A Novel and Efficient Synthesis of δ-Sultones

Article abstract of DOI:10.1055/s-0033-1340785

A class of sulfonic ionic liquids, with the supremacy of methylsulfonylimidazolium triflate hydrochloride, was found to be efficient for catalyzing the self-condensation of acetophenone derivatives and sulfonating the resulting condensates in a one-pot rapid reaction leading to the synthesis of 4,6-diaryl-1,2-oxathiine-2,2-dioxides (diaryl-δ-sultones). Georg Thieme Verlag Stuttgart, New York. Georg Thieme Verlag Stuttgart, New York.

Full text of DOI:10.1055/s-0033-1340785

LETTER
▌827
lAetteNr ovel and Efficient Synthesis of δ-Sultones
A Novel and Efficient Synthesis of δ-Sultones
Kurosh Rad-Moghadam,* Saeedeh Toorchi Roudsari, Mehdi Sheykhan
Department of Chemistry, University of Guilan, P.O. Box 41335-19141, Rasht, Iran
Fax +98(131)3220066; E-mail: radmm@guilan.ac.ir
Received: 01.11.2013; Accepted after revision: 15.01.2014
O
O
Abstract: A class of sulfonic ionic liquids, with the supremacy of
methylsulfonylimidazolium triflate hydrochloride, was found to be
efficient for catalyzing the self-condensation of acetophenone de-
rivatives and sulfonating the resulting condensates in a one-pot rap-
id reaction leading to the synthesis of 4,6-diaryl-1,2-oxathiine-2,2-
dioxides (diaryl-δ-sultones).
S
O
O
[MSIm]HSO4⋅HCl
25 °C
Cl
Cl
Cl
Me
1a
2a
N
MSIm =
N
SO3H
Key words: δ-sultone, ionic liquid, sulfonation, domino reaction,
oxathiine-2,2-dioxide
Scheme 1 Trial condensation of 4′-chloroacetophenone with
[MSIm]HSO4·HCl; MSIm = 1-methyl-3-sulfonyl imidazolium cation
Ionic liquids are really a new generation of solvents with
valuable properties such as non-volatility, non-flamma-
bility and tuned solubility, characteristic to their both ion-
ic and organic natures.1 Owing to diverse interaction
regimes with solutes,2 they have found progressive appli-
cations as catalysts either simply3 or by incorporating
functional components.4 The ionic liquids made up of
acidic ions proved to be valuable catalysts for diverse or-
ganic syntheses.5 Despite these promising benefits, ionic
liquids have received rare applications as reagents in the
synthesis of organic compounds.6
1,2-Oxathiine-2,2-dioxides, also commonly named as δ-
sultones, have proved to be useful in sulfoalkylation and
several other unique synthetic organic transformations.8
Valuable synthetic applications and potential biological
activities9 of sultone derivatives added to the paucity of
general routes for synthesis of this heterocyclic system,
spurring investigation of multistep syntheses using im-
proved and innovative reactions. Recent and fine develop-
ments in this arena employing ring-closing metathesis,10
Pd-catalyzed intramolecular coupling reaction,11 Rh-cata-
lyzed C–H insertion cyclization,12 Rh-catalyzed carbene
cyclization cycloaddition cascade,13 intramolecular
Diels–Alder cycloaddition,14 intramolecular olefin sulfo-
nation,15 and nucleophilic addition reactions9a are well
documented. To the best of our knowledge the single pre-
ceding pseudo-three-component synthesis of fully unsatu-
rated δ-sultones, parallel to the present one-pot approach,
was reported by Rogachev et al. Their method involving
the reaction of arylacetylenes with either of sulfur triox-
ide, oleum or trimethylsilyl chlorosulfonate in dioxane so-
lution gave moderate yields and is supposed to proceed
through the formation of β-sultone intermediates.16
Recently we were testing the self-condensation of aceto-
phenone derivatives under acidic ionic-liquid catalysis
with the expectation to obtain triarylbenzenes. The ionic
liquids we employed for this purpose were prepared from
treatment of 1-methylimidazole with chlorosulfonic acid
followed by addition of an equimolar amount of a strong
acid to the in situ generated 1-methyl-3-sulfonylimidazo-
lium chloride ([MSIm]Cl).7 At the onset of this investiga-
tion, we found that treatment of 4′-chloroacetophenone
with most of the prepared sulfonic ionic liquids gave an
unanticipated product which was structurally different
from the expected 1,3,5-tri(4-chlorophenyl)benzene.
Spectroscopic identification of the product disclosed that
it was comprised of sulfone functionality displaying
strong absorptions at around 1350 cm–1 and 1160 cm–1.
The missing structural puzzle of the product was readily
deduced from its mass spectral molecular ion peak to be-
ing the Knoevenagel condensate of two 4′-chloroaceto-
phenone molecules. Based on these facts and the NMR
spectral data, the product was identified to be 4,6-di(4-
chlorophenyl)-1,2-oxathiine-2,2-dioxide (2a; Scheme 1).
Our preliminary results on the reaction of methylsulfo-
nylimidazolium
hydrogensulfate
hydrochloride,
[MSIm]HSO4·HCl, with 4′-chloroacetophenone in terms
of yields and reaction times were encouraging with re-
spect to those reported by Rogachev et al. To this end, we
aimed at screening a set of sulfonic ionic liquids for any
similar smooth sulfonating abilities and to find out an op-
timal ionic liquid for reaction with 4′-chloroacetophe-
none, as the model substrate.
As Table 1 shows, the best result was obtained with the
ionic liquid methylsulfonylimidazolium triflate hydro-
chloride, [MSIm]TfO·HCl, at room temperature.17 The el-
emental microanalysis and NMR spectral data of this
ionic liquid are consistent with its formulation as
[MSIm]TfO·HCl. Moreover, the stoichiometric content
of chloride ion in the ionic liquid was confirmed by quan-
SYNLETT 2014, 25, 0827–0830
Advanced online publication: 10.02.2014
DOI: 10.1055/s-0033-1340785; Art ID: ST-2013-D1024-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

828
K. Rad-Moghadam et al.
LETTER
titative measurement of this ion.18 The ionic liquids this method from readily available acetophenone deriva-
[MSIm]ClSO3·HCl, [MSIm]ClO4·HCl and tives.
[MSIm]HSO4·HCl were alternatively prepared by adding
an equimolar amount of the corresponding acids ClSO3H,
HClO4 or H2SO4 to dichloromethane solution of
[MSIm]Cl. These three ionic liquids, unlike their precur-
sor, [MSIm]Cl, are capable of realizing the trial reaction
and give the δ-sultone 2a, however, with slightly lower
yields than that achieved by using [MSIm]TfO·HCl. In-
Table 2 The Synthesized δ-Sultone Derivatives
O
O
O
S
O
[MSIm]TfO⋅HCl
Ar
Ar
Ar
25 °C
1a–f
2a–f
creasing the reaction temperature up to 25 °C is associated Product
Ar
Time (min)
Yield (%)a
with the acceleration of δ-sultone formation, but at higher
temperatures unidentified side reactions come into com-
petition with the δ-sultone synthesis and the yield of prod-
uct is decreased. It is noteworthy that application of
chlorosulfonic acid, either pure or as CH2Cl2 solution, in
reaction with acetophenones leads to the formation of
multiple by-products. Likewise, side products predomi-
nate when 4′-chloroacetophenone is treated with a 1:1
2a
4-ClC6H4
4-FC6H4
4-BrC6H4
3-ClC6H4
4-MeC6H4
Ph
5
10
5
91
81
93
90
87
87
2b
2c
2d
5
2e
5
(v/v) solution of chlorosulfonic acid and a strong acid (Ta-
ble 1, entries 11 and 12).
2f
5
a Yields of isolated products.
Table 1 The Reagents and Solvents used for Optimizations
O
O
It should be noted that resolution of the δ-sultone product
derived from 4-methoxyacetophenone (based on TLC
monitoring) needs additional investigations due to its
probable decomposition during separation.16b
O
S
O
reagent
Cl
Cl
Cl
1a
2a
A rapid equilibrium seems to be established under strong
acidic condition of the sulfonic ionic liquids, through
which 1-methyl-3-sulfonylimidazolium cation 3a is re-
versibly desulfonated to furnish 1H-3-methylimidazolium
cation 3c. As outlined in Scheme 2, these cations presum-
ably interconvert through intermediacy of the protonated
1-methyl-3-sulfonylimidazolium cation 3b. This interme-
diate likely serves as the main sulfonating agent of the
ionic liquid and is formed rapidly by the equilibrium sys-
tem. 1H NMR spectrum of the ionic liquid displays the ex-
istence of two imidazole species in the ratio of 74:26
along with a sharp singlet at the lowest field which was in-
terpreted to be arisen from the proton of the added
CF3SO3H. The other acidic proton of the ionic liquid ap-
peared as two peaks corresponding to its partition between
intermediates 3a and 3c. The protons of the minor imidaz-
olium cation 3a are more deshielded than those of the ma-
jor counterpart 3c. The presence of these two cationic
species is also apparent in 13C NMR spectrum of the ionic
liquid. Likewise, all the carbons of the minor cation 3a are
more deshielded than those of the major one 3c, due to
electron-withdrawing effect of the sulfonyl group. How-
ever, detailed mechanism of the present reaction has not
been proven, it is likely a domino tandem reaction pro-
ceeding through a sequence of sulfonation and condensa-
tion steps. As is suggested in Scheme 2, initial self-
condensation of acetophenones under the acidic and dehy-
drating condition of the ionic liquid [MSIm]TfO·HCl
gives the intermediate enone 4. This intermediate subse-
quently undergoes sulfonation at the carbonyl group fol-
lowed by liberation of a proton to yield the dienylsulfate
intermediate 5. At this stage, once again sulfonation oc-
Entry Reagent
Solvent
Temp Time
Yielda
(%)
(°C)
(min)
1
2
[MSIm]OTf·HCl
[MSIm]OTf·HCl
none
none
THF
25
40
25
25
25
25
40
25
25
25
25
25
5
5
91
76
3
[MSIm]OTf·HCl
[MSIm]OTf·HCl
[MSIm]ClSO3·HCl
[MSIm]ClO4·HCl
[MSIm]Cl
5
67
4
CH2Cl2
none
none
none
none
none
CH2Cl2
none
none
5
71
5
5
89
6
5
86
7
20
10
25
25
25
25
trace
62
8
[MSIm]HSO4·HCl
ClSO3H
b
9
–
b
10
11
12
ClSO3H
–
b
ClSO3H–H2SO4
ClSO3H–CF3SO3H
–
21b
a Yields of isolated products.
b In each case pure chlorosulfonic acid (0.4 mL) or its 1:1 (v/v) mix-
ture with a strong acid was applied to 4′-chloroacetophenone (1
mmol), whereupon multiple products formed.
Further exploration of this synthetic approach was con-
ducted by examining several acetophenone derivatives in
reaction with [MSIm]TfO·HCl, as the superior sulfonat-
ing agent. According to Table 2, fairly high yields of 4,6-
diaryl-1,2-oxathiine-2,2-dioxides could be obtained by
Synlett 2014, 25, 827–830
© Georg Thieme Verlag Stuttgart · New York

LETTER
A Novel and Efficient Synthesis of δ-Sultones
829
curs, this time at CH2-termini of the diene intermediate 5,
where the electron-donating conjugation of the ester oxy-
gen with the diene segment makes the carbon atom at this
end of the intermediate more suitable for electrophilic at-
tack. The double sulfonated intermediate 6 then under-
goes cyclization followed by elimination of a molecule of
sulfuric acid to deliver the product 2.
References and Notes
(1) (a) Wasserscheid, P. In Ionic Liquids in Synthesis, 2nd ed.;
Wasserscheid, P.; Welton, T., Eds.; Wiley-VCH: Weinheim,
Baden-Württemberg, 2008. (b) Hallett, J. P.; Welton, T.
Chem. Rev. 2011, 111, 3508.
(2) Rebelo, L. P. N.; Canongiav Lopes, J. N.; Esperanca, J. M.
S. S.; Guedes, H. J. R.; Łachwa, J.; Najdanovic-Visak, V.;
Visak, Z. P. Acc. Chem. Res. 2007, 40, 1114.
(3) (a) Rad-Moghadam, K.; Youseftabar-Miri, L. Tetrahedron
2011, 67, 5693. (b) Raha Roy, S.; Chakraborti, A. K. Org.
Lett. 2010, 12, 3866. (c) Bica, K.; Gaertner, P. Org. Lett.
2006, 8, 733. (d) Yang, Q.; Robertson, A.; Alper, P. Org.
Lett. 2008, 10, 5079. (e) Patel, R.; Srivastava, V. P.; Yadav,
L. D. S. Synlett 2010, 1797.
H
N
SO3H
N H
N
SO3H
N
N
N
Me
Me
Me
3a
3b
3c
–
CF3SO3H
CF3SO3
CF3SO3H
Cl–
Cl–
ClSO3
–
(4) (a) Sarma, R.; Prajapati, D. Synlett 2008, 3001.
(b) Hagiwara, H.; Okunaka, N.; Hoshi, T.; Suzuki, T. Synlett
2008, 1813.
Ar
O
3
O
(5) (a) Moosavi-Zare, A. R.; Zolfigol, M. A.; Zarei, M.; Zare,
A.; Khakyzadeh, V. J. Mol. Liquid 2013, 186, 63. (b) Kore,
R.; Srivastava, R. Tetrahedron Lett. 2012, 53, 3245.
(c) Chen, Z.; Zhu, Q.; Su, W. Tetrahedron Lett. 2011, 52,
2601. (d) Liu, S.; Xie, C.; Yu, S.; Liu, F.; Ji, K. Catal.
Commun. 2008, 9, 1634. (e) Zolfigol, M. A.; Khazaei, A.;
Moosavi-Zare, A. R.; Zare, A.; Kruger, H. G.; Asghari, Z.;
Khakyzadeh, V. J. Org. Chem. 2012, 77, 3640.
Ar
4
1
Ar
Ar
3
3b
H
Ar
Ar
SO3H
O
HO3S
N
– H+
N
O
Me
SO3H
(6) Shirini, F.; Rad-Moghadam, K.; Akbari-Dadamahaleh, S. In
Green Solvents II, 1st ed.; Inamuddin, A. M., Ed.; Springer:
Dordrecht, 2012.
Ar
O
5
–
CF3SO3
O
H
H
O
O
(7) General Method for the Synthesis of 1,2-Oxathiine-2,2-
dioxides 2a–f: An acetophenone derivative (1 mmol in each
case) was mixed with the ionic liquid 1-methyl-3-sulfonyl-
imidazolium triflate hydrochloride (0.2 mL) in a glass vial
containing a stirring bar. The mixture was stirred for
appropriate time until disappearance of acetophenone at r.t.
(see Table 2). Progress of the reaction was monitored by
TLC using silica gel coated sheets and EtOAc–petroleum
ether (1:4) as eluents. After completion of the reaction
(usually less than 5 min), cold distilled H2O (10 °C, 10 mL)
was added to the reaction mixture and the precipitated solids
were filtered, dried in air and recrystallized from EtOH
(95%).
S
Ar
Ar
S
OH
– H2SO4
– H+
O
O
6
SO3H
O
SO3H
Ar
Ar
–
O
CF3SO3
Cl–
Ar
S
O
O
N H
N
2
Me
Ar
Scheme 2 A suspected mechanism for formation of the δ-sultones
In conclusion, the mild sulfonating ability and acid cata-
lytic effect of [MSIm]TfO·HCl created a harmony of effi-
cient C–C, C–S and C–O bond formations leading to a
novel pseudo-three-component synthesis of fully unsatu-
rated δ-sultones. This domino reaction certainly proceeds
through a complex cascade of sulfonation and condensa-
tion steps, very likely amenable to be modulated for other
reacting systems. This letter is the first report, as far as this
publication goes, on using ionic liquids as mild and easy-
to-handle sulfonating agents and attempts to give new in-
sights into the potential reactions of such reactive sulfo-
nating ionic liquids. Further studies are currently
underway in our laboratory to extend the utility of these
reactive ionic liquids.
Selected Data for 4,6-Di(4-fluorophenyl)-[1,2]-oxathiine-
2,2-dioxide (2b): mp 220 °C. IR (KBr): 3100, 1622, 1350,
1160, 780, 840 cm–1. 1H NMR (400.2 MHz, CDCl3): δ = 7.84
(dd, 3JHH = 8.6 Hz, 4JFH = 4.8 Hz, 2 H), 7.60 (dd, 3JHH = 8.6
Hz, 4JFH= 5.2 Hz, 2 H), 7.23 (t, J = 8.6 Hz, 2 H), 7.22 (t, J =
8.6 Hz, 2 H), 6.72 (s, 1 H), 6.66 (s, 1 H). 13C NMR (100.63
MHz, CDCl3): δ = 164.7 (d, J = 254 Hz), 164.3 (d, J = 253
Hz), 155.8, 146.7, 131.6 (d, J = 4 Hz), 128.9 (d, J = 9 Hz),
128.3 (d, J = 9 Hz), 127.1 (d, J = 3 Hz), 116.6 (d, J = 22 Hz),
116.4 (d, J = 22 Hz), 113.0, 101.9. MS (70 eV): m/z (%) =
320 (52) [M+], 256 (83) [M+ – SO2], 227 (100) [M+ –
CHSO3], 207 (19), 149 (22), 133 (40), 123 (45), 95 (62).
Anal. Calcd for C16H10F2O3S (320.31): C, 60.00; H, 3.15.
Found: C, 60.13; H, 3.21.
Selected Data for 4,6-Di(4-bromophenyl)-[1,2]-
oxathiine-2,2-dioxide (2c): mp 216 °C. IR (KBr): 3095,
1620, 1355, 1165, 770, 820 cm–1. 1H NMR (400.2 MHz,
CDCl3): δ = 7.70 (d, J = 9.2 Hz, 2 H), 7.68 (d, J = 9.2 Hz, 2
H), 7.66 (d, J = 8.8 Hz, 2 H), 7.45 (d, J = 8.4 Hz, 2 H), 6.77
(d, J = 1 Hz, 1 H), 6.70 (d, J = 1 Hz, 1 H). 13C NMR (100.63
MHz, CDCl3): δ = 155.9, 146.5, 134.3, 129.7, 126.4, 125.6,
132.7, 132.4, 128.4, 127.3, 113.7, 102.1. MS (70 eV): m/z
(%) = 444 (12) [M+, 81Br 81Br], 442 (22) [M+, 81Br 79Br], 440
(12) [M+, 79Br 79Br], 380 (42) [M+ – SO2, 81Br 81Br], 378 (53)
[M+ – SO2, 81Br 79Br], 376 (22) [M+ – SO2, 79Br 79Br], 359
(37), 351 (5) [M+ – CHSO3, 81Br 81Br], 349 (10) [M+ –
Acknowledgment
We gratefully acknowledge the financial support of this work by the
Research Council of the University of Guilan.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SuppInigfor
m
o
ti
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 827–830

830
K. Rad-Moghadam et al.
LETTER
(17) Typical Procedure for Synthesis of 1-Methyl-3-
CHSO3, 81Br 79Br], 347 (6) [M+ – CHSO3, 79Br 79Br], 301
(60), 299 (62), 271 (32) [M+ – BrC6H4O, 81Br], 269 (33) [M+
– BrC6H4O, 79Br], 220 (66), 189 (81), 157 (50), 155 (52),
115 (100). Anal. Calcd for C16H10Br2O3S (442.12): C, 43.47;
H, 2.28. Found: C, 43.41; H, 2.33.
sulfonylimidazolium Triflate Hydrochloride: To a flask,
kept in an ice-cooled bath and equipped with a magnetic
stirring bar, was added a solution of 1-methylimidazole,
(0.8 mL, 10 mmol), in anhyd CH2Cl2 (15 mL). To this
continuously stirred solution was added dropwise
chlorosulfonic acid, (0.66 mL, 10 mmol), whereupon white
suspended solids were produced. After an hour of additional
stirring at r.t., trifluoromethanesulfonic acid (0.88 mL, 10
mmol) was added slowly over a period of 15 min to the flask.
Within this time, the suspended white salt inside the flask
was completely dissolved. Stirring of this solution was
continued for about 30 min at r.t., and then the solvent was
removed under reduced pressure. According to this
procedure, the ionic liquid [MSIm]OTf·HCl was obtained as
a viscous yellow oil. Treatment of aqueous solution of the
ionic liquid with AgNO3 (aq) resulted in the formation
and precipitation of AgCl solids. Anal. Calcd for
(8) (a) Mondal, S. Chem. Rev. 2012, 112, 5339. (b) Koch, F. M.;
Peters, R. Synlett 2008, 1505. (c) Buglass, A. J.; Tillett, J. G.
In The Chemistry of Sulfonic Acids, Esters and their
Derivatives, 1st ed.; Patai, S.; Rappoport, Z., Eds.; Wiley:
New York, 1991, 789. (d) Enders, D.; Iffland, D. Synthesis
2007, 1837. (e) Merten, J.; Wang, Y.; Krause, T.; Kataeva,
O.; Metz, P. Chem. Eur. J. 2011, 17, 3332. (f) Flohic, A. L.;
Meyer, C.; Cossy, J. Tetrahedron 2006, 62, 9017.
(9) (a) Camarasa, M. J.; Pérez-Pérez, M. J.; San-Félix, A.;
Balzarini, J.; De Clercq, E. J. Med. Chem. 1992, 35, 2721.
(b) Pérez-Pérez, M. J.; Balzarini, J.; Hosoya, M.; De Clercq,
E.; Camarasa, M. J. J. Bioorg. Med. Chem. Lett. 1992, 2,
647. (c) Castro, S. D.; García-Aparicio, C.; Andrei, G.;
Snoeck, R.; Balzarini, J.; Camarasa, M. J.; Velázquez, S.
J. Med. Chem. 2009, 52, 1582.
[MSIm]OTf·HCl: C, 17.22; H, 2.31; N, 8.03. Found: C,
17.33; H, 2.43; N, 7.87.
(10) Karsch, S.; Schwab, P.; Metz, P. Synlett 2002, 12, 2019.
(11) Majumdar, K. C.; Mondal, S.; Ghosh, D. Tetrahedron Lett.
2009, 50, 4781.
(12) (a) Wolckenhauer, S. A.; Devlin, A. S.; Du Bois, J. Org.
Lett. 2007, 9, 4363. (b) John, J. P.; Novikov, A. V. Org. Lett.
2007, 9, 61.
(13) Shi, B.; Merten, S.; Wong, D. K. Y.; Chu, J. C. K.; Liu, L.
L.; Lam, S. K.; Jäger, A.; Wong, W. T.; Chiu, P.; Metz, P.
Adv. Synth. Catal. 2009, 351, 3128.
(14) Metz, P.; Fleischer, M.; Fröhlich, R. Tetrahedron 1995, 51,
711.
(15) Ender, D.; Harnying, W.; Vignola, N. Eur. J. Org. Chem.
2003, 20, 3939.
(16) (a) Rogachev, V. O.; Yusubov, M. S.; Filimonov, V. D.
Russ. J. Org. Chem. 1999, 35, 415. (b) Gaitzsch, J.;
Rogachev, V.; Metz, P.; Filimonov, V. D.; Zahel, M.;
Kataeva, O. J. Sulfur Chem. 2011, 32, 3.
NMR Data for [MSIm]OTf·HCl: 1H NMR (400.13 MHz,
CDCl3): δ (minor component) = 12.62 (s, 1 H), 12.52 (s, 1
H), 8.75 (s, 1 H, CH), 7.60 (s, 1 H, CH), 7.31 (s, 1 H, CH),
4.00 (s, 3 H, Me). 1H NMR (400.13 MHz, CDCl3): δ (major
component) = 12.62 (s, 1 H), 11.49 (s, 1 H, NH), 8.44 (s, 1
H, CH), 7.36 (s, 1 H, CH), 7.31 (s, 1 H, CH), 3.90 (s, 3 H,
Me). 13C NMR (100.6 MHz, CDCl3): δ (minor component)
= 135.15 (CH), 123.90 (CH), 12.30 (CH), 118.93
(q, 1JC–F = 317 Hz, CF3), 36.29 (Me). 13C NMR (100.6 MHz,
CDCl3): δ (major component) = 134.77 (CH), 123.19 (CH),
119.90 (CH), 118.93 (q, 1JC–F = 317 Hz, CF3), 35.84 (Me).
(18) For quantitative measurement of chloride ion by Mohr’s
method, see: Skoog, D. A.; West, D. M.; Holler, F. J.
Fundamentals of Analytical Chemistry, 7th ed; Thomson
Learning Inc: Stamford (CT, USA), 1996.
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