Regioselective synthesis of benzofuran-annulated six-membered sulfur heterocycles by aryl radical cyclization

Article abstract of DOI:10.1080/00397910701471246

The tin hydride-mediated cyclization of a number of sulfides and sulfones under mild and neutral conditions has been investigated. The sulfides were in turn derived from 3(2H) benzothiofuranone and 2-bromobenzyl bromides by phase-transfer-catalyzed reacti

Full text of DOI:10.1080/00397910701471246

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Regioselective Synthesis of
Benzofuran‐Annulated Six‐Membered Sulfur
Heterocycles by Aryl Radical Cyclization
K. C. Majumdar a , B. Chattopadhyay a & S. Nath a
a Department of Chemistry , University of Kalyani , Kalyani, India
Published online: 30 Aug 2007.
To cite this article: K. C. Majumdar , B. Chattopadhyay & S. Nath (2007) Regioselective Synthesis
of Benzofuran‐Annulated Six‐Membered Sulfur Heterocycles by Aryl Radical Cyclization, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:17,
2907-2915, DOI: 10.1080/00397910701471246
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Synthetic Communicationsw, 37: 2907–2915, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701471246
Regioselective Synthesis of
Benzofuran-Annulated Six-Membered
Sulfur Heterocycles by Aryl Radical
Cyclization
K. C. Majumdar, B. Chattopadhyay, and S. Nath
Department of Chemistry, University of Kalyani, Kalyani, India
Abstract: The tin hydride–mediated cyclization of a number of sulfides and sulfones
under mild and neutral conditions has been investigated. The sulfides were in turn
derived from 3(2H) benzothiofuranone and 2-bromobenzyl bromides by phase-
transfer-catalyzed reaction, and the corresponding sulfones were prepared by
treatment of the corresponding sulfides with m-CPBA at room temperature. The
sulfides and sulfones were then reacted with nBu3SnH-AIBN to afford regioselectively
benzofuran-annulated six-membered sulfur heterocycles.
Keywords: 6-endo trig, radical cyclization, regioselective, sulfur heterocycles,
tri-n-butyltin hydride
INTRODUCTION
Recently, aryl radical cyclization has been developed as a powerful method
for constructucting carbon-carbon bonds in organic synthesis.[1] These
radical cyclization protocols commonly have several advantages over nonra-
dical methods; for example, radical cyclization can be carried out in neutral
solutions. Previously, we synthesized several novel heterocyclic systems con-
taining sulfur by sigmatropic rearrangement.[2] During the course of our
studies, we have noted the unusual formation of [6,6]pyranothiopyrans
during the second Claisen rearrangement step.[3] An examination of the
Received January 19, 2007
Address correspondence to K. C. Majumdar, Department of Chemistry, University
of Kalyani, Kalyani, 741235, W.B., India. E-mail: kcm_ku@yahoo.co.in
2907

2908
K. C. Majumdar, B. Chattopadhyay, and S. Nath
literature[4] reveals only scattered examples of the synthesis of sulfur-based
heterocycles via radical methods. These include cyclizations with sulfur func-
tionality in the hexenyl chain at the 3-[5] and 4-positions[6] and an example of a
sulfonylated radical in the 2-position.[7] Several useful radical cyclizations
with sulfur functionality located a to the radical center have also been
reported, but these yield cyclic products with the sulfur functionality
external to the ring.[8] We also have recently succeeded in the regioselective
synthesis of [6,6] fused cyclic sulfur heterocycles by tri-n-butyltin hydride–
mediated cyclization.[9] In this context, we undertook a study on the radical
cyclization of the sulfides (4a,b) and their corresponding sulfones (5a,b).
Here we report the results.
RESULTS AND DISCUSSIONS
The starting materials 4a,b were synthesized in 80–95% yield by the
phase-transfer-catalyzed alkylation of 3(2H) benzothiofuranone 2, with
either 2-bromobenzyl bromide 3a or 2-bromo-5-methoxybenzyl bromide 3b,
in dichloromethane and 1% aq. NaOH solution in the presence of benzyl-
triethylammonium chloride (BTEAC) at room temperature for 30 min. The
other starting materials 4c,d were prepared from the corresponding sulfides
4a,b, by treatment with 2 equiv. of m-CPBA in dry dichloromethane
solution at room temperature. The compound 2, 3(2H)-benzothiofuranone,
was in turn prepared by the reaction of 3(2H) benzofuranone[10] with P2S5
in dry THF solution in the presence of sodium bicarbonate (Scheme 1).
Scheme 1. Reagents and conditions: (i) dry THF, P2S5, NaHCO3, stirring, rt, 30 min;
(ii) PTC, 1% NaOH(aq), CH2Cl2, 30 min; (iii) m-CPBA (2-equiv), CH2Cl2, stirring, rt,
3–4 h.

Cyclization of Sulfides and Sulfones
2909
Compounds 4 were characterized from their elemental analyses and
spectroscopic data. The IR spectrum of 4c showed characteristic S55O
stretching frequencies at nmax ¼ 1314 (sym.) and 1125 (asym.) cm21. The
two-proton singlet due to S-CH2 protons of 4a at dH ¼ 4.04 was shifted by
0.66 ppm downfield in 4c (i.e., the two proton singlet of SO2-CH2 protons of
4c appeared at dH ¼ 4.70). The substrate 4a was treated at 808C in dry
n
degassed toluene under a nitrogen atmosphere with Bu3SnH in the presence
of AIBN as radical initiator for 4 h to afford the cyclic product 5a (yield
90%), accompanied by a small amount of the debrominated product 6a (10%
yield). This debromination was inhibited by the conversion of the sulfides to
the corresponding sulfones. Exposure of the sulfone 4c to nBu3SnH under the
same reaction conditions as described previously afforded the cyclized hetero-
cyclic sulfone 5c in 96% yield. The structure of the compounds 5 (X ¼ S, SO2)
were readily elucidated by 1 H NMR spectroscopy, which exhibited a one-
proton doublet at dH ¼ 4.80–4.82 (J ¼ 8.0 Hz) and another one proton
doublet at dH ¼ 5.82–5.84 (J ¼ 8.0 Hz) due to ring junction protons Hc and
Hd respectively, for 5a (X ¼ S), whereas the same two protons resonated at
dH ¼ 4.69–4.70 (J ¼ 8.8 Hz) and 6.06–6.09 (J ¼ 8.8 Hz), respectively, for
5c (X ¼ SO2). The stereochemistry of the ring fusion of the cyclic system
can be surmised from the molecular model (Dreiding model), which shows a
trans-arrangement, and also from the high coupling constants of 5a (X ¼ S,
J ¼ 8.0 Hz) and 5c (X ¼ SO2, J ¼ 8.8 Hz). The 13CNMR spectrum of 5c
(X ¼ SO2, R ¼ H) also supported the proposed structure. The 13C NMR
chemical shift as well as the multiplicity of the signals in compound 5c was
established by a distortionless enhancement of polarization transfer (DEPT)
experiment. There are 11 protonated carbons, one CH2 and ten CH moieties.
The mass spectrum of compounds 5a and 5c showed molecular ion peaks at
m/z ¼ 240 and 270 (Mþ) respectively. The generality of the reaction was
tested by subjecting two other substrates 4b and 4d under the same reaction
conditions to give the products 5b and 5d in excellent yields (Scheme 2).
Scheme 2. Reagents and conditions: (i) nBu3SnH, AIBN, toulene, N2 atm, reflux, 4 h.

2910
K. C. Majumdar, B. Chattopadhyay, and S. Nath
Scheme 3.
The mechanistic pathway for the formation of six-membered sulfur
heterocycles 5 from the substrates 4 is outlined in Scheme 3. The aryl
n
radical 7 is generated in the reaction by Bu3SnH and AIBN. The radical
7 on addition of a hydrogen radical from tri-n-butyl tin hydride afforded
the debrominated product 6a. The formation of six-membered sulfur hetero-
cyclic ring in products 5a–d from the substrates 4a–d may be explained by
the initial formation of the aryl radical 7 followed by a 6-endo trig ring
closure to give a tertiary radical 10, which may then accept a hydrogen
radical to afford the final products 5a–d. In an alternative route, the aryl
radical 7 may undergo a 5-exo trig ring closure to generate a spiro hetero-
cyclic radical 8,[11] which may be converted to the tertiary radical 10 via
radical 9 by a neophyl[12] rearrangement. The reaction pathway, whether a
6-endo or a 5-exo ring closure, can be predicted by the application of the
frontier molecular orbital (FMO) theory. According to the theory, the aryl
radicals are a high-energy species and have nucleophilic character. The
presence of a high-election-withdrawing SO2 group confers considerable
electrophilic character to the C2-position of the benzofuran moiety. In the
nucliophilic radical 7, FMO theory suggests that the mode of ring closure
should largely be determined by the interaction between the radical singly
occupied molecular orbital (SOMO) (i.e., highest occupied molecular
orbital (HOMO)) and alkene lowest unoccupied molecular orbital
(LUMO) of the acceptor (election-deficient center), and accordingly more
favorable bond formation should occur between the radical center (nucleo-
philic) and C2 of the benzofuran moiety, leading to 6-endo trig cyclization.
Again, if we compare the two paths (i.e., 6-endo trig and 5-exo trig cycliza-
tion) for the resulting intermediate products radical 10 and 8, the stability of
the intermediate product radical 10 is higher than the stability of the inter-
mediate product radical 8. Because in intermediate 10, the radical is at the
immediate vicinity of the strong electron-withdrawing SO2, the S group

Cyclization of Sulfides and Sulfones
2911
therefore is highly stabilized[13] and expected to follow the 6-endo trig
cyclization mode, giving the six-membered sulfur heterocyclic compounds
5. One interesting observation is that the usual oxidation does not occur
at the present instance and the dihydro compound is isolated in excellent
yield. The usual course during this type of cyclization is that the initially
formed dihydro products give oxidized products by aerial oxidation, i.e.,
n
an oxidation step in Bu3SnH-mediated cyclization.[14]
n
In conclusion, we have performed the Bu3SnH-mediated radical cycli-
zation methodology to the regioselective synthesis of tetracyclic sulfur hetero-
cyclic compounds solely by the 6-endo trig cyclization path. The mildness of
the reaction conditions and the high degree of the chemoselectivity allow this
radical cyclization to serve as a powerful synthetic tool. This methodology is a
general one and is attractive by its simplicity.
EXPERIMENTAL
General Remarks
Melting points are determined in open capillaries and are uncorrected. IR
spectra were recorded on a Perkin Elmer L120-000A spectrometer (nmax in
cm21) using samples as neat; solid samples were recorded in KBr disks.
NMR spectra were recorded on Bruker DPX-400 and Bruker DPX-500 spec-
trometers in CDCl3 (chemical shifts in d) with TMS as the internal standard.
Silica gel (60–120 mesh, E-mark, India) was used for chromatographic
separation. Silica gel G (E-mark, India) was used for thin-layer chromatography
(TLC). Petroleum ether refers to the fraction boiling between 60 and 808C.
3(2H)-Benzofuranone was prepared according to the published
procedure.[10]
General Procedure for the Preparation of
3(2H)-Benzothiofuranone
To a magnetically stirred solution of 3(2H)-benzofuranone (1) (0.53 g,
4 mmol) and phosphorous pentasulfide (0.89 g, 4 mmol) in dry THF,
sodium bicarbonate (2 g) was added slowly. After the addition was
complete, the stirring was continued for an additional 2 h. The mixture
was filtered and concentrated. This was then extracted with CH2Cl2
(3 ꢀ 20 ml). The dichloromethane solution was washed with water
(2 ꢀ 10 ml) and dried (Na2SO4). Attempts to evaporate dichloromethane
led to considerable decomposition of the compound 2. Therefore, this dichloro-
methane solution was directly used in the next phase-transfer-catalyzed
alkylation step.

2912
K. C. Majumdar, B. Chattopadhyay, and S. Nath
General Procedure for the Preparation of the Precursors 4(a,b)
To a stirred solution of 3(2H)-benzothiofuranone (2) (0.32 g, 2 mmol) and
2-bromobenzyl bromide 3a,b (2 mmol) in dichloromethane solution (20 ml),
a
solution of benzyltriethylammonium chloride (BTEAC) (0.33 g,
1.2 mmol) in 1% aq. NaOH solution (10 ml) was added, and the mixture
was stirred for about 30 min at room temperature. It was diluted with water
(20 ml). The dichloromethane layer was washed with dilute HCl followed
by brine and dried (Na2SO4). The crude mass obtained on evaporation of
CH2Cl2 was subjected to column chromatography over silica gel. Elution of
the column with ethyl acetate–pet. ether (1%) afforded compounds 4a,b.
Data
Compound 4a
Yield 90%, viscous liquid. IR(neat): nmax ¼ 1460, 1592 cm21
;
1H NMR
(CDCl3, 400 MHz): dH ¼ 4.04 (s, 2H, -SCH2), 6.88–6.90 (m, 1H, ArH),
7.05–7.08 (m, 2H, ArH), 7.24–7.33 (m, 2H, ArH), 7.43 (s, 1H, 55CCH),
7.46–7.48 (m, 1H, ArH), 7.52–7.56 (m, 2H, ArH). MS: m/z ¼ 318, 320
(Mþ). Anal. calcd. for C15H11BrOS: C, 56.44; H, 3.47%. Found: C, 56.51;
H, 3.48%.
Compound 4b
1
Yield 88%, viscous liquid. IR (neat): nmax ¼ 1463, 1593 cm21; H NMR
(CDCl3, 400 MHz): dH ¼ 3.79 (s, 3H, -OCH3), 3.98 (s, 2H, -SCH2), 6.61–
6.64 (dd, 1H, ArH, J ¼ 8.76 Hz, J ¼ 3.0 Hz), 6.71–6.74 (dd, 1H, ArH,
J ¼ 8.76 Hz, J ¼ 3.0 Hz), 6.97–6.98 (d, 1H, ArH, J ¼ 3.0 Hz), 7.31–7.35
(m, 2H, ArH), 7.46 (s, 1H, 55CCH), 7.53–7.60 (m, 2H, ArH). MS:
m/z ¼ 348, 350 (Mþ). Anal. calcd. for C16H13BrO2S: C, 55.03; H, 3.75%.
Found: C, 55.21; H, 3.80%.
General Procedure for the Synthesis of the Sulfones 4(c,d)
To a well-stirred solution of compound 3 (200 mg, 0.63 mmol) in dry dichlor-
omethane (10 ml), a solution of m-CPBA (77%, 0.22 g, 1.2 mmol, 2 eq.) was
added at 08C over a period of 30 min. After complete addition of m-CPBA, the
reaction was refluxed for 1 h to complete the oxidation. The mixture was
cooled and washed with saturated sodium carbonate solution (3 ꢀ 10 ml),
followed by brine, and dried (Na2SO4). The dichloromethane was removed,
and the crude mass obtained was purified by column chromatography over
silics gel and by eluting the column with 5% ethyl acetate–pet. ether to
afford the sulfones as white solids 4c,d.

Cyclization of Sulfides and Sulfones
2913
Data
Compound 4c
Yield 92%, white solid, mp 1158C. IR (KBr): nmax ¼ 1125, 1314 cm21
.
1H NMR (CDCl3, 400 MHz): dH ¼ 4.70 (s, 2H, SO2-CH2), 7.14–7.23
(m, 2H, ArH), 7.32–7.38 (m, 4H, ArH), 7.52–7.55 (m, 2H, ArH), 7.94
(s, 1H, 55CCH). 13C NMR (CDCl3, 125 MHz): dC ¼ 62.5, 112.4, 120.6,
121.8, 123.5, 125.0, 126.3, 126.5, 128.1, 128.4, 130.9, 133.5, 133.6, 151.2,
155.7; MS: m/z ¼ 350, 352 (Mþ). Anal. calcd. for C15H11BrO3S: C, 51.30;
H, 3.16%. Found: C, 51.39; H, 3.11%.
Compound 4d
Yield 92%, white solid, mp 1208C. IR (KBr): nmax ¼ 1129, 1311 cm21
.
1H NMR (CDCl3, 400 MHz): dH ¼ 3.75 (s, 3H, -OCH3), 4.65 (s, 2H,
SO2-CH2), 6.71–6.79 (dd, 2H, J ¼ 8.8 Hz, J ¼ 3.0 Hz, ArH), 7.03–7.04
(d, 1H, J ¼ 3.0 Hz, ArH), 7.30–7.35 (m, 2H, ArH), 7.36–7.40 (m, 2H,
ArH), 7.96 (s, 1H, 55CCH); MS: m/z ¼ 380, 382 (Mþ). Anal. calcd. for
C16H13BrO4S: C, 50.41; H, 3.44%. Found: C, 50.30; H, 3.41%.
General Methods for the Synthesis of the Compounds 5a–d and 6a
To a magnetically stirred suspension of the compounds 4a,d (0.2 mmol) and
azobisisobutyronitrile (AIBN) (0.5 eq.) in dry degassed toluene (8 ml),
tri-n-butyltin hydride was added slowly all at once under a nitrogen atmos-
phere. The reaction mixture was heated at 808C for 4–5 h. After the
complete conversion of the starting materials (TLC observation), the
solvent was removed under reduced pressure. The liquid mass obtained was
dissolved in CH2Cl2 (5 ml) and stirred with 10% aqueous potassium
fluoride solution (8 ml) for 2–3 h. The white precipitate was separated by
filtration, and the aqueous phase was extracted with dichloromethane
(3 ꢀ 15 ml). The combined extract was washed with brine and dried
(Na2SO4). Dichloromethane was distilled off, and the residual mass was
subjected to column chromatography over silica gel. The column was eluted
with ethyl acetate–pet. ether (2%) to give the products 5a–d and 6a.
Data
Compound 5a
Yield 90%, white solid, mp 1188C. IR (KBr): nmax ¼ 2851, 2922, 2958 cm21
.
1H NMR (CDCl3, 400 MHz): dH ¼ 3.45–3.49 (d, 1H, J ¼ 14.8 Hz, -SCHaHb),

2914
K. C. Majumdar, B. Chattopadhyay, and S. Nath
3.66–3.70 (d, 1H, J ¼ 14.8 Hz, -SCHaHb), 4.80–4.82 (d, 1H, J ¼ 8.0 Hz,
-CHdCHc), 5.82–5.84 (d, 1H, J ¼ 8.0 Hz, -CHdCHc), 6.78–6.80 (d, 1H,
J ¼ 8.0 Hz, ArH), 6.95–6.99 (m, 1H, ArH), 7.17–7.21 (m, 2H, ArH),
7.29–7.34 (m, 2H, ArH), 7.36–7.40 (m, 1H, ArH), 7.50–7.52 (d, 1H,
J ¼ 7.05 Hz, ArH). MS: m/z ¼ 240 (Mþ). Anal. calcd. for C15H12OS: C,
74.97; H, 5.03%. Found: C, 75.01; H, 5.00%.
Compound 5b
Yield 95%, white solid, mp 1208C. IR (KBr): nmax ¼ 2853, 2923, 2958 cm21
.
1H NMR (CDCl3, 400 MHz): dH ¼ 3.38–3.42 (d, 1H, J ¼ 14.8 Hz,
-SCHaHb), 3.64–3.68 (d, 1H, J ¼ 14.8 Hz, -SCHaHb), 3.81 (s, 3H, -OCH3),
4.77–4.79 (d, 1H, J ¼ 8.0 Hz, -CHdCHc), 5.82–5.84 (d, 1H, J ¼ 8.0 Hz,
-CHdCHc), 6.75–6.78 (m, 2H, ArH), 6.84–6.87 (m, 1H, ArH), 6.94–6.98
(m, 1H, ArH), 7.15–7.17 (m, 1H, ArH), 7.37–7.42 (m, 2H, ArH). MS:
m/z ¼ 270 (Mþ). Anal. calcd. for C16H14O2S: C, 71.08; H, 5.22%. Found:
C, 71.19; H, 5.22%.
Compound 5c
Yield 95%, white solid, mp. 1958C. IR (KBr): nmax ¼ 2849, 2920, 2955 cm21
.
1H NMR (CDCl3, 400 MHz): dH ¼ 3.93–3.97 (d, 1H, J ¼ 15.0 Hz,
-SCHaHb), 4.18–4.22 (d, 1H, J ¼ 15.0 Hz, -SCHaHb), 4.67–4.70 (d, 1H,
J ¼ 8.8 Hz, -CHdCHc), 6.06–6.09 (d, 1H, J ¼ 8.8 Hz, -CHdCHc), 6.84–
6.86 (d, 1H, J ¼ 8.11 Hz, ArH), 7.04–7.07 (t, 1H, J ¼ 7.4 Hz, ArH), 7.24–
7.26 (d, 1H, J ¼ 5.94 Hz, ArH), 7.29–7.33 (m, 1H, ArH), 7.46–7.50
(m, 2H, ArH), 7.54–7.57 (m, 1H, ArH), 7.61–7.63 (d, 1H, J ¼ 7.57 Hz,
ArH). 13CNMR (CDCl3, 125 MHz): dC ¼ 51.7, 63.2, 85.1, 109.9, 120.0,
122.0, 128.3, 129.3, 130.0, 130.3, 130.4, 130.7, 131.0, 131.2, 160.7; DEPT
(CDCl3, 125 MHz): dC ¼ 51.7, 63.2, 85.0, 109.9, 122.0, 128.3, 129.3,
130.3, 130.4, 131.0, 131.2; MS: m/z ¼ 272 (Mþ). Anal. calcd. for
C15H12O3S: C, 66.16; H, 4.44%. Found: C, 66.29; H, 4.37%.
Compound 5d
Yield 93%, white solid, mp. 1908C. IR (KBr): nmax ¼ 2850, 2921, 2954 cm21
.
1H NMR (CDCl3, 400 MHz): dH ¼ 3.83 (s, 3H, -OCH3), 3.86–3.90 (dd, 1H,
J ¼ 15.04 Hz, J ¼ 2.76 Hz, -SCHaHb), 4.17–4.20 (d, 1H, J ¼ 15.04 Hz,
-SCHaHb), 4.63–4.66 (dd, 1H, J ¼ 8.92 Hz, J ¼ 2.44 Hz, -CHdCHc), 6.03–
6.05 (d, 1H, J ¼ 8.92 Hz, -CHdCHc), 6.77 (d, 1H, J ¼ 2.2 Hz, ArH),
6.82–6.84 (d, 1H, J ¼ 8.12 Hz, ArH), 6.94–6.97 (dd, 1H, J ¼ 8.32 Hz,
J ¼ 2.4 Hz, ArH), 7.02–7.06 (t, 1H, J ¼ 7.56 Hz, ArH), 7.29–7.32 (t, 1H,
J ¼ 7.52 Hz, ArH), 7.45–7.47 (d, 1H, J ¼ 8.4 Hz, ArH), 7.59–7.61 (d, 1H,
J ¼ 7.52 Hz, ArH). MS: m/z ¼ 302 (Mþ). Anal. calcd. for C16H14O4S: C,
63.56; H, 4.67%. Found: C, 63.61; H, 4.66%.

Cyclization of Sulfides and Sulfones
ACKNOWLEDGMENT
2915
We thank the CSIR (New Delhi) for financial assistance, and one of us
(B. Chattopadhyay) is thankful to the CSIR (New Delhi) for a junior
research fellowship. We also thank the Department of Science and Technol-
ogy (DST) (New Delhi) for providing UV-VIS and FT-IR spectrometers
under the DST-FIST program.
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