Multicomponent reactions for the synthesis of new 3′-indolyl substituted heterocycles under microwave irradiation

Article abstract of DOI:10.1016/j.tetlet.2008.02.101

A series of polysubstituted (3′-indolyl)pyrazolo[3,4-b]pyridine and (3′-indolyl)benzo[h]quinoline derivatives were synthesized via one-pot multicomponent reactions of aldehydes, 3-cyanoacetyl indoles with 5-aminopyrazol or naphthylamine under microwave irradiation. Particularly valuable features of this method include high yields of products, broad substrate scope, short reaction time and straightforward procedure.

Full text of DOI:10.1016/j.tetlet.2008.02.101

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2578–2582
Multicomponent reactions for the synthesis of new 30-indolyl
substituted heterocycles under microwave irradiation
a,
a
a
Song-Lei Zhu a,b, Shun-Jun Ji , Kai Zhao , Yu Liu
*
a Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry and Chemical Engineering,
Suzhou(Soochow) University, Suzhou 215123, People’s Republic of China
b Department of Chemistry, Xuzhou Medical College, Xuzhou 221002, People’s Republic of China
Received 5 December 2007; revised 14 February 2008; accepted 18 February 2008
Available online 21 February 2008
Abstract
A series of polysubstituted (30-indolyl)pyrazolo[3,4-b]pyridine and (30-indolyl)benzo[h]quinoline derivatives were synthesized via
one-pot multicomponent reactions of aldehydes, 3-cyanoacetyl indoles with 5-aminopyrazol or naphthylamine under microwave
irradiation. Particularly valuable features of this method include high yields of products, broad substrate scope, short reaction time
and straightforward procedure.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Indole; Heterocycle; Multicomponent reactions; Microwave irradiation
Indole moiety has been found in various pharmacologi-
H2N
H
cally and biologically active compounds.1 Many indole
alkaloids are recognized as one of the rapidly growing
groups of marine invertebrate metabolites for their broad
spectrum of biological properties.2 For example, five novel
indole alkaloids,3 meridianins A–E, have been isolated
from the tunicate Splidium meridiaum. They show cyto-
toxicity toward murine tumor cell lines and have potent
inhibition against several protein kinases.4
N
R3
N
N
R1
N
R1
X
R2
R3
H
N
N
R2
O
H
R4
Aplysinopsins
Meridianins A-E
Fig. 1. Examples of 30-indolyl substituted heterocycles.
Along with these, the substitution at 3-position of the
indole ring can take place by connecting an extra hetero-
cyclic ring, such as imidazole (topsentins5 and nortopsen-
tins6), dihydroimidazole (discodermindole7), oxazole
(martefrgin,8 amazol9), oxadiazine (alboinon10), maliemide
(didemidines11), and piperazine (dragmacidon12). These 3-
substituted indoles represent a promising structural class
of marine alkaloids and therefore, are interesting synthetic
targets (Fig. 1).
However, the methods for synthesis of these important
compounds often suffer from tedious synthetic routes, long
reaction time, drastic reaction conditions, as well as narrow
application scope of substrates. In addition, to the best of
our knowledge, there have been few reports about synthesis
of indole derivatives including pyrazolo[3,4-b]pyridine
moieties. While, pyrazolo[3,4-b]pyridines are also attractive
targets in organic synthesis due to their interesting biolog-
ical and pharmacological properties such as vasodialotors,
hypoglycemic, anti-inflammatory, analgesic, and anti-pyr-
itic activities.13
*
Corresponding author. Fax: +86 512 65880307.
E-mail address: chemjsj@suda.edu.cn (S.-J. Ji).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.101

S.-L. Zhu et al. / Tetrahedron Letters 49 (2008) 2578–2582
2579
Table 1
Multicomponent reactions (MCRs) are of increasing
Optimization of reaction conditions of compound 4aa
importance in organic and medicinal chemistry.14 The
strategies of MCRs offer significant advantages over con-
ventional linear-type syntheses for their high degree of
atom economy, convergence, ease of execution, excellent
yields and broad application characters, which are particu-
larly useful to generate diverse chemical libraries of ‘drug-
like’ molecules for biological screening.15
Entry
Solventb
T (°C)
Time (min)
Yieldc
1
2
3
4
5
6
7
8
9
H2O
90
90
90
90
90
110
130
140
150
160
150
20
20
15
15
15
12
10
9
None
32
50
42
57
65
72
78
85
EtOH
HOAc
DMF
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
EtOH
Recently, we described a new and efficient synthesis of 3-
(2-furanyl)indole derivative by three-component reactions
of isocyanides, 3-cyanoacetyl indole, and aromatic alde-
hydes in good yields (Scheme 1).16
Due to our interest in the multicomponent syntheses and
in continuation to our work on the synthesis of indole
derivatives,17 herein, we reported a simple and facile proto-
col for the synthesis of a series of indole derivative-incorpo-
rated pyrazolo[3,4-b]pyridine units.
8
8
12
10
11
a
84
75
The reaction was carried out under MW.
The amount of solvent was 2 mL.
Isolated yields.
b
c
The target compounds 4-substituted-6-(1H-indol-3-
yl)-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbo-
nitrile 4 were synthesized by one-pot reactions of 3-cyano-
acetyl indoles 1, aldehydes 2 and 3-methyl-1-phenyl-1H-
pyrazol-5-amine 3 under microwave irradiation conditions
(Scheme 2).
heterocycles, with which a 30-indolyl group binding in posi-
tion 6 of the pyrazolo[3,4-b]pyridine nucleus were synthe-
sized in good yields. The results were summarized in
Table 2.
As shown in Table 2, this protocol can be applied not
only to aromatic aldehydes with either electron-withdraw-
ing groups or electron-donating groups, but also to hetero-
cyclic and aliphatic aldehydes. Furthermore, a series of
substituted 3-cyanoacetyl indoles were used in this reac-
tion, which all gave excellent results. We have also
observed delicate electronic effects: that is, arylaldehydes
with electron-withdrawing groups reacted rapidly and gave
higher yields; while substitutions of electron-rich groups on
the benzene ring, as well as heterocyclic and aliphatic alde-
hydes, required longer reaction times and got lower yields.
This was probably due to the decreased reactivities.
Although the detailed mechanism of the above reaction
remains to be fully clarified, according to the literatures,18
compound 4 could be formed from the intermediate 5 via
Michael addition reaction with 5-aminopyrazol 3 followed
by intramolecular cyclization and elimination. Intermedi-
ate 5 could be prepared by condensation of 3-cyanoacetyl
indole 1 and aldehyde 2 (see Scheme 3).
In our initial study, various reaction conditions includ-
ing solvents and temperatures were tested in the one-pot
three-component synthesis of 4a under MW irradiation.
Among different polar solvents, such as ethanol, acetic
acid, glycol, DMF and water, the best yield was found
when glycol was employed (Table 1, entries 1–5, 9 and 11).
To further optimize the reaction temperature, the reac-
tions were carried out at temperatures ranging from 90 to
160 °C. It was found that, as the reaction temperature
was increased, the yield of 4a was improved and the reac-
tion time was shortened. The yields plateaued when tem-
perature was further increased to 150 and 160 °C (Table
1, entries 5–10). Therefore, 150 °C was chosen as the reac-
tion temperature for all further microwave-assisted
reactions.
Under these optimal microwave experimental conditions
(150 °C, glycol), a series of new 30-indolyl substituted
R3
HN
O
CN
O
R2
CN
O
CH3COONH4(20 mol %)
NC
+
+
R3
R1
R2
H
EtOH reflux
N
R1
H
N
H
Scheme 1.
R'
O
CN
CN
MW
150 oC
+
+
N
R'CHO
N
NH2
N
N
N
N
glycol,
R
H
Ph
Ph
N
R
H
1
2
4
3
Scheme 2.

2580
S.-L. Zhu et al. / Tetrahedron Letters 49 (2008) 2578–2582
Table 2
Synthesis of 4 under microwave irradiation and conventional heating at 150 °C
Entry
Product
R0
R
Time
Yielda (%)
MWb
Mp (°C)
MWb (min)
CHc (h)
CHc
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
a
4a
4b
4c
4d
4e
4f
4-ClC6H4
C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
8
10
10
10
9
8
8
6
10
12
12
12
13
14
15
15
7
10
11
12
12
10
8
8
8
9
10
11
11
14
14
16
16
8
87
82
80
82
86
85
85
88
76
78
80
80
75
70
74
67
88
83
75
86
72
70
65
72
72
74
75
73
69
68
71
72
65
60
62
60
75
73
68
72
>300
>300
>300
>300
284–286
276–277
>300
2-ClC6H4
2-BrC6H4
3-BrC6H4
3-NO2C6H4
4-NO2C6H4
4-FC6H4
4-CH3C6H4
4-CH3OC6H4
4-OHC6H4
4-OH-3-OCH3C6H3
2-Thienyl
2-Pyridyl
n-Propyl
n-Butyl
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4g
4h
4i
>300
296–297
274–275
>300
>300
>300
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
>300
297–299
275–276
298–299
>300
>300
198–200
2-CH3
4-CH3
7-CH3
2-Ph
8
8
8
8
8
8
Isolated yields.
b
c
The time and yields under microwave irradiation (MW) conditions.
The time and yields under conventional heating (CH) conditions.
NH2
O
O
CN
CN
Ph
N
N
+
R'CHO
3
N
H
N
R
R'
R
H
1
2
5
R'
R'
NC
NC
-H2O,
-2H
N
N
N
N
N
O
H2N
Ph
N
Ph
R
H
N
4
R
H
Scheme 3.
Evidence supporting this proposed mechanism came
To further expand the scope of the present method, the
from the observation that the intermediate 5 can be
isolated from the reaction, and when 5a was prior pre-
pared19 and subsequently reacted with 3 under the same
condition, the expected product 4a was obtained in a yield
similar to that obtained in one-pot reaction (Scheme 4).
Moreover, the synthesis of 4 was performed under both
MW and classical heating at 150 °C. The reactions were
efficiently promoted by MW with increased yields, and
the reaction times were strikingly shortened to minutes
from hours required under traditional heating. Therefore,
microwave irradiation exhibited several advantages over
the conventional heating by significantly reducing the
reaction time and improving the reaction yield, owing to
a specific nonthermal microwave effect.
replacement of 3-methyl-1-phenyl-1H-pyrazol-5-amine
with naphthylamine was examined. Under the above-opti-
mized MW conditions, the reactions were performed
smoothly and a variety of the desired products 2-(1H-
indol-3-yl)-4-arylbenzo[h]quinoline-3-carbonitrile 7 were
obtained in moderate to good yields (Scheme 5 and Table
3). However, when this three-component reaction was car-
ried out in oil bath condition, only little amount of desired
product 7 was isolated (yield: <20%), accompanied by large
amount of condensation product 5 in 70% yield.
In this study, the products were characterized by melting
point, IR, NMR and HRMS spectral data. Furthermore,
the structures of 4c and 7d were established by X-ray crys-
tallographic analysis (Figs. 2 and 3).20

S.-L. Zhu et al. / Tetrahedron Letters 49 (2008) 2578–2582
2581
Cl
NC
O
MW
CN
+
N
Cl
N
glycol
N
NH2
N
N
N
H
Ph
Ph
N
H
5a
3
4a
Scheme 4.
O
Ar
CN
NH2
NC
MW
glycol, 150 oC
+
ArCHO
+
N
N
H
N
H
1
2
6
7
Scheme 5.
Table 3
Synthesis of 7 under microwave irradiation
Product
Ar
Time (min)
Yielda (%)
Mp (°C)
7a
7b
7c
7d
7e
a
C6H5
12
12
14
15
15
67
72
70
72
68
>300
>300
>300
>300
>300
4-ClC6H4
4-BrC6H4
4-CH3C6H4
3-BrC6H4
Isolated yields.
Fig. 3. The crystal structure of 7d.
Acknowledgments
This work was partially supported by the Key Labora-
tory of Organic Synthesis of Jiangsu Province at Suzhou
University (No. S8109108), the Natural Science Founda-
tion of Jiangsu Province (No. BK2006048) and the
National Science Foundation of China (No. 20472062,
20672079), Nature Science Key Basic Research of Jiangsu
Province for Higher Education (No. 06KJA15007,
05KJB150116), Jiangsu Provincial Key Laboratory of Fine
Petrochemical Technology (KF0402), and a Research
Grant from the Innovation Project for Graduate Student
of Jiangsu Province.
Fig. 2. The crystal structure of 4c.
In summary, we have demonstrated a simple and effi-
cient approach for synthesis of highly functionalized indole
derivatives via one-pot three-component reactions. This
method incorporates both indole and pyrazolo[3,4-b]pyr-
idine or benzo[h]quinoline moieties into a single molecule.
In view of those molecules having either functionality,
these novel compounds may potentially have enhanced
biological activities.
References and notes
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(4-Chlorophenyl)-6-(1H-indol-3-yl)-3-methyl-1-phenyl-1H-pyrazolo[3,
4-b]pyridine-5-carbonitrile: Light yellow solid; mp: >300 °C; IR
(KBr): m 3303, 3063, 2971, 2199, 1613, 1481, 1304, 1234, 1041,
Matsumura, K.; Kawasaki, T. Tetrahedron 2005, 61, 2309; (d)
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740 cmꢀ1 1H NMR (400 MHz, DMSO-d6): d 11.92 (br s, 1 H, NH),
;
8.44–8.45 (m, 2H), 8.27 (d, J = 8.0 Hz, 2H), 7.73–7.74 (m, 4H), 7.62
(t, J = 7.6 Hz, 2H), 7.56 (d, J = 8.0 Hz, 1H), 7.42 (t, J = 7.2 Hz, 1H),
7.26 (t, J = 7.6 Hz, 1H), 7.20 (t, J = 7.6 Hz, 1H), 2.06 (s, 3H, CH3);
13C NMR (100 MHz, DMSO-d6): d 155.3, 151.2, 149.9, 143.6, 138.2,
136.3, 134.6, 132.7, 130.8, 129.4, 129.1, 128.5, 126.4, 125.7, 122.6,
121.4, 121.0, 120.9, 118.5, 112.6, 112.1, 111.4, 99.5, 14.3; HRMS
found: m/z 469.1265 (M+), calcd for C28H18N5Cl: M, 459.1251.
Preparation of compound 4a under conventional heating condition:
An equimolar (1 mmol) mixture of 3-cyanoacetyl indole 1, aldehyde 2,
and 5-amino-3-methyl-1-phenylpyrazol 3 was subsequently intro-
duced into a 25 mL flask, and stirred in 5 mL glycol at 150 °C (oil
7. Sun, H. H.; Sakemi, J. J. Org. Chem. 1991, 56, 4307.
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bath temperature) for a given time. The subsequent work-up
procedure was the same as in the microwave irradiation reactions.
Preparation of compound 5a. A mixture containing 3-cyanoacetyl
indole (1 mmol), 4-chlorobenzaldehyde (1 mmol), and glycol (2.0 mL)
was introduced into a 10 mL reaction vial and irradiated at the
maximum power of 250 W and 150 °C for 10 min. The reaction
mixture was allowed to cool to room temperature. The solid was
filtered and washed with water and EtOH (95%) to give pure product
5a. 3-(4-Chlorophenyl)-2-(1H-indole-3-carbonyl)acrylonitrile: Yellow
solid; mp: 224–225 °C; IR (KBr): m 3356, 3175, 3056, 2929, 2221, 1602,
1512, 1489, 1433, 1312, 1234, 1182, 1091, 1013, 867, 823, 751, 645,
9. Takahashi, S.; Matsunaga, T.; Hasegawa, C.; Saito, H.; Fujita, D.;
Kiuchi, F.; Tsuda, Y. Chem. Pharm. Bull. 1998, 46, 1527.
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N’Diaye, Y.; Pietra, F. Helv. Chim. Acta 1999, 77, 1994.
11. Bergmann, T.; Schories, D.; Steffan, B. Tetrahedron 1997, 53, 2055.
12. Kohmoto, S.; Kashman, Y.; McConnell, O. J.; Rinehart, K. L.;
Wright, A., Jr.; Koehn, F. J. Org. Chem. 1988, 53, 3116.
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Marta, M. J. Heterocycl. Chem. 1991, 28, 1301.
527 cmꢀ1 1H NMR (400 MHz, DMSO-d6): d 12.39 (br s, 1H, NH),
;
8.46 (s, 1H, CH), 8.23 (s, 1H, Indolyl-H), 8.17 (d, J = 6.8 Hz, 1H,
Indolyl-H), 8.05 (d, J = 8.4 Hz, 2H, Ar-H), 7.68 (d, J = 8.4 Hz, 2H,
Ar-H), 7.55 (d, J = 7.2 Hz, 1H, Indolyl-H), 7.26–7.31 (m, 2H, Indolyl-
H); 13C NMR (100 MHz, DMSO-d6): d 181.2, 150.6, 136.9, 136.8,
136.3, 131.9, 131.4, 129.3, 126.1, 123.6, 122.5, 121.4, 117.5, 113.5,
112.6, 112.1; HRMS found: m/z 306.0564 (M+), calcd for
14. (a) Tietze, L. F. Chem. Rev. 1996, 96, 115; (b) Do¨mling, A.; Ugi, I.
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C
18H11N2OCl: M, 306.0560.
16. Sun, C.; Ji, S.-J. Tetrahedron Lett. 2007, 48, 8987.
Preparation of compound 7b. In a 10 mL EmrysTM reaction vial, 3-
cyanoacetyl indole (1 mmol), 4-chlorobenzaldehyde (1 mmol), naph-
thalen-1-amine 6 (1 mmol) and glycol (2 mL) were mixed and then
capped. The mixture was irradiated for the given time at the
maximum power of 250 W and 150 °C. Upon completion, monitored
by TLC, the reaction mixture was allowed to cool to room
temperature and then poured into cold water (100 mL). The solid
product was filtered, washed with EtOH (95%). The product was
purified by column chromatography (silica gel, 200–300 mesh, PE–
acetone, 4:1) to give (7b). 4-(4-Chlorophenyl)-2-(1H-indol-3-yl)benzo-
[h]quinoline-3-carbonitrile: Brown solid; mp: >300 °C; IR (KBr): m
3252, 3058, 2979, 2187, 1608, 1519, 1479, 1399, 1241, 951, 807, 755,
17. (a) Ji, S.-J.; Wang, S.-Y.; Zhang, Y.; Loh, T.-P. Tetrahedron 2004, 60,
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´
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Chem. 2005, 25, 987.
19. Typical experimental procedure:
600 cmꢀ1 1H NMR (400 MHz, DMSO-d6): d 11.86 (br s, 1H, NH),
;
Preparation of compound 4a under MW condition: The reaction was
performed in a monomodal EmrysTM Creator from Personal Chem-
istry, Uppsala, Sweden. In a 10 mL EmrysTM reaction vial, 3-cyano-
acetyl indole (1 mmol), 4-chlorobenzaldehyde (1 mmol), 3-methyl-1-
phenyl-1H-pyrazol-5-amine 3 (1 mmol) and glycol (2 mL) were mixed
and then capped. The mixture was irradiated for the given time at the
maximum power of 250 W and 150 °C. Upon completion, monitored
by TLC, the reaction mixture was allowed to cool to room
temperature and then poured into cold water (100 mL). The solid
product was filtered, washed with EtOH (95%). The product was
9.35 (d, J = 10.0 Hz, 1H), 8.68 (d, J = 10.4 Hz, 1H), 8.45 (s, 1H,
Indolyl-H), 8.07–8.09 (m, 2H), 7.88–7.90 (m, 4H), 7.33–7.52 (m, 6H);
13C NMR (100 MHz, DMSO-d6): d 153.4, 152.9, 149.8, 138.3, 136.4,
134.8, 132.6, 132.4, 130.6, 129.2, 128.8, 128.5, 127.8, 127.0, 126.8,
126.5, 126.1, 122.9, 122.4, 122.2, 120.7, 119.2, 118.2, 112.2, 111.9,
104.5; HRMS found: m/z 429.1019 (M+), calcd for C28H16N3Cl: M,
429.1033.
20. Crystallographic data for the structures of 4a and 7d reported in this
paper have been deposited at the Cambridge Crystallographic Data
Centre with No. CCDC-632865 and 631307, respectively.