Synthesis of sulphur compounds: Regioselective synthesis of thieno[2,3-b]thiophenes by sulfoxide rearrangement

Article abstract of DOI:10.1080/10426500601047438

A number of thieno[2,3-b]thiophenes have been successfully synthesized in a 50-55% yield by m-chloroperoxybenzoic acid-mediated tandem cyclization involving [2, 3] and [3, 3] sigmatropic rearrangements and an intramolecular Michael addition. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon
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Synthesis of Sulphur
Compounds: Regioselective
Synthesis of Thieno[2,3-
b]thiophenes by Sulfoxide
Rearrangement
K. C. Majumdar a , N. Pal a & S. K. Samanta a
a Department of Chemistry , University of Kalyani ,
Kalyani, West Bengal, India
Published online: 30 Jan 2007.
To cite this article: K. C. Majumdar , N. Pal & S. K. Samanta (2007) Synthesis of
Sulphur Compounds: Regioselective Synthesis of Thieno[2,3-b]thiophenes by Sulfoxide
Rearrangement, Phosphorus, Sulfur, and Silicon and the Related Elements, 182:3,
667-675, DOI: 10.1080/10426500601047438
To link to this article: http://dx.doi.org/10.1080/10426500601047438
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Phosphorus, Sulfur, and Silicon, 182:667–675, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500601047438
Synthesis of Sulphur Compounds: Regioselective
Synthesis of Thieno[2,3-b]thiophenes by Sulfoxide
Rearrangement
K. C. Majumdar
N. Pal
S. K. Samanta
Department of Chemistry, University of Kalyani, Kalyani, West Bengal,
India
A number of thieno[2,3-b]thiophenes have been successfully synthesized in a 50–55%
yield by m-chloroperoxybenzoic acid–mediated tandem cyclization involving [2, 3]
and [3, 3] sigmatropic rearrangements and an intramolecular Michael addition.
Keywords [2,3] Sigmatropic rearrangement; [3,3] sigmatropic rearrangement; Sulfox-
ide rearrangement; thieno[2,3-b]thiophene
INTRODUCTION
Thieno[2,3-b]thiophenes have been synthesized1 for different purposes
in the pharmaceutical field and were tested as potential antiviral,2
antibiotic,3 antiglucoma,4 analgesic, and antipyretic5 drugs. A tandem
cyclization methodology for the synthesis of benzo[b]thiophene and in-
dole derivatives was reported by Majumdar and Thyagrajan6a,6b and
El-Osta et al.6c Later this methodology was successfully utilized for
the synthesis of some heterocyclic systems.7 The methodology involves
two consecutive sigmatropic rearrangements—a [2,3] shift followed by
a [3,3] shift in arylprop-2-ynyl sulphoxides.8 The sulfoxide rearrange-
ments usually demand relatively high activation energy and require
reaction conditions at r.t. to refluxing carbon tetrachloride (78◦C). The
same protocol was also applied to arylpropynyl amine oxide for the syn-
thesis of fused pyrroles.9 In continuation of our work in the area of
Received April 30, 2006; accepted August 19, 2006.
The authors thank Council of Scientific and Industrial Research (New Delhi, India) for
financial assistance. We also thank Department of Science and Technology (New Delhi,
India) for providing us with UV-Vis and an FTIR spectrometer under the FIST program.
N. Pal is grateful to CSIR for a Senior Research Fellowship.
Address correspondence to K. C. Majumdar, Department of Chemistry, University of
Kalyani, Kalyani-741235, West Bengal, India. E-mail: kcm ku@yahoo.co.in
667

668
K. C. Majumdar et al.
annulated heterocycles,10 we became interested in exploring the viabil-
ity of synthesizing thieno[2,3-b]thiophene derivatives by the application
of the aforementioned tandem reaction, the sulfoxide rearrangement.
RESULTS AND DISCUSSION
The starting material for this study, 2-[4-aryloxybut-2-ynylsulfo-
nyl]thiophenes 2, were prepared in 75–85% yields by the reaction of
thiophene, n-butyl lithium, sulphur powder, and 1-aryloxy-4- chlorobut-
2-yne under a nitrogen atmosphere11 (Scheme 1).
OAr
(i) n-BuLi, Et2O
(ii) S powder, 0–5°C
(iii) ClCH2−C C−CH2OAr,
Et2O, r.t.; 16h
S
S
S
2 a–f
1
2 a Ar = 4-ClC6H4
b Ar = 4-MeC6H4
c Ar = -C6H5
d Ar = 4-t-BuC6H4
e Ar = 3,5-Me2C6H3
f Ar = 4-MeOC6H4
SCHEME 1
Substrates 2a–f are endowed with an aryloxy propynyl and
a thioprop-2-ynyl moiety. In order to form thiophene annulated
heterocycles, substrate 2a was treated with one equivalent of
m−chloroperoxybenzoic acid in chloroform at 0–5◦C. TLC indicated the
formation of the highly polar sulfoxide 3a. Compound 2a was completely
converted to the new sulfoxide 3a in 30 min. During hydrolytic work-
up, another new spot appeared on TLC, but the conversion was only
partial. As sulfoxide is labile, no attempt was made to isolate it in pure
form. The solvent was removed, and the crude 3a was refluxed in CCl4
for a period of 4 h (Scheme 2).
A colorless solid (55%), m.p. 86◦C, was obtained. This was shown to
be 2-(4-chlorophenoxy)-1[thieno-[2,3-b]thiophene-3-yl]ethanone 4a by
its elemental analysis and spectral data. The 1H NMR spectrum of

Synthesis of Thieno[2,3-b]thiophenes
669
OAr
3-Chloroperoxybenzoic acid
0–5°C, 30 min
2a-f
S
3a–f
S
O
CCl4, reflux, 4 h
O
OAr
S
S
H
4a–f
SCHEME 2
compound 4a displayed a singlet at δ 5.10 for OCH2 protons. The
aromatic protons appeared as two multiplets, one at δ 6.87–6.91 and
another at δ 7.21–7.26. For the thiophene protons, three signals were
observed: a doublet of doublets at δ 7.48 (J = 5.2, 1.0 Hz), a doublet
at δ 7.79 (J = 5.2 Hz), and a doublet at δ 8.38 Hz (J = 1.0 Hz). The IR
spectrum of compound 4a displayed νmax at 1685 cm−1 for the side chain
carbonyl group. The mass spectrum of the compound showed molecular
ion peaks at m/z 308 and 310. Sulfoxides 3b–f obtained from 2b–f in an
analogous way were treated similarly to give products 4b–f in 50–55%
yields (Scheme 2).
The formation of thieno[2,3-b]thiophenes 4 from sulfoxides 3 may be
mechanistically rationalized as shown in Scheme 3. A [2,3] sigmatropic
rearrangement of sulfoxides 3 may give allenyl sulphenate derivatives
5. An instantaneous [3,3] sigmatropic rearrangement through two het-
eroatoms, oxygen and sulphur, may generate intermediates 6, which
may then undergo tautomerization to 7. Intermediates 7 possess a nu-
cleophilic –SH functionality favorably situated to a α,β-enone moiety to
allow an intramolecular Michael–type addition to afford compounds 4
(pathway a, Scheme 3). Another mode of cyclization (pathway b) could

670
K. C. Majumdar et al.
SCHEME 3
also have occurred in intermediates 7. Thus –SH could have delivered
its nucleophilic attack to the carbonyl carbon of the enone moiety lead-
ing to allylic alcohols 9. The SN2ꢀ attack of water or m-chlorobenzoate
ion on 9 could have produced compounds 10. However, we could not
detect compound 10 in any of the cases studied so far. The reaction dis-
played generality and regioselectivity. This is a mild and direct method
for the synthesis of thieno[2,3-b]thiophene derivatives.
EXPERIMENTAL
Melting points were determined in an open capillary and are uncor-
rected. IR spectra were recorded with a Perkin-Elmer L 120-000A spec-
trometer (νmax in cm−1). UV absorption spectra were recorded in EtOH

Synthesis of Thieno[2,3-b]thiophenes
671
with a Shimadzu UV-2401PC spectrophotometer (λmax in nm). 1H NMR
(300 MHz, 500 MHz) and 13C NMR (125.7 MHz) spectra were recorded
with a Bruker DPX-300 and Bruker DPX-500 spectrometer in CDCl3
with TMS as an internal standard. Elemental analyses and mass spec-
tra were recorded on a Perkin Elmer 2400 Series-II CHN analyzer and
a JEOL JMS-600 instrument. Silica gel ([60–120 mesh]), Spectrochem)]
was used for chromatographic separation. Silica gel G [(E-Merck)] was
used for TLC. Petroleum ether refers to the fraction boiling between
60◦C and 80◦C.
General Procedure for the Preparation of Compounds 2a–f
1.6 M n-BuLi solution in heptane (6.2 mL, 9.92 mmol) was added to
thiophene (0.83 g, 9.92 mmol) in anhydrous ether at r.t. under a nitro-
gen atmosphere. The reaction mixture was then refluxed for 30 min,
and sulphur powder (0.32 g, 9.92 mmol) was added portionwise at 0–
5◦C. The reaction mixture was then allowed to attain r.t., 1-aryloxy-4-
chlorobut-2-yne (9.92 mmol) in ether was added during 30 min, and the
mixture was stirred overnight. The reaction mixture was poured into
NH4Cl solution (50 mL). The organic layer was washed with water (2 ×
50 mL), brine (50 mL), and dried (Na2SO4). Evaporation of the solvent
left a gummy residue, which was subjected to column chromatography.
Elution of the column with EtOAc/petroleum ether (1:50) afforded com-
pounds 2a–f.
2-[4-(4-Chlorophenoxy)but-2-ynylsulfanyl]thiophene (2a)
Yield 83%; viscous liquid; UV (EtOH) λmax: 229, 250, 257, 267 nm; IR
1
(neat) νmax: 1218, 1489, 1594, 2225 cm−1; H NMR (CDCl3): δ 3.50 (t,
J = 2.0 Hz, 2H, SCH2), 4.67 (t, J = 2 Hz, 2H, OCH2), 6.83–7.41 (m, 7H,
ArH); MS m/z 294, 296 (M+); anal. calcd for C14H11OS2Cl: C, 57.03; H,
3.76; found; C, 57.30; H, 3.63%.
2-[4-(p-Tolyloxy)but-2-ynylsulfanyl]thiophene (2b)
Yield 77%; viscous liquid; UV (EtOH) λmax: 228, 250, 257, 267 nm; IR
1
(neat) νmax: 1216, 1509, 1610, 2224 cm−1; H NMR (CDCl3): δ 2.30 (s,
3H, CH3), 3.50 (t, J = 2.0 Hz, 2H, SCH2), 4.67 (t, J = 2.0 Hz, 2H, OCH2),
6.81–7.40 (m, 7H, ArH); MS m/z 274 (M+); anal. calcd. for C15H14OS2:
C, 65.66; H, 5.14; found; C, 65.42; H, 4.88%.

672
K. C. Majumdar et al.
2-[4-Phenoxybut-2-ynylsulfanyl]thiophene (2c)
Yield 80%; viscous liquid; UV (EtOH) λmax: 220, 237, 262, 269 nm; IR
1
(neat) νmax: 1215, 1494, 1598, 2225 cm−1; H NMR (CDCl3): δ 3.51 (t,
J = 2.0 Hz, 2H, SCH2), 4.70 (t, J = 2.0 Hz, 2H, OCH2), 6.93–7.40 (m,
8H, ArH); MS m/z 260 (M+); anal. calcd for C14H12OS2: C, 64.58; H,
4.65; found; C, 64.80; H, 4.78%.
2-[4-(4-tert-Butylphenoxy)but-2-ynylsulfanyl]thiophene (2d)
Yield 85%; viscous liquid; UV (EtOH) λmax: 226, 250, 257, 267 nm; IR
1
(neat) νmax: 1222, 1512, 1608, 2224 cm−1; H NMR (CDCl3): δ 1.23 (s,
9H, CH3), 3.43 (t, J = 2.1 Hz, 2H, SCH2), 4.60 (t, J = 2.1 Hz, 2H, OCH2),
6.77–7.30 (m, 7H, ArH); MS m/z 316 (M+); anal. calcd. for C18H20OS2:
C, 68.31; H, 6.37; found; C, 68.52; H, 6.48%.
2-[4-(3,5-Dimethylphenoxy)but-2-ynylsulfanyl]thiophene (2e)
Yield 75%; viscous liquid; UV (EtOH) λmax: 231, 250, 257, 267 nm; IR
1
(neat) νmax: 1217, 1471, 1613, 2224 cm−1; H NMR (CDCl3): δ 2.31 (s,
6H, CH3), 3.52 (t, J = 1.9 Hz, 2H, SCH2), 4.68 (t, J = 1.9 Hz, 2H, OCH2),
6.59–7.40 (m, 7H, ArH); MS m/z 288 (M+); anal. calcd. for C16H16OS2:
C, 66.63; H, 5.59; found; C, 66.89; H, 5.77%.
2-[4-(4-Methoxylphenoxy)but-2-ynylsulfanyl]thiophene (2f)
Yield 83%; viscous liquid; UV (EtOH) λmax: 230, 250, 257, 267 nm; IR
(neat) νmax: 1207, 1509, 1610, 2224 cm−1; 1H NMR (CDCl3): δ 3.50 (t, J =
2.0 Hz, 2H, SCH2), 3.77 (s, 3H, CH3), 4.64 (t, J = 2.0 Hz, 2H, OCH2),
6.80–7.39 (m, 7H, ArH); MS m/z 290 (M+); anal. calcd. for C15H14O2S2:
C, 62.04; H, 4.86; found; C, 61.81; H, 5.01%.
General Procedure for the Oxidation and Subsequent
Rearrangement of 2-[4-Aryloxybut-2-ynylsulfanyl]thiophenes
(2a–f)
A solution of m-CPBA (2.43 mmol, 600 mg [70%]) in chloroform (30 mL)
was slowly added to a well-stirred solution of sulphides 2a–f (2.43 mmol)
in chloroform (30 mL) at 0–5◦C over a period of 15 min. The reaction
mixture was stirred at r.t. for an additional 30 min. The resulting so-
lution was then washed with 10% aq. Na2CO3 (3 × 25 mL), brine (25
mL), and dried (Na2SO4). Removal of the solvent gave a gummy mass,
which was then dissolved in carbon tetrachloride and refluxed for 4 h.

Synthesis of Thieno[2,3-b]thiophenes
673
After evaporation of the carbon tetrachloride, the crude product was
purified by column chromatography. Elution of the column with 2%
EtOAc/petroleum ether gave compounds 4a–f.
2-(4-Chlorophenoxy)-1[thieno-[2,3-b]thiophene-3-yl]ethanone
(4a)
Yield 50%; colorless solid, m.p. 86◦C; UV (EtOH) λmax: 229, 250, 256,
1
287 nm; IR (KBr) νmax: 1187, 1429, 1490, 1583, 1685 cm−1; H NMR
(CDCl3): δ 5.14 (s, 2H, OCH2), 6.87–6.91 (m, 2H, aryl-H), 7.21–7.26 (m,
2H, aryl-H), 7.48 (dd, J = 5.2, 1.0 Hz, 1H, thiophene-H), 7.79 (d, J = 5.2
Hz, 1H, thiophene-H), 8.38 (d, J = 1.0 Hz, 1H, thiophene-H); MS m/z
308, 310 (M+); anal. calcd. for C14H9O2S2Cl: C, 54.45; H, 2.94; found;
C, 54.60; H, 3.11%.
2-(p-Tolyloxy)-1[thieno-[2,3-b]thiophene-3-yl]ethanone (4b)
Yield 55%; colorless powder, m.p. 94◦C; UV (EtOH) λmax: 230, 251, 257,
1
286 nm; IR (KBr) νmax: 1177, 1429, 1509, 1610, 1686 cm−1; H NMR
(CDCl3): δ 2.27 (s, 3H, CH3), 5.11 (s, 2H, OCH2), 6.84 (d, J = 8.4 Hz,
2H, aryl-H), 7.07 (d, J = 8.4 Hz, 2H, aryl-H), 7.46 (dd, J = 5.2, 1.0 Hz,
1H, thiophene-H), 7.80 (d, J = 5.2, Hz, 1H, thiophene-H), 8.43 (d, J =
1.0 Hz, 1H, thiophene-H); 13C NMR (CDCl3): δ 20.8, 72.2, 114.9, 122.0,
130.4, 130.5, 131.1, 131.5, 137.8, 138.0, 145.7, 156.2, 189.6; MS m/z 288
(M+); anal. calcd for C15H12O2S2: C, 62.47; H, 4.19; found; C, 62.73; H,
4.35%.
2-Phenoxy-1[thieno-[2,3-b]thiophene-3-yl]ethanone (4c)
Yield 55%; viscous liquid; UV (EtOH) λmax: 217, 250, 256, 288 nm; IR
1
(neat) νmax: 1186, 1429, 1495, 1598, 1686 cm−1; H NMR (CDCl3): δ
5.15 (s, 2H, OCH2), 6.95–7.02 (m, 3H, aryl-H), 7.27–7.33 (m, 2H, aryl-
H), 7.47 (dd, J = 5.3, 1.0 Hz, 1H, thiophene-H), 7.81 (d, J = 5.3 Hz, 1H,
thiophene-H), 8.44 (d, J = 1.0 Hz, 1H, thiophene-H);MS m/z 274 (M+);
anal. calcd. for C14H10O2S2: C, 61.29; H, 3.67; found; C, 61.51; H, 3.39%.
2-(4-tert-Butylphenoxy)-1[thieno-[2,3-b]thiophene-3-
yl]ethanone (4d)
Yield 52%; colorless powder, m.p. 100◦C; UV (EtOH) λmax: 225, 245,
1
250, 282 nm; IR (KBr) νmax: 1183, 1429, 1512, 1608, 1686 cm−1; H
NMR (CDCl3): δ 1.27 (s, 9H, CH3), 5.11 (s, 2H, OCH2), 6.87 (d, J =

674
K. C. Majumdar et al.
8.8 Hz, 2H, aryl-H), 7.28 (d, J = 8.8 Hz, 2H, aryl-H), 7.45 (dd, J = 5.2,
0.9 Hz, 1H, thiophene-H), 7.80 (d, J = 5.2, Hz, 1H, thiophene-H), 8.43
(d, J = 0.9 Hz, 1H, thiophene-H); MS m/z 330 (M+); anal. calcd. for
C18H18O2S2: C, 65.42; H, 5.49; found; C, 65.63; H, 5.25%.
2-(3,5-Dimethylphenoxy)-1[thieno-[2,3-b]thiophene-3-
yl]ethanone (4e)
Yield 50%; colorless powder, m.p. 78◦C; UV (EtOH) λmax: 234, 251, 257,
1
288 nm; IR (KBr) νmax: 1169, 1429, 1504, 1594, 1686 cm−1; H NMR
(CDCl3): δ 2.28 (s, 6H, CH3), 5.10 (s, 2H, OCH2), 6.59 (s, 2H, aryl-H),
6.64 (s, 1H, aryl-H), 7.47 (dd, J = 5.2, 1.1 Hz, 1H, thiophene-H), 7.82
(d, J = 5.2 Hz, 1H, thiophene-H), 8.45 (d, J = 1.1 Hz, 1H, thiophene-H);
MS m/z 302 (M+); anal. calcd. for C16H14O2S2: C, 63.55; H, 4.67; found;
C, 63.83; H, 4.46%.
2-(4-Methoxyphenoxy)-1[thieno-[2,3-b]thiophene-3-
yl]ethanone (4f)
Yield 53%; viscous liquid; UV (EtOH) λmax: 226, 250, 256, 288 nm; IR
(neat) νmax: 1185, 1430, 1506, 1594, 1687 cm−1; 1H NMR (CDCl3): δ 3.76
(s, 3H, OCH3) 5.09 (s, 2H, OCH2), 6.81–6.86 (m, 2H, aryl-H), 6.87–6.93
(m, 2H, aryl-H), 7.47 (dd, J = 5.2, 1.1 Hz, 1H, thiophene-H), 7.81 (d,
J = 5.2, Hz, 1H, thiophene-H), 8.44 (d, J = 1.1 Hz, 1H, thiophene-H);
MS m/z 304 (M+); anal. calcd. for C15H12O3S2: C, 59.19; H, 3.97; found;
C, 59.37; H, 3.79%.
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