Oxidative ring opening of 1,3-diarylbenzo[c]heterocycles using m-CPBA: Preparation of 1,2-diaroylbenzenes

Article abstract of DOI:10.1002/ejoc.201200311

An unprecedented oxidative cleavage of benzo[c]heterocycles using m-CPBA is reported. The reaction of 1,3-diaryl benzo[c]heterocycles with m-CPBA (meta-chloroperoxybenzoic acid) at room temperature for 5 min led to the formation of 1,2-diaroylbenzenes in good to excellent yields.

Full text of DOI:10.1002/ejoc.201200311

FULL PAPER
DOI: 10.1002/ejoc.201200311
Oxidative Ring Opening of 1,3-Diarylbenzo[c]heterocycles Using m-CPBA:
Preparation of 1,2-Diaroylbenzenes
Meganathan Nandakumar,[a] Ramakrishnan Sivasakthikumaran,[a] and
Arasambattu K. Mohanakrishnan*[a]
Keywords: Synthetic methods / Heterocycles / Fused-ring systems / Oxidative cleavage / Selenium
An unprecedented oxidative cleavage of benzo[c]hetero-
cycles using m-CPBA is reported. The reaction of 1,3-diaryl
benzo[c]heterocycles with m-CPBA (meta-chloroperoxy-
benzoic acid) at room temperature for 5 min led to the forma-
tion of 1,2-diaroylbenzenes in good to excellent yields.
ever, the photochemical degradation of the 1,3-di-
arylbenzo[c]thiophene analogs is considered a drawback, li-
miting their optical applications.[3b,9] Hence, there is plenty
of opportunity to further explore the optical applications
of these heterocycles, provided the stability issue is suitably
addressed.[10]
On the basis of theoretical calculations, the oxidation of
oligothiophene analogs has been proposed to increase their
stability.[11] Furthermore, it has been established that the
oxidation of one thiophene unit of a linear thienyl oligomer
increases the electron deficiency, which in turn increases the
electron delocalization, electron transport, and injection
properties of the material.[12] With this concept in mind,
and also to increase the stability of the benzo[c]thiophene
analogs, the oxidation of 1,3-diarylbenzo[c]thiophene was
explored.
Introduction
Although benzo[c]thiophene (1) has been known for
some time,[1] its first report as a precursor of low band gap
poly(benzo[c]thiophene) by Wudl and co-workers[2] led to
the synthesis and electrochemical polymerization of a more
stable analog, namely, 1,3-dithienylbenzo[c]thiophene (2).[3]
During the last ten years, the synthesis and characterization
of symmetrical as well as unsymmetrical 1,3-diarylbenzo-
[c]thiophene 3 (Scheme 1) have been explored by our
group.[4] A detailed study of the photophysical properties
of these benzannelated thienyl oligomers has also been car-
ried out. The HOMO, LUMO, and band gap values of
these heterocycles were correlated with their electronic
structures.[4d,5] A noteworthy application of 1,3-dithienyl-
benzo[c]thiophene includes its use as a fluorescent label to
aid in understanding the structure and dynamics of nucleic
acids.[6] Thelakkat et al. outlined the syntheses of di-
arylamine-capped 1,3-dithienylbenzo[c]thiophene analogs
for use as a promising light-harvesting and hole-trans-
porting material in organic solar cells.[7]
Results and Discussion
Accordingly, as a representative case, the reaction of 1,3-
diphenylbenzo[c]thiophene (4a) in DCM (dichloromethane)
with a preformed mixture of KF/m-CPBA[13] (meta-chloro-
peroxybenzoic acid) in CH3CN/H2O led to the disappear-
ance of the fluorescent color of the heterocycle (Scheme 2).
Scheme 1. Benzo[c]thiophenes 1–3.
Swager and co-workers reported the syntheses and char-
acterizations of benzo[c]furan and benzo[c]thiophene-based
push-pull systems for use as near-IR fluorophores.[8] How-
[a] Department of Organic Chemistry, University of Madras,
Guindy Campus,
Chennai 600025, Tamil Nadu, India
Fax: +91-44-22352494
E-mail: mohanakrishnan@unom.ac.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200311.
Scheme 2. Oxidative cleavage of benzo[c]heterocycle 5a and 6a.
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M. Nandakumar, R. Sivasakthikumaran, A. K. Mohanakrishnan
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However, the usual workup followed by purification by col- the best of our knowledge, the cleavage of benzo[c]furan
umn chromatography afforded only diketone 5a in 70% using m-CPBA was not known until now. Finally, the struc-
yield. This unexpected cleavage of the nonclassical thio- tures of representative diketones 11b (Figure 1) and 24c
phene framework prompted us to explore the reaction using (Figure 2) were confirmed by single-crystal X-ray structure
m-CPBA.
To our surprise, the reaction of benzo[c]thiophene 4a
analyses.[18]
with 1.5 equiv. of m-CPBA in DCM at room temp. for
5 min afforded diketone 5a in a better yield (84%). It
should be noted that even with 1 equiv. of m-CPBA, the
thio analog of diketone 5a was not observed (confirmed
by 13C NMR), and only 5a was isolated. Under identical
conditions, benzo[c]furan 6a furnished diketone 5a in 92%
yield. Although the cleavage of 6a is known to use singlet
oxygen,[14] recent reports on organic thin-film transistors
(OTFT) and other optical applications of aroyl-capped
oligothiophenes[15] encouraged us to undertake a systematic
study of this facile oxidative ring-opening reaction with
other heterocycles.
Table 1. Oxidative cleavage of 1,3-diarylbenzo[c]heterocycles.
To our delight, a large number of symmetrical as well
as unsymmetrical benzo[c]heterocycles underwent a smooth
oxidative cleavage with m-CPBA to furnish diketones in ex-
cellent yields (Table 1).
In the case of 1,3-dithienylbenzo[c]thiophene (4b), the m-
CPBA-mediated cleavage led to the isolation of unsymmet-
rical diketone 5bЈ formed through oxidation of a thiophene
unit into a S,S-dioxide moiety (Table 1, Entry 1). However,
the reaction with diarylbenzo[c]thiophenes 4c–4e afforded
their respective diketones in excellent yields (Table 1, En-
try 1). The oxidative ring-opening reaction of 1,3-di-
arylbenzo[c]furans 6a–6i proceeded with relatively better
yields than their respective benzo[c]thiophenyl heterocycles
(Table 1, Entry 2). Diarylbenzo[c]selenophenes 7a and 7b
also underwent oxidative cleavage to furnish the corre-
sponding diketones 5a and 5c in slightly reduced yields
(Table 1, Entry 3). In the case of thiophene-tethered
benzo[c]furans 8a–8c, oxidation of the thiophene unit was
not observed (Table 1, Entry 4). Thus, the observed oxi-
dation of the classical thiophene unit into the S,S-dioxide
was found to be specific to the case of benzo[c]thiophene
4b. As expected, the benzo[c]furans linked to the dihexyl-
fluorene, pyrene, and diphenylmethane groups also led to
their respective diketones in excellent yields (Table 1, En-
tries 5–7). The dibenzoheterocycle-tethered benzo[c]hetero-
cycles also underwent smooth oxidative cleavage with m-
CPBA to give the expected diketones (Table 1, Entries 8–
11). The N-hexylcarbazole-capped benzo[c]thiophenes 18a–
18c furnished diketones 19a–19c in moderate yields
(Table 1, Entry 9). The triphenylamine-linked diarylbenzo-
[c]furans 23a–23d led to the diketones 24a–24d in 77–83%
yields. The oxidative ring-opening reaction of the dibenzo-
thiophene-linked benzo[c]furans 25 and 27 with m-CPBA
also gave products formed by the oxidation of sulfur into
sulfoxide as well as sulfone (Table 1, Entries 13 and 14).
The reaction of 2,5-diarylfurans 29a–29c with m-CPBA
in DCM at room temperature led to the isolation of cis-
diaroylethylenes 30a–30c in 68–92% yields (Table 1, En-
try 15). It should be noted that the cleavage of furan deriva-
tives using m-CPBA was observed earlier.[17] However, to
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Preparation of 1,2-Diaroylbenzenes
Table 1. (Continued).
Figure 1. X-ray crystal structure of compound 11b.
Figure 2. X-ray crystal structure of compound 24c.
To further understand the efficacy of this m-CPBA-medi-
ated oxidative cleavage protocol, a comparative study was
planned with the reported procedure for the cleavage of
nonclassical furans using lead tetraacetate (LTA).[3b] Ac-
cordingly, the oxidative cleavage of some representative 1,3-
diarylbenzo[c]furans was performed using LTA/m-CPBA,
and the yields of the respective diaroylbenzenes isolated are
presented in Table 2. In the case of the simple 1,3-di-
arylbenzo[c]furans 6d, 6e, and 6h, the yields of the respec-
tive diaroylbenzenes were almost comparable between m-
CPBA and LTA (Table 2, Entries 1–3). However, when
treated with LTA, the carbazole- and triphenylamine-teth-
ered benzo[c]furans furnished the corresponding diketones
[a] Isolated yields. [b] For Entries 13 and 14, 2 equiv. of m-CPBA
was used.
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M. Nandakumar, R. Sivasakthikumaran, A. K. Mohanakrishnan
FULL PAPER
in low yields (Table 2, Entries 3–6). In the case of the di- metrical 1,2-diaroylbenzenes as intermediates for the syn-
benzothiophene-tethered benzo[c]furan 25 and 27, the oxi- theses of 9-arylanthracenes and annulated hetero-
dative cleavage with LTA was selective, affording the respec- cycles.[19]
tive diaroylbenzenes 26a and 28a in better yields than with
the m-CPBA protocol.
Experimental Section
Table 2. Comparative study of oxidative cleavage of 1,3-di-
General Methods: The solvents were dried by standard procedures.
arylbenzo[c]furans using m-CPBA and LTA.
All of the experiments were carried out under a nitrogen atmo-
Entry Benzo[c]furan Diketone
Yield [%][a]
Yield [%][b]
sphere unless otherwise stated. The progression of all of the reac-
tions was monitored by TLC, using a hexanes/ethyl acetate mixture
as the eluent. Column chromatography was carried out on silica
1
2
3
4
5
6
7
8
9
6d
6e
5d
5e
93
86
88
82
80
81
83
41
44
76
85
84
25
21
23
24
80
83
1
6h
5h
gel (230–400 mesh, Merck) by increasing the solvent polarity. H,
13C, and DEPT-135 spectroscopic data were recorded at room tem-
perature in CDCl3 using TMS as an internal standard with a
Bruker 300 MHz spectrometer. The chemical shift values were
quoted in parts per million (ppm), and the coupling constants (J)
were quoted in Hertz (Hz). Elemental analysis data were recorded
with a Elementar Vario Series Analyser (IISER, Pune) instrument.
HRMS data were recorded with a MS001300-FINNIGAN MAT-
95XL instrument. Commercially available m-CPBA (77%), pur-
chased from Sigma–Aldrich, was used in the reactions. The re-
quired benzo[c]thiophenes 4a–4e, benzo[c]furans 6a–6i, and
benzo[c]selenophenes 7a and 7b were prepared using reported pro-
cedures.[3a,3b,3e,4a,20]
20a
20c
23c
23d
25
19a
19c
24c
24d
26a
28a
27
[a] The yield obtained using m-CPBA. [b] The yield obtained using
LTA.
A plausible mechanism for the oxidative cleavage of
benzo[c]furan using m-CPBA has been outlined in
Scheme 3. The benzo[c]furan unit undergoes an electro-
philic hydroxylation at the 1-position through the aromati-
zation of the benzene ring to form intermediate 31, which
upon ring opening may lead to the formation of the diaroyl-
benzenes.
Oxidative Cleavage of Benzo[c]thiophene 4a Using KF/m-CPBA
Protocol:[13] To a solution of KF (0.15 g, 2.62 mmol) in acetonitrile/
water (10 mL, 2 mL) was added 77% m-CPBA (0.58 g, 2.62 mmol),
and the mixture was stirred at 0 °C for 30 min. To this, 1,3-di-
phenylbenzo[c]thiophene (4a, 0.30 g, 1.05 mmol) in DCM was
added, and the mixture was stirred at room temperature for 5 min.
The reaction mixture was then poured into a saturated sodium hy-
drogen carbonate solution, and the resulting mixture was extracted
with DCM (3ϫ15 mL). The combined organic extracts were
washed with water (2ϫ30 mL) and dried with Na2SO4. Removal
of solvent followed by purification by column chromatography [sil-
ica gel, 5% EA (ethyl acetate)/hexane] afforded diketone 5a[14d]
(0.21 g, 70%) as a colorless solid; m.p. 138 °C. 1H NMR (300 MHz,
CDCl3): δ = 7.63 (d, J = 7.2 Hz, 4 H, Ar), 7.54 (s, 4 H, Ar), 7.44
(t, J = 7.2 Hz, 2 H, Ar) 7.32–7.27 (m, 4 H, Ar) ppm. 13C NMR
(75 MHz, CDCl3): δ = 196.6, 140.0, 137.2, 133.0, 130.4, 129.8,
129.7, 128.3 ppm. Dept-135 (75 MHz, CDCl3): δ = 133.0, 130.4,
129.8, 129.7, 128.3 ppm.
Scheme 3. Mechanism for cleavage of nonclassical heterocycle.
Oxidative Cleavage of Benzo[c]thiophene 4a Using m-CPBA: To a
solution of benzo[c]thiophene 4a (0.3 g, 1.05 mmol) in DCM
(15 mL) was added m-CPBA (0.35 g, 1.57 mmol), and the reaction
mixture was stirred at room temperature for 5 min. It was then
poured into a saturated sodium hydrogen carbonate solution, and
the resulting mixture was extracted with DCM (3ϫ15 mL). The
combined organic extracts were washed with water (2ϫ30 mL) and
dried with Na2SO4. Removal of solvent followed by purification by
column chromatography (silica gel, 5% EA/hexane) afforded di-
ketone 5a (0.25 g, 84%).
Conclusions
In summary, for the first time, a facile m-CPBA-mediated
oxidative cleavage of a wide variety of 1,3-disubstituted
benzo[c]furans was achieved at room temperature. The reac-
tion was also found to be successful with nonclassical thio-
phene and selenophene analogs. Compared to the existing
methods available for the cleavage of benzo[c]furans,[3a,9,14]
the present protocol using m-CPBA is simple, widely appli-
cable, and also less time-consuming. In the case of the nitro-
gen-containing benzo[c]furans, the present method was
found to be better than the method with LTA. The various
types of heteroaroyl oligothiophenes reported herein may
find application in the area of field-effect transistors as well
as in organic solar cell (OSC) applications.[15] There is
Oxidative Cleavage of Benzo[c]furan 6a Using m-CPBA: To a solu-
tion of benzo[c]furan 6a (0.5 g, 1.85 mmol) in DCM (15 mL) was
added m-CPBA (0.62 g, 2.78 mmol), and the reaction mixture was
stirred at room temperature for 5 min. It was then poured into a
saturated sodium hydrogen carbonate solution, and the resulting
mixture was extracted with DCM (3ϫ30 mL). The combined or-
ganic extracts were washed with water (2ϫ30 mL) and dried with
Na2SO4. Removal of solvent followed by purification by column
chromatography (silica gel, 5% EA/hexane) afforded diketone 5a
plenty of chemistry to explore with readily available unsym- (0.46 g, 92%).
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Preparation of 1,2-Diaroylbenzenes
5bЈ: The oxidative cleavage of 1,3-dithienylbenzo[c]thiophene
(4b,[3b] 0.50 g, 1.77 mmol) using m-CPBA (0.79 g, 2.66 mmol) fol-
lowed by the usual workup and purification by column chromatog-
raphy (silica gel, 10% EA/hexane) gave diketone 5bЈ (0.35 g, 60%)
Oxidative Cleavage of Benzo[c]furan 6e: Benzo[c]furan 6e (0.4 g,
1.19 mmol) was treated with m-CPBA (0.31 g, 1.79 mmol) followed
by the usual workup and purification by column chromatography
to give diketone 5e (0.38 g, 91%) as a colorless solid.
1
as a thick, brown liquid; H NMR (300 MHz, CDCl3): δ = 7.75–
5f: The oxidative cleavage of benzo[c]furan 6f (0.50 g, 1.35 mmol)
using m-CPBA (0.45 g, 2.03 mmol) followed by the usual workup
and purification by column chromatography afforded diketone 5f
(0.45 g, 90%) as a colorless solid; m.p. 128–130 °C, ref.[22] m.p. 130–
131 °C. 1H NMR (300 MHz, CDCl3): δ = 8.05 (d, J = 8.4 Hz, 2
H, Ar), 7.73 (d, J = 8.4 Hz, 2 H, Ar) 7.61–7.52 (m, 6 H, Ar), 7.40
(d, J = 6.9 Hz, 2 H, Ar), 7.29–7.16 (m, 6 H, Ar) ppm. 13C NMR
(75 MHz, CDCl3): δ = 197.9, 141.6, 135.3, 133.7, 133.2, 131.1,
131.0, 130.8, 130.2, 127.9, 127.8, 126.4, 125.5, 123.8 ppm. Dept-
135 (75 MHz, CDCl3): δ = 133.2, 131.1, 131.0, 130.2, 127.9, 127.8,
126.4, 125.5, 123.8 ppm.
7.64 (m, 5 H, Ar), 7.59–7.54 (m, 1 H, Ar), 7.42 (d, J = 3.6 Hz, 1
H, Ar), 7.35 (d, J = 3.9 Hz, 1 H, Ar), 7.08–7.04 (m, 2 H, Ar) ppm.
13C NMR (75 MHz, CDCl3): δ = 187.6, 179.6, 143.8, 140.2, 138.2,
135.4, 135.2, 132.7, 132.1, 131.1, 130.8, 130.4, 129.3, 128.4, 128.0,
126.3 ppm. Dept-135 (75 MHz, CDCl3): δ = 135.4, 135.3, 132.7,
132.1, 131.1, 130.8, 130.4, 129.3, 128.0, 126.3 ppm. HRMS: calcd.
for C16H10O4S2 [M + H]+ 331.0099; found 330.9935.
5c: The oxidative cleavage of benzo[c]thiophene 4c (0.55 g,
1.75 mmol) using m-CPBA (0.59 g, 2.63 mmol) followed by the us-
ual workup and purification by column chromatography gave di-
ketone 5c (0.48 g, 87%) as a colorless solid; m.p. 188–189 °C, ref.[21]
5g: The required benzo[c]furan 6g was prepared using the pro-
cedure similar to that of 6f. Benzo[c]furan 6g (0.3 g, 0.67 mmol)
was treated with m-CPBA (0.22 g, 1.00 mmol, 1.5 equiv.) in DCM
to give diketone 5g (0.27 g, 87%) as a thick, orange liquid. 1H
NMR (300 MHz, CDCl3): δ = 7.63–7.60 (m, 2 H, Ar), 7.52–7.50
(m, 2 H, Ar), 7.18 (d, J = 3.6 Hz, 2 H, Ar), 6.66 (d, J = 3.3 Hz, 2
H, Ar), 2.72 (t, J = 7.5 Hz, 4 H, CH2) 1.60–1.53 (m, 4 H, CH2),
1
m.p. 189.7–191.6 °C. H NMR (300 MHz, CDCl3): δ = 7.62–7.59
(m, 8 H, Ar), 7.17 (d, J = 7.5 Hz, 4 H, Ar), 2.37 (s, 6 H, CH3) ppm.
13C NMR (75 MHz, CDCl3): δ = 196.3, 143.8, 140.2, 134.7, 130.1,
130.0, 129.5, 129.0, 21.7 ppm. Dept-135 (75 MHz, CDCl3): δ =
130.2, 130.0, 129.5, 129.0, 21.7 ppm.
5d: The oxidative cleavage of benzo[c]thiophene 4d (0.50 g,
1.58 mmol) using m-CPBA (0.53 g, 2.37 mmol) followed by the us-
ual workup and purification by column chromatography gave di-
ketone 5d (0.42 g, 83%) as a colorless solid; m.p. 122–123 °C, ref.[21]
1.32–1.16 (m, 12 H, CH2), 0.81 (t, J = 6.0 Hz, 6 H, CH3) ppm. 13
C
NMR (75 MHz, CDCl3): δ = 188.1, 157.3, 141.6, 139.4, 135.8,
130.4, 129.1, 125.6, 31.5, 31.3, 30.7, 28.7, 22.5, 14.1 ppm. Dept-135
(75 MHz, CDCl3): δ = 135.8, 130.4, 129.1, 125.6, 31.5, 31.3, 30.7,
28.7, 22.5, 14.1 ppm. C28H34O2S2 (466.70): calcd. C 72.06, H 7.34,
S 13.74; found C 71.81, H 7.39, S 13.68.
1
m.p. 118.6–119.1 °C. H NMR (300 MHz, CDCl3): δ = 7.62–7.58
(m, 5 H, Ar), 7.51–7.50 (br. s, 3 H, Ar) 7.41 (t, J = 6.9 Hz, 1 H,
Ar), 7.30–7.25 (m, 2 H, Ar), 6.76 (d, J = 8.1 Hz, 2 H, Ar), 3.74 (s,
3 H, OCH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.7, 195.3,
163.6, 140.4, 139.8, 137.3, 133.0, 132.2, 130.4, 130.2, 130.0, 129.8,
129.6, 129.4, 129.1, 128.3, 113.6, 55.5 ppm. Dept-135 (75 MHz,
CDCl3): δ = 133.0, 132.20, 130.3, 130.0, 129.8, 129.6, 129.4, 128.3,
113.6, 55.5 ppm.
5h: Benzo[c]furan 6h[16b] (0.50 g, 1.81 mmol) was treated with m-
CPBA (0.61 g, 2.72 mmol, 1.5 equiv.) in DCM to give diketone 5h
(0.44 g, 88%) as a brown solid; m.p. 134–135 °C, ref.[21] m.p. 132.5–
1
133.3 °C. H NMR (300 MHz, CDCl3): δ = 7.49 (d, J = 7.5 Hz, 3
H, Ar), 7.56–7.53 (m, 4 H, Ar), 7.45–7.39 (m, 2 H, Ar), 7.31–7.26
(m, 2 H, Ar), 6.99–6.96 (m, 1 H, Ar) ppm. 13C NMR (75 MHz,
CDCl3): δ = 196.6, 188.3, 144.1, 139.8, 139.6, 137.2, 135.0, 134.9,
133.1, 130.5, 130.5, 129.8, 129.7, 129.1, 128.3, 128.0 ppm. Dept-
135 (75 MHz, CDCl3): δ = 135.0, 134.9, 133.1, 130.6, 130.5, 129.9,
129.8, 129.1, 128.4, 128.0 ppm.
5e: The oxidative cleavage of benzo[c]thiophene 4e (0.45 g,
1.29 mmol) using m-CPBA (0.43 g, 1.91 mmol) followed by the us-
ual workup and purification by column chromatography gave di-
ketone 5e (0.39 g, 86%) as a colorless solid; m.p. 139–140 °C, ref.[21]
1
m.p. 139.7–140.4 °C. H NMR (300 MHz, CDCl3): δ = 8.34 (d, J
= 8.4 Hz, 1 H, Ar), 7.99–7.94 (m, 2 H, Ar), 7.84 (d, J = 8.7 Hz, 1
H, Ar), 7.73–7.28 (m, 9 H, Ar), 7.08 (d, J = 7.8 Hz, 2 H, Ar), 2.33
(s, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 197.5, 196.7,
143.9, 141.2, 140.5, 135.3, 134.9, 133.6, 132.9, 131.5, 130.9, 130.8,
130.5, 130.1, 129.6, 129.0, 128.1, 127.5, 126.4, 125.8, 125.8, 124.0,
21.6 ppm.
5i: The ring-opening reaction of 3-(3,4-dimethylphenyl)isobenzo-
furan-1(3H)-one with freshly prepared p-anisylmagnesium bromide
followed by acidic workup gave benzo[c]furan 6i as a fluorescent,
yellow solid. Benzo[c]furan 6i (0.50 g, 1.52 mmol) was treated with
m-CPBA (0.51 g, 2.29 mmol, 1.5 equiv.) in DCM to give diketone
5i (0.47 g, 94%) as a colorless solid; m.p. 132 °C. 1H NMR
(300 MHz, CDCl3): δ = 7.61 (d, J = 8.4 Hz, 2 H, Ar), 7.50 (s, 4 H,
Ar), 7.41 (s, 1 H, Ar), 7.35 (d, J = 7.8 Hz, 1 H, Ar), 7.03 (d, J =
7.5 Hz, 1 H, Ar), 6.76 (d, J = 8.7 Hz, 2 H, Ar), 3.74 (s, 3 H, OCH3),
2.19 (s, 3 H, CH3), 2.13 (s, 3 H, CH3) ppm. 13C NMR (75 MHz,
CDCl3): δ = 196.6, 195.3, 163.5, 142.6, 140.3, 140.2, 136.7, 135.1,
132.2, 130.9, 130.3, 130.0, 130.0, 129.5, 129.3, 127.8, 113.6, 55.5,
20.0, 19.6 ppm. Dept-135 (75 MHz, CDCl3): δ = 132.2, 130.9,
130.1, 130.0, 129.5, 129.3, 127.8, 113.6, 55.5, 20.1, 19.7 ppm.
5b: The oxidative cleavage of benzo[c]furan 6b[3b] (0.53 g,
1.88 mmol) using m-CPBA (0.63 g, 2.82 mmol) followed by the
usual workup and purification by column chromatography
afforded diketone 5b (0.47 g, 89%) as a brown solid; m.p. 152–
153 °C, ref.[3b] m.p. 149–150 °C. IR (KBr): ν = 1638, 1408,
˜
1287 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.66–7.64 (m, 2 H,
Ar), 7.58–7.56 (m, 4 H, Ar), 7.39–7.38 (m, 2 H, Ar), 6.99–6.96 (m,
2 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 188.3, 144.0,
139.4, 135.1, 135.0, 130.6, 129.2, 128.1 ppm. Dept-135 (75 MHz,
CDCl3): δ = 135.2, 135.0, 130.6, 129.2, 128.1 ppm.
Oxidative Cleavage of Benzo[c]selenophene 7a: Benzo[c]selenophene
7a (0.30 g, 0.90 mmol) was treated with m-CPBA (0.30 g,
1.35 mmol) in DCM to give diketone 5a (0.19 g, 75%) as a colorless
solid.
Oxidative Cleavage of Benzo[c]furan 6c: Benzo[c]furan 6c (0.35 g,
1.17 mmol) was treated with m-CPBA (0.32 g, 1.85 mmol) followed
by the usual workup and purification by column chromatography
to give diketone 5c (0.35 g, 94%) as a colorless solid.
Oxidative Cleavage of Benzo[c]selenophene 7b: Benzo[c]selenophene
7b (0.32 g, 1.34 mmol) was treated with m-CPBA (0.30 g,
1.33 mmol) in DCM to give diketone 5c (0.22 g, 78%) as a colorless
solid.
Oxidative Cleavage of Benzo[c]furan 6d: Benzo[c]furan 6d (0.4 g,
1.33 mmol) was treated with m-CPBA (0.36 g, 2.1 mmol) followed
by the usual workup and purification by column chromatography
to give diketone 5d (0.39 g, 93%) as a colorless solid.
9a: Benzo[c]furan 8a (0.50 g, 1.34 mmol) was treated with m-CPBA
(0.45 g, 2.02 mmol, 1.5 equiv.) in DCM to give diketone 9a (0.46 g,
Eur. J. Org. Chem. 2012, 3647–3657
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3651

M. Nandakumar, R. Sivasakthikumaran, A. K. Mohanakrishnan
FULL PAPER
88%) as a brown solid; m.p. 140–142 °C. 1H NMR (300 MHz,
CDCl3): δ = 7.67–7.64 (m, 1 H, Ar), 7.56–7.50 (m, 5 H, Ar), 7.30–
7.18 (m, 3 H, Ar), 7.1 (d, J = 8.1 Hz, 2 H, Ar), 7.03 (d, J = 3.9 Hz,
10 H, CH2CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.7,
195.4, 163.6, 152.1, 150.8, 146.0, 140.0, 139.8, 135.8, 132.3, 130.3,
130.1, 129.8, 129.7, 129.2, 128.4, 127.0, 124.0, 123.1, 120.7, 119.2,
1 H, Ar), 6.98–6.94 (m, 1 H, Ar), 2.31 (s, 3 H, CH3) ppm. 13C 113.6, 55.4, 55.2, 40.1, 31.5, 29.7, 23.7, 22.6, 14.0 ppm. C40H44O3
NMR (75 MHz, CDCl3): δ = 196.2, 187.9, 146.7, 144.0, 142.1,
(572.79): calcd. C 83.88, H 7.74; found C 84.19, H 7.91. For the
139.8, 139.4, 136.2, 135.9, 134.7, 130.4, 130.3, 130.0, 129.6, 129.0,
single-crystal X-ray structure analysis of 11b, all of the calculations
128.9, 128.2, 126.6, 125.8, 124.1, 21.7 ppm. Dept-135 (75 MHz, were performed with the SHELXL-97 program.[23] Crystal data of
CDCl3): δ = 135.9, 130.4, 130.3, 130.0, 129.6, 129.0, 128.9, 128.2,
126.6, 125.8, 124.1, 21.7 ppm. HRMS (EI): calcd. for C23H16O2S2
388.0592; found 388.0587.
11b: C40H44O3, M = 572.75 gmol–1, monoclinic crystal system,
space group P 21/n, Z = 4, a = 16.7593(10) Å, b = 11.4989(6) Å, c
= 17.1460(5) Å, α = γ = 90°, β = 90.449(2)°, V = 3304.2(3) Å3, and
Dcalcd. = 1.151 Mgm–3. In total, 5850 independent reflections were
collected, of which 3875 were considered as observed [I Ͼ 2σ(I)].
The structure was solved by direct methods and refined by full-
matrix least-squares procedures to a final R value of 4.96%.
9b: Benzo[c]furan 8b (0.50 g, 1.34 mmol) was treated with m-CPBA
(0.45 g, 2.02 mmol) in DCM to give diketone 9b (0.35 g, 85%) as
1
a thick, brown liquid. H NMR (300 MHz, CDCl3): δ = 8.27–8.26
(m, 1 H, Ar), 7.84 (d, J = 8.1 Hz, 1 H, Ar), 7.76–7.65 (m, 2 H,
Ar), 7.57–7.48 (m, 3 H, Ar), 7.44 (d, J = 6.6 Hz, 1 H, Ar), 7.33– CCDC-867083 (for 11b) contains the supplementary crystallo-
7.27 (m, 3 H, Ar), 7.20–7.15 (m, 2 H, Ar), 7.04 (d, J = 3.3 Hz, 1 graphic data for this paper. These data can be obtained free of
H, Ar), 6.94–6.91 (m, 2 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): charge from The Cambridge Crystallographic Data Centre via
δ = 197.6, 188.2, 146.8, 142.5, 140.3, 136.1, 135.8, 135.2, 133.6,
133.1, 131.5, 130.8, 130.7, 130.5, 128.6, 128.2, 128.2, 127.7, 126.7,
126.5, 124.8, 125.6, 124.2, 124.0 ppm. Dept-135 (75 MHz, CDCl3):
δ = 135.8, 133.1, 131.5, 130.8, 130.7, 130.5, 128.6, 128.2, 128.2,
127.7, 126.7, 126.5, 125.8, 125.6, 124.2, 124.0 ppm. C26H16O2S2
(424.53): calcd. C 73.56, H 3.80, S 15.11; found C 73.83, H 3.71, S
15.02.
www.ccdc.cam.ac.uk/data_request/cif.
11c: The ring-opening reaction of 9,9-dihexyl-9H-fluoren-2-yl lact-
one[5c] with freshly prepared 1-naphthylmagnesium bromide fol-
lowed by acidic workup gave benzo[c]furan 10c as a fluorescent,
thick, yellow liquid. Benzo[c]furan 10c (0.53 g, 0.92 mmol) was
treated with m-CPBA (0.31 g, 1.39 mmol, 1.5 equiv.) in DCM to
give diketone 11c (0.44 g, 80%) as a thick, yellow liquid. 1H NMR
(300 MHz, CDCl3): δ = 8.37–8.34 (m, 1 H, Ar), 7.82 (d, J = 8.1 Hz,
1 H, Ar), 7.74–7.48 (m, 10 H, Ar), 7.40–7.37 (m, 2 H, Ar), 7.30–
7.23 (m, 4 H, Ar), 1.84–1.78 (m, 4 H, CH2), 1.02–0.91 (m, 12 H,
CH2), 0.67–0.46 (m, 10 H, CH2CH3) ppm. 13C NMR (75 MHz,
CDCl3): δ = 197.6, 196.8, 152.1, 150.9, 146.0, 141.5, 140.9, 139.7,
9c: The required benzo[c]furan 8c was prepared by adopting a pro-
cedure similar to that for 8b. Benzo[c]furan 8c (0.3 g, 0.82 mmol)
was treated with m-CPBA (0.27 g, 1.24 mmol) to afford diketone
9c (0.27 g, 86%) as a brown solid; m.p. 146–147 °C. 1H NMR
(300 MHz, CDCl3): δ = 7.68–7.66 (m, 2 H, Ar), 7.60–7.55 (m, 3 H,
Ar), 7.40 (d, J = 3.9 Hz, 1 H, Ar), 7.28–7.22 (m, 3 H, Ar), 7.05– 135.6, 135.3, 133.6, 132.7, 131.0, 131.0, 130.8, 130.4, 130.0, 129.3,
6.99 (m, 3 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 188.3,
187.8, 147.0, 144.1, 142.0, 139.3, 139.1, 136.2, 136.1, 135.2, 135.0,
130.7, 130.6, 129.2, 129.1, 128.3, 128.0, 126.7, 125.9, 124.2 ppm.
129.1, 128.4, 128.1, 127.7, 127.0, 126.5, 125.9, 123.9, 123.8, 123.1,
120.7, 119.2, 55.2, 40.0, 31.5, 29.6, 23.7, 22.6, 14.0 ppm. Dept-135
(75 MHz, CDCl3): δ = 132.7, 131.0, 130.8, 130.5, 130.0, 129.9,
Dept-135 (75 MHz, CDCl3): δ = 136.1, 135.2, 135.0, 130.7, 130.6, 128.4, 128.1, 127.8, 127.0, 126.5, 125.9, 123.9, 123.8, 123.1, 120.7,
129.3, 129.1, 128.3, 128.0, 126.8, 125.9, 124.2 ppm. C20H12O2S3
(380.49): calcd. C 63.13, H 3.18, S 25.28; found C 63.26, H 3.13, S
25.35.
119.2, 40.0, 31.5, 29.6, 23.7, 22.6, 14.0 ppm. C43H44O2 (592.82):
calcd. C 87.12, H 7.48; found C 87.51, H 7.76.
11d: The ring-opening reaction of 9,9-dihexyl-9H-fluoren-2-yl lact-
one[5c] with freshly prepared 2-thienylmagnesium bromide followed
by acidic workup gave benzo[c]furan 10d as a fluorescent, thick,
orange liquid. Benzo[c]furan 10d (0.45 g, 0.81 mmol) was treated
with m-CPBA (0.21 g, 1.21 mmol, 1.5 equiv.) in DCM to give di-
ketone 11d (0.36 g, 78%) as a thick, yellow liquid. 1H NMR
(300 MHz, CDCl3): δ = 7.66–7.51 (m, 9 H, Ar), 7.38 (dd, 1J =
1.2 Hz, 2J = 3.9 Hz, 1 H, Ar), 7.28–7.26 (br. s, 3 H, Ar), 6.98–6.95
(m, 1 H, Ar), 1.88–1.83 (m, 4 H, CH2), 1.05–0.95 (m, 12 H, CH2),
11a: The ring-opening reaction of 9,9-dihexyl-9H-fluoren-2-yl lact-
one[5c] with freshly prepared p-tolylmagnesium bromide followed
by acidic workup gave benzo[c]furan 10a as a thick, orange liquid.
The oxidative ring-opening reaction of benzo[c]furan 10a (0.5 g,
0.93 mmol) with m-CPBA (0.31 g, 1.39 mmol) gave diketone 11a
(0.43 g, 83%) as a pale yellow solid; m.p. 90 °C. IR (KBr): ν =
˜
2923, 1654, 1599, 1456, 1292 cm–1. 1H NMR (300 MHz, CDCl3):
δ = 7.64–7.53 (m, 10 H, Ar), 7.26 (br. s, 3 H, Ar), 7.06 (d, J =
7.5 Hz, 2 H, Ar), 2.27 (s, 3 H, CH3), 1.87–1.82 (m, 4 H, CH2), 0.71–0.47 (m, 10 H, CH2CH3) ppm. 13C NMR (75 MHz, CDCl3):
1.04–0.95 (m, 12 H, CH2), 0.70–0.44 (m, 10 H, CH2CH3) ppm. 13
C
δ = 196.5, 188.4, 152.1, 150.8, 146.1, 144.3, 139.86, 139.81, 139.72,
135.8, 135.1, 134.7, 130.54, 130.49, 130.04, 129.9, 129.0, 128.5,
127.9, 127.0, 123.9, 123.1, 120.8, 119.2, 55.3, 40.1, 31.5, 29.6, 23.7,
22.6, 14.0 ppm. Dept-135 (75 MHz, CDCl3): δ = 135.1, 134.7,
NMR (75 MHz, CDCl3): δ = 196.5, 196.3, 152.1, 150.8, 146.0,
143.8, 140.4, 140.2, 139.8, 135.8, 134.8, 130.3, 130.1, 129.7, 129.4,
129.0, 128.4, 127.0, 124.0, 123.1, 120.7, 119.2, 55.2, 40.1, 31.5, 29.6,
23.7, 22.6, 21.6, 14.0 ppm. Dept-135 (75 MHz, CDCl3): δ = 130.3, 130.54, 130.49, 130.04, 129.9, 129.0, 128.5, 127.9, 127.0, 123.9,
130.1, 130.0, 129.7, 129.4, 129.0, 128.4, 127.0, 124.0, 123.1, 120.7,
119.2, 40.1, 31.5, 29.6, 23.7, 22.6, 21.6, 14.0 ppm. C40H44O2
(556.79): calcd. C 86.29, H 7.97; found C 86.61, H 7.83.
123.1, 120.8, 119.2, 40.1, 31.5, 29.6, 23.7, 22.6, 14.0 ppm.
C37H40O2S (548.78): calcd. C 80.98, H 7.35, S 5.84; found C 81.26,
H 7.48, S 5.71.
11b: The ring-opening reaction of 9,9-dihexyl-9H-fluoren-2-yl lact-
one[5c] with freshly prepared p-anisylmagnesium bromide followed
by acidic workup gave benzo[c]furan 10b. The oxidative ring-open-
ing reaction of benzo[c]furan 10b with m-CPBA (1.5 equiv.) gave
13: Benzo[c]furan 12 (0.4 g, 0.98 mmol) was treated with m-CPBA
(0.33 g, 1.47 mmol) in DCM to give diketone 13 (0.39 g, 94%) as
a pale yellow solid; m.p. 65 °C. H NMR (300 MHz, CDCl3): δ =
8.43 (d, J = 9.3 Hz, 1 H, Ar), 8.02–7.79 (m, 8 H, Ar), 7.56–7.37
(m, 6 H, Ar), 6.86 (d, J = 8.1 Hz, 2 H, Ar), 2.10 (s, 3 H, CH3) ppm.
13C NMR (75 MHz, CDCl3): δ = 1.97, 196.7, 143.8, 141.2, 140.8,
134.7, 133.7, 131.6, 131.2, 130.8, 130.34, 130.29, 129.8, 129.5,
129.0, 128.9, 128.5, 126.9, 126.2, 126.1, 126.0, 124.6, 124.4, 123.9,
1
1
diketone 11b (0.46 g, 81%) as a pale yellow solid; m.p. 130 °C. H
NMR (300 MHz, CDCl3): δ = 7.65–7.54 (m, 10 H, Ar), 7.28 (br.
s, 3 H, Ar), 6.76 (d, J = 8.4 Hz, 2 H, Ar), 3.75 (s, 3 H, OCH3),
1.88–1.83 (m, 4 H, CH2), 1.05–0.95 (m, 12 H, CH2), 0.71–0.48 (m,
3652
www.eurjoc.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 3647–3657

Preparation of 1,2-Diaroylbenzenes
123.4, 21.4 ppm. DEPT-135 (75 MHz, CDCl3): δ = 130.2, 129.8, 19b: The known benzo[c]thiophene 18b[5a] (0.5 g, 1.02 mmol) was
128.9, 128.5, 128.0, 127.9, 127.6, 125.9, 125.2, 125.1, 125.0, 123.7,
122.4, 20.5 ppm. C31H20O2 (424.50): calcd. C 87.71, H 4.75; found
C 87.94, H 4.61.
treated with m-CPBA (0.34 g, 1.53 mmol) to give diketone 19b
(0.33 g, 65%) as a pale yellow solid; m.p. 173–174 °C. 1H NMR
(300 MHz, CDCl3): δ = 8.37 (s, 1 H, Ar), 7.92 (d, J = 7.8 Hz, 1 H,
Ar), 7.78 (d, J = 8.4 Hz, 1 H, Ar), 7.60–7.58 (m, 3 H, Ar), 7.51–
7.49 (m, 3 H, Ar), 7.36–7.27 (m, 2 H, Ar), 7.21 (d, J = 8.7 Hz, 1
H, Ar), 7.15–7.13 (m, 1 H, Ar), 6.69 (d, J = 8.7 Hz, 2 H, Ar), 4.15
(t, J = 7.2 Hz, 2 H, NCH2), 3.66 (s, 3 H, OCH3), 1.75–1.71 (m, 2
H, CH2), 1.26–1.17 (m, 6 H, CH2), 0.75 (t, J = 6.6 Hz, 3 H,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.3, 195.5, 163.5,
143.3, 141.1, 140.9, 140.5, 132.2, 130.4, 129.9, 129.8, 129.6, 129.3,
128.6, 128.1, 126.4, 123.8, 123.2, 122.5, 120.7, 119.9, 113.5, 109.2,
108.3, 55.4, 43.3, 31.5, 28.9, 26.9, 22.5, 14.0 ppm. DEPT-135
(75 MHz, CDCl3): δ = 132.2, 129.9, 129.8, 129.6, 129.3, 128.1,
126.4, 123.8, 120.9, 119.9, 113.5, 109.2, 108.3, 55.4, 43.3, 31.5, 28.8,
26.9, 22.5, 14.0 ppm. C33H31NO3 (489.61): calcd. C 80.95, H 6.38,
N 2.86; found C 80.72, H 6.46, N 2.98.
19c: The known benzo[c]thiophene 18c[5a] (0.5 g, 1.09 mmol) was
treated with m-CPBA (0.37 g, 1.63 mmol) to give diketone 19c
(0.32 g, 64%) as a thick, yellow liquid. 1H NMR (300 MHz,
CDCl3): δ = 8.47 (s, 1 H, Ar), 8.02 (d, J = 7.8 Hz, 1 H, Ar), 7.87
(dd, J = 1.5 Hz, J = 8.7 Hz, 1 H, Ar), 7.72–7.69 (m, 3 H, Ar), 7.67–
7.59 (m, 3 H, Ar), 7.49–7.39 (m, 3 H, Ar), 7.37–7.23 (m, 4 H, Ar),
4.25 (t, J = 7.2 Hz, 2 H, NCH2), 1.85–1.81 (m, 2 H, CH2), 1.42–
1.26 (m, 6 H, CH2), 0.85 (t, J = 6.9 Hz, 3 H, CH3) ppm. 13C NMR
(75 MHz, CDCl3): δ = 196.9, 196.2, 143.3, 141.1, 140.1, 137.4,
132.9, 130.3, 129.9, 129.86, 129.7, 129.6, 128.6, 128.3, 128.1, 126.4,
123.8, 123.2, 122.5, 120.7, 119.9, 109.3, 108.3, 43.3, 31.5, 28.9, 26.9,
22.5, 14.0 ppm. DEPT-135 (75 MHz, CDCl3): δ = 132.8, 130.3,
129.9, 129.8, 129.7, 129.6, 128.3, 128.1, 126.4, 123.8, 120.7, 119.9,
109.3, 108.4, 43.3, 31.6, 28.9, 26.9, 22.5, 14.0 ppm. HRMS (EI):
calcd. for C32H29NO2 459.2198; found 459.2206.
15: Benzo[c]furan 14 (0.4 g, 1.07 mmol) was treated with m-CPBA
(0.36 g, 1.60 mmol) in DCM to give diketone 15 (0.38 g, 91%) as
1
a thick, yellow liquid. H NMR (300 MHz, CDCl3): δ = 7.54–7.47
(m, 8 H, Ar), 7.20–7.03 (m, 9 H, Ar), 3.88 (s, 2 H, CH2), 2.25 (s,
3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.2, 146.6,
143.7, 140.1, 139.9, 135.2, 134.5, 130.1, 130.0, 129.9, 129.43,
129.40, 128.9, 128.83, 128.79, 128.5, 126.3, 41.8, 21.6 ppm. DEPT-
135 (75 MHz, CDCl3): δ = 130.1, 130.0, 129.9, 129.4, 128.9, 128.83,
128.79, 128.5, 126.3, 41.8, 21.6 ppm. C28H22O2 (390.48): calcd. C
86.13, H 5.68; found C 86.47, H 5.49.
17a: Benzo[c]furan 16a (0.4 g, 0.89 mmol) was treated with m-
CPBA (0.30 g, 1.33 mmol) in DCM to give diketone 17a (0.33 g,
80%) as a thick, red liquid. 1H NMR (300 MHz, CDCl3): δ = 8.25
(s, 1 H, Ar) 7.84–7.76 (m, 2 H, Ar), 7.70–7.67 (m, 1 H, Ar), 7.61–
7.53 (m, 3 H, Ar), 7.49–7.36 (m, 3 H, Ar), 7.30–7.23 (m, 2 H, Ar),
6.66 (d, J = 3.6 Hz, 1 H), 2.65 (t, J = 7.5 Hz, 2 H, CH2), 1.51–1.44
(m, 2 H, CH2), 1.26–1.09 (m, 6 H, CH2), 0.78 (t, J = 6.4 Hz, 3
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.1, 187.9, 158.9,
157.5, 156.8, 141.5, 140.0, 139.8, 135.7, 132.6, 130.5, 130.3, 129.5,
129.1, 127.9, 125.6, 124.4, 123.7, 123.3, 123.2, 121.1, 111.9, 111.5,
31.4, 31.2, 30.7, 28.7, 22.5, 14.1 ppm. DEPT-135 (75 MHz, CDCl3):
δ = 135.7, 130.5, 130.3, 129.6, 129.1, 127.9, 125.6, 123.3, 123.2,
121.1, 111.9, 111.5, 31.4, 31.2, 30.7, 28.7, 22.5, 14.1 ppm.
C30H26O3S (466.59): calcd. C 77.22, H 5.62, S 6.87; found C 77.41,
H 5.48, S 6.96.
17b: The ring-opening reaction of dibenzofuranyl lactone[5a] with
freshly prepared 1-naphthylmagnesium bromide followed by acidic
workup gave benzo[c]furan 16b as a thick, yellow liquid. Benzo[c]-
furan 16b (0.4 g, 0.97 mmol) was treated with m-CPBA (0.33 g,
1.46 mmol) in DCM to give diketone 17b (0.35 g, 83%) as a thick,
yellow liquid. 1H NMR (300 MHz, CDCl3): δ = 8.07–8.02 (m, 2
H, Ar), 7.74–7.66 (m, 2 H, Ar), 7.61–7.37 (m, 8 H, Ar), 7.30–7.06
(m, 6 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 197.4, 196.3,
158.8, 156.8, 141.2, 140.6, 135.2, 133.5, 132.9, 131.6, 130.9, 130.8,
130.5, 130.3, 129.15, 129.12, 128.1, 127.9, 127.4, 126.4, 125.5, 124.5
124.0, 123.6, 123.3, 122.7, 121.0, 111.8, 111.4 ppm. DEPT-135
(75 MHz, CDCl3): δ = 132.9, 131.6, 130.9, 130.6, 130.3, 129.15,
129.12, 128.1, 127.9, 127.4, 126.4, 125.5, 124.0, 123.3, 122.7, 121.0,
111.8, 111.4 ppm. HRMS (EI): calcd. for C30H18O3 426.1256;
found 426.1252.
Oxidative Ring-Opening Reaction of Benzo[c]furan 20a: Benzo[c]-
furan 20a (0.5 g, 1.09 mmol) was treated with m-CPBA (0.37 g,
1.64 mmol) to give diketone 19a (0.42 g, 82%) as a thick, yellow
liquid.
Oxidative Ring-Opening Reaction of Benzo[c]furan 20b: The reac-
tion of N-hexylcarbazole lactone[5a] with p-anisylmagnesium brom-
ide followed by acidic workup, as mentioned above, gave benzo[c]-
furan 20b. The oxidative ring-opening reaction of benzo[c]furan
20b (0.5 g, 1.06 mmol) with m-CPBA (0.36 g, 1.58 mmol) afforded
diketone 19b (0.42 g, 82%) as a thick, yellow liquid.
Oxidative Ring-Opening Reaction of Benzo[c]furan 20c: The reac-
tion of N-hexylcarbazole lactone[5a] with phenylmagnesium brom-
ide followed by acidic workup, as mentioned above, gave benzo-
[c]furan 20c. The oxidative ring-opening reaction of benzo[c]furan
20c (0.5 g, 1.13 mmol) with m-CPBA (0.38 g, 1.69 mmol) gave dike-
tone 19c (0.41 g, 80%) as a thick, yellow liquid.
19a: The known benzo[c]thiophene 18a[5a] (0.5 g, 1.06 mmol) was
treated with m-CPBA (0.36 g, 1.58 mmol) to give diketone 19a
(0.347 g, 67%) as a thick, yellow liquid. 1H NMR (300 MHz,
CDCl3): δ = 8.38 (s, 1 H, Ar), 7.94 (d, J = 7.8 Hz, 1 H, Ar), 7.79
(d, J = 8.7 Hz, 1 H, Ar), 7.63–7.52 (m, 6 H, Ar), 7.37 (d, J =
19d: The reaction of N-hexylcarbazole lactone[5a] with 1-naphth-
ylmagnesium bromide followed by acidic workup, as mentioned
7.5 Hz, 1 H, Ar), 7.31 (d, J = 8.1 Hz, 1 H, Ar), 7.23 (d, J = 8.7 Hz, above, gave benzo[c]furan 20d. Benzo[c]furan 20d (0.45 g,
1 H, Ar), 7.15 (t, J = 7.4 Hz, 1 H, Ar), 7.03 (d, J = 8.1 Hz, 2 H, 0.91 mmol) was treated with m-CPBA (0.31 g, 1.37 mmol) to give
Ar), 4.18 (t, J = 7.2 Hz, 2 H, NCH2), 2.24 (s, 3 H, ArCH3), 1.78– diketone 19d (0.38 g, 81%) as a pale yellow solid; m.p. 136–138 °C.
1.71 (m, 2 H, CH2), 1.22–1.20 (m, 6 H, CH2), 0.77 (t, J = 6.8 Hz,
3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.5, 196.2,
143.7, 143.2, 141.0, 140.9, 140.2, 134.8, 130.0, 129.9, 129.8, 129.5,
129.4, 128.9, 128.5, 128.0, 126.3, 123.7, 123.1, 122.4, 120.7, 119.8,
109.1, 108.2, 43.2, 31.46, 28.8, 26.8, 22.4, 21.6, 13.9 ppm. DEPT-
1H NMR (300 MHz, CDCl3): δ = 8.25 (s, 1 H, Ar), 8.1 (d, J =
8.7 Hz, 1 H, Ar), 7.98 (d, J = 7.5 Hz, 1 H, Ar), 7.86 (d, J = 7.8 Hz,
1 H, Ar), 7.77 (d, J = 7.5 Hz, 1 H, Ar), 7.69–7.56 (m, 6 H, Ar),
7.50–7.45 (m, 1 H, Ar), 7.40–7.31 (m, 2 H, Ar), 7.27–7.22 (m, 2 H,
Ar), 7.15–7.08 (m, 2 H, Ar), 4.21 (t, J = 7.0 Hz, 2 H, NCH2), 1.84–
135 (75 MHz, CDCl3): δ = 130.0, 129.9, 129.8, 129.5, 129.4, 128.9, 1.78 (m, 2 H, CH2), 1.42–1.26 (m, 6 H, CH2), 0.89–0.85 (m, 3 H,
128.0, 126.3, 123.7, 120.6, 119.8, 109.1, 108.2, 43.2, 31.4, 28.8, 26.8,
22.4, 21.6, 13.9 ppm. C33H31NO2 (473.61): calcd. C 83.69, H 6.60,
N 2.96; found C 83.54, H 6.56, N 3.03.
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 197.7, 196.5, 143.2,
141.8, 141.0, 140.7, 135.4, 133.6, 133.5, 132.7, 131.4, 130.8, 130.6,
130.5, 130.2, 129.9, 129.2, 129.0, 127.9, 127.5, 127.1, 126.3, 136.1,
Eur. J. Org. Chem. 2012, 3647–3657
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3653

M. Nandakumar, R. Sivasakthikumaran, A. K. Mohanakrishnan
FULL PAPER
125.5, 124.0, 123.3, 123.1, 122.5, 120.7, 119.9, 109.1, 108.2, 43.3,
31.5, 29.0, 26.9, 22.5, 14.0 ppm. DEPT-135 (75 MHz, CDCl3): δ =
132.7, 131.4, 130.6, 130.56, 129.9, 129.2, 127.9, 127.5, 127.1, 126.4,
126.1, 125.5, 124.0, 123.3, 120.7, 119.9, 109.1, 108.2, 43.3, 31.5,
77%) as a pale yellow solid; m.p. 140–141 °C. IR (KBr): ν = 1646,
˜
1582, 1489, 1278 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.65–7.40
(m, 8 H, Ar), 7.26–7.19 (m, 5 H, Ar), 7.07–7.00 (m, 6 H, Ar), 6.82
(d, J = 8.7 Hz, 2 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ =
29.0, 26.9, 22.5, 14.0 ppm. HRMS (EI): calcd. for C36H31NO2 194.9, 188.5, 152.2, 146.4, 144.3, 140.3, 139.6, 135.1, 134.7, 131.6,
509.2355; found 509.2353.
130.4, 130.0, 129.65, 129.6, 129.4, 129.0, 128.0, 126.0, 124.7,
119.4 ppm. DEPT-135 (75 MHz, CDCl3): δ = 135.1, 134.7, 131.6,
130.4, 130.0, 129.6, 129.5, 129.0, 128.0, 126.0, 124.7, 119.4 ppm.
HRMS (EI): calcd. for C30H21NO2S 459.1293; found 459.1283.
19e: Benzo[c]furan 20e (0.50 g, 1.11 mmol) was treated with m-
CPBA (0.37 g, 1.67 mmol) to give diketone 19e (0.40 g, 78%) as a
thick, yellow liquid. 1H NMR (300 MHz, CDCl3): δ = 8.37 (s, 1
H, Ar), 7.88 (d, J = 7.8 Hz, 1 H, Ar), 7.78 (d, J = 8.7 Hz, 1 H,
Ar), 7.64–7.55 (m, 2 H, Ar), 7.51–7.47 (m, 2 H, Ar), 7.41–7.39 (m,
2 H, Ar), 7.33–7.24 (m, 2 H, Ar), 7.18 (d, J = 8.7 Hz, 1 H, Ar),
7.10 (t, J = 7.2 Hz, 1 H, Ar), 6.87 (t, J = 4.3 Hz, 1 H, Ar), 4.10 (t,
J = 6.9 Hz, 2 H, NCH2), 1.72–7.65 (m, 2 H, CH2), 1.16–1.10 (m,
6 H, CH2), 0.72 (t, J = 6.9 Hz, 3 H, CH3) ppm. 13C NMR
24b: The ring-opening reaction of triphenylamino lactone[5a] with
freshly prepared 5-hexyl-2-thienylmagnesium bromide followed by
acidic workup gave benzo[c]furan 23b as a thick, orange liquid.
Benzo[c]furan 23b (0.40 g, 0.76 mmol) was treated with m-CPBA
(0.26 g, 1.14 mmol) to give diketone 24b (0.32 g, 79%) as a pale
yellow liquid. 1H NMR (300 MHz, CDCl3): δ = 7.56–7.53 (m, 1
(75 MHz, CDCl3): δ = 195.9, 188.4, 144.05, 143.1, 140.9, 140.5, H, Ar),7.52–7.43 (m, 5 H, Ar), 7.26–7.19 (m, 5 H, Ar), 7.07–7.05
139.6, 134.8, 134.4, 130.3, 129.8, 129.6, 128.9, 128.3, 127.9, 127.8,
126.2, 123.6, 122.9, 120.5, 119.8, 109.1, 108.2, 43.1, 31.4, 28.7, 26.7,
(m, 6 H, Ar), 6.82 (d, J = 8.7 Hz, 2 H, Ar), 6.68 (d, J = 3.4 Hz, 1
H, Ar), 2.74 (t, J = 7.5 Hz, 2 H, CH2), 1.64–1.54 (m, 2 H, CH2),
22.4, 13.9 ppm. DEPT-135 (75 MHz, CDCl3): δ = 134.8, 134.4, 1.30–1.20 (m, 6 H, CH2), 0.80 (t, J = 6.3 Hz, 3 H, CH3) ppm. 13C
130.3, 129.8, 129.6, 128.9, 127.9, 127.8, 126.2, 123.6, 120.5, 119.8,
109.1, 108.2, 43.1, 31.4, 28.7, 26.7, 22.4, 13.9 ppm. C30H27NO2S
(465.61): calcd. C 77.39, H 5.84, N 3.01, S 6.89; found C 77.15, H
5.97, N 3.19, S 6.74.
NMR (75 MHz, CDCl3): δ = 195.0, 188.1, 157.1, 152.1, 146.4,
141.7, 140.2, 139.6, 135.7, 131.5, 130.2, 129.9, 129.8, 129.5, 129.3,
128.9, 126.0, 125.6, 124.6, 119.5, 31.4, 31.3, 30.7, 28.7, 22.5,
14.0 ppm. DEPT-135 (75 MHz, CDCl3): δ = 135.7, 131.4, 130.2,
129.9, 129.5, 129.3, 128.9, 125.9, 125.5, 124.6, 119.5, 31.4, 31.3,
30.7, 28.7, 22.5, 14.0 ppm. C36H33NO2S (543.72): calcd. C 79.52,
H 6.12, N 2.58, S 5.90; found C 79.36, H 6.17, N 2.63, S 5.84.
22a: The oxidative ring-opening reaction of benzo[c]furan 21a
(0.5 g, 1.06 mmol) with m-CPBA (0.36 g, 1.59 mmol) gave diketone
22a (0.43 g, 83%) as a thick, yellow liquid. 1H NMR (300 MHz,
CDCl3): δ = 8.38 (s, 1 H), 7.96 (d, J = 7.8 Hz, 1 H, Ar), 7.79 (d, J
= 8.7 Hz, 1 H, Ar), 7.63–7.56 (m, 6 H, Ar), 7.43–7.31 (m, 3 H, Ar),
7.27–7.15 (m, 4 H, Ar), 4.09 (d, J = 7.2 Hz, 2 H, NCH2), 2.01–
24c: The ring-opening reaction of triphenylamino lactone[5a] with
freshly prepared phenylmagnesium bromide followed by acidic
workup gave benzo[c]furan 23c as a thick, yellow liquid. Benzo-
1.91 (m, 1 H, CH), 1.33–1.18 (m, 8 H, CH2), 0.85–0.76 (m, 6 H, [c]furan 23c (0.52 g, 1.19 mmol) was treated with m-CPBA (0.40 g,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.9, 196.2, 143.8,
141.6, 141.1, 140.0, 137.4, 132.8, 130.3, 129.9, 129.8, 129.7, 129.6,
128.6, 128.2, 128.1, 126.4, 123.7, 123.1, 122.5, 120.7, 119.9, 109.5,
108.6, 47.6, 39.4, 29.7, 28.8, 24.4, 23.0, 14.0, 10.9 ppm. DEPT-135
1.78 mmol) to give diketone 24c (0.44 g, 81%) as a pale yellow
solid; m.p. 174–175 °C. H NMR (300 MHz, CDCl3): δ = 7.66 (d,
1
J = 7.2 Hz, 2 H, Ar), 7.55–7.45 (m, 7 H, Ar), 7.35–7.30 (m, 2 H,
Ar), 7.26–7.19 (m, 5 H, Ar), 7.07–7.04 (m, 5 H, Ar), 6.82 (d, J =
(75 MHz, CDCl3): δ = 132.8, 130.3, 129.9, 129.8, 129.7, 129.6, 8.7 Hz, 2 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.8,
128.2, 128.1, 126.4, 123.7, 120.7, 119.9, 109.5, 108.6, 47.6, 39.4,
29.7, 28.8, 24.4, 23.0, 14.0, 10.9 ppm. C34H33NO2 (487.64): calcd.
C 83.74, H 6.82, N 2.87; found C 83.98, H 6.75, N 2.94.
194.9, 152.2, 146.4, 140.7, 139.8, 137.3, 133.0, 131.6, 130.3, 129.9,
129.87, 129.63, 129.53, 129.38, 128.3, 126.1, 124.8, 119.4 ppm.
DEPT-135 (75 MHz, CDCl3): δ = 133.0, 131.6, 130.3, 129.94,
129.87, 129.63, 129.53, 129.38, 128.3, 126.1, 124.8, 119.4 ppm.
C32H23NO2 (453.54): calcd. C 84.74, H 5.11, N 3.09; found C
84.36, H 4.90, N 3.24. For the single-crystal X-ray structure analy-
sis of 24c, all of the calculations were performed with the
22b: Benzo[c]thiophene 21b (0.4 g, 0.51 mmol) was treated with m-
CPBA (0.17 g, 0.77 mmol) followed by workup to give diketone
22b (0.31 g, 78%) as a thick, yellow liquid. 1H NMR (300 MHz,
CDCl3): δ = 8.44 (s, 1 H, Ar), 8.30 (s, 1 H, Ar), 8.25 (s, 1 H, Ar),
8.10 (d, J = 7.5 Hz, 1 H, Ar), 7.83 (d, J = 8.7 Hz, 1 H, Ar), 7.76
(d, J = 8.4 Hz, 1 H, Ar), 7.70 (d, J = 8.4 Hz, 1 H, Ar), 7.66–7.64
(m, 1 H, Ar), 7.57–7.54 (m, 5 H, Ar) 7.41–7.31 (m, 4 H, Ar), 7.25
(d, J = 8.1 Hz, 1 H, Ar), 7.20–7.14 (m, 1 H, Ar), 7.04 (d, J =
8.1 Hz, 2 H, Ar), 4.10 (d, J = 6.9 Hz, 4 H, NCH2), 2.23 (s, 3 H,
CH3), 2.07–1.95 (m, 2 H, CH), 1.33–1.17 (m, 16 H, CH2), 0.89–
0.77 (m, 12 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.5,
196.2, 144.1, 143.7, 141.3, 141.0, 140.5, 140.3, 140.1, 134.8, 134.5,
132.7, 130.1, 130.0, 129.8, 129.6, 129.4, 128.9, 128.6, 128.0, 126.1,
125.6, 125.3, 123.8, 123.7, 123.3, 122.9, 122.6, 122.6, 120.3, 119.1,
118.81, 118.7, 109.7, 109.2, 109.1, 108.7, 47.7, 47.5, 39.4, 31.0,
30.96, 28.81, 28.78, 24.39, 24.34, 23.04, 23.0, 21.6, 14.0, 10.9,
10.87 ppm. DEPT-135 (75 MHz, CDCl3): δ = 130.15, 130.06,
129.9, 129.7, 129.5, 129.0, 128.1, 126.2, 125.7, 125.7, 125.4, 123.9,
120.4, 119.2, 118.9, 118.8, 109.7, 109.3, 109.1, 108.8, 47.7, 47.5,
39.48, 39.45, 31.05, 31.0, 28.9, 28.8, 24.44, 24.4, 23.1, 23.07, 21.68,
14.11, 11.0, 10.9 ppm. HRMS (EI): calcd. for C55H58N2O2
778.4498; found 778.4498.
SHELXL-97 program.[23] Crystal data of 24c: C32H23NO2, M =
–1
¯
453.51 gmol , triclinic crystal system, space group P1, Z = 4, a =
10.7599(3) Å, b = 13.0389(3) Å, c = 17.9453(5) Å, α = 90.447(2)°,
β = 98.415(2)°, γ = 108.904(2)°, V = 2352.13(11) Å3 and Dcalcd.
=
1.281 Mgm–3. In total, 8280 independent reflections were collected,
of which 6006 were considered as observed [I Ͼ 2σ(I)]. The struc-
ture was solved by direct methods and refined by full-matrix least-
squares procedures to a final R value of 3.64%.
CCDC-867364 (for 24c) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
24d: The ring-opening reaction of triphenylamino lactone[5a] with
freshly prepared p-anisylmagnesium bromide followed by acidic
workup gave benzo[c]furan 23d as an orange solid. The oxidative
ring-opening reaction of benzo[c]furan 23d (0.50 g, 1.07 mmol)
with m-CPBA (0.36 g, 1.61 mmol) gave diketone 24d (0.43 g, 83%)
as a pale yellow solid; m.p. 144–145 °C. 1H NMR (300 MHz,
CDCl3): δ = 7.64 (d, J = 8.7 Hz, 2 H, Ar), 7.52–7.44 (m, 6 H, Ar),
7.24–7.19 (m, 4 H, Ar), 7.05–7.02 (m, 6 H, Ar), 6.82–6.77 (m, 4 H,
24a: Benzo[c]furan 23a (0.50 g, 1.13 mmol) was treated with m-
CPBA (0.38 g, 1.69 mmol) in DCM to give diketone 24a (0.4 g,
3654
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 3647–3657

Preparation of 1,2-Diaroylbenzenes
Ar), 3.76 (s, 3 H, OCH3) ppm. 13C NMR (75 MHz, CDCl3): δ = C30H18O2S 442.1028; found 442.1024. Data for 28b: Thick, light
195.5, 195.0, 163.5, 152.1, 146.4, 140.4, 140.2, 132.3, 131.6, 130.3,
129.9, 129.8, 129.7, 129.6, 129.3, 129.2, 129.0, 124.7, 119.4, 113.6,
brown liquid (224 mg, 31%). 1H NMR (300 MHz, CDCl3): δ =
8.08 (d, J = 8.4 Hz, 1 H, Ar), 7.89 (d, J = 8.1 Hz, 1 H, Ar), 7.83
55.5 ppm. DEPT-135 (75 MHz, CDCl3): δ = 132.3, 131.6, 129.9, (s, 1 H, Ar), 7.73 (d, J = 7.5 Hz, 2 H, Ar), 7.68–7.55 (m, 6 H, Ar),
129.8, 129.6, 129.3, 129.2, 126.0, 124.7, 119.4, 113.6, 55.5 ppm.
HRMS (EI): calcd. for C33H25NO3 483.1834; found 483.1828.
7.52–7.44 (m, 4 H, Ar), 7.34 (t, J = 7.5 Hz, 2 H, Ar), 7.25 (d, J =
7.3 Hz, 1 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.9,
195.6, 142.3, 140.8, 140.1, 139.9, 137.8, 134.8, 134.0, 133.5, 133.3,
132.2, 131.3, 131.2, 131.0, 130.9, 130.8, 130.6, 129.0, 128.3, 128.1,
126.7, 125.0, 124.0, 122.2, 122.1, 122.0, 121.5 ppm. DEPT-135
(75 MHz, CDCl3): δ = 134.1, 133.3, 132.2, 131.3, 131.2, 131.0,
130.9, 130.8, 129.0, 128.3, 128.1, 126.7, 125.0, 124.0, 122.2, 122.1,
122.0, 121.5 ppm. HRMS (EI): calcd. for C30H18O4S 474.0926;
found 474.0883. Data for 28c: Thick, light brown liquid (126 mg,
Oxidative Cleavage of Benzo[c]furan 25: Benzo[c]furan 25 (0.60 g,
1.57 mmol) was treated with m-CPBA (0.70 g, 3.14 g) in DCM fol-
lowed by workup and purification by column chromatography (sil-
ica gel, 5% EA/hexane for 26a, 15% EA/hexane for 26b, 25% EA/
hexane for 26c) to give diketones 26a–26c. Data for 26a: Pale yel-
low solid (256 mg, 41%); m.p. 110–112 °C. 1H NMR (300 MHz,
CDCl3): δ = 8.43 (s, 1 H, Ar), 8.04–8.01 (m, 1 H, Ar), 7.79–7.71
(m, 5 H, Ar), 7.63–7.58 (m, 2 H, Ar), 7.54–7.52 (m, 1 H, Ar), 7.47–
7.45 (m, 1 H, Ar), 7.42–7.37 (m, 2 H, Ar), 7.00–6.98 (m, 1 H,
Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 196.2, 188.2, 144.7,
144.0, 139.9, 139.8, 139.6, 135.4, 135.1, 135.0, 134.8, 133.7, 130.6,
130.4, 129.7, 129.2, 128.0, 127.8, 127.7, 127.4, 125.8, 124.8, 122.6,
122.0 ppm. DEPT-135 (75 MHz, CDCl3): δ = 135.0, 134.9, 130.6,
130.4, 129.7, 129.2, 128.0, 127.7, 127.4, 124.8, 123.3, 122.8, 122.6,
122.0 ppm. HRMS (EI): calcd. for C24H14O2S2 398.0435; found
398.0440. Data for 26b: Pale brown solid (196 mg, 29%); m.p. 173–
1
18%). H NMR (300 MHz, CDCl3): δ = 8.12 (d, J = 8.1 Hz, 1 H,
Ar), 7.89–7.85 (m, 3 H, Ar), 7.79 (d, J = 8.1 Hz, 1 H, Ar), 7.71–
7.68 (m, 1 H, Ar), 7.66–7.57 (m, 5 H, Ar), 7.55–7.51 (m, 2 H, Ar),
7.50–7.41 (m, 2 H, Ar), 7.34–7.30 (m, 3 H, Ar) ppm. 13C NMR
(75 MHz, CDCl3): δ = 196.9, 195.9, 148.7, 145.1, 141.1, 140.2,
140.1, 137.4, 136.0, 134.8, 133.4, 133.1, 132.7, 132.0, 131.2, 130.8,
130.7, 130.6, 130.2, 130.0, 129.0, 128.1, 128.0, 127.4, 127.2, 126.5,
125.2, 124.0, 122.3, 121.8 ppm. DEPT-135 (75 MHz, CDCl3): δ =
133.2, 132.7, 132.1, 131.2, 130.9, 130.6, 130.3, 130.1, 129.1, 128.1,
128.0, 127.5, 127.3, 126.6, 125.3, 124.0, 122.4, 121.9 ppm.
175 °C. IR (KBr): ν = 1659, 1618, 1408, 1296, 1162 cm–1. 1H NMR
˜
30a:[24] 2,5-Diphenylfuran (29a, 0.50 g, 2.27 mmol) was treated
with m-CPBA (0.76 g, 3.41 mmol) to give cis-1,4-diphenylbut-2-
ene-1,4-dione[11] (30a, 0.49 g, 92%) as a pale yellow solid; m.p.
(300 MHz, CDCl3): δ = 8.11 (s, 1 H, Ar), 8.0–7.94 (m, 1 H, Ar),
7.76–7.69 (m, 4 H, Ar), 7.64–7.55 (m, 4 H, Ar), 7.50–7.48 (m, 2 H,
Ar), 7.07–7.01 (m, 1 H, Ar), 6.93–6.90 (m, 1 H, Ar) ppm. 13C NMR
(75 MHz, CDCl3): δ = 195.1, 187.7, 143.6, 141.9, 139.5, 138.9,
137.8, 135.5, 135.2, 134.2, 132.2, 131.7, 131.2, 131.1, 130.9, 129.9,
129.6, 129.5, 128.9, 128.2, 128.1, 126.0, 122.3, 122.1 ppm. DEPT-
135 (75 MHz, CDCl3): δ = 135.5, 135.2, 134.2, 131.7, 131.2, 131.1,
131.0, 129.7, 129.6, 129.5, 128.2, 126.0, 122.3, 122.1 ppm. HRMS
(EI): calcd. for C24H14O4S2 430.0334; found 430.0335. Data for
125 °C, ref.[24] m.p. 133.5–134.5 °C. IR (KBr): ν = 1649, 1588, 1444,
˜
1294, 1185 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.85 (d, J =
7.8 Hz, 4 H, Ar), 7.50–7.46 (m, 2 H, Ar), 7.39–7.34 (m, 4 H, Ar),
7.07 (s, 2 H, Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 192.5,
136.1, 135.6, 133.6, 128.8, 128.6 ppm. DEPT-135 (75 MHz,
CDCl3): δ = 135.6, 133.6, 128.8, 128.6 ppm.
26c: Pale brown solid (123 mg, 19%); m.p. 162 °C. IR (KBr): ν =
˜
30b: The oxidative ring cleavage of known 2,5-bis(thiophen-2-yl)
furan[25] (29b, 0.48 g, 2.07 mmol) using m-CPBA (0.535 g,
3.10 mmol) gave cis-1,4-bis(thiophen-2-yl)but-2-ene-1,4-dione (30b,
0.35 g, 68%) as a brown solid; m.p. 178–180 °C. 1H NMR
(300 MHz, CDCl3): δ = 7.87–7.85 (m, 4 H, Ar), 7.71 (d, J = 4.8 Hz,
2 H, Ar), 7.15 (t, J = 4.0 Hz, 2 H, Ar) ppm. 13C NMR (75 MHz,
CDCl3): δ = 181.4, 144.7, 135.8, 134.2, 133.6, 128.7 ppm. DEPT-
135 (75 MHz, CDCl3): δ = 135.8, 134.2, 133.6, 128.7 ppm.
1640, 1408, 1288, 1024 cm–1. 1H NMR (300 MHz, CDCl3): δ =
8.12 (s, 1 H, Ar), 7.94–7.89 (m, 2 H, Ar), 7.77–7.70 (m, 3 H, Ar),
7.66–7.55 (m, 5 H, Ar), 7.52–7.47 (m, 2 H, Ar), 7.06–7.03 (m, 1 H,
Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 195.5, 187.8, 149.0,
145.2, 143.7, 140.8, 139.6, 139.1, 137.5, 136.1, 135.3, 135.2, 132.8,
131.1, 131.0, 130.8, 130.1, 129.6, 129.4, 128.2, 127.6, 127.4, 122.6,
122.5 ppm. DEPT-135 (75 MHz, CDCl3): δ = 135.3, 135.2, 132.8,
131.1, 131.0, 130.8, 130.1, 129.7, 129.5, 128.2, 127.6, 127.4, 122.6,
122.5 ppm. HRMS (EI): calcd. for C24H14O3S2 414.0384; found
414.0380.
30c: 2-(4-Chlorophenyl)-5-phenylfuran (29c, 0.60 g, 2.36 mmol)
was treated with m-CPBA (0.61 g, 3.53 mmol) to give cis-1-(4-chlo-
rophenyl)-4-diphenylbut-2-ene-1,4-dione (30c, 0.57 g, 89%) as a
pale yellow solid; m.p. 84–85 °C, ref.[26] m.p. 83 °C. 1H NMR
(300 MHz, CDCl3): δ = 7.86 (d, J = 7.8 Hz, 2 H, Ar), 7.80 (d, J =
8.4 Hz, 2 H, Ar), 7.50 (t, J = 7.2 Hz, 1 H, Ar), 7.41–7.33 (m, 4 H,
Ar), 7.13 (d, J = 12 Hz, 1 H, Ar), 7.01 (d, J = 11.7 Hz, 1 H,
Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 191.9, 191.8, 140.0,
136.0, 135.9, 135.2, 134.4, 133.7, 130.0, 129.1, 128.8, 128.6 ppm.
DEPT-135 (75 MHz, CDCl3): δ = 135.9, 135.2, 133.7, 130.0, 129.1,
128.8, 128.7 ppm.
Oxidative Ring Opening of Benzo[c]furan 27: The ring-opening re-
action of dibenzothiophenyl lactone[5a] with freshly prepared 1-
naphthylmagnesium bromide followed by acidic workup gave
benzo[c]furan 27 as a thick, yellow liquid. Benzo[c]furan 27 (0.66 g,
1.55 mmol) was treated with m-CPBA (0.694 g, 3.09 mmol) in
DCM followed by purification by column chromatography (silica
gel, 5% EA/hexane for 28a, 15% EA/hexane for 28b, 25% EA/
hexane for 28c) to give diketones 28a–28c. Data for 28a: Pale yel-
low solid (296 mg, 44%); m.p. 88 °C. IR (KBr): ν = 1655, 1419,
˜
Oxidation of Benzo[c]furan 6d Using LTA: To a stirred suspension
of benzo[c]furan 6d (2.41 g, 8.03 mmol) in dry THF (40 mL) was
added lead tetraacetate (3.56 g, 8.03 mmol), and the reaction mix-
ture was stirred for 30 min at 50 °C. The reaction mixture was then
poured into water (200 mL), and the resulting mixture was ex-
tracted with ethyl acetate (2ϫ30 mL). The combined organic ex-
tracts were washed with a brine solution and dried with Na2SO4.
Removal of solvent in vacuo followed by crystallization from meth-
anol gave 5d (1.94 g, 76%) as a colorless solid.
1246, 1066 cm–1. 1H NMR (300 MHz, CDCl3): δ = 8.12 (s, 1 H,
Ar), 7.99 (d, J = 8.1 Hz, 1 H, Ar), 7.91–7.88 (m, 1 H, Ar), 7.78 (d,
J = 8.1 Hz, 1 H, Ar), 7.73–7.68 (m, 2 H, Ar), 7.64–7.52 (d, 5 H,
Ar), 7.48–7.43 (m, 2 H, Ar), 7.40–7.31 (m, 2 H, Ar), 7.28–7.23 (m,
1 H, Ar), 7.16–7.10 (m, 2 H, Ar) ppm. 13C NMR (CDCl3,
75 MHz): δ = 197.3, 196.6, 144.7, 140.9, 140.5, 139.4, 135.4, 135.1,
134.9, 134.1, 133.3, 132.9, 131.6, 130.8, 130.6, 130.4, 129.1, 127.9,
127.3, 127.2, 127.1, 126.2, 125.1, 124.7, 123.9, 122.7, 122.5, 122.4,
121.8 ppm. DEPT-135 (75 MHz, CDCl3): δ = 133.0, 131.7, 130.9,
130.7, 130.5, 129.2, 128.0, 127.4, 127.3, 127.2, 126.3, 125.2, 124.8,
Oxidation of Benzo[c]furan 6e Using LTA: Following a similar pro-
123.9, 122.8, 122.6, 122.5, 121.9 ppm. HRMS (EI): calcd. for cedure to that of 6d, the oxidation of benzo[c]furan 6e (0.75 g,
Eur. J. Org. Chem. 2012, 3647–3657
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3655

M. Nandakumar, R. Sivasakthikumaran, A. K. Mohanakrishnan
FULL PAPER
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R. M. Metzger, J. Org. Chem. 1998, 63, 3105–3112.
2.24 mmol) using LTA (0.99 g, 2.23 mmol) gave diketone 5e (0.65 g,
85%) as a colorless solid.
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Oxidation of Benzo[c]furan 6h Using LTA: Following a similar pro-
cedure to that of 6d, the oxidation of benzo[c]furan 6h (1.0 g,
3.62 mmol) using LTA (1.60 g, 3.62 mmol) gave diketone 5h (0.88 g,
84%) as a brown solid.
Oxidation of Benzo[c]furan 20a Using LTA: Following a similar
procedure to that of 6d, the oxidation of benzo[c]furan 20a (0.66 g,
1.44 mmol) using LTA (0.64 g, 1.44 mmol) gave diketone 19a
(0.26 g, 25%) as a brown solid.
[5]
Oxidation of Benzo[c]furan 20c Using LTA: Following a similar pro-
cedure to that of 6d, the oxidation of benzo[c]furan 20c (0.72 g,
1.62 mmol) using LTA (0.72 g, 1.62 mmol) gave diketone 19a
(0.16 g, 21%) as a thick, yellow liquid.
[6]
[7]
Oxidation of Benzo[c]furan 23c Using LTA: Following a similar pro-
cedure to that of 6d, the oxidation of benzo[c]furan 23c (0.59 g,
1.35 mmol) using LTA (0.61 g, 1.35 mmol) gave diketone 24c
(0.14 g, 23%) as a pale yellow solid.
[8]
Oxidation of Benzo[c]furan 23d Using LTA: Following a similar
procedure to that of 6d, the oxidation of benzo[c]furan 23d (0.56 g,
1.20 mmol) using LTA (0.53 g, 1.20 mmol) gave diketone 24d
(0.14 g, 24%) as a pale yellow solid.
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2.61 mmol) using LTA (1.16 g, 2.61 mmol) gave diketone 26a
(0.83 g, 80%) as a pale yellow solid.
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Oxidation of Benzo[c]furan 27 Using LTA: Following a similar pro-
cedure to that of 6d, the oxidation of benzo[c]furan 27 (2.13 g,
5.0 mmol) using LTA (2.21 g, 5.0 mmol) gave diketone 28a (1.83 g,
83%) as a colorless solid.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures for benzo[c]heterocycles 6f, 7a, 7b,
8a, 8b, 12, 14, 16a, 20a, 20e, 21a, 21b, 23a, 25, and 29a–29c. Copies
of the 1H NMR, 13C NMR, and DEPT 135 spectra for benzo-
[c]heterocycles 8a, 8b, 12, 14, 16a, and 21a. Copies of 1H NMR,
13C NMR, DEPT 135, and HRMS spectra for all of the 1,2-di-
aroylbenzenes produced.
[13]
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Acknowledgments
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We thank the Department of Science and Technology (DST), New
Delhi for financial support. M. N. thanks the DST, New Delhi for a
fellowship. R. S. thanks the University Grants Commission (UGC),
New Delhi for a fellowship. The authors thank the Department of
Science and Technology Funds for the Improvement of Science and
Technology (DST-FIST) for the 300 MHz NMR facility.
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Published Online: May 23, 2012
Eur. J. Org. Chem. 2012, 3647–3657
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3657