Design, preparation, and characterization of Fe3O4 nanoparticles encapsulating β-cyclodextrin-bearing guanidine as a highly efficient and reusable heterogeneous base catalyst for synthesis of 3,4-dihydropyrano[3,2-c]chromenes

Article abstract of DOI:10.1002/aoc.5862

Chromene substructure is an important structural motif present in a variety of medicines, natural products, and materials showing biological activities. Here, a simple and convenient procedure for the synthesis of 3,4-dihydropyrano[3,2-c]chromene derivati

Full text of DOI:10.1002/aoc.5862

Received: 26 February 2020
DOI: 10.1002/aoc.5862
Revised: 29 April 2020
Accepted: 8 May 2020
F U L L P A P E R
Design, preparation, and characterization of Fe O
3
4
nanoparticles encapsulating β-cyclodextrin-bearing
guanidine as a highly efficient and reusable
heterogeneous base catalyst for synthesis
of 3,4-dihydropyrano[3,2-c]chromenes
Soraya Rahmatinejad | Hossein Naeimi
Department of Organic Chemistry,
Chromene substructure is an important structural motif present in a variety
of medicines, natural products, and materials showing biological activities.
Here, a simple and convenient procedure for the synthesis of 3,4-dihydropyrano
Faculty of Chemistry, University of
Kashan, 87317-51167, Kashan, Islamic
Republic of Iran, Iran
Correspondence
[3,2-c]chromene derivatives is described. For this purpose, Fe O nanoparticles
3 4
Hossein Naeimi, Department of Organic
Chemistry, Faculty of Chemistry,
University of Kashan, Kashan,
supported on β-cyclodextrin-guanidine were successfully prepared and used as
catalyst. The structure of this catalyst was assigned by Fourier transform
infrared spectroscopy, X-ray diffraction, scanning electron microscopy, thermal
gravimetric analysis, and vibrating sample magnetometer techniques. The pre-
pared nanocomposites were used as a highly active, heterogeneous, and reusable
nanocatalyst for the one-pot, three-component reaction of 4-hydroxycoumarin,
aromatic aldehydes, and ethyl cyanoacetate. This method has advantages
such as mild conditions, high yields, easy workup and simple purification
of products, little catalyst loading, cost efficiency, and reusability of the catalyst.
87317-51167, Islamic Republic
of Iran, Iran.
Email: naeimi@kashanu.ac.ir
Funding information
University of Kashan, Grant/Award
Number: Grant No. 159148/89
K E Y W O R D S
cyclodextrin, dihydropyrano[3,2-c]chromene, Fe
nanoparticles
3 4
O , heterogeneous catalyst, magnetic
[
7]
1
| INTRODUCTION
variety of catalysts such as L-prolin/SiO , SO -SBA-15,
2 3
[
8]
[9]
GO,
SBA-Py-Diph,
La-MOF, SBA-15@DABCO(1,-
Chromenes, common heterocyclic motifs containing
an oxygen atom, exist extensively in nature and
show several biological and therapeutic activities,
4-diazabicyclo[2.2.2]octane), silica-bonded 2-HEAA, and
[
10]
p-TSA.
In addition, dihydropyrano[3,2-c]chromene and
[1]
[2]
[11]
including antiacetylcholinesterase, antiproliferative,
its derivatives show antimicrobial, antifungal,
Acetylcholinesterase (AChE) inhibitory
and
[
3]
[4]
[5]
[12]
antibacterial, antioxidant, and antitumor proper-
ties. Chromenes and their derivatives isolated from plants
are widely used to treat diverse ailments including hyper-
activities.
Therefore, the development of efficient methods for
the synthesis of these important heterocycles has attracted
researchers' attention. Various methods have been
reported for the synthesis of these compounds in the pres-
[
6]
tension, diabetes, cancer, arthritis, and diarrhea. The
chemical structures of some bioactive chromenes are
shown in Scheme 1. Multicomponent synthesis of
chromene and its derivatives has been developed using a
[
13]
ence of a variety of catalysts, such as thiourea dioxide,
DABCO,
[
14]
[15]
DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene),
Appl Organomet Chem. 2020;e5862.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.5862

2
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RAHMATINEJAD AND NAEIMI
SCHEME 1 Some examples of
bioactive chromenes
[
12]
dehydroabietylamine–cinchonine–squaramide,
mag-
magnetic nanoparticles can serve as a highly efficient cat-
alyst supports. Magnetically supported catalysts are con-
sidered as green catalysts because they can be readily
separated from the reaction media by a simple magnetic
netic nanoparticles tag: piperidinium benzene-1,-
[
16]
3
-disulfonate ionic liquid,
polypyrrole/Fe O /Carbon
3 4
[17]
[18]
nanotube (CNT),
functionalized
benzenesulfinates,
ammonium acetate,
Li -montmorillonite,
Ni(II)-
+
[19]
[39,40]
sodium
separation step without any additional purification.
[20]
[21]
[22]
sodium malonate,
nano-SiO2,
potassium phthalimide,
crown ether complex cation ionic liquids,
There are various reports on applying Fe O
3
4
[
23]
[24]
o-benzene disulfonimide,
nanoparticle-supported β-cyclodextrin as catalyst in
[
25]
and
organic synthesis that indicate the efficiency of hybrid
[26]
[41–44]
N,N-dimethylbenzylamine.
magnetic catalysts.
Cyclodextrins (CDs) are cyclic oligomers made up
of α-(1,4)-linked D-glucopyranose units. The practically
important CDs are α-, β-, and γ-CD, which are composed
of six, seven, and eight units, respectively. All CDs
possess a truncated cone or torus structure. Due to the
presence of hydroxyl groups, the exterior surface of each
CD is hydrophilic, while the interior cavity is hydropho-
Herein, considering the importance of chromenes
and with the aim of developing an efficient,
reusable, heterogeneous, cost-effective, and environmen-
tally friendly catalyst for the synthesis of these com-
pounds, amino-functionalized β-CD was prepared and
grafted onto Fe O nanoparticles via the chemical co-
precipitation approach. The prepared catalyst's effi-
ciency was explored in the synthesis of dihydropyrano
[3,2-c]chromenes.
3
4
[27]
bic and nonpolar.
CDs have the ability to encapsulate
a variety of guest molecules, such as aliphatic and aro-
matic hydrocarbons, acids, amines, alcohols, and small
[
28]
inorganic anions, through host–guest interactions.
In
addition, the hydroxyl groups of CDs make them appro-
2 | EXPERIMENTAL
2.1 | General
[29]
priate for modification and functionalization.
CDs
have therefore been used as catalysts in various organic
[30–33]
reactions.
Guanidine and its derivatives have been
used in different organic reactions as a strong organobase
All commercially available reagents were used without
further purification and purchased from Merck, Fluka,
and Aldrich in high purity. The solvents used were
purified by standard procedure. Fourier transform
infrared (FT-IR) spectra were obtained as KBr pellets
on a Perkin Elmer 550 spectrometer in the range
[34–36]
catalyst.
Additionally, guanidines are found to be
asymmetric catalysts in various reactions, including
Henry, aza-Michael, phospha-Michael, Diels–Alder, pro-
tonation, electrophilic amination, and nucleophilic
[37]
epoxidation reactions.
Separation and recycling of the
400–4,000 cm− (Massachusetts, USA). H NMR spectra
1
1
catalyst are important factors that have been highly
[38]
regarded in recent years.
Immobilization of homoge-
were recorded in CDCl solvent on a Bruker DRX-400
3
neous organic catalysts on various support materials is a
sustainable method to recover and recycle catalysts.
Owning to their unique structures and properties,
spectrometer (Bruker Daltonics, Bremen Germany)
with tetramethylsilane as the internal reference. A
BANDELIN ultrasonic HD 3200 with probe model KE

RAHMATINEJAD AND NAEIMI
3 of 12
7
6 (Berlin, Germany) was used to produce ultrasonic irra-
Briefly, FeCl .6H O (1.10 g), FeCl .4H O (0.43 g) and
3
2
2
2
diation and homogenize the reaction mixture. Field-
emission scanning electron microscopy (FT-SEM) of the
nanocatalyst was performed on an AIS2100 instrument
β-CD-GA (0.7 g) were dissolved in distilled water (30 ml)
and the mixture was heated to 90 C with vigorous stirring.
Then, concentrated ammonia (25%) was added dropwise to
ꢀ
(
Seoul, Korea). Thermogravimetric analysis (TGA) was
the reaction mixture under an N atmosphere. Stirring was
continued for 60 min at 90 C. The resulting nanoparticles
were separated from the solution using an external
magnetic field, washed several times with deionized water,
2
ꢀ
carried out with a STA1500 (Rheometric Scientific,
Piscataway, NJ) instrument at a heating rate of 10 C/min
under an inert nitrogen atmosphere. X-ray diffraction
ꢀ
(
XRD) patterns of nanostructures were taken using Cu Kα
radiation (λ = 1.5406A) on a Holland Philips Xpert X-ray
Almelo, Holland) powder diffraction diffractometer. The
and dried in a vacuum oven. Bare Fe O4 Magnetic
nanoparticles (MNPs) were prepared using this method
without the addition of β-CD.
3
(
magnetic properties of nanoparticles were measured by a
vibrating sample magnetometer (VSM, PPMS-9 T, San
Diego, CA, USA) at 300 K at the University of Kashan,
Kashan, Iran. Determination of the trace metal ions was
carried out using a simultaneous Inductively coupled
plasma-optical emission spectrometry (ICP-OES) (Varian
Vista-Pro, Springvale, Australia) coupled to a V-groove
nebulizer and equipped with a charge coupled device
2.4 | General procedure for the synthesis
of dihydropyrano[3,2-c]chromene
derivatives catalyzed by Fe O -β-CD-GA
3
4
A mixture of aromatic aldehyde (1 mmol), 4-hydro-
xycoumarin (1 mmol, 0.16 g), and ethyl cyanoacetate
(1.1 mmol, 0.11 ml) in the presence of Fe O -β-CD-GA
(CCD). The elemental analyses (C, H, N) were obtained
3
4
from a Carlo ERBA Model EA 1108 analyzer (Milan, Italy).
Melting points were obtained using a Yanagimoto micro
melting point apparatus (Kyoto, Japan) and are
uncorrected. The purity determination of the substrates
and reaction monitoring were accomplished by thin-layer
chromatography (TLC) on silica-gel polygram SILG/UV
nanoparticles (0.05 g) in 5 ml of ethanol was refluxed for
the appropriate time as indicated in Table 4. The progress
of the reaction was monitored by TLC. At the end of the
reaction and after cooling to room temperature the catalyst
was separated with an external magnet. Then, distilled
water (5 ml) was added to the mixture and the solid prod-
uct was filtered off and washed with water and cold diethyl
ether to give the pure product. All of the products were
identified by physical and spectroscopic data (For IR and
2
54 plates (Merck Company).
1
2.2 | Preparation of β-cyclodextrin-
H NMR spectra, see Supporting Information)."
guanidine
2-Amino-4-(4-nitrophenyl)
3-carboethoxy-4H,5H-
(4a): Yellow solid,
pyrano[3,2-c]chromene-5-one
ꢀ
ꢀ
[47]
β-cyclodextrin-guanidine (β-CD-GA) was prepared
m.p. = 235–236 C (m.p. = 232–233 C ). IR (KBr)/ν
[
45,46]
−1
according to the previously reported procedure.
(cm ): 3440, 3,326, 1,719, 1,688, 1,661, 1,611, 1,512,
1
First, a solution of β-CD (1 g) and NaOH (30 ml, 22%
1,376, 1,284. H NMR (CDCl , 400 MHz)/δ (ppm): 1.15
3
m/v) was vigorously stirred at room temperature for
(3H, t, J = 7.2, CH ), 4.04–4.10 (2H, m, CH ), 5.03
3
2
2
4 hr to deprotonate the hydroxyl groups of β-CD. Then,
(1H, s, CH), 6.54 (2H, broad s, NH ), 7.34 (1H, d,
2
0
.4 g of guanidine powder (GN) was dissolved in 5 ml of
J = 7.6 Hz), 7.39 (1H, d, J = 7.6 Hz), 7.54 (2H, d,
J = 8.8 Hz), 7.58–7.62 (1H, m), 7.86 (1H, dd, J = 8,
1.2 Hz), 8.12 (2H, d, J = 8.8 Hz).
deionized water. The guanidine (GA) solution was added
dropwise to the β-CD solution and the mixture was
stirred for 2 hr. Next, 0.07 ml of epichlorohydrin (EPH)
was added and the reaction mixture was stirred for
another 3 hr. The mixture was concentrated to about
2-Amino-4-(3-nitrophenyl)
3-carboethoxy-4H,5H-
(4b): Yellow solid,
pyrano[3,2-c]chromene-5-one
ꢀ
ꢀ
[48]
m.p. = 235–236 C (m.p. = 228–230 C ). IR (KBr)/ν
−1
1
5 ml and cold ethanol was added until a gummy
(cm ): 3432, 3,313, 1,710, 1,687, 1,657, 1,610, 1,535,
1
precipitate appeared. The precipitate was washed with
ethanol and acetone and dried in an oven at 50 C.
1,374, 1,277. H NMR (CDCl , 400 MHz)/δ (ppm): 1.16
3
ꢀ
(3H, t, J = 7.2, CH ), 4.06–4.10 (2H, m, CH ), 5.03 (1H, s,
3
2
CH), 6.49–6.62 (2H, broad s, NH ), 7.33–7.44 (3H, m),
2
7.60 (1H, t, J = 7.6 Hz), 7.77 (1H, d, J = 7.6 Hz), 7.87 (1H,
2
.3 | Preparation of Fe O nanoparticle-
d, J = 8 Hz), 8.04 (1H, d, J = 7.6 Hz), 8.17 (1H, s). Anal.
3
4
supported β-CD-GA
calcd for C H N O : C 61.77, H 3.95, N, 6.86%; found: C
21
16 2 7
61.90, H 3.95, N 6.88%.
Fe O -β-CD-GA was prepared by a one-step co-precipitation
method according to a previously reported procedure.
2-Amino-4-(4-chlorophenyl) 3-carboethoxy-4H,5H-pyrano
[3,2-c]chromene-5-one (4c): White solid, m.p. = 189–191 C
3
4
[46]
ꢀ

4
of 12
RAHMATINEJAD AND NAEIMI
ꢀ
[47]
−1
1
(
m.p. = 188–190 C ). IR (KBr)/ν (cm ): 3466, 3,313,
1,690, 1,651, 1,610, 1,522, 1,372, 1,284, 1,198. H NMR
1
1
,717, 1,688, 1,660, 1,613, 1,528, 1,489, 1,373, 1,285,
(CDCl , 400 MHz)/δ (ppm): 1.19 (3H, t, J = 7.2, CH ),
3
3
1
,196. H NMR (CDCl , 400 MHz)/δ (ppm): 1.78 (3H, t,
4.06 (2H, q, J = 7.2, CH ), 5.06 (1H, s, CH), 6.46–6.51
3
2
J = 7.2, CH ), 4.03–4.12 (2H, m, CH ), 4.90 (1H, s, CH),
(2H, broad s, NH ), 6.90–6.94 (1H, m), 7.03 (1H, t,
3
2
2
6
7
.42–6.58 (2H, broad s, NH ), 7.21 (2H, d, J = 8.4 Hz),
.29–7.36 (4H, m), 7.55–7.59 (1H, m), 7.83 (1H, dd,
J = 7.4 Hz), 7.12–7.17 (1H, m), 7.31–7.41 (3H, m), 7.56
(1H, t, J = 8.0 Hz), 7.85 (1H, d, J = 8.0 Hz). C NMR
2
1
3
J = 8, 1.6 Hz).
(CHCl , 100 MHz)/δ ppm: 14.11, 31.35, 60.05, 78.59,
3
2
-Amino-4-(2-chlorophenyl)
3-carboethoxy-4H,5H-
(4d): White solid,
105.85, 113.41, 115.69, 116.92, 122.32, 123.57, 124.31,
128.58, 130.42, 131.84, 132.27, 152.72, 153.74, 158.10,
160.25, 160.64, 162.71, 168.78.
pyrano[3,2-c]chromene-5-one
ꢀ
ꢀ
[48]
m.p. = 218–220 C (m.p. = 221–223 C ). IR (KBr)/ν
−
1
(
1
(
cm ): 3397, 3,282, 1,716, 1,689, 1,648, 1,610, 1,527,
2-Amino-4-(2-bromophenyl)-3-carboethoxy-4H,5H-
1
,373, 1,283, 1,198. H NMR (CDCl , 400 MHz)/δ
ppm): 1.16 (3H, t, J = 7.2, CH ), 4.03–4.11 (2H, m,
pyrano[3,2-c]chromene-5-one
m.p. = 225–227 C. IR (KBr)/ν (cm ): 3397, 3,283, 1,717,
1,688, 1,649, 1,610, 1,527, 1,372, 1,283, 1,197. H NMR
(4i):
White
solid,
3
ꢀ
−1
3
1
CH ), 5.25 (1H, s, CH), 6.52 (2H, broad s, NH ),
2
2
7
(
.08–7.18 (2H, m), 7.25 (1H, s), 7.30–7.35 (2H, m), 7.39
1H, d, J = 6.8 Hz), 7.53–7.58 (1H, m), 7.84 (1H, d,
J = 8.0 Hz).
(CDCl , 400 MHz)/δ (ppm): 1.16 (3H, t, J = 7.2, CH ),
3
3
4.05–4.12 (2H, m, CH ), 5.27 (1H, s, CH), 6.50–6.52 (2H,
2
broad s, NH ), 6.99–7.04 (1H, m), 7.18–7.22 (1H, m),
2
2
-Amino-4-(3-chlorophenyl)
3-carboethoxy-4H,5H-
(4e): White solid,
7.31–7.38 (3H, m), 7.47 (1H, dd, J = 1.2, 8.0 Hz),
1
3
pyrano[3,2-c]chromene-5-one
m.p. = 214–216 C. IR (KBr)/ν (cm ): 3406, 3,306, 1,728,
,688, 1,655, 1,611, 1,527, 1,372, 1,287, 1,196. H NMR
CDCl , 400 MHz)/δ (ppm): 1.19 (3H, t, J = 7.2 Hz, CH ),
.05–4.10 (2H, m, Hz, CH ), 5.00 (1H, s, CH), 6.45 (2H,
broad s, NH ), 6.90–6.94 (1H, m), 7.02–7.05 (1H,
7.55–7.59 (1H, m), 7.85 (1H, dd, J = 1.2, 8.0 Hz).
C
ꢀ
−1
NMR (CHCl , 100 MHz)/δ ppm: 14.34, 36.45, 60.06,
3
1
1
(
4
79.11, 106.02, 113.23, 116.92, 122.33, 124.00, 124.26,
126.93, 128.20, 132.32, 132.85, 133.34, 142.07, 152.73,
153.51, 157.99, 160.32, 168.85. Anal. calcd for
C H NO Br: C 57.03, H 3.65, N 3.17%; found: C 57.11,
3
3
2
2
21 16
5
J = 7.6 Hz), 7.31–7.41 (3H, m), 7.54–7.58 (1H, m), 7.85
H 3.52, N 3.18%.
13
(
1H, d, J = 8 Hz). C NMR (CHCl , 100 MHz)/δ ppm:
2-Amino-4-(4-bromophenyl)-3-carboethoxy-4H,5H-
3
1
1
1
4.19, 35.45, 60.16, 79.68, 107.21, 113.36, 116.95, 122.31,
24.41, 126.98, 127.00, 128.58, 129.24, 132.42, 133.85,
46.34, 152.67, 153.39, 157.93, 160.65, 168.53. Anal. calcd
pyrano[3,2-c]chromene-5-one
(4j):
White
solid,
ꢀ
ꢀ
[49]
m.p. = 190–192 C (m.p. = 193–194 C ). IR (KBr)/ν
−1
(cm ): 3419, 3,295, 1,717, 1,690, 1,651, 1,610, 1,520,
1
for C H NO Cl: C 63.40, H 4.05, N 3.52%; found:
1,374, 1,279, 1,197. H NMR (CDCl , 400 MHz)/δ (ppm):
21
16
5
3
C 63.70, H 4.05, N 3.73%.
-Amino-4-(2,4-dichlorophenyl)
H,5H-pyrano[3,2-c]chromene-5-one (4f): White solid,
1.17 (3H, t, J = 7.2, CH ), 4.03–4.12 (2H, m, CH ), 4.88
3
2
2
3-carboethoxy-
(1H, s, CH), 6.46–6.49 (2H, broad s, NH ), 7.24 (1H, d,
2
4
J = 8.4 Hz), 7.32–7.37 (3H, m), 7.54–7.58 (1H, m), 7.83
(1H, d, J = 8.0 Hz).
ꢀ
ꢀ
[49]
m.p. = 198–199 C (m.p. = 200–201 C ). IR (KBr)/ν
−
1
(
1
cm ): 3411, 3,291, 1,721, 1,691, 1,654, 1,611, 1,523,
2-Amino-4-(3-methoxyphenyl)-3-carboethoxy-4H,5H-
1
,374, 1,280, 1,198. H NMR (CDCl , 400 MHz)/δ
pyrano[3,2-c]chromene-5-one
m.p. = 210–212 C. IR (KBr)/ν (cm ): 3412, 3,305, 1,695,
1,658, 1,634, 1,605, 1,539, 1,377, 1,272, 1,194. H NMR
(4k):
White
solid,
3
ꢀ
−1
(
ppm): 1.18 (3H, J = 7.2 Hz, CH ), 4.03–4.14 (2H, m,
3
1
CH ), 5.20 (1H, s, CH), 6.53 (2H, broad s, NH ), 7.15
2
2
(
7
1H, dd, J = 8.4, 2 Hz), 7.28 (1H, d, J = 2 Hz),
.32–7.37 (3H, m), 7.56–7.60 (1H, m), 7.84 (1H, d,
J = 6.8 Hz).
(CDCl , 400 MHz)/δ (ppm): 1.18 (3H, t, J = 7.2 Hz, CH ),
3
3
3.76 (3H, s, OMe), 4.07 (2H, q, J = 7.2 Hz, CH ), 4.90
2
(1H, s, CH), 6.42–6.51 (2H, broad s, NH ), 6.70 (1H, d,
2
2
-Amino-4-(4-fluorophenyl)-3-carboethoxy-4H,5H-
J
=
5.4 Hz), 6.92–6.97 (2H, m),7.12–7.17 (1H,
pyrano[3,2-c]chromene-5-one
(4g):
White
solid,
m),7.24–7.33 (2H, m), 7.52–7.56 (1H, m), 7.79–7.81 (1H,
ꢀ
ꢀ
[49]
13
m.p. = 223–224 C (m.p. = 223–225 C ). IR (KBr)/ν
d, J = 7.2 Hz). C NMR (CHCl , 100 MHz)/δ ppm: 14.27,
3
−
1
(
1
cm ): 3483, 3,315, 1,717, 1,687, 1,662, 1,610, 1,529,
35.44, 55.20, 60.05, 80.11, 107.90, 111.72, 113.53, 114.69,
116.84, 121.01, 122.20, 124.28, 132.17, 145.97, 152.57,
153.24, 158.04, 159.36, 160.80, 168.74. Anal. calcd for
C H NO : C 67.17, H 4.87, N 3.56%; found: C 67.42, H
1
,510, 1,374, 1,288, 1,200. H NMR (CDCl , 400 MHz)/δ
3
(ppm): 1.17 (3H, t, J = 7.2, CH ), 4.04–4.12 (2H, m, CH ),
3
2
4
.91 (1H, s, CH), 6.44 (2H, broad s, NH ), 6.92 (2H, t,
2
22 19
6
J = 8.8 Hz), 7.31–7.37 (4H, m), 7.55–7.59 (1H, m), 7.84
1H, dd, J = 7.6, 1.6 Hz).
-Amino-4-(2-fluorophenyl)-3-carboethoxy-4H,5H-
4.87, N 3.53%.
2-Amino-4-(4-methoxyphenyl)-3-carboethoxy-4H,5H-
(
2
pyrano[3,2-c]chromene-5-one
(4l):
White
solid,
ꢀ
ꢀ
[49]
pyrano[3,2-c]chromene-5-one
(4h):
White
solid,
m.p. = 165–167 C (m.p. = 160–162 C ). IR (KBr)/ν
ꢀ
−1
−1
m.p. = 223–224 C. IR (KBr)/ν (cm ): 3393, 3,271, 1,718,
(cm ): 3399, 3,297, 1,695, 1,654, 1,608, 1,534, 1,510,

RAHMATINEJAD AND NAEIMI
1
5 of 12
1
1
,376, 1,282, 1,192. H NMR (CDCl , 400 MHz)/δ (ppm):
3 | RESULTS AND DISCUSSION
3
.19 (3H, t, J = 7.2, CH ), 3.75 (3H, s, OMe), 4.07–4.10
3
(
6
2H, m, CH ), 4.88 (1H, s, CH), 6.41 (2H, broad s, NH ),
.78 (2H, d, J = 8.4 Hz), 7.29–7.35 (4H, m), 7.53–7.57
3.1 | Preparation and characterization of
catalyst
2
2
(1H, m), 7.82 (1H, d, J = 8.0 Hz).
2
-Amino-4-(2-methylphenyl) 3-carboethoxy-4H,5H-
The synthetic strategy for the preparation of magnetic
nanoparticle-supported β-CD-GA is shown in Scheme 2.
In the beginning, deprotonation of hydroxyl groups
on CDs was performed in an alkaline aqueous solution at
room temperature. Then, GN was added to the obtained
solution to activate GA. After adding EPH to the solution,
substitution of chlorine and ring-opening of epoxide
occurred. A simple co-precipitation method was applied
for the preparation of Fe O -supported β-CD-GA. The
pyrano[3,2-c]chromene-5-one
m.p. = 221–224 C. IR (KBr)/ν (cm ): 3400, 3,290,1,686,
,653, 1,609, 1,533, 1,376, 1,282, 1,195. H NMR (CDCl ,
00 MHz)/δ (ppm): 1.20 (3H, t, J = 7.2, CH ), 2.29 (3H, s,
(4m):
White
solid,
ꢀ
−1
1
1
4
3
3
CH ), 4.04–4.13 (2H, m, CH ), 4.90 (1H, s, CH), 6.43 (2H,
3
2
broad s, NH ), 6.97 (1H, d, J = 6.8 Hz), 7.11–7.16 (3H,
m), 7.31–7.36 (2H, m), 7.53–7.58 (1H, m), 7.84 (1H, dd,
J = 8, 1.2 Hz). C NMR (CHCl , 100 MHz)/δ ppm: 14.21,
2
13
3
3 4
2
1
1
1.50, 35.43, 60.03, 80.37, 108.07, 113.59, 116.85, 122.24,
24.27, 125.62, 127.57, 127.93, 129.20, 132.13, 137.55,
44.19, 152.60, 153.21, 157.96, 160.81, 168.80. Anal. calcd
catalyst was obtained from FeCl and FeCl as iron pre-
2 3
cursors, and β-CD-GA that mixed together in the reaction
medium. After successful preparation of Fe O -β-CD-GA,
3
4
for C H NO : C 70.02, H 5.07, N 3.71%; found: C 70.31,
the catalyst was characterized by FT-IR, SEM, XRD,
22
16
5
H 5.29, N 3.90%.
TGA, and VSM techniques.
2-Amino-4-phenyl-3-carboethoxy-4H,5H-pyrano
The FT-IR spectra of the β-CD-GA and Fe O -
3
4
[
3,2-c]chromene-5-one
(4n):
White
solid;
β-CD-GA are shown in Figure 1. The spectrum of
β-CD-GA (Figure 1a) shows an absorption band at
3463 cm that is related to the overlap between water
ꢀ
ꢀ
[47]
m.p. = 208–209 C (m.p. = 209–211 C ). IR (KBr)/ν
−
1
−1
(cm ): 3413, 3,307, 1,688, 1,655, 1,609, 1,543, 1,492,
1
1
,377, 1,278, 1,192, 1,053. H NMR (CDCl , 400 MHz)/δ
molecules, the primary and secondary O–H groups of
β-CD, and the N–H stretching of guanidine. The
absorption peaks at 1661 and 1,440 cm− are related to
C=N and C–N of guanidine, respectively. Characteristic
peaks of β-CD at 1156 and 1,034 (attributed to the C–
O of the secondary alcohol and C–O–C, respectively)
are present with a small shift compared to the same
3
(ppm): 1.12 (3H, t, J = 7.2, CH ), 4.10 (2H, q, J = 6.8 Hz
3
1
CH ), 4.90 (1H, s, CH), 6.18–6.53 (2H, broad s, NH ), 7.16
2
2
(
7
1H, d, J = 6.8 Hz), 7.22–7.25 (3H, m), 7.30–7.37 (3H, m),
.55 (1H, t, J = 8.0 Hz), 7.83 (1H, d, J = 8.0 Hz). Anal.
calcd for C H NO : C 64.41, H 4.72, N 3.55%; found: C
21
17
5
6
4.70, H 4.99, N 3.58%.
3 4
SCHEME 2 Schematic presentation of β-CD-GA grafting onto Fe O nanoparticles

6
of 12
RAHMATINEJAD AND NAEIMI
FIGURE 1 IR spectra of (a) β-CD-GA,
(
b) Fe
3
O
4
3 4
-β-CD-GA, and (c) Fe O
[
45]
peaks in pure CD.
The peak at 569 in the spectrum
To find the morphological characteristics of the
nanoparticles SEM was carried out (Figure 3). The SEM
image of Fe O -β-CD-GA shows semi spherical particles
of Fe O4 (Figure 1c) is related to the Fe–O bond,
which is shifted to 574 cm after being supported with
β-CD-GA (Figure 1b).
3
−
1
3
4
with average size of about 49 nm.
The XRD patterns of Fe O nanoparticles and Fe O -
The magnetic properties of the prepared Fe O
3
4
3
4
3
4
β-CD-GA are illustrated in Figure 2. All of the peaks are
in good agreement with the standard XRD pattern of
Fe O (JCPDS card No. 75–0449) in terms of position and
nanoparticles and final catalyst were characterized by
VSM (Figure 4). Comparison of the room temperature
magnetization curves of Fe O4 nanoparticles and
3
4
3
relative intensity. No obvious difference between the two
XRD patterns reveals that the grafting of β-CD-GA onto
Fe O nanoparticles did not result in a phase change of
Fe O -β-CD-GA indicates that the saturation magnetiza-
3
4
tion of Fe O -β-CD-GA because of the nonmagnetic
3
4
properties β-CD-GA is less than that of the bare Fe O
3
4
3 4
Fe O4 nanoparticles. The average crystallite size of
nanoparticles. The saturation magnetization values were
3
Fe O -β-CD-GA by applying the Scherrer formula was
found to be 52.29 and 29.53 emu/g for nano Fe O and
3
4
3 4
calculated 32 nm.
Fe O -β-CD-GA, respectively.
3
4
FIGURE 2 XRD patterns of (a) nano-Fe
and (b) Fe -β-CD-GA
3 4
O
3 4
O

RAHMATINEJAD AND NAEIMI
7 of 12
3 4
FIGURE 3 SEM image of Fe O -β-CD-GA
FIGURE 4 VSM curve of analysis of
a) Fe nanoparticles and (b) Fe -β-CD-GA
(
3
O
4
3 4
O
The thermal stability and composition of Fe O -β-CD-
presence of Fe O -β-CD-GA nanoparticles as a highly
3
4
3 4
GA were analyzed by TGA (Figure 5). The weight loss at
temperatures below 200 C is most likely due to the
efficient catalyst was examined (Scheme 3).
ꢀ
Initially, in an attempt to develop an optimal
catalytic system for the synthesis of dihydropyrano
[3,2-c]chromene derivatives the reaction of 2-chloro-
benzaldehyde (1 mmol), 4-hydroxycoumarin (1 mmol),
and ethyl cyanoacetate (1.1 mmol) was selected as a
model reaction. The reaction was carried out in the
presence of different quantities of Fe O -β-CD-GA in
elimination of the adsorbed water or trapped solvent. The
ꢀ
other degradation stage between about 220 and 600 C
with a weight loss of approximately 12% can be attributed
to the decomposition of organic groups.
3
4
3
.2 | Catalytic activity of the prepared
catalyst in the synthesis of dihydropyrano
3,2-c]chromene derivatives
ethanol as a solvent. The results are summarized in
Table 1. The results of the controlled blank reaction
showed that in the absence of a catalyst, no product
was obtained even after 4 hr (Table 1, entry 1). It was
found that the best results in terms of time and prod-
uct yield were obtained in the presence of 0.05 g of
[
In this study, the reaction of aromatic aldehydes,
-hydroxycoumarin, and ethyl cyanoacetate in the
4

8
of 12
RAHMATINEJAD AND NAEIMI
3 4
FIGURE 5 TGA curve of Fe O -β-CD-GA
SCHEME 3 Synthesis of
dihydropyrano[3,2-c]chromene derivatives
the catalyst (Table 1, entry 4), which was not
improved by increasing the catalyst amount to 0.07 g
cyanoacetate in 5 ml of EtOH was performed in the pres-
ence of bare Fe O nanoparticles, bare β-CD, and guani-
3
4
(Table 1, entry 5).
dine separately, and their catalytic activities were
To examine the role of Fe O , β-CD, and guanidine
compared with that of Fe O -β-CD-GA (Table 2). The
3
4
3 4
skeletons of the catalyst in the reaction, the condensation
reaction using these skeletons as catalyst afforded the
of 2-chlorobenzaldehyde, 4-hydroxycoumarin, and ethyl
products in lower yields and longer reaction times
a
TABLE 1
Optimization of the catalyst amount for the synthesis of dihydropyrano[3,2-c]chromenes
b
Entry
Catalyst amount (g)
Time (min)
Yield (%)
1
2
3
No catalyst
0.02
240
80
80
80
80
0
69
80
89
89
0.03
4
5
0.05
0.07
a
General reaction conditions: mixture of 2-chlorobenzaldehyde (1 mmol), 4-hydroxycoumarin (1 mmol), and ethyl cyanoacetate (1.1 mmol)
in 5 ml of EtOH was refluxed.
b
Isolated yields.

RAHMATINEJAD AND NAEIMI
9 of 12
TABLE 2
3 4
Study of the role of Fe O nanoparticles, β-CD, and guanidine skeletons of the catalyst on the synthesis of dihydropyrano
a
[
3,2-c]chromenes
b
Entry
Catalyst/catalyst amount
Fe -β-CD-GA (0.05 g)
Nano Fe (0.04 g)
Time (min)
Yield (%)
1
2
3
4
3
O
4
80
89
30
42
58
3
O
4
180
180
180
β-CD (0.19 g)
Guanidine (0.01 g)
a
General reaction conditions: mixture of 2-chlorobenzaldehyde (1 mmol), 4-hydroxycoumarin (1 mmol), and ethyl cyanoacetate (1.1 mmol)
in 5 ml of EtOH was refluxed.
b
Isolated yields.
compared with our method (Table 2, entries 2–4).
These results can be attributed to the synergistic effects
of β-CD, GA, and Fe O nanoparticles, which increase
catalysts because they efficiently disperse active cata-
lytic sites in the reaction medium.
To survey the effects of temperature and solvent, the
reaction was performed in the presence of optimum
amounts of Fe O -β-CD-GA in CH Cl , CH CN, H O,
3
4
the efficacy of the catalyst. The β-CDs are well known
as host molecules with notable inclusion capabilities
for a variety of small organic guests. In the prepared
Fe O -β-CD-GA, the catalytic property of CD is due to
3
4
2
2
3
2
and EtOH solvents at room temperature and under reflux
conditions (Table 3). The best results were obtained in
EtOH under reflux conditions (Table 3, entry 6). There-
fore, 0.05 g of the catalyst, EtOH as solvent, and reflux
conditions were chosen as optimum terms.
In another study, to extend the scope of the
reaction, the reaction of 4-hydroxycoumarine and ethyl
cyanoacetate with a range of aromatic aldehydes was
performed according to the general experimental proce-
dure (Table 4). As shown in Table 4, various aromatic
aldehydes with electron-withdrawing and electron-
donating groups reacted well to give the corresponding
3
4
the inclusion complex formation between reactants
(aldehyde and ethyl cyanoacetate) and β-CD via hydro-
gen bonding between the carbonyl oxygen of the reac-
tants and the outwardly hydroxyl groups of β-CD,
which activates the reactants. Also, the formation of
this complex can enhance the local amount of the sub-
strates. In addition, functionalization of CD with GA
brings the active catalytic center closer to substrates,
leading to an increase in the reaction rate. Moreover,
magnetic nanoparticles are appropriate supports for
a
TABLE 3
Effect of solvent and temperature on the synthesis of dihydropyrano[3,2-c]chromenes
b
Entry
Solvent
Conditions
Reflux
Reflux
r.t.
Time (min)
Yield (%)
1
2
3
4
CH
CH
2
3
Cl
2
180
120
300
120
180
80
33
47
35
50
65
89
CN
H
H
2
2
O
O
Reflux
r.t.
5
6
EtOH
EtOH
Reflux
a
General reaction conditions: 2-chlorobenzaldehyde (1 mmol), 4-hydroxycoumarin (1 mmol) and ethyl cyanoacetate (1.1 mmol) and 0.05 g of
3 4
Fe O -β-CD-GA in 5 ml of solvent.
b
Isolated yields.
a
Note. r.t., room temperature.

10 of 12
RAHMATINEJAD AND NAEIMI
a
TABLE 4
Preparation of dihydropyrano[3,2-c]chromenes under the optimized conditions
[
b]
4
4
a–4n, time (min), yield (%) , TON
a: 75, 90%, 56.2
4b: 100, 83%, 51.8
4e: 90, 87%, 54.3
4 h: 90, 86%, 53.7
4 k: 140, 78%, 48.7
4n: 90, 85%, 53.1
4b: 100, 83%, 51.8
4
d: 80, 89%, 55.6
4f: 85, 88%, 55
4
g: 80, 89%, 55.6
4i: 95, 83%, 51.8
4
4
j: 85, 85%, 53.1
4 l: 120, 76%, 47.5
m: 110, 80%, 50.0
Abbreviation: TON, Turn-Over Number.
a
General reaction conditions: aldehyde (1 mmol), 4-hydroxycoumarin (1 mmol), ethyl cyanoacetate (1.1 mmol), and 0.05 g of Fe
in 5 ml of ethanol.
Isolated yield.
3
O
4
–βCD-GA
b
a
TABLE 5
Recyclability studies of the catalyst system
products in high yields and short reaction times. These
results demonstrate that this method is an efficient
approach for one-pot multicomponent synthesis of
dihydropyrano[3,2-c]chromene derivatives. As men-
tioned, the synergism between the components of
Fe O -β-CD-GA enhances the catalytic efficiency. We
conclude that the encapsulation of the substrates within
the cavity of β-CD and activation of substrates, the high
basicity of GA, and the close proximity between sub-
strates and active sites of GA facilitated the reaction.
b
Entry
Number of cycles
First
Yield (%)
1
2
3
4
89
87
85
83
80
Cycle 1
Cycle 2
3
4
Cycle 3
5
Cycle 4
a
General reaction conditions: mixture of 2-chlorobenzaldehyde
(
(
1 mmol), 4-hydroxycoumarin (1 mmol), and ethyl cyanoacetate
1.1 mmol) using 0.05 g of catalyst in 5 ml of EtOH was refluxed for
The super-paramagnetic nature of the Fe O -β-CD-GA
nanocatalyst leads to simple recovery of the catalyst
3
4
80 min.
Isolated yields.
b

RAHMATINEJAD AND NAEIMI
11 of 12
SCHEME 4 Proposed reaction pathway for
the synthesis of dihydropyrano[3,2-c]chromenes
3 4
using Fe O -β-CD-GA as catalyst
using an external magnet. The reactions were clean and
did not require chromatographic purification. The prod-
ucts were isolated by a simple work-up and obtained in
high purity. The structure of products was characterized
through tautomerization forms the final product 4. In this
reaction, guanidine acts as an organic base and β-CD
forms a host–guest complex through hydrogen bonding
with the reactants, thereby activating them.
1
by spectroscopic data such as FT-IR, H NMR, and
melting point, and the results were consistent with
[
47–49]
those of authentic samples.
4 | CONCLUSION
To investigate the reusability of the catalyst, after
completion of the model reaction, Fe O -β-CD-GA was
easily separated from reaction media by an external
In summary, we developed an environmentally friendly,
safe, easily accessible, highly efficient, inexpensive,
magnetically separable, and recyclable Fe O -β-CD-GA
catalyst for one-pot three-component coupling of aromatic
aldehyde, 4-hydroxycoumarin, and ethyl cyanoacetate.
The reaction was efficiently performed under mild and
clean conditions, giving the corresponding products in
good to high isolated yields. This protocol also offers some
advantages, such as short reaction times, low catalyst
requirement and very easy work-up.
3
4
magnetic field. The separated catalyst was washed with
3
4
ꢀ
ethanol and acetone several times, dried at 50 C for 10 hr
and reused for the next cycle. It was found that the cata-
lyst could be reused at least for five runs without
a remarkable decrease in its catalytic performance
(Table 5). Also, the inductively coupled plasma analysis
of the reaction mixture after separation of the catalyst did
not reveal any detectable leaching of Fe.
A
probable mechanism for the synthesis of
dihydropyrano[3,2-c]chromenes in the presence of Fe O -
3
4
β-CD-GA nanocatalyst is proposed in Scheme 4.
Knoevenagel condensation of activated ethyl cyanoacetate
and aldehyde followed by Michel addition of 4-hydro-
xycoumarin with the Knoevenagel product 1 gives inter-
mediate 2. Then cyclization of intermediate 2 in the
presence of the catalyst produces intermediate 3, which
ACKNOWLEDGMENTS
The authors are grateful to the University of Kashan for
supporting this work by Grant No. 159148/89.
ORCID
Hossein Naeimi https://orcid.org/0000-0002-9627-596X

12 of 12
RAHMATINEJAD AND NAEIMI
REFERENCES
[31] B. Kaboudin, Y. Abedi, T. Yokomatsu, Eur. J. Org. Chem. 2011,
2011, 6656.
[
[
[
[
1] Y. Bouazizi, A. Romdhane, H. B. Jannet, Eur. J. Chem. 2014,
, 457.
2] A. M. El-Agrody, A. H. Halawa, A. M. Fouda, A.-A. M. Al-
Dies, J. Saudi, Chem. Soc. 2017, 21, 82.
3] P. Naveen, B. Susmitha, G. Jhansi, G. Chaitanya, B. Anupama,
K. Chenchu Lakshmi, In. J. Res. Pharm. Chem. 2013, 3, 808.
4] M. Mladenovi ꢀc , M. Mihailovi ꢀc , D. Bogojevi ꢀc , S. Mati ꢀc ,
N. Ni ꢀc iforovi ꢀc , V. Mihailovi ꢀc , N. Vukovi ꢀc , S. Sukdolak,
S. Soluji ꢀc , Int. J. Mol. Sci. 2011, 12, 2822.
[
[
[
[
[
32] N. S. Krishnaveni, K. Surendra, K. Rama Rao, Adv. Synth.
5
Catal. 2004, 346, 346.
33] G. Floresta, C. Talotta, C. Gaeta, M. De Rosa, U. Chiacchio,
P. Neri, A. Rescifina, J. Org. Chem. 2017, 82, 4631.
34] E. C. Santos, T. C. dos Santos, R. B. Guimar ~a es, L. Ishida,
R. S. Freitas, C. M. Ronconi, RSC Adv. 2015, 5, 48031.
35] J. Han, Y. Xu, Y. Su, X. She, X. Pan, Catal. Commun. 2008,
9
, 2077.
36] T. Kita, A. Georgieva, Y. Hashimoto, T. Nakata, K. Nagasawa,
Angew. Chem. Int. Ed. 2002, 41, 2832.
[
5] H. Gourdeau, L. Leblond, B. Hamelin, C. Desputeau, K. Dong,
I. Kianicka, D. Custeau, C. Boudreau, L. Geerts, S.-X. Cai, Mol.
Cancer Ther. 2004, 3, 1375.
[
[
37] D. Leow, C. H. Tan, Chem. – Asian J. 2009, 4, 488.
38] S. Shylesh, V. Schünemann, W. R. Thiel, Angew. Chem. Int.
Ed. 2010, 49, 3428.
[
[
6] R. Pratap, V. J. Ram, Chem. Rev. 2014, 114, 10476.
7] N. Majumdar, N. D. Paul, S. Mandal, B. de Bruin, W. D. Wulff,
ACS Catal. 2015, 5, 2329.
[
[
39] J. Govan, Y. K. Gun'ko, Nanomaterials 2014, 4, 222.
40] T.-J. Yoon, W. Lee, Y.-S. Oh, J.-K. Lee, New J. Chem. 2003,
[
8] O. Mohammadi, M. Golestanzadeh, M. Abdouss, New J. Chem.
27, 227.
2
017, 41, 11471.
[
[
41] J. Zhu, P.-c. Wang, M. Lu, J. Brazil. Chem. Soc. 2013, 24, 171.
42] B. Kaboudin, R. Mostafalu, T. Yokomatsu, Green Chem. 2013,
[
9] Q. Yang, R. Guo, J. Wang, Asian J. Org. Chem. 2019, 8, 1742.
[
[
[
[
[
10] R. Fareghi-Alamdari, F. Mansouri, M. Golestanzadeh,
N. Zekri, Curr. Org. Chem. 2018, 22, 1373.
11] C. Sangani, D. Mungra, M. Patel, R. Patel, Open Chem. 2011,
15, 2266.
[
[
43] M. Wang, G. Fang, P. Liu, D. Zhou, C. Ma, D. Zhang, J. Zhan,
Appl. Catal. B. 2016, 188, 113.
44] S. Sayyahi, S. Mozafari, S. J. Saghanezhad, Res. Chem.
Intermediat 2016, 42, 511.
9, 635.
12] J. Zheng, M. He, B. Xie, L. Yang, Z. Hu, H.-B. Zhou, C. Dong,
Org. Biomol. Chem. 2018, 16, 472.
13] S. S. Mansoor, K. Logaiya, K. Aswin, P. N. Sudhan, J. Taibah,
Univ. Sci. 2015, 9, 213.
14] S. Jain, D. Rajguru, B. S. Keshwal, A. D. Acharya, ISRN Org.
Chem. 2013, 2013, 1.
[
[
45] B. G. Mathapa, V. N. Paunov, J. Mater. Chem. B 2013, 1, 3588.
46] A. Z. M. Badruddoza, Z. B. Z. Shawon, W. J. D. Tay,
K. Hidajat, M. S. Uddin, Carbohydr. Polym. 2013, 91, 322.
47] M. G. Dekamin, M. Eslami, A. Maleki, Tetrahedron 2013, 69,
[
[
[
1074.
[
[
15] J. M. Khurana, B. Nand, P. Saluja, Tetrahedron 2010, 66, 5637.
16] R .G horbani-Vaghei, J. Mahmoodi, Y. Maghbooli, A. Shahriari,
Curr. Org. Chem. 2017, 14, 904.
48] S. Tu, H. Jiang, F. Fang, Y. Feng, S. Zhu, T. Li, X. Zhang,
D. Shi, J. Chem. Res. 2004, 2004, 396.
49] D.-Q. Shi, N. Wu, Q.-Y. Zhuang, J. Chem. Res. 2008, 2008, 542.
[
[
[
17] S. F. Hojati, A. Amiri, N. Moeini Eghbali, S. Mohamadi, Appl.
Organomet. Chem. 2018, 32, 4235.
18] S. Kanakaraju, B. Prasanna, S. Basavoju, G. Chandramouli,
Arabian J. Chem. 2017, 10, S2705.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
19] H. Azizi, A. Khorshidi, K. Tabatabaeian, J. Iran. Chem. Soc.
2
018, 15, 1.
[
[
[
[
[
[
[
20] M. Abaszadeh, M. Seifi, Acta Chem. Iasi. 2017, 25, 38.
21] H. Kiyani, M. Tazari, Res. Chem. Intermed. 2017, 43, 6639.
22] E. Mollashahi, M. Nikraftar, J. Saudi, Chem. Soc. 2017, 22, 42.
23] B. Maleki, Org. Prep. Proced. Int. 2016, 48, 303.
24] H. Kiyani, F. Ghorbani, Res. Chem. Intermed. 2015, 41, 4031.
25] M. Abaszadeh, M. Seifi, Res. Chem. Intermed. 2015, 41, 7715.
26] H. Kiyani, M. Sadat Jalali, Comb. Chem. High. T. Scr. 2016,
How to cite this article: Rahmatinejad S,
Naeimi H. Design, preparation, and
characterization of Fe O nanoparticles
3
4
encapsulating β-cyclodextrin-bearing guanidine as
a highly efficient and reusable heterogeneous base
catalyst for synthesis of 3,4-dihydropyrano[3,2-c]
chromenes. Appl Organomet Chem. 2020;e5862.
https://doi.org/10.1002/aoc.5862
1
9, 275.
[
[
[
27] T. Loftsson, M. E. Brewster, J. Pharm. Sci. 1996, 85, 1017.
28] J. Szejtli, Chem. Rev. 1998, 98, 1743.
29] A. R. Khan, P. Forgo, K. J. Stine, V. T. D'Souza, Chem. Rev.
1
998, 98, 1977.
[
30] J. D. Senra, L. F. B. Malta, M. E. da Costa, R. C. Michel,
L. Aguiar, A. B. Simas, O. E. A. E. C. Antunes, Adv. Synth.
Catal 2009, 351, 2411.