Boric acid-catalyzed multi-component reaction for efficient synthesis of 4H-isoxazol-5-ones in aqueous medium

Article abstract of DOI:10.1007/s11164-013-1411-x

Abstract The one-pot three-component reaction of aryl aldehydes with hydroxylamine hydrochloride and ethyl 3-oxobutanoate/ethyl 4-chloro-3-oxobutanoate/ethyl 3-oxo-3-phenylpropanoate in the presence of boric acid, H₃BO₃, in water lea

Full text of DOI:10.1007/s11164-013-1411-x

Res Chem Intermed
DOI 10.1007/s11164-013-1411-x
EDITORIAL
Boric acid-catalyzed multi-component reaction
for efficient synthesis of 4H-isoxazol-5-ones
in aqueous medium
•
Hamzeh Kiyani Fatemeh Ghorbani
Received: 8 June 2013 / Accepted: 17 September 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract The one-pot three-component reaction of aryl aldehydes with hydrox-
ylamine hydrochloride and ethyl 3-oxobutanoate/ethyl 4-chloro-3-oxobutanoate/
ethyl 3-oxo-3-phenylpropanoate in the presence of boric acid, H3BO3, in water leads
to 4H-isoxazol-5(4H)-ones in high yields. The merits of this method are efficiency,
simplicity, clean, green, easy work-up, high yields, and shorter reaction times.
Keywords Isoxazol Á Three-component reaction Á Boric acid Á Green
Introduction
Multi-component reactions (MCRs) have been proven to be a very powerful method
for the synthesis of highly functionalized molecules in a single reaction vessel.
These reactions have a particular importance in the synthesis of biologically active
compounds and natural products due to their significant features such as high
efficiency, minimization of waste, versatility, facial completing, atom economy, and
excellent functional group compatibility. Also, MCRs in water can be visualized as
a well-designed synthetic method to attain a wide range of diverse molecular
frameworks [1–3].
On the other hand, the synthesis of small heterocyclic systems containing an
isoxazol ring occupies a significant place in pharmaceutical chemistry and organic
synthesis. One of the best methods for achieving these systems is the use of MCRs.
This five-membered heterocyclic unit represents a class of heterocyclic compounds
with biological activities and medicinally useful agents such as anticonvulsant
[4], antifungal [5], analgesic [6], antitumor [7], antioxidant [8], antimicrobial [9],
H. Kiyani (&) Á F. Ghorbani
School of Chemistry, Damghan University, 36715-364 Damghan, Iran
e-mail: hkiyani@du.ac.ir
123

H. Kiyani, F. Ghorbani
COX-2 inhibitory [10], anti-inflammatory [11], antiviral [12], and antimycobacterial
[13], as well as being used for the treatment of leishmaniasis [14] and of patients
with active arthritis [15]. Furthermore, the isoxzolon core can also be used as the
basis for the design and construction of merocyanine dyes, which are used in optical
recording and nonlinear optical research [16].
Boric acid, H3BO3, is a white solid moderately soluble in water, and is
considered a very weak acid, with a presented ionization constant of about
pKa = 9.2 [17]. It is a very stable compound and relatively benign to humans [18].
The development of mild, low-cost and high-performance acid catalysts has
attracted much interest for green chemistry [19]. Recently, the organic synthesis of
boric acid has attracted great interest because it is commercially available,
environmentally benign, inexpensive, and easy to handle. Recently, this reagent has
been used in various reactions including aza-Michael reaction [20], transesterifi-
cation of ethyl acetoacetate [21], oxidation of sulfides [22], Biginelli reaction [23],
and Mannich reaction [24]. In addition, it has also been used for the preparation of
compounds such as the synthesis of 1,5-benzodiazepine derivatives [25], 2-amino-
3,5-dicarbonitrile-6-thio-pyridines [26], 1-amidoalkyl-2-naphthols [27], dibenzox-
anthenes [28], a-aminophosphonates [29], b-acetamido ketones [30], and
a-hydroxyamides [31].
As recently reported, arylmethylene isoxazol-5(4H)-ones were prepared by using
sodium benzoate [32], sodium sulfide [33], and sodium silicate [34] as the catalyst.
Also, we synthesized the same arylmethylene isoxazol-5(4H)-ones by using of
sodium ascorbate as the catalyst [35]. With this methodology as background, we
attempted to develop an alternative catalyst for the preparation of arylmethylene
isoxazol-5(4H)-ones. Although 4H-isoxazol-5-ones have so far been synthesized
[31–38], to the best of our knowledge, no reports that include the use of boric acid
for condensation of aromatic aldehydes (1), b-ketoesters (3), and hydroxylamine
hydrochloride (2) have been reported. We here report the preparation of 4H-
isoxazol-5-ones (4a-x) by one-pot three-component condensation of aryl aldehydes
(1), hydroxylamine hydrochloride (2), and ethyl 3-oxobutanoate (3a)/ethyl 4-chloro-
3-oxobutanoate (3b)/ethyl 3-oxo-3-phenylpropanoate (3c) using boric acid, H3BO3,
as a green and reusable catalyst in water at room temperature (Scheme 1).
Experimental
All the reagents and chemicals were obtained from commercial sources and used
without further purification. Melting points were measured on a Buchi 510 melting
point apparatus and are uncorrected. IR spectra were recorded on a Shimadzu FT-IR
1
8300 Spectrophotometer using the KBr pellets technique. H NMR and 13C NMR
spectra were recorded at ambient temperature on a BRUKER AVANCE DRX-
400 MHz spectrophotometer using CDCl3 or DMSO-d6 as the solvent and TMS as
an internal standard. The purity of newly synthesized compounds and the
development of reactions were monitored by thin layer chromatography (TLC) on
Merck pre-coated silica gel 60 F254 aluminum sheets, visualized by UV light.
123

Boric acid-catalyzed multi-component reaction
O
Ar
R
Boric acid
10 mol%
O
R
NH2OH.HCl
N
+
+
H2O, r.t.
Ar
H
O
O
O
O
3a
3b
)
R = H ( ), Cl (
Ph (3c)
4a-x
1a-p
2
3
Scheme 1 One-pot three-component condensation of aryl aldehydes (1a–p), hydroxylamine
hydrochloride (2), and b-ketoesters (3a, b, c) using boric acid
General procedure for preparation of 4H-isoxazole-5(4H)-ones (4a-x)
A mixture of equimolar quantities of hydroxylamine hydrochloride 2 (0.07 g,
1 mmol) and b-ketoester 3 (0.130 g, 1 mmol) were stirred in 5 mL of water for
5 min. Then, aryl aldehyde 1a–p (1 mmol) and 10 mol % of boric acid was added
and the reaction mixture was stirred at room temperature for the indicated time
shown in Table 2 (below). The solid product formed was isolated by simple
filtration and washed with water (5 mL). The products obtained were shown to be
pure by TLC and spectral techniques. The filtrate was evaporated to remove water,
leaving the catalyst. The same catalyst was employed to synthesize further
derivatives. If necessary, further purification was performed by recrystallization
from suitable solvents such as EtOH to give the desired compounds in high yields.
The products are known and their identity was confirmed by comparison of their
physical and spectroscopic data with those available in recent papers [31–38].
Selected spectral data
1
4-benzylidene-3-methylisoxazol-5(4H)-one (4a): H NMR (400 MHz, CDC13): d
2.34 (s, 3H), 7.46 (s, 1H), 7.55 (t, J = 7.8 Hz, 2H), 7.61–7.64 (m, 1H), 8.38 (dd,
J = 1.3, 7.4 Hz, 2H).
3-methyl-4-(4-methylbenzylidene)isoxazol-5(4H)-one (4c): 1H NMR (400 MHz,
CDC13): d 2.33 (s, 3H), 2.48 (s, 3H), 7.36 (d, J = 8.0 Hz, 2H), 7.42 (s, 1H), 8.32 (d,
J = 8.4 Hz, 2H); 13C NMR (100 MHz, CDC13): d 11.6, 22.1, 118.2, 129.8, 129.9,
134.2, 145.8, 150.2, 161.4, 168.3.
4-(3-hydroxybenzylidene)-3-methylisoxazol-5(4H)-one
(4f):
1H
NMR
(400 MHz, DMSO-d6): d 2.28 (s, 3H), 7.08 (d, J = 8.0 Hz, 1H), 7.39 (t,
J = 8.0 Hz, 1H), 7.79 (d, J = 7.6 Hz, 1H), 7.85 (s, 1H), 7.95 (s, 1H), 9.96 (s,
1H); 13C NMR (100 MHz, DMSO-d6): d 11.7, 118.9, 119.9, 121.8, 125.8, 130.2,
134.1, 152.3, 157.8, 162.6, 168.2.
3-methyl-4-(thiophen-2-ylmethylene)isoxazol-5(4H)-one (4 m): 1H NMR
(400 MHz, CDC13): d 2.32 (s, 3H), 7.29 (t, J = 4.8 Hz, 1H), 7.64 (s, 1H), 7.95
(d, J = 4.8 Hz, 1H), 8.13 (d, J = 3.6 Hz, 1H); 13C NMR (100 MHz, CDC13): d
11.5, 114.6, 128.9, 136.5, 139.2, 139.6, 141.5, 160.7, 168.7.
3-methyl-4-(thiophen-3-ylmethylene)isoxazol-5(4H)-one (4n): 1H NMR
(400 MHz, CDC13): d 2.29 (s, 3H), 7.42 (dd, J = 5.2, 2.8 Hz, 1H), 7.49 (s, 1H),
123

H. Kiyani, F. Ghorbani
7.95 (dd, J = 4.8, 1.6 Hz, 1H), 8.99 (dd, J = 2.8, 0.8 Hz, 1H); 13C NMR
(100 MHz, CDC13): d 11.6, 117.0, 126.8, 131.5, 135.2, 139.4, 140.9, 161.3, 168.5.
3-(chloromethyl)-4-(4-hydroxybenzylidene)isoxazol-5(4H)-one (4v): 1H NMR
(400 MHz, DMSO-d6): d 4.90 (s, 2H), 7.10 (d, J = 8.4 Hz, 2H), 8.04 (s, 1H), 8.48
(d, J = 8.8 Hz, 2H), 11.28 (s, 1H); 13C NMR (100 MHz, DMSO-d6): d 35.6, 110.7,
116.9, 124.9, 138.6, 153.3, 162.2, 164.9, 168.9.
3-(chloromethyl)-4-(4-(dimethylamino)benzylidene)isoxazol-5(4H)-one
(4w):
1H NMR (400 MHz, CDC13): d 3.22 (s, 6H), 4.56 (s, 2H), 6.78 (d, J = 8.6 Hz,
2H), 7.55 (s, 1H), 8.48 (d, J = 7.2 Hz, 2H); 13C NMR (100 MHz, CDC13): d 35.7,
40.3, 107.3, 111.7, 121.7, 138.3, 150.4, 154.8, 160.6, 169.9.
1
3-(chloromethyl)-4-(thiophen-2-ylmethylene)isoxazol-5(4H)-one (4x): H NMR
(400 MHz, CDC13): d 4.60 (s, 2H), 7.34–7.36 (m, 1H), 7.98 (s, 1H), 8.06 (d,
J = 4.8 Hz, 1H), 8.19 (d, J = 3.6 Hz, 1H); 13C NMR (100 MHz, CDC13): d 35.3,
111.1, 129.2, 136.5, 140.9, 141.0, 142.6, 159.7, 168.3.
Results and discussion
The synthesis of arylmethylidene-isoxazole-5(4H)-ones is outlined in Scheme 1. In
the beginning, equimolar quantities of benzaldehyde (1a), hydroxylamine hydro-
chloride (2), and ethyl acetoacetate (3a) were treated at room temperature in the
presence of catalytic amounts of boric acid in water. In this reaction, compound 4a
was achieved in high yield. Compound 4a has been described in the literature [31–
34]. The structure of the compound 4a was established from the spectral and
physical data. The 1H NMR spectrum of 4a displayed two singlet peaks at d = 2.34
for the methyl group and d = 7.46 ppm for the olfinic proton of the C=C double
bond. Aromatic protons of 4a resonate as a triplet at d = 7.55, a doublet of doublets
at d = 8.38 and multiplets in the region of d = 7.61–7.64 ppm. Also, the melting
point of the compound 4a was recorded and fixed with authentic samples showing
that 4a has been formed.
With the intention of investigating the optimum amounts of catalyst, the reaction
of 4-hydroxybenzaldehyde (1 g) hydroxylamine hydrochloride (2), and ethyl
acetoacetate (3a) as the model was conducted in water with different amounts of
boric acid ranging from 5 to 20 mol % at room temperature (Table 1). As shown in
Table 1, when the amount of boric acid was increased from 5 to 10 mol %, the yield
of 4 g was improved from 83 to 93 % (Table 1, entries 1–2). However, when the
amount of boric acid was further raised to 20 mol %, remarkable increases in the
yield of 4 g were not observed (Table 1, entries 3–4). Consequently, the amount of
10 mol % boric acid was selected as the catalyst for these reactions. In addition, to
search for the optimal solvent, the synthesis of 4 g was accomplished by using
solvents of water, acetone, 1,4-dioxane, ethanol, hexane, and a mixture of water–
ethanol (V:V, 1:1) at room temperature (Table 1, entries 5–9). As can be seen in
Table 1, the reaction in water led to higher yields and shorter reaction times than the
others. As a consequence, water was nominated as the suitable solvent. Therefore,
all further reactions were carried out using 10 mol % of boric acid in water at room
temperature.
123

Boric acid-catalyzed multi-component reaction
Table 1 Synthesis of 4g in the presence of different solvents and amounts of catalysta
Entry
Solvent
Amounts of
catalyst (mol %)
Time
(min)
Yield
(%)b
1
2
3
4
5
6
7
8
9
10
a
H2O
5
10
15
20
10
10
10
10
10
10
100
100
100
100
100
150
150
120
100
100
83
93
90
91
75
35
45
40
88
25
H2O
H2O
H2O
EtOH
1,4-Dioxane
Hexane
Acetone
H2O:EtOH (1:1)
Solvent free
Reaction conditions: 4-hydroxybenzaldehyde 1 g (1 mmol), hydroxylamine hydrochloride 2 (1 mmol),
ethyl acetoacetate 3 (1 mmol), water (5 mL), room temperature
b
Isolated yield of product
With the optimized reaction conditions in hand, the expediency of this method
was well evaluated using a variety of aryl aldehydes and b-ketoesters, and a series
of compounds 4 were synthesized with this simple approach. The results are
summarized in Table 2. The nature and position of the functional groups on the
phenyl ring affected the yields of the products and the reaction times. The results
indicated that aromatic aldehydes bearing electron-donating groups, such as –OCH3,
–CH3, –OH, and –N(Me)2-, are suitable for this reaction and the corresponding
products were prepared in high yields (Table 2, entries 2–8, 18–23). However, the
2-hydroxybenzaldehyde yielded the corresponding isoxazol-5(4H)-one derivative in
relatively lower yield, presumably due to its higher crowded steric (Table 2, entry
5). On the other hand, aryl aldehydes containing electron-withdrawing functional
groups, such as –Cl or –NO2, were ineffective and did not proceed under the
optimized reaction conditions. Therefore, they are not suitable for this reaction
(Table 2, entries 9–11). The current reaction was further examined using electron-
rich heterocyclic aryl aldehydes such as furan-2-carbaldehyde, thiophene-2-
carbaldehyde, and thiophene-3-carbaldehyde (Table 2, entries 12–14, 24). In this
case, the reaction proceeded well with high yields. Interestingly, when 4-hydroxy-3-
nitrobenzaldehyde was used, compound 4p was obtained in high yield and shorter
reaction time (Table 2, entry 16).
The structures of the title products were deduced from their spectra analysis. For
1
instance, the H NMR spectrum of 3-(chloromethyl)-4-(thiophen-2-ylmethylene)-
123

H. Kiyani, F. Ghorbani
Table 2 Synthesis of 4H-isoazol-5(4H)-ones (4)a
Entry
R
Product Time
(min)
Yield (%)b Mp (°C)
Found
Reportedc
1
H
4a
4b
100
50
90
140–142 141–143
1a
2
H
92
174–175 174–176
1b
3
4
H
H
4c
75
85
90
95
136–137 135–136
214–215 211–214
1c
4d
1d
5
H
4e
140
82
198–199 198–201
1e
6
7
H
H
4f
100
100
94
93
201–203 202–203
214–215 214–216
1f
4 g
1g
8
H
4 h
90
93
226–228 227–228
1h
123

Boric acid-catalyzed multi-component reaction
Table 2 continued
Entry
R
H
Product
Time
(min)
Yield (%)b
Mp (°C)
Found
Reportedc
–
9
4i
900
900
Trace
–
–
–
1i
10
11
H
H
4j
Trace
Trace
–
–
1j
4 k
1,440
1k
12
13
H
H
4 l
90
75
89
91
239–241
146–147
238–241
146–147
1l
4 m
1m
1n
14
15
H
H
4n
4o
80
90
90
145–147
242–244
146–147
120
241
1o
16
H
4p
50
86
266–267
267–268
1p
123

H. Kiyani, F. Ghorbani
Table 2 continued
Entry
R
Product Time
(min)
Yield (%)b Mp (°C)
Found
Reportedc
17
Ph 4q
100
70
85
213–215 215–216
1a
18
Ph 4r
91
168–170 168–169
1b
19
Ph 4 s
75
90
90
88
195–196 194–196
1c
20
Ph 4t
209–211 210
1g
21
Ph 4u
95
92
195–197 194–196
1h
22
Cl
Cl
4v
70
80
90
92
183–186
179–180
–
–
1g
23
4w
1h
123

Boric acid-catalyzed multi-component reaction
Table 2 continued
Entry
24
R
Product
Time
(min)
Yield (%)b
Mp (°C)
Found
Reportedc
–
Cl
4x
70
89
146–148
1m
a
Reagents and conditions: aryl aldehyde 1 (1 mmol), hydroxylamine hydrochloride 2 (1 mmol), b-
ketoester 3 (1 mmol), water (5 mL), room temperature
b
All yields are of pure products after filtrated and recrystallization from ethanol
c
Melting points are listed in refs. [32–39]
isoxazol-5(4H)-one (4x) revealed the structural characteristics of both thiophene
and chloromethyl (Fig. 1). The 1H NMR spectrum revealed distinctive singlet
signals at d = 4.60 and 7.98 ppm for chloromethylene (—CH2Cl) and olfinic
([C=CH—Ar) protons, respectively. Also, protons of the thiophene heterocyclic
unit appear at d = 8.06 and 8.19 ppm, and in the region of d = 7.34–7.36 ppm. The
Fig. 1 1H NMR (400 MHz, CDCl3) spectrum of 3-(chloromethyl)-4-(thiophen-2-ylmethylene)isoxazol-
5(4H)-one (4x)
123

H. Kiyani, F. Ghorbani
13C NMR spectrum of compound 4x showed characteristic signals at d = 35.3 ppm
for —CH2Cl, as well as 111.1, 159.7, and 168.3 ppm, for [C=CH—Ar, C=N, and
C=O of the isoxazol ring. The remaining five carbon resonances appearing between
d = 129.2 and 142.6 ppm are due to the carbon atoms of the thiophene ring and
C-b.
A plausible mechanism for the formation of the title compounds is shown in
Scheme 2. To evaluate the proposed mechanism, we first tried to prepare the
isoxazolon product with equivalent molar amounts of ethyl acetoacetate and
hydroxylamine hydrochloride in water solvent in the presence of boric acid. The
reaction was pursued by TLC. After completion of the reaction, the product was
isolated, and then its infrared spectrum was recorded. The following peaks were
observed on the IR-spectrum; 3,460, 1,695, and 1,600 cm-1. The peak in the region
of 3,460 cm-1 confirms the hydroxyl functional group in the intermediate. Also, the
ester carbonyl group appeared in the region 1,695 cm-1 due to the hydrogen bond
being shifted to lower frequencies. In isoxazolon intermediate products, the
frequency of the carbonyl group should be shifted to a higher region, and the peak of
OH eliminated. Accordingly, the isoxazolon product was not obtained. By adding an
aromatic aldehyde to the mixture of ethyl acetoacetate and hydroxylamine
hydrochloride, the final products (4a–x) were formed. This result probably reflects
the formation of the oxime intermediate (B), and probably also represents the role of
acid. Therefore, the formation of the title compounds proceeds via the Intramole-
cular cyclization intermediacy of a Knoevenagel adduct (D) of the preformed oxime
of ethyl acetoacetate followed by ring closure in the presence of boric acid.
B(OH)3 + 2 H2O
B(OH)4- + H3O+
HO
HO
O
O
OH
N
O
O
OH
H3O+
N
NH2OH
-H2O
R
R
R
R
O
O
O
O
H
B
O
3
A
OH
Ar
H
B(OH)3 + 2 H2O
B(OH)4- + H3O+
Ar
H
knoevenagel
condensation
H
O
HO
HO
OH
O
N
H
O
N
N
Intramolecular
cyclization
O
-
B(OH)4
R
R
R
O
O
-H2O, - B(OH)3
OH
Ar
Ar
Ar
E
D
C
H
O
O
O
N
N
R
R
O
O
Ar
Ar
F
4a x
-
Scheme 2 Proposed mechanism for the formation of title compounds 4a-x
123

Boric acid-catalyzed multi-component reaction
Also, in our exploration of the reusability of the catalyst in water after
completion of the reaction, the filtrate that remained as catalyst was subjected to
evaporation under reduced pressure, and the recovery solid catalyst was applied as
such for the consecutive runs in five series of the same model reaction under the
optimized conditions for up to five runs (1st use: 90 %; isolated yield; 2nd use:
87 % isolated yield; 3rd use: 82 % isolated yield; 4th use: 76 % isolated yield; 5th
use: 70 % isolated yield). Decreasing the yield is probably related to the slight
reduction in the catalytic activity of the catalyst or the decreasin the the amount of
catalyst recovery which is attributed to the handling.
Conclusion
In summary, the efficient and clean synthesis of 4-arylidene-3-methyl-isoxazol-
5(4H)-ones (4a–h and 4l–p), 4-arylidene-3-phenyl-isoxazol-5(4H)-ones (4q–u), as
well as 3-(chloromethyl)-4-arylidene-isoxazol-5(4H)-ones (4v–x) via a one-pot
three-component condensation of b-ketoesters (3a–c), aryl aldehydes, and hydrox-
ylamine hydrochloride in the presence of boric acid as the catalyst at room
temperature has been developed. The reaction exhibited such merits as mild
conditions, easy work-up, completion of reaction in shorter reaction times, reuse of
catalyst, safety, and use of water with its ecologically friendly point of view.
Acknowledgment The authors are grateful to the Research Council of Damghan University.
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