Synthesis of benzo[b]benzo[2,3-d]thiophen-6,9-diones via palladium(II) acetate-mediated cyclization of 5-arylthiocyclohexa-2,5-diene-1,4-diones

Article abstract of DOI:10.1080/00397910600941208

Palladium(II)-mediated oxidative cyclization of 5-arylthiocyclohexa-2,5- diene-1,4-diones 4 giving biologically important benzo[b]benzo[2,3-d]thiophene- 6,9-diones 2 has been performed with a stoichiometric amount of palladium(II) acetate in distillated acetic acid. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600941208

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Synthesis of
Benzo[b]benzo[2,3‐d]thiophen‐6,9‐diones via
Palladium(II) Acetate–Mediated Cyclization of
5‐Arylthiocyclohexa‐2,5‐diene‐1,4‐diones
Chung‐Kyu Ryu a , Ik Hwa Choi, a & Rae Eun Park a
a College of Pharmacy, Ewha Womans University , Seoul, Korea
Published online: 23 Nov 2006.
To cite this article: Chung‐Kyu Ryu , Ik Hwa Choi, & Rae Eun Park (2006) Synthesis of
Benzo[b]benzo[2,3‐d]thiophen‐6,9‐diones via Palladium(II) Acetate–Mediated Cyclization of
5‐Arylthiocyclohexa‐2,5‐diene‐1,4‐diones, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:22, 3319-3328, DOI: 10.1080/00397910600941208
To link to this article: http://dx.doi.org/10.1080/00397910600941208
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Synthetic Communicationsw, 36: 3319–3328, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600941208
Synthesis of Benzo[b]benzo[2,3-d]thiophen-
6,9-diones via Palladium(II)
Acetate–Mediated Cyclization of
5-Arylthiocyclohexa-2,5-diene-1,4-diones
Chung-Kyu Ryu, Ik Hwa Choi, and Rae Eun Park
College of Pharmacy, Ewha Womans University, Seoul, Korea
Abstract: Palladium(II)-mediated oxidative cyclization of 5-arylthiocyclohexa-2,5-
diene-1,4-diones 4 giving biologically important benzo[b]benzo[2,3-d]thiophene-6,9-
diones 2 has been performed with a stoichiometric amount of palladium(II) acetate
in distillated acetic acid.
Keywords: 5-Arylthiocyclohexa-2,5-diene-1,4-dione, arylthiol, benzo[b]benzo[2,3-
d]thiophene-6,9-dione, palladium acetate
Quinonoid compounds represent an important class of biologically
active molecules that are widespread in nature.[1,2] The heterocyclic 9H-
carbazole-1,4-dione derivatives 1 have received attention because of their
potential biological activities[3–5] (Fig. 1). This fact prompted us to
consider a bioisosteric substitution of the 50-nitrogen (NH) by sulfur (S).
The quinone analogues containing a sulfur such as benzo[b]benzo[2,3-
d]thiophene-6,9-diones 2 could have similar activity as compounds 1
because sulfur is isoelectronic with nitrogen. There have not been any
reports on benzo[b]benzo[2,3-d]thiophene-6,9-dione derivatives to the best
of our knowledge. Therefore, we synthesized the compounds 2 as a bioisostere
of the compounds 1.
Received in Japan October 11, 2005
Address correspondence to Chung-Kyu Ryu, College of Pharmacy, Ewha Womans
University, Seoul 120-750, Korea. E-mail: ckryu@ewha.ac.kr
3319

3320
C.-K. Ryu, I. H. Choi, and R. E. Park
Figure 1. 9H-Carbazole-1,4-diones and benzo[b]benzo[2,3-d]thiophene-6,9-diones.
Palladium-mediated annulation methodology has been effectively employed
for the preparation of new heterocyclic systems.[6] As shown in Scheme 1,
compounds 2 should be synthesized by either one-pot (Path A) or two-step
synthesis (Path B) from 2,3-dimethylcyclohexa-2,5-diene-1,4-dione (4) and appro-
priate arylthiols 6 with palladium(II) acetate. Although the one-pot synthesis of
compounds 2 seem to be much more attractive, the two-step synthesis is much
easier than the one-pot synthesis as a result of the difficulty in both the substitution
reaction and the C-C coupling. Herein, we report efficient two-step synthesis of
compounds 2 via palladium(II) acetate–mediated cyclization of 5-arylthio-
2,3-dimethyl-cyclohexa-2,5-diene-1,4-diones 7.
The first step in Path B synthesis was preparation of 5-arylthio-
2,3-dimethyl-cyclohexa-2,5-diene-1,4-diones 7 (Scheme 1). For example,
the substitution of 4-methylbenzenethiol (6a) to the quinone 4 in the
presence of 0.05 equivalent of palladium(II) acetate as a catalyst in air at
room temperature for 4 h afforded 2,3-dimethyl-5-(4-methylphenylthio)cyclo-
hexa-2,5-diene-1,4-dione (7a) in good yield (Scheme 2). Otherwise, reactions
Scheme 1. Reagents and conditions: a) EtOH, 0.05M Pd(OAc)2, rt, 4 h; b)
CH3COOH, 1M Pd(OAc)2, reflux, Ar, 24 h.

Synthesis of Benzo[b]benzo[2,3-d]thiophen-6,9-diones
3321
Scheme 2. Substitution of 4-methylbenzenethiol (6a) to the quinone 4.
in absence of the palladium(II) acetate provided the compound 7a with poor
yields along with 5,6-bis(4-methylphenylthio)-2,3-dimethylcyclohexa-2,5-
diene-1,4-dione (8a) as a by-product (Table 1). THF for the reaction seems
to be the best solvent. Acetic acid and ethanol were also tried, and in those
solvents the product 7a was observed with lower yield than in THF.
A number of 5-arylthio-2,3-dimethylcyclohexa-2,5-diene-1,4-diones 7
were synthesized by employing the optimized reaction condition (Scheme 2).
The compounds 7 were prepared by the substitution of appropriate arylthiols
6 to the quinone 4 in the presence of 0.05 equivalent of palladium(II) acetate
as a catalyst in air at room temperature for 4 h in THF, respectively (Table 2).
The second step in Path A synthesis was palladium(II)-mediated oxidative
cyclization of compounds 7 in acetic acid, which provided the desired products
2 (Scheme 1). For example, we refluxed the compound 7a with stoichiometric
amounts of palladium(II) acetate in the distilled acetic acid under an argon
atmosphere for about 24 h to afford 2,7,8-trimethylbenzo[b]benzo[2,3-
Table 1. Substitution of 4-methylbenzenethiol (6a) to the quinone 4
Entry
Reaction condition
C2H5OH, air, reflux, 10 h
HOAc, air, reflux, 4 h
7a (%)a
8a (%)
1
2
3
3
4
5
55
53
39
72
59
83
24
21
42
0
THF, air, reflux, 4 h
0.05 equiv. Pd(OAc)2, C2H5OH, air, reflux, 4 h
0.05 equiv. Pd(OAc)2, HOAc, air, reflux, 4 h
0.05 equiv. Pd(OAc)2, THF, air, reflux, 4 h
0
0
aIsolated yield.

3322
C.-K. Ryu, I. H. Choi, and R. E. Park
Table 2. Structure of afforded 5-arylthio-2,3-dimethylcyclohexa-2,5-
diene-1,4-diones 7
Compounds
R1
R2
R3
Yield (%)
7a
7b
7c
7d
7e
7f
H
H
H
H
F
H
H
H
H
H
H
Cl
CH3
F
83
45
52
55
64
55
59
Cl
H
F
H
H
CH3O
H
7g
Notes: Reaction condition: 1.0 mmol quinone 4, 1.1 mmol arylthiol 6,
0.05 mmol Pd(OAc)2, THF, air, rt, reflux, 4 h.
d]thiophene-6,9-dione (2a) (Scheme 3). The acetic acid seems to be the best
solvent for the oxidative cyclization. The purity of acetic acid is of primary
importance for cyclization,[7,8] but the reason for this is not known. Ethanol and
THF were also tried, but in those solvents no cyclization was observed (Table 3).
Furukawa described the first palladium(II)-mediated oxidative cyclization
of 3-phenylamino-1,4-benzoquinones to carbazole-1,4-quinones.[9] In the
¨
synthesis of benzo[b]carbarzole-6,9-diones, Knolker demonstrated that this
reaction becomes catalytic in palladium by reoxidation of Pd(0) to Pd(II)
with Cu(II).[10] We also tried, but the catalytic condition by reoxidation with
copper(II) acetate was not useful for getting the cyclized product 2a (Table 3).
To determine the versatility of this intramolecular C-C coupling process
on a quinone system, a number of benzo[b]benzo[2,3-d]thiophene-6,9-diones
2 were synthesized by employing the optimized reaction condition (Scheme 3
Scheme 3. Cyclization of 2,3-dimethyl-5-(4-methylphenylthio)cyclohexa-2,5-diene-
1,4-dione (7a).

Synthesis of Benzo[b]benzo[2,3-d]thiophen-6,9-diones
3323
Table 3. Oxidative cyclization of the quinone 7a to get the cyclized product 2a
Entry
Solvent
Cu(OAc)2
Time
Atmosphere
Yield (%)
1
2
3
4
5
6
7
HOAc
HOAc
HOAc
HOAc
C2H5OH
C2H5OH
THF
—
—
10 h
24 h
40 h
24 h
24 h
24 h
24 h
Argon
Argon
Air
30
51
—
35
Trace
0
2.5 equiv.
2.5 equiv.
—
Argon
Argon
Argon
Argon
0
Trace
—
Note: We refluxed the 0.2 mmol quinone 7a with 0.2 mmol of Pd(OAc)2 in various
conditions.
and Table 3). The various compounds 2 was obtained by cyclization of
5-arylthio-2,3-dimethylcyclohexa-2,5-diene-1,4-diones 7 with stoichiometric
amounts of palladium(II) acetate in refluxed acetic acid under an argon
atmosphere for about 24 h (Table 4).
As shown in Scheme 1, compounds 2 could be also synthesized by
one-pot synthesis of the quinone 4 with arylthiols 6 in both substitution
reaction and the C-C coupling (Path A). For an example of the one-pot
synthesis, the quinone 4 was reacted with 4-fluorothiophenol (6b) and stoi-
chiometric amounts of palladium(II) acetate in refluxed acetic acid under an
argon atmosphere for about 2 h. As a result of unexpected discovery, the
one-pot synthesis provided 2,3-dimethyl-5-(4-fluorophenyl)cyclohexa-2,5-
Table 4. Structure of benzo[b]benzo[2,3-d]thiophene-6,9-
diones 2
Compounds
R1
R2
R3
Yield (%)
2a
2b
2c
2d
2e
2f
H
H
H
H
F
H
H
H
H
H
H
Cl
CH3
F
51
23
47
37
43
39
40
Cl
H
F
H
H
CH3O
H
2g
Note: Reaction condition: quinone 7, Pd(OAc)2, HOAc, Ar,
reflux, 24 h.

3324
C.-K. Ryu, I. H. Choi, and R. E. Park
Scheme 4. One-pot synthesis.
diene-1,4-dione (9) along with the desired product 2b in low yields
(Scheme 4). Although the one-pot synthesis seems to be much more attractive
than the two-step route synthesis, the latter is more favorable for getting the
cyclized product 2b than the one-pot synthesis, because both intra- and
intermolecular coupling occur all at once (Schemes 3 and 4).
In conclusion, we achieved an efficient palladium-mediated synthesis of
benzo[b]benzo[2,3-d]thiophene-6,9-diones 2, and we suggest that the
palladium-mediated coupling has potential for even wider application to the
quinone series or sulfur chemistry than described in the present work.
EXPERIMENTAL
All melting points were measured in open capillary tubes with a Bu¨chi
melting-point B-545 apparatus and are uncorrected. The thin-layer chromato-
graphy (TLC) was performed on precoated silica gel (60G 254, Merck) using
chloroform as a solvent. The compounds were detected under UV light
(254 nm) or by heating to 1108C after spraying with a 30% H2SO4–vanillin
solution. Column chromatography was performed on silica gel G60 (70–230
mesh, ASTM, Merck). 1H NMR spectra were recorded on Unity Varian
Inova 400-MHz FT-NMR spectrometer using CDCl3 as a solvent, and
chemical shifts were given in ppm with TMS as a standard. High-resolution
mass (HRMS EI) spectra were taken with a Jeol JMS AX505 WA. Arylthiols,
palladium(II) acetate, CDCl3, and other reagents were purchased from Aldrich
Chemical Co. Acetic acid was used after purification by the distillation. 2,3-
Dimethylcyclohexa-2,5-diene-1,4-dione (4) was prepared by the oxidation
from 2,3-dimethylhydroquinone with ammonium cerium(IV) nitrate.
General Procedure for Synthesis of 5-Arylthio-2,3-
dimethylcyclohexa-2,5-diene-1,4-diones 7a–g
A solution of 2,3-dimethyl-cyclohexa-2,5-diene-1,4-dione (4) (136 mg,
1.0 mmol) and palladium(II) acetate (112 mg, 0.05 mmol) in 20 mL of THF
was added to a solution of arylthiols 6 (1.1 mmol) in 10 mL of THF, and

Synthesis of Benzo[b]benzo[2,3-d]thiophen-6,9-diones
3325
then the mixture was stirred at room temperature for 4 h. After the reaction,
the precipitate was collected by the filtration. The crude product was
purified by silica-gel column chromatography with EtOAc/n-hexane (1:2)
or crystallized from 95% ethanol.
2,3-Dimethyl-5-(4-methylphenylthio)cyclohexa-2,5-diene-1,4-dione (7a):
yield: 83%. Mp: 68–708C. 1H NMR (CDCl3): d 7.35 (d, 2H, J ¼ 7.2,
benzene), 7.26 (d, 2H, J ¼ 7.2, 2H, benzene), 5.83 (s, 1H, benzene), 2.41
(s, 3H, benzene), 2.06 (s, 3H, methyl), 2.00 (s, 3H, methyl). MS (m/z): 258 (Mþ).
2,3-Dimethyl-5-(4-fluorophenylthio)cyclohexa-2,5-diene-1,4-dione (7b):
yield: 45%. Mp: 82–848C. 1H NMR (CDCl3): d 7.47 (d, 2H, J ¼ 7.4,
benzene), 7.17 (d, 2H, J ¼ 7.4, benzene), 5.79 (s, 1H, benzene), 2.06
(s, 3H, methyl), 2.00 (s, 3H, methyl). MS (m/z): 262 (Mþ).
5-(4-Chlorophenylthio)-2,3-dimethylcyclohexa-2,5-diene-1,4-dione (7c):
yield: 52%. Mp: 70–718C. 1H NMR (CDCl3): d 7.45 (d, 2H, J ¼ 7.2,
benzene), 7.43 (d, 2H, J ¼ 7.2, benzene), 5.82 (s, 1H, benzene), 2.06
(s, 3H, methyl), 2.00 (s, 3H, methyl). MS (m/z): 278 (Mþ).
5-Phenylthio-2,3-dimethylcyclohexa-2,5-diene-1,4-dione (7d): yield: 55%.
1
Mp: 90–918C. H NMR (CDCl3): d 7.45 (m, 5H, benzene), 5.84 (s, 1H,
benzene), 2.05 (s, 3H, methyl), 1.99 (s, 3H, methyl). MS (m/z): 244 (Mþ).
5-(2,4-Difluorophenylthio)-2,3-dimethylcyclohexa-2,5-diene-1,4-dione (7e):
1
yield: 64%. Mp: 76–788C. H NMR (CDCl3): d 7.50 (m, 1H, benzene), 7.00
(m, 2H, benzene), 2.06 (s, 3H, methyl), 2.01 (s, 3H, methyl). MS (m/z): 280 (Mþ).
2,3-Dimethyl-5-(4-methoxyphenylthio)cyclohexa-2,5-diene-1,4-dione (7f):
1
yield: 59%. Mp: 109–1108C. H NMR (CDCl3): d 7.38 (d, 2H, J ¼ 7.1,
benzene), 6.97 (d, 2H, J ¼ 7.1, benzene), 5.81 (s, 1H, benzene), 3.85 (s, 3H,
benzene), 2.05 (s, 3H, methyl), 2.00 (s, 3H, methyl). MS (m/z): 274 (Mþ).
5-(3-Chlorophenylthio)-2,3-dimethylcyclohexa-2,5-diene-1,4-dione (7g):
1
yield: 56%. Mp: 82–838C. H NMR (CDCl3): d 7.3–7.0 (m, 4H, benzene),
6.82 (s, 1H, benzoquinone), 1.97 (s, 3H, methyl), 1.91 (s, 3H, methyl), MS
(m/z): 278 (Mþ).
Synthesis of 5,6-bis(4-Methylphenylthio)-2,3-dimethylcyclohexa-
2,5-diene-1,4-dione (8a)
A solution of the quinone 4 (136 mg, 1.0 mmol) in 20 mL of THF was added
to a solution of 4-methylbenzenethiol 6a (2.1 mmol) in 10 mL of THF, and
then the mixture was stirred at room temperature for 4 h. After the reaction,
the precipitate was collected by the filtration. The filtrated crude product

3326
C.-K. Ryu, I. H. Choi, and R. E. Park
was purified by silica-gel column chromatography with CHCl3. The
compound 7a was obtained with moderate yields (39%) along with the
compound 8a as a by-product (42%).
5,6-bis(4-Methylphenylthio)-2,3-dimethylcyclohexa-2,5-diene-1,4-dione (8a):
yield: 42%. Mp: 155–1568C. 1H NMR (CDCl3): d 7.12 (m, 4H, benzene), 6,91
(m, 4H, benzene), 2.30 (s, 6H, methyl), 1.94 (s, 6H, methyl), MS (m/z): 380 (Mþ).
General Procedure for Synthesis of Benzo[b]benzo[2,3-d]thiophene-
6,9-diones 2a–g
A mixture of compounds 7 (2.0 mmol) and Pd(OAc)2 (448 mg, 2.0 mmol) in
10 mL of acetic acid was refluxed under an argon atmosphere for about 24 h.
The insoluble matter was filtered off and washed with acetic acid. The filtrate
was evaporated in vacuum. The residue was purified by silica-gel column
chromatography with CH3Cl/n-hexane or crystallized from 95% ethanol to
give the compounds 2a–g.
2,7,8-Trimethylbenzo[b]benzo[2,3-d]thiophen-6,9-dione (2a): orange powder
1
(51%). Mp: 190–1928C. H NMR (CDCl3): d 8.53 (s, 1H, benzothiophene),
7.78 (d, 1H, J ¼ 7.1, benzothiophene), 7.33 (d, 1H, J ¼ 7.1, benzothiophene),
2.52 (s, 3H, methyl), 2.15 (s, 6H, methyl), MS (m/z): 256 (Mþ). HRMS calcd.
for C15H12O2S: 256.05581; found: 256.0559.
7,8-Dimethyl-2-fluorobenzo[b]benzo[2,3-d]thiophen-6,9-dione (2b): yellow
powder (23%). Mp: 140–1428C. 1H NMR (CDCl3): d 8.41 (d, 1H, J ¼ 7.6, ben-
zothiophene), 7.85 (d, 1H, J ¼ 7.6, benzothiophene), 7.00 (m, 1H, benzothio-
phene), 2.52 (s, 6H, methyl), MS (m/z): 260 (Mþ). HRMS calcd. for
C14H9FO2S: 260.0307; found: 260.0308.
2-Chloro-7,8-dimethylbenzo[b]benzo[2,3-d]thiophen-6,9-dione (2c): orange
powder (47%). Mp: 189–1908C. 1H NMR (CDCl3): d 8.72 (s, 1H, benzothio-
phene), 7.83 (d, 1H, J ¼ 7.6, benzothiophene), 7.45 (d, 1H, J ¼ 7.6, benzothio-
phene), 2.19 (s, 6H, methyl), MS (m/z): 276 (Mþ). HRMS calcd. for
C14H9ClO2S: 276.0012; found: 276.0012.
7,8-Dimethylbenzo[b]benzo[2,3-d]thiophen-6,9-dione (2d): bright brown
powder (37%). Mp: 140–1428C. 1H NMR (CDCl3): d 8.72 (s, 1H, benzothio-
phene), 7.92 (d, 1H, benzothiophene), 7.53 (m, 2H, benzothiophene), 2.16
(s, 6H, methyl), MS (m/z): 242 (Mþ). HRMS calcd. for C14H10O2S:
242.0402; found: 242.0401.
2,4-Difluoro-7,8-dimethylbenzo[b]benzo[2,3-d]thiophen-6,9-dione (2e):
1
yellow powder (43%). Mp: 142–1448C. H NMR (CDCl3): d 8.25 (m, 1H,

Synthesis of Benzo[b]benzo[2,3-d]thiophen-6,9-diones
3327
benzothiophene), 7.04 (m, 1H, benzothiophene), 2.17 (s, 6H, methyl), MS
(m/z): 278 (Mþ). HRMS calcd. for C14H8F2O2S: 278.0213; found: 278.0212.
7,8-Dimethyl-2-methoxybenzo[b]benzo[2,3-d]thiophen-6,9-dione (2f): yellow
powder (39%). Mp: 192–1948C. 1H NMR (CDCl3): d 8.18 (s, 1H, J ¼ 7.1, ben-
zothiophene), 7.74 (d, 1H, J ¼ 7.12, benzothiophene), 7.14 (d, 1H, J ¼ 7.1, ben-
zothiophene), 3.93 (s, 3H, methyl), 2.13 (s, 6H, methyl), MS (m/z): 272 (Mþ).
HRMS calcd. for C15H12O3S: 272.0507; found: 272.0508.
3-Chloro-7,8-dimethylbenzo[b]benzo[2,3-d]thiophen-6,9-dione (2g): orange
powder (40%). Mp: 228–2298C. 1H NMR (CDCl3): d 8.66 (m, 1H, benzothio-
phene), 7.90 (s, 1H, benzothiophene), 7.50 (m, 1H, benzothiophene), 2.17
(s, 6H, methyl), MS (m/z): 276 (Mþ). HRMS calcd. for C14H9ClO2S:
276.0012; found: 276.0013.
Procedure for One-Pot Synthesis of the 7,8-Dimethyl-2-
fluorobenzo[b]benzo[2,3-d]thiophen-6,9-dione (2b)
A mixture of compound 4 (272 mg, 2.0 mmol) and Pd(OAc)2 (448 mg,
2.0 mmol) in 10 mL of acetic acid was added to a solution of 4-fluorothio-
phenol (6b) (2.1 mmol) in 10 mL of acetic acid, and the mixture was refluxed
for about 2 h under an argon atmosphere. The filtrate was evaporated in a
vacuum. After evaporation, the compound 2b (29%) and compound 9 were
separated by silica-gel column chromatography with CH3Cl/n-hexane and
crystallized from 95% ethanol.
2,3-Dimethyl-5-(4-fluorophenyl)cyclohexa-2,5-diene-1,4-dione (9): Yellow
powder (45%). Mp: 104–1068C. 1H NMR (CDCl3): d 7.08 (m, 2H, benzene),
6.93 (m, 2H, benzene), 6.24 (S, 1H, p-benzoquinone), 2.23 (s, 3H methyl),
2.20 (s, 3H, methyl). MS (m/z): 230 (Mþ).
ACKNOWLEDGMENTS
This study was supported by a grant of the Research Institute of
Pharmaceutical Science, Ewha Womans University (2005).
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