Analgesic activity of acylated 2-benzoxazolinone derivatives

Article abstract of DOI:10.1016/S0014-827X(98)00111-6

Ten new benzoxazolinone derivatives having a disubstituted benzoyl group at the six position of the ring were synthesized by reacting 2-benzoxazolinone with aromatic carboxylic acids in the presence of polyphosphoric acid. The structure of the compounds were elucidated by IR, ¹H NMR, MS and elemental analysis. Analgesic activity was evaluated by a modified Koster test. Seven compounds showed analgesic activities higher than that of acetylsalicylic acid.

Full text of DOI:10.1016/S0014-827X(98)00111-6

Il Farmaco 54 (1999) 112–115
Analgesic activity of acylated 2-benzoxazolinone derivatives
Nesrin Go¨khan a,*, Hakkı Erdog˘an a, N. Tug˘ba Durlu b, Ru¨meysa Demirdamar b
a Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Hacettepe Uni6ersity, Ankara, Turkey
b Department of Pharmacology, Faculty of Pharmacy, Hacettepe Uni6ersity, Ankara, Turkey
Received 17 June 1998; accepted 20 December 1998
Abstract
Ten new benzoxazolinone derivatives having a disubstituted benzoyl group at the six position of the ring were synthesized by
reacting 2-benzoxazolinone with aromatic carboxylic acids in the presence of polyphosphoric acid. The structure of the
1
compounds were elucidated by IR, H NMR, MS and elemental analysis. Analgesic activity was evaluated by a modified Koster
test. Seven compounds showed analgesic activities higher than that of acetylsalicylic acid. © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: 2-Benzoxazolinone; Acylation; Analgesic activity
In particular, 6-acylbenzoxazolinones exhibit anal-
gesic properties much greater than those of the parent
1. Introduction
heterocycle and are indeed the latest development in the
Since the first report on the hypnotic properties of
field of central nervous system drugs. Electrophilic sub-
2-benzoxazolinone [1,2], a number of derivatives have
stitution, such as chlorination, sulfonation and nitra-
been tested for various activities including anticonvul-
tion was achieved using classical reagents but acylation
sant [3], antipyretic, analgesic [4–10], cardiotonic
yielding 6-acyl derivatives was found to require particu-
[11,12], antiulcer [13], antineoplastic [14] or antibacte-
rial, antimicrobial and antifungal [15–20] effects. 6-
lar conditions. Acylation of benzoxazolinone was ac-
complished earlier by two methods in the literature, in
Methoxybenzoxazolinone is
a
natural occurring
which either DMF–AlCl3 and acid chloride [26] or
polyphosphoric and carboxylic acids [4] were used. It
was reported that both methods gave the same product
(6-acylbenzoxazolinone) at very similar yields [27].
The aim of the present work is to synthesize deriva-
tives carrying benzoyl moieties at 6 position of the
benzoxazolinone (Scheme 1) and to examine their anal-
gesic activities.
compound that acts as an agonist of melatonin and also
serves as an environmental stimulus for reproductive
activities [21,22]. This role emphasizes the potential of
the benzoxazolinone structure. However, it is interest-
ing to note that the benzoxazolinone ring could be
considered as a cyclic bioisoster of pyrocatechol. This
analogy was confirmed by the biological study of the
compounds including a benzoxazolinone ring linked to
phenylethylamine chains and these compounds were
accepted as potent agonists or antagonists of dopamine
or norepinephrine [23–25].
2. Experimental
Numerous similar works are currently in progress to
obtain a better understanding of the subject and it is
clear that choosing a useful starting substance such as
the acyl derivatives of benzoxazolinone is the key step
of the medicinal chemistry.
2.1. Chemistry
All chemicals and solvents used in the present study
were purchased from Merck AG and Aldrich Chemi-
cals. Melting points were determined using a Thomas
Hoover capillary melting point apparatus (Philadelphia,
USA) and were uncorrected. IR spectra were recorded
* Corresponding author.
0014-827X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0014-827X(98)00111-6

N. Go¨khan et al. / Il Farmaco 54 (1999) 112–115
113
Scheme 1. Synthesis of 6-acyl-2-benzoxazolinone.
Table 1
Analytical data for the synthesized compounds
Comp.
R
R1/R2
Crystallization solvents
M.p. (°C)
Yield (%)
Formula
Analysis
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
2,5-F
2,5-F
2,3-F
2,3-F
3,4-F
3,4-F
3,5-F
3,5-F
2,6-F
2,6-F
Ethanol–water
Acetonitrile
161–163
192–194
176–177
151–154
170–172
175–177
182–184
153–156
181–183
139–141
58
62
57
67
51
64
72
59
68
74
C14H7O3NF2
C14H6O3NClF2
C14H7O3NF2
C14H6O3NClF2
C14H7O3NF2
C14H6O3NClF2
C14H7O3NF2
C14H6O3NClF2
C, N, H
C, N, H
C, N, H
C, N, H
C, N, H
C, N, H
C, N, H
C, N, H
C, N, H
C, N, H
Acetonitrile–water
Acetonitrile–water
Ethyl acetate–n-hexane
Ethyl acetate–n-hexane
Ethanol–water
Ethanol
Acetonitrile–water
Acetonitrile–water
C
14H7O3NF2
2j
C14H6O3NClF2
Table 2
Spectral data of 6-acyl-benzoxazolinone derivatives
Comp. IR (cm−1
)
1H NMR (ppm)
N–H 2-Benzoxazolinone H4
Aromatic ketone Lactam CꢀO
CꢀO
2-Benzoxazolinone H5,7
Aromatic ring (benzene)
2a
2b
2c
2d
2e
2f
2g
2h
2I
2j
1657
1618
1620
1623
1650
1615
1621
1622
1623
1621
1747
1777
1757
1778
1789
1771
1810
1779
1771
1792
3264 6.81–6.83 (s, 1H, J=8.24 Hz) 7.63–7.69 (m, 2H)
3293 6.87–6.89 (s, 1H, J=8.24 Hz) 7.14–7.43 (m, 4H)
3118 6.86–6.88 (s, 1H, J=8.24 Hz) 7.57–7.66 (m, 2H)
3295 6.84–6.86 (s, 1H, J=8.24 Hz) 7.05–7.44 (m, 4H)
3186 6.81–6.83 (s, 1H, J=8.24 Hz) 7.63–7.70 (m, 2H)
5155 6.90–6.92 (s, 1H, J=8.24 Hz) 7.16–7.60 (m, 4H)
3108 6.91–6.93 (s, 1H, J=8.24 Hz) 7.55–7.61 (m, 2H)
3161 6.82–6.83 (s, 1H, J=8.24 Hz) 7.05–7.58 (m, 4H)
3258 6.84–6.86 (s, 1H, J=8.24 Hz) 7.64–7.40 (m, 2H)
3197 6.82–6.84 (s, 1H, J=8.24 Hz) 7.14–7.56 (m, 4H)
7.13–7.26 (m, 3H)
7.14–7.43 (m, 4H)
7.08–7.32 (m, 3H)
7.05–7.44 (m, 4H)
7.13–7.26 (m, 3H)
7.16–7.60 (m, 4H)
7.10–7.30 (m, 3H)
7.05–7.58 (m, 4H)
6.95–7.11 (m, 3H)
7.14–7.56 (m, 4H)
8
on a Perkin–Elmer 457 IR spectrophotometer (Uber-
lingen, Germany). H NMR spectra were recorded on a
2.1.1. 6-Acyl-2-benzoxazolinone
1
6-Acyl-2-benzoxazolinone was obtained by reacting
2-benzoxazolinone and/or 5-chloro-2-benzoxazolinone
(chlorzoxazone) with the appropriate carboxylic acid in
polyphosphoric acid (PPA) as reported by Pilli et al. [7].
Bruker AC 400 MHz spectrometer (Karlsruhe, Ger-
many) using dimethylsulfoxide-d6 with chemical shifts
reported as l (ppm) with TMS as internal standard.
Mass spectra were taken on a VG Analytical 70-250S
or HP II Plus 5890 mass spectrometer with electron
ionization (EI).
2.2. Pharmacology
Microanalyses were performed in the laboratory of
the Turkish Scientific and Technical Research Council
(Ankara, Turkey). The purity of the compounds was
assessed by TLC on silica gel HF254+366 (E. Merck,
Darmstadt, Germany).
Locally-bred, female albino mice weighing 2292 g
were used. The animals were transferred to the labora-
tory at least 2 days before the experiments to acclima-
tize them to their new environment. They were placed

114
N. Go¨khan et al. / Il Farmaco 54 (1999) 112–115
4. Discussion
in groups of six in glass cages, and fed with food and
water ad libitum.
Due to the fact that both the 3-nitrogen and 2-oxygen
atoms were electron-donating, their 5th- and 6th-positions
were activated and therefore, the regioselectivity in the
C-acylation of benzoxazolones could not be easily pre-
dicted. The existence of 5-acylation products was re-
ported earlier [29], but this report was not substantiated
by any spectroscopic data. It was found that the spectral
region corresponding to the aromatic protons extended
over a wider range of chemical shifts (7.11–7.92 ppm) in
the 6-series, and the 4- and 6-protons, which were in the
vicinity of the carbonyl group, were well separated in the
5-series but overlapping occurred in the 6-series.
2.2.1. Analgesic acti6ity [28]
Analgesic activity of the compounds was examined by
employing a modified Koster test. Each compound was
dispersed in carboxymethyl cellulose and then a 100
mg/kg dose was administered orally to a group of six
mice. Exactly 1 h later, a 3% solution of acetic acid was
injected intraperitoneally at a dose of 300 mg/kg and 5
min after the injections stretches were observed in the IR
spectra for a further 10 min. Pure carboxymethyl cellu-
lose was given to the control group (n=6). As a standard
analgesic, acetylsalicylic acid (ASA) was used. The anal-
gesic activity was calculated using the following formula:
In order to clarify this situation, we reproduced the
acylation in PPA (which unambiguously leads to 6-acyl
derivatives) and also discovered that the isolated materi-
als definitively belonged to the 6-acyl series.
% analgesic activity=(n−n%)/n×100
where n and n% indicate the average number of the
stretches for the control and test groups, respectively.
In the IR spectra, absorption bands were detected at
ca. 3200 cm−1, thus indicating the presence of a N–H
group. The lactam and ketone CꢀO stretching bands
were seen at ca. 1810–1747 and 1657–1618 cm−1
,
3. Results
respectively. Other observed stretches confirmed the
structures of these compounds.
1
The molecular formulae, melting points, % yields and
crystallization solvents used in this study are listed in
Table 1. The compounds were obtained by reacting
2-benzoxazolinone or 5-chloro-2-benzoxazolinone with
disubstituted fluorobenzoic acid derivatives in PPA
(Scheme 1). The structure of the compounds were eluci-
As was reported earlier, the H NMR spectral region
extended over a wider range of chemical shifts (6.81–
7.70) and so H5 and H7 protons (7.05–7.70), which were
in close proximity to the carbonyl group, overlapped in
the acylated products. The 2-benzoxazolinone H4 proton
was observed at ca. 6.81–6.89 ppm (J=8.2 Hz). Other
aromatic protons were seen as expected chemical shift
and integral values.
The mass spectroscopic fragmentation of the 6-di-
fluorobenzoyl-(5-chloro)-2-benzoxazolinone ring was
studied under electron ionization. Molecular ion peaks
were found to appear for 2-benzoxazolinone derivatives.
Base peaks resulted from a loss of a difluorophenyl
radical and gave a peak at m/z 162. A second cleavage
occurred between the carbonyl and 2-benzoxazolinone
ring. As a result, the remaining radical gave a peak at
m/z 113. Molecular ion peaks were not found to appear
for 5-chloro-2-benzoxazolinone (chlorzoxazone). Base
peaks were seen at m/z 169, resulting from the loss of a
difluorobenzoyl radical. Intense peaks were also seen as
a result of CO2 and 2CO loss from the molecular ions.
The results of microanalyses were within 90.4% of
theoretical values.
It was reported that the introduction of an acyl group
into the 6th position of the benzoxazolinone ring caused
an increase in activity [4]. These results led us to screen
the compounds for analgesic activity. As seen in Table
3 (with the exception of 2c, 2e and 2h), all the compounds
showed analgesic activity greater than that of ASA
(PB0.01). In addition, substituting a chlorine atom into
the 5th position of a benzoxazolinone also improved the
analgesic activity of the compounds mentioned herein.
1
dated by IR, H NMR, MS and microanalyses and all
spectral data were in accordance with the assigned
structure (Table 2). The analgesic activity of the com-
pounds was screened by means of the modified Koster
test (Table 3).
Table 3
Analgesic activities of 6-acyl-2-benzoxazolinone derivatives
Comp.a
R
R1/R2
% Analgesic activityb
2a
2b
2c
2d
2e
2f
2g
2h
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
–
2,5-F
2,5-F
2,3-F
2,3-F
3,4-F
3,4-F
3,5-F
3,5-F
2,6-F
2,6-F
–
76.07
64.26
38.22
78.63
19.86
63.40
70.25
–*
2i
2j
53.26
68.37
45.24
Aspirin
a 100 mg/kg p.o.
b Results are expressed as their mean values.
* No activity, n=6, PB0.01.

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