Ligand-free, copper-catalyzed ullmann-type C-N coupling: Regioselective synthesis of azole-substituted imidazo[1,2-a]pyridines

Article abstract of DOI:10.1055/s-0033-1339927

A simple and highly efficient protocol for the regioselective synthesis of azole-substituted imidazo[1,2-a]pyridines has been developed using a ligand-free, copper-catalyzed Ullmann-type C-N coupling of 2-(2-bromophenyl) imidazo[1,2-a]pyridines with different azoles and in situ generated 1,2,3-triazoles. The reactions proceeded smoothly to furnish azolo-imidazo[1,2-a]pyridines in good to excellent yields (65-96%). Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1339927

LETTER
▌2751
letter
Ligand-Free, Copper-Catalyzed Ullmann-Type C–N Coupling: Regioselective
Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines
Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines
Pinku Kaswan, Kasiviswanadharaju Pericherla, Anil Kumar*
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333031, India
Fax +91(1596)244183; E-mail: anilkumar@pilani.bits-pilani.ac.in
Received: 30.07.2013; Accepted after revision: 15.09.2013
ders), alpidem, necopidem and saripidem (anxiolytics),
Abstract: A simple and highly efficient protocol for the regioselec-
zolimidine (treatment of peptic ulcers), olprinone (cardio-
tive synthesis of azole-substituted imidazo[1,2-a]pyridines has
tonic agent), and miroprofen (analgesic).
been developed using a ligand-free, copper-catalyzed Ullmann-type
C–N coupling of 2-(2-bromophenyl)imidazo[1,2-a]pyridines with
different azoles and in situ generated 1,2,3-triazoles. The reactions
proceeded smoothly to furnish azolo-imidazo[1,2-a]pyridines in
aminations of these privileged motifs are poorly studied.5
good to excellent yields (65–96%).
Although some ligand-promoted C–N couplings have
been reported on imidazo[1,2-a]pyridines, ortho-directed
In a continuation of our efforts towards designing novel
Key words: copper, ligand-free C–N coupling, imidazo[1,2-a]pyr-
heterocycles containing imidazo[1,2-a]pyridines,16 we
idines, 1,2,3-triazole, CuAAC, tandem reaction
wish to report a regioselective, ligand-free, copper-cata-
lyzed C–N coupling for the synthesis of azole-substituted
imidazo[1,2-a]pyridines by the reaction of 2-(2-bromo-
Copper-catalyzed Ullmann-type couplings are traditional
phenyl)imidazo[1,2-a]pyridines with different azoles and
methods for forming carbon–heteroatom bonds.1 Howev-
in situ generated 1,2,3-triazoles (Scheme 1).
er, these protocols are less favored as they require the use
2-(2-Bromophenyl)imidazo[1,2-a]pyridine (1a) and 1H-
imidazole (2a) were chosen as a model substrates for the
initial studies (Scheme 1). After extensive examinations,
we were gratified to find that 20 mol% of CuI and K2CO3
(2.0 equiv) gave a clean reaction in N,N-dimethylfor-
mamide (DMF) at 150 °C without any external ligand to
afford the coupled product, 2-[2-(1H-imidazol-1-yl)phe-
nyl]imidazo[1,2-a]pyridine (3a) in excellent yield (92%,
of stoichiometric amounts of copper reagents, harsh reac-
tion conditions, strong bases, and extended reaction times.
During recent decades, great efforts have been made to
overcome these shortcomings. Although the palladium-
catalyzed Buchwald–Hartwig reaction is an effective al-
ternative for the C–N coupling, copper-catalyzed proto-
cols of Ullmann-type aminations still have advantages
such as low cost and toxicity of the reagents and high
Table 1, entry 1). The structure of 3a was confirmed by its
functional-group tolerance.2 Improvements include the
1H NMR and 13C NMR data.
use of organic ligands such as diamines,3 amino acids,4
After optimization of the reaction conditions, the scope
and generality of this copper-catalyzed Ullmann-type C–N
coupling reaction was explored using a series of azoles
such as 2-methyl-1H-imidazole, 4-methyl-1H-imidazole,
and 1H-benzimidazole. The results are summarized in Ta-
ble 1. The reactions were carried out under the optimized
conditions without any special precautions. As can be
seen from Table 1, the coupling proceeded smoothly with
azoles 2a–g to give the corresponding products in good
diols,5 1,10-phenanthrolines,6 and other heteroatom-
containing compounds7 for this reaction.
Imidazo[1,2-a]pyridines possess biological properties
such as antifungal,8 antiviral,9 anticonvulsant activity,10
topoisomerase II inhibition,11 anti-ulcer,12 anti-inflamma-
tory,13 antiprotozoal,14 and antiretroviral activity.15 This
motif is found in commercially available drugs such as
zolpidem (treatment of insomnia and certain brain disor-
R3
X
X
N
N
R2
X
N
N
N
[Cu]
[Cu]
N
N
N
NaN3 (4)
R3
N
X
X
R1
N
R1
NH
R1
1a–d
3a–k
6a–k
R3 = alkyl or aryl
X
5
2a–g
a R1 = H, R2 = Br
b R1 = Me, R2 = Br
c R1 = Br, R2 = Br
d R1 = Br, R2 = H
X = CH or N
Scheme 1 Synthesis of azole-substituted imidazo[1,2-a]pyridines
SYNLETT 2013, 24, 2751–2757
Advanced online publication: 06.11.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339927; Art ID: ST-2013-D0717-L
© Georg Thieme Verlag Stuttgart · New York

2752
P. Kaswan et al.
LETTER
yields (Table 1, entries 2–6). 1H-1,2,4-Triazole and 1H- case of 4-methyl-1H-imidazole as pair of isomers are pos-
tetrazole also reacted well under the reaction conditions to sible in this case (Table 1, entry 2).17 The use of an inex-
give the coupled products in good yields (Table 1, entries pensive catalytic system, high efficiency, and simple
7–9). When 1H-indole was used as an amine partner, a methodology make this protocol of considerable practical
good yield of 2-[2-(1H-indol-1-yl)phenyl]-6-methylimid- utility. All the synthesized compounds were well charac-
azo[1,2-a]pyridine was obtained (Table 1, entry 10), albe- terized by 1H NMR, 13C NMR, and mass spectral data (see
it the reaction required six hours for completion. Supporting Information).
Unexpectedly, only a single isomer was observed in the
Table 1 Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines through Ullmann-Type C–N Coupling20,22
Entry
Substrate 1
Substrate 2
Product 3
Yield (%)a,b
N
Br
N
N
N
N
1
92c
N
N
H
N
2a
1a
1a
3a
3b
N
N
N
N
N
H
2
3
90
85
N
2b
N
N
N
N
H
1a
N
N
2c
3c
3d
3e
N
Br
N
N
N
4
5
2a
87
65
N
N
1b
N
N
N
N
N
H
1b
1b
N
2d
2c
N
N
6
92
N
N
3f
Synlett 2013, 24, 2751–2757
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines
2753
Table 1 Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines through Ullmann-Type C–N Coupling20,22 (continued)
Entry
Substrate 1
Substrate 2
Product 3
Yield (%)a,b
N
N
N
N
N
N
N
7
1a
85
H
N
2e
3g
3h
N
N
N
N
8
9
1b
1b
2e
87
88
N
N
N
N
N
N
N
N
N
N
H
N
2f
3i
N
N
N
H
10
11
1b
74c
N
N
2g
2a
3j
Br
N
N
N
85
N
Br
N
Br
1c
3k
a Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), CuI (0.2 mmol), K2CO3 (2.0 mmol), DMF, 150 °C, 2 h.
b Isolated yield.
c Reaction time was 6 h.
Encouraged by the success of this Ullmann amination Next, we investigated substrate scope for this tandem syn-
with azoles, we next explored this coupling towards the thesis of 1,2,3-triazole-substituted imidazo[1,2-a]pyri-
tandem synthesis of 1,2,3-triazole-substituted imid- dines (Table 2). In general, the scope of this reaction was
azo[1,2-a]pyridines. In a typical experiment 2-(2-bromo- found to be wide as phenylacetylenes with substitutions
phenyl)imidazo[1,2-a]pyridine (1a), sodium azide (4), such as 4-t-Bu, 4-Me, 4-OMe, and 4-F gave the coupled
and phenylacetylene (5a) were reacted under the opti- products in good yields (Table 2, entries 2–7). Aliphatic
mized conditions. Again, we were gratified to observe that terminal alkynes also efficiently participated to form nov-
2-[2-(4-phenyl-1H-1,2,3-triazol-1-yl)phenyl]imidazo-
el 1,2,3-triazolo-imidazo[1,2-a]pyridines in good yields
[1,2-a]pyridine (6a) was isolated in 55% yield. The struc- (Table 2, entries 8–10).
ture of 6a was elucidated by analysis of H NMR, 13C
1
The regioselectivity of the coupling reaction was estab-
NMR, and mass spectra (see Supporting Information). To
improve the yield of 6a, the reaction conditions were var-
ied by examining catalysts such as CuBr, CuCl, CuO,
CuCl2·2H2O, and CuSO4·5H2O while keeping other pa-
rameters unchanged. Among the copper salts screened,
CuCl2·2H2O provided best yields at 100 °C within three
hours (Table 2, entry 1).
lished by performing control experiments on 6-bromo-2-
(2-bromophenyl)imidazo[1,2-a]pyridine (1c) and 6-bro-
mo-2-phenylimidazo[1,2-a]pyridine (1d). When 1c and
1d were treated with 1H-imidazole (2a) under the opti-
mized conditions, monocoupled product 2-[2-(1H-imid-
azol-1-yl)phenyl]-6-bromoimidazo[1,2-a]pyridine (3k)
was obtained in 85% yield (Table 1, entry 11) from 1c
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2751–2757

2754
P. Kaswan et al.
LETTER
while 1d did not react under these conditions. It is worth corresponding monosubstituted product 6-bromo-2-[2-(4-
mentioning that no double C–N coupled product was de- phenyl-1H-1,2,3-triazol-1-yl)phenyl]imidazo[1,2-a]pyri-
tected from 1c in this reaction. Similarly, when 1c and 1d dine (6k) was isolated in 82% yield (Table 2, entry 11)
were reacted with 4 and 5a using CuCl2·5H2O in DMF the from 1c whereas 1d was unreactive.
Table 2 Synthesis of 1,2,3-Triazolo-imidazo[1,2-a]pyridines through Tandem Click–Ullmann C–N Coupling Reaction21,22
Entry
Substrate 1
Substrate 5
Product 6
Yield (%)a,b
N
N
N
1
1a
76
N
5a
N
6a
N
N
N
2
1a
84
N
5b
N
6b
MeO
N
N
MeO
N
3
4
5
6
1a
1a
1b
1b
75
84
80
87
N
5c
N
6c
F
N
N
N
N
F
N
N
5d
N
N
N
6d
6e
6f
N
N
5a
N
N
N
N
5e
Synlett 2013, 24, 2751–2757
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines
2755
Table 2 Synthesis of 1,2,3-Triazolo-imidazo[1,2-a]pyridines through Tandem Click–Ullmann C–N Coupling Reaction21,22 (continued)
Entry
Substrate 1
Substrate 5
Product 6
Yield (%)a,b
MeO
N
N
N
7
1b
5c
96c
N
N
6g
N
N
N
N
8
9
1b
1b
1b
1c
78
80
84
82
5f
N
6h
N
N
N
N
5g
N
6i
N
N
N
N
10
11
5h
5a
N
6j
N
Ph
N
N
N
N
Br
6k
a Reaction conditions: 1 (1.0 mmol), 4 (1.2 mmol), 5 (1.2 mmol), CuCl2·2H2O (0.2 mmol), K2CO3 (2.0 mmol), DMF, 100 °C, 3 h.
b Isolated yield.
c Yield with CuI was 55%.
These results are particularly interesting as the products B. Subsequently, the halide group on the copper can be
can be further functionalized by transition-metal-cata- exchanged with 1H-imidazole in the presence of base to
lyzed C–C or C–heteroatom cross-coupling reactions. To give intermediate C which, on reductive elimination,
illustrate this we performed a Suzuki reaction with 3k and leads to the target molecule with the regeneration of cop-
6k (Scheme 2). When 3k and 6k (1 mmol) were separate- per catalyst. In the case of the tandem 1,2,3-triazole cou-
ly treated with phenylboronic acid (1.1 mmol), Pd(OAc)2 pling reaction, Cu(I) can be generated in situ from its
(5 mol%), Ph3P (10 mol%), Cs2CO3 (1.5 mmol) in DMF precursor Cu(II) by reacting with phenylacetylene in a ho-
at 140 °C for two hours, the corresponding cross-coupled mocoupling reaction (Scheme 3).19
products 2-[2-(1H-imidazol-1-yl)phenyl]-6-phenylimid-
azo[1,2-a]pyridine (8) and 6-phenyl-2-[2-(4-phenyl-1H-
We further investigated the sequence of the tandem 1,2,3-
triazole coupling as there are two pathways possible for
this reaction (path A and path B; Scheme 4). To under-
stand the sequence of reactions, 1a was treated with sodi-
1,2,3-triazol-1-yl)phenyl]imidazo[1,2-a]pyridine
(9)
were obtained in 86% and 77% yield, respectively.
Based on literature precedent2a,18 the proposed mechanism um azide (4) in the absence of 5a under the optimized
of the coupling reaction is shown in Scheme 3. It is pro- reaction conditions with the expectation that azide would
posed that the imidazo[1,2-a]pyridine–copper complex A undergo substitution in the presence of copper to give 2-
{through N1-nitrogen atom of imidazo[1,2-a]pyridine} on (2-azidophenyl)imidazo[1,2-a]pyridine (10, path A).
oxidative addition with C–Br bond leads to intermediate However, no conversion was observed in the absence of
© Georg Thieme Verlag Stuttgart · New York
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2756
P. Kaswan et al.
LETTER
N
N
HO
OH
Pd(OAc)2 (5 mol%)
Ph3P (10 mol%)
B
N
N
N
N
+
Cs2CO3 (1.5 mmol)
DMF, 140 °C
N
N
Ph
Br
3k
7
8, 86%
N
N
Ph
N
Ph
N
N
HO
OH
N
N
Pd(OAc)2 (5 mol%)
Ph3P (10 mol%)
B
N
N
N
+
Cs2CO3 (1.5 mmol)
DMF, 140 °C
Br
Ph
9, 77%
6k
7
Scheme 2 Functionalization of azole-substituted imidazo[1,2-a]pyridines with Suzuki reaction
X
X
X
X
Br
N
NH
X
N
X
N
N
CuX
K2CO3
N
1
3 or 6
CuX
Br
– CuX
X
X
X
X
Br
X
X
N
X
X
Cu
Cu
N
Cu
NH
oxidative
addition
N
N
N
X
N
N
K2CO3
– HBr
A
B
C
Scheme 3 Plausible mechanism for ligand-free, copper-catalyzed Ullmann-type C–N coupling
5a, and 1a was recovered quantitatively. In another exper- imidazo[1,2-a]pyridines by ligand-free, copper-catalyzed
iment 5a was treated with sodium azide and Cu(II) in Ullmann-type C–N coupling. Under the optimized condi-
DMF to give 4-phenyl-1H-1,2,3-triazole (11, path B). Af- tions, reactions proceeded smoothly to give azolo-imid-
ter complete conversion as indicated by TLC, 1a and azo[1,2-a]pyridines in good to excellent (65–96%) yields.
K2CO3 were added and stirred at 100 °C for additional two The strategy has been extended towards the tandem syn-
hours. From this sequence 6a was isolated in 65% yield, thesis of 1,2,3-triazole-substituted imidazo[1,2-a]pyri-
confirming that the sequence of reactions in this tandem dines through in situ formation of 1,2,3-triazoles by the
protocol is as shown in path B.
reaction between terminal alkynes and sodium azide fol-
lowed by C–N bonding. Further functionalization of halo-
substituted, azole-substituted imidazo[1,2-a]pyridines
In summary, we have developed a simple and practical
protocol for regioselective synthesis of azole-substituted
N
Ph
N
Br
N3
Ph
path A
N
N
N
5a
X
N
N
N
CuCl2⋅2H2O
N
1a
NaN3, K2CO3
10
6a, 0%
DMF, 100 °C, 6 h
Br
N
N
Ph
N
N
Ph
N
Ph
N
N
5a
1a
path B
N
N
+
CuCl2⋅2H2O
DMF, r.t., 30 min
K2CO3
100 °C, 2 h
N
NaN3
4
H
11
6a, 65%
Scheme 4 Possible sequence of the reaction for the tandem 1,2,3-triazolo-imidazo[1,2-a]pyridine 6 formation
Synlett 2013, 24, 2751–2757
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Azole-Substituted Imidazo[1,2-a]pyridines
2757
(18) Sperotto, E.; van Klink, G. P. M.; van Koten, G.; de Vries, J.
G. Dalton Trans. 2010, 39, 10338.
was achieved via Suzuki–Miyaura cross-coupling. Thus,
the synthesized novel azole-substituted imidazo[1,2-a]-
pyridines have synthetic potential.
(19) (a) Hu, Y.-Y.; Hu, J.; Wang, X.-C.; Guo, L.-N.; Shu, X.-Z.;
Niu, Y.-N.; Liang, Y.-M. Tetrahedron 2010, 66, 80.
(b) Eglinton, G.; Galbraith, A. R. J. Chem. Soc. 1959, 889.
(20) General Procedure for the Synthesis of Compound 3
A round-bottom flask (10 mL) was charged with 2-(2-
bromophenyl)imidazo[1,2-a]pyridine (1, 0.36 mmol), azole
2 (0.439 mmol), CuI (0.07 mmol), K2CO3 (0.732 mmol), and
DMF (2.0 mL). The resulting solution was stirred at 150 °C
in an oil bath for 2 h under nitrogen atmosphere. On
completion, the reaction mass was filtered through Celite
and washed with EtOAc. The filtrate was washed with H2O,
dried over anhydrous Na2SO4, filtered, and concentrated
under vacuum. The crude residue was purified by column
chromatography (EtOAc–hexane, 2:3) to obtain the product
3.
Acknowledgment
Authors thank the Council of Scientific and Industrial Research
(CSIR), New Delhi for financial support. P.K. and K.P. are grateful
to CSIR, New Delhi and the University Grant Commission (UGC),
New Delhi, respectively, for providing Junior Research Fellow-
ships.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
(21) General Procedure for the Synthesis of Compound 6
A round-bottom flask (10 mL) was charged with 2-(2-
bromophenyl)imidazo[1,2-a]pyridine (1, 0.36 mmol),
sodium azide (4, 0.439 mmol), phenylacetylene (5a, 0.439
mmol), CuCl2·2H2O (0.073 mmol), K2CO3 (0.732 mmol),
and DMF (2.0 mL). The resulting solution was stirred at
100 °C in an oil bath for 3 h. On completion, the reaction
mass was filtered through Celite and washed with EtOAc.
The filtrate was washed with H2O, dried over anhydrous
Na2SO4, filtered, and concentrated under vacuum. The crude
residue was purified by column chromatography (EtOAc–
hexane, 2:3) to obtain product 6.
References and Notes
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(22) Spectroscopic Data of Selected Compounds
2-[2-(1H-Imidazol-1-yl)phenyl]H-imidazo[1,2-
a]pyridine (3a)
Yield 92%; colorless solid; mp 149–151 °C. 1H NMR (400
MHz, CDCl3): δ = 8.42 (dd, J = 7.9, 1.4 Hz, 1 H), 7.89 (dt, J
= 6.8, 1.1 Hz, 1 H), 7.65–7.55 (m, 3 H), 7.44 (td, J = 7.6, 1.5
Hz, 1 H), 7.34 (dd, J = 7.8, 1.2 Hz, 1 H), 7.29 (s, 1 H), 7.18–
7.14 (m, 1 H), 7.06 (s, 1 H), 6.73 (td, J = 6.8, 1.1 Hz, 1 H),
6.29 (s, 1 H). 13C NMR (126 MHz, CDCl3): δ = 144.86,
140.10, 137.36, 134.31, 131.38, 129.95, 129.78, 129.67,
128.37, 127.71, 125.82, 125.06, 120.48, 117.43, 112.53,
+
109.70. HRMS: m/z calcd for C16H13N4 : 261.1135; found:
261.1119 [M + H]+.
2-[2-(4-Methyl-1H-imidazol-1-yl)phenyl]H-imidazo [1,2-
a]pyridine (3b)
(10) Ulloora, S.; Shabaraya, R.; Adhikari, A. V. Bioorg. Med.
Chem. Lett. 2013, 23, 3368.
Yield 90%; colorless; mp 174–177 °C. 1H NMR (300 MHz,
CDCl3): δ = 8.41 (dd, J = 7.9, 1.4 Hz, 1 H), 7.90 (dt, J = 6.8,
1.0 Hz, 1 H), 7.62–7.52 (m, 2 H), 7.47 (s, 1 H), 7.40 (td, J =
7.6, 1.5 Hz, 1 H), 7.32–7.26 (m, 1 H), 7.18–7.11 (m, 1 H),
6.76 (s, 1 H), 6.72 (td, J = 6.8, 1.1 Hz, 1 H), 6.40 (s, 1 H),
2.34 (s, 3 H). 13C NMR (75 MHz, CDCl3): δ = 144.82,
140.26, 138.83, 136.50, 134.55, 131.26, 129.88, 129.43,
128.30, 127.79, 125.84, 124.96, 117.43, 116.77, 112.46,
(11) Baviskar, A. T.; Madaan, C.; Preet, R.; Mohapatra, P.; Jain,
V.; Agarwal, A.; Guchhait, S. K.; Kundu, C. N.; Banerjee, U.
C.; Bharatam, P. V. J. Med. Chem. 2011, 54, 5013.
(12) Kaminski, J. J.; Bristol, J. A.; Puchalski, C.; Lovey, R. G.;
Elliott, A. J.; Guzik, H.; Solomon, D. M.; Conn, D. J.;
Domalski, M. S. J. Med. Chem. 1985, 28, 876.
(13) Almirante, L.; Polo, L.; Mugnaini, A.; Provinciali, E.;
Rugarli, P.; Biancotti, A.; Gamba, A.; Murmann, W. J. Med.
Chem. 1965, 8, 305.
(14) Ismail, M. A.; Brun, R.; Wenzler, T.; Tanious, F. A.; Wilson,
W. D.; Boykin, D. W. J. Med. Chem. 2004, 47, 3658.
(15) Chaouni-Benabdallah, A.; Galtier, C.; Allouchi, H.;
Kherbeche, A.; Debouzy, J.-C.; Teulade, J.-C.; Chavignon,
O.; Witvrouw, M.; Pannecouque, C.; Balzarini, J.; De
Clercq, E.; Enguehard, C.; Gueiffier, A. Arch. Pharm. 2001,
334, 224.
+
109.88, 13.76. HRMS: m/z calcd for C17H15N4 : 275.1291;
found: 275.1278 [M + H]+.
2-[2-(4-Phenyl-1H-1,2,3-triazol-1-yl)phenyl]H-
imidazo[1,2-a]pyridine (6a)
Yield 76%; off-white solid; mp 189–192 °C. 1H NMR (300
MHz, DMSO-d6): δ = 8.89 (s, 1 H), 8.43 (d, J = 6.8 Hz, 1 H),
8.29 (d, J = 7.8 Hz, 1 H), 7.93 (d, J = 7.3 Hz, 2 H), 7.79–7.69
(m, 1 H), 7.59 (d, J = 3.1 Hz, 2 H), 7.51 (d, J = 4.8 Hz, 1 H),
7.46 (d, J = 7.7 Hz, 2 H), 7.36 (t, J = 7.3 Hz, 1 H), 7.27–7.15
(m, 1 H), 6.93 (s, 1 H), 6.80 (t, J = 6.7 Hz, 1 H). 13C NMR
(75 MHz, DMSO-d6): δ = 147.98, 145.06, 140.34, 134.62,
131.26, 130.65, 130.56, 130.40, 128.86, 128.44, 128.29,
127.47, 126.15, 125.84, 125.00, 122.05, 117.55, 112.53,
(16) (a) Pericherla, K.; Khungar, B.; Kumar, A. Tetrahedron
Lett. 2012, 53, 1253. (b) Pericherla, K.; Khedar, P.;
Khungar, B.; Kumar, A. Chem. Commun. 2013, 49, 2924.
(c) Khedar, P.; Pericherla, K.; Kumar, A. Synlett 2012, 23,
2609.
+
110.39. HRMS: m/z calcd for C21H16N5 : 338.1400; found:
338.1405 [M + H]+.
(17) Sun, M.; Wu, H.; Zheng, J.; Bao, W. Adv. Synth. Catal.
2012, 354, 835.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2751–2757

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