 Substrate scope of N-aryl-2-aminopyridinesa,b
3
)
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
2
O
(
Cp*RhCl2)2 5 mol%
H
N
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
-
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
N
Cu(OAc)2 2.5 equiv.
N
R1
R1
N
NBn2
Na2C2O4 4 equiv
THF, 120°C, air, 24 h
1
2a
3
N
N
N
N
N
N
N
N
3
N
Ph
Ph
Ph
Ph
O
K
2
CO
NaHCO
Na
3
3a 82%
3b 85%
3c 80%
3d 80%
3
4
2
C
2
O
N
N
N
N
N
N
Cl
Cl
N
N
K
2
C
2
O
4
Ph
Ph
Ph
Ph
F3C
F
Na
2
CO
CO
3
O
3e 76%
3f 43%
3g 25%
3h 51%
Cs
2
3
c
d
e
f
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
Na
2
C
C
C
C
C
C
C
C
C
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
4
4
4
4
4
4
4
4
4
N
N
N
N
N
Cl
N
N
N
Ph
Ph
Ph
Ph
Cl
Br
Ph
THF
THF
THF
THF
THF
THF
THF
3i 78%
3j 69%
3k 65%
3l 64%
g
h
i
N
N
N
N
N
N
Ph
Ph
Ph
3m 33%
3n 68%
3o 73%
f
f
aReaction conditions: 1 (0.2 mmol), 2a (3.0 equiv), [Cp*RhCl2]2 (5 mol%),
[Cp*RhCl
]
2 2
0
Cu(OAc)
2
(2.5 equiv) and Na
2
C
2
O
4
(4.0 equiv) in THF (1.0 mL) were stirred at
aGeneral conditions: 1a (0.2 mmol), 2a (0.4 mmol), catalyst (5 mol%), additive
b
1
20 °C for 24 h in the air. Isolated yields.
(
2.5 equiv) and base (4.0 equiv) in solvent (1.0 mL) were stirred at 120 °C for 24
b
c
d
h in the air. Isolated yields. Under N
used. Na
2
atmosphere. Na
(4.0 equiv) was used. 2a (3.0 equiv) was used. Reaction
2
C
2
O
4
(3.0 equiv) was
e
f
g
Next, the substrate scope of N-aryl-2-aminopyridines was
2 2
C O
4
h
i
investigated. As shown in Scheme 2, various functional groups
substituted on the phenyl ring were well tolerated, leading to
the corresponding products in moderate to good yields.
Electron-donating substituents seemed to be more beneficial to
the reaction. For example, para methyl or methoxyl substituted
substrates give products above 80% yields (3a, 3b), while only
temperature 100 °C. Reaction temperature 110 °C. Reaction temperature
30 °C.
1
Initially, N-(p-tolyl)pyridin-2-amine (1a) and N,N-dibenzyl-
,3-diphenylprop-2-yn-1-amine (2a) were chosen as model
1
substrates for the optimization of reaction conditions. As shown
in Table 1, when [Cp*RhCl (5 mol%) was used as catalyst in
the presence of Cu(OAc) (2 equiv) in 1,4-dioxane at 120 °C for
4 h, the desired prodcut 3a could be obtained in 48% yield
Table 1, entry 1). Screening other metal catalysts including
2 2
]
4
3% yields was obtained in the case of trifluoromethyl
2
substituted substrate (3f). For the steric effect, o-methyl
substituted substrate was converted to be the corresponding
product 3m in only 33% yield, indicating an apparent steric
2
(
2
| J. Name., 2012, 00, 1-3
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(a)
hindrance in this transformation. Moreover, when bromo-
substituted arylamine was employed as substrate, the
corresponding product 3k could be obtained in 65% yield,
suggesting the possibility to derive more complex molecules.
Furthermore, a variety of substituted propargylic amines
were also screened. As shown in Scheme 3, the substituents on
the phenyl ring of propargylic amines markedly affected the
reaction outcome. Electron-donating substituents were found
again to be favorable for the reaction (3q, 3r, 3s, 3u). Steric
effects seemed to have some influence on the results. For
example, propargylic amines substituted with p-methyl group
gave 3p in 82% yield, whereas meta or ortho-substituted
substrates resulted in 77% and 69% yields, respectively (3v, 3w).
Propargylic amine bearing thienyl group (3y) was also smoothly
H
(Cp*RhCl2)2 5 mol%
Cu(OAc)2 2.5 equiv
NView Article Online
N
N
DOI: 10.1039/C9CC09777E
N
NBn2
Na2C2O4 4 equiv
THF, 120°C, air, 24 h
1
a 1.10 g 6 mmol
2a 6.97 g 18 mmol
3a 1.26g 74%
(b)
(
0
1) MeOTf, DCM
H
N
N
C, rt, 12h
N
Ph
(
2) NaOH (2 equiv)
MeOH, 60 C, 12h
Ph
3n
4 82%
(c)
N
N
Pd(OAc)2 ( 2 mol%)
N
N
PhI(OAc)2, KOAc
CH3CN,70 C
Ph
Ph
O
O
5 40%
3
a
X-ray structure of 5
(
d)
N
(COCl)2 (3.0 equiv)
Et2O, 0C to rt
N
N
Ph
N
CH3OH
Ph
O
O
O
3a
6
71%
converted to the corresponding product, however, alkyl alkyne Scheme 4 Transformations of product.
and terminal alkyne substrates (3aa) resulted in no desired
(a)
products. The catalytic system could also be applied for the
secondary amine substrates, N-alkyl substituted substrates and
allylic amine substrates, albeit with lower yields (2aa-2af).
N
H
(Cp*RhCl2)2 5 mol%
Cu(OAc)2 2.5 equiv.
N
N
2a
Na2C2O4 4 equiv.
THF, 120°C, air, 24 h
3
a N.D.
(b)
(
Cp*RhCl2)2 5 mol%
Cu(OAc)2 2.5 equiv.
Scheme 3 Substrate scope of propargylic aminesa,b
1a
3a
Na2C2O4 4 equiv.
THF, 120°C, air, 24 h
N.D.
7 89%
(
Cp*RhCl2)2 5 mol%
H
N
N
Cu(OAc)2 2.5 equiv.
N
(c)
Cu(OAc)2 1 equiv.
1a
2a
N
R2
NBn2
Na2C2O4 4 equiv.
THF, 120°C, air, 24 h
R2
Na2C2O4 4 equiv.
THF, 120°C, air, 24 h
7 53%
1a
2
3
(d)
(Cp*RhCl2)2 5 mol%
Additive Yield
TEMPO 82%
N
N
Cu(OAc)2 2.5 equiv.
N
N
N
N
1a
2a
3.0 equiv.)
Na2C2O4 4 equiv.
N
N
(
THF, 120°C, air, 24 h
additive (1 equiv.)
BHT
76%
3
a
3
p 82%
3q 80%
3r 78%
Scheme 5 Studies of the reaction pathway.
N
N
F
N
N
N
N
.14 Its structure was confirmed by
F
Cl
reaction of 3a with PhI(OAc)
2
3
s 54%
3t 40%
3u 56%
X-ray crystallographic analysis (Scheme 4c). Furthermore, the
reaction between 3a and oxalyl chloride afforded methyl 2-(5-
methyl-2-phenyl-1-(pyridin-2-yl)-1H-indol-3-yl)-2-oxoacetate (6)
in 71% yield, which was reported as the key building block of
the monoindole alkaloid from the marine sponge Spongosorites
N
N
N
N
N
N
Cl
3v 77%
3w 69%
3x 48%
1
5
N
(Scheme 4d).
N
N
N
N
S
N
R
N
To get a better understanding of the possible reaction
3
y 44%
3z 0%
3aa 0%
R=H, CH3
pathway, a series of control experiments were then carried out.
Firstly, no product was found when 4-methyl-N-phenylaniline
was used instead of 1a under the standard reaction conditions,
suggesting the necessity of pyridine directing group (Scheme
5a). Next, only Glaser coupling product 7 was obtained when
propargylic amine 2a was replaced by phenylacetylene,
suggesting the necessity of using 2 as substrates (Scheme 5b).
NnHex2
NHBn
HN
2
aa
2ab
2ae
2ac
1
a + 2aa : 60%
Ph
NBn2
1a + 2ab : 48%
N
NBn2
1a + 2ac : 48%
1
1
a + 2ad : 37%
a + 2ae : 25%
2
ad
2af
1a + 2af : 21%
aReaction conditions: 1a (0.2 mmol), 2 (3.0 equiv), [Cp*RhCl
(5 mol%),
Meanwhile, 7 was also obtained in 53% yield when Cu(OAc)2
2
]
2
Cu(OAc)
2
(2.5 equiv) and Na
2
C
2
O
4
(4.0 equiv) in THF (1.0 mL) were stirred at
was used as sole catalyst (Scheme 5c), which further confirmed
the reaction would involve the pathway of Cu(OAc) -mediated
2
b
1
20 °C for 24 h in the air. Isolated yields.
C-C bond cleavage of 2a. At last, addition of 2,2,6,6-
tetramethylpiperidine-N-oxyl (TEMPO) or 2,6-di-tert-butyl-4-
methylphenol (BHT) as radical scavengers would result in 82%
and 76% yields for 3a, which could exclude the radical pathway
during reaction (Scheme 5d).
To illustrate the practical utility of this method, a scale-up
experiment (6.0 mmol) was performed by using 1a and 2a as
substrates, affording the desired product 3a in 74% yield under
the standard conditions (Scheme 4a). Also, the directing group
could be easily removed from 3n in good yields by treatment
with MeOTf and subsequent NaOH to form free NH indole
Based on these observations and literature precedents,
7
,8,10,16
a plausible catalytic cycle was proposed. As shown in
(
Scheme 4b).13 Meanwhile, 2-phenyl-3-acetoxyl-5-methyl N-
pyridine indole 5, which was reported as precursor for potential
-HT receptor ligands, could also be achieved by the
Scheme 6, a pyridine-directed rhodacycle (Int-A) was firstly
2
formed from substrate 1a with [Cp*RhCl
]
2 2
via C(sp )-H
5
6
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3
2
(a) S. Kher, K. Lake, I. Sircar, M. Pannala, F. Bakir,VJie.wZaArpticf,leKO.nXlinue,
activation. Meanwhile, copper-catalyzed C(sp)-C(sp ) bond
cleavage assisted by the nitrogen lone-pair electrons happened,
affording alkynylcopper species while releasing an iminium
molecule 8, which was not stable and decomposed to be
S. H. Zhang, J. Liu, L. Morera, N. Sakurai, R. Jack and J. F. Cheng,
DOI: 10.1039/C9CC09777E
Bioorg. Med. Chem. Lett., 2007, 17, 4442; (b) M. B. Hadimani,
M. T. Macdonough, A. Ghatak, T. E. Strecker, R. Lopez, M.
Sriram, B. L. Nguyen, J. J. Hall, R. J. Kessler, A. R. Shirali, L. Liu,
C. M. Garner, G. R. Pettit, E. Hamel, D. J. Chaplin, R. P. Mason,
M. L. Trawick and K. G. Pinney, J. Nat. Prod., 2013, 76, 1668.
(a) M. Murakami and T. Matsuda, Chem. Commun., 2011, 47,
benzaldehyde and dibenzylamine (Detected by GC-MS) in this
work.10 Then, Int-A underwent transmetalation with
3
alkynylcopper species to generate Int-B, which went through
reductive elimination to afford Int-C. A final nucleophilic attack
of the amino group to the activated alkynyl moiety promoted
by Lewis acid and thereby intramolecular cyclization produces
the desired product 3a. The oxidation of Rh(I) to Rh(III) by Cu(II)
closed the catalytic cycle.
1
1
100; (b) M. Murakami and N. Ishida, J. Am. Chem. Soc., 2016,
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4
5
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H
N
N
N
N
Cu (0)
1a
3a
Cp*Rh (III)
Cu (II)
6
H
Ph
M = Rh or Cu
N
N
M
Rh
L
Cp
NH
Int-A
7
8
9
1
1
X. Yan, R. Ye, H. Sun, J. Zhong, H. Xiang and X. Zhou, Org. Lett.,
2019, 21, 7455.
N
Cu (II)
Ph
Ph
Ph
Int-C
T. Sugiishi, A. Kimura and H. Nakamura, J. Am. Chem. Soc.,
2010, 132, 5332.
H
N
CuX
Bn2N
Ph
2a
N
Rh
NBn2
Y. Shao, F. Zhang, J. Zhang and X. Zhou, Angew. Chem., Int. Ed.,
Cp
CuX2
8
2
016, 55, 11485.
Ph
Int-B
0 J. Ye, A. Limouni, S. Zaichuk and M. Lautens, Angew. Chem.,
Int. Ed., 2015, 54, 3116.
PhCHO
HNBn2
Scheme 6 Proposed catalytic cycle.
1 (a) Z. Q. Jiang, Q. Huang, S. Chen, L. S. Long and X. G. Zhou,
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Conclusions
1
In conclusion, a novel cross-coupling reaction between
propargylic amine and arylamine has been developed in the
presence of rhodium/copper catalyst system. The selective
cleavage of the carbon-carbon bond of the propargylic amine
and the carbon-hydrogen bond of arylamine occurred during
the reaction, which was followed by an intramolecular
cyclization procedure, affording a wide variety of substrates 2- 13 (a) V. K. Tiwari, N. Kamal and M. Kapur, Org. Lett., 2015, 17,
arylindoles with good functional group tolerance. Further
investigations are currently underway in our laboratory.
(
e) J. W. Chen, H. F. Xiang, L. Yang and X. G. Zhou, Rsc. Adv.,
2
017, 7, 7753.
1
766; (b) L. Jie, L. Wang, D. Xiong, Z. Yang, D. Zhao and X. Cui,
J. Org. Chem., 2018, 83, 10974.
1
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7
6, 80; (b) K. Alex, N. Schwarz, V. Khedkar, I. A. Sayyed, A.
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Conflicts of interest
There are no conflicts to declare.
1
1
5 (a) C. Liu, J.-C. Yi, L.-X. Dai and S.-L. You, Asian J. Org. Chem., ,
2
017, 6, 1201; (b) A. Longeon, B. R. Copp, E. Quevrain, M.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Grant No. 21472128) and Sichuan Science and
Technology Program (No. 2018JY560) for financial support.
Roue, B. Kientz, T. Cresteil, S. Petek, C. Debitus and M. L.
Bourguet-Kondracki, Mar. Drugs., 2011, 9, 879.
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6, 2074; (b) J. Gao, Y. Shao, J. Zhu, J. Zhu, H. Mao, X. Wang
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Notes and references
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(a) M. Bandini and A. Eichholzer, Angew. Chem., Int. Ed., 2009,
4
8, 9608; (b) D. Goyal, A. Kaur and B. Goyal, Chem Med Chem.,
018, 13, 1275.
2
4
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