Hypervalent iodine(III)-promoted: N -incorporation into N -aryl vinylogous carbamates to quinoxaline diesters: Access to 1,4,5,8-tetraazaphenanthrene

Article abstract of DOI:10.1039/c6ob00447d

A novel oxidative N-incorporation strategy for synthesis of quinoxaline diesters under metal-free conditions is described for the first time. The mild reaction conditions allow for this transformation via the formation of two C(sp²)-N bonds utilizing cheaply available NaN₃ as the N-atom source. N-Aryl vinylogous carbamates in this study undergo azidation at enamino C(sp²)-H selectively. The robustness of this strategy is further demonstrated by the synthesis of a valuable 1,4,5,8-tetraazaphenanthrene derivative using a mild and convenient approach.

Full text of DOI:10.1039/c6ob00447d

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Hypervalent iodine(III)-promoted N-incorporation
into N-aryl vinylogous carbamates to quinoxaline
diesters: access to 1,4,5,8-tetraazaphenanthrene†
Cite this: DOI: 10.1039/c6ob00447d
Received 26th February 2016,
Accepted 24th March 2016
A. Sagar, Shinde Vidaycharan, Anand H. Shinde and Duddu S. Sharada*
DOI: 10.1039/c6ob00447d
www.rsc.org/obc
A novel oxidative N-incorporation strategy for synthesis of qui- forming reactions on arenes involving easily accessible azides
noxaline diesters under metal-free conditions is described for the are of great importance for the synthesis of azaheterocycles.6
first time. The mild reaction conditions allow for this transform- Despite numerous methods for construction of aromatic C–N
ation via the formation of two C(sp2)–N bonds utilizing cheaply bonds which lead to azaheterocycles being reported,7 methods
available NaN3 as the N-atom source. N-Aryl vinylogous carba- via incorporation of the N-atom into arene molecules using
mates in this study undergo azidation at enamino C(sp2)–H selec- azides as the N-source under metal-free conditions are rare.8
tively. The robustness of this strategy is further demonstrated by However a few research groups like Glorius, Ellman and Jiao
the synthesis of a valuable 1,4,5,8-tetraazaphenanthrene derivative et al. have developed prominent reactions for the synthesis of
using a mild and convenient approach.
azaheterocycles via N-incorporation into arene molecules by
using azides and employing transition metal catalysts
(Scheme 1).9 More recently, a new approach was described by
Yu’s group for oxidative N-incorporation into N-arylenamines
under copper catalysis.10 However, these N-incorporation strat-
egies utilized transition metal catalysts and organic azides as
the N-source.9,10 Recently, Jiao et al. have developed a metal-
free expeditious nitrogenation of 2-acetylbiphenyls leading to
phenanthridines under acidic conditions (Scheme 1).8 Our
interests in the development of metal-free protocols for the
synthesis of heterocyclic scaffolds,11 persuaded us to design a
strategy for the synthesis of azaheterocycles via N-incorpor-
ation from nitrogen functionality free substrates. Herein, we
Introduction
Of all the C–N bond formation reactions, aromatic C–N bond
forming methods under oxidative conditions are the most fas-
cinating approaches and highly explored in recent years,1 due
to the wide occurrence of azaheterocycles in natural products,
biologically active molecules and materials science.2 Over the
past few decades, most of the literature procedures and con-
ventional methods seem to suggest that a substrate with nitro-
gen functionality is essential for aromatic C–N bond formation
for the synthesis of azaheterocycles.3 Such procedures often
require multisteps to bring nitrogen functionality into the sub-
strate and have limited generality. On the other hand, most of
the aromatic C–N bond transformations have been achieved
via toxic and expensive metal-based catalysts.4
Though cheap and environmentally benign catalytic
systems based on Cu and Fe have also been used for C–N bond
formation in heterocyclic synthesis,4f–h recently chemists have
mainly directed their efforts towards designing green economy
strategies for expeditious synthesis of azaheterocycles from
nitrogen functionality free substrates under metal-free con-
ditions5 which is highly challenging and desirable. C–N bond
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285,
Sangareddy, Telangana, India. E-mail: sharada@iith.ac.in; Fax: +(040) 2301 6032;
Tel: +(040) 2301 7058
†Electronic supplementary information (ESI) available. CCDC 1453940. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6ob00447d
Scheme 1 Intermolecular nitrogen incorporation strategies to syn-
thesize azaheterocycles.
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dichloroethane (DCE) as the solvent at 0 °C to room tempera-
ture (27 °C) for 6 h. We were delighted to find that the desired
product diethyl quinoxaline-2,3-dicarboxylate 2a was indeed
observed in 21% yield (Table 1, entry 1). This interesting result
motivated us to improve the yield of the desired product 2a.
On switching from PIFA to less potent PIDA, the yield of the
product increased to 38% (Table 1, entry 2). Next, we screened
the reaction by varying the equivalents of PIDA and NaN3
(Table 1, entries 3–11). We were pleased to find quinoxaline 2a
in 78% yield with 2 equiv. of PIDA and 2 equiv. of NaN3 in
DCE solvent (Table 1, entry 6). Among the various solvents
tested, DCE was found to be effective (Table 1, entry 6). Our
attempts to further improve the yield by replacing NaN3 with
other N-sources were shown to be unsuccessful (Table 1,
entries 13 & 14). Continuing our zest, we screened other
oxidant sources, unfortunately our attempts were in vain
(Table 1, entries 15 & 16).
Under the optimal reaction conditions (Table 1, entry 6),
the scope of the nitrogenation reaction of N-aryl vinylogous
carbamates bearing electron-donating substituents (–Me,
–OMe) provided the desired products in very good yields
(Table 2, 2a–2j). Electron-donating substituents at any position
of the ring were well tolerated, whereas weak electron-donating
groups like halogens on the aryl ring of vinylogous carbamate
derivatives greatly affected the reactivity (Table 3). When we
employed our standard conditions to ortho halo substituted
N-aryl vinylogous carbamates, we observed alkyl N-aryloxamate
as the byproduct in 18% yield (see the ESI,† Table 1, 2k-2,
entry 1) along with our desired product 2k-1.
Fig. 1 Biologically important compounds and TAP containing a qui-
noxaline core.
are delighted to report for the first time an unprecedented
metal-free hypervalent iodine(III) promoted dehydrogenative
nitrogenation of N-aryl vinylogous carbamates using sodium
azides as the N-source via the formation of two C(sp2)–
N bonds leading to quinoxalines in a cascade fashion.
Hypervalent iodine(III) derivatives have been recognized as
very promising reagents for metal-free oxidative dehydrogena-
tive transformations in synthetic organic chemistry,12 due to
their low toxicity, environmentally friendly nature and easy
handling.
Among azaheterocycles, quinoxaline containing com-
pounds have found use as potential anticancer,13 antiviral,
antibiotic and anti-inflammatory agents14 (Fig. 1) and appli-
cations in diverse areas of materials science. In particular, a
quinoxaline core bearing ester functionalities is a key struc-
tural unit of n-type semiconductors (Fig. 1).15 Not surprisingly,
many efforts have been directed for the efficient synthesis of
quinoxalines. Generally, the most common strategy to build
up quinoxaline rings relies on the condensation of o-pheny-
lenediamine with 1,2-dicarbonyls or their equivalents.16
However, these methods have some drawbacks such as gener-
ality and multisteps with a narrow substrate scope. Owing to
this, we aimed to develop a metal-free protocol for the syn-
thesis of quinoxalines via a dehydrogenative N-incorporation
strategy using azides as the N-source with hypervalent
iodine(III).
Table 1 Optimization of the reaction conditions for the synthesis of
diethyl quinoxaline-2,3-dicarboxylate (2a)a
Entry
Iodine(III) (equiv.)
[N] source
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
PIFA (1.0)
PIDA (1.0)
PIDA (1.5)
PIDA (1.5)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
—
NaN3 (1.0)
NaN3 (1.0)
NaN3 (1.0)
NaN3 (1.5)
NaN3 (1.5)
NaN3 (2.0)
NaN3 (2.0)
NaN3 (2.0)
NaN3 (2.0)
NaN3 (2.0)
NaN3 (2.0)
TMSN3 (2.0)
IBA-N3
DCE
DCE
DCE
DCE
DCE
DCE
DCM
HFIP
DMF
CH3CN
THF
DCE
DCE
TFE
21
38
50
52c
68
78
75
13
63
50
17
22
ND
ND
ND
Results and discussion
Having been inspired by the very recent applications of dehy-
drogenative nitrogenation reactions involving azides as the
source of the N-atom in the synthesis of natural products and
biologically active compounds,9 we have selected the inter- 10
11
13
14
molecular cyclization reaction between diethyl 2-(phenyl-
amino)maleate 1a and azides as the target for the formation of
two C(sp2)–N bonds with hypervalent iodine reagents, which 15
PhIO
PhI(OH)(OTs)
NaN3 (2.0)
NaN3 (2.0)
16
DCE
lead to quinoxalines.
a Reaction conditions 1a (0.19 mmol), PIDA (0.38 mmol), NaN3
(0.38 mmol), DCE (3 mL), 0 °C–27 °C, 6 h. b Yield of the isolated
product after column chromatography. c Reaction was carried out at
Accordingly, we started our study with the readily prepared
diethyl 2-(phenylamino)maleate 1a (see the ESI†) to probe the
feasibility of the reaction. Initially, the nitrogenation of 1a was
investigated by using 1 equiv. of sodium azide (NaN3) as the
N-source and 1 equiv. of bis(trifluoroacetate) (PIFA) in 1,2-
40 °C, IBA-N3
=
1-azido-1,2-benziodoxol-3-(1H)-one, TMSN3
=
trimethylsilyl azide, HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol, TFE =
2,2,2-triflouroethanol, ND = not detected.
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2k-1 selectively (Table 3, 2k-2). When we extended these con-
ditions to 1l-1x, we obtained the desired quinoxalines 2l–2q,
2r-1, 2s, 2t-1, 2u, 2v, 2w-1 and 2x-1 in moderate yields, except
in four cases (2r-2, 2t-2, 2w-2 & 2x-2) where the formation of
the byproduct was identified, albeit in negligible yields
(Table 3). To test the scope of this method, we performed a
control experiment wherein we replaced α-ester functionality
in vinylogous carbamate 1a with –CH3 (1z) and –CF3 (1aa) and
subjected it to standard conditions, unfortunately this failed
to give the desired products rather compound 1z yielded the
byproduct N-phenylacetamide (2z-2) in 30% yield (Table 4,
eqn (1)).
To probe the mechanism of this dehydrogenative nitrogena-
tion, preliminary control experiments were conducted
(Table 4). No nitrogenation product 2a was observed in the
absence of PIDA as an oxidant even under thermal conditions
(Table 4, eqn (2)), which indicates the key role played by PIDA
in the dehydrogenative nitrogenation. To obtain more insight
into the mechanism, we performed the reaction in the pres-
ence of organic azide TMSN3 resulting in 2a (22%), thus indi-
cating the low reactivity of TMSN3 (Table 4, eqn (3)). To probe
whether the reaction proceeds through a radical pathway, we
performed the standard reaction in the presence of radical sca-
vengers like 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), di-
tert-butyl peroxide (DTP) and tert-butyl peroxybenzoate (TBPB)
(Table 4, eqn (4)–(6)), which showed no significant decrease in
the yield of the product 2a, thus confirming the ionic mechan-
ism for the reaction. Accordingly, we envisioned that the
enamine C(sp2)–H azidation product may serve as the key
intermediate III in this transformation. To verify this possi-
bility, we conducted another control experiment with ortho
azido N-acyl vinylogous carbamate 1y in the absence of NaN3
Table 2 Nitrogenation reaction of N-aryl vinylogous carbamates
bearing electron donating groupsa,b
R1
R2
R3
R4
R5
2
Yield (%)
H
H
H
H
H
H
H
Me
Me
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
Et
2a
2b
2c
2d
2e
2f
2g
2h
2i
78
72
67
80
75
65
68
64c
62
61
Me
Me
Et
Me
Me
Me
Et
OMe
OMe
OMe
H
Me
Me
H
H
Me
Me
Me
Me
Me
H
H
Et
Me
H
2j
a Reaction conditions 1a (0.19 mmol), PIDA (0.38 mmol), NaN3
(0.38 mmol), DCE (3 mL), 0 °C–27 °C, 4–6 h. b Yield of the isolated
product after column chromatography. c Compound 2h (CCDC
1453940) was further confirmed by single crystal XRD (see the ESI).
Table 3 Nitrogenation reaction of N-aryl vinylogous carbamates
bearing halogensa,b
R1
R2
R3
R4
R5
2
Yield (%)
I
I
Br
Br
F
H
H
H
H
Cl
H
H
Br
Br
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
I
Br
Br
Cl
H
F
H
H
H
H
H
H
H
H
H
H
H
H
Br
H
Me
Et
Et
Me
Et
Et
Et
Me
Et
Me
Et
Me
Me
Et
2k-1
2l
2m
2n
2o
2p
2q
2r-1
2s
2t-1
2u
2v
2k-2
58, 0
59
61
58
57
61
Table 4 Test on the scope of the method and control experiments
64
2r-2
2t-2
51c, 4c
61
60, 10
62
F
H
Me
64
2w-1
2x-1
2w-2
2x-2
54c, 3c
48c, 5c
a Reaction conditions 1a (0.19 mmol), PIDA (0.38 mmol), NaN3
(0.76 mmol), DCE (4 mL), 0 °C–27 °C, 4–6 h. b Yield of the isolated
product after column chromatography. c Ratio was determined by
1H NMR.
In order to reduce the formation of by-products, we
screened the reaction with different equivalents of NaN3, we
found 4 equivalents of NaN3 were furnishing the quinoxaline a Trace amount of 2a was observed in TLC.
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(TAP) derivative involving the formation of four new C–N
bonds and two rings in a single step. Further investigations on
the more detailed mechanism and applications of the present
protocol are currently underway in our lab.
Acknowledgements
Financial support by the Council of Scientific Industrial
Research [(CSIR), No. 02(0097)/12/EMR-II], New Delhi and the
Indian Institute of Technology (IIT) Hyderabad is gratefully
acknowledged. A. S. thanks CSIR, S. V. C and A. H. S thank
UGC, New Delhi for the award of research fellowship.
Fig. 2 Plausible reaction mechanism for 2.
Notes and references
1 For selected reviews on C–N bond formation, see:
(a) C. Sambiagio, S. P. Marsden, A. J. Blacker and P. C. Mc
Gowan, Chem. Soc. Rev., 2014, 43, 3525; (b) G. Y. Song,
F. Wang and X. W. Li, Chem. Soc. Rev., 2012, 41, 3651;
(c) T. A. Ramirez, B. G. Zhao and Y. Shi, Chem. Soc. Rev.,
2012, 41, 931; (d) Y. Zhang, X. Yang, Q. Yao and D. Ma, Org.
Lett., 2012, 14, 3056; (e) C. Fischer and B. Koenig, Beilstein
J. Org. Chem., 2011, 7, 59; (f) D. S. Surry and
S. L. Buchwald, Chem. Sci., 2011, 2, 27; (g) F. Monnier and
M. Taillefer, Angew. Chem., Int. Ed., 2009, 48, 6954.
2 (a) T. Henkel, R. M. Brunne, H. Müller and F. Reichel,
Angew. Chem., Int. Ed., 1999, 111, 688; (b) F. Fache,
E. Schulz, M. L. Tommasino and M. Lemaire, Chem. Rev.,
2000, 100, 2159; (c) M. Feher and J. M. Schmidt, J. Chem.
Inf. Comput. Sci., 2002, 43, 218; (d) R. Hili and A. K. Yudin,
Nat. Chem. Biol., 2006, 2, 284; (e) N. Ramkumar and
R. Nagarajan, J. Org. Chem., 2014, 79, 736;
(f) D. B. Ramachary and J. Sangeeta, Org. Biomol. Chem.,
2011, 9, 1277; (g) M. Bakthadoss, D. Kannan, J. Srinivasan
and V. Vinayagam, Org. Biomol. Chem., 2015, 13, 2870.
3 (a) C. Venkatesh, B. Singh, P. K. Mahata, H. Ila and
H. Junjappa, Org. Lett., 2005, 7, 2169; (b) Y. Kikugawa,
A. Nagashima, T. Sakamoto, E. Miyazawa and M. Shiiya,
J. Org. Chem., 2003, 68, 6739; (c) X. Ban, Y. Pan, Y. Lin,
S. Wang, Y. Du and K. Zhao, Org. Biomol. Chem., 2012, 10,
3606.
Scheme 2 Synthesis of tetramethyl pyrazino[2,3-f]quinoxaline-2,3,8,9-
tetracarboxylate (2ab).
which resulted in 2a in a trace amount and the corresponding
alkyl N-aryl oxamate 2y in 13% yield (Table 4, eqn (7)).
Together, these observations indicated that an ionic mech-
anism may be involved in this transformation. Based on these
experiments, we have proposed a plausible mechanism as
shown in Fig. 2, where the intermolecular reaction of 1a and
PIDA generated the intermediates I (pathway a) and II
(pathway b) by the loss of acetic acid. Afterwards, the azide
intermediate III formed by either cleavage of C–I or N–I bonds
would undergo the Friedel–Crafts reaction followed by aroma-
tization leading to the product 2a.
After having successfully synthesized the quinoxalines, to
check further applicability of our protocol, the reaction was
extended to synthesize tetramethyl pyrazino[2,3-f]quinoxaline-
2,3,8,9-tetracarboxylate (2ab) from bis N-aryl vinylogous carba-
mates (1ab) which afforded 2ab, albeit in moderate yield (52%)
under standard conditions in a single step (Scheme 2). It is
worth mentioning here that, this straightforward method is
highly superior to what is found in the literature report17 for
the synthesis of such a valuable scaffold, since it avoids mul-
tiple steps, metal-catalysts and harsh conditions.
4 (a) K. Shin, H. Kim and S. Chang, Acc. Chem. Res., 2015, 48,
1040; (b) M. A. Mcgowan, J. L. Henderson and
S. L. Buchwald, Org. Lett., 2012, 14, 1432; (c) C.-L. Sun and
Z.-J. Shi, Chem. Rev., 2014, 114, 9219; (d) A. F. Larsen and
T. Ulven, Chem. Commun., 2014, 50, 4997; (e) S. Ueda and
S. L. Buchwald, Angew. Chem., Int. Ed., 2012, 51, 10364;
(f) X.-X. Guo, D.-W. Gu, Z. Wu and W. Zhang, Chem. Rev.,
2015, 115, 1622; (g) F. S. Melkonyan, D. E. Kuznetsov,
M. A. Yurovskaya and A. V. Karchava, RSC Adv., 2013, 3,
8388; (h) A. Sh. El-Etrawy and A. A.-H. Abdel-Rahman,
Chem. Heterocycl. Compd., 2010, 46, 1033.
Conclusions
In conclusion, we have presented an unprecedented metal-free
hypervalent iodine(III)-promoted dehydrogenative N-incorpor-
ation into N-aryl vinylogous carbamates for synthesis of qui-
noxaline diesters. This protocol involves mild reaction
conditions for N-incorporation via the formation of two
C(sp2)–N bonds in a cascade fashion utilizing cheaply available
azide as the N-source. The robustness of this strategy is
demonstrated by the synthesis of a tetraazaphenanthrene
5 R. Samanta, K. Matcha and A. P. Antonchick, Eur. J. Org.
Chem., 2013, 5769.
Org. Biomol. Chem.
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6 Organic Azides: Syntheses and Applications, ed. S. Bräse and
K. Banert, Wiley & Sons, 2010.
(f) J. P. Brand, D. F. Gonzalez, S. Nicolai and J. Waser,
Chem. Commun., 2011, 47, 102; (g) Y. He, J. Huang,
D. Liang, L. Liu and Q. Zhu, Chem. Commun., 2013, 49,
7352; (h) F. Zhao, X. Liu, R. Qi, D. Zhang-Negrerie,
J. Huang, Y. Du and K. Zhao, J. Org. Chem., 2011, 76,
10338; (i) A. Yoshimura and V. V. Zhdankin, Chem. Rev.,
2016, 116, 3328.
7 (a) K. Seth, M. Nautiyal, P. Purohit, N. Parikh and
A. K. Chakraborti, Chem. Commun., 2015, 51, 191;
(b) F. Collet, R. H. Dodd and P. Dauban, Chem. Commun.,
2009, 45, 5061; (c) J. Y. Kim, S. H. Park, J. Ryu, S. H. Cho,
S. H. Kim and S. Chang, J. Am. Chem. Soc., 2012, 134, 9110.
8 C. Tang, Y. Yuan and N. Jiao, Org. Lett., 2015, 17, 2206.
9 (a) D.-G. Yu, M. Suri and F. Glorius, J. Am. Chem. Soc., 2013,
13 (a) L. Marcus, N. Kingi and J. Bergman, J. Med. Chem.,
2008, 51, 7744.
135, 8802; (b) Y. Lian, J. R. Hummel, R. G. Bergman and 14 (a) J. K. Landquist, in Comprehensive Heterocyclic Chemistry,
J. A. Ellman, J. Am. Chem. Soc., 2013, 135, 12548;
(c) Q.-Z. Zheng, P. Feng, Y.-P. Liang and N. Jiao, Org. Lett.,
2013, 15, 4262.
ed. A. R. Katritzky and C. W. Rees, Pergamon, Oxford, 1984,
vol. 1, p. 166; (b) L. E. Seitz, W. J. Suling and
R. C. Reynolds, J. Med. Chem., 2002, 45, 5604; (c) Y. B. Kim,
Y. H. Kim, J. Y. Park and S. K. Kim, Bioorg. Med. Chem.
Lett., 2004, 14, 541; (d) W. He, M. R. Meyers, B. Hanney,
A. Spada, G. Blider, H. Galzeinski, D. Amin, S. Needle,
K. Page, Z. Jayyosi and H. Perrone, Bioorg. Med. Chem. Lett.,
2003, 13, 3097; (e) C. W. Lindsley, Z. Zhao, W. H. Leister,
R. G. Robinson, S. F. Barnett, D. Defeo-Jones, R. E. Jones,
G. D. Hartman, J. R. Huff, H. E. Huber and M. E. Duggan,
Bioorg. Med. Chem. Lett., 2005, 15, 761.
10 H. Ma, D. Li and W. Yu, Org. Lett., 2016, 18, 868.
11 (a) S. Vidyacharan, A. Sagar, N. C. Chaitra and D. S. Sharada,
RSC Adv., 2014, 4, 34232; (b) S. Vidyacharan, N. C. Chaitra,
A. Sagar and D. S. Sharada, Synth. Commun., 2015, 45, 898;
(c) S. Vidyacharan, A. Sagar and D. S. Sharada, Org. Biomol.
Chem., 2015, 13, 7614; (d) S. Vidyacharan, A. H. Shinde,
B. Satpathi and D. S. Sharada, Green Chem., 2014, 16, 1168;
(e) A. H. Shinde, S. Vidyacharan and D. S. Sharada, Tetra-
hedron Lett., 2014, 55, 3064; (f) A. Sagar, V. N. Babu and 15 (a) P. Gawrys, T. Marszalek, E. Bartnik, M. Kucinska,
D. S. Sharada, RSC Adv., 2015, 5, 29066; (g) A. Sagar, J. Ulanski and M. Zagorska, Org. Lett., 2011, 13, 6091.
V. N. Babu and D. S. Sharada, Tetrahedron Lett., 2015, 56, 16 (a) S. Okumura, Y. Takeda, K. Kiyokawa and S. Minakata,
2710.
Chem. Commun., 2013, 49, 9266; (b) Z.-J. Yang, C.-Z. Liu,
B.-L. Hu, C.-L. Deng and X.-G. Zhang, Chem. Commun.,
2014, 50, 14554; (c) S. Shi, T. Wang, W. Tang, M. Ruldoph
and A. S. Hashmi, Chem. – Eur. J., 2013, 19, 6576.
12 (a) Z. Zheng, S. Ma, L. Tang, D. Zhang-Negrerie, Y. Du and
K. Zhao, J. Org. Chem., 2014, 79, 4687; (b) V. V. Zhdankin
and P. J. Stang, Chem. Rev., 2002, 102, 2523; (c) H. Tohma
and Y. Kita, Adv. Synth. Catal., 2004, 346, 111; 17 (a) S. C. Shim and S. K. Lee, Synthesis, 1980, 2, 116;
(d) V. V. Zhdankin and P. J. Stang, Chem. Rev., 2008, 108,
5299; (e) T. Dohi and Y. Kita, Chem. Commun., 2009, 2073;
(b) R. N. Hinkens, M. B. Vamos and D. Maetens, J. Hetero-
cycl. Chem., 1980, 17, 873.
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