MCM-41-SO3H: an efficient, reusable, heterogeneous catalyst for the one-pot, three-component synthesis of pyrano[3,2-b]pyrans

Article abstract of DOI:10.1007/s11164-015-2275-z

MCM-41-SO₃H, ordered mesoporous silica material MCM-41 with covalently anchored sulfonic acid groups, was used as a solid acid catalyst for the convenient, efficient, and ‘green’ synthesis of 2-amino-6-(hydroxymethyl)-8-oxo-4-aryl-4,8-dihydropy

Full text of DOI:10.1007/s11164-015-2275-z

Res Chem Intermed
DOI 10.1007/s11164-015-2275-z
MCM-41-SO3H: an efficient, reusable, heterogeneous
catalyst for the one-pot, three-component synthesis
of pyrano[3,2-b]pyrans
Yaghoub Sarrafi1 Ebrahim Mehrasbi1
•
•
Seyyedeh Zohreh Mashalchi1
Received: 30 March 2015 / Accepted: 12 September 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract MCM-41-SO3H, ordered mesoporous silica material MCM-41 with
covalently anchored sulfonic acid groups, was used as a solid acid catalyst for the
convenient, efficient, and ‘green’ synthesis of 2-amino-6-(hydroxymethyl)-8-oxo-4-
aryl-4,8-dihydropyrano[3,2-b]pyrans via a one-pot, three-component reaction of
5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one, aldehydes and malononitrile in
aqueous media.
Keywords 2-Amino-6-(hydroxymethyl)-8-oxo-4-aryl-4 Á 8-Dihydropyrano[3,2-b]
pyran Á MCM-41-SO3H Á Multicomponent reactions Á Aqueous media
Introduction
Multicomponent reactions (MCRs) have proved to be very influential and efficient
bond-forming tools for the synthesis of biologically active compounds. MCRs are
extremely convergent, producing a remarkably high increase of molecular
complexity in just one step [1–3].
Kojic acid [5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one], which was first
isolated from the mycelia of Aspergillus oryzae grown on steamed rice (koji), is
widely used as a food additive for preventing enzymatic browning of raw crabs and
shrimps, and as a skin-whitening agent because of its tyrosinase inhibitory activity
on melanin synthesis. Kojic acid derivatives possess wide-ranging pharmacological
activities such as antifungal [4], anti-neoplastic [5], antiproliferative [6], anti-HIV
&
Yaghoub Sarrafi
Ysarrafi@umz.ac.ir
1
Faculty of Chemistry, University of Mazandaran, Babolsar, Iran
123

Y. Sarrafi et al.
[7], anticonvulsant [8], anti-inflammatory [9], antioxidative [10], antibacterial [11],
and tyrosinase inhibitory activities [12]. Because of these valuable properties, kojic
acid derivatives are of great interest for the pharmaceutical chemistry. On the other
hand, 2-amino-4H-pyran derivatives are the core structure of many pharmacological
agents and are favorite compounds for medicinal chemists due to their potential
biomedical application as a novel anticancer agent for human use. Giving attention
to the synthetic feasibility of 2-amino-4H-pyran and kojic acid derivatives it
appeared useful to fuse both these scaffolds as a single molecular entity in view of
enhanced biological activity.
In 1997, Piao et al. [13] reported the two step synthesis of 2-amino-6-
(hydroxymethyl)-8-oxo-4-aryl-4,8-dihydropyrano[3,2-b]pyranes using amine-based
catalyst. This method is associated with some disadvantages such as the use of a
toxic and non-recyclable catalyst, high reaction temperature, and two step
synthesis. Litvinov et al. have described a three-component Et3N-catalyzed
procedure for the synthesis of similar compounds [14]. Recently, ultrasound-
assisted synthesis of these compounds has also been reported [15]. However, only
a few methods have been developed for the synthesis of this class of compounds,
but, also, 2-amino-6-(hydroxymethyl)-8-oxo-4-aryl-4,8-dihydropyrano[3,2-b]pyr-
anes have not been widely explored. Thus, the development of environmental,
efficient, and facile process for the synthesis of these compounds is of current
interest.
In recent years, replacement of hazardous and corrosive homogeneous catalysts
with environmental compatible and reusable heterogeneous catalysts has been one
of the key areas of green chemistry [16]. In this context, organically functionalized
ordered mesoporous silica with a tunable pore structure and tailored composition
have gained much research interest because of their several advantages such as
reusability, environmental compatibility, non-corrosiveness and easy separation,
compared to homogeneous counterparts [16–22]. Ever since the discovery of MCM-
41 mesoporous silica, these structurally ordered materials have been regarded as the
ideal solid support for various catalysts due to their high surface areas and tunable
pore size.
In continuation of our efforts for the developing of new and eco-friendly
synthetic methodologies [23–27], herein, we disclose the application of sulfonic
acid functionalized MCM-41 as a recyclable catalyst for the synthesis of 2-amino-6-
(hydroxymethyl)-8-oxo-4-aryl-4,8-dihydropyrano[3,2-b]pyranes (Scheme 1).
O
N
N
C
C
H2N
-
-
O
M M 41
H
SO3
C
O
O
H
O
2
H
O
H
O
N
C
O
aryl
hetary
H
O
aryl
hetaryl
H2 90 °
O
C
,
l
1
3
4
Scheme 1 MCM-41-SO3H-catalyzed synthesis of 2-amino-dihydropyrano[3,2-b]pyrane derivatives
123

MCM-41-SO3H: an efficient, reusable, heterogeneous…
Experimental
All the chemicals were purchased from Merck, Fluka, and Sigma-Aldrich. The
reactions were monitored by thin layer chromatography (TLC). Melting points were
measured on an electrothermal 9100 apparatus. NMR spectra were recorded with a
1
Bruker DRX-400 AVANCE instrument (400.1 MHz for H and 100.6 MHz for
13C). IR spectra were recorded on a FT-IR Bruker vector 22 spectrometer. Mass
spectra were measured on a Finnigan-Matt 8430 mass spectrometer operating at an
ionization potential of 30 eV.
Synthesis of sulfonic acid functionalized MCM-41
The MCM-41 (60.0 g) were dispersed in dry CH2Cl2 (5 mL) and ClSO3H (81.13 g,
0.70 mol) was then added dropwise over a period of 60 min at r.t. Hydrogen
chloride gas evolved from the reaction vessel immediately. After completion of the
addition, the mixture was shaken for 30 min and the obtained MCM-41-SO3H was
washed three times with dry CH2Cl2 to remove the unattached substrates. The solid
material was finally dried at 60 °C overnight [28]. The amount of sulfonic acid
groups of MCM-41-SO3H, which were determined by acid–base titration, was
found to be 0.95 mmol g-1
.
General procedure for preparation of 2-amino-4,8-dihydropyrano[3,2-
b]pyran-3-carbonitriles
A mixture of benzaldehyde (1 mmol), malonitrile (1 mmol), kojic acid (1 mmol),
and MCM-41-SO3H (30 mg) in water (5 mL) was stirred at 90 °C. After completion
of the reaction (as monitored by TLC), the catalyst was filtered out using filter paper
and washed with hot ethanol. Then, the solvent of the filtrate was evaporated, and
the crude product was purified by recrystallization from ethanol. The new
compounds were identified by their 1H and 13CNMR spectra and elemental
analysis. The known products were characterized by comparison of their physical
and spectroscopic data with the literature [13–15].
Physical and spectral data of new products
2-amino-4,8-dihydro-6-(hydroxymethyl)-4-(4-nitrophenyl)-8-oxopyrano[3,2-b]pyran-3-
carbonitrile (4c) MP: 230–232 °C; IR (KBr, cm-1) mmax: 3538, 3452, 3329, 2195,
1
1650, 1521, 1352; H NMR (400 MHz, DMSO-d6) d: 4.17 (2H, ABqd, J = 16.4,
6 Hz, CH2), 5.08 (1H, s, CHaliph), 5.69 (1H, t, J = 6 Hz, OH), 6.35 (1H, s, CHvinyl),
7.39 (2H, s, NH2), 7.62 (2H, d, J = 8.8 Hz, ArH), 8.26 (2H, d, J = 8.8 Hz, ArH);
13C NMR (100 MHz, DMSO-d6) d: 55.12, 59.51, 111.96, 124.64, 129.82, 137.15,
?
147.62, 148.19, 148.44, 159.82, 168.81, 169.99; [M] 341.
2-amino-4-(4-cyanophenyl)-4,8-dihydro-6-(hydroxymethyl)-8-oxopyrano[3,2-b]pyran-
3-carbonitrile (4d) MP: 228–230 °C; IR (KBr, cm-1) mmax: 3520, 3424, 3325,
123

Y. Sarrafi et al.
1
2229, 2191, 1653; H NMR (400 MHz, DMSO-d6) d: 4.16 (2H, ABqd, J = 16,
6 Hz, CH2), 4.99 (1H, s, CHaliph), 5.68 (1H, t, J = 6 Hz, OH), 6.34 (1H, s, CHvinyl),
7.35 (2H, s, NH2), 7.53 (2H, d, J = 8 Hz, ArH), 7.88 (2H, d, J = 8.4 Hz, ArH); 13
C
NMR (100 MHz, DMSO-d6) d: 55.20, 59.52, 111.26, 111.96, 119.00, 119.46,
129.46, 133.44, 137.15, 146.55, 148.28, 159.83, 168.75, 169.98; [M] 321.
?
2-amino-4-(4-(trifluoromethyl)phenyl)-4,8-dihydro-6-(hydroxymethyl)-8-oxopyrano
[3,2-b]pyran-3-carbonitrile (4e) MP: 240–242 °C; IR (KBr, cm-1) mmax: 3408,
2197, 1639; 1H NMR (400 MHz, DMSO-d6) d: 4.10–4.23 (2H, m, CH2), 4.99 (1H,
s, CHaliph), 5.68 (1H, t, J = 6 Hz, OH), 6.34 (1H, s, CHvinyl), 7.33 (2H, s, NH2),
7.55 (2H, d, J = 8 Hz, ArH), 7.77 (2H, d, J = 8.4 Hz, ArH); 13C NMR (100 MHz,
DMSO-d6) d: 40.43, 55.45, 59.54, 111.94, 119.54, 124.59 (q,
3
2
1J = 270.6 Hz),128.89 (q, J = 3.8 Hz),129.26,129.89 (q, J = 31.3 Hz), 137.08,
?
145.80, 148.57, 159.79, 168.78, 170.01; [M] 364.
2-amino-4-(3-chlorophenyl)-4,8-dihydro-6-(hydroxymethyl)-8-oxopyrano[3,2-b]pyran-
1
3-carbonitrile (4g) MP: 238–240 °C; IR (KBr, cm-1) mmax: 3317, 2201, 1644; H
NMR (400 MHz, DMSO-d6) d: 4.10–4.24 (2H, m, CH2), 4.99 (1H, s, CHaliph), 5.68
(1H, t, J = 6 Hz, OH), 6.34 (1H, s, CHvinyl), 7.27 (1H, dt, J = 7.6, 1.6 Hz, ArH),
7.30 (2H, s, NH2), 7.37–7.45 (3H, m, ArH); 13C NMR (100 MHz, DMSO-d6) d:
55.64, 59.56, 111.95, 127.11, 128.12, 128.44, 131.38, 133.94, 137.01, 143.71,
?
148.67, 159.76, 168.70, 170.02; [M] 330.
2-amino-4,8-dihydro-6-(hydroxymethyl)-4-(3-methoxyphenyl)-8-oxopyrano[3,2-b]pyran-
3-carbonitrile (4h) MP: 225-227 °C; IR (KBr, cm-1) mmax: 3189, 2202, 1630,
1
1260, 1032; H NMR (400 MHz, DMSO-d6) d: 3.34 (1H, s, CHaliph), 4.07–4.22
(2H, m, CH2), 4.99 (1H, s, CH), 5.66 (1H, t, J = 6 Hz, OH), 6.32 (1H, s, CHvinyl),
6.96 (1H, td, J = 7.6, 0.8 Hz, ArH), 7.05 (1H, d, J = 8 Hz, ArH), 7.11 (2H, s,
NH2), 7.13 (1H, dd, J = 7.6, 1.6 Hz, ArH), 7.28–7.32 (1H, m, ArH); 13C NMR
(100 MHz, DMSO-d6) d: 35.84, 55.59, 56.28, 59.54, 111.82, 112.48, 119.83,
121.42, 128.91, 129.67, 129.77, 137.39, 149.77, 157.53, 160.12, 168.50, 170.01;
?
[M] 326.
2-amino-4,8-dihydro-6-(hydroxymethyl)-4-(2-nitrophenyl)-8-oxopyrano[3,2-b]pyran-3-
carbonitrile (4i) MP: 223–225 °C; IR (KBr, cm-1) mmax: 3378, 3318, 3210, 2192,
1678, 1529, 1354; 1H NMR (400 MHz, DMSO-d6) d: 4.03–4.19 (2H, m, CH2), 5.50
(1H, s, CHaliph), 5.67 (1H, t, J = 6 Hz, OH), 6.32 (1H, s, CHvinyl), 7.38 (2H, s,
NH2), 7.57–7.63 (2H, m, ArH), 7.77 (1H, td, J = 7.8, 1.2 Hz, ArH), 7.98 (1H, dd,
J = 8, 1.2 Hz, ArH); 13C NMR (100 MHz, DMSO-d6) d: 36.55, 54.16, 59.42,
111.85, 119.26, 125.20, 130.03, 131.64, 133.92, 134.44, 137.10, 147.94, 149.68,
?
160.18, 168.75, 169.92; [M] 342.
2-amino-4-(2-chloro-5-nitrophenyl)-4,8-dihydro-6-(hydroxymethyl)-8-oxopyrano[3,2-
b]pyran-3-carbonitrile (4j) MP: 260–262 °C; IR (KBr, cm-1) mmax: 3425, 3310,
1
3209, 2200, 1623, 1527, 1348; H NMR (400 MHz, DMSO-d6) d: 4.17 (2H, ABq,
123

MCM-41-SO3H: an efficient, reusable, heterogeneous…
J = 15.6 Hz, CH2), 5.50 (1H, s, CHaliph), 5.66 (1H, s, OH), 6.35 (1H, s, CHvinyl),
7.44 (2H, s, NH2), 7.84 (2H, d, J = 8.8 Hz, ArH), 8.21 (2H, dd, J = 8.8, 2.8 Hz,
ArH), 8.26 (2H, d, J = 2.8 Hz, ArH); 13C NMR (100 MHz, DMSO-d6) d: 38.95,
53.88, 59.48, 111.99, 119.18, 125.17, 126.28, 132.31, 137.61, 139.32, 140.09,
?
147.13, 147.36, 160.24, 168.83, 169.89; [M] 375.
2-amino-4-(4-(2-amino-3-cyano-4,8-dihydro-6-(hydroxymethyl)-8-oxopyrano[3,2-
b]pyran-4-yl)phenyl)-4,8-dihydro-6-(hydroxymethyl)-8-oxopyrano[3,2-b]pyran-3-
1
carbonitrile (4m) MP: 270–272 °C; IR (KBr, cm-1) mmax: 3320, 2191, 1644; H
NMR (400 MHz, DMSO-d6) d: 4.12 (2H, ABqd, J = 16.8, 6 Hz, CH2), 4.81 (1H, s,
CHaliph), 5.69 (1H, t, J = 6 Hz, OH), 6.33 (1H, s, CHvinyl), 7.25 (2H, s, NH2), 7.30
(2H, s, ArH); 13C NMR (100 MHz, DMSO-d6): dc 55.92, 59.58, 111.93, 119.78,
?
128.75, 136.98, 140.81, 149.31, 159.85, 168.71, 170.03; [M] 510.7
Results and discussion
Powder X-ray diffraction patterns of MCM-41, MCM-41-SO3H, and reused MCM-
41-SO3H are shown in Fig. 1. For MCM-41, four well-defined Brag peaks at low
angles can be seen, which can be indexed to the (1 0 0), (1 1 0), (2 0 0), and (2 1 0)
reflections corresponding to a hexagonal lattice (i.e., MCM-41). When the surface
of mesoporous silica is functionalized with the sulfonic groups, a decrease in the
intensity of the peaks is observed; however, the two-dimensional hexagonal array of
mesoporous structure is retained. Also, the used MCM-41-SO3H XRD diagram
revealed that mesopore channels during catalytic reaction were stable as physical
structure damage had not been observed.
As illustrated in Fig. 2, N2 adsorption–desorption measurement for the sulfonated
MCM-41 demonstrated a type IV isotherm, which is the characteristic of
mesoporous materials. The specific surface area and total pore volume of the
catalyst determined from the BET equation were 390 m2/g and 0.38 cm3/g,
respectively. Furthermore, the TEM micrograph of MCM-41-SO3H (Fig. 3) showed
an ordered hexagonal pore system.
1600
1400
1200
1000
800
600
400
200
0
800
700
600
500
400
300
200
100
0
(100)
(100)
MCM-41-SO3H
MCM-41
used MCM-41-SO3H
(110)
(200)
(110)
(200)
(210)
(210)
1
3
5
7
9
1
3
5
7
9
2
/(°)
2
/(°)
Fig. 1 XRD patterns of MCM-41, MCM-41-SO3H and used MCM-41-SO3H
123

Y. Sarrafi et al.
300
200
100
0
Fig. 2 N2 adsorption–
desorption isotherm of the
MCM-41-SO3H
ADS
DES
a
V
0
0/1 0/2 0/3 0/4 0/5 0/6 0/7 0/8 0/9
p0
1
p
/
Fig. 3 The TEM image of
MCM-41-SO3H
Our study began with a one-pot three-component reaction of 4-chloro-benzalde-
hyde 1a, malononitrile 2, and kojic acid 3 in EtOH at room temperature. The choice
of the catalyst played a crucial role and the use of solid catalysts was at the center of
our study. As shown in Table 1, the best result was obtained with the MCM-41-SO3H
catalyst in terms of yield and reaction time (entry 7). The commercial ion-exchange
sulfonic resins such as Amberlist-15 gave moderate yields of the corresponding
product (entry 1). The Alum-catalyzed reaction gave a much lower yield. The
observed meaningful catalytic activity of MCM-41-SO3H compared to the
mentioned acid catalysts may be due to the higher surface area of the MCM-41-
SO3H, which enhanced the chemical accessibility of substrates to the anchored acid
groups. The use of SiO2, TiO2 and Al2O3 nanoparticles and H2SO4 were also
examined whereby a moderate yield of product was obtained in a relatively longer
reaction time (entries 3, 4, 5, 9). This reaction was also carried out with MCM-41 that
required longer reaction times and resulted in a significantly lower yield (entry 6).
123

MCM-41-SO3H: an efficient, reusable, heterogeneous…
Table 1 Effect of different catalysts on the reaction of 4-chloro-benzaldehyde 1a, malononitrile 2, and
kojic acid 3 in EtOH at room temperature
Entry
Cat. (0.02 g)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
a
Amberlyst-15
Alum
12
12
9
50
25
Nano-SiO2
Nano-Tio2
Nano-Al2O3
MCM-41
MCM-41-SO3H
–
45
10
10
5.5
3
40
43
65
75
12
10
Trace
45
H2SO4
Isolated yields
Table 2 Effect of solvent on the MCM-41-SO3H -catalyzed synthesis of 2-amino-dihydropyrano[3,2-
b]pyrane 4a
Entry
Solventa,b
Time (h)
Yield (%)c
1
2
3
4
5
a
H2O
2.5
3
80
EtOH
MeOH
MeCN
CHCl3
75
6
70
24
24
Trace
Trace
Reaction conditions: aromatic aldehydes (1 mmol), malononitrile (1 mmol), kojc acid (1 mmol) and
MCM-41-SO3H (0.02 g) at RT
b
Solvent: 5 ml
c
Isolated yields
123

Y. Sarrafi et al.
Table 3 Effect of temperature on the MCM-41-SO3H-catalyzed synthesis of 2-amino-dihydropy-
rano[3,2-b]pyrane 4aa
Entry
Temperature (°C)
Time (h)
Yield (%)b
1
2
3
4
a
RT
50
2.5
2
80
80
85
85
90
1
100
1
Reaction conditions: aromatic aldehydes (1 mmol), malononitrile (1 mmol), kojc acid (1 mmol), and
MCM-41-SO3H (0.02 g) in H2O (5 mL)
b
Isolated yields
Table 4 Influence of the amount of MCM-41-SO3H in synthesis of 2-amino-dihydropyrano[3,2-b]pyr-
ane 4aa
Entry
MCM-41-SO3H (g)
Time
Yield (%)b
1
2
3
4
a
0.04
0.03
0.02
–
60 min
40 min
1 h
96
97
85
25
10 h
Reaction conditions: aromatic aldehydes (1 mmol), malononitrile (1 mmol), and kojc acid (1 mmol) in
H2O (5 mL) at 90 °C
b
Isolated yields
When the reaction was attempted without a catalyst, trace product formation was
observed.
We then tried to screen the reaction in various solvents in order to optimize the
reaction conditions using MCM-41-SO3H as catalyst (Table 2). The results revealed
that solvents show great effect on the catalytic activity of MCM-41-SO3H. The
highest yield was obtained with water as solvent.
During the optimization of the reaction condition, the effect of temperature on
the reaction rate as well as the yields of products was investigated (Table 3). In this
regard, the reactions were carried out at different temperatures ranging from room
temperature to 100 °C. We found that the yield of product 3a was improved, and the
reaction time was shortened as the temperature was increased to 90 °C (Table 3,
entry 3).
We also evaluated the amount of catalyst required for this transformation. It was
found that 0.03 g of MCM-41-SO3H was enough to afford 3a with 97 % isolated
yield in suitable time (Table 4, entry 2). The yield remained nearly unaffected when
the catalyst loading was increased to 0.04 g (Table 4, entry 1). However, the yield
was dramatically decreased when the catalyst-free reaction was performed (Table 2,
entry 4).
123

MCM-41-SO3H: an efficient, reusable, heterogeneous…
Table 5 Synthesis of different 2-amino-dihydropyrano[3,2-b]pyrane derivatives 4 catalyzed by MCM-
41-SO3Ha
Aromatic aldehydes
Entry
1
Product
Time (min) Yield (%)b
Mp °C
40
45
97
91
235–237
4a
4b
247–249
230–232
228–230
2
3
35
35
45
50
98
98
94
90
4c
4d
4e
4f
4
5
6
240–242
221–223
7
50
95
238–240
4g
123

Y. Sarrafi et al.
Table 5 continued
Time (min) Yield (%)b
Mp °C
Aromatic aldehydes
Entry
Product
8
45
10
50
50
50
89
95
89
89
87
225–227
223–225
260–262
222–224
234–236
4h
4i
9
10
11
12
4j
4k
4l
13
45
95
270–272
4m
a
Reaction conditions: aromatic aldehydes (1 mmol), malononitrile (1 mmol), kojc acid (1 mmol) and
MCM-41-SO3H (0.03 g) in H2O (5 mL) at 90 °C
b
Isolated yields
To evaluate the versatility of this methodology, a series of aromatic aldehydes
were studied under the optimal reaction conditions and the results are collected in
Table 5. In all cases, the reactions gave the products in good to excellent yields in
very short reaction times. The aromatic aldehydes with both electron-withdrawing
123

MCM-41-SO3H: an efficient, reusable, heterogeneous…
Scheme 2 A plausible reaction mechanism
and electron-donating substituents such as CN, NO2, halogens, and CH3 were
performed smoothly under optimized conditions to afford the corresponding
products in high yields (Table 5). However, the aromatic aldehydes having a strong
electron-donating substituent such as p-OCH3 provided the lower yields of product.
Acid sensitive electron-rich heterocyclic aldehydes such as furfural and thiophene-
2-carbaldehyde also reacted very efficiently with no side reaction (entries 11 and
12).
The suggested mechanism for the synthesis of 2-amino-4H-pyran derivatives 4 is
described in (Scheme 2). The unsaturated nitrile 7 is formed via a Knoevenagel
123

Y. Sarrafi et al.
100
90
80
70
60
50
40
30
20
10
0
Fig. 4 Reusability study of the
catalyst in the model reaction
1
2
3
4
Number of cycles
condensation reaction of benzaldehyde 1 and malononitrile 2. This step was
regarded as a rapid Knoevenagel condensation. Compound 7 is gained through
Michael addition in which kojc acid 3 is employed as nucleophile attacks on
unsaturated nitriles 7. As the result of the intramolecular addition reaction, including
the hydroxyl group and the cyano group in compound 11, the imine 14 is generated.
Finally, following a tautomeric proton shift of imine 14, 2-amino-6-(hydrox-
ymethyl)-8-oxo-4-aryl-4,8-dihydropyrano[3,2-b]pyrane 4 is formed.
The recovery and reuse of MCM-41-SO3H were studied by using the preparation
of 4a as a model. The result indicated that the catalyst could be reused several times
without significant loss of its activity and any prolonging of reaction time (Fig. 4).
Furthermore, leaching of sulfur during the catalytic reaction was found to be
negligible and decreased activity was attributed to catalyst weight loss in recycling.
The amount of sulfonic acid groups of the catalyst before reaction indicated
0.95 mmol g-1. After reaction at 90 °C, the amount of sulfonic acid groups of
MCM-41-SO3H was found to be 0.93 mmol g-1
.
Conclusion
In summary, we have demonstrated an environmentally benign and simple protocol
for the synthesis of pyrano[3,2-b]pyrans derivatives by a one-pot three-component
reaction of aldehydes or malononitrile, and kojic acid in the presence of MCM-41-
SO3H as efficient and recoverable catalyst. The catalyst can be recycled and reused
without losing the original activity.
Acknowledgments The financial support from University of Mazandaran is gratefully acknowledged.
Compliance with ethical standards
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MCM-41-SO3H: an efficient, reusable, heterogeneous…
We confirm that the manuscript has been submitted solely to this journal and is not published, in press, or
submitted elsewhere. We confirm that all the research meets the ethical guidelines, including adherence to
the legal requirements of the study country.
Conflicts of interest We certify that we have no affiliation with or financial involvement with any
organization or entity with a direct financial or any other interest in the subject matter or materials
discussed in the manuscript. We declare no conflicts of interest.
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