Synthesis and photooxygenation of (S)-p-tolylsulfinylfuran derivatives

Article abstract of DOI:10.1016/S0957-4166(00)00051-3

The synthesis of different (S)-1,4-dicarbonyl-2-(p-tolylsulfinyl)-2-alkenes by photooxygenation of enantiomerically pure (S)-p-tolylsulfinylfurans is reported. Copyright (C) 2000.

Full text of DOI:10.1016/S0957-4166(00)00051-3

Tetrahedron: Asymmetry 11 (2000) 1183±1191
Synthesis and photooxygenation of (S)-p-tolylsul®nylfuran
derivatives
Yolanda Arroyo,a M. Carmen Carreno,b,* Jose L. Garcõa Ruano,b,*
Justo F. Rodrõguez Amo,a,* Mercedes Santosa and M. Ascension Sanz Tejedora
a
Â
Departamento de Quõmica Organica, E. T. S. de Ingenieros Industriales, Paseo del Cauce s/n, 47011 Valladolid, Spain
Â
Departamento de Quõmica Organica (C-1), Universidad Autonoma, Cantoblanco, 28049 Madrid, Spain
b
Â
Â
Â
Received 23 December 1999; accepted 3 February 2000
Abstract
The synthesis of di€erent (S)-1,4-dicarbonyl-2-(p-tolylsul®nyl)-2-alkenes by photooxygenation of enantio-
merically pure (S)-p-tolylsul®nylfurans is reported. # 2000 Published by Elsevier Science Ltd. All rights
reserved.
1. Introduction
Substituted furans are key architectural features in a wide range of natural products1 and have
been frequently used in organic synthesis2 as versatile building blocks. One of their most useful
transformations is the oxidative ring opening3 giving rise to 1,4-dicarbonyl compounds which can
be further transformed into complex molecules taking advantage of their high reactivity as
dienophiles and Michael acceptors. Applications in asymmetric synthesis require the accessibility
of enantiomerically pure derivatives.4 In connection with a program directed to developing new
applications of sulfoxides,5,6 we reasoned that introduction of an enantiopure sul®nyl group into
the furan ring would allow access to chiral sul®nyl substituted 1,4-diketo derivatives.7 Therefore,
we focused on p-tolylsul®nylfurans 1 as a source of sul®nyl derivatives bearing 1,4-dicarbonyl-2-
alkenes such as 2 (Fig. 1) which could be obtained by oxidative ring opening of the former
compounds.
Although some sul®nylfuran derivatives that could act as dienes in Diels±Alder reactions are
known,8 their use in asymmetric synthesis has been rare probably as a consequence of the inherent
diculties encountered in the synthesis of such di€erently substituted chiral targets. A few recent
reports have revealed the ability of some sul®nylfuryl carboxaldehydes to behave as chiral hetero-
dienophiles9 and, when supporting an a,b-unsaturated ketone, as dienophiles.8a A bibliographic
* Corresponding author.
0957-4166/00/$ - see front matter # 2000 Published by Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00051-3

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Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
Figure 1.
search of compounds of type 2, revealed no reports of such highly functionalized analogues. The
wide interest in both families of compounds prompted us to undertake their synthesis. We report
herein a regiocontrolled approach to enantiomerically pure p-tolylsul®nylfurans 1a±e and the
study of their sensitized photooxygenation as a direct access to compounds of type 2.
2. Results and discussion
The simplest and most widely used approach to enantiomerically pure sulfoxides is Andersen's
synthesis10 based on the reaction of an organometallic species with menthyl p-toluenesul®nate.
Application of this procedure for the preparation of p-tolylsul®nylfurans involves the regio-
controlled formation of metallated furan precursors. Although regioselective directed metallation
of furans is highly dependent on the substitution on the aromatic ring as well as on experimental
conditions,11 preferred reaction occurs at C-2 when it is unsubstituted. The synthesis of 3-sul®nyl
substituted derivatives was then planned starting from the corresponding 3-bromofurans to allow
the selective formation of the required 3-metallated species through a Br metal exchange. Com-
pound 1a8b had already been obtained following this methodology from 3-bromofuran 3. When
we applied the reported procedure for 3-(p-tolylsul®nyl)furan 1a generating the organolithium
intermediate by addition of 3-bromofuran 3 to a stirred solution of n-BuLi, and subsequent
12
.
addition of MgBr2 Et2O and (SS)-menthyl-p-toluenesul®nate,
3-bromo-2-(p-tolylsul®nyl)-
furan8b was obtained in a 73% yield. Enantiomerically pure derivative 1a (Scheme 1) could be
synthesized in 82% yield applying a slight modi®cation of this procedure: addition of the n-BuLi
solution to the bromo derivative 3 to generate the organolithium (see Experimental). (SS)-4-
Bromo-3-(p-tolylsul®nyl)furan 1b was prepared in a highly regiocontrolled manner by successive
treatment of 3,4-dibromofuran 413 with n-BuLi (1 equiv.), an excess of MgBr2 (4 equiv.) and
( )-(SS)-menthyl-p-toluenesul®nate at 78ꢀC (Scheme 1). Under these conditions, 1b was
isolated after ¯ash chromatography in a 74% yield and in enantiomerically pure form.14 Synthesis
of 1c15 was similarly achieved in 72% yield starting from 2-methylfuran 5. As previously observed
by us16 and corroborated by other authors,8c the use of MgBr2 is essential to avoid partial
racemization of the ®nal sulfoxide that sometimes occurred when the initially formed organo-
lithium reagent directly reacted with the sul®nylating agent.

Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
1185
Scheme 1.
In a similar way, 3,4-dibromo-2,5-dimethyl furan 617 a€orded enantiomerically pure (SS)-4-
bromo-2,5-dimethyl-3-(p-tolylsul®nyl)furan 1d in a 81% yield. Further debromination of 1d, that
was achieved after n-BuLi/bromo exchange by adding water, provided (SS)-2,5-dimethyl-3-(p-
tolylsul®nyl)furan 1e in a 30% yield. The moderate yield of this reaction was due to the compe-
titive formation of n-butyl-p-tolyl sulfoxide18 that was isolated in a 23% yield. This sulfoxide could
only be formed in a nucleophilic attack of n-BuLi to the sul®nyl group of the furan ring of 1d
and/or 1e. In order to improve the yield of 1e, we attempt to introduce the sul®nyl group at C-3
by direct metallation of 2,3-dimethylfuran 7 (Scheme 2). In all trials, (SS,SS)-2-(1,10-bis-p-tolyl-
sul®nyl)methyl-5-methylfuran 1f was formed and no traces of 1e were detected. The best yield of
1f (54%) was achieved by sequentially adding 2,5-dimethylfuran 7 in THF and the sul®nylating
agent over a solution of LDA.
Scheme 2.
Transformation of compounds 1 into 1,4-dicarbonyl alkenyl sulfoxides 2 had to take into
account the presence of the sul®nyl group which is sensitive to oxidation. Among the wide range
of procedures reported to oxidize the furan ring,3 we ®rst tried Br2 in the presence of pyridine.19
We investigated the behaviour of 4-bromo-2,5-dimethyl-3-(p-tolylsul®nyl)furan 1d in order to
®nd the best conditions to isolate the presumably more stable 1,4-diketone 2d. Unfortunately, all
the experiments e€ected with this oxidizing reagent using di€erent solvents (acetone:H2O,
CH3CN:H2O, MeOH) were unsuccessful, in that the starting material decomposed to a complex
reaction mixture. The use of 1O23b±d,20 and methylene blue as sensitizer, working under very mild
conditions (MeOH as solvent), gave rise, after treatment with Me2S, to a clean reaction mixture

1186
Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
where (Z,SS)-4-bromo-3-(p-tolylsul®nyl)-3-hexen-2,5-dione 2d and DMSO were the only products
detected (Scheme 3). The use of MeOH as solvent promoted the transformation of the initial
endoperoxide to a new intermediate hydroperoxide21 which is stable at low temperature and
allowed the formation of cis-1,4-dicarbonyl compound3a in the presence of Me2S.
Scheme 3.
The reaction of (SS)-3-(p-tolylsul®nyl)-2,5-dimethylfuran 1d in similar conditions allowed the
isolation of (E,SS)-3-(p-tolylsul®nyl)-3-hexen-2,5-dione 2d in 86% yield. Dialdehyde 2a resulting
from the photooxidation of 1a could be generated using CH2Cl2 as solvent since in the presence
of MeOH a complex reaction mixture was formed. Compound 2a was characterized from the
crude reaction mixture that revealed the clean formation of 2a and DMSO. Dialdehyde 2a could
not be puri®ed by chromatography due to its easy decomposition in the presence of silica gel.22
The stereochemistry of the double bonds of 2a, 2d and 2e was assigned as indicated in Scheme 3
from the accepted mechanism of these processes. 3a When photosensitized oxidations were carried
out on 1b, 1c and 1f under the same conditions as above, a complex reaction mixture resulted
after the starting materials had disappeared. The use of di€erent solvents (MeOH, CH2Cl2) and
temperatures did not result in the presence of compounds 2b, 2c and 2f being detected in the
reaction mixtures. From these results, only furans bearing the sulfoxide at C-3 of the ring are able
to give stable products after photooxidation.23
In summary, we have successfully synthesized four new enantiomerically pure (S)-p-tolylsul®nyl
substituted furans 1b±e and shown that the MB sensitized photooxidation a€orded (S)-3-(p-
tolylsul®nyl)-1,4-dicarbonyl alkenes 2a, 2d and 2e when the sulfoxide in the furan precursor
was situated at C-3. We are now studying the reactivity of such chiral dicarbonyl compounds as
dienophiles and Michael acceptors. The results will be reported in due course.
3. Experimental
1
Melting points were obtained in open capillary tubes and are uncorrected. H and 13C NMR
spectra were recorded at 300 MHz and 75.5 MHz in CDCl3. All reactions were monitored by
TLC which was performed on precoated sheets of silica gel 60, and ¯ash chromatography was
carried out with silica gel 60 (230±400 mesh) of Merck. Eluting solvents are indicated in the text.
The apparatus for inert atmosphere experiments was dried by ¯aming it in a stream of dry argon.
THF and Et2O were distilled over sodium and benzophenone ketyl radical anion. CH2Cl2 was

Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
1187
dried over P2O5 and stored over molecular sieves. Absolute methanol was purchased from
Fluka. For routine workup, hydrolysis was carried out with saturated aqueous NH4Cl, extrac-
tions with CH2Cl2 and ethyl acetate and solvent drying with MgSO4. Optical rotations were
1
measured with a 141 Perkin±Elmer polarimeter. Speci®c rotations are given in units of 10 deg
1
cm2 g .
3.1. General procedure for the synthesis of p-tolylsul®nylfurans: method A
A solution of n-BuLi in hexane was added dropwise to a solution of the corresponding furan
ꢀ
.
3±6 in THF at 78 C. After 1 h stirring, Br2Mg Et2O was slowly added, and the resulting
mixture was warmed to 60ꢀC and stirred for 2 h. A solution of (SS)-menthyl-p-toluenesul®nate
in THF was then added. After conversion of all the starting furan, work-up a€orded a crude
mixture that was puri®ed by ¯ash chromatography.
3.2. (+)-(SS)-3-(p-Tolylsul®nyl)furan 1a
Compound 1a (2.33 g, 11.3 mmol, 82% yield) was obtained following method A from 3-bromo-
.
furan 3 (13.9 mmol) in THF (20 ml) using Br2Mg Et2O (41.6 mmol) and ( )-(SS)-menthyl-p-
toluenesul®nate (13.9 mmol) in THF (10 ml). Eluent: hexane:Et2O, 1:2, Rf=0.20. White solid.
20
20
D
‰ꢀŠ +63.3 (c 1.00, CHCl3); ‰ꢀŠ +43.5 (c 1.00, acetone). Lit.8b ‰ꢀŠD20+31. (c 2.50, acetone).
D
NMR data are superimposable with those previously reported.
3.3. (+)-(SS)-4-Bromo-3-(p-tolylsul®nyl)furan 1b
Compound 1b (464 mg, 1.6 mmol, 74% yield) was obtained following method A from 3,4-
.
dibromofuran 4 (500 mg, 2.2 mmol) in THF (8 ml) using Br2Mg Et2O (6.6 mmol) and ( )-(SS)-
menthyl-p-toluenesul®nate (2.2 mmol) in THF (10 ml). Eluent: hexane:Et2O, 3:1, Rf=0.18. White
20
solid. Mp: 47±48ꢀC; ‰ꢀŠ +53.0 (c 1.03, CHCl3); 1H NMR ꢁ: 2.42 (3H, s, CH3-Tol), 7.47 (1H, d,
D
J2,5 1.7 Hz, H-C5), 7.63 (H, d, J2,5 1.7, H-C2), 7.32 and 7.64 (4H, system AA0BB0, H-arom); 13C
NMR ꢁ: 21.4 (CH3-Tol), 98.1 (C-4), 125.5 and 129.9 (CH-arom), 130.6 (C-3), 139.0 and 142.2
(C-arom), 143.3 and 145.0 (C-2 and C-5); MS [m/z (relative intensity)]: 286 (35), 284 (31), 268 (3),
236 (100), 139 (21), 91 (43), 65 (40). HRMS (EI) calcd for C11H9O2BrS: 283.95099, found:
283.95032. Anal. calcd for C11H9O2BrS: C, 46.34; H, 3.18. Found: C, 46.35; H, 3.17.
3.4. (+)-(SS)-5-Methyl-2-(p-tolylsul®nyl)furan 1c
Compound 1c (317 mg, 1.4 mmol, 72%) was obtained following method A from 2-methylfuran
.
5 (114.0 ml, 2 mmol) in THF (5 ml) using Br2Mg Et2O (6.6 mmol) and ( )-(SS)-menthyl-p-
20
D
toluenesul®nate (588.0 mg, 2.0 mmol) in THF (5 ml). Eluent hexane:Et2O, 1:3; Rf=0.35. ‰ꢀŠ
20
D
20
D
1
+153.5 (c 0.96, CHCl3); ‰ꢀŠ +195 (c 1.00, acetone); Lit.8c ‰ꢀŠ +165 (c 1.00, acetone); H
NMR ꢁ: 2.29 (3H, s, CH3-C5), 2.42 (3H, s, CH3-Tol), 6.04 (1H, dd, J3,4 3.3, J3,Me 0.8, H-C4), 6.68
(1H, d, J4,3 3.3, H-C3), 7.32 and 7.58 (4H, system AA0BB0, H-arom); 13C NMR ꢁ: 13.8 (CH3-C5),
21.4 (CH3-Tol), 107.6 and 117.7 (C-3 and C-4), 124.9 and 129.7 (CH-arom), 138.3 and 141.4 (C-
arom), 151.2 and 158.1 (C-2 and C-5); MS [m/z (relative intensity)]: 220 (1), 204 (3), 172 (100),
129 (42), 97 (31), 91 (16), 43 (26). HRMS (EI) calcd for C12H12O2S: 220.05580, found: 220.05510.
Anal. calcd for C12H12O2S: C, 65.42; H, 5.49. Found: C, 65.32; H, 5.51.

1188
Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
3.5. ( )-(SS)-4-Bromo-2,5-dimethyl-3-(p-tolylsul®nyl)furan 1d
Compound 1d (370 mg, 1.2 mmol, 81%) was obtained following method A from 3,4-dibromo-
.
2,5-dimethylfuran 6 (370 mg, 1.5 mmol) in THF (5 ml) using Br2Mg Et2O (4.4 mmol) and
( )-(SS)-menthyl-p-toluenesul®nate (429.2 mg, 1.5 mmol) in THF (10 ml). Eluent: hexane:Et2O,
1
20
D
3:1, Rf=0.32. White solid. Mp: 92±93ꢀC; ‰ꢀŠ
57.3 (c 1.04, CHCl3); H NMR ꢁ: 2.18 (3H, s,
CH3-C5), 2.39 (3H, s, CH3-Tol), 2.46 (3H, s, CH3-C2), 7.26 and 7.48 (4H, system AA0BB0,
H-arom.); 13C NMR ꢁ: 11.5 and 12.7 (CH3-furan), 21.3 (CH3-Tol), 94.6 (C4), 121.9 (C3), 124.8
and 129.6 (CH-arom), 139.6 and 140.7 (C-arom), 149.4 and 154.9 (C-2 and C-5); MS [m/z (rel-
ative intensity)]: 314 (10), 312 (19), 266 (14), 295 (47), 173 (33), 139 (32), 91 (37), 43 (100). HRMS
(EI) calcd for C13H13O2BrS: 311.98196, found: 311.98294. Anal. calcd for C13H13O2BrS: C,
49.85; H, 4.18. Found: C, 49.70; H, 4.18.
3.6. ( )-(SS)-2,5-Dimethyl-3-(p-tolylsul®nyl)furan 1e
A solution of n-BuLi (1.76 mmol) in hexane was added dropwise to a solution of ( )-(SS)-4-
bromo-2,5-dimethyl-3-(p-tolylsul®nyl)furan 1d (500 mg, 1.6 mmol) in THF (12 ml) at 78ꢀC.
The mixture was stirred for 30 min at 78ꢀC and then distilled water (6 ml) was added. The
reaction mixture was warmed to rt and extracted with Et2O and ethyl acetate. The combined
organic layers were washed with brine, dried over MgSO4, and the solvents were removed
under reduced pressure. The resulting crude reaction was puri®ed by ¯ash chromatography
76.6 (c 0.50,
20
D
(hexane:Et2O, 1:3, Rf=0.38) a€ording 1e as a yellow oil in 30% yield. ‰ꢀŠ
1
CHCl3); H NMR ꢁ: 2.17 (3H, s, CH3-C5), 2.40 (3H, s, CH3-pTol), 2.52 (3H, s, CH3-C2), 5.78
(1H, s, H-C4), 7.28 and 7.49 (4H, system AA0BB0, H-arom); 13C NMR ꢁ: 12.3 and 13.4
(CH3-furan), 21.3 (CH3-Tol), 102.8 (C-4), 124.2 (CH-arom), 125.1 (C-3), 129.7 (CH-arom), 140.7
and 142.2 (C-arom), 152.2 and 154.0 (C-2 and C-5); MS [m/z (relative intensity)]: 234 (16), 217
(60), 186 (35), 143 (31), 91 (26), 43 (100). (RS)-n-Butyl-p-tolyl sulfoxide24 was also obtained in a
1
23% yield as a white solid. H NMR ꢁ: 0.86 (3H, t, Jvic 7.2, CH3-CH2), 1.33±1.43 (2H, m,
CH3-CH2-), 1.52±1.58 (2H, m, -CH2-), 2.36 (3H, s, CH3-Tol), 2.70±2.75 (2H, d, -CH2S(O)Tol),
7.26 and 7.47 (4H, system AA0BB0, H-arom). HRMS (EI) calcd for C13H14O2S: 234.07145,
found: 234.07204.
3.7. (SS,SS)-2-(1,10-Bis-p-tolylsul®nyl)methyl-5-methylfuran 1f
To a solution of LDA (2.2 mmol) in THF (10 ml), at 78ꢀC, 2,5-dimethylfuran 7 (212 ml, 2.0
mmol) in THF (5 ml) was added. The mixture was warmed to rt, stirred for 5 h and then cooled
at 78ꢀC. A solution of ( )-(SS)-menthyl-p-toluenesul®nate (294 mg, 1.0 mmol) in THF (5 ml)
was added. The resulting mixture was stirred at 78ꢀC for an additional hour and left to warm to
rt. Work-up led to a crude that was puri®ed by column chromatography (hexane:Et2O, 2:1) to
yield pure 1f (101 mg, 0.3 mmol, 54%) as a white solid. Eluent: hexane:Et2O, 1:3, Rf=0.28. Mp:
20
D
90±91ꢀC; ‰ꢀŠ +134.4 (c 0.96, CHCl3); H NMR ꢁ: 1.95, (3H, s, CH3-C5), 2.34 and 2.35 (two s,
1
3H each other, CH3-Tol), 4.64 (1H, s, CH-(SOTol)2), 5.94 (1H, d, Jvic 3.2, H-C4), 6.58 (1H, d, Jvic
3.2, H-C5), 7.15, 7.18, 7.23 and 7.31 (8H, two systems AA0BB0, H-arom); 13C NMR ꢁ: 13.2 (CH3-
C5), 21.3 (CH3-Tol), 89.7 (-CH-(SOTol)2, 107.4 and 117.7 (C-3 and C-4), 117.7 124.9, 129.5 and
129.7 (CH-arom), 136.0, 136,9, 138.6 and 141.9 (C-arom), 142.2 and 154.3 (C-2 and C-5). Anal.
calcd for C20H20O3S2: C, 64.48; H, 5.41. Found: C, 64.55; H, 5.39.

Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
1189
3.8. General procedure for the photosensitized oxidation of p-tolylsul®nylfurans: method B
A solution of the corresponding p-tolylsul®nylfuran in a dry solvent (CH3OH or CH2Cl2) was
irradiated with a halogen lamp (Tungsram, 500 W) in the presence of MB. During the irradiation,
dry oxygen was bubbled through the solution which was cooled at 40ꢀC. Progress of the
reaction was checked by TLC (hexane:Et2O, 1:3). When the reaction was completed (20 min) the
solution was degassed at 0ꢀC by bubbling Ar through (15 min), and then Me2S was added. The
reaction mixture was left to reach rt, and stirred for 3 h. As soon as the reduction was completed,
the solvent was removed under reduced pressure, Cl4C (30 ml) was added and the mixture was
®ltered to remove MB. Evaporation of the solvents gave a clean mixture of compounds 2 and
dimethyl sulfoxide.
3.9. (+)-(2E,SS)-2-(p-Tolylsul®nyl)-2-buten-1,4-dial 2a
Compound 2a (222 mg, 0.8 mmol, 84%) was obtained following method B from 1a (200 mg,
1.0 mmol) in CH2Cl2 (20 ml), MB (7Â10 3 mmol) and Me2S (155 mml, 1.9 mmol). The yield was
determined from the crude reaction consisting of a mixture of (E)-2a and dimethyl sulfoxide in a
1
58:42 ratio by H NMR. All attempts to purify 2a by chromatographic methods failed since it
20
D
decomposes on contact with the silica gel. Eluent: hexane:Et2O, 1:3, Rf=0.40. ‰ꢀŠ +225.0
0
0
1
(c 0.67, CHCl3); H NMR 2.39 (3H, s, CH3-Tol), 7.29 (2H, system AA BB , H-arom), 7.47 (1H,
d, J3,4 4.3, H-C3), 7.65 (2H, system AA0BB0, H-arom), 10.25 (1H, s, H-Cl), 10.32 (1H, d, J4,3 4.3
Hz, H-C4). IR (KBr, liquid ®lm): 1759, 1648; MS [m/z (relative intensity)]: 193 ([M+ 29], 6), 139
(100), 123 (86), 91 (85), 82 (12).
3.10. ( )-(3Z,SS)-4-Bromo-3-(p-tolylsu®nyl)-3-hexen-2,5-dione 2d
Compound 2d (240 mg, 0.9 mmol, 94%) was obtained following method B from 1d (290 mg,
3
0.9 mmol) in CH3OH (20 ml), MB (7Â10 mmol) and Me2S (135 ml, 1.8 mmol). The yield was
determined from the crude reaction consisting of a mixture of 2d and DMSO in a 67:33 ratio by
1H NMR. All attempts to purify 2d by chromatographic methods failed since it decomposes on
20
D
1
contact with the silica gel. Eluent: hexane:Et2O, 1:3. ‰ꢀŠ
396.1 (c 2.80, CHCl3); H NMR ꢁ:
2.31 (3H, s, H-C6), 2.42 (3H, s, CH3-Tol), 2.51 (3H, s, H-C1), 7.34 and 7.69 (4H, system
AA0BB0, H-arom); 13C NMR ꢁ: 21.6 (CH3-Tol), 28.2 and 32.5 (C-1 and C-6), 99.4 (C-4), 123.5
(C-3), 125.2 and 130.3 (CH-arom), 137.9 and 143.2 (C-arom.), 193.2 and 195.6 (C-2 and C-5); IR
(KBr, liquid ®lm): 1710, 1620; MS [m/z (relative intensity)]: 314 ([M+ 15], 8), 312 (9), 269 (13),
191 (13), 161 (18), 139 (100), 123 ( 15), 91 (33), 43 (63).
3.11. ( )-(3E,SS)-3-(p-Tolylsul®nyl)-3-hexen-2,5-dione 2e
Compound 2e (57 mg, 0.2 mmol, 86%) was obtained following method B from 1e (50 mg, 0.2
3
mmol) in CH3OH (20ml), MB (1.6Â10 mmol) and Me2S (30 ml, 0.4 mmol). The yield was
determined from the crude reaction consisting of a mixture of 2c and DMSO in a 50:50 ratio by
1H NMR. All attempts to purify 2c by chromatographic methods failed since it decomposes on
20
D
contact with the silica gel. Eluent: hexane:Et2O, 1:3. ‰ꢀŠ
377.3 (c 2.00, CHCl3); 1H NMR ꢁ 2.36
(3H, s, H-C6), 2.39 (3H, s, CH3-Tol), 2.41 (3H, s, H-Cl), 6.93 (1H, s, H-C4), 7.32 and 7.50 (4H,
system AA0BB0, H-arom); 13C-NMR ꢁ: 21.9 (CH3-Tol), 30.3 and 31.0 (C-1 and C-6), 125.0, 125.9

1190
Y. Arroyo et al. / Tetrahedron: Asymmetry 11 (2000) 1183±1191
and 130.5 (C-4 and CH-arom), 129.9 (C-3), 136.9 and 143.4 (C-arom), 195.4 and 198.6 (C-2 and
C-5); IR (KBr, liquid ®lm): 1715, 1625; MS [m/z (relative intensity)]: 234 ([M+ 16], 15), 139
(100), 123 (87), 91 (85), 43 (48).
Acknowledgements
We thank DGICYT (grants PB98-0062 and PB96-0078) for ®nancial support.
References
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1191
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