Bronsted acidic ionic liquid catalyzed tandem reaction of 4-hydroxy-1-methyl-2-quinolone with chalcone: Regioselective synthesis of pyrano[3,2-c]quinolin-2-ones

Article abstract of DOI:10.1039/c4ra03221g

A new methodology has been developed for the synthesis of pyrano[3,2-c]quinolin-2-one derivatives by the tandem cyclization of 4-hydroxy-1-methyl-2-quinolone with chalcones using a catalytic amount of task specific ionic liquid [1-methyl-3-(4-sulfobutyl)imidazolium-4- methylbenzenesulfonate] under solvent-free conditions. The developed protocol is applicable for the construction of pyrano[3,2-c]quinolin-2-ones from easily accessible chalcones having aryl and hetero aryl substituents. This reaction possibly proceeds through Michael addition followed by cyclization. The feasibility of the catalyst recycling is also demonstrated. This method produced only water as the byproduct and represents a green synthetic protocol.

Full text of DOI:10.1039/c4ra03221g

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Brønsted acidic ionic liquid catalyzed tandem
reaction of 4-hydroxy-1-methyl-2-quinolone with
chalcone: regioselective synthesis of pyrano[3,2-c]
quinolin-2-ones†
Cite this: RSC Adv., 2014, 4, 23287
Received 10th April 2014
Accepted 15th May 2014
*
Avik Kumar Bagdi and Alakananda Hajra
DOI: 10.1039/c4ra03221g
www.rsc.org/advances
A new methodology has been developed for the synthesis of pyrano narrow substrates scope and are only applicable for the
[3,2-c]quinolin-2-one derivatives by the tandem cyclization of 4- synthesis of pyrano[3,2-c]quinoline-2,5-dione type derivatives.
hydroxy-1-methyl-2-quinolone with chalcones using
a catalytic In addition, these methodologies bear many drawbacks like
amount of task specific ionic liquid [1-methyl-3-(4-sulfobutyl)imida- requirement of high temperature, poor yield, use of toxic
zolium-4-methylbenzenesulfonate] under solvent-free conditions. solvents, generation of signicant amounts of waste materials,
The developed protocol is applicable for the construction of pyrano and special efforts for the preparation of the starting materials
[3,2-c]quinolin-2-ones from easily accessible chalcones having aryl etc. As such, it is still desirable to develop an efficient and
and hetero aryl substituents. This reaction possibly proceeds through selective protocol for the diversied synthesis of pyrano[3,2-c]
Michael addition followed by cyclization. The feasibility of the catalyst quinolin-2-one derivatives with broad substrate scopes under
recycling is also demonstrated. This method produced only water as environmentally friendly reaction conditions.
the byproduct and represents a green synthetic protocol.
a,b-Unsaturated ketones are easily accessible from readily
available aldehydes and ketones by the condensation reaction.
These are good Michael acceptor and have been used for the
construction of biologically important scaffolds by exploring
their bielectrophilic properties in cascade fashion.6 During the
studies on the construction of pyranocoumarin derivatives,6d we
envisioned that pyrano[3,2-c]quinolin-2-one derivatives might
be synthesized by the tandem reaction between 4-hydrox-
yquinolone and a,b-unsaturated ketones under suitable reac-
tion conditions (Fig. 2).
The pyranoquinolinone moiety is found in many alkaloids
which are generally seen in the plant family Rutaceae (Fig. 1).
These derivatives exhibit a wide range of biological activities
such as antibacterial, anticoagulant, antitumor, antihyperten-
sive, antifungal, anti-algal, anti-inammatory, and antimalarial
activities.1 In addition to these bioactivities, some of them show
cancer cell growth inhibitory activity and have been found to be
potential anti-cancer agents.2 These are also useful intermedi-
ates for the construction of azo dyestuffs.3 They are oen used
as synthetic precursors for the preparation of other natural
products such as dimeric quinoline alkaloids and other poly-
cyclic heterocycles.4
Several methodologies have been developed for the synthesis
of pyranoquinolinone derivatives.5 Most of the methodologies
were developed via the reaction of 4-hydroxyquinolin-2-one with
various 1,3-dielectrophiles like diethyl malonate,5b b-keto
ester,5c a,b-unsaturated aldehyde,5d arylidenemalononitrile/ary-
lidenecyanoacetate,5e etc. (Scheme 1). But most of these bear
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235,
India. E-mail: alakananda.hajra@visva-bharati.ac.in; Fax: +91 3463 261526; Tel: +91
3463 261526
† Electronic supplementary information (ESI) available: Experimental procedure,
E-factor calculations, crystallographic data (3g) and NMR spectra for all
compounds. CCDC 964212. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ra03221g
Fig. 1 Some alkaloids containing pyranoquinolinone moiety.
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Scheme 2 Synthesis of pyranoquinolinones catalyzed by Brønsted
acidic ionic liquid.
Scheme 1 Synthetic strategies for the pyranoquinolinones.
reaction was carried out in various organic solvents such as
DMSO, DMF, PEG, toluene, 1,4-dioxane, DCE, methanol,
ethanol etc. as well as in aqueous medium. The results are
summarized in Table 1. Either no formation or trace amount of
the desired product was observed in DMSO, DMF, H2O,
dioxane, DCE, toluene (Table 1, entries 1–3, 7–9). In case of
ethanol, methanol and PEG, the desired product was obtained
in 27%, 24%, 15% yields respectively (Table 1, entries 4–6). The
targeted product was obtained with maximum yield (82%)
ꢀ
Fig. 2 Plan of our work.
under solvent-free conditions at 110 C for 6 h (Table 1, entry
10). These results indicate the detrimental effect of the solvents
for this transformation. Next the effects of reaction time and
temperature on the reaction were also investigated. The yield of
the reaction did not improve by increasing the reaction time
from 6 h to 8 h (Table 2, entry 5) and the best operating reaction
temperature was found to be 110 ꢀC (Table 2, entry 3).
Increasing the temperature was not benecial (Table 2, entry 4)
while decreasing the reaction temperature decreased the yield
of the reaction (Table 2, entries 2 & 3). No signicant amount of
Ideally, running a reaction in the absence of solvent presents
the most environmentally friendly scenario, as a consequence
signicant interest has been focused on the development of
new protocols with solvent minimization. On the other hand,
catalytic applications of the ionic liquids have gained much
interest due to their environmental compatibility, reusability,
greater selectivity, operational simplicity, non-corrosiveness,
and ease of isolation.7 Combining the useful characteristics of
solid acids and mineral acids, Brønsted acidic ionic liquids
(BAILs) have been applied to replace traditional mineral liquid
acids such as hydrochloric acid and sulfuric acid in chemical
reactions. BAILs are more useful compare to the traditional
ionic liquids as these combine strong acidity with the use of
nonvolatile ionic liquids. Aer the report by Forbes and Davis,
phosphonium- and imidazolium ion-based ionic liquids
equipped with a pendant acidic sulfonic acid moiety have
drawn considerable attention as a Brønsted acidic ionic liquid.8
We are actively engaged with the catalytic application of ionic
liquids and molten salts in various organic transformations9
and our works also demonstrate that the imidazolium ion-
based Brønsted acidic ionic liquid show remarkable catalytic
activity in various syntheses.10 Herein we report a new, signi-
cantly simplied, and environmentally friendly approach to the
pyrano[3,2-c]quinolin-2-one derivatives having different aryl
substituents on the pyran ring under solvent- and metal-free
reaction conditions (Scheme 2).
Table 1 Screening of the solvents effectsa
Entry
Catalyst (10 mol%)
Solvents
Yieldsb (%)
1
2
3
4
5
6
7
8
9
10
BAIL-1
BAIL-1
BAIL-1
BAIL-1
BAIL-1
BAIL-1
BAIL-1
BAIL-1
BAIL-1
BAIL-1
DMSO
DMF
2
0
0
27
24
15
0
0
5
82
H2Oc
EtOHc
MeOHc
PEG
Dioxanec
DCEc
To nd out the suitable reaction conditions, we commenced
our study by choosing the reaction of 4-hydroxy-1-methyl-2-
quinolone with 1, 3-diphenyl-propenone as the model reaction
employing 10 mol% 1-methyl-3-(4-sulfobutyl)imidazolium-4-
methylbenzenesulfonate (BAIL-1) as the catalyst. Initially the
Toluene
Neat
a
Reaction conditions: mixture of 1 (0.5 mmol) and 2a (0.5 mmol) was
heated at 110 ꢀC for 6 h. Isolated yield. c Under reux.
b
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Table 2 Effect of temperature on the reactiona
mol% (Table 3, entries 8–10). Lewis acids such as Cu(OTf)2,
Zn(OTf)2, In(OTf)3, La(OTf)3 were not so effective and afforded
30 to 40% yields. Thus the optimal yield (81%) was obtained
when the reaction was carried out employing 2 mol% BAIL-1 at
Entry
Temp. (ꢀC)
Time
Yieldsb (%)
1
2
3
4
5
6
RT
80
100
110
110
120
24
6
6
6
8
Trace
51
75
82
82
ꢀ
110 C under solvent-free conditions for 6 h.
Aer getting the optimized reaction conditions, various
chalcones were reacted with the 4-hydroxy-1-methyl-2-quino-
lone to prove the general applicability of this methodology. The
results are summarized in Table 4. In the most cases the desired
products were obtained with good yields (3a–k). The chalcones
bearing electron donating substituents like –Me, –OMe (3b–f)
and electron withdrawing substituents such as –Cl, –NO2 (3h,
3i) reacted well to afford the corresponding pyranoquinolinone
derivatives. The acid sensitive group containing chalcones (3g,
3j) were unaffected under the present reaction conditions which
signify the mildness of the reaction conditions. The hetero aryl
substituted a,b-unsaturated ketone also produced the
6
83
a
Reaction conditions: mixture of 1 (0.5 mmol), 2a (0.5 mmol) and BAIL-
1 (10 mol%). b Isolated yield.
the desired product was formed when the reaction was carried
out at room temperature (Table 2, entry 1).
The effect of various ionic liquids were also studied which
are summarized in Table 3. The acidic ionic liquid BAIL-1
afforded the maximum yield (Table 3, entry 1). Neutral ionic
liquid (IL-2) was totally ineffective for this transformation
(Table 3, entry 2) and the BAIL-3 also produced the desired
products with comparable yields (Table 3, entry 3). The yields of
the reaction decreased when the chain length of the ionic liquid
(BAIL-4) was decreased (Table 3, entry 4).11 The methyl group at
the C-2 position of the imidazolium ring (BAIL-5) decreased the
yield of the reaction (Table 3, entry 5). This results suggest that
C2–H of imidazolium ring plays a signicant role to catalyze the
reaction probably through the formation of H-bonding.7f
However lower yield was obtained by employing 10 mol% p-
TsOH as the catalyst while AcOH was not effective at all (Table 3,
entries 6 & 7). The catalyst loading was also optimized. 2 mol%
acidic ionic liquid is the optimum amount as further decreasing
the catalyst loading decreased the yield while the signicant
increment of the yield was not observed with 5 mol% and 10
Table 4 Substrate scopesa,b
Table 3 Effect of various ionic liquids and catalyst loadinga
Entry
Catalyst
mol%
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
BAIL-1
IL-2
10
10
10
10
10
10
10
5
82
—
80
69
57
40
—
82
81
70
BAIL-3
BAIL-4
BAIL-5
p-TsOH
AcOH
BAIL-1
BAIL-1
BAIL-1
2
1
a
Reaction conditions: mixture of 1 (0.5 mmol) and 2 (0.5 mmol) was
a
b
Reaction conditions: 1 (0.5 mmol) and 2a (0.5 mmol) was heated at 110 heated at 110 ꢀC for 6 h in presence of 2 mol% BAIL-1. All are
b
ꢀC for 6 h. Isolated yield.
isolated yields and obtained as a racemic mixture.
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Scheme 3 Probable mechanism.
Fig. 3 The single crystal XRD structure of compound 3g.
adduct A is formed. The intermediate A on intramolecular
cyclization afforded the intermediate B which nally afforded
the pyranoquinolinone on subsequent removal of water. The
acidic ionic liquid activates the unsaturated ketone through
protonation of the carbonyl group and increased the electro-
philic character at the b-carbon which promote the Michael
adduct formation. In addition the acidic ionic liquid also helps
to facilitate the intramolecular cyclization step by the proton-
ation of carbonyl group of intermediate A.
corresponding pyranoquinolinone (3k) derivative having three
different heterocycles in moderate yield. However, the a,b-
unsaturated aldehydes (cinnamaldehyde and crotonaldehyde)
produced a mixture of unidentied product under the present
reaction conditions. All the prepared compounds have been
characterized by spectral and analytical data. Finally the single-
crystal X-ray analysis12 of 3g was performed to conrm the
substituent pattern on pyran moiety of pyranoquinolinone
(Fig. 3). It is interesting to note that two aryl substituents on
pyran moiety are orthogonal to each other.
Conclusions
Next, we turn our attention towards the recovery and reus-
ability of the catalyst. For this purpose, we have chosen the
reaction of 4-hydroxy-1-methyl-2-quinolone (1) with 1, 3-
diphenyl-propenone (2a) in presence of 2 mol% acidic ionic
In summary, we have successfully developed an efficient and
regioselective methodology for the synthesis of pyrano[3,2-c]
quinolin-2-one derivatives by the coupling of 4-hydroxy-1-
methyl-2-quinolone with chalcones catalyzed by Brønsted acidic
ionic liquid (BAIL-1). This tandem reaction possibly proceeds
through the Michael addition followed by annulation reaction
and only water is produced as the byproduct. A library of pyr-
anoquinolinone derivatives having variety of aryl and hetero
aryl substituents on the pyran moiety have been synthesized
employing this atom efficient methodology. Clean reaction,
ease of product isolation/purication, easily accessible reac-
tants, metal and solvent-free and environmentally friendly
reaction conditions are the notable advantages of the present
methodology and these features makes this procedure to be a
green synthetic protocol. We believe that our present method-
ology will open up a new route for the synthesis of bioactive
pyrano[3,2-c]quinolin-2-one derivatives.
ꢀ
liquid at 110 C as the model reaction. Aer completion of the
reaction water was added to the reaction mixture. The reaction
mixture was then ltered off and the ionic liquid was recovered
by evaporating the water. Pure product was obtained by
recrystallizing the residue from hot ethanol. The recovered ionic
liquid was reused for a subsequent fresh batch of the reaction
aer reactivation. The catalytic activity was found to be retained
up to a h cycle (Table 5). This newly developed protocol also
bears low E-factor13 of 0.29 and 0.30 in the cases of synthesizing
3a and 3f respectively (see ESI†).
The probable mechanism of the reaction is outlined in
Scheme 3. First step of this tandem reaction is either electro-
philic substitution or the Michael addition of 4-hydroxy-1-
methyl-2-quinolone to the a,b-unsaturated ketone and the
Acknowledgements
Table 5 Reuse of BAIL for the synthesis of pyranoquinolinonesa
A.H. acknowledges the nancial support from CSIR, New Delhi
(Grant no. 02(0168)/13/EMR-II). A.K.B thanks CSIR for his
fellowship.
Entry
Use
Yieldb (%)
1
2
3
4
5
1st
81
81
80
78
77
2nd
3rd
4th
5th
Notes and references
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