4-(Succinimido)-1-butane sulfonic acid as a Br?nsted acid catalyst for the synthesis of pyrano[4,3-b]pyran derivatives using thermal and ultrasonic irradiation

Article abstract of DOI:10.1016/S1872-2067(14)60307-7

4-(Succinimido)-1-butane sulfonic acid was shown to be an efficient and reusable Br?nsted acid catalyst for the synthesis of pyrano[4,3-b]pyran derivatives using thermal and ultrasonic conditions. The catalyst was prepared by mixing succinimide and 1,4-bu

Full text of DOI:10.1016/S1872-2067(14)60307-7

Chinese Journal of Catalysis 36 (2015) 728–733
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
4
-(Succinimido)-1-butane sulfonic acid as a Brönsted acid catalyst
for the synthesis of pyrano[4,3-b]pyran derivatives using thermal
and ultrasonic irradiation
Nader Ghaffari Khaligh *, Sharifah Bee Abd Hamid
Nanotechnology & Catalysis Research Centre, NANOCAT, University Malaya, Kuala Lumpur, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history:
4-(Succinimido)-1-butane sulfonic acid was shown to be an efficient and reusable Brönsted acid
catalyst for the synthesis of pyrano[4,3-b]pyran derivatives using thermal and ultrasonic conditions.
The catalyst was prepared by mixing succinimide and 1,4-butanesultone, which is simpler and safer
than the preparation of succinimide sulfonic acid. This method has the advantages of high yield,
clean reaction, simple methodology, and short reaction time. The catalyst can be recycled without
loss of activity.
Received 7 January 2015
Accepted 29 January 2015
Published 20 May 2015
Keywords:
Brönsted acid
Pyrano[4,3-b]pyran
©
2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
4-Hydroxy-6-methylpyran-2-one
Three-component reaction
One-pot condensation
1.
Introduction
Homogeneous inorganic acids and alkalis such as H
2
SO ,
4
KOH, and NaOH are catalysts in organic transformations. How-
ever, these catalysts have disadvantages: they are strongly
corrosive and nonrenewable and easily cause environmental
pollution through wastewater or sludge discharging. Solid acids
as economical and ecologically benign catalysts offer unique
properties and important advantages over the homogeneous
inorganic acids in organic synthesis, for example, operational
simplicity, environmental compatibility, nontoxicity, low cost,
and ease of isolation [12–17]. However, they have some disad-
vantages. For example, although zeolites demonstrate high
activity, their reactions typically give a variety of undesired
byproducts due to the higher temperature employed and metal
triflates are costly and moisture sensitive. Also, some catalysts
require a special effort to prepare [18,19]. Ion exchange resins
are limited in application because they are thermally unstable
above 120 °C in the acid form [20].
Pyridone and pyran structural units occur widely in various
molecules exhibiting a wide range of biological activities and
serve as a specific nonnucleoside reverse transcriptase inhibi-
tor of HIV-1 [1,2], inotropic and vasodilatatory drugs [3], anti-
tumors and antioxidants [4,5], rhinovirus 3C protease inhibi-
tors [6], anticancers [5,7–9], potential antiviral and antileish-
manial agents [10]. Compounds containing a 2-pyridone moiety
fused with a substituted pyran ring are reported as a Ca2+ in-
hibitor [11], which is active against multidrug resistant KB-VI
cancer cells and has a selective cytotoxicity profile [12]. There-
fore, a variety of synthetic strategies have been developed for
the preparation of dihydropyrano[4,3-b]pyran derivatives
through the formation of the intermediate Knoevenagel prod-
ucts and their subsequent reactions with 4-hydroxy-6-
methylpyran-2-one [13] or a multicomponent reaction of py-
rone with malononitrile and various aromatic aldehydes [11].
Green chemistry with its twelve principles was proposed to
*
Corresponding author. Tel: +98-2166431738; Fax: +98-2166934046; E-mail: ngkhaligh@guilan.ac.ir
DOI: 10.1016/S1872-2067(14)60307-7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 5, May 2015

Nader Ghaffari Khaligh et al. / Chinese Journal of Catalysis 36 (2015) 728–733
729
increase the efficiency of synthetic methods, to use less haz-
ardous solvents, reduce the stages of the synthetic routes and
minimize waste as far as practically possible [21]. One of the
key areas of green chemistry is the replacement of the tradi-
tional thermal methods with eco-environmental technology
such as the use of ultrasound. The use of ultrasound gives en-
hanced reaction rates, formation of purer products, improved
yields, suppression of side products, increased selectivities,
easier experimental procedures, and use of milder conditions
obtained. The resulting SBSA was dried to constant weight in
vacuum at 60 °C. White needles were obtained by crystalliza-
tion in a mixture of ethanol and water using a slow evaporation
technique (2.12 g, yield 90.2%). M.p. 222 °C (dec.); IR (KBr):
ν
max 3140, 3090, 2980, 2940, 1740, 1640, 1600, 1460, 1380,
–1 1
1190, 1120, 1040 cm ; H NMR (300 MHz, D
(m, 2H, –CH –), 2.03–1.91 (m, 2H, –CH –), 2.64 (s, 4H,
–CH –CH –, succinimide), 2.95 (t, J = 7.4 Hz, 2H, –CH –S), 4.23
(t, J = 6.9 Hz, 2H, –CH O): δ 22.3
–N) ppm; 13C NMR (75 MHz, D
(C of butane), 28.2 (C of butane), 29.3 (CH of succinimide),
49.3 (N–CH ), 51.2 (S–CH ), 186.5 (C=O) ppm; HRMS (ENI):
m/z calcd. for C S [M-1] : 234.0436; Found: 234.0431.
2
O): δ 1.75–1.68
2
2
2
2
2
2
2
[22–24]. A survey of the literature reveals that a number of
2
3
2
researchers have demonstrated the efficacy of sonication under
solvent-free conditions, but at least one of the phases of the
reaction mixture was a liquid [25,26]. In addition, the use of a
heterogeneous catalyst in a dry medium can improve the pro-
duction process, eliminating or transforming unwanted and/or
toxic byproducts and avoiding the need for tedious separation.
Recently, succinimide sulfonic acid was synthesized and
their application in the variety of organic transformations was
investigated [27–30]. Here, a new Brönsted acid, namely,
2
2
+
8
H12NO
5
2.3. Preparation of 2-amino-4-aryl-7-methyl-5-oxo-4,5-
dihydropyrano[4,3-b]pyran-3-carbonitriles (2)
In a 25-mL round bottom flask, a mixture of 4-hydroxy-6-
methylpyran-2-one (1.0 mmol), aromatic aldehyde (1.0 mmol),
and malononitrile (1.0 mmol) were mixed in the presence of
4-(succinimido)-1-butane sulfonic acid (10 mg) at 60 °C under
solvent-free condition and ultrasound irradiation for an ap-
propriate time. After completion of the reaction (monitored by
TLC), the reaction mixture was cooled to room temperature
and water was added, and the solid precipitated was filtered to
separate the catalyst. Water was evaporated under reduced
pressure and the catalyst was recovered and used for the next
run. The solid product was recrystallized from ethanol to yield
the pure product.
4-(succinimido)-1-butane sulfonic acid (SBSA) is introduced
and its application in the promotion of the synthesis of dihy-
dropyrano[4,3-b]pyran derivatives is described. The present
study developed a new preparative procedure for this class of
heterocyclic scaffolds by utilizing SBSA under solvent-free con-
ditions.
2
.
Experimental
.1. General
Chemicals were purchased from Fluka AG, Merck, and Syn-
2
2c: 2-amino-4-(4-fluorophenyl)-7-methyl-5-oxo-4,5-dihy-
dropyrano[4,3-b]pyran-3-carbonitrile. Colorless solid; M.p. =
221–223 °C; IR (KBr) νmax = 3369, 3317, 3195, 2924, 2194,
1715, 1678, 1641, 1618, 1591, 1378, 1259, 1138, 1091, 1032,
thetic Chemicals Ltd. Reaction monitoring and purity determi-
nation of the products were accomplished by TLC or GC-MS on
an Agilent GC-Mass-6890 instrument under 70 eV conditions.
Fouriter transform infrared (FTIR) spectra were obtained using
a Perkin-Elmer spectrometer 781 and Bruker Equinox 55 using
KBr pellets for solid samples and neat liquid samples in the
–1 1
978 cm ; H NMR (400 MHz, DMSO-d
6
): δ = 2.19 (s, 3H, CH
3
),
2
4.28 (s, 1H, CH), 6.31 (s, 1H, CH), 7.19 (brs, 2H, NH ), 7.19–7.22
(m, 2H, ArH), 7.31–7.34 (m, 2H, ArH) ppm.
2d: 2-amino-4-(4-bromophenyl)-7-methyl-5-oxo-4,5-dihy-
dropyrano[4,3-b]pyran-3-carbonitrile. Colorless solid; M.p. =
225–227 °C; IR (KBr) νmax = 3381, 3322, 3197, 2921, 2204,
1712, 1676, 1643, 1611, 1596, 1384, 1263, 1141, 1095, 1036,
–1
1
range of 4000–400 cm . In all the cases, the H NMR spectra
were recorded with a Bruker Avance 400 MHz instrument.
Mass spectra (MS) and high resolution mass spectrometry
–1 1
972 cm ; H NMR (400 MHz, DMSO-d
4.31 (s, 1H, CH), 6.27 (s, 1H, CH), 7.18 (d, 2H, J = 8.0 Hz, ArH),
7.25 (s, 2H, NH ), 7.46 (d, 2H, J = 8.0 Hz, ArH) ppm.
6
): δ = 2.21 (s, 3H, CH
3
),
(HRMS) were recorded with PESciex Model API 3000 and Wa-
ters-Micromass Q-ToF Micro instruments, respectively. Micro-
analysis was performed on a Perkin-Elmer 240-B microanalyz-
er. Melting points (M.p.) were recorded on a Büchi B-545 ap-
paratus using open capillary tubes. Sonication was performed
in a Bandelin Sonorex reactor with a frequency of 35 kHz and a
nominal power of 200 W, and built-in heating to 30–80 °C
which was thermostatically adjustable. The reaction vessel was
placed inside the ultrasonic bath containing water.
2
2m: 4,4'-(1,4-phenylene)bis(2-amino-7-methyl-5-oxo-4,5-
dihydropyrano[4,3-b]pyran-3-carbonitrile). Colorless solid;
M.p. = 256–258 °C; IR (KBr): νmax = 3372, 3317, 3196, 2196,
1699, 1673, 1614, 1463, 1383 cm ; 1H NMR (400 MHz,
DMSO-d ): δ = 2.16 (s, 6H, 2CH ), 4.19 (s, 2H, 2CH), 6.22 (s, 2H,
2CH), 7.06 (s, 4H, Ar–H), 7.13 (brs, 4H, 2NH
) ppm; 13C NMR
(100 MHz, DMSO-d ): δ = 18.9, 35.5, 57.8, 98.0, 119.4, 127.5,
30.1, 136.6, 142.4, 158.6, 161.2, 161.9, 162.8, 174.8 ppm;
–1
6
3
2
6
1
+
2.2. Synthesis of SBSA
MS(ESI): m/z [M+1] = 483; Anal. Calcd. for C26
H
18
N
4
O : C,
6
64.73%; H, 3.73%; N, 11.62%. Found: C, 64.62%; H, 3.83%; N,
Succinimide (0.99 g, 10 mmol) was added to 1,4-butane
11.78%.
sultone (1.5 mL, 14.4 mmol) and stirred continuously for 10 h
at 40–50 °C using solar energy to obtain SBSA as a white solid.
The viscous liquid was washed with diethyl ether three times
to remove unreacted starting materials, and a white solid was
3. Results and discussion
An aim of our research is to introduce an eco-efficient

730
Nader Ghaffari Khaligh et al. / Chinese Journal of Catalysis 36 (2015) 728–733
methodology that allows decreasing the amount of waste and
less use of hazardous materials. In the traditional preparation
of succinimide-N-sulfonic acid, chlorosulfonic acid is stirred
with succinimide to generate gaseous HCl [27–30]. This has the
disadvantage of using chlorosulfonic acid which causes severe
burns and reacts exothermically and violently with water pro-
O
O
O
S
O
Solar energy (40-60 oC)
O
S
NH +
O
N
OH
8 h
O
O
O
Scheme 1. Synthesis of 4-(succinimido)-1-butane sulfonic acid (SBSA)
using solar energy.
ducing H
2
SO , HCl, and large quantities of dense white acid
4
NO2
fumes. It is very toxic on inhalation and corrosive to metals.
The present catalyst prepared by mixing succinimide and
OH
O
NO2
O
1
4
,4-butane sultone is simpler and safer. The synthesis of
-(succinimido)-1-sulfonic acid involved stirring the same
CN
CN
CN
SBSA
O
+
+
Different parameters
H3C
O
3
H C
O
NH2
equivalents of succinimide and 1,4-butane sultone at 40–50 °C
for 6 h. The present method does not use a traditional heater.
Instead, 10 mirrors reflected sunlight onto the 25 mL round
bottom flask. When the concentrated sunlight strikes the round
bottom flask, it heats the reaction mixture to 40–60 °C (Scheme
CHO
1
e
2e
Scheme 2. Synthesis of 2-amino-4-(4-nitrophenyl)-7-methyl-5-oxo-4,5-
dihydropyrano[4,3-b]pyran-3-carbonitrile derivatives in the presence
of SBSA under various reaction conditions.
1). The viscous liquid was washed with diethyl ether, and a
white solid was obtained. The resulting SBSA was dried to con-
stant weight in vacuum. The structure was confirmed by IR, H
NMR, and 13C NMR. The content of water of the SBSAwas 5.4%
from a Karl-Fischer titration method. SBSA is soluble in DMSO,
DMF, water, methanol, and ethanol. It is immiscible with dieth-
[4,3-b]pyran-3-carbonitrile was obtained in the presence of 4.2
mol% SBSA at 60 °C in 60 min. Higher temperatures caused
more spots on the TLC and it was deduced that byproducts
were obtained. The reaction was not complete at 60 °C even
after 4 h in the absence of SBSA, and only 38% of 2e was ob-
tained. The model reaction was also carried out under the op-
timized conditions in the presence of succinimide-N-sulfonic
acid (SuSA), but only 54% of 2e was obtained. It was deduced
that this reaction was catalyzed by SBSA more than by succin-
imide-N-sulfonic acid.
1
yl ether, ethyl acetate, and CH
2
Cl . So the catalyst can be sepa-
2
rated conveniently from the products by a simple phase sepa-
ration.
To evaluate the effect of the amount of SBSA, the condensa-
tion of 4-hydroxy-6-methylpyran-2-one, 4-nitrobenzaldehyde
(
1e), and malononitrile was carried out in the presence of dif-
To study the effect of ultrasonic irradiation, the model reac-
tion was performed using SBSA at room temperature under
ultrasonic irradiation. In general, ultrasound has chemical and
mechanical effects. The main advantages of the uses of ultra-
sound in the processing of liquids are directly related to the
physical effects of acoustic cavitation [31]. Because this work
used a heterogeneous catalytic system in a dry medium, the
main effect of ultrasound was due to its mechanical effect. The
mechanical effects of ultrasound lead to a reduction of particle
ferent amounts of catalyst (2.1, 4.2, and 8.5 mol%) under sol-
vent-free conditions (Scheme 2). It was observed that 4.2 mol%
of SBSA was an optimum amount for this model reaction to
furnish the desired product in high yield. Increasing the
amount of the catalyst beyond 4.2 mol% did not increase the
yield noticeably. The different reaction temperatures (r.t.–80
°C) were compared and the results showed that 88% of
2
-amino-4-(4-nitrophenyl)-7-methyl-5-oxo-4,5-dihydropyrano
Table 1
a
Synthesis of 2-amino-4-aryl-7-methyl-5-oxo-4,5-dihydropyrano[4,3-b]pyran-3-carbonitriles in the presence of SBSA .
Thermal condition (60 °C)
Time (min) Yield b (%)
Ultrasound irradiation
Time (min) Yield b (%)
M.p. (°C)
Found
236
Entry
Substrate (1)
–CHO
Product (2)
Ref.
Reported
236–238
231–232
223–225
218–220
216–218
223–225
205–207
216
1
2
3
4
5
6
7
8
9
C
6
H
5
a
b
c
d
e
f
g
h
i
80
75
70
80
60
90
95
70
62
95
80
80
90
120
80
84
82
84
88
72
70
69
74
70
78
74
80
—
15
12
12
14
10
15
15
15
12
18
15
15
20
120
92
90
92
90
95
90
88
90
93
90
92
90
94
—
[35]
[39]
[10]
[10]
[37]
[38]
[35]
[36]
[37]
[36]
[5]
4-Cl–C
6
H
4
–CHO
–CHO
–CHO
–CHO
–CHO
–CHO
–CHO
–CHO
–CHO
228–230
221–223
225–227
220–222
218–220
210–212
217–219
218–220
198–202
118–120
238–240
256–258
—
4-F–C
6
H
4
4-Br–C
6
H
4
4-NO
4-CH
4-CH
3-Br–C
3-NO –C
3,4-(CH O)
Furfural
2-Thiophene carbaldehyde
1,4-(CHO) –C
4-Dimethylaminobenzaldehyde
2
–C
–C
O–C
6
H
4
3
6
H
4
3
6
H
4
6
H
4
2
6
H
4
234–236
198–202
223–224
242–244
—
1
1
1
1
0
1
2
3
4
3
2
–C
6
H
3
j
k
l
m
n
[5]
—
—
2
6
H
4
1
—
a
Reaction conditions: 4-hydroxy-6-methylpyran-2-one 1.0 mmol, aromatic aldehyde 1.0 mmol, malononitrile 1.0 mmol, SBSA 4.2 mol%, sol-
vent-free.
b
Isolated yield.

Nader Ghaffari Khaligh et al. / Chinese Journal of Catalysis 36 (2015) 728–733
731
size and increase of surface area and accelerated the motion of
suspended particles to allow better mass transfer [32]. Fre-
quencies below 50 kHz are generally preferred for heteroge-
neous systems due to the more intense mechanical effects [33].
Hence, in the present method, 35 kHz was selected for maxi-
mum sonication. The model reaction was subjected to ultra-
sonic irradiation initially for 2 min in the presence of SBSA (4.2
mol%) at room temperature, but no product was detected (TLC
monitoring). Then sonication was continued and product for-
mation (2e) was noticed. After 10 min, the product formation
reached 95% yield. Continuation of sonication for 15 min did
not affect the yield. It was deduced that the ultrasound irradia-
tion accelerated the reaction. The need of just one third of the
reaction time (10 min versus 60 min) and the lower tempera-
ture (r.t. versus 60 °C) showed that ultrasonic chemical activa-
tion clearly affected the course of the reaction, increasing its
energy efficiency.
O
H
O
H
N
N
H C
3
CH3
H C
3
CH3
Scheme 3. Tautomerisation with formation of a quinonoid structure.
Table 2
Reusability of SBSA for the synthesis of 2-amino-4-(4-nitrophenyl)-7-
methyl-5-oxo-4,5-dihydropyrano[4,3-b]pyran-3-carbonitrile (2e) un-
der optimized conditions.
Thermal condition (60 °C)
Ultrasound irradiation
Run
Time
(min)
60
Isolated yield
Time
(min)
10
Isolated yield
(%)
88
88
88
86
86
(%)
95
94
94
93
93
1
2
3
4
5
60
10
60
60
62
12
12
12
In order to evaluate the generality of this model reaction, a
range of 2-amino-4-aryl-7-methyl-5-oxo-4,5-dihydropyrano
[
4,3-b]pyran-3-carbonitriles 2a–m were prepared under the
optimized reaction conditions in the presence of SBSA (Table
). Aryl aldehydes with electron-donating and electron-with-
derivative and the starting materials were quantitatively re-
covered under the same conditions (Table 1, entry 14). The
explanation for this result may be that the strong elec-
tron-donating dimethylamino group reduced the reactivity. A
degree of tautomerisation occurred with the formation of a
quinonoid structure as shown in Scheme 3, which thus de-
creased the reactivity of the aldehyde group [34].
1
drawing substituents and heteryl aldehydes provided the de-
sired dihydropyrano[4,3-b]pyrans in good to high yields with-
out any side product (Table 1, entries 1–13). However, aliphatic
aldehydes did not undergo pyranization even after a long reac-
tion time and at an elevated temperature. TLC and GC-MS anal-
ysis of the reaction mixture showed numerous products. Elec-
tron-donating substituents caused lower yields and needed
longer reaction times than electron-withdrawing substituents
New products were characterized by IR, H NMR, 13C NMR,
1
MS spectra, and elemental analysis, and known products were
1
characterized by IR and H NMR and comparison of their M.p.
(
Table 1, entries 5, 7, 9, and 10). 4,4'-(1,4-Phenylene)
bis(2-amino-7-methyl-5-oxo-4,5-dihydropyrano[4,3-b]pyran-3
carbonitrile 2m was obtained in 80% yield when terephthal-
dehyde 1m was reacted with malononitrile and 4-hydroxy-
-methylpyran-2-one in the molar ratio 1.0:2.0:2.0 under the
with those of reference samples.
SBSA was isolated and could be recycled up to five times
without any significant loss of activity (Table 2).
A proposed mechanism for the formation of the product via
tandem Knoevenagel-cyclo condensation is outlined in Scheme
4. The carbonyl group of the aldehyde was activated by the
Brönsted acid SBSA (1). Next, nucleophilic attack of the malo-
-
6
optimized reaction conditions (Table 1, entry 13). 4-Dime-
thylaminobenzaldehyde failed to give the corresponding pyran
Intermediate
H+ from SBSA for
H+ from SBSA for
N
activating of intermediate
activating of aldehyde
O
NC
C
O
R
H
CN
Knoevenagel condensation
O
O
H
+
CN -H O and regeneration of H+
2
R
6
0 oC or )))))
(
1)
CH3
R
R
R
O
H
O
O
O
Intramolecular
CN condensation
H
CN
60 oC or )))))
H3C
O
O
0 oC or )))))
CN
6
H C
3
O
NH2
O
H3C
O
NH
C
(
2)
HN
Scheme 4. Proposed mechanism for the synthesis of 2-amino-4-aryl-7-methyl-5-oxo-4,5-dihydropyrano[4,3-b]pyran-3-carbonitriles in presence of
BBMIm](HSO at 60 °C.
[
)
4 2

732
Nader Ghaffari Khaligh et al. / Chinese Journal of Catalysis 36 (2015) 728–733
Table 3
Comparison of the present method with other reported strategies for the synthesis of 2-amino-4-phenyl-7-methyl-5-oxo-4,5-dihydropyra-
no[4,3-b]pyran-3-carbonitrile.
Entry
Catalyst
OAc (10 mol%)
Conditions
Neat, r.t.
80 °C
MeOH, reflux
EtOH, r.t.
100 °C
Time (min) Yield (%)
Ref.
[5]
[10]
[37]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
this work
this work
1
2
3
4
5
6
7
8
9
NH
4
10
180
60
94
85
79
76
77
65
89
94
94
92
88
95
[bmim][BF
Piperidine (1–2 drops)
KF-Al
4
] (1.5 g)
a
2
O
3
480
60
1,1,3,3-N,N,N,N-Tetramethylguanidinium trifluoroacetate (TMGT) (1 mol%)
—
H
2
O, 80 °C
O/EtOH, reflux
O, reflux
Neat, 60 °C
O, 80 °C
Neat, 60 °C
Neat, ultrasound irradiation
630
30
MgO (0.25 g)
H
2
H
6
P
2
W
18
O
62·18H
2
O (1 mol%)
(500 mg)
H
2
60
35
[BBMIm](HSO
4
)
2
1
1
1
0
1
2
Thiourea dioxide (TUD)
SBSA (4.2 mol%)
H
2
40
60
10
SBSA (4.2 mol%)
nonitril on the carbonyl carbon formed the intermediate aryli-
dene malononitrile. Subsequent Michael addition of
rivatives under thermal and ultrasonic conditions. To prepare
the catalyst, solar energy was applied and a hazardous materi-
al, namely, chlorosulfonic acid, was avoided. The current
method has the advantages of a simple experimental proce-
dure, use of ultrasound irradiation as an eco-environmental
technology, gave good to high yield of the products, and has
reusability of the catalyst.
4-hydroxy-6-methylpyran-2-one followed by cyclization af-
forded the product (2).
To validate the proposed mechanism, the synthesis of 2b
was carried out in two steps. First, 4-chlorobenzylidene malo-
nonitrile was prepared by the condensation of 4-chlorobenzal-
dehyde 1b and malononitrile in the presence of SBSA under
thermal or ultrasonic conditions. Then the product of the first
step was reacted with 4-hydroxy-6-methylpyran-2-one in the
presence of SBSA to give the product 2b in 87% and 92% yield
in 45 and 8 min in the presence of SBSA under thermal or ul-
trasonic conditions, respectively. This provided the evidence in
support of the intermediate arylidene malononitrile in the
proposed pathway.
Acknowledgments
The authors are thankful to the financial support from Uni-
versity Malaya to Project GC 001A-14-AET.
References
The comparison of the present methodology with previous-
ly reported procedures for the synthesis of 2a is shown in Ta-
ble 3. As can be seen, the reaction catalyzed by SBSA at 60 °C
gave a comparable yield, required less catalyst and less time
than the other protocols, and also the catalyst was reusable.
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Graphical Abstract
Chin. J. Catal., 2015, 36: 728–733 doi: 10.1016/S1872-2067(14)60307-7
-(Succinimido)-1-butane sulfonic acid as a Brönsted acid catalyst
for the synthesis of pyrano[4,3-b]pyran derivatives using thermal
and ultrasonic irradiation
O
O
4
O
S
O
Solar energy (40-60 oC)
O
S
NH
+
O
N
OH
8
h
O
O
O
Nader Ghaffari Khaligh *, Sharifah Bee Abd Hamid
4-(Succinimido)-1-butane sulfonic acid (SBSA)
University Malaya, Malaysia
R
O
4
-(Succinimido)-1-butane sulfonic acid as an efficient and reusable
OH
Brönsted acid catalyzed the synthesis of pyrano[4,3-b]pyran derivatives
under ultrasound irradiation. The preparation of 4-(Succinimido)-1-
butane sulfonic acid is more simple and safer than of succinimide sul-
fonic acid.
CN
CN
SBSA
O
+
R
+
CN Neat, 60 oC or )))))
H3C
O
O
H3C
O
NH2
CHO

Nader Ghaffari Khaligh et al. / Chinese Journal of Catalysis 36 (2015) 728–733
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