One-pot synthesis of dihydropyrano[2,3-c]chromenes via a three component coupling of aromatic aldehydes, malononitrile, and 3-hydroxycoumarin catalyzed by nano-structured ZnO in water: A green protocol

Article abstract of DOI:10.1016/j.tetlet.2011.06.101

An efficient and green protocol for the synthesis of dihydropyrano[2,3-c] chromene derivatives by one-pot, three component coupling reaction of aromatic aldehyde, malononitrile, and 3-hydroxycoumarin has been developed using nano-structured ZnO as the cat

Full text of DOI:10.1016/j.tetlet.2011.06.101

Tetrahedron Letters 52 (2011) 4636–4641
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-pot synthesis of dihydropyrano[2,3-c]chromenes via a three component
coupling of aromatic aldehydes, malononitrile, and 3-hydroxycoumarin
catalyzed by nano-structured ZnO in water: a green protocol
Sanjay Paul a, Pranabes Bhattacharyya a, Asish R. Das a,
a Department of Chemistry, University of Calcutta, Kolkata 700009, India
⇑
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and green protocol for the synthesis of dihydropyrano[2,3-c]chromene derivatives by one-
pot, three component coupling reaction of aromatic aldehyde, malononitrile, and 3-hydroxycoumarin
has been developed using nano-structured ZnO as the catalyst. The formation of self-assembled organic
nanofiber arising from low-molecular-weight molecules has also been performed. The optical properties
of the nano-structured organic molecules were studied.
Received 26 May 2011
Revised 20 June 2011
Accepted 27 June 2011
Available online 2 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Dihydropyrano[2,3-c]chromene
Green protocol
Organic nanofibers
Self aggregation
Multicomponent coupling reaction (MCR) is a powerful syn-
thetic tool for the synthesis of biologically active compounds.1
Development of such multicomponent coupling reaction strategies
in aqueous medium has been of considerable interest, as they pro-
vide simple and rapid access to a large number of organic mole-
cules through a sustainable path.2 One important aspect of green
chemistry is the elimination of solvents in chemical processes or
the replacement of hazardous solvents with relatively benign sol-
vents. Water being the most environmentally benign, cleanest,
cheapest, nonflammable, and naturally occurring solvent having
high specific heat capacity is the primary choice. Moreover, in
water, significant rate enhancement was observed in many reac-
tions,3 because of hydrophobic interactions that induce a favorable
aggregation of polar components in water.4
Pyrano[3,2-c]chromene derivatives have received considerable
attention of synthetic and medicinal chemists because of their
broad spectrum of biological and pharmaceutical activities. These
derivatives show antihyperglycemic and antidyslipidemic,5
cytotoxic,6 molluscicidal,7 antiinflammatory,8 and antifungal activ-
ities.9 These compounds also exhibit a wide spectrum of biological
activities,10 including anticancer,11 antimalarial12 and are also
widely employed as cosmetics, pigments,13 and potential biode-
gradable agrochemicals.14 Again [3,2-c]chromene derivatives are
components of numerous natural products like calanolides,
calanone, calophyllolides etc.15
In view of the great importance of chromene[3,2-c] derivatives,
in recent years efforts have been made in developing new method-
ologies for the synthesis of these compounds. A variety of reagents,
such as heteropolyacids,16 ionic liquid,17 magnesium oxide,18
diammonium hydrogen phosphate,19 tetrabutylammoniumbro-
mide,20 KF/Al2O3,21 and triethylbenzylammonium chloride22 have
been employed to achieve the synthesis of these molecules. The
conventional protocol involves a three component condensation
of 4-hydroxycoumarin, malononitrile, and aldehyde in presence
of solvent or water and under elevated temperature.16–21 However,
there is a maiden report for the synthesis of dihydropyrano[2,3-
c]chromene derivatives from 3-hydroxycoumarin and condensed
product of the malononitrile and aldehyde.23 But it is a two step
process and it takes longer time for completion of the reaction.
Herein, we disclose a general, rapid, high yielding, green synthetic
protocol24 for a variety of chromene[2,3-c] derivatives starting
from less explored 3-hydroxycoumarin25 with an objective to find
useful applications of dihydropyrano[2,3-c]chromene derivatives.
Since 4-hydroxycoumarin (pKa 0.735 at 70 °C in water) is enol
tautomer of 1,3-dicarbonyl compound, in presence of acid or base
it can act as a nucleophile and easily forms chromene[3,2-c] deriva-
tives. But, 3-hydroxycoumarin (pKa 4.23 at 70 °C in water) may be
considered as the enol tautomer of 1,2-dicarbonyl compound and
hence it is a very poor Michael donor under acidic or basic conditions
and the formation of chromene[2,3-c] derivatives from it is rather
difficult. As nanomaterials are of much potential in organic reaction
owing to their high surface to volume ratio and large number of
active sites, we considered that catalysis by nanomaterial may take
the pivotal role in the synthesis of [2,3-c]chromene derivative.
⇑
Corresponding author. Tel.: +91 3323501014/9433120265; fax: +91 332351
9754.
E-mail addresses: ardchem@caluniv.ac.in, ardas66@rediffmail.com (A.R. Das).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.101

S. Paul et al. / Tetrahedron Letters 52 (2011) 4636–4641
4637
Accordingly, we achieved the synthesis of dihydropyrano[2,3-
c]chromene derivatives using nano ZnO as catalyst (Scheme 1) and
inducing 3-hydroxycoumarin as the potential Michael donor to par-
ticipate in multicomponent coupling reaction.
In the preliminary stage of investigation we focused on system-
atic evaluation of different catalysts for the model reaction of 4-
nitrobenzaldehyde, malononitrile, and 3-hydroxycoumarin at
70 °C in aqueous medium. A wide variety of catalysts including
at a higher temperature there might be some sort of polymeriza-
tion of the Knoevenagel condensation product which lowers the
yield of the desired product.
Various solvents were also screened to test their efficiency at
70 °C and the results are summarized in Table 2. It is noteworthy
to mention that the polar solvents afforded better yield than the
nonpolar ones and the best result was obtained in aqueous med-
ium in which nano ZnO catalyst worked most efficiently by phas-
ing out of the desired product. This also revealed that quantity of
the catalyst plays a vital role in realizing the optimized product
yield. An increase in the amount of nano ZnO from 3 to 10 mol %
increased the yield of the desired product to a great extent (54–
91%, Table 2).
On the basis of the above results during the optimization of cat-
alyst and solvent, we then devised a novel three-component one
pot synthesis of functionalized pyrano[2,3-c]chromene derivatives
in aqueous medium in presence of nano ZnO under thermal condi-
tions. To study the scope and limitations of this protocol, we have
employed a wide range of aldehydes with 3-hydroxycoumarin and
malononitrile. As mentioned in Table 3 (following Scheme 1), the
reaction proceeded smoothly with unsubstituted benzaldehyde,
electron-withdrawing, and also with electron-releasing para-
substituted benzaldehydes.
With these excellent results in hand, we are now in a position to
propose the mechanism of the reaction which involves, Knoevena-
gel condensation, Michael addition, and then intra molecular cycli-
zation catalyzed by nano ZnO as presented in Scheme 2. In first
step, the Knoevenagel condensation is catalyzed by amphoteric
nano ZnO which during abstraction of acidic proton from malono-
nitrile unit acts as a base and during dehydration, it shows acidic
nature. At the time of cyclization, the particular catalyst plays
the key role where ZnO acts as a mild acid and not only minimizes
the 1,2 dipolar repulsion but also activates the nitrile group by
polarizing the same very efficiently for the intramolecular nucleo-
philic attack by the OH group leading to the ring closure which
ultimately forms the final product and again the catalyst enters
into the catalytic cycle.
In addition, when the reaction was scaled up to 2.5 g with
10 mol % nano ZnO at 70 °C in aqueous medium, excellent results
were still obtained in 3 h. To take advantage of the elaborated
green protocol, solid product was separated from water phase by
filtration, and after crystallisation from ethanol, pure chromene
derivatives were obtained. This procedure excluded use of expen-
sive silica gel chromatography, and the application of small
amount of ethanol instead of chromatographic eluents seems to
be promising from green chemistry perspective.
nano aluminum oxide (Al2O3), H6P2W18O62,18H2O, L-proline, alum,
tetrabutylammonium bromide, zeolites, bulk ZnO, and nano ZnO
were employed to improve the yield for the specific synthesis of
dihydropyrano[2,3-c]chromene derivative. The results are pre-
sented in Table 1. Interestingly, when the reaction was carried
out in the presence of nano ZnO; it led to the desired product in
91% yield in 2.5 h (Table 1).
During the optimization of the reaction condition, the effect of
temperature was monitored (Fig. 1). At 70 °C the yield of the prod-
uct is maximum. Due to the unavailability of activation energy, the
rate of the reaction is very slow at a temperature below 70 °C and
CN
Ar
NH2
OH
Nano ZnO
H2O , 70oC
O
CN
CN
+
Ar CHO
+
O
O
O
O
Scheme 1. Synthesis of dihydropyrano[2,3-c]chromene derivatives over nano ZnO.
Table 1
Influence of different catalysts for the reaction of 4-nitrobenzaldehyde, malononitrile,
and 3-hydroxycoumarin at 70 °C in aqueous medium
Entry
Catalyst
Time (h)
Yielda (%)
1
2
3
Nano aluminum oxide (Al2O3)
H6P2W18O62, 18H2O
6
7
6
29
48
70
L-Proline
4
5
6
7
8
Alum
8
6
8
6
53
31
-
51
91
Tetrabutylammonium bromide
Zeolites
Bulk ZnO
Nano ZnO
2.5
a
Isolated yield of the pure compound.
100
90
80
70
60
50
40
30
20
It is noteworthy to mention that in recent years nanofibers have
gained a tremendous attention in material science.26 Nanofiber-
based materials are considered to be very promising for broad
spectrum of applications, such as, templating of inorganic
Table 2
Solvent effects on the three-component coupling reaction for the synthesis of
dihydropyrano[2,3-c]chromene derivatives
Entry
Catalysta (mol %)
Solvent
Temperature (°C)
Yieldb
1
2
3
4
5
6
7
8
9
Nano ZnO (3 mol %)
Nano ZnO (6 mol %)
Nano ZnO (10 mol %)
Nano ZnO (15 mol %)
Nano ZnO (10 mol %)
Nano ZnO (10 mol %)
Nano ZnO (10 mol %)
Nano ZnO (10 mol %)
Nano ZnO (10 mol %)
H2O
H2O
H2O
H2O
Toluene
Acetonitrile
DMF
DMSO
Ethanol
70
70
70
70
70
70
70
70
70
54
63
91
83
49
52
67
69
70
20
30
40
50
60
70
80
90
100
110
temperature (0C)
a
Reaction time: 2.5 h.
Isolated yield of the pure compound.
Figure 1. Effect of temperature on the reaction of 4-nitrobenzaldehyde, malono-
nitrile and 3-hydroxycoumarin at 70 °C in aqueous medium.
b

4638
S. Paul et al. / Tetrahedron Letters 52 (2011) 4636–4641
Table 3
Nano ZnO catalyzed three component coupling of dihydropyrano[2,3-c]chromene derivative (Scheme 1)
b
Entry
Aldehyde
Product
Time (h)
Yield (%)
Reference
CHO
CN
NH2
O
1
3
87
23
O
O
3a
CHO
NO2
O2N
H3CO
Br
CN
NH2
O
2
3
4
5
2.5
91
78
89
83
23
23
23
—
O
O
3b
CHO
CN
NH2
O
OCH3
3
O
O
3c
CHO
Br
CN
NH2
O
2.5
O
O
3d
CHO
CH3
H3C
CN
NH2
O
a
3
O
O
3e
CHO
CN
NH2
O
6
2.5
89
—
O
O
3f
CHO
F
F
CN
NH2
O
7
2.0
90
—
O
O
3g
CHO
CN
NC
CN
NH2
O
8
9
2.5
90
75
—
—
O
O
3h
CN
NH2
S
S
CHO
O
3
O
O
3i

S. Paul et al. / Tetrahedron Letters 52 (2011) 4636–4641
4639
Table 3 (continued)
b
Entry
Aldehyde
Product
Time (h)
Yield (%)
Reference
CHO
H2N
NC
O
O
O
O
10
4
65
—
CHO
O
O
CN
NH2
3j
a
Single crystals were grown from hexane/ethyl acetate solution (Supplementary data).
b
Products (3a–j) are characterized from 1H NMR, 13C NMR, IR, HR-MS data.
Conventional nomenclature distinguishes between red-shifted
J- and blue-shifted H-aggregate transitions, arising from a parallel
and perpendicular alignment of transition dipoles, respectively.
The J-aggregates are characterized by very narrow, intense and
red shifted absorption relative to the monomer. The UV-absorption
spectrum of the self aggregated nanofiber has shown a bathochro-
mic shift and a decrease in the bandwidth relative to the monomer
as shown in the Figure 3 and it indicates that the J-aggregation of
the compound 3j leads to the formation of the highly ordered stack
like architecture. The spectral properties of the self-assembled or-
ganic nanofiber are furthermore emphasized in the fluorescence
titration study (6 ꢀ 10ꢁ3 M to 18 ꢀ 10ꢁ3 M) as shown in the Fig-
ure 4 and an obvious and remarkable enhancement of the fluores-
cence intensity was observed when the same experiment was
performed for the self-aggregated nano-crystalline compound. This
dramatic enhancement of fluorescence as well as the red shift is
attributed to the extensive H-bonding interaction between the
suitably oriented NH2 groups of the molecules and a unidirectional
aggregation of the monomers leading to J-stacking which is quite
presumable due to the asymmetric nature of the molecule.
At the end we must emphasize how intriguing is the formation
of fibers from the coumarin derivative. Different type of organic
systems have been reported to self organize into fluorescent solid
micro- or nano-fibers.34–36 Concerning the reprecipitation method,
the formation of nanowire crystals has already been reported for
1,3-diphenyl-2-pyrazoline37 and a phenylenediamine derivative,38
as well as for the polydiacetylene studied by Nakanishi et al.39 But
in all cases the reprecipitation was done either at high temperature
or by adding surfactant micelles to the medium40 to obtain nano-
structures but the formation of this type of nanofiber from the or-
ganic molecule of low molecular weight is, therefore, rather an
uncommon and challenging job which we have achieved very suc-
cessfully with ease.
O
CN
CN
+
H
Ar
H
CN
Ar
nano
ZnO
H
CN
O
Water
nano
ZnO
CN
Ar
NH2
O
O
O
O
O
O
Ar
CN
CN
H
Ar
N C
H
N C
O
O
OH
Scheme 2. Proposed mechanism for the formation of dihydropyrano[2,3-c]chro-
mene derivatives over nano ZnO.
materials,27 textile application,28 filtration,29 and catalysis.30Fur-
thermore, nanofibers can be used as potential functional materials
for photovoltaics, optoelectronics,31 optics32 etc. Thus we focused
our attention to generate self aggregated organic-nanofibers involv-
ing the synthesised dihydropyrano[2,3-c]chromene derivative.
The application of ‘bottom up’ method and more specifically
‘reprecipitation method’ is very much extensively used for the syn-
thesis of organic nanofibers.33 Therefore; we become fascinated in
exploring the possibility of formation of materials of specific mor-
phology with the synthesized compounds. Unfortunately only the
compound 3j had shown the self-aggregating property. Scanning
electron microscopic (SEM) images of the dried fibrous materials
of the compound 3j grown slowly from ethylacetate–hexane mix-
ture (1:1) clearly demonstrate that the aggregates in the solid state
are bunches of flat linear fiber like structures (Fig. 2) having
300 nm widths.
The nano ZnO catalyst was also prepared by the bottom up
method. It was characterized by SEM, X-ray diffraction study. From
the SEM image (Supplementary data) it is evident that the particles
have a rod like morphology. The exact crystal plane and particle
size were estimated by X-ray diffraction analysis (Supplementary
data). In the wide angle study, characteristic peaks due to the crys-
tal planes appeared. The peaks correspond to the (1 0 0), (0 0 2),
(1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes of ZnO which are
Figure 2. SEM image of the organic nanofiber.

4640
S. Paul et al. / Tetrahedron Letters 52 (2011) 4636–4641
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
80
60
40
20
0
1
2
3
4
5
200
300
400
500
600
No of run
Wave Length(nm)
Figure 5. Reusability study of ZnO nano-rod with entry 2, Table 3.
Figure 3. UV absorption spectrum of 3j in self aggregated state (
very dilute methanol solution monomer ( ).
) and its
Acknowledgments
We gratefully acknowledge the financial support from U.G.C
and Calcutta University. S.P. thanks U.G.C, New Delhi, India for
the grant of Junior Research fellowship. P.B. thanks CSIR for the Se-
nior Research Fellowship.
Self aggregated state
2500
2000
1500
1000
500
18 X 10 -6 (M)
15 X 10 -6 (M)
12 X 10 -6 (M)
9 X 10 -6 (M)
6 X 10 -6 (M)
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.06.101.
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A mixture of an aromatic aldehyde (1.3 mmol), malononitrile (1.3 mmol), 3-
hydroxycoumarin (1.0 mmol), nano ZnO (10 mol %) in water (5 mL) was stirred
at 70 °C for a required period of time (TLC). After completion of the reaction,
the reaction mixture was filtered in hot to remove the catalyst and the filtrate
was allowed to cool at room temperature. The solid thus appeared was
collected by filtration, washed with chilled aqueous ethanol and finally
recrystallized from ethanol to get pure product. 3-Amino-5-oxo-1-p-tolyl-1,5-
dihydro-pyrano[2,3-c]chromene-2-carbonitrile(3e): Yield: (0.275 g, 83%);
characteristic: yellow crystalline solid; Mp: 231 °C (from EtOH); IR
(KBr):3430 (NH), 3296 (NH), 3173, 2198 (CN), 1727(C@O), 1659, 1597, 1406,
1295, 1165, 1116, 753 cmꢁ1; dH (300 MHz, DMSO-d6; Me4Si): d 2.14 (s, 3H,
CH3), 4.96 (s, 1H, CH), 7.01–7.44 (m, 10H, Ar, NH2); dC (75 MHz, DMSO-d6;
Me4Si): d 20.7, 37.3, 57.3, 116.4, 117.0, 119.6, 124.7, 125.2, 126.7, 127.4, 129.6,
130.3, 134.1, 136.7, 140.2, 150.3, 154.1, 158.9; HR-MS Calcd for C20H14N2O3
([M+H]+): 331.1083 found: 331.1082.
35. Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev.
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36. (a) Balzer, F.; Bordo, V. G.; Simonsen, A. C.; Rubahn, H.-G. Phys. Rev. B 2003, 67,
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37. Fu, H.; Xiao, D.; Yao, J.; Yang, G. Angew. Chem., Int. Ed. 2003, 42, 2883–2886.
38. Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004, 108, 10887–10892.
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Rescaled procedure for the preparation of dihydropyrano [2,3-c]chromene
derivatives:
(8.5 mmol), 3-hydroxycoumarin (8.0 mmol), nano ZnO (10 mol %) in water
(30 mL) was stirred at 70 °C for required period of time (TLC). After
A mixture of an aromatic aldehyde (8.5 mmol), malononitrile
a
completion of the reaction, the reaction mixture was filtered in hot to remove
the catalyst and the filtrate was allowed to cool at room temperature. The solid
thus appeared was collected by filtration, washed with chilled aqueous ethanol
and finally recrystallized from ethanol to get pure product.
25. (a) Miky, J. A. A.; Nadia, S. H.; Shmeiss, N. M. M. J. Indian Chem. Soc. 1997, 74,
814; (b) Ghantwal, S. R.; Samant, S. D. J. Indian Chem. Soc. 2000, 77, 100; (c)
40. Fu, H.; Xiao, D.; Yao, J.; Yang, G. Angew. Chem., Int. Ed. 2003, 42, 2883–2886.