Synthesis of 3-arylcoumarins by FeCL3-promoted cyclization of orto-methoxy-substituted (E)-2,3-diphenylpropenoic acids or their methyl esters

Article abstract of DOI:10.1007/s10593-014-1442-2

3-Arylcoumarins were conveniently synthesized in excellent yields by an iron(III) chloride-promoted tandem reaction using methoxy-substituted (E)-2,3-diphenylpropenoic acids or their methyl esters. The structure of 3-(4-methoxyphenyl)coumarin was establis

Full text of DOI:10.1007/s10593-014-1442-2

Chemistry of Heterocyclic Compounds, Vol. 50, No. 1, April, 2014 (Russian Original Vol. 50, No. 1, January, 2014)
SYNTHESIS OF 3-ARYLCOUMARINS
BY FeCL3-PROMOTED CYCLIZATION
OF orto-METHOXY-SUBSTITUTED
(E)-2,3-DIPHENYLPROPENOIC ACIDS
OR THEIR METHYL ESTERS
1,2
1
2
*
H. Y. Ge , Z. J. Fang , and B. H. Qian
3-Arylcoumarins were conveniently synthesized in excellent yields by an iron(III) chloride-promoted
tandem reaction using methoxy-substituted (E)-2,3-diphenylpropenoic acids or their methyl esters. The
structure of 3-(4-methoxyphenyl)coumarin was established from X-ray crystallographic data.
Keywords: 3-arylcoumarin, iron(III) chloride, synthesis, tandem reaction.
Coumarins (2H-chromen-2-ones) are a large group of compounds found as natural products. They are
also widely prepared synthetically because of their applications in medicine and the perfume and dye industries
[1]. In particular, 3-aryl coumarins are used as fluorescent brightening agents, dispersed fluorescents, and laser
dyes due to the stability of trans-stilbene skeleton under light irradiation [2].
The main synthetic strategy to construct 3-arylcoumarins is based on the ring-closure reaction of active
methylene compounds and aryl aldehydes [3, 4], which suffers from drawbacks like the requirement of using
activated phenols and expensive catalysts and has potential environmental issues as well. Although there are a
few methods involving the use aryl methyl ethers to synthesize coumarins, the processes require harsh reaction
conditions for ether bond cleavage in the cyclization step [5].
On the other hand, the direct functionalization of coumarins at the C-3 atom to construct
3-arylcoumarins is based on the Suzuki reaction of coumarinyl halides and boronic acids or esters, which still
requires prefunctionalization of the coumarin moiety and suffers from a lack of step economy [6]. An alternative
method is zincation of coumarin at the C-3 atom followed by a Negishi cross coupling [7]. Although a direct
3-arylation of coumarins via Pd-catalyzed coupling of coumarins and iodoarenes was recently discovered, the
process was limited by its low functional group tolerance and low yields of the adducts [8].
_______
*To whom correspondence should be addressed, e-mail: zjfang@njust.edu.cn.
1School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094,
P. R. China.
2School of Chemical Engineering, Huaihai Institute of Technology, Lianyungang 222005, P. R. China;
qianbh@hhit.edu.cn.
_________________________________________________________________________________________
Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 17-23, 2014. Original article
submitted November 21, 2013.
12
0009-3122/14/5001-0012©2014 Springer Science+Business Media New York

Different Lewis acids as reaction promoters or catalysts have been explored for coumarin synthesis in
the ring-closure step, such as ZnCl2, TiCl4, and NiCl2, etc. [9]. Outside of these contributions, there have been
no focused efforts on using FeCl3 as promoter. Recently, Maiti reported a reaction route synthesizing coumarin
derivatives under dual catalysis with piperidine and FeCl3 in refluxing toluene [10]; the presence of a secondary
amine is essential for the success of this reaction.
To the best of our knowledge, no synthesis of coumarin using FeCl3 as a sole promoter has been
reported. In this study, we report a convenient method for the synthesis of 3-arylcoumarins at room temperature
by an FeCl3-promoted tandem reaction with methoxy-substituted (E)-2,3-diphenylpropenoic acids or their
methyl esters as the starting materials.
Using appropriate methoxy-substituted benzaldehydes 1a,b and phenylacetic acid 2 as starting
materials, we prepared methoxy-substituted (E)-2,3-diphenylpropenoic acids 3a,b by a modified Perkin
condensation reaction [11, 12]. Previous studies have shown that such condensations invariably yield a mixture
of E-isomer as the main product and Z-isomer as the minor product [11, 12], which can be separated by
precipitation at different pH and recrystallization. The acids 3a,b can be converted to their methyl esters 4a,b.
MeO
MeO
OMe
CHO
OMe
CO2H
CO2Me
Ac2O
MeOH
+
Et3N
, 24 h
H2SO4
, 6 h
R1
OMe
OMe
HO2C
R1
3a,b
R1
4a,b
1a,b
2
OMe
5
4
3
3'
FeCl3, CH2Cl2
room temp.
6
7
3,4 a,b
2'
2
1
O
O
8
R1
5a,b
1–5 a R1 = H, b R1 = OMe
Construction of coumarins 5a,b from the methoxy-substituted 2,3-diphenylpropenoic acids 3a,b or their
methyl esters 4a,b required the cleaving of the methyl ether bond. Various choices were available to effect the
cleavage. The reagents usually employed are Brønsted acids, Lewis acids (those of the type BX3 and AlX3 most
of all), alkaline reagents, nucleophiles, as well as oxidizing or reducing reagents. However, the cleavage of
methyl ether, which has been extensively studied, is usually carried out under harsh conditions [13]. Reactions
using FeCl3 as a cleavage reagent need to be carried out in the presence of acyl chloride or acid anhydride,
yielding esterified compounds [14]. The use of FeCl3 as a sole cleavage reagent of methyl ethers has not so far
been discussed. In this study, the cleavage capacity of FeCl3 for aryl methyl ethers was tested by the
investigation of model reactions using anisole and o-methoxybenzaldehyde as substrates (Table 1).
As shown in Table 1, the cleavage cannot proceed substantially at room temperature even with
increased FeCl3 dosage and prolonged reaction times. When we prolonged the reaction temperature, the yield
slightly increased, but was still very low. When o-methoxybenzaldehyde was used as the substrate, the electron-
withdrawing effect of ortho-aldehyde group causes the ether bond to be cleaved more easily [15]. Nevertheless,
the yield was only slightly higher than using anisole.
13

TABLE 1. Effect of FeCl3 on Ether Cleavage
Entry
FeCl3, equiv.
Reaction conditions
Anisole
Conversion, % (HPLC)
1
0.1
1
room temp., CH2Cl2, 2 h
room temp., CH2Cl2, 2 h
room temp., CH2Cl2, 24 h
room temp., CH2Cl2, 24 h
80°C, 1,2-dichloroethane, 8 h
o-Anisaldehyde
—
2
3
4
5
Trace
1
2
3
5
3
3
6
0.1
1
room temp., CH2Cl2, 2 h
room temp., CH2Cl2, 2 h
room temp., CH2Cl2, 24 h
room temp., CH2Cl2, 24 h
80°C, 1,2-dichloroethane, 8 h
—
7
Trace
8
1
4
5
8
9
3
10
3
The synthesis of coumarins with active phenols as the starting materials and Lewis acids as catalysts,
also called lactonization or ester exchange reaction, has been reported in a number of studies [1]. Construction
of coumarins 5a,b from methoxy-substituted 2,3-diphenylpropenoic acids 3a,b or their methyl esters 4a,b by a
FeCl3-promoted reaction was carried out for the first time; hence, as a starting point, we chose ester 4a as the
substrate to optimize the reaction conditions for the desired lactonization reaction (Table 2).
As shown in Table 2, by using 0.1 equiv. of FeCl3 as the promoter, the reaction gave only a trace
conversion (entry 1). With increased amount of FeCl3 (entries 2-5), the conversion increased gradually, and a
plateau of conversion was achieved when using FeCl3 triple excess or more (entries 4-5), which implies that
FeCl3 may participate in the reaction as a stoichiometric reagent. The reaction conversion also increased with
prolonged reaction times and reached its maximum after 8 h (entries 4, 6-7). The survey of solvents indicated
that this reaction is sensitive to the solvent medium (entries 4, 8-10), and a comparably good conversion can be
obtained in halogenated alkane solvents such as CH2Cl2, CHCl3, and 1,2-dichloroethane. Various temperatures
were tested for the reaction (entries 11-12), which shows that the reaction can be performed in a wide range of
temperatures with good degree of conversion. For operational simplicity, the reactions were carried out at room
temperature. A further investigation of the reaction mixture showed the presence of chloromethane [16], which
further demonstrated that FeCl3 participated in the ether cleavage reaction.
The acids 3a,b and esters 4a,b produced by Perkin condensation and esterification, respectively, were
1
E-isomers [11], as indicated by H NMR spectroscopy. Previous reports have shown that the chemical shifts of
hydrogen on the double bond of (E)-2,3-diphenyl-substituted acrylic acids or esters are slightly downfield with
respect to the Z-isomers. The E-isomers display a characteristic singlet resonance at about 8.0 ppm in the
1H NMR spectrum; by contrast, the Z-isomers display a resonance at about 7.0 ppm [12]. In the current work,
the chemical shifts of hydrogen on the double bond of acids 3a,b and corresponding esters 4a,b displayed a
resonance at about 8.0 ppm. The double bond of the coumarin ring has Z-configuration, which requires the
configuration inversion of the double bond upon the cyclization. In general, a double bond cannot freely rotate,
so it must first be transformed to a single bond to accomplish the rotation.
Based on the previous studies [17] and the present experimental results, a coupling mechanism for the
present tandem reaction was proposed. Firstly, FeCl3 forms an oxonium salt complex, which assists in cleaving
the ether bond. Secondly, one electron is abstracted by Fe3+ from the substrate to give a radical cationic species,
which enables the configuration inversion. Finally, the cyclization occurs with the cleavage of the ester and
release of FeCl2OR2 to give the corresponding coumarin ring.
14

TABLE 2. Effects of Reaction Conditions on the Lactonization Reaction
of Ester 4a into Coumarin 5a
Entry FeCl3, equiv.
Solvent
Time, h
Temp., °C Conversion, % (HPLC)
1
2
3
4
5
6
7
8
0.1
1
2
3
4
3
3
3
3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
24
8
8
8
8
4
24
8
8
8
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
10
5
24
58
88
90
71
90
87
88
21
86
89
9
1,2-Dichloroethane
MeOH
CH2Cl2
10
11
12
3
3
3
8
8
CH2Cl2
40
To further confirm the structure of the obtained products, X-ray structural analysis of compound 5a was
performed (Fig. 1). Within the molecule, the C(7)−C(6), C(8)−C(10), and C(8)−C(9) bond lengths are 1.4306,
1.4830, and 1.4637 Å, respectively. These bonds are significantly shorter than the typical C−C single bond
(1.53 Å), indicating a certain extent of conjugation involving the double bond, carbonyl group, and two benzene
rings. The pyran ring is almost planar with the torsion angles C(7)−C(8)−C(9)−O(1), O(1)−C(1)−C(6)−C(7),
and O(2)−C(9)−O(1)−C(1) being 0.34(18)°, -0.80(17)°, and 179.36(12)°, respectively.
Fig. 1. Molecular structure of compound 5a with atoms represented
as thermal vibration ellipsoids at the 50% probability level.
15

In conclusion, a new and simple method of synthesizing 3-arylcoumarins in excellent yields by
FeCl3-promoted tandem reaction was developed. Although it is challenging to change to new substrate at this
stage, the method still served as convenient and effective method for the synthesis of coumarin. The advantages
include the use of easily available starting materials, mild reaction conditions, environmental friendliness, and
simple operation. Thus, studies focused on further diversifying this moiety are currently under way in our
laboratory.
EXPERIMENTAL
IR spectra were recorded on a Bruker Tensor 27 FTIR in KBr. 1H and 13C NMR spectra were recorded
on a Bruker DRX-400 spectrometer (400 and 100 MHz, respectively) with TMS as internal standard. Mass
spectra were recorded on a QP2010 plus GC-MS instrument with chemical ionization. Elemental analyses were
performed on a Perkin Elmer 2400 series elemental analyzer. Melting points were determined on a WPS-1B
melting point apparatus and were uncorrected. The conversion was measured on a Shimadzu LC-10AVP HPLC
system with a fluorescence detector (excitation wavelength 236 nm, emission wavelength 306 nm) using a
Diamonsil C18 (250 × 4.6 mm, 5 μm) column; the mobile phase consisted of MeCN–0.05 M H3PO4, 40:60.
Unless otherwise stated, all reactions were performed in an oven-dried vessel under nitrogen atmosphere. All
anhydrous solvents were dried and purified by standard techniques prior to use. FeCl3 was commercially
obtained from Aladdin Chemical Co., Ltd (China) and used without further purification. Silica GF254 for TLC
and silica gel (200-300 mesh) for column chromatography were purchased from Qingdao Marine Chemical
Company (Qingdao, China).
Trace amounts of residual chloromethane in the reaction mixture were determined by a modified
method [16] using an Agilent 6890 capillary gas chromatography system. The sample (0.3 ml) was dissolved
in DMF–H2O, 1:2 (5 ml) and separated on an Agilent DB-624 capillary column (30 m×0.32 mm×1.80 μm) in
the headspace sampling mode with hydrogen flame ionization detector (FID). The column temperature was
maintained at 60°C for 10 min, then raised to 180°C at the rate of 20°C·min-1 and maintained at 180°C for
2 min. The FID temperature was 250°C, and the injection port temperature was 200°C. The retention time of
chloromethane was 4.1 min.
(E)-3-(2-Methoxyphenyl)-2-(4-methoxyphenyl)propenoic acid (3a). A mixture of 4-methoxyphenyl-
acetic acid (2) (16.6 g, 0.1 mol), 2-methoxybenzaldehyde (1a) (13.6 g, 0.1 mol), Ac2O (40 ml), and Et3N
(20 ml) was heated under reflux for 24 h. The red solution was allowed to cool to 60°C before water (10 ml)
was added, and the mixture was stirred for 1 h at room temperature. The mixture was then poured into aqueous
Na2CO3 solution (80 g in 300 ml of water) and stirred at 70°C until nearly all resinous material was dissolved.
The solution was cooled to room temperature, extracted with Et2O (3×100 ml), and carefully acidified with 6 M
HCl to pH 3-4 with the precipitation of a yellow solid. The mixture was filtered, and the solid was washed with
MeOH and recrystallized three times from MeOH. Yield 18.6 g (66%), light-yellow solid, mp 202-204°C (mp
1
202-203°C [18]). IR spectrum, ν, cm-1: 1672 (C=O), 1599 (C=C). H NMR spectrum (DMSO-d6), δ, ppm
(J, Hz): 3.75 (3H, s, CH3); 3.84 (3H, s, CH3); 6.61-6.67 (2H, m, H Ar); 6.88 (2H, d, J = 8.8, H Ar); 7.00-7.24
(4H, m, H Ar); 7.94 (1H, s, H-3); 12.61 (1H, s, COOH). Found, %: C 71.69; H 5.61. C17H16O4. Calculated, %:
C 71.82; H 5.67.
(E)-3-(2,3-Dimethoxyphenyl)-2-(4-methoxyphenyl)propenoic acid (3b) was obtained from compounds
1b and 2 following the above procedure. Yield 63%, light-yellow solid, mp 172-174°C (mp 170-171°C [19]). IR
1
spectrum, ν, cm-1: 3422 (O–H), 1679 (C=O), 1612 (C=C). H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 3.75
(3H, s, CH3); 3.79 (6H, s, 2CH3); 6.23 (1H, d, J = 7.8, H Ar); 6.73 (1H, t, J = 8.0, H Ar); 6.87 (2H, d, J = 8.6,
H Ar); 6.91 (1H, d, J = 8.1, H Ar); 7.05 (2H, d, J = 8.6, H Ar); 7.89 (1H, s, H-3); 12.67 (1H, s, COOH). Found, %:
C 68.59; H 5.71. C18H18O5. Calculated, %: C 68.78; H 5.77.
16

Methyl (E)-3-(2-methoxyphenyl)-2-(4-methoxyphenyl)propenoate (4a). Concentrated sulfuric acid
(10 ml) was added to a solution of acid 3a (5.7 g, 0.02 mol) in MeOH (150 ml). Then the reaction mixture was
heated under reflux for 6 h and concentrated under reduced pressure. The residue was dissolved in CH2Cl2
(100 ml) and subsequently washed with water (100 ml), aqueous Na2CO3 solution (10%, 2×100 ml), and brine.
The organics were dried over Na2SO4, filtered, and concentrated in vacuum. The obtained crude product was
recrystallized from MeOH. Yield 5.1 g (86%), colorless needles, mp 131-133°C. IR spectrum, ν, cm-1: 1702
(C=O), 1601 (C=C). 1H NMR spectrum (CDCl3), δ, ppm (J, Hz). 3.80 (6H, s, 2CH3); 3.86 (3H, s, CH3); 6.60 (1H, t,
J = 7.5, H Ar); 6.74 (1H, d, J = 7.6, H Ar); 6.83-7.20 (6H, m, H Ar); 8.10 (1H, s, H-3). Mass spectrum, m/z (Irel, %):
298 [M]+ (100). Found, %: C 72.28; H 6.04. C18H18O4. Calculated, %: C 72.47; H 6.08.
Methyl (E)-3-(2,3-dimethoxyphenyl)-2-(4-methoxyphenyl)propenoate (4b) was obtained from
compound 3b similarly to compound 4a. Yield 87%, light-yellow solid, mp 96-98°C. IR spectrum, ν, cm-1: 1705
1
(C=O), 1609 (C=C). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 3.80 (3H, s, CH3); 3.81 (3H, s, CH3); 3.84
(3H, s, CH3); 3.91 (3H, s, CH3); 6.32 (1H, d, J = 7.8, H Ar); 6.69 (1H, t, J = 8.0, H Ar); 6.77 (1H, d, J = 7.7, H
Ar); 6.85 (2H, d, J = 8.6, H Ar); 7.12 (2H, d, J = 8.6, H Ar); 8.06 (1H, s, H-3). Mass spectrum, m/z (Irel, %): 328
[M]+ (100). Found, %: C 69.24; H 6.09. C19H20O5. Calculated, %: C 69.50; H 6.14.
3-(4-Methoxyphenyl)-2H-chromen-2-one (5a). Anhydrous FeCl3 (4.9 g, 0.03 mol) was added to a
solution of acid 3a (2.8 g, 0.01 mol) or ester 4a (3.0 g, 0.01 mol) in CH2Cl2 (200 ml). The black reaction
mixture was stirred at room temperature for 6 to 8 h. The progress of the reaction was monitored by TLC. After
the completion of reaction, the mixture was washed with HCl (1 mol/l, 3×200 ml). Then, the organic phase was
sequentially washed with aqueous Na2CO3 solution (10%, 2×100 ml), water, and brine. The organics were dried
over Na2SO4, filtered, and concentrated in vacuum. The residue was purified by silica gel (CH2Cl2). Yield 2.0 g
(78%, from compound 3a) and 2.1 g (85%, from compound 4a), light-yellow sheet crystals, mp 143-145°C
1
(mp 140°C [4]). IR spectrum, ν, cm-1: 3063 (=C–H); 1717 (C=O), 1608 (C=C). H NMR spectrum (CDCl3), δ,
ppm (J, Hz): 3.85 (3H, s, CH3); 6.97 (2H, d, J = 8.7, H-3',5'); 7.28-7.53 (4H, m, H-5,6,7,8); 7.68 (2H, d, J = 8.7,
H-2',6'); 7.76 (1H, s, H-4). Mass spectrum, m/z (Irel, %): 253 [M+H]+ (18), 252 [M]+ (100). Found, %: C 75.96;
H 4.71. C16H12O3. Calculated, %: C 76.18; H 4.79.
8-Methoxy-3-(4-methoxyphenyl)-2H-chromen-2-one (5b) was obtained following the above
procedure. Yield 75% (from 3b) and 79% (from 4b), light-yellow needles, mp 148-150°C (mp 145°C [20]). IR
1
spectrum, ν, cm-1: 3072 (=C-H); 1704 (C=O), 1608 (C=C). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 3.85
(3H, s, CH3); 3.98 (3H, s, CH3); 6.97 (2H, d, J = 8.7, H-3',5'); 7.06 (1H, d, J = 8.0, H-5); 7.10 (1H, d, J = 7.2,
H-7); 7.21 (1H, dd, J = 8.0, J = 7.2, H-6); 7.69 (2H, d, J = 8.7, H-2',6'); 7.74 (1H, s, H-4). Mass spectrum, m/z
(Irel, %): 282 [M]+ (100). Found, %: C 72.16; H 4.91. C17H14O4. Calculated, %: C 72.33; H 5.00.
X-ray Structural Investigation of Compound 5a. Crystals of compound 5a were obtained from the
methanol solution by a slow evaporation process. A suitable single crystal of compound 5a was selected for data
collection performed on a Bruker APEX II spectrometer with graphite monochromator with MoKα radiation (λ
= 0.71073 Å) using an ω–2θ scan mode at 296 K. Colorless crystal (C16H12O3, M 252.26); at 296 K it was
monoclinic, space group P21/c; a 6.1015(6), b 21.738(2), c 9.9137(10) Å; β 107.1310(10)°; V 1256.6(2) Å3; Z 4;
dcalc 1.333 g/cm3. In a series of three scannings, 11262 reflections were selected. The structure was solved by
direct method with SHELXS-97 and refined on F2 by full-matrix least-square techniques with the SHELXS-97
program [21]. All non-hydrogen atoms were refined with anisotropic thermal parameters, and hydrogen atoms,
isotropically refined with rider model position parameters, were located from different Fourier maps and added
theoretically. Crystallographic data for compound 5a have been deposited at the Cambridge Crystallographic
Data Center (deposit CCDC 910488).
This project was supported by the open project of the Jiangsu Research Institute of Marine Resources
(JSIMR11B04), Lianyungang United Research Project of Production-University Institute (CXY1212), and
Industrial Promotion Project of Jiangsu University (JHB2011-60).
17

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18