Ring-Opening of Indoles: An Unconventional Route for the Transformation of Indoles to 1 H-Pyrazoles Using Lewis Acid

Article abstract of DOI:10.1021/acscombsci.8b00071

An unusual transformation of indoles to pyrazoles via an aromatic ring-opening strategy has been developed. The salient feature of this strategy involves the C2-N1 bond opening and concomitant cyclization reaction of the C2=C3 bond of the indole moiety with the tosylhydrazone, which proceeds under transition-metal and ligand free conditions. This ring-opening functionalization of indoles provides a wide scope of differently substituted pyrazoles.

Full text of DOI:10.1021/acscombsci.8b00071

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Letter
Ring-opening of Indoles: An Unconventional Route for the
Transformation of Indoles to 1H-Pyrazoles using Lewis Acid
Subhankar Panda, Nirmalya Pradhan, and Debasis Manna
ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.8b00071 • Publication Date (Web): 10 Sep 2018
Downloaded from http://pubs.acs.org on September 11, 2018
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Ring-opening of Indoles: An Unconventional Route
for the Transformation of Indoles to 1H-Pyrazoles
using Lewis Acid
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Subhankar Panda, Nirmalya Pradhan and Debasis Manna*
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
KEYWORDS: Cyclization • indole • Lewis acid • 1H-pyrazole • ring-opening.
ABSTRACT: An unusual transformation of indoles to pyrazoles via an aromatic ring-opening
strategy has been developed. The salient feature of this strategy involves the C2-N1 bond
opening and concomitant cyclization reaction of the C2=C3 bond of the indole moiety with the
tosylhydrazone which proceeds under transition-metal and ligand free conditions. This ring-
opening functionalization of indoles provides a wide scope of differently substituted pyrazoles.
The indole scaffold represents one of the most important structural subunits present in various
pharmaceutically active and naturally occurring products. Conventional ways of indole
functionalization follow nucleophilic and electrophilic reactions in which the indole scaffold
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1-3
retains its aromaticity. Transition-metal catalyzed C-H functionalization of both the benzene
and pyrrole rings of indole has also been widely studied and considered as one of the fascinating
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strategies for arene functionalization in modern chemistry. Ring-opening functionalization of
indole is uncommon and more challenging. The aromaticity of indole restricts such ring-opening
functionalization strategy. Recently, unconventional opening of C2-N1 bond of indoles was
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described (Scheme 1a).
On the other hand, the usage of C2=C3 bond as electrophile for the
in-situ cyclization reaction followed by ring-opening of indole is unknown.
In our previous report, we described the regioselective synthesis of 1H-pyrazoles through the
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2
coupling reaction between the ambiphilic tosylhydrazones and alkenes or alkynes. Other
references also describe that the aryl hydrazones are an important structural unit for the
13-24
transformation of alkene or alkyne to pyrazoles.
We and other research groups demonstrated
that tosylhydrazone have tremendous application in synthetic organic chemistry, especially in
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2, 25-26
cyclization reactions. However, its reactivity towards indole has not been greatly explored.
Recent studies described that the reaction of tosylhydrazone with indole favors N-alkylation or
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N-vinylation of indole, rather than cyclization reactions with C2=C3 bond (Scheme 1b).
We
hypothesized that C2-N1 bond opening could be one of the driving forces for C2=C3 bond
activation.
It is well documented that the carbonyl and sulfonyl groups form stable adducts with Lewis
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0
acids. Hence, we judiciously introduced the carbonyl or sulfonyl containing directing groups
like acyl, benzoyl and tosyl at the N1-position of indoles assuming that the complexation
between Lewis acids and these directing groups would induce the C2-N1 bond opening (Scheme
1
0, 29, 31-40
1
c).
Therefore, the decrease in electron density of the C2=C3 bond would lead to a
cyclization reaction with tosylhydrazones leading to the formation of pyrazoles. They are known
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Scheme 1. C2-N1 bond opening and the reactivity of indole towards hydrazone
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to display a wide spectrum of biological activities including, anti-microbial, anti-fungal, anti-
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tubercular, anti-inflammatory, anti-convulsant, anti-cancer, anti-viral, neuroprotective.
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pyrazole is a versatile synthetic intermediate and potent medicinal scaffold.
The synthesis of
pyrazoles still continues to attract considerable attention because of its applications in
pharmaceutical and agrochemical industries. We reveal herein transition-metal and ligand free
ring-opening functionalization and regioselective transformation of indole to 1H-pyrazoles. The
product, 1H-pyrazole was obtained with moderate to excellent yield. The inhibitory activities of
these synthesized compounds were performed against purified human immunosuppressive
enzyme indoleamine 2,3-dioxygenase 1 (IDO1).
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Our initial investigations commenced with the aim of enhancing the electrophilicity of C2=C3
bond of the indole moiety, so that the C2-N1 bond opening would become more facile leading to
the annulation reaction with tosylhydrazone. For this purpose, we decided to study the reaction
under mild acidic conditions using 1-acyl-1H-indole (1{1}) and tosylhydrazone (2{7}) as model
substrates. To our delight, tosylhydrazone (2{7}) in the presence of BF .OEt (catalytic amount)
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provided the regioisomeric 1H-pyrazole (3{1,7} under ambient temperature (Table 1, entry1). As
expected, the model reaction in the absence of BF .OEt failed to provide the desired product
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even at higher temperature (Table 1, entry 2). This model reaction was also performed at higher
temperatures and with different equivalents of BF .OEt . Reaction at 50 °C with 0.3 equivalents
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of BF .OEt provided the target product in higher yield (Table 1, entry 4). Other Lewis and
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Bronsted acids such as AlCl , FeCl , Zn(OTf) , I , AcOH, TsOH, TfOH and TFA were found to
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be ineffective (Table 1). Solvent screening was also carried out, but dichloroethane (DCE)
remained the best solvent for successful formation of pyrazole from indole (Table 1). This ring-
opening reaction was unsuccessful under the basic reaction conditions.
These optimized reaction conditions for the unusual indole ring-opening and cyclization
reaction encouraged us to explore the scope and limitations of the synthesis of pyrazoles from
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the corresponding indoles (Figure 1). We hypothesized that the transformation of an indole into a
pyrazole strongly depends on the C2-N1 bond opening proficiency of the indoles. The electronic
effects of the substituents could play an important role for the formation of desired pyrazoles.
Reaction between 1-acyl-1H-indole and electron-neutral phenyl tosylhydrazone provided the
targeted pyrazole with moderate yield (55%, Table 2, 3{1,1}).
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a
Table 1. Optimization of the reaction conditions for the synthesis of 1H-pyrazole (3{1,7})
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Entry Acid / Base (equiv.) Solvent
Time (h) Temperature
°C)
Yield (%)
(
c
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BF .OEt (0.3)
DCE
DCE
DCE
DCE
DCE
DCE
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DCE
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RT
RT → 50
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-
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d
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BF .OEt (0.1)
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BF .OEt (0.5)
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BF .OEt (0.3)
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AlCl (0.3)
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FeCl (0.3)
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Iodine (0.3)
AcOH (0.3)
TsOH (0.3)
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Et N (0.3)
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Cs CO (0.3)
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All the reactions were performed using 0.1 mmol (1 equiv) of 1{1} and 0.11 mmol (1.1 equiv)
b
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of 2{7} under acidic conditions. Isolated yield of product (1H-pyrazole). 5-60% starting
material 1{1} and 2{7} was recovered. 100% Starting materials 1{1} and 2{7} were recovered.
d
Alternation in the electronic environments on both indole and hydrazone moieties affected the
reaction yield under the optimized reaction conditions. The aryl-tosylhydrazones bearing
electron-rich substituents such as –Me and –OMe produced the corresponding pyrazoles in lower
yields (3{1,2}, 3{1,3}). While, the presence of an electron-deficient aryl-tosylhydrazone allowed
the formation of corresponding pyrazoles (3{1,4-8}, 3{2,7}, 3{3,7}) in higher yields. It is
important to mention that the synthesis of pyrazoles containing the nitro-, cyano- and halogen
groups are highly significant as these functional groups could be deployed in transition-metal
catalyzed cross-coupling reactions. The use of tosylhydrazones with such reactive groups was
well tolerated in the presence of BF .OEt thus providing the opportunity for further
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H-indoles 1:
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{4}
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{5}
1{1}
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Tosylhydrazones 2:
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{7}
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Figure 1. Substrates used in the synthesis of pyrazole derivatives
functionalization of these pyrazoles. Interestingly, tosylhydrazone with alkene as substituent
shows high selectivity for the synthesis of regioselective pyrazole 3{1,9} with only a moderate
yield. Reactions with substituted indoles also displayed quite interesting results. The presence of
electron-withdrawing group (–NO ) in the benzene ring of the indole produced the corresponding
2
pyrazole in higher yield. However, the desired pyrazole was obtained in lower yields when an
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electron-donating group (-OMe) was present in the benzene ring of the indole. The reaction of
methyl substituted (at C2 or C3 position) indoles was unsuccessful. A combination of both
a
Table 2. Substrate scope for the synthesis of 1H-pyrazoles .
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3{1,1}; 55%
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{1,2}; 52%
{1,6}; 88%
3{1,3}; 48%
3{1,4}; 90%
3{1,8}; 86%
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{1,5}; 85%
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{1,7}; 92%
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{1,9}; 60%
{3,7}; 94%
3{2,2}; 42%
3{3,13}; 45%
3{3,2}; 65%
3{4,3}; 42%
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{2,7}; 65%
{4,1}; 52%
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{4,10}; 82%
{5,2}; 42%
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{4,11}; 85%
{5,12}; 74%
3{4,7};86%
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{4,5}; 78%
{5,1}; 48%
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3{5,10}; 74%
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{5,6}; 78%
3{5,11}; 80%
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{5,7}; 83%
a
All the reactions were performed using 0.2 mmol of 1{1-5} (1 equiv) and 0.22 mmol of 2{1-12}
1.1 equiv) in the presence of 0.06 mmol of BF .OEt (0.3 equiv) in 2 mL of DCE at 50 °C.
(
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electron deficient indole and tosylhydrazone provided the targeted pyrazoles with maximum
yield (3{3,7}). The reaction with tosylhydrazones bearing pyridine ring produced the target
pyrazole with only moderate yield. Unfortunately, the reaction of C2/C3-methyl substituted N-
acyl indole failed to provide the targeted pyrazole.
This uncommon reactivity of 1-acyl-indole with aryl-tosylhydrazone prompted us to extend
our heterocycles compound library using various N1-substituted indoles. The N1-benzoyl and
N1-tosyl indoles successfully participated in this ring-opening and concomitant cyclization
reaction. The reaction between N1-benzoyl and N1-tosyl indoles with both electron-rich and
electron-deficient aryl-tosylhydrazone showed similar reactivity trends (Table 2) as that with the
acyl group, but the yields of the desired pyrazoles were reduced to some extent. The XRD
analysis of the compound 3{4,5} confirmed the structure of the desired pyrazole (Figure S1 and
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Table S1). Unfortunately, N1-methyl and N1-benzyl containing indole or unsubstituted (N1-H)
indole failed to provide the corresponding pyrazole under these optimized reaction conditions
(most of the starting materials were recovered). The reaction between 1-acyl-1H-indole and the
acetophenone tosylhydrazone under the similar experimental conditions failed to provide the
target product.
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Based on the reported literatures and our results, we propose the following atypical
mechanistic pathways for the synthesis of 1H-pyrazoles from indoles (Scheme 2). The
complexation of Lewis acid (BF .OEt ) with the oxygen of N-acyl / N-benzoyl / N-tosyl indoles
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sequestrated the nitrogen lone-pair and allowing the activation of C2=C3 bond.
The
Scheme 2. Plausible coupling reaction of activated indoles with tosylhydrazones.
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formation of activated N-acyl indole (I) in the presence of BF .OEt was supported by the C
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NMR and 2D-NMR (HSQC) experiments (Figure S2 and S3). The unsuccessful ring-opening
reaction of alkyl or aryl substituted indoles also support this hypothesis. The nucleophilic attack
of the NH of Lewis acid-tosylhydrazone complex at the C2 of activated indole would lead to the
formation of intermediate IIA followed by the cleavage of C2-N1 bond (Path A).46 Isolation of
pyrazole 3{14} from compound 2{14} supported both the nucleophilic attack at the C2-centre
and C2-N1 bond cleavage (Scheme 3). However, we failed to isolate any intermediates,
indicating stronger reactivity of the reactants/intermediates under the optimized experimental
reaction conditions. The intramolecular cyclization of intermediate IIA would lead to the
formation of intermediate IIIA which could produce the desired pyrazoles through
aromatization.
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The other probable pathway (Path B) describe the 3+2 cycloaddition reaction between the
tosylhydrazone, (complexed with BF .OEt ) and activated indole (intermediate I). Then the
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aromatization through cleavage of indole ring of intermediate IIIB would lead to the desired
pyrazole. However, the intramolecular cyclization of compound 2{14} failed to produce the
desired pyrazole 3{14}′ indicating the preference for Path A over Path B mechanistic pathway
The cyclization reaction strongly depends on the electrophilic nature of imine bond of
hydrazone. The withdrawal of electron density from the C=N bond of the hydrazones should be
facile for the electron deficient aryl group. Hence, the cyclization reactions for hydrazones with
electron withdrawing groups are more effective in comparison with the electron rich hydrazones.
We also performed the intramolecular cyclization of compound 2{15}, which is the reduced
product of compound 2{14}. However, the compound 2{15} also produced the pyrazole 3{14},
which could be due to the oxidized aromatization.
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Scheme 3. Plausible mode of reaction of compound 2{14} for the synthesis of 1H-pyrazole
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The mechanistic studies suggested that the ring opening reaction is directed by the complex
formation ability of the N-acyl (or tosyl or benzoyl) indole with the Lewis acid. The formation of
complex of the Lewis acid (BF .OEt ) with the –C=O (or sulphonyl) group of the protected
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indole results in the cleavage of C2-N1 bond of the indole ring. The absence of such carbonyl or
sulphonyl group in N1-methyl, N1-benzyl or unsubstituted (N1-H) indole failed to undergo
through the ring opening and concomitant cyclization reaction with tosylhydrazone.
Interestingly, these observations also demonstrate the importance of carbonyl or sulphonyl group
at N1 position of the indole. We hypothesize that the formation of intermediate IIIA requires the
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presence of H-atom at the imine carbon for the final aromatization reaction. For this reason, the
hydrazone of acetophenone failed to produce the target product.
To demonstrate further the synthetic utility of these pyrazoles, we performed iodination
reactions. The newly generated amide or sulphonamide group can act as powerful directing
groups for the functionalization of the C5-position of pyrazoles (Table 3). To our delight, the
selective C-H iodination of the pyrazole over the two other aromatic rings was performed with
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excellent yield (4{1,2}, 4{4,11}, 4{4,7} and 4{5,11}; 40 – 98%). The carbon- iodine bond has
47-48
tremendous synthetic applications in modern chemistry.
The presence of sulphonamide and
benzamide with the aryl ring of pyrazoles shows better result in comparison with the acetamide
group for this iodination reaction, which indicates the importance of these directing groups. The
directing group of compound 3{4,11} was deprotected under basic condition using ethanolic
solution at high temperature (Scheme 4). The free amine group of compound 4{4,11} can be
49
utilized for further functionalization.
Table 3. Synthetic applications of this novel class of pyrazoles.
4
{1,2}; 40%
4{4,7}; 77%
4
{4,11}; 75%
4{5,11}; 98%
a
All the reactions were performed using 1 equiv of 3{1,2} or 3{5,11} or 3{4,11} or 3{4,7}, 1
equiv. of iodine, 2 equiv. of KI and 2 equiv. of K CO in the presence of 0.3 equiv. of BF .OEt
2
3
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in 1 mL of DMSO at 100 °C.
Synthesis of substituted pyrazoles using such quick and easy to perform mild reaction
conditions is a part of our on-going heterocyclic drug discovery program for cancer
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immunotherapy. Immunotherapeutic approach is being considered as one of the most promising
50-51
approaches in the battle against cancer.
In this regard, we explored the IDO1 inhibitory
activity of these compounds (HPLC purified). The IC values were measured against purified
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human IDO1 enzyme using standard spectrophotometric method (Table 4 and S2). Compounds,
{2,2} (IC50 = 40.33 μM) and 3{5,1} (IC50 = 25.0 μM) showed moderate IDO1 inhibitory
activity, which could be due to the presence of substituted aryl containing pyrazole and
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2-53
sulfonamide moieties.
Nonetheless, the inhibitory activity study suggests that this pyrazole
scaffold can be used as the lead compound and its modifications may lead to the development of
potent IDO1 inhibitors with favorable biochemical and biophysical properties.
Scheme 4. Removal of the directing group
In summary, we described an unusual ring-opening and cyclization reaction of a stable
aromatic heterocyclic compound. This transition-metal and ligand free synthesis of pyrazoles
from the corresponding indoles through the C2-N1 bond cleavage is unknown. Complexation of
the Lewis acid with indoles and tosylhydrazones could be the driving force for this reaction. This
method could be a useful alternative to the existing methods for regioselective synthesis of
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pyrazoles. The inhibitory activities of these synthesized pyrazoles were tested against immune
suppressive enzyme, IDO1. The preliminary results suggest that the pyrazole moiety can be used
as synthetically amenable lead for future development of IDO1 inhibitors.
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Table 4. Inhibitory activity of the 1H-pyrazoles against purified human IDO1 enzyme.
Compound
IDO1
IC (μM)
Compound
IDO1
IC (μM)
a
a
50
50
3
3
3
{1,4}
{2,2}
{4,3}
84.50 ± 3.6
40.33 ± 0.3
74.17 ± 2.3
3{5,1}
3{5,2}
3{5,6}
25.0 ± 3.5
115.5 ± 5.8
96.8 ± 5.8
a
IC values are the mean of three independent assays.
50
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at
DOI:
Spectroscopic data for all compounds; crystallographic information for compounds 3{4,5}
(PDF)
X-ray data for compounds 3{4,5} (CIF).
AUTHOR INFORMATION
Corresponding Author
*
E-mail: dmanna@iitg.ernet.in
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Author Contributions
The manuscript was written through contributions of all authors.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
We are thankful to Science and Engineering Research Board, Govt. of India
(EMR/2016/005008) for financial support; CIF, IIT Guwahati for instrumental support; Dr.
Sreeparna Das and Mr. Arup Tarai from IIT Guwahati for technical support.
ABBREVIATIONS
IDO1, indoleamine 2,3-dioxygenase; DCE, dichloroethane.
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Ring-opening: Lewis acid catalyzed ring-opening of the indoles afforded regeoselective
pyrazole in the presence of tosylhydrazones. In-situ cyclization after the cleavage of C2-N1 bond
of the indoles proceeds in the absence of any transition-metal and ligand.
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Ring-opening: Lewis acid catalyzed ring-opening of the indoles afforded regeoselective pyrazole in the
presence of tosylhydrazones. In-situ cyclization after the cleavage of C2-N1 bond of the indoles proceeds in
the absence of any transition-metal and ligand.
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C2-N1 bond opening and the reactivity of indole towards hydrazone.
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Plausible coupling reaction of activated indoles with tosylhydrazones.
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Plausible mode of reaction of compound 2{14} for the synthesis of 1H-pyrazole.
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