Regioselective synthesis of 3-(aryloxyacetyl)-2,3-dihydrothieno-[2,3-b][1] benzothiopyran-4-ones via tandem cyclization

Article abstract of DOI:10.1081/SCC-120038494

2-[4-Aryloxybut-2-ynylthio][1]benzothiopyran-4-ones on treatment with m-chloroperoxy benzoic acid in chloroform at 0°C to room temperature afforded 3-(aryloxyacetyl)-2,3-dihydrothieno[2,3-b]benzothiopyrans in 65-70% yield. The sigmatropic [2,3] followed b

Full text of DOI:10.1081/SCC-120038494

SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 12, pp. 2159–2167, 2004
Regioselective Synthesis of
3-(Aryloxyacetyl)-2,3-dihydrothieno-
[2,3-b][1]benzothiopyran-4-ones
via Tandem Cyclization
*
K. C. Majumdar, A. Bandyopadhyay, and S. K. Ghosh
Department of Chemistry, University of Kalyani,
Kalyani, West Bengal, India
ABSTRACT
2-[4-Aryloxybut-2-ynylthio][1]benzothiopyran-4-ones on treatment with
m-chloroperoxy benzoic acid in chloroform at 08C to room temperature
afforded 3-(aryloxyacetyl)-2,3-dihydrothieno[2,3-b]benzothiopyrans in
65–70% yield. The sigmatropic [2,3] followed by [3,3] rearrangement,
are attributed to this transformation.
Key Words: 2-[4-Aryloxybut-2-ynylthio]benzothiopyran-4-ones; Claisen
rearrangement; Heterocycles; Phase-transfer catalysis; Regioselective
synthesis; [2,3] Sigmatropic rearrangement.
*
Correspondence: K. C. Majumdar, Department of Chemistry, University of Kalyani,
Kalyani 741 235, West Bengal, India; Fax: þ91-33-25828282; E-mail: kcm@klyuniv.
ernet.in.
2159
DOI: 10.1081/SCC-120038494
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

ORDER
REPRINTS
2160
Majumdar, Bandyopadhyay, and Ghosh
INTRODUCTION
Thieno[2,3-b]benzothiopyran-4-one skeleton [1] has been used as an inter-
mediate for the syntheses of a series of anti-psychotic drugs. Literature search
[2]
revealed that the synthesis
a long route. Few years ago, we reported
of this important intermediate was achieved by
[3]
a simple synthesis of thieno-
[2,3-b]benzothiopyran-4-one and thiopyrano[2,3-b]benzothiopyran-5(2H)-one
skeleton. Construction of the five-membered heterocyclic rings in benzo(b)-
[4–7]
[8–10]
thiophenes and indoles through the sulphoxide
and amine oxide
rearrangements, respectively, was reported by Majumdar and Thyagarajan.
Both amine oxide and sulphoxide rearrangement involve the intermediacy of
a [2,3] followed by a [3,3] sigmatropic rearrangement. Application of the
amine oxide rearrangement [11–13] in heterocyclic substrates [14,15] for the synth-
eses of a number of tricyclic skeletons has been reported from this laboratory.
Recently, we have also reported the regioselective synthesis of thieno[2,3-f]-
[16,17]
quinolin-7(6H)-one and pyrano[3,2-f]benzo[b]thiophene derivatives
by
the application of the sulphoxide rearrangement. This mild and simple reaction
for the construction of the five-membered heterocyclic ring has drawn our inter-
est to undertake a study on the sulphoxide rearrangement of 2-[4-aryloxybut-
2-ynylthio][1]benzothiopyran-4-ones (3a–f). The results are reported here.
RESULTS AND DISCUSSION
The substrates 2-[4-aryloxybut-2-ynylthio][1]benzothiopyran-4-ones (3a–f)
(Sch. 1) were prepared in 80–85% yield according to published procedure.[18]
Scheme 1. Reagents and conditions: (i) NaOH, CHCl3, benzyl triethyl ammonium
chloride (BTEAC), r.t, 4 hr.

ORDER
REPRINTS
3-(Aryloxyacetyl)-2,3-dihydrothieno[2,3-b][1]benzothiopyrans
2161
The sulphides 3a–f were oxidized to the corresponding sulphoxides 4a–f
by the addition of one equivalent of m-chloroperoxybenzoic acid (Sch. 2). The
reaction was monitored by TLC. Formation of a highly polar sulphoxide was
indicated by the appearance of a spot much below (Rf value 0.1 in benzene)
that of starting sulphide (Rf value 0.4 in benzene). The sulphoxides 4a–f are
quite unstable. One experiment was carried out in an NMR tube for the charac-
terization of the sulphoxide. After mixing the substrate 3e with m-CPBA in
CDCl3 at 0–58C for 30min the NMR spectrum was recorded. The spectrum
displayed a two proton quartet at d 3.98 due to the sulphoxido-methylene proton
(2O . Sþ– CH2). The –S–CH2– in the starting sulphide appears as a triplet at
d 3.83. This clearly indicates the formation of sulphoxide.
The sulphoxides seem to rearrange even during the work up of the reac-
tion mixture. Therefore, these crude sulphides were directly subjected to ther-
mal rearrangement without further purification. The sulphoxide 4a after
removal of chloroform, was refluxed in carbon tetrachloride to give the
compound 5a in 70% yield (Sch. 3).
Compound 5a was characterized from its elemental analysis and spectral
data. The 1H NMR spectrum of 5a showed three-proton singlet at d 2.27 due to
one –CH3 substituent, two one-proton doublets of a doublet at d 3.60
(J ¼ 9.10, 11.22 Hz) and at 3.68 (J ¼ 7.20, 11.22 Hz) due to –SCH2, one
proton doublet of a doublet at d 4.95 (J ¼ 7.20, 9.10 Hz) due to one proton
of the dihydrothiophene ring, two one-proton doublets at d 4.90
(J ¼ 15.10 Hz) and 5.01 (J ¼ 15.10 Hz) due to –O–CH2–, six proton multi-
plet at d 6.83–7.58 and one proton doublet at d 8.37 (J ¼ 8.51 Hz). 13C-NMR
showed 20 carbons present in the molecule (vide experimental). Mass
spectrum of compound 5a showed a molecular ion peak at m/z 404, 402
(Mþ). To test the generality of the reaction five other substrates 4b–f
were subjected to sulphoxide rearrangement under the same reaction condi-
tion. All the substrates gave similar products 5b–f in 65–70% yield.
The formation of the products 5a–f from the sulphoxides 4a–f may be
explained by the initial [2,3] sigmatropic rearrangement of the sulphoxides
Scheme 2. Reagents and conditions: (i) m-CPBA, CHCl3, 0–58C, 30 min.

ORDER
REPRINTS
2162
Majumdar, Bandyopadhyay, and Ghosh
Scheme 3. Reagents and conditions: (i) CCl4, 4 h.
4 to give the intermediate allenylsulphenates (7) followed by a [3,3] sigma-
tropic rearrangement and enolisation leading to the intermediates 9, contain-
ing an enone moiety favourably juxtaposed to a –SH function for an
internal Michael addition of the thiol to the enone moiety to yield 5a–f
(Sch. 4).
[16,17]
The intermediates 9 could have yielded 5. This method is found
to be general for the synthesis of 3-(aryloxyacetyl)-2,3-dihydrothieno
[2,3-b][1]benzothiopyran-4-ones (5a–f) in good yield. The reaction is
Scheme 4.

ORDER
REPRINTS
3-(Aryloxyacetyl)-2,3-dihydrothieno[2,3-b][1]benzothiopyrans
2163
chemoselective. m-Chloroperoxybenzoic acid seems to react selectively
with only one sulphur leaving the other reactive centre unaffected. The
formation of the dihydrothieno[2,3-b]benzothiopyran-4-ones system by an
intramolecular radical addition has recently been reported by Boivin et al.[19]
EXPERIMENTAL
Melting points were measured on a sulphuric acid bath. UV absorption
spectra were recorded on a UV-VIS Spectrophotometer Shimadzu Model
No. UV-2401PC (absolute ethanol). IR spectra were run on KBr discs on a
Perkin–Elmer 1330 apparatus and FTIR spectrophotometer Perkin–Elmer
Model No. L120-000A. 1H-NMR spectra were recorded in CDCl3 with
TMS as internal standard on a Bruker DPX-300 (300 MHz) and Bruker
DPX-500 (500 MHz) spectrometers. Mass spectra were recorded on a
[JEOL D-300 (El)] instrument and elemental analyses (agreeing within
the error with calculated values) at RSIC (CDRI) Lucknow. Silica gel
(60–120 mesh), Spectrochem, India, was used for chromatographic separa-
tion. Petroleum ether refers to the fraction boiling between 608C and 808C.
General Procedure for the Alkylation of
7-Chloro-4-hydroxydithiocoumarin
To a mixture of 7-chloro-4-hydroxydithiocoumarin (6mmol) and 1-aryloxy-
4-chlorobut-2-yne (9 mmol) in chloroform (50 mL) is added a solution of
TBAB (0.25 mmol) or BTEAC (0.9 mmol) in 1% aq. NaOH (50 mL) and
the mixture was stirred at room temperature for a period of 5 hr. The mixture
was then diluted with water (100 mL) and extracted with CHCl3 (3 ꢀ 25 mL),
brine (3 ꢀ 25 mL), and dried (Na2SO4). The solvent was removed in vacuo
and the residual crude mass was purified by chromatography over silica gel
using benzene/petroleum ether (1 : 2) as eluent to afford the following
compounds.
The sulphides 3a,[20] 3e,[20] and 3f[18] were reported previously.
Compound 3b. Yield (82%); white solid; m.p. 1028C; lmax (nm) 219
(log 1 ¼ 4.00), 271 (log 1 ¼ 4.20), 335 (log 1 ¼ 3.85); n
(cm21) 1600,
¯
max
1580, 1230; dH (300 MHz) 3.83 (t, J ¼ 1.50 Hz, 2H, SCH2), 4.70 (t, J ¼
1.50 Hz, 2H, OCH2), 6.84–7.48 (m, 8H, ArH), 8.39 (d, J ¼ 8.51 Hz, 1H,
ArH); m/z ¼ 374, 372 (Mþ); Anal. calcd for C19H13ClO2S2: C, 61.29; H,
3.49; Found C, 61.51; H, 3.35%.
Compound 3c. Yield (85%); white solid; m.p. 838C; lmax (nm) 219
(log 1 ¼ 4.02), 271 (log 1 ¼ 4.24), 336 (log 1 ¼ 3.82); n
(cm21) 1600,
¯
max

ORDER
REPRINTS
2164
Majumdar, Bandyopadhyay, and Ghosh
1580, 1280; dH (300 MHz) 2.18 (s, 3H, CH3), 3.83 (t, J ¼ 1.50 Hz, 2H, SCH2),
4.72 (t, J ¼ 1.50 Hz, 2H, OCH2), 6.80–7.48 (m, 7H, ArH), 8.39 (d, J ¼
8.51 Hz, 1H, ArH); m/z ¼ 388, 386 (Mþ); Anal. calcd for C20H15ClO2S2:
C, 62.18; H, 3.89; Found C, 62.26; H, 3.75%.
Compound 3d. Yield (82%); white solid; m.p. 998C; lmax (nm) 219
(log 1 ¼ 4.00), 271 (log 1 ¼ 4.22), 335 (log 1 ¼ 3.81); n
(cm21) 1600,
¯
max
1580, 1230; dH (500 MHz) 2.10 (s, 3H, CH3), 2.22 (s, 3H, CH3), 3.83
(t, J ¼ 1.50 Hz, 2H, SCH2), 4.69 (t, J ¼ 1.50 Hz, 2H, OCH2), 6.72–7.47
(m, 6H, ArH), 8.38 (d, J ¼ 8.51 Hz, 1H, ArH); m/z ¼ 402, 400 (Mþ);
Anal. calcd for C21H17ClO2S2: C, 63.00; H, 4.25; Found C, 63.18; H, 4.08%.
General Procedure for the Preparation of Compounds 5a–f
m-Chloroperoxybenzoic acid (50%, 142 mg, 0.41 mmol) in chloroform
(20 mL) was slowly added to a well-stirred solution of the sulphide 3a
(150 mg, 0.4 mmol) in chloroform (20 mL) at 0–58C over a period of
30 min. The reaction mixture was stirred for additional 30 min. Chloroform
solution was washed with saturated sodium carbonate solution (3 ꢀ 20 mL)
followed by water (3 ꢀ 20 mL) and dried (Na2SO4). The solvent was removed
and the residue was refluxed in carbon tetrachloride (25 mL) for 4 hr. Then
carbon tetrachloride was removed and a viscous liquid was obtained. It was
then chromatographed over silica gel using benzene as eluent to give a solid
5a, m.p. 1508C; yield 70%.
Compound 5a. Yield (70%); white solid; m.p. 1508C; lmax (nm) 230
(log 1 ¼ 4.02), 270 (log 1 ¼ 4.25), 344 (log 1 ¼ 3.80); n
(cm21) 1720,
¯
max
1600, 1575, 1220; dH (300 MHz) 2.27 (s, 3H, CH3), 3.60 (dd, J ¼ 9.10,
11.22 Hz, 1H, SCH2), 3.68 (dd, J ¼ 7.20, 11.22 Hz, 1H, SCH2), 4.95
(dd, J ¼ 7.20, 9.10 Hz, 1H, CH), 4.90 (d, J ¼ 15.20 Hz, 1H, OCH2), 5.01
(d, J ¼ 15.20 Hz, 1H, OCH2), 6.83 (d, J ¼ 8.40 Hz, 2H, ArH), 7.07
(d, J ¼ 15.10 Hz, 2H, ArH), 7.45–7.58 (m, 2H, ArH), 8.37 (d, J ¼ 8.51 Hz,
1H, ArH); dC (75.5 MHz) 20.45 (40CH3), 35.37 (C2), 65.01 (C3), 73.00
0
(C11), 114.46 (C2 ), 114.59 (C6 ), 125.00 (C9a), 125.195 (C8), 127.56 (C6),
0
0
128.45 (C5), 128.83 (C4 ), 130.07 (C3 ), 130.18 (C5 ), 130.96 (C4a), 131.04
0
0
0
(C7), 137.50 (C8a), 138.02 (C3a), 155.65 (C1 ), 174.50 (C10), 204.87 (C4);
m/z ¼ 404, 402 (Mþ); Anal. calcd for C20H15ClO3S2: C, 59.70; H, 3.73;
Found C, 58.91; H, 3.58%.
Compound 5b. Yield (70%); white solid; m.p. 1808C; lmax (nm) 220
(log 1 ¼ 4.00), 272 (log 1 ¼ 4.21), 340 (log 1 ¼ 3.75);
n
(cm21
)
¯
max
1720, 1600, 1575, 1220; dH (300 MHz) 3.62 (dd, J ¼ 9.10, 11.22 Hz,
1H, SCH2), 3.70 (dd, J ¼ 7.20, 11.22 Hz, 1H, SCH2), 4.95 (dd, J ¼ 7.20,
9.10 Hz, 1H, CH), 4.91 (d, J ¼ 15.20 Hz, 1H, OCH2), 5.02 (d, J ¼ 15.20 Hz,

ORDER
REPRINTS
3-(Aryloxyacetyl)-2,3-dihydrothieno[2,3-b][1]benzothiopyrans
2165
1H, OCH2), 6.93–7.53 (m, 7H, ArH), 8.39 (d, J ¼ 8.51 Hz, 1H, ArH); m/
z ¼ 390, 388 (Mþ); Anal. calcd for C19H13ClO3S2: C, 58.76; H, 3.35;
Found C, 58.57; H, 3.52%.
Compound 5c. Yield (67%); white solid; m.p. 1388C; lmax (nm) 223
(log 1 ¼ 3.61), 230 (log 1 ¼ 3.74), 269 (log 1 ¼ 3.93); n
(cm21) 1710,
¯
max
1580, 1560, 1220; dH (500 MHz) 2.29 (s, 3H, CH3), 3.63 (dd, J ¼ 9.10,
11.22 Hz, 1H, SCH2), 3.70 (dd, J ¼ 7.20, 11.22 Hz, 1H, SCH2), 5.01
(dd, J ¼ 7.20, 9.10 Hz, 1H, CH), 4.87 (d, J ¼ 15.20 Hz, 1H, OCH2), 5.02
(d, J ¼ 15.20 Hz, 1H, OCH2), 6.76–7.53 (m, 6H, ArH), 8.38
(d, J ¼ 8.51 Hz, 1H, ArH); m/z ¼ 404, 402 (Mþ); Anal. calcd for
C20H15ClO3S2: C, 59.70; H, 3.73; Found C, 59.62; H, 3.58%.
Compound 5d. Yield (65%); white solid; m.p. 1208C; lmax (nm) 220
(log 1 ¼ 3.59), 270 (log 1 ¼ 3.75), 340 (log 1 ¼ 3.84); n
(cm21) 1710,
¯
max
1580, 1560, 1220; dH (300 MHz) 2.17 (s, 3H, CH3), 2.27 (s, 3H, CH3), 3.62
(dd, J ¼ 9.10, 11.22 Hz, 1H, SCH2), 3.69 (dd, J ¼ 7.20, 11.22 Hz, 1H,
SCH2), 5.01 (dd, J ¼ 7.20, 9.10 Hz, 1H, CH), 4.85 (d, J ¼ 15.20 Hz, 1H,
OCH2), 5.00 (d, J ¼ 15.20 Hz, 1H, OCH2), 6.63–7.53 (m, 6H, ArH), 8.38
(d, J ¼ 8.51 Hz, 1H, ArH); m/z ¼ 418, 416 (Mþ); Anal. calcd for
C21H17ClO3S2: C, 60.58; H, 4.09; Found C, 60.49; H, 4.25%.
Compound 5e. Yield (68%); white solid; m.p. 1618C; lmax (nm) 220
(log 1 ¼ 3.51), 272 (log 1 ¼ 3.67), 336 (log 1 ¼ 3.81); n
(cm21) 1710,
¯
max
1600, 1570, 1280; dH (300 MHz) 2.17 (s, 3H, CH3), 2.23 (s, 3H, CH3), 3.60
(dd, J ¼ 9.10, 11.22 Hz, 1H, SCH2), 3.67 (dd, J ¼ 7.20, 11.22 Hz,
1H, SCH2), 4.99 (dd, J ¼ 7.20, 9.10 Hz, 1H, CH), 4.88 (d, J ¼ 15.20 Hz,
1H, OCH2), 5.06 (d, J ¼ 15.20 Hz, 1H, OCH2), 6.55–7.52 (m, 6H, ArH),
8.38 (d, J ¼ 8.51 Hz, 1H, ArH); m/z ¼ 418, 416 (Mþ); Anal. calcd for
C21H17ClO3S2: C, 60.58; H, 4.09; Found C, 60.43; H, 4.23%.
Compound 5f. Yield (70%); white solid; m.p. 1308C; lmax (nm) 233
(log 1 ¼ 4.33), 267 (log 1 ¼ 4.34), 342 (log 1 ¼ 4.06); n
(cm21) 1720,
¯
max
1590, 1570, 1210; dH (300 MHz) 3.61 (dd, J ¼ 9.10, 11.22 Hz, 1H, SCH2),
3.69 (dd, J ¼ 7.20, 11.22 Hz, 1H, SCH2), 3.73 (s, 3H OCH3), 5.01 (dd,
J ¼ 7.20, 9.10 Hz, 1H, CH), 4.88 (d, J ¼ 15.20 Hz, 1H, OCH2), 4.95
(d, J ¼ 15.20 Hz, 1H, OCH2), 6.80–7.58 (m, 7H, ArH), 8.47
(d, J ¼ 8.51 Hz, 1H, ArH); m/z ¼ 420, 418 (Mþ); Anal. calcd for
C21H17ClO3S2: C, 57.42; H, 3.83; Found C, 57.83; H, 3.91%.
ACKNOWLEDGMENTS
We are thankful to CSIR (New Delhi) for financial assistance. One of us
(SKG) is grateful to CSIR (New Delhi) for a fellowship.

ORDER
REPRINTS
2166
Majumdar, Bandyopadhyay, and Ghosh
REFERENCES
1. Two or more heteroatoms in different fused rings: thienobenzothiopyran
derivatives. Chem. Abstr. 1965, 63, 13265, Sandoz Ltd. Neth. Appl.
6,411,476 (C1, C O7d), 1965 April 9, Swiss Appl 1963 Oct 8, and
1964 July 29, 12pp.
2. Two or more heteroatoms in different fused rings: 7-(R-substituted)-4-
oxothieno[2,3-b][1]benzothiopyrans. Chem. Abstr. 1965, 63, 13265,
Sandoz Ltd. Neth. Appl. 6,411,477 (C1, C O7d), (1965) April 9, Swiss
Appl (1963) Oct 8, and (1964) July 29, 7pp.
3. Majumdar, K.C.; Khan, A.T.; Saha, S. Facile regioselective alkylative
cyclization: synthesis of 2-methyl-4H-thieno[2,3-b][1]-benzothiopyran-
5(4H)-one. Synthlett. 1991, 595.
4. Thyagarajan, B.S.; Majumdar, K.C. Unusual ortho Claisen rearrangement
of aryl prop-2-ynylsulfoxide. J. Chem. Soc. Chem. Commun. 1972, 83.
5. Majumdar, K.C.; Thyagarajan, B.S. Studies in Claisen rearrangement.
Part XI. A novel synthesis of benzo(b)thiophene and derivatives from
aryl-2-propynylsulfoxides. Int. J. Sulfur Chem. 1972, 2A, 93.
6. Majumdar, K.C.; Thyagarajan, B.S. A new direct synthesis of methyl-3-
benzothienylketones from dihydrobenzothiophenes. Int. J. Sulfur Chem.
1972, 2A, 67.
7. El-Osta, B.; Majumdar, K.C.; Thyagarajan, B.S. Synthesis of 2-aryloxy-
methyl-3-aminomethylbenzo[b]thiophene and 2,3-diaryloxymethylbenzo-
[b]thiophenes. J. Heterocycl. Chem. 1973, 10, 107.
8. Hillard, J.B.; Thyagarajan, B.S.; Reddy, K.V.; Majumdar, K.C. A novel
synthesis of indole derivatives via a Claisen rearrangement. Tetrahedron
Lett. 1974, 1999.
9. Hillard, J.B.; Reddy, K.V.; Majumdar, K.C.; Thyagarajan, B.S. A new
synthesis of 2,3 disubstituted indoles. J. Heterocycl. Chem. 1974, 11, 369.
10. Thyagarajan, B.S.; Majumdar, K.C. Synthesis of indole derivatives
part III. Extension of the amine oxide rearrangement(1). J. Heterocycl.
Chem. 1975, 12, 43.
11. Majumdar, K.C.; Chattopadhyay, S.K. Studies in amine oxide rear-
rangement: an unusual product from the reaction of 1-aryloxy-4-tetra-
hydroquinolylbut-2-yne with m-chloroperoxybenzoic acid. J. Chem.
Soc. Chem. Commun. 1987, 524.
12. Majumdar, K.C.; Chattopadhyay, S.K.; Khan, A.T. One pot synthesis of
2-aryloxymethyl-1-cyanomethyl-5,6-dihydro4H-pyrrolo[3,2,1-ij]quinolinevia
rearrangement of the N-oxide from 1-aryloxy-4-tetrahydro-1-quinolylbut-
2-yne. J. Chem. Soc. Perkin Trans. 1 1989, 1285.

ORDER
REPRINTS
3-(Aryloxyacetyl)-2,3-dihydrothieno[2,3-b][1]benzothiopyrans
2167
13. Majumdar, K.C.; Ghosh, S.K. Studies on amine oxide rearrangements:
regioselective synthesis of pyrano[3,2-e]indole-7-one. J. Chem. Soc.
Perkin Trans. 1 1994, 2889.
14. Majumdar, K.C.; Das, U.; Jana, N.K. Studies on pyrimidine annelated
heterocycles: synthesis of pyrrolo[3,2-d]pyrimidines by amine oxide
rearrangement. J. Org. Chem. 1998, 63, 3550.
15. Majumdar, K.C.; Biswas, P.; Jana, G.H. Studies on amine oxide
rearrangements: regioselective synthesis of pyrrolo[3,2-f ]quinolin-
7-ones. J. Chem. Res. (S) 1997, 310 (S), 2068 (M).
16. Majumdar, K.C.; Biswas, P. Studies in sulfoxide rearrangement: regio-
selective synthesis of thieno[3,2-f ]quinolin-7(6H)-one. Tetrahedron
1998,
54, 11603.
17. Majumdar, K.C.; Biswas, P. Studies of bioactive heterocycles: one pot
regioselective synthesis of pyrano[3,2-f ]benzo[b]thiophene derivatives.
Tetrahedron 1999, 55, 1449.
18. Majumdar, K.C.; Khan, A.T.; Saha, S. Regioselective synthesis of
4-(aryloxymethyl)thiopyrano[2,3-b][1]benzothiopyran-5(2H)-ones. Synth.
Commun. 1992, 22, 901.
19. Boivin, J.; Boutillier, P.; Zard, S.Z. An unusual and highly efficient
access to thieno [2,3-b]benzothiopyran structures. Tetrahedron Lett.
1999, 40, 2529.
20. Majumdar, K.C.; Bandyopadhyay, A.; Biswas, A. Regioselective
synthesis of pentacyclic heterocycles by sequential [3,3]sigmatropic
rearrangement of 2-(40aryloxybut-20-ynylmercapto)thiochromen-4-ones.
Tetrahedron 2003, 59, 5289.
Received in India February 4, 2004

Copyright of Synthetic Communications is the property of Taylor & Francis Ltd and its content may not be
copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written
permission. However, users may print, download, or email articles for individual use.