Oxazolones: New tyrosinase inhibitors; synthesis and their structure-activity relationships

Article abstract of DOI:10.1016/j.bmc.2006.05.014

The tyrosinase inhibitory potential of seventeen synthesized oxazolone derivatives has been evaluated and their structure-activity relationships developed in the present work. All the synthesized derivatives, 3-19, demonstrated excellent in vitro tyrosina

Full text of DOI:10.1016/j.bmc.2006.05.014

Bioorganic & Medicinal Chemistry 14 (2006) 6027–6033
Oxazolones: New tyrosinase inhibitors; synthesis and their
structure–activity relationships
Khalid Mohammed Khan,a,* Uzma Rasool Mughal,a Mahmud Tareq Hassan Khan,a
Zia-Ullah,a Shahnaz Perveenb and Muhammad Iqbal Choudharya
aH.E.J. Research Institute of Chemistry, International Center for Chemical Sciences, University Karachi, Karachi-75270, Pakistan
bPCSIR Laboratories Complex, Off University Road, Karachi-75280, Pakistan
Received 21 March 2006; revised 3 May 2006; accepted 4 May 2006
Available online 5 June 2006
Abstract—The tyrosinase inhibitory potential of seventeen synthesized oxazolone derivatives has been evaluated and their structure–
activity relationships developed in the present work. All the synthesized derivatives, 3–19, demonstrated excellent in vitro tyrosinase
inhibitory properties having IC50 values in the range of 1.23 0.37–17.73 2.69 lM, whereas standard inhibitors L-mimosine and
kojic acid have IC50 values 3.68 0.02 and 16.67 0.52 lM,, respectively. Compounds 4–8 having IC50 values 3.11 0.95,
3.51 0.25, 3.23 0.66, 1.23 0.37, and 2.15 0.75, respectively, were found to be very active members of the series, even better
than both the standard inhibitors. However, compounds 3, 9–11, 13, 14, 16, 17, and 19 were found to be better than kojic acid but
not L-mimosine. (2-Methyl-4-[E,2Z)-3-phenyl-2-propenyliden]-1,3-oxazol-5(4H)-one (7) bearing a cinnamyol residue at C-4 of oxa-
zolone moiety and an IC50 = 1.23 0.37 lM was found to be the most active one among all tested compounds. These studies reveal
that the substitution of functional group (s) at C-4 and C-2 positions plays a vital role in the activity of this series of compounds. It is
concluded that compound 7 may act as a potential lead molecule to develop new drugs for the treatment of tyrosinase based
disorders.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Therefore, many tyrosinase inhibitors that suppress
melanogenesis have been widely studied with the aim
of developing preparations for the treatment of
hyperpigmentation.6
Tyrosinase (EC 1.14.18.1), a multifunctional copper-
containing enzyme, is widely distributed in the plant
and animal kingdom. It is responsible for catalyzing
ortho-hydroxylation of phenols and ortho-phenol oxida-
tion to corresponding quinones.1–4 Tyrosinase inhibitors
are clinically useful for the treatment of skin diseases
associated with melanin hyperpigmentation and applied
in cosmetics for whitening and depigmentation after
In insects, several enzymes have been reported7 to gener-
ate o-diphenols and quinones that cure pigmentation
heal wounds, encapsulate parasite or help sclerotize.
Such enzymes may be an alternative target site for the
control of insect pests. In food industry, tyrosinase is
effective for the enzymatic browning reactions in dam-
aged fruits during post-harvest handling and processing.
Controlling enzymatic browning is essential during fruit
pulp manufacturing process.
sunburn.5 Melanin is
a heteropolymer of indole
compounds and is produced inside melanosomes by
the action of the tyrosinase enzyme on the tyrosinase
precursor material in melanocytes. It has recently been
discovered that some other factors such as metal ions
and the TRP-1 and TRP-2 enzymes also contribute to
the production of melanin. However, tyrosinase plays
a critical role in the regulation of melanin biosynthesis.
Oxazolones exhibit a wide spectrum of pharmacological
activities including anticancer, antimicrobial, antifungal,
antagonistic, sedative, etc. Oxazolones that are internal
anhydrides of acyl amino acids make an important class
of five-membered heterocycles. These are highly versa-
tile intermediates used for the synthesis of several organ-
ic molecules, including amino acids, peptides,8–13
antimicrobial or antitumor compounds,14,15 immuno-
modulators,16 heterocyclic precursors,17–20 for biosen-
Keywords: Tyrosinase inhibition; Oxazolones; Structure–activity rela-
tionship; Lead compounds.
*
Corresponding author. Tel.: +92 21 4824910; fax: +92 21 4819018
19; e-mail: hassaan2@super.net.pk
0968-0896/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2006.05.014

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K. M. Khan et al. / Bioorg. Med. Chem. 14 (2006) 6027–6033
sors coupling, and/or photosensitive composition devic-
es for proteins.21 Some oxazolones have shown a wide
range of pharmaceutical properties.22 These can easily
be prepared from N-acyl amino acids by dehydration.
yields.28–30 All the oxazolones were isomerized by heat-
ing them with polyphosphoric acid on a water bath at
80–90 °C for 2 h according to literature procedure.31
A
general structure of oxazolone is shown in Scheme 1
along with its numbering. The structures were deter-
mined by using different spectroscopic methods like H
1
In continuation of our research work on oxazolone
derivatives as potential lead compounds in our drug dis-
covery program,23–27 we synthesized a variety of oxazol-
ones and screened them randomly for their tyrosinase
inhibitory properties.
NMR, EI, IR, and UV, and purity was confirmed by
CHN analysis.
2.2. Biology
All the synthesized oxazolones were subjected to tyrosi-
nase inhibitory assays, according to literature protocol7
(Table 1, Fig. 1).
2. Results and discussion
2.1. Chemistry
The oxazolones 3–19 demonstrated excellent in vitro
tyrosinase inhibitory properties having IC50 values in
the range of 1.23 0.37–17.73 2.69 lM, whereas stan-
dard inhibitors, L-mimosine and kojic acid, have IC50 val-
ues 3.68 0.02 and 16.67 0.52 lM, respectively.
Compounds 3–8 having IC50 values 10.51 1.04,
3.11 0.95, 3.51 0.25, 3.23 0.66, 1.23 0.37, and
2.15 0.75 lM, respectively, were found to be very
In recent past, we investigated a variety of classes of
compounds for their potential use in medicinal chemis-
try. In the present study, seventeen oxazolones 3–19
were synthesized from commercially available glycine
with acetic anhydride or benzoyl chloride in the presence
of anhydrous sodium acetate followed by Erlenmeyer
condition with appropriate aldehyde in very high
O
O
O
5
b,c
R2CHO
a
4
OH
OH
1
O
R1
HN
R2
H2N
R1
Cl
N
O
2
O
3
R1
1
2
3-19
a) Ac2O or BzCl, 10% NaOH, H2O; b) Ac2O, NaOAc, reflux, c) PPA, 80-90 ˚C
R1
R2
R2
Compound
Compound
12
R1
OCOCH3
3
N
COCH3
OCOCH3
CH3
13
14
15
4
5
CH3
CH3
S
OCOCH3
NO2
NO2
NO2
6
7
CH3
CH3
OCH3
16
17
OCH3
OCH3
H3COCO
OH
8
9
OCOCH3
OCH3
18
19
CH3
NHCOCH3
10
NHCOCH3
NO2
O
11
CH3
O
Scheme 1. Reagents and conditions: (a) Ac2O or BzCl, 10% NaOH, H2O; (b) Ac2O, NaOAc, reflux; (c) PPA, 80–90 °C.

K. M. Khan et al. / Bioorg. Med. Chem. 14 (2006) 6027–6033
6029
Table 1. Tyrosinase inhibitory activities of the oxazolones 3–19, as
compared to the standard inhibitors
and oxazolone moiety to the phenyl ring present at
C-2. On the other hand, compound 12 likewise com-
pound 18 having phenyl moieties at C-2 seemingly
fulfill the requirement for extension of conjugation
through indole and naphthyl residues, respectively,
but found to be inactive in terms of tyrosinase inhib-
itory potential. This shows that extension of conjuga-
tion through an aliphatic double bond could be the
prerequisite for activity rather than extension through
an aromatic ring. In the case of nitrophenyl contain-
ing derivatives 5, 6, 15, and 19, the difference in their
activity seems to follow supposition of extension of
conjugation. The active compound 6, containing
Compound
IC50 SEMa (lM)
3
10.51 1.04
3.11 0.95
3.51 0.25
3.23 0.66
1.23 0.37
2.15 0.75
8.72 0.47
6.35 1.06
5.21 1.22
NAb
4
5
6
7
8
9
10
11
12
13
14
13.39 0.99
6.20 2.37
17.73 2.69
5.67 0.64
4.55 0.55
NA
nitrophenyl residue (IC50 3.23 0.66 lM), has
a
phenyl ring at C-2 and a 4-nitrophenyl at C-4,
whereas a sharp decline in the activity of compound
5 (IC50 3.51 0.25 lM) might be due to the presence
of methyl residue at C-2. A decrease in the activity of
compound 19 (IC50 16.45 1.96 lM) as compared to
compound 6 was due to the change in the position of
nitro group in phenyl ring present at C-4. The least
activity of compound 15 (IC50 17.73 2.69 lM) may
be because of change in the position of nitro group
in phenyl ring present at C-4 and substituent at C-
2. Compound 8 (IC50 2.15 0.75 lM) was found to
be second highly active member of the present series
of oxazolones. Its excellent activity may be due to the
presence of a phenyl group at its perfect position,
that is, C-2 and the presence of an acetyl group at
C-4 of second phenyl group, which meets the criteria
for achieving extension of conjugation in addition to
some effect of phenolic hydroxyl present at C-3 of the
same phenyl ring. Though structurally similar, dra-
matic decline in the activities of compounds 3 (IC50
10.51 1.04 lM), 13 (IC50 1.39 0.99 lM), and 14
(IC50 6.20 2.37 lM) as compared to 8 may be due
to either of the above-mentioned reasons. Compounds
9 (IC50 8.72 0.47 lM) and 10 (IC50 6.35 1.06 lM)
are structurally similar to compound 8 except where
oxygen is replaced by nitrogen. Interestingly, com-
pounds 16 and 17 having IC50 values 5.67 0.64
and 4.55 0.55 lM, respectively, showed excellent
activity. The activities may be due to the presence
of methoxy groups on phenyl ring at C-4, and differ-
ence in their activity is due to the obvious reason of
presence of substituents at C-2. The enhanced activity
15
16
17
18
19
Kojic acidc
L-Mimosinec
16.45 1.96
16.67 0.52
3.68 0.02
a SEM is the standard error of the mean.
b Not active.
c Standard inhibitors of the enzyme tyrosinase.
active members of the series, even better than both the
standard inhibitors. However, compounds 9–11, 13, 14,
16, 17, and 19 were found better than the standard kojic
acid but not L-mimosine. Interestingly only compound
15 (IC50 17.73 2.69 lM) was found a little less active
than standard kojic acid and no tyrosinase inhibitory
activity was observed for compounds 12 and 18. 2-Meth-
yl-4-[(E)-3-phenyl-2-propenyliden]-1,3-oxazol-5(4H)-one
(7) bearing a cinnamyol residue at C-4 of oxazolone moi-
ety was found to be the most active one having
IC50 = 1.23 0.37 lM among all tested compounds.
Comparing the activities with the structures of com-
pounds, it turns out that the tyrosinase activity is
mainly dependent on the substituents present at C-2
and C-4 positions of oxazolone ring. When tyrosinase
inhibitory activity of the most active compound 7
was compared with other compounds, it was observed
that it has an extension of conjugation from distant
phenyl ring passing through aliphatic double bonds
20
15
10
5
0
3
4
5
6
7
8
9
10
11
13
14
15
16
17
19
LM
KA
Figure 1. Comparative graphical presentation of the tyrosinase inhibitory potentials of the oxazolones 3–19.

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K. M. Khan et al. / Bioorg. Med. Chem. 14 (2006) 6027–6033
of compound
explained on the basis of above-mentioned reasons.
4
(IC50 3.11 0.95 lM) could be
and L-mimosine were used as standard inhibitors for
the tyrosinase. All the chemicals and reagents were pur-
chased from Sigma Chem. Co., MO, USA.
3. Conclusion
4.2. General procedure for the synthesis of compounds 3–19
Present studies suggested that the substitutions of func-
tional group (s) at C-4 and C-2 positions of oxazolone
are crucial for tyrosinase inhibitory activity of this class
of compounds. It is concluded that an extension of conju-
gation through an aliphatic double bond present at C-4
position of oxazolone moiety and a phenyl ring present
at C-2play apivotalrolein activity. Ofthisseries, themost
active compound 7 may act as a potential lead molecule
for the future research in the field of tyrosinase inhibition.
A
mixture of N-acylglycine or benzoyl glycine
(1.2 mmol), aldehyde (1 mmol), acetic anhydride
(3 mmol), and fused sodium acetate (1.5 mmol) was
heated to liquification and heating was continued for
additional 3–4 h. After completion of reaction (TLC
analysis), EtOH (5 ml) was added and the product was
left in refrigerator for overnight. It was filtered to obtain
solid mass which was isomerized by mixing and heating
with polyphosphoric acid on a steam bath at 80–90 °C
for 2 h and finally pouring into water.31 The resultant
solid was collected, washed with cold ethanol, hot water,
ethanol, and small amount of hexane and then dried to
afford title compounds 3–19.
4. General experimental
Melting points were determined on a Buchi 434 melting
¨
point apparatus and are uncorrected. NMR was per-
formed on a Bruker AM 300 and 500 MHz, respectively.
CHN analysis was performed on a Carlo Erba Strumen-
tazion-Mod-1106, Italy. Ultraviolet (UV) spectra were
recorded on Perkin-Elmer Lambda-5 UV/vis spectrome-
ter for MEOH. Infrared (IR) spectra were recorded on
JASCO IR-A-302 Spectrometer. Electron impact mass
spectra (EIMS) were recorded on a Finnigan MAT-
311A, Germany. Thin-layer chromatography (TLC)
was performed on precoated silica gel glass plates (Kie-
selgel 60, 254, E. Merck, Germany). Chromatograms
were visualized by UV at 254 and 365 nm or by iodine
vapors.
4.2.1. 3-{[5-Oxo-2-phenyl-1,3-oxazol-4-(5H)-ylidene]methyl}
phenylacetate (3). Yield: 53%; mp: 123 °C; Rf: 0.48 (ethyl
acetate/hexane, 3:7); UV (MEOH): kmax 190 (loge = 5.5)
nm. IR (KBr): mmax 3072, 1797, 1656, 1216, 1170 cmꢀ1
.
1H NMR (400 MHz, CDCl3): d 7.20 (s, 1H, H-6), 8.17
(d, J = 7.2 Hz, 2H, H-20/60), 7.47 (t, J = 7.2 Hz, 2H, H-
30/50), 7.53 (t, J = 7.2 Hz, 1H, H-40), 8.16 (d,
J = 1.5 Hz, 1H, H-200), 8.17 (d, J = 7.3 Hz, 1H, H-400),
7.61 (t, J = 7.3 Hz, 1H, H-500), 7.84 (d, J = 7.3 Hz, 1H,
H-600), 2.35 (s, 3H, COCH3); EIMS: m/z (rel. abund.
%) 307 (M+, 2.0), 265 (2.42), 105 (100), 76 (84), 51
(90); Anal. Calcd for C18H13NO4: C, 70.35; H, 4.26;
N, 4.56; O, 20.83. Found: C, 70.39; H, 4.28; N, 4.59;
O, 20.86.
4.1. Tyrosinase inhibition assay
4.2.2. 2-Methyl-4-[(E)-2-thienylmethylidene]-1,3-oxazol-
5-one (4). Yield: 62%; mp: 175 °C; Rf: 0.49 (ethyl ace-
tate/hexane 3:7); UV (MEOH): kmax 357 (loge = 4.40)
nm. IR (KBr): mmax 3268, 3078, 2924, 1796, 1651,
Tyrosinase inhibition assays were performed in 96-well
microplate format using SpectraMaxÒ 340 microplate
reader (Molecular Devices, CA, USA) according to the
method Hearing7 protocol. Briefly, first the compounds
were screened for the o-diphenolase inhibitory activity
of tyrosinase using L-DOPA as substrate. All the active
inhibitors from the preliminary screening were subjected
to IC50 studies. Compounds were dissolved in methanol
to a concentration of 2.5%. 30 U of mushroom tyrosi-
nase (28 nM) was pre-incubated with the compounds
in 50 nM Na-phosphate buffer (pH 6.8) for 10 min at
25 °C. Then the L-DOPA (0.5 mM) was added to the
reaction mixture and the enzyme reaction was moni-
tored by measuring the change in absorbance at
475 nm (at 37 °C) due to the formation of the DOPA
chrome for 10 min. The percent inhibition of the enzyme
1264 cmꢀ1 1H NMR (400 MHz, CDCl3) d 7.30 (s,
.
1H, H-6), 2.39 (s, 3H, CH3), 7.54 (d, J = 3.5 Hz, 1H,
H-30), 7.12 (dd, J = 4.7, 3.5 Hz, 1H, H-40), 7.66 (d,
J = 4.7 Hz, 1H, H-50); EIMS: m/z (%) 193, (M+, 43),
165 (17), 123 (100), 96 (34), 83 (26); Anal. Calcd for
C9H7NO2S: C, 55.94; H, 3.65; N, 7.25; O, 16.56; S,
16.06. Found: C, 55.96; H, 3.68; N, 7.27; O, 16.58; S,
16.09.
4.2.3. 2-Methyl-4-[(E)-(4-nitrophenyl)methylidene]-1,3-
oxazol-5(4H)-one (5). Yield: 62%; mp: 273 °C; Rf: 0.59
(ethyl acetate/hexane, 3:7); UV (MEOH): kmax 193
(loge = 4.8) nm. IR (KBr): mmax 3387, 2926, 1663,
1
was calculated as follows, by using MS ExcelÒ 2000
1343, 1168 cmꢀ1. H NMR (400 MHz, CDCl3) d 7.11
TM
(Microsoft Corp., USA) based program developed for
this purpose:
(s, 1H, H-6), 2.44 (s, 3H, CH3), 8.23 (d, J = 8.9 Hz,
2H, H-20/60), 8.26 (d, J = 8.9 Hz, 2H, H-30/50); EIMS:
m/z (%) 232, (M+, 100), 204 (10), 162 (33), 132 (26),
116 (35), 89 (90), 63 (78); Anal. Calcd for C11H8N2O4:
C, 56.90; H, 3.47; N, 12.06; O, 27.56. Found: C, 56.95;
H, 3.49; N, 12.11; O, 27.60.
Percent inhibitionð%Þ ¼ ½B ꢀ S=Bꢁ ꢂ 100
Here, the B and S represent absorbance values for the
blank and samples, respectively. After screening of the
compounds, median inhibitory concentration (IC50)
was also calculated. All the experiments were carried
out at least in triplicate and the results represent
means SEM (standard error of the mean). Kojic acid
4.2.4. 4-[(E)-(4-Nitrophenyl)methylidene]-2-phenyl-1,3-
oxazol-5(4H)-one (6). Yield: 83%; mp: 113 °C; Rf: 0.60
(ethyl acetate/hexane, 3:7); UV (MEOH): kmax 201

K. M. Khan et al. / Bioorg. Med. Chem. 14 (2006) 6027–6033
6031
(loge = 4.1) nm. IR (KBr): mmax 3633, 3104, 2925, 1797,
4.2.9. 2-Methyl-4-[(E)-(4-oxo-4H-chromen-3-yl)methyli-
dene]-1,3-oxazol-5-one (11). Yield: 91%; mp: 239 °C;
Rf: 0.58 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
194 (loge = 5.1) nm. IR (KBr): mmax 3074, 2926, 1716,
1
1520, 1343, 1296 cmꢀ1. H NMR (400 MHz, CDCl3): d
7.24 (s, 1H, H-6), 8.20 (d, J = 7.3 Hz, 2H, H-20/60), 7.5
(t, J = 7.3 Hz, 2H, H-30/50), 7.66 (t, J = 7.3 Hz, 1H, H-
40), 8.30 (d, J = 7.3 Hz, 2H, H-200/600), 8.36 (d,
J = 8.8 Hz, 2H, H-300/500); EIMS: m/z (%) 294 (M+ 10),
105 (100), 77 (54); Anal. Calcd for C16H10N2O4: C,
65.31; H, 3.43; N, 9.52; O, 21.75. Found: C, 65.35; H,
3.49; N, 9.55; O, 21.77.
1
1653, 1531 cmꢀ1. H NMR (400 MHz, CDCl3): d 7.12
(s, 1H, H-6), 2.38 (s, 3H, CH3), 8.1 (s, 1H, H-20), 7.50
(d, J = 8.3 Hz, 1H, H-50), 7.45 (td, J = 8.0, 1.0 Hz, 1H,
H-60), 7.70 (t, J = 8.0 Hz, 1H, H-70), 8.27 (dd, J = 8.0,
1.6 Hz, 1H, H-80); EIMS: m/z (%) 255 (M+, 27.22),
185 (100), 120 (22), 92 (51); Anal. Calcd for
C14H9NO4: C, 65.88; H, 3.55; N, 5.49; O, 25.07. Found:
C, 65.91; H, 3.58; N, 5.52; O, 25.09.
4.2.5. 2-Methyl-4-[(E)-3-phenyl-2-propenyliden]-1,3-oxa-
zol-5(4H)-one (7). Yield: 80%; mp: 105 °C; Rf: 0.61 (eth-
yl acetate/hexane, 3:7); UV (MEOH): kmax 195
(loge = 5.39) nm. IR (KBr): mmax 3030, 1787, 1639
4.2.10.
4-[(E)-(1-Acetyl-1H-indol-3-yl)methylidene]-2-
1
1296 cmꢀ1. H NMR (400 MHz, CDCl3): d 7.1 (s, 1H,
phenyl-1,3-oxazol-5-(4H)-one (12). Yield: 52%; mp:
215 °C Rf: 0.59 (ethyl acetate/hexane, 3:7); UV
(MEOH): kmax 194 (loge = 5.6) nm. IR (KBr): mmax
H-6), 7.65 (d, J = 11.6 Hz, H-7), 7.69 (d, J = 11.6 Hz,
1H, H-7), 8.13 (dd, J = 8.1, 1.0 Hz, 2H, H-20/60), 7.50
(t, J = 8.1 Hz, 2H, H-30/50), 7.60 (td, J = 8.1, 1.0 Hz,
1H, H-40), 8.05 (dd, J = 8.2, 1.5 Hz, 2H, H-200/600), 7.38
(m, 3H, H-300/500); EIMS: m/z (%) 275 (M+, 32), 247
(8.0), 105 (100), 77 (64); Anal. Calcd for C18H13NO2:
C, 78.53; H, 4.76; N, 5.09; O, 11.62: Found: C, 78.55;
H, 4.79; N, 5.11; O, 11.65.
3420, 2926, 1789, 1645, 1526, 1448, 1207 cmꢀ1 1H
.
NMR (400 MHz, CDCl3): d 7.42 (s, 1H, H-6), 8.14 (d,
J = 8.6 Hz, 2H, H-20/60), 7.54 (t, J = 8.6 Hz, 2H, H-30/
50), 7.62 (t, J = 8.6 Hz, 1H, H-40), 7.52 (s, 1H, H-200),
8.39 (d, J = 8.0 Hz, 1H, H-400), 8.27 (m, 2H, H-500/600),
8.48 (dd, J = 8.6, 1.4 Hz, 1H, H-700), 2.81 (s, 3H,
COCH3); EIMS: m/z (%) 330, (M+, 11.8), 288 (9.4),
105 (100), 77 (36); Anal. Calcd for C20H14N2O3: C,
72.72; H, 4.27; N, 8.48; O, 14.53. Found: C, 72.76; H,
4.29; N, 8.50; O, 14.55.
4.2.6. 3-(Acetoyloxy)-2-hydroxy-4-{[5-oxo-2-phenyl-1,3-
oxazol-4(5H)-ylidene]methyl}phenylacetate (8). Yield:
75%; mp: 169 °C; Rf: 0.45 (ethyl acetate/hexane, 3:7),
UV (MEOH): kmax 195 (loge = 5.7) nm. IR (KBr): mmax
3094, 3058, 1774, 1369, 1211 cmꢀ1. 1H NMR (400 MHz,
CDCl3): d 7.1 (s, 1H, H-6), 7.54 (d, J = 7.2 Hz, 2H, H-20/
60), 7.44–7.31 (m, 3H, H-30-50), 7.9(d, J = 7.6 Hz, 1H, H-
200), 8.16 (d, J = 7.6 Hz 1H, H-300), 2.29 (s, 3H, COCH3),
2.32 (s, 3H, COCH3); EIMS: m/z (%) 381, (M+, 1.32),
339 (3.04), 297 (22), 105 (100), 82(42); Anal. Calcd for
C20H15NO7: C, 62.99; H, 3.96; N, 3.67; O, 29.37. Found:
C, 62.98; H, 3.98; N, 3.69; O, 29.39.
4.2.11.
4-{[2-Methyl-5-oxo-1,3-oxazol-4(5H)-ylidene]-
methyl} phenylacetate (13). Yield: 45%; mp: 203 °C; Rf:
0.47 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
195 (loge = 5.5) nm. IR (KBr): mmax 3296, 3000, 2926,
1758, 1656, 1225, 1376 cmꢀ1 1H NMR (500 MHz,
.
CDCl3): d 7.10 (s, 1H, H-6), 2.39 (s, 3H, CH3), 7.16
(d, J = 8.6 Hz, 2H, H-20/60), 8.09 (d, J = 8.6 Hz, 2H,
H-30/50), 2.30 (s, 3H, COCH3); EIMS: m/z (%) 245
(M+, 45), 203 (100), 133 (73), 77 (47); Anal. Calcd for
C13H11NO4: C, 63.67; H, 4.52; N, 5.71; O, 26.10. Found:
C, 63.69; H, 4.56; N, 5.74; O, 26.13.
4.2.7. N-(3-{[2-Methyl-5-oxo-1,3-oxazol-4(5H)-ylidene]-
methyl}phenyl)acetamide (9). Yield: 52%; mp: 210 °C;
Rf: 0.53 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
192 (loge = 5.12) nm. IR (KBr): mmax 3822, 3302, 1799,
4.2.12. 4-{[5-Oxo-2-phenyl-1,3-oxazol-4-(5H)-ylidene]-
methyl} phenylacetate (14). Yield: 56%; mp: 236 °C; Rf:
0.48 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
194 (loge = 5.5) nm. IR (KBr): mmax 3423, 3075, 2923,
1
1659, 1552, 1260 cmꢀ1. H NMR (500 MHz, CDCl3):
d 7.1 (s, 1H, H-6), 2.40 (s, 3H, CH3), 7.89 (d,
J = 7.5 Hz, 2H, H-20/60), 8.19 (d, J = 7.5 Hz, 2H, H-30/
50), 8.39 (s, 1H, NH), 2.19 (s, 3H, COCH3); EIMS: m/
z (%) 244 (M+, 11), 202 (2.06), 174 (77), 132 (100), 82
(40); Anal. Calcd for C13H12N2O3: C, 63.93; H, 4.95;
N, 11.47; O, 19.65. Found: C, 63.95; H, 4.98; N,
11.49; O, 19.67.
1795, 1656, 1217, 1165 cmꢀ1 1H NMR (400 MHz,
.
CDCl3): d 7.20 (s, 1H, H-6), 7.59 (d, J = 7.4 Hz, 2H,
H-20/60), 7.52 (m, 3H, H-30-50), 8.16 (d, J = 8.7 Hz,
2H, H-200/600), 8.23 (d, J = 8.7 Hz, 2H, H-300/500), 2.32 (s,
3H, CH3); EIMS: m/z (%) 307, (M+, 16), 265 (41), 104
(100), 76 (65); Anal. Calcd for C18H13NO4: C, 70.35;
H, 4.26; N, 4.56; O, 20.83. Found: C, 70.36; H, 4.28;
N, 4.57; O, 20.84.
4.2.8. N-3-{[5-Oxo-2-phenyl-1,3-oxazol-4(5H)-ylidene]-
methyl}acetamide (10). Yield: 57%; mp: 193 °C; Rf:
0.55 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
196 (loge = 6.2) nm. IR (KBr) mmax 3299, 3087, 2923,
4.2.13. 2-Methyl-4-[(E)-(3-nitrophenyl)methylidene]-1,3-
oxazol-5(4H)-one (15). Yield: 40%; mp: 238 °C; Rf:
0.59 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
270 (loge = 4.9) nm. IR (KBr): mmax 3086, 2930, 1801,
1
1790, 1658, 1158 cmꢀ1. H NMR (500 MHz, CDCl3):
d 7.2 (s, 1H, H-6), 7.70 (d, J = 7.7 Hz, 2H, H-20/60),
7.42 (t, J = 7.7 Hz, 2H, H-30/50), 7.53 (t, J = 7.7 Hz,
2H, H-40), 7.89 (d, J = 7.5 Hz, 2H, H-200/600), 8.19 (d,
J = 7.5 Hz, 2H, H-30/50), 8.31 (s, 1H, NH), 2.22 (s, 3H,
COCH3); EIMS: m/z (%) 306 (M+, 2.97), 105 (100), 77
(52); Anal. Calcd for C18H14N2O3: C, 70.58; H, 4.61;
N, 9.15; O, 5.67. Found: C, 70.60; H, 4.64; N, 9.16; O,
5.69.
1662, 1351, 1296, 1263 cmꢀ1 1H NMR (400 MHz,
.
CDCl3): d 7.12 (s, 1H, H-6), 2.44 (s, 3H, CH3), 8.28 (s,
1H, H-2), 8.31 (d, J = 7.8 Hz, 1H, H-4), 8.13 (d,
J = 8.1 Hz, 1H, H-5), 8.23 (m, 1H, H-6); EIMS: m/z
(%) 232, (M+, 19.6), 204 (5.4), 187 (26), 144 (100), 83
(54); Anal. Calcd for C11H8N2O4: C, 56.90; H, 3.47;

6032
K. M. Khan et al. / Bioorg. Med. Chem. 14 (2006) 6027–6033
N, 12.06; O, 27.56. Found: C, 56.94; H, 3.49; N, 12.09;
O, 27.59.
References and notes
1. Hider, C. R.; Lerch, K. Biol. Chem. J. 1998, 257, 289.
2. Kubo, I.; Kinst-Hori, I.; Yokokawa, Y. J. Nat. Prod.
1994, 4, 545.
3. Sakuma, K.; Ogawa, M.; Sugibayashi, K.; Yamada, K.;
Yamamoto, K. Arch. Pharmcol. Res. 1999, 4, 335.
4. Yang, F.; Boissy, R. E. Pigment Cell Res. 1999, 4, 237.
5. Lee, H. S. J. Agric. Food. Chem. 2002, 50, 1400.
6. Masamoto, Y.; Ando, H.; Murata, Y.; Shimoishi, Y.;
Tada, M.; Takahata, K. Biosci., Biotechnol., Biochem.
2003, 67, 631–634.
7. Hearing, V. J. Methods in Enzymology; Academic Press:
New York, 1987, Vol. 142, p 154.
8. Gottwald, K.; Seebach, D. Tetrahedron 1999, 55, 723–738.
9. Meiwes, J.; Schudock, M.; Kretzschmar, G. Tetrahedron:
Asymmetry 1997, 8, 527–536.
4.2.14.
4-[(E)-(3,4-Dimethoxyphenyl)methylidene]-2-
methyl-1,3-oxazol-5 (4H)-one (16). Yield: 75%; mp:
190 °C;Rf: 0.50 (ethyl acetate/hexane, 3:7); UV
(MEOH): kmax 194 (loge = 4.8) nm. IR (KBr): mmax
3080, 3003, 2933, 1790, 1656, 1270 cmꢀ1 1H NMR
.
(400 MHz, CDCl3); d 7.07 (s, 1H, H-6), 2.38 (s, 3H,
CH3), 3.93 (s, 3H, OCH3), 3.94 (s, 3H, OCH3), 6.89
(d, J = 8.4 Hz, 2H, H-20/50), 7.50 (d, J = 8.4 Hz, 1H,
H-60); EIMS: m/z (%) 248, (M+, 100), 177 (100), 162
(90), 82 (12); Anal. Calcd for C13H13NO4: C, 63.15; H,
5.30; N, 5.67; O, 25.88. Found: C, 63.20; H, 5.35; N,
5.69; O, 25.92.
10. Seebach, D.; Jaeschke, G.; Gottwald, K.; Matsuda, K.;
Formisano, R.; Chaplin, D. A. Tetrahedron 1997, 53,
7539–7556.
4.2.15. 4[(E)-(3,4-Dimethoxyphenyl)methylidene]-2-phen-
yl-1,3-oxazol-5(4H)-one (17). Yield: 80%; mp: 196 °C;Rf:
0.52 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax 197
(loge = 4.9) nm. IR (KBr): mmax 3264, 3062, 2949, 1787,
´
´
11. Cativiela, C.; Dıaz-de-Villegas, M. D.; Garcıa, J. I.;
´
Jimenez, A. I. Tetrahedron 1997, 53, 4479–4486.
1
1655, 1516, 1250 cmꢀ1. H NMR (400 MHz, CDCl3): d
´
12. Bun˜uel, E.; Cativiela, C.; Dıaz-de-Villegas, M. D. Tetra-
hedron 1995, 32, 8923–8934.
7.21 (s, 1H, H-6), 8.12 (dd, J = 8.6 Hz, 2H, H-200/600),
7.44 (t, J = 8.6 Hz, 2H, H-30/50), 7.55 (t, J = 8.6 Hz,
1H, H-40), 6.93 (d, J = 8.4 Hz, 2H, H-200/500), 7.50 (m,
1H, H-600), 3.95 (s, 3H, OCH3), 4.02 (s, 3H, OCH3);
EIMS: m/z (%) 309, (M+, 42), 177 (4.1), 105 (100), 77
(22); Anal. Calcd for C18H15NO4: C, 69.89; H, 4.89;
N, 4.53; O, 20.69. Found: C, 69.95; H, 4.94; N, 4.56;
O, 20.75.
13. Donati, D.; Garzon-Aburbeh, A.; Natalini, B.; Marchi-
oro, C.; Pellicciari, R. Tetrahedron 1996, 52, 9901–
9908.
14. Martinez, A. P.; Lee, W. W.; Goodman, L. Tetrahedron
1964, 20, 2763–2771.
15. Gelmi, M. L.; Clerici, F.; Melis, A. Tetrahedron 1997, 53,
1843–1854.
16. Mesaik, M. A.; Rahat, S.; Khan, K. M.; Choudhary, M.
I.; Shahnaz, M.; Ismaeil, Z.; Atta-ur-Rahman; Ahmad,
A.; Zia-Ullah Bioorg. Med. Chem. 2004, 12, 2049–2057.
17. Croce, P. D.; Ferraccioli, R.; Rosa, C. L. J. Chem. Soc.,
Perkin Trans. 1 1994, 2499–2502.
18. Cannella, R.; Clerici, F.; Gelmi, M. L.; Penso, M.; Pocar,
D. J. Org. Chem 1996, 61, 1854–1856.
19. Bossio, R.; Marcaccini, S.; Pepino, R.; Paoli, P. J.
Heterocycl. Chem. 1994, 31, 729–732.
20. Arenal, I.; Bernabe, M.; Alvarez, E. F.; Izquierdo, M. L.;
4.2.16. 4-[(E)-1-Naphthylmethylidene]-2-phenyl-1,3-oxa-
zol-5(4H)-one (18). Yield: 48%; mp: 278 °C; Rf: 0.60
(ethyl acetate/hexane, 3:7); UV (MEOH): kmax 203
(loge = 4.4) nm. IR (KBr): mmax 3289, 3057, 1723,
1
1648, 1249 cmꢀ1. H NMR (400 MHz, CDCl3): d 7.2
(s, 1H, H-6), 7.97 (d, J = 8.1 Hz, 2H, H-20/60), 7.54 (m,
3H, H-30-50), 8.31 (d, J = 8.3 Hz, 1H, H-200), 8.20 (d,
J = 8.3 Hz, 1H, H-300), 7.63 (m, 4H, H-400/700), 8.14 (s,
1H, H-800); EIMS: m/z (%) 298, (M+, 43), 210 (63), 105
(100), 77 (54); Anal. Calcd for C20H13NO2: C, 80.25;
H, 4.38; N, 4.68; O, 10.69. Found: C, 80.28; H, 4.40;
N, 4.70; O, 10.62.
´
Penades, S. J. Heterocycl. Chem. 1983, 20, 607–613.
21. Kojima, S.; Ohkawa, H.; Hirano, T.; Maki, S.; Niwa, H.;
Ohashi, M.; Inouye, S.; Tsuji, F. I. Tetrahedron Lett. 1998,
39, 5239–5242.
´
´
22. Cativiela, C.; Fraile, J. M.; Garcıa, J. J.; Lopez, M. P.;
Mayoral, J. A.; Pires, E. Tetrahedron: Asymmetry 1996, 7,
2391–2394.
4.2.17. 4-[(E)-(3-Nitrophenyl)methylidene]-2-phenyl-1,3-
oxazol-5 (4H)-one (19). Yield: 54%; mp: 133 °C; Rf:
0.62 (ethyl acetate/hexane, 3:7); UV (MEOH): kmax
196 (loge = 4.9) nm. IR (KBr): mmax 3092, 2863, 1792,
23. Khan, K. M.; Maharvi, G. M.; Khan, M. T. H.; Perveen,
S.; Choudhary, M. I.; Atta-ur-Rahman Mol. Divers. 2005,
9, 15–26.
24. (a) Khan, M. T. H.; Choudhary, M. I.; Khan, K. M.;
Rani, M.; Atta-ur-Rahman Bioorg. Med. Chem. 2005, 13,
3385–3395; (b) Khan, K. M.; Maharvi, G. M.; Perveen, S.;
Khan, M. T. H.; Abdel-Jalil, R. J.; Shah, S. T. A.; Fecker,
M.; Choudhary, M. I.; Atta-ur-Rahman; Voelter, W.
Chem. Biodivers. 2005, 2, 470–476.
25. Khan, K. M.; Saify, Z. S.; Khan, M. T. H.; Butt, N.;
Maharvi, G. M.; Perveen, S.; Ambreen, N.; Choudhary,
M. I.; Atta-ur-Rahman; Supran, C. T. J. Enzyme Inhib.
Med. Chem. 2005, 20, 401–407.
26. Mesaik, M. A.; Khan, K. M.; Rahat, S.; Zia-Ullah;
Choudhary, M. I.; Murad, S.; Abdullah, N. R.; Ismail, Z.;
Atta-ur-Rahman; Ahmad, A.; Siddiqui, R. A. Lett. Drug
Design Discovery 2005, 2, 490–496.
1
1503, 1349, 1167 cmꢀ1. H NMR (500 MHz, CDCl3):
d 7.24 (s, 1H, H-6), 8.21 (d, J = 7.8 Hz, 2H, H-20/60),
7.56 (t, J = 7.8 Hz, 2H, H-30/60), 7.66 (t, J = 7.8 Hz,
1H, H-40), 8.28 (d, J = 1.2 Hz, 1H, H-200), 8.40 (d,
J = 7.8 Hz, 1H, H-400), 8.21 (d, J = 7.8 Hz, 1H, H-500),
8.27 (dd, J = 7.8 Hz, 1H, H-600); EIMS: m/z (%) 294,
(M+, 58), 105 (100), 77 (37), 51(6.2). Anal. Calcd for
C16H10N2O4: C, 65.31; H, 3.43; N, 9.52 O, 21.75.
Found: C, 65.35; H, 3.45; N, 9.60; O, 21.78.
Acknowledgment
27. (a) Khan, K. M.; Maharvi, G. M.; Khan, M. T. H.;
Shaikh, A. J.; Perveen, S.; Begum, S.; Choudhary, M. I.
Bioorg. Med. Chem. 2006, 14, 344–351; (b) Mesaik, M. A.;
Khan, K. M.; Zia-Ullah; Rahat, S.; Choudhary, M. I.;
This work was financially supported by Pakistan Science
Foundation (PSF) under ‘Research Grant To Active
Scientist/Technologist of Pakistan.’

K. M. Khan et al. / Bioorg. Med. Chem. 14 (2006) 6027–6033
6033
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Chem. 2004, 12, 2049–2057.
28. McCapra, F.; Razavi, Z.; Neary, A. P. J. Chem. Soc.,
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30. (a) Cleaver, C. S.; Pratt, B. C. J. Am. Chem. Soc. 1955, 77,
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22, 35–42; (c) Afifi, A. A.; El-Hashash, M. A.; El-Kady, S.
S. Rev. Roum. Chim. 1983, 28, 849–855.
29. Iwakura, Y.; Toda, F.; Torii, Y. Tetrahedron 1967, 23,
3363–3373.
31. (a) A ratio of 1:10 of the oxazolones to PPA was used to get
E-isomers; (b) Rao, Y. S. J. Org. Chem. 1976, 41, 722–725.