Synthesis and antifungal activity of 5-arylamino-4,7-dioxobenzo[b] thiophenes

Article abstract of DOI:10.1016/j.bmcl.2005.03.042

5-Arylamino-4,7-dioxobenzo[b]thiophenes 3-6 were synthesized and tested for in vitro antifungal activity against Candida and Aspergillus species. 5-Arylamino-6-chloro-2-(methoxycarbonyl)-4,7-dioxobenzo[b]thiophenes 5 showed, in general, more potent antifungal activity against Candida species than the other 4,7-dioxobenzo[b]thiophenes 3, 4 and 6. The results suggest that 5-arylamino-4,7-dioxobenzo[b]thiophenes would be potent antifungal agents.

Full text of DOI:10.1016/j.bmcl.2005.03.042

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2617–2620
Synthesis and antifungal activity of
5-arylamino-4,7-dioxobenzo[b]thiophenes
Chung-Kyu Ryu,* Su-Kyung Lee, Ja-Young Han, Ok-Jai Jung,
Jung Yoon Lee and Seong Hee Jeong
College of Pharmacy, Ewha Womans University, Seodaemun-ku, Seoul 120-750, South Korea
Received 18 January 2005; revised 9 March 2005; accepted 10 March 2005
Available online 9 April 2005
Abstract—5-Arylamino-4,7-dioxobenzo[b]thiophenes 3–6 were synthesized and tested for in vitro antifungal activity against Candida
and Aspergillus species. 5-Arylamino-6-chloro-2-(methoxycarbonyl)-4,7-dioxobenzo[b]thiophenes 5 showed, in general, more potent
antifungal activity against Candida species than the other 4,7-dioxobenzo[b]thiophenes 3, 4 and 6. The results suggest that 5-aryl-
amino-4,7-dioxobenzo[b]thiophenes would be potent antifungal agents.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
O
O
O
O
H
N
(CH2)10CH3
OH
R1
R2
N
S
N
S
H3C
Heterocyclic quinone compounds are an attractive 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 Sac-
charomyces cerevisiae.2 UHDBT (1) has been reported
as an inhibitor of mitochondrial cytochrome complex
in yeast3 and bacteria.4 In our previous report,5 5-aryl-
amino-4,7-dioxobenzothiazoles 2, which could be ana-
logues of UHDBT, have demonstrated potent anti-
fungal activity against pathogenic fungi (Fig. 1). A
variety of heterocyclic quinones with different substitu-
ents could exhibit biological activities through different
action and sometimes improve the activities. The pres-
ence of arylamino, arylthio, alkyl group or halogen
atoms substituted in quinones was a considerably
important factor to affect their antifungal activity.5
Based on this speculation, we further extended to syn-
thesize 5-arylamino-4,7-dioxobenzo[b]thiophenes 3–6,
which would be bioisosteres of quinones 2 and evaluated
their antifungal activity.
R3
UHDBT (1)
2 : R1, R2 , R3= H, F, Cl, ...
O
O
H
N
R2
S
X
R1
R1 = H, F, Cl, ..
3: R2 = CO2Me; X=H
4: R2 = CH2OH; X=H
5: R2 = CO2Me; X=Cl
6: R2 = CH2OH; X=Cl
Figure 1. Antifungal 4,7-dioxobenzo[b]thiophene derivatives.
activity against Trypanosome cruzi and strains of Leish-
mania. However, the inhibitory activity of 4,7-dioxo-
benzo[b]thiophenes on the antifungal properties has
not been reported to the best of our knowledge. There-
fore, the 4,7-dioxobenzo[b]thiophenes 3–6 with various
substituents were designed and synthesized to elucidate
their contribution to the antifungal activity. The in vitro
antifungal activity of the 4,7-dioxobenzo[b]thiophenes
3–6 against pathogenic fungi was determined by the
2-fold broth dilution method. Additional data for
There have been a few reports6,7 on 4,7-dioxo-
benzo[b]thiophene derivatives, exhibiting antiprotozoal
Keywords: 4,7-Dioxobenzo[b]thiophene; Antimicrobial compounds;
Antifungal; Fungi; Substitution effects.
*
Corresponding author. Tel.: +82 2 3277 3027; fax: +82 2 3277
3051; e-mail: ckryu@mm.ewha.ac.kr
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.03.042

2618
C.-K. Ryu et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2617–2620
The MIC (minimum inhibitory concentration) values
O
O
O
H
N
were determined by comparison with 5-fluorocytosine
as a standard agent. As indicated in Table 1, the more
active potential among the 4,7-dioxobenzo[b]thiophene
series 3–6 was found for 5-arylamino-6-chloro-2-(meth-
oxycarbonyl)-4,7-dioxobenzo[b]thiophenes 5, which
showed generally good activity against all tested Can-
dida species and A. niger. The 5-arylamino-4,7-dioxo-
d
R
R2
S
S
H
R1
O
9 : R = CO2Me
10 : R = CH2OH
R1 = H, F, Cl, ..
3a-i : R2 = CO2Me
4a-i : R2 = CH2OH
b
benzo[b]thiophenes 3,
4
and
6
did not show
OMe
significant antifungal activity against C. albicans and
A. niger, although many compounds of them exhibited
good activity against C. tropicalis and C. krusei. Most
of the 5-arylamino-2-hydroxymethyl-4,7-dioxobenzo-
[b]thiophenes 4 showed potent antifungal activity
against C. tropicalis, C. krusei and A. niger. Most of
compounds 3–6 were superior or comparable to those
of 5-fluorocytosine against C. tropicalis. Actually, the
activity of 4,7-dioxobenzo-[b]thiophenes 5a and 6a
was superior to those of 5-fluorocytosine against all
tested fungi. Compounds 5a and 6a completely inhibi-
ted the growth of all fungal species tested at the MIC
level of 12.5 lg/mL.
7 : R=CO2Me
8 : R=CH2OH
R
a
S
OMe
c
O
H
O
N
Cl
Cl
d
R2
R
S
S
Cl
R1
O
O
11 : R = CO2Me
12 : R = CH2OH
R1 = H, F, Cl, ..
5a-i : R2 = CO2Me
6a-i : R2 = CH2OH
In terms of structure activity relationship, the 5-aryl-
amino-6-chloro-2-(methoxycarbonyl)-4,7-dioxobenzo[b]-
thiophenes 5 showed, in general, a more potent anti-
fungal activity than the other 5-arylamino-4,7-dioxo-
benzo[b]thiophenes 3, 4 and 6. The 5-arylamino-6-
Scheme 1. Synthesis of 4,7-dioxobenzo[b]thiophenes 3–6. Reagents
and conditions: (a) LiAlH4/ether/rt; (b) CAN/AcCN/H2O/rt; (c)
HNO3/HCl/80 ꢁC; (d) arylamine/CeCl3/EtOH/reflux/10 h.
chloro-2-(methoxycarbonyl)-compounds
5
exhibited
properties and antifungal activity of 4,7-dioxobenzo[b]-
thiophenes are provided (Scheme 1).
the greatest activity, indicating a correlation that may
offer an insight into the mode of action of these com-
pounds. The 6-chloro- and 2-methoxycarbonyl moieties
of compounds 5 appears to contribute partially towards
biological potency. In contrast, 2-hydroxymethyl moi-
ety of compounds 4 and 6 did not improve their anti-
2. Chemistry
A method for the synthesis of 5-arylamino-4,7-dioxo-
benzo[b]thiophenes 3–6 (Table 1) is shown in Scheme 1.
4,7-Dimethoxy-2-(methoxycarbonyl)-benzo[b]thiophene
(7) was prepared according to a reported method8 with
minor modification. Cyclization of 2,5-dimethoxy-6-
nitrobenzaldehyde9 with methyl thioglycolate gave
compound 7. 4,7-Dimethoxy-2-(hydroxymethyl)benzo-
[b]thiophene (8) was synthesized by reduction of com-
pound 7 with LiAlH4 in 57% yield. The preparation of
4,7-dioxobenzo[b]thiophenes 9 and 10 by oxidative
demethylation of 4,7-dimethoxybenzo[b]thiophenes 7
and 8 was carried out with ammonium cerium(IV) ni-
trate (CAN) in AcCN/water solution.8 5,6-Dichloro-4,
7-dioxobenzo[b]thiophenes 11 and 12 were synthesized
by oxidizing compounds 7 and 8 with HNO3/HCl vari-
ation. 5-Arylamino-4,7-dioxobenzo[b]thiophenes 3–6
were synthesized by nucleophilic substitution of com-
pounds 9–12 with the appropriate arylamines. Most of
these substitutions went as expected and had overall
high yields of 76–94%. Experimental details and data
for this procedure are cited in Refs. 10–14.
fungal activity in comparison to compounds
significantly.
5
In addition, the 4,7-dioxobenzo[b]thiophenes 9 and 10
without a 5-arylamino group exhibited no or poor, if
any, antifungal activity. Thus, 5-arylamino moiety of
4,7-dioxobenzo[b]thiophenes 3–6 partially improves the
antifungal activity. The structure activity relationship
may not exist between properties of substituents (R1:
F,Cl,Br,. . .) for the 5-arylamino moieties of the 4,7-
dioxobenzo[b]thiophenes 3–6.
4. Conclusion
The 5-arylamino-4,7-dioxobenzo[b]thiophenes 3–6 were
synthesized by nucleophilic substitution of the 4,7-dioxo-
benzo[b]thiophenes 9–12 with the appropriate aryl-
amines. 5-Arylamino-6-chloro-2-(methoxycarbonyl)-4,7-
dioxobenzo[b]thiophenes
5 showed generally more
potent antifungal activity than 4,7-dioxobenzo[b]thioph-
enes 3, 4 and 6. The 6-chloro moiety of compounds 5
improved their antifungal activity significantly. The
results suggest that the 4,7-dioxobenzo[b]thiophenes
would be potent antifungal agents. Moreover, the
results should encourage the synthesis of 4,7-dioxo-
benzo[b]thiophenes analogues for improving antifungal
properties.
3. Antifungal activity
The synthesized 4,7-dioxobenzo[b]thiophenes 3–6 were
tested in vitro for their growth inhibitory activity
against pathogenic fungi by the standard method.15

C.-K. Ryu et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2617–2620
Table 1. Structures and in vitro antifungal activity for 4,7-dioxobenzo[b]thiophenes 3–6
2619
O
H
N
R2
S
X
R1
O
Compds
R1
R2
X
MICa (lg/mL)
C. tropicalis
6.3
25
C. albicansb
C. krusei
A. niger
3a
3b
H
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
H
H
50
>50
50
12.5
25
50
>50
50
F
Cl
3c
3d
H
6.3
0.2
0.2
3.2
6.3
0.4
12.5
0.8
12.5
12.5
12.5
1.6
25
25
Br
I
H
50
50
12.5
6.3
6.3
25
50
50
3e
H
3f
3g
CH3
OCH3
OH
CF3
H
H
50
50
H
>50
50
>50
50
3h
H
12.5
1.6
6.3
>25
>25
3.2
25
3i
4a
H
50
50
50
H
0.8
0.8
25
4b
4c
F
Cl
H
50
H
>50
50
4d
4e
Br
I
H
3.2
3.2
3.2
3.2
0.8
0.8
12.5
12.5
25
H
50
4f
4g
CH3
OCH3
OH
CF3
H
H
>50
3.2
0.8
0.8
0.8
12.5
12.5
0.8
6.3
0.8
3.2
12.5
12.5
6.3
12.5
12.5
50
25
25
H
0.8
0.8
0.8
3.2
6.3
6.3
3.2
1.6
12.5
6.3
0.8
1.6
3.2
0.4
0.4
0.8
1.6
1.6
1.6
25
4h
H
12.5
12.5
6.3
25
4i
5a
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
5b
5c
F
Cl
25
25
5d
5e
Br
I
25
25
25
3.2
50
5f
5g
CH3
OCH3
OH
CF3
H
3.2
6.3
25
50
50
5h
5i
6a
50
1.6
1.6
1.6
0.8
1.6
0.8
3.2
12.5
3.2
>50
50
12.5
50
6b
6c
F
Cl
>50
6d
6e
Br
I
50
50
50
50
6f
6g
CH3
OCH3
OH
CF3
50
50
25
6h
6i
>50
50
>50
50
0.4
25
9
10
>50
>50
6.3
>50
50
25
12.5
5-Fluorocytosine
6.3
12.5
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 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 lg/mL and 100 lg/mL.
b Fungi tested: Candida albicans Berkout KCCM 50235, Candida tropicalis Berkout KCCM 50662, Candida krusei Berkout KCCM 11655 and
Aspergillus niger KCTC 1231.
2. Roberts, H.; Choo, W. M.; Smith, S. C.; Mrzuki, S.;
Linnane, A. W.; Porter, T. H.; Folkers, K. Arch. Biochem.
Biophys. 1978, 191, 306.
3. Di Rigo, J.-P.; Bruel, C.; Graham, L. A.; Sonimski, P.;
Trumpower, B. L. J. Biol. Chem. 1996, 271, 15341.
4. Musser, S. M.; Stowell, M. H. B.; Lee, H. K.; Rumbley, J.
N.; Chan, S. I. Biochemistry 1997, 36, 894.
Acknowledgments
This work was supported by the Korea Research Foun-
dation Grant (KRF-2002-041- E00292).
References and notes
5. Ryu, C.-K.; Kang, H.-Y.; Yi, Y.-J.; Shin, K.-H.; Lee,
B.-H. Bioorg. Med. Chem. Lett. 2000, 10, 1589.
6. Valderrama, J. A.; Fournet, A.; Valderrama, C.; Bastias,
S.; Astudillo, C.; de Arias, A. R.; Inchausti, A.; Yaluff, G.
Chem. Pharm. Bull. 1999, 47, 1221.
1. Middleton, R. W.; Parrick, J. In The Chemistry of the
Quinonoid Compounds; Patak, S., Rappoport, Z., Eds.;
John Wiley & Sons: London, 1988; p 1019.

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C.-K. Ryu et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2617–2620
1
7. Valderrama, J. A.; Astudillo, C.; Tapia, R. A.; Prina, E.;
Estrabaud, E.; Mahieux, R.; Fournet, A. Chem. Pharm.
Bull. 2002, 50, 1215.
8. Valderrama, J. A.; Valderrama, C. Synth. Commun. 1997,
27, 2143.
yellow powder (79%); mp 140–141 ꢁC; H NMR (CDCl3)
d 3.87 (s, 3H, OCH3), 7.09 (d, 1H, J = 9.2), 7.28 (d, 1H,
J = 9.2), 7.29 (s, 1H); HRMS calcd for C10H6O4S:
221.9987. Found: 221.9988. 2-(Hydroxymethyl)benzo[b]-
thiophene-4,7-dione (10): yellow powder (61%); mp 106–
1
9. Shaikh, I. A.; Johnson, F.; Grollman, A. P. J. Med. Chem.
1986, 29, 1339.
108 ꢁC; H NMR (CDCl3) d 2.14 (t, 1H, OH, J = 5), 4.95
(d, 2H, CH2, J = 5), 6.77 (d, 1H, J = 10), 6.85 (d, 1H,
J = 10), 7.29 (d, 1H, J = 5); HRMS calcd for C9H6O3S:
194.0038. Found: 194.00377.
10. Experimental: all melting points were measured with Buchi
¨
melting point B-545 and were uncorrected. 1H NMR
spectra were recorded on Varian Unity INOVA 400 MHz
FT-NMR spectrometer using CDCl3 with TMS. High-
resolution mass spectra (HRMS EI) were taken with Jeol
JMS AX505 WA. 2,5-Dimethoxybenzaldehyde and other
reagents were purchased from Aldrich Chemical Co.
11. 4,7-Dimethoxy-2-(hydroxymethyl)benzo[b]thiophene (8):
to a solution of LiAlH4 (1.0 g, 26.4 mmol) in dry ether (10
mL) was added to a solution of 4,7-dimethoxy-2-(meth-
oxycarbonyl)benzo[b]thiophene8 (7, 0.20 g, 0.80 mmol) in
dry ether (30 mL). The mixture was stirred for 3 h at rt.
The resulting mixture was acidified with AcOH and then
extracted with EtOAc. The extract was washed with
aqueous Na2CO3, water, dried and evaporated. The
residue was chromatographed on silica gel and eluted
with EtOAc to afford pure alcohol 8 as a white needle
13. General procedure for synthesis of 5,6-dichlorobenzo[b]thio-
phene-4,7-diones (11 and 12): 5 mL of concd HNO3 was
added over a period of 1 h to a stirred suspension of
compounds 7 or 8 (10 mmol) in 15 mL of concd HCl at 80–
90 ꢁC. The mixture was stirred at rt for 2 h and was
extracted twice with ether. The extract was evaporated and
crystallization form EtOH afforded compounds 11 and 12,
respectively. 5,6-Dichloro-2-(methoxycarbonyl)benzo[b]thio-
phene-4,7-dione (11): yellow needle (79%); mp 186–188 ꢁC;
1H NMR (CDCl3) d 3.98 (s, 3H, OCH3), 8.19 (s, 1H);
HRMS calcd for C10H4Cl2O4S: 289.9207. Found: 289.9208.
5,6-Dichloro-2-(hydroxymethyl)benzo[b]thiophene-4,7-dione
(12): yellow powder (48%); mp 140–141 C; 1H NMR
(CDCl3) d 2.14 (t, 1H, OH, J = 5), 4.95 (d, 2H, CH2,
J = 5), 7.39 (s, 1H); HRMS calcd for C9H4Cl2O3S:
261.9258. Found: 261.9259.
1
(0.103 g, 57%): mp 132–136 ꢁC; H NMR (CDCl3) d 1.90
(br s, 1H, OH), 3.96 (s, 3H, OCH3), 4.00 (s, 3H, OCH3),
4.97 (s, 2H, CH2), 6.72 (s, 2H), 7.43 (s, 1H); HRMS calcd
for C11H12O3S: 224.0507. Found: 224.0508.
14. General procedure for synthesis of 5-arylamino-4,7-dioxo-
benzo[b]thiophenes (3–6): a solution of compounds 9–11
or 12 (1 mmol) and CeCl3Æ7H2O (0.01 mmol) in 20 mL of
12. General procedure for synthesis of 4,7-dioxobenzo[b]thio-
phenes (9 and 10): a solution of CAN (540 mg, 1.0 mmol)
in AcCN/water (4:1, 5 mL) was added dropwise to
compounds 7 or 8 (0.48 mmol) dissolved in AcCN
(10 mL). The mixture was kept at rt for 10 min, diluted
with water (70 mL) and extracted with EtOAc
(3 · 15 mL). The extract was washed with brine, dried
and evaporated. The residue was chromatographed on
silica gel (1:1 EtOAc/petroleum ether) to afford pure
quinones 9 and 10, respectively.
EtOH was added to a solution of the arylamine
(1.1 mmol) in 10 mL EtOH and stirred at rt for 2 h and
then refluxed for 4–10 h. After the reaction mixture was
kept overnight, the precipitate was collected by the
filtration. The crude product was purified by silica gel
column chromatography with CHCl3/EtOAc or crystal-
lized from EtOH afforded 5-arylamino-4,7-dioxo-
benzo[b]thiophenes 3–5 or 6, respectively.
15. McGinnis, M. R.; Rindali, M. G. In Antibiotics in
Laboratory Medicine; 4th ed.; Lorian, V., Ed., Williams
and Wilkins: Baltimore, 1996; pp 176–211.
2-(Methoxycarbonyl)-benzo[b]thiophene-4,7-dione
(9):