Synthesis of Erlenmeyer-Pl?chl azlactones promoted by 5-Sulfosalicylic acid

Article abstract of DOI:10.3987/COM-17-13711

5-Sulfosalicylic acid was found as an efficient catalyst in the synthesis of 4-arylidene-2-phenyl-5(4H)-oxazolones by condensation and cyclodehydration of aromatic aldehydes with hippuric acid and acetic anhydride at room temperature. The catalyst was eas

Full text of DOI:10.3987/COM-17-13711

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HETEROCYCLES, Vol. 94, No. 7, 2017
HETEROCYCLES, Vol. 94, No. 7, 2017, pp. 1314 - 1321. © 2017 The Japan Institute of Heterocyclic Chemistry
Received, 5th April, 2017, Accepted, 26th May, 2017, Published online, 8th June, 2017
DOI: 10.3987/COM-17-13711
SYNTHESIS OF ERLENMEYER-PLÖCHL AZLACTONES PROMOTED
BY 5-SULFOSALICYLIC ACID
Hamzeh Kiyani* and Shiva Aslanpour
School of Chemistry, Damghan University, 36719-41167 Damghan, Iran; E-mail:
hkiyani@du.ac.ir
Abstract – 5-Sulfosalicylic acid was found as an efficient catalyst in the synthesis
of 4-arylidene-2-phenyl-5(4H)-oxazolones by condensation and cyclodehydration
of aromatic aldehydes with hippuric acid and acetic anhydride at room
temperature. The catalyst was easily recycled from the filtrate via evaporation of
solvent and reused. This process is green, simple to handle, commercially
available starting materials and catalyst, mild conditions, as well as it does not
require to hazardous organic solvents, heating, and microwave and ultrasound
irradiations.
Nitrogen- and oxygen-containing five-membered heterocycles are important compounds that can be
found in a wide range of drug-likes and biologically relevant molecules. Some examples of these
five-membered heterocycles are 4-arylidene-2-phenyl-5(4H)-oxazolones (Erlenmeyer-Plöchl azlactones
or oxazol-5(4H)-ones), which synthesis of them was started in 1893 by Erlenmeyer for the first time.1
Some azlactones and their derivatives exhibited biological activities including, antimicrobial,2
antiproliferative,3 tyrosinase inhibitory,4 antitubercular,5 antioxidant,6 anti-inflammatory,7 anti-HIV,8
anticonvulsant,9 and monoacylglycerol lipase (MAGL) inhibitory.10 These important heterocyclic
compounds have been found to be generally useful intermediates for the synthesis imidazolones,11
N-benzoylamino-3-arylacrylates,12 oxazole-4-carboxamides,13 α-amino acids,14-15 optically active
spiro-fused cyclohexanone/5-oxazolone derivatives,16 peptides,17 and some heterocyclic compounds.18
Thus, the synthesis of this heterocyclic unit is of particular interest. Erlenmeyer-Plöchl azlactones can be
synthesized through two steps from readily available N-substituted glycines and aldehydes in acetic
anhydride, using anhydrous sodium acetate as a basic catalyst.19 Various reagents and reaction conditions
have been previously used for the cyclization of aldehydes and hippuric acid in presence of acetic
anhydride to form azlactones such as POCl3,20 Al2O3,21 Bi(OAc)3,22 Bi(OTf)3,23 silica-supported
heteropolyacids,24 Yb(OTf)3,25 Ca(OAc)2,26 supported KF,27 anhydrous ZnCl2,28 Fe2O3,29 ZnO,30

HETEROCYCLES, Vol. 94, No. 7, 2017
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Al2O3-H3BO3,31 2,4,6-trichloro-1,3,5-triazine, catalytic triphenylphosphine, and sodium carbonate,32
MgO/Al2O3,33 cellulose supported ionic liquid phase,34 2-chloro-4,6-dimethoxy-1,3,5-triazine/
N-methylmorpholine,35 ionic liquid [Et3NH][HSO4],36 2-aminopyridine, supported on nano-sphere SiO2,37
Hünig’s base,38 nano-TiO2,39 triphenylphosphine (PPh3),40 zeolite NaY,41 and N,N-dimethylacetamide
(DMAC)/N,N-dicyclohexylcarbodiimide (DCC) under microwave irradiation.42 The organic
transformations for synthesis of isoxazol-5(4H)-ones,43 pyran annulated heterocycles,44 and
aminopyrazoles45 are of particular interest to our laboratory and this report describes synthesis of some
derivatives of oxazol-5(4H)-ones in presence of 5-sulfosalicylic acid (SSA, 4) under green conditions
(Scheme 1).
O
O
O
O
O
Ar
O
H
N
OH
SSA (4, 20 mol%)
O
S
+
+
Ar
HO
O
H
HO
HO
O
N
O
water (5 mL), rt
O
1a-r
(1 mmol)
3
4
2
(1 mmol)
(1 mmol)
5-sulfosalicylic acid (SSA)
5a-r
Ar: C6H5 (1a), 4-Me-C6H4 (1b), 4-HO-C6H4 (1c), 3-HO-C6H4 (1d), 2-HO-C6H4 (1e), 4-MeO-C6H4 (1f), 3-MeO-C6H4
(1g), 4-Me2N-C6H4 (1h), 4-HO-3-MeO-C6H3 (1i), 2,4-(MeO)2-C6H3 (1j), 2-Cl-C6H4 (1k), 4-Cl-C6H4 (1l),
2,4-diCl-C6H4 (1m), 4-NO2-C6H4 (1n), 3-NO2-C6H4 (1o), 2-NO2-C6H4 (1p), 2-thienyl (1q), 2-furyl (1r)
Scheme 1. SSA-catalyzed synthesis of 4-arylidene-2-phenyl-5(4H)-oxazolones (5a-r)
Initially, to find the best reaction conditions for the synthesis of azlactones, the reaction of benzaldehyde
(1a), hippuric acid (2) and acetic anhydride (3) was chosen as a model. The results are shown in Table 1.
When the reaction was carried out without any catalyst, the formation of the product was not observed.
Considering the fact that we had already used the SSA (4) in the synthesis of 1-amidoalkyl-2-naphthols and
3,4-disubstituted isoxazol-5(4H)-ones,46 we decided to use it as a catalyst in the synthesis of azlactones. In
the direction of optimize the catalyst loading, the model reaction was conducted in 5 mol%, 10 mol%, 15
mol%, and 20 mol% catalyst loadings under aqueous media at rt (Table 1, entries 2-5). The reaction
proceeded sluggishly with SSA (4) at 5 mol% loading but, at 20 mol% loading, furnished 90% yield of
corresponding product (5a) for 30 min (Table 1, entry 5). 25 mol% Catalyst loading furnished 91% yield of
5a product. Subsequently 20 mol% of catalyst was selected as the optimum loading. Several other solvents
such as EtOH, EtOAc, CHCl3, MeOH, Ph-Me, and a mixed of EtOH-H2O was screened and gave trace to
good yields of product (5a) (Table 1, entries 7-12). The reaction was also conducted under solvent-free
conditions and gave 40% isolated yield for 120 min (Table 1, entry 13). Furthermore, the reaction
temperature was investigated. The implementation of the model using the optimal amount of catalyst (20
mol%) at temperatures higher than rt, the product was formed in 84-90% isolated yields (Table 1, entries
14-16). Implementation of model reaction using 5 mol% and 10 mol% SSA at 60 °C leads to 70 and 79%

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HETEROCYCLES, Vol. 94, No. 7, 2017
isolated yields, respectively (Table 1, entries 17, 18). Finally, 20 mol% of SSA as the catalyst, water as
solvent, and rt as reaction temperature were selected as optimized conditions (Table 1, entry 5).
Table 1. Optimization of the reaction conditions for the synthesis of 4-benzylidene-
2-phenyloxazol-5(4H)-one (5a)
O
O
O
O
H
N
O
O
SSA (4)
+
N
+
HO
O
solvent (5 mL), temp.
H
O
2 (1 mmol)
3 (1 mmol)
1a (1 mmol)
5a
Entry Catalyst loading/mol%
Solvent
H2O
H2O
Temp/°C
rt
rt
Time/min Isolated yields/%
1
-
120
60
trace
65
2
5
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
10
15
20
25
20
20
20
20
20
20
20
20
20
20
5
H2O
H2O
H2O
H2O
EtOH
EtOAc
CHCl3
MeOH
Ph-Me
EtOH-H2O (1:1)
no solvent
H2O
rt
rt
rt
rt
rt
rt
rt
rt
rt
50
40
30
27
120
120
120
120
70
120
120
60
68
75
90
91
trace
78
85
trace
96
15
40
84
87
90
70
79
rt
rt
40
50
60
60
60
H2O
H2O
H2O
H2O
45
30
40
40
10
Optimized conditions shown in bold.
After optimization of conditions for synthesis of Erlenmeyer products, our study was focused on different
substituted benzaldehydes containing both electron-withdrawing and electron-donating groups and
heteroaromatic aldehydes such as thiophene-2-carboxaldehyde and furan-2-carboxaldehyde towards
synthesis of diversity azlactones. The results showed that the corresponding products (5a-r) were
synthesized within 82-98% isolated yields in 28-53 min. In this study, the catalyst was recovered and
reused for synthesis of 5a. After completion of the reaction, the solid product was filtered and washed
with water. The catalyst was recovered by simple evaporation from the filtrate, dried in the air and reused.
The remained SSA showed good catalytic activity as the product 5a obtained in 90%, 88%, 84%, and
70% isolated yields in four successive runs, respectively.

HETEROCYCLES, Vol. 94, No. 7, 2017
1317
Table 2. SSA-catalyzed synthesis of Erlenmeyer-Plöchl azlactones (5a-r) based on the optimized
conditions according to the Scheme 1
Structure of Erlenmeyer-Plöchl azlactones
Time Isolated
(min) yields
(%)
Mp (°C)
Reported
O
Found
(Melting
points are matched
with data from the
O
Ar
N
literature33,34,39,42,47
)
5a-r
Ar
C6H5 (5a)
30
53
40
35
40
34
42
28
40
45
37
32
46
42
32
45
40
45
90
95
97
94
96
86
88
89
90
87
85
86
82
94
98
94
90
86
171-173 169-170
143-144 144-145
171-173 172-173
145-156 144-145
138-139 138-139
154-156 153-154
102-103 101-102
212-214 213-214
159-191 157-158
154-156 151-152
162-163 163-164
188-190 198
157-159 159-160
240-241 238-240
169-171 167-168
164-165 158-159
172-174 173-175
165-167 169-170
4-Me-C6H4 (5b)
4-HO-C6H4 (5c)
3-HO-C6H4 (5d)
2-HO-C6H4 (5e)
4-MeO-C6H4 (5f)
3-MeO-C6H4 (5g)
4-(Me)2N-C6H4 (5h)
4-HO-3-MeO-C6H3 (5i)
2,4-diMeO-C6H3 (5j)
2-Cl-C6H4 (5k)
4-Cl-C6H4 (5l)
2,4-diCl-C6H3 (5m)
4-NO2-C6H4 (5n)
3-NO2-C6H4 (5o)
2-NO2-C6H4 (5p)
thiophen-2-yl (5q)
furan-2-yl (5r)
A plausible reaction mechanism for this transformation is outlined in Scheme 2. The reaction occurs
through initial formation of intermediate A by the reaction between the activated hippuric acid (2) using
SSA catalyst and acetic anhydride (3) followed by cyclization to 2-phenyloxazol-5(4H)-one (B). Enol (C)
is formed via protonation using acid catalyst and deprotonation by conjugate base of the SSA and this enol
form is then condensed with aldehyde (1) to form intermediate E. In the last step, protonation and
dehydration leads to the formation of the corresponding heterocyclic products (5a-r).
In conclusion, this work describes an efficient, mild and SSA-catalyzed condensation and
cyclodehydration reaction of hippuric acid, a wide range of aldehydes, and acetic anhydride to form
4-arylidene-2-phenyl-5(4H)-oxazolone derivatives in good to excellent yields. This method is easy to
implement and was successfully carried out with satisfactory results in a green medium.

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Scheme 2. Proposed reaction mechanism
EXPERIMENTAL
All chemicals, unless otherwise specified, were purchased from commercial sources and were used
without further purification, with the exception of liquid aldehydes, which were distilled before using.
1
Melting points were measured on a Buchi 510 melting point apparatus and are uncorrected. H NMR and
13C NMR spectra were recorded on a Bruker AVANCE DRX spectrometer. FT-IR spectra were recorded
on a Perkin Elmer RXI spectrometer. The development of reactions was monitored by thin layer
chromatography (TLC) analysis on Merck pre-coated silica gel 60 F254 aluminum sheets, visualized by
UV light. Elemental microanalyses were performed on Elementar Vario EL III analyzer at Damghan
University, Damghan, Iran. All of the targeted products are reported in the literature and are characterized
by comparison of their spectral and physical data on the basis of literature descriptions.
General procedure for the synthesis of 4-arylidene-2-phenyl-5(4H)-oxazolones (5a-r) catalyzed by
SSA. The appropriate aldehyde (1, 1 mmol), hippuric acid (2, 1 mmol), acetic anhydride (3, 1 mmol),
water (5 mL), and SSA as a catalyst (20 mol%) was stirred at rt for the appropriate time (Table 2). After
completion of the reaction as indicated by TLC (hexane:EtOAc; 70:30) analysis, the reaction mixture
allowed to stand for 5 h at rt. The resulting precipitated products were filtered off, washed with ice water
and dried at room temperature. The crude products were purified by recrystallization from absolute EtOH,
if required. The filtrate containing the catalyst was used as such for exploring the reusability of the
catalyst. Spectral data for 5a and 5n were as follows:
4-Benzylidene-2-phenyloxazol-5(4H)-one (5a). 1H NMR (CDCl3, 400 MHz): δ = 7.27 (s, 1H), 7.43-7.61
13
(m, 6H), 8.18-8.24 (m, 4H) ppm; C NMR (CDCl3, 100 MHz): δ = 113.2, 125.7, 128.2, 128.5, 128.7,

HETEROCYCLES, Vol. 94, No. 7, 2017
1319
129.2, 129.8, 131.2, 131.7, 135.5, 160.9, 166.0; Anal. Calcd for C16H11NO2: C, 77.10; H, 4.45; N, 5.62.
Found: C, 77.07; H, 4.41; N, 5.66.
4-(4-Nitrobenzylidene)-2-phenyloxazol-5(4H)-one (5n). 1H NMR (CDCl3, 400 MHz): δ = 7.28 (s, 1H),
7.61 (dd, J = 4.7, 10.9 Hz, 2H), 7.71-7.73 (m, 1H), 8.24 (dd, J = 1.2, 8.6 Hz, 2H), 8.34-8.37 (m, 2H),
13
8.29-8.32 (m, 2H); C NMR (CDCl3, 100 MHz): δ = 112.8, 122.2, 127.5, 128.9, 129.3, 129.8, 131.2,
131.3, 142.0, 146.7, 160.5, 166.3; Anal. Calcd for C16H10N2O4: C, 65.31; H, 3.43; N, 9.52. Found: C,
65.27; H, 3.39; N, 9.48.
ACKNOWLEDGEMENTS
The authors are grateful to the Research Council of Damghan University.
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