A facile synthesis of 2H-indazoles under neat conditions and further transformation into aza-γ-carboline alkaloid analogues in a tandem one-pot fashion

Article abstract of DOI:10.1039/c4ra06838f

We have described a facile, microwave-assisted, catalyst-free and solvent-free approach to 2H-indazoles and further developed a robust tandem one-pot metal-free strategy for C-C bond formation at the C-3 position of 2H-indazoles leading to a unique class

Full text of DOI:10.1039/c4ra06838f

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A facile synthesis of 2H-indazoles under neat
conditions and further transformation into aza-g-
carboline alkaloid analogues in a tandem one-pot
fashion†
Cite this: RSC Adv., 2014, 4, 34232
*
Shinde Vidyacharan, A. Sagar, N. C. Chaitra and Duddu S. Sharada
We have described a facile, microwave-assisted, catalyst-free and solvent-free approach to 2H-indazoles
and further developed a robust tandem one-pot metal-free strategy for C–C bond formation at the C-3
Received 9th July 2014
Accepted 22nd July 2014
position of 2H-indazoles leading to
a unique class of aza-g-carboline alkaloid analogues. This
DOI: 10.1039/c4ra06838f
straightforward expedient synthesis constitutes an interesting alternative to the existing conventional
transition metal catalyzed reactions.
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however, selective synthesis of 2H-indazoles is difficult.13,14
Hence, considering the potent bioactivities of compounds
Introduction
possessing a 2H-indazole core, development of newer and
economic methods for the selective synthesis of 2H-indazoles
is necessary. Even though few promising strategies have been
established for the selective synthesis of 2H-indazole, like (1)
Pd-catalyzed domino reaction of 2-halophenyl acetylenes with
hydrazines,15 (2) reaction of 2-chloromethylarylzinc reagents
and aryldiazonium salts,16 (3) Cu-catalyzed intramolecular
N–N bond formation of 2-azidobenzaldehyde and primary
amine,17 (4) Fe-catalyzed N–N bond formation of 2-azido-
phenyl ketoximes,18 (5) [3 + 2] cycloaddition of arynes and
sydnones,19 (6) DBSA catalysed indazole synthesis,20 (7)
reductive cyclization of 2-nitrobenzylamines induced by low-
valent titanium reagent,21a (8) synthesis of 2-(cyclic amino)-2H-
indazoles21b most of these methods suffer from drawbacks
such as use of transition metal catalyst, longer reaction time,
requirement for an inert atmosphere, low yield, tedious
procedures, generation of large amount of toxic metal-con-
taining waste, and the use of bronsted acids, stoichiometric or
relatively expensive reagents.
Heterocycles are widely distributed in natural compounds and
have a vast number of applications.1 Among them, nitrogen-
containing heterocycles play a key role as alkaloids in the plant
kingdom and have biological signicance.2 Nowadays, the
demand for developing alternative routes that are environ-
mentally benign for the synthesis of novel heterocyclic mole-
cules is increasing. Several ways for achieving this includes new
and innovative methodologies using neither solvent nor catalyst
and employing alternative energy inputs, such as microwave,
mechanochemical mixing, high-speed ball milling and sonica-
tion etc.3 Among these, microwave technology is growing rapidly
over the decade, within the pharmaceutical and academic
domains.4 Therefore, there is a great demand for developing
facile, efficient and non-polluting synthetic procedures to build
novel heterocycles reducing the use of organic solvents and
toxic reagents.5
Indazoles being N-heterocycles, are gaining
a lot of
importance in the eld of medicinal and organic chemistry as
they exhibit a broad spectrum of pharmacological and bio-
logical activities.6 2H-Indazoles are widely used in pharma
sector than 1H-indazoles due to their potent bioactivities like
anti-tumor,7 anti-microbial,8 anti-inammatory,9 HIV protease
inhibition10 etc. For example, drugs like MK-4827 (anti-
cancer),11 and pazopanib (tyrosine kinase inhibitor),12 incor-
porate this basic scaffold. A number of synthetic methods have
been reported in the literature to build indazole scaffolds,
Apart from this, there are few microwave-assisted reactions
known in the literature, which suffer from draw backs such as
23
use of triethylphosphite,22 MoO2Cl2(dmf)2 and Ph3P23 as
catalyst etc. Thus there is a great demand for developing effi-
cient and green protocol for the synthesis of 2H-indazole.
As part of our research program on the development of
sustainable chemistry for the synthesis of novel heterocylic
molecules,24 we envisaged to develop an efficient and green
protocol for the synthesis of 2H-indazole via a consecutive
formation of C–N and N–N bonds without catalyst and solvent.
To the best of our knowledge there are no such reports so far in
the literature.
Department of Chemistry, Indian Institute of Technology, ODF-Estate,
Yeddumailaram-502205, Medak District, Telangana, India. E-mail: sharada@iith.
ac.in
† Electronic supplementary information (ESI) available. CCDC 994876. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra06838f
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Table 1 Investigation of optimum reaction condition for the synthesis
of 2H-indazole 3aa
Entry
Temperature (ꢀC)
Time (min)
Yieldb (%)
Fig. 3 Single X-ray crystal structure of 3k (ORTEP diagram). Thermal
ellipsoids are drawn at 25% probability level CCDC 994876.†
1
2
3
4
5
6
40
70
90
100
120
140
60
60
60
50
40
20
0
10
50
94
95
97
Table 2 Reaction scope of amine in 2H-indazole 3b,c formationa
a
b
Reaction conditions: 1a (1 mmol), 2a (1 mmol). Isolated yield aer
recrystallization using EtOH. MW condition: 200 psi, 300 W, 3–5 min
run time.
Fig. 1 Biologically active molecules having 2H-Indazole core and g-
carboline alkaloid.
Results and discussion
To achieve our target, we performed the reaction between 2-
azidobenzaldehyde and aniline under catalyst- and solvent-free
conditions at various temperatures. To begin with, the reaction
was performed at 40 ꢀC under microwave condition, which
failed to give expected product. Without losing hope we pro-
ceeded further by increasing the temperature gradually,
a
b
Reaction conditions: 1 (1 mmol), 2 (1 mmol). Isolated yield aer
c
recrystallization using EtOH. All the compounds were conrmed by
1H NMR, 13C NMR & HR-MS spectral analyses. Isolated yield aer
d
e
f
column chromatography. Reaction carried at 85 ꢀC. Further
conrmed by XRD (Fig. 3). MW condition: 200 psi, 300 W, 3–5 min
run time.
ꢀ
however, no satisfactory results were obtained up to 90 C. On
further increasing the temperature, to our surprise, excellent
yields (>90%) were obtained at 100 ꢀC, 120 ꢀC and 140 ꢀC, aer
recrystallization using EtOH. In order to check the scope of our
ꢀ
reaction, we chose 100 C as optimum condition (Table 1 and
Fig. 2).
To prove the efficiency and applicability of our protocol, we
treated 2-azidobenzaldehyde with various amines like aromatic,
heteroaromatic, aliphatic and benzylic amines, which offered
wide range of 2H-indazoles in excellent yields (86–97%)
(Table 2). To further demonstrate the scope of the reaction, we
turned our attention towards the synthesis of di- and
Fig. 2 Reaction temperature vs. yield of the 2H-indazole.
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Table 3 Scope of substituted 2-azidobenzaldehyde and amine in 2H- Table 4 Synthesis of indazolo[2,3-a]quinoxaline 6a,d
indazole 3b,c formationa
a
b
Reaction conditions: 1 (1 mmol), 2 (1 mmol). Isolated yield aer
c
recrystallization using EtOH. All the compounds were conrmed by
1H NMR, 13C NMR and HR-MS spectral analyses. Isolated yield aer
d
e
f
column chromatography. Reaction carried out at 85 ꢀC. Reaction
carried out at 60 ꢀC. MW condition: 200 psi, 300 W, 3–5 min run time.
trisubstituted 2H-indazoles. Accordingly, we treated various
substituted ortho-azidobenzaldehydes with amines to obtain
expected products in good to excellent yields (68–93%). To our
delight, we have got some interesting and attractive tricyclic 2H-
indazole compounds (3w and 3x) (Table 3).
a
Reaction conditions: 1 (1 mmol), 2 (1 mmol), 4 (1 mmol). b Stepwise.
Synthetic application
c
Isolated yields aer column chromatography. One-pot isolated
yields. d All the compounds were conrmed by H NMR, 13C NMR and
1
Functionalization of heteroarenes, especially indazoles via C–H
activation is a powerful alternative approach to the cross
coupling reactions resulting in privileged structural motifs
which are of great relevance in further modications into
diverse bioactive molecular scaffolds, which are of importance
HR-MS spectral analyses. MW condition: 200 psi, 300 W, 3–5 min run
time.
in agrochemicals and pharmaceuticals. As a result, extensive compounds, we envisaged to report an efficient, metal-free
efforts have been directed for C–C bond formation at C-3 posi- route for C-3 funtionalization. In this context we rationalized to
tion of indazoles followed by subsequent functionalization reach our target via Pictet–Spengler strategy by choosing
resulting in annulated indazoles, such as (1) direct arylation of bifunctional amine component (2) to react with 2-azido-
2H-indazole,25 (2) synthesis of annulated 2H-indazoles by benzaldehyde (1) under CF & SF conditions. This afforded us the
palladium-catalyzed direct arylation,26 (3) Pd(II)-catalyzed C-3 desired 2H-indazole (3) in very good yield which could be
arylation of indazoles,13a (4) zincation of indazoles using further explored for functionalization at C-3 position resulting
TMP2Zn and Negishi cross-coupling with aryl and heteroaryl in annulated product.
iodides;27 however each of the above methods employ either
To execute our idea, when we treated indazole (3) with 2-
expensive transition metal catalysts along with ligands, bases or methoxybenzaldehyde in 1% TFA in toluene at 110 ꢀC under
additives.28 Hence there exists a great challenge to contrive a microwave condition to our delight resulted in 6-(2-methoxy-
simple route for C–C bond formation at C-3 position of inda- phenyl)-5,6-dihydroindazolo[2,3-a]quinoxaline (5), which when
zoles resulting in annulated products.
oxidized with DDQ yielded (96%) 6-(2-methoxyphenyl)indazolo
In continuation of our interest in developing new and effi- [2,3-a]quinoxaline (6a). It is very appropriate to mention here
cient routes for the synthesis of diverse annulated heterocyclic the importance of g-carboline frame work, which is an integral
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alkaloid analogues through
reaction.
a
multicomponent coupling
Acknowledgements
We gratefully acknowledge Council of Scientic and Industrial
Research (CSIR), New Delhi and Indian Institute of Technology,
Hyderabad for nancial support. We also thank Dr T. K. Panda
and Dr G. P. Sanker for X-ray analysis. S.V.C. thank UGC and A.S
thank CSIR, New Delhi for the award of research fellowship.
Notes and references
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In summary, we have described a facile, microwave-irradiated,
catalyst-free and solvent-free approach to 2H-indazoles. We
have further successfully developed an expedient tandam one-
pot, metal-free route for the synthesis of aza-g-carboline alka-
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indazole, which is an interesting alternative to the exciting
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