Synthesis and antifungal activity of benzo[d]oxazole-4,7-diones

Article abstract of DOI:10.1016/j.bmcl.2009.08.062

Benzo[d]oxazole-4,7-diones were synthesized and tested for in vitro antifungal activity against fungi. Among them tested, many compounds showed good antifungal activity. The results suggest that benzo[d]oxazole-4,7-diones would be potent antifungal agents.

Full text of DOI:10.1016/j.bmcl.2009.08.062

Bioorganic & Medicinal Chemistry Letters 19 (2009) 5924–5926
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and antifungal activity of benzo[d]oxazole-4,7-diones
*
Chung-Kyu Ryu , Ra-Young Lee, Na Young Kim, Yang Hui Kim, Ae Li Song
College of Pharmacy, Ewha Womans University, Seodaemun-ku, Seoul 120-750, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzo[d]oxazole-4,7-diones were synthesized and tested for in vitro antifungal activity against fungi.
Among them tested, many compounds showed good antifungal activity. The results suggest that
benzo[d]oxazole-4,7-diones would be potent antifungal agents.
Received 26 June 2009
Accepted 14 August 2009
Available online 21 August 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Benzo[d]oxazole-4,7-diones
Antimicrobial compounds
Antifungal
Fungi
Substitution effects
Heterocyclic quinone scaffolds represent often an important
class of biologically active molecules.1 The quinones such as 5-n-
undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT, 1) blockade
a mitochondrial electron transport in Saccaromyces cerevisiae.2
UHDBT (1) has been reported as an inhibitor of mitochondrial cyto-
chrome complex in yeast3 and bacteria.4
considerably important factor to improve their antifungal activ-
ity.5,6,8 Therefore, 5-arylamino-6-bromo-2-ethylbenzo[d]oxazole-
4,7-diones 3a–m and 5,6-bisarylthio-2-ethylbenzo[d]oxazole-4,7-
diones 4a–e with various substituents were designed and synthe-
sized to elucidate their contribution to the antifungal activity
(Scheme 1). The in vitro antifungal activity of compounds 3a–m
and 4a–e against pathogenic fungi was determined by the twofold
broth dilution method. Additional data for properties and anti-
fungal activity of compound 6 is provided.
In our previous reports,5 4,7-dioxobenzothiazoles 2 which have
30-sulfur (S) as analogues of UHDBT, demonstrated potent
antifungal activity against pathogenic fungi (Fig. 1). This fact
prompted us to consider a bioisosteric substitution of the 30-sulfur
by oxygen (O). The quinone analogues containing oxygen such as
benzo[d]oxazole-4,7-diones 3 could have similar activity as com-
pounds 2 since oxygen is isoelectronic with sulfur.
A convenient method for the synthesis of benzo[d]oxazole-4,7-
diones 3a–m is shown in Scheme 1 and Table 1.
Structure–activity relationship studies from quinonoid com-
pounds indicated that the number and position of nitrogen (N),
sulfur (S) or oxygen atoms (O) substituted in the heterocyclic ring
were considerably important factors to affect the biological activi-
ties.5,6 We speculated that bioisosteric compounds 3 would be a
new pharmacophore with a different biological profile from com-
pounds 2. Based on this information, we further extended to syn-
O
O
OH
R1
R2
S
N
S
N
R3
(CH2)10CH3
O
O
1
2
3
UHDBT (1)
2 : R , R , R = H, X,
thesize benzo[d]oxazole-4,7-diones
bioisosteres of 4,7-dioxobenzothiazoles 2, and evaluated their anti-
fungal activity (Fig. 1).
There have been only a report on benzo[d]oxazole-4,7-diones as
inhibitors of the dual specificity protein phosphatase CDC25C.7
However, the antifungal activity of compounds 3 against fungi
has not been reported to the best of our knowledge. The presence
of arylamino, arylthio or halo moieties of quinones was
3
which would be
amino, thio, Ph, ..
O
O
R1
R2
O
R3
N
1
2
3
3 : R , R = X, amino, R = Et
4 : R = R = thio, R = Et
1
2
3
* Corresponding author. Tel.: +82 2 3277 3027; fax: +82 2 3277 3051.
E-mail address: ckryu@ewha.ac.kr (C.-K. Ryu).
Figure 1. Antifungal heterocyclic quinone compounds.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.08.062

C.-K. Ryu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5924–5926
5925
O
O
O
N
O
N
Br
Br
O
O
N
a
c
b
N
NH
2
NO
2
O
O
8
5
6
7
e
O
O
O
O
1
H
N
S
R
1
2
O
N
R
d
O
N
2
R
S
R
Br
1
R
2
1
2
R
3a-m: R ,R = H, F, OH,..
1
2
4a-e: R , R = H, Br, CH ,..
3
Scheme 1. Synthesis of benzo[d]oxazole-4,7-dione derivatives. Reagents and conditions: (a) H2, PtO2/EtOH/3 psi/rt/3 h/78%; (b) Fremy’s salt (2 equiv) in 0.3 M KH2PO4/
acetone/rt/4 h/ 62%; (c) Br2, AcONa/AcOH/rt/8 h/89%; (d) arylamine (1 equiv)/EtOH/5 h/reflux/45–93%; (f) arylthiol (2 equiv)/EtOH/rt/5 h/56–85%.
Table 1
Structures and in vitro antifungal activity for benzo[d]oxazole-4,7-diones
O
O
O
O
O
H
N
R1
R2
S
S
R1
R2
O
N
O
N
S
O
N
R1
Br
S
R2
O
3a-m
R1
4a-d
4-e
R2
Compound
R1
R2
MICa
(l
g/mL)
b
C. albicans
C. tropicalis
C. krusei
C. neoformans
A. niger
A. flavus
3a
3b
3c
3d
3e
3f
3g
3h
H
F
H
Cl
H
H
H
OH
H
H
H
F
H
Cl
Br
I
H
OH
H
1.6
12.5
12.5
6.3
6.3
0.8
6.3
50
50
3.2
3.2
6.3
6.3
25
3.2
6.3
6.3
12.5
6.3
50
3.2
6.3
6.3
6.3
6.3
3.2
3.2
25
1.6
6.3
6.3
3.2
6.3
1.6
3.2
25
1.6
6.3
6.3
3.2
3.2
0.8
6.3
50
3.2
6.3
3.2
3.2
3.2
1.6
3.2
25
3i
3j
3k
25
50
50
12.5
12.5
25
50
12.5
25
25
6.3
25
CN
H
12.5
50
CN
3l
3m
4a
4b
4c
4d
4e
6
H
H
H
Br
CH3
CH2CH3
OCH3
OCH3
NO2
H
6.35
50
12.5
50
25
12.5
50
12.5
50
12.5
50
12.5
25
50
1.6
50
12.5
12.5
3.2
50
50
100
3.2
3.2
25
12.5
12.5
12.5
25
50
100
3.2
3.2
25
12.5
25
12.5
25
12.5
>100
1.6
6.3
25
12.5
25
6.3
25
12.5
>100
1.6
Br
CH3
CH2CH3
OCH3
—
>100
3.2
50
3.2
5-Fluorocytosine
—
a
The MIC value was defined as the lowest concentration of the antifungal agent. MIC values were read after 1 day for Candida species and C. neoformans, and 2 days for A.
niger in 37 °C. The inoculum sizes contained approximately 1 ꢀ 105 cells/mL. Culture media tested were the modified Sabouraud dextrose broth (Difco Lab.). The final
concentration of antifungal agents was between 0.2 and 100 lg/mL.
b
Fungi tested: Candida albicans Berkout KCCM 50235, Candida tropicalis Berkout KCCM 50662, Candida krusei Berkout KCCM 11655, Cryptococcus neoformans KCCM 50564,
Aspergillus niger KCTC 1231 and Aspergillus flavus KCCM 11899.
2-Ethyl-4-nitrobenzo[d]oxazole (5)7 was synthesized by cycli-
zation of commercially available 2-amino-3-nitrophenol with para
toluene sulfonic acid (PTSA) and triethylorthopropionate in 87%
yield according to the reported method9 with minor modification.
The nitro group of compound 5 was reduced to 2-ethyl-
benzo[d]oxazol-4-amine (6) by catalytic hydrogenation with H2
and PtO2 in EtOH in 78% yield. The preparation of 2-ethyl-
benzo[d]oxazole-4,7-dione (7) by oxidation of compound 6 was

5926
C.-K. Ryu et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5924–5926
carried out with Fremy’s salt [potassium nitrosodisulfonate,
(KO3S)2NO] in 62% yield.
5,6-Dibromo-2-ethylbenzo[d]oxazole-4,7-dione (8) was syn-
thesized by bromination of compound 7 with the Br2, AcONa/AcOH
variation in 89% yield.
5-Arylamino-6-bromo-2-ethylbenzo[d]oxazole-4,7-diones 3a–
m were synthesized by nucleophilic substitution of compound 8
with appropriate arylamines. When compound 8 with equivalent
amount of appropriate arylamines in EtOH were refluxed for 5 h,
compounds 3a–m were formed. Most of these substitutions went
as expected and had overall high yields of 45–93%.
In similar manner, 5,6-bisarylthio-2-ethyl benzo[d]oxazole-4,7-
diones 4a–d and 5,6-bisethylthio-2-ethylbenzo[d]oxazole-4,7-
dione (4e) were synthesized by nucleophilic substitution on com-
pound 8 with 2 equiv of appropriate aryl or alkylthiols in EtOH.
Most of the substitutions went as expected and had an overall yield
of 56–85%.
diones 3 and 4 showed, in general, more potent antifungal activity
than compound 6. Thus, the quinone moiety in compounds 3 and 4
could be essential for the activity, for example, as non-quinonoid
compound 6 losts the activity. The structure–activity relationship
may not exist between properties of subsistent (R1 and R2 and
R3) for the 5- or 6-aryl moieties of compounds 3 and 4.
In conclusion, compound 3a–m was synthesized by nucleo-
philic substitution of compound 8 with appropriate arylamines.
Compounds 4a–e were synthesized by nucleophilic substitution
on compound 8 with 2 equiv of appropriate thiols in EtOH. Most
of the substitutions went as expected and had an overall yield.
Among them tested, many of compounds 3a–m and 4a–e
showed potent antifungal activity against pathogenic fungi. These
benzo[d]oxazole-4,7-diones may thus be promising leads for the
development of antifungal agents. Moreover, the results should
encourage the synthesis of these analogs for improving antifungal
properties.
The synthesized compounds 3a–m and 4a–e were tested
in vitro for their growth inhibitory activity against pathogenic fun-
gi by the standard method.10 As indicated in Table 1, the MIC (min-
imum inhibitory concentration) values were determined by
comparison with 5-fluorocytosine as standard agent.
Among tested 5-arylamino-benzo[d]oxazole-4,7-diones 3a–m,
many compounds generally showed potent antifungal activity
against the tested pathogenic fungi. Actually, the activity of com-
pounds 3a and 3f was superior or comparable to those of 5-fluoro-
cytosine against all tested pathogenic fungi. The compounds 3a
and 3l completely inhibited the growth of all fungal species tested
Acknowledgment
This study was supported by a Grant of the Korea Healthcare
Technology R&D Project, Ministry for Health, Welfare and Family
Affairs, Republic of Korea (A08-0414-AA1723-08N1-00010A).
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In addition, 2-ethylbenzo[d]oxazol-4-amine (6) exhibited no or
poor, if any, antifungal activity. Quinonoid benzo[d]oxazole-4,7-