Reaction of 1,2-Dithiete with Alkenes and Alkynes: Experimental and Theoretical Study

Article abstract of DOI:10.1021/jo9908086

Reactions of 3,4-bis(methoxycarbonyl)-1,2-dithiete (1) with various alkenes or alkynes formed 2,3-dihydro-1,4-dithiins or thiophenes, respectively. The reactions with alkenes were stereospecific, which indicates the concerted reaction between 1,2-bis(methoxycarbonyl)ethane-1,2-dithione (13), the valence isomer of the 1,2-dithiete, and the dienophiles. Theoretical study confirmed the reactions of 13 with alkenes and alkynes are of reverse electron demand hetero Diels-Alder type. The MO calculations showed the 1,2-dithiete 1 was 5.8 kcal mol^-1 more stable than the corresponding ethane-1,2-dithione 13, and the tautomerization energy between the 1,2-dithiete and the ethane-1,2-dithione was also calculated to be 28.5 kcal mol^-1 from the 1,2-dithiete, which suggests the tautomerization from 1,2-dithiete 1 to ethane-1,2-dithione 13 is possible at least at high temperature. Reaction of 3,4,7,8-tetrakis(methoxycarbonyl)-1,2,5,6-tetrathiocin (2) or (Z,Z,Z,Z)-3,4,7,8,11,12,15,16-octakis-(methoxycarbonyl)-1,2,5,6,9,10,13,14- octathiacyclohexadeca-3,7,11,15-tetraene (3) with ethyl vinyl ether also formed the 2,3-dihydro-1,4-dithiin derivative, which is the same compound obtained by the reaction of 1,2-dithiete 1 with the ether. 1,2,5,6-Tetrathiocin 2 and 16-membered cyclic compound 3 also reacted with diphenylacetylene to give the thiophene derivative.

Full text of DOI:10.1021/jo9908086

J . Org. Chem. 1999, 64, 8489-8494
8489
Rea ction of 1,2-Dith iete w ith Alk en es a n d Alk yn es: Exp er im en ta l
a n d Th eor etica l Stu d y
Toshio Shimizu, Hideyuki Murakami, and Nobumasa Kamigata*
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Minami-ohsawa,
Hachioji, Tokyo 192-0397, J apan
Received May 17, 1999
Reactions of 3,4-bis(methoxycarbonyl)-1,2-dithiete (1) with various alkenes or alkynes formed 2,3-
dihydro-1,4-dithiins or thiophenes, respectively. The reactions with alkenes were stereospecific,
which indicates the concerted reaction between 1,2-bis(methoxycarbonyl)ethane-1,2-dithione (13),
the valence isomer of the 1,2-dithiete, and the dienophiles. Theoretical study confirmed the reactions
of 13 with alkenes and alkynes are of reverse electron demand hetero Diels-Alder type. The MO
calculations showed the 1,2-dithiete 1 was 5.8 kcal mol-1 more stable than the corresponding ethane-
1,2-dithione 13, and the tautomerization energy between the 1,2-dithiete and the ethane-1,2-dithione
was also calculated to be 28.5 kcal mol-1 from the 1,2-dithiete, which suggests the tautomerization
from 1,2-dithiete 1 to ethane-1,2-dithione 13 is possible at least at high temperature. Reaction of
3,4,7,8-tetrakis(methoxycarbonyl)-1,2,5,6-tetrathiocin (2) or (Z,Z,Z,Z)-3,4,7,8,11,12,15,16-octakis-
(methoxycarbonyl)-1,2,5,6,9,10,13,14-octathiacyclohexadeca-3,7,11,15-tetraene (3) with ethyl vinyl
ether also formed the 2,3-dihydro-1,4-dithiin derivative, which is the same compound obtained by
the reaction of 1,2-dithiete 1 with the ether. 1,2,5,6-Tetrathiocin 2 and 16-membered cyclic compound
3 also reacted with diphenylacetylene to give the thiophene derivative.
In tr od u ction
by X-ray crystallographic analysis.9 Some reactions of 1,2-
dithietes with alkenes have been examined to give 2,3-
dihydro-1,4-dithiin derivatives.3,6 However, it has not
been clarified whether the alkenes react with 1,2-dithiete
or ethane-1,2-dithione, which will be formed from the 1,2-
dithiete. Recently, we started to examine the reaction of
the 1,2-dithiete with various alkenes and alkynes. In this
paper, we report the reactions of 1,2-dithiete 1 with
alkenes and alkynes, and it was found that the reaction
proceeded between ethane-1,2-dithione and the unsatur-
ated compounds via a concerted path. Theoretical study
also supports the reaction of reverse electron demand
hetero Diels-Alder type. Activation energy for tautomer-
ization between the 1,2-dithiete and the ethane-1,2-
dithione is also discussed on the basis of ab initio MO
calculations.
Among the unsaturated cyclic compounds possessing
disulfide linkage, the chemistry of 1,2-dithietes has been
extensively studied and many reports have been pub-
lished.1-8 Especially, the relation with their valence
isomers, ethane-1,2-dithiones, has been focused on.5-8
Previously, we have succeeded in synthesizing 3,4-bis-
(methoxycarbonyl)-1,2-dithiete (1), 1,2,5,6-tetrathiocin 2,
and 16-membered cyclic compound 3 by oxidation of the
titanocene dithiolene complex with sulfuryl chloride, and
the crystal structure of 1,2-dithiete 1 could be determined
(4) (a) Bergson, G. Arkiv. Kemi 1962, 19, 181. (b) Bergson, G. Arkiv.
Kemi 1962, 19, 265.
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10.1021/jo9908086 CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/12/1999

8490 J . Org. Chem., Vol. 64, No. 23, 1999
Shimizu et al.
Sch em e 1
Resu lts a n d Discu ssion
When a benzene solution of 1,2-dithiete 1 and ethyl
vinyl ether was stirred at room temperature, 2,3-dihydro-
1,4-dithiin 4 was obtained in 65% yield, as shown in
Scheme 1. Although the reactions of 1 with (Z)- and (E)-
bis(benzylthio)ethenes were very slow at room temper-
ature, the reactions proceeded smoothly and stereospe-
cifically in refluxing benzene to give cis- and trans-
cycloadducts 5 and 6, respectively. The cis-geometry of
5 was confirmed by X-ray crystallographic analysis.
Similarly, reactions of 1 with (E)-anethole and (E)-
cinnamyl methyl ether also afforded only one of the
stereoisomers 7 and 8, respectively, though the reactions
were slow even in refluxing benzene. The reaction of 1
with N-phenylmaleimide also formed cycloadduct 9 al-
though the yield was low (11%) together with tetrakis-
(methoxycarbonyl)thiophene (10) in 38% yield. However,
1,2-dithiete 1 did not react with maleic anhydride even
in refluxing benzene, and thiophene 10 was obtained in
26% yield after refluxing for 24 h.
Some mechanisms are considered for the reactions of
1,2-dithiete 1 with the alkenes. Two mechanisms are via
zwitterion intermediates 11 and 12 generated by the
the dithione 13, in which the energy level of the HOMO
of maleic anhydride is lower than those of the other
alkenes.
Under this situation, the possibility of the formation
of ethane-1,2-dithione 13 from 1,2-dithiete 1 arises to be
a problem. 1H and 13C NMR spectra of 1 only showed the
1,2-dithiete structure, and no signal assigned to the
ethane-1,2-dithione could be detected even at high tem-
perature (80 °C in toluene-d8). However, it is no wonder
because a trace amount of formation of ethane-1,2-
dithione in the equilibrium may allow the reaction in the
solution. There are some theoretical studies concerning
stability of 1,2-dithietes and ethane-1,2-dithiones.4,6,8
However, these older studies used low levels of theory,
and the many calculations used the simple model mol-
ecules substituted by a hydrogen or methyl group. There-
fore, we attempted the comparison of the potential energy
of 1,2-dithiete 1 and ethane-1,2-dithione 13 with the
cisoid-form and estimated the activation energy of the
interconversion between the isomers. The structural
optimization and energy calculation were performed by
the second-order Møller-Plesset perturbation method
with a 6-31G(d) basis set. The calculation showed 1,2-
dithiete 1 is 5.8 kcal mol-1 more stable than ethane-1,2-
dithione 13, and the tautomerization energy was obtained
to be 28.5 kcal mol-1 from the 1,2-dithiete 1 (Figure 2).
This value of the activation energy supports the pos-
sibility of the tautomerization between 1,2-dithiete
1 and ethane-1,2-dithione 13 at least at high tempera-
ture.
reaction of 1 or ethane-1,2-dithione 13 with the alkenes,
and another is a concerted reaction of 13, the valence
isomer of the 1,2-dithiete, with the alkenes. The present
reactions are stereospecific, indicating the reactions are
proceeding via a concerted path.
To clarify the mechanism, we calculated the FMO
energy levels of the ethane-1,2-dithione 13 as a cisoid-
form and the alkenes used for the reaction. The calcula-
tions were performed by the Hartree-Fock method
with a 6-31G(d) basis set. The calculated FMO energy
levels are summarized in Figure 1. Differences of energy
levels between the LUMO of 13 and the HOMO of
the alkenes are smaller than those of the π orbital of
13 and the LUMO of the alkenes. This result indicates
the reaction is of reverse electron demand hetero
Diels-Alder type due to the low energy level of the
LUMO (π*) for ethane-1,2-dithione 13, and it can also
explain the inert reactivity of maleic anhydride toward
(8) (a) Calzaferri, G.; Gleiter, R. J . Chem. Soc., Perkin Trans. 2
1975, 559. (b) Haddon, R. C.; Wasserman, S. R.; Wudl, F.; Williams,
G. R. J . J . Am. Chem. Soc. 1980, 102, 6687. (c) Yu, H.; Chan, W.-T.;
Goddard, J . D. J . Am. Chem. Soc. 1990, 112, 7529. (d) J onas, V.;
Frenking, G. Chem. Phys. Lett. 1991, 177, 175. (e) Maier, G.; Schrot,
J .; Reisenauer, H. P.; Frenking, G.; J onas, V. Chem. Ber. 1992, 125,
265. (f) Gonza´lez, L.; Mo´, O.; Ya´n˜ez, M. Chem. Phys. Lett. 1996, 263,
407.
When a p-xylene solution of 1,2-dithiete 1 was re-
fluxed for 8 h, the thiophene 10 was obtained in 31%
yield (Scheme 2). Therefore, the formation of thio-
phene 10 in the reaction of 1,2-dithiete 1 with N-
phenylmaleimide or maleic anhydride can be explained
(9) Shimizu, T.; Murakami, H.; Kobayashi, Y.; Iwata, K.; Kamigata,
N. J . Org. Chem. 1998, 63, 8192.

Reaction of 1,2-Dithiete with Alkenes and Alkynes
J . Org. Chem., Vol. 64, No. 23, 1999 8491
F igu r e 1. FMO energy levels of ethane-1,2-dithione 13 as a cisoid-form and alkenes calculated by the Hartree-Fock method
with a 6-31G(d) basis set.
Sch em e 2
as the result of reaction of 1,2-dithiete 1 with ethane-
1,2-dithione 13 because the desulfurization of the cy-
cloadduct 14 probably forms 1,4-dithiin derivative fol-
lowed by rearrangement and subsequent desulfurization7,10
to give thiophene 10. The calculated energy difference
between the LUMO of 13 and the HOMO of 1 is 8.9 eV,
and that between the π of 13 and the LUMO of 1 is 12.4
eV. Therefore, the reaction of 1,2-dithiete 1 with ethane-
1,2-dithione 13 is also considered to be of reverse electron
demand hetero Diels-Alder type. These results also
clarified that 1,2-dithiete 1 and N-phenylmaleimide
compete as the dienophiles toward the ethane-1,2-
dithione 13 in the reaction of 1 with N-phenylmaleimide.
Reactions of 1,2-dithiete 1 with acetylenes were also
examined in refluxing p-xylene. The 1,4-dithiin derivative
which is expected to be formed from the Diels-Alder
F igu r e 2. Geometry and energy diagram (kcal mol-1) of
tautomerization between 1,2-dithiete and ethane-1,2-
dithione 13 calculated by the second-order Møller-Plesset
perturbation method with a 6-31G(d) basis set.
1
reaction of ethane-1,2-dithione 13 with the acetylene
was not obtained in either reaction of 1 with dimethyl
acetylenedicarboxylate or diphenylacetylene, and tetra-
kis(methoxycarbonyl)thiophene (10) (25%) and 2,3-bis-
(methoxycarbonyl)-4,5-diphenylthiophene (15)11 (44%)
were yielded after 2 h (Scheme 3). Lack of isolation of
(10) (a) Parham, W. E.; Traynelis, V. J . J . Am. Chem. Soc. 1954,
76, 4960. (b) Grigg, R.; Hayes, R.; J ackson, J . L. J . Chem. Soc., Chem.
Commun. 1969, 1167. (c) Kobayashi, K.; Mutai, K.; Kobayashi, H.
Tetrahedron Lett. 1979, 5003. (d) Nakayama, J .; Shimomura, M.;
Iwamoto, M.; Hoshino, M. Heterocycles 1985, 23, 1907.
(11) Schrauzer, G. N.; Mayweg, V. Z. Naturforsch. 1964, 19b, 192.

8492 J . Org. Chem., Vol. 64, No. 23, 1999
Shimizu et al.
Sch em e 4
with the ether. The reactions of 2 and 3 with dipheny-
lacetylene in refluxing p-xylene also yielded thiophene
derivative 15 in 49 and 41% yields, respectively, while
the reaction did not proceed at room temperature. In
these reactions, 1,2-dithiete 1 is considered to be formed
by heating since the compounds 2 and 3 are already
known to be stable in nonpolar solvents at room temper-
ature,9 although the ionic reaction via betaine intermedi-
ates 16 and 17 could not be completely excluded. How-
F igu r e 3. FMO energy levels of ethane-1,2-dithione 13 as a
cisoid-form and alkynes calculated by the Hartree-Fock
method with a 6-31G(d) basis set.
Sch em e 3
the 1,4-dithiin derivatives is also due to the high reactiv-
ity of the 1,4-dithiin derivatives under the conditions.7,10
The reaction of 3,4-di-tert-butyl-1,2-dithiete with acety-
lene was reported by Nakayama et al.7 to form two types
of thiophenes; however, the reaction of 1 with dipheny-
lacetylene yielded only one isomer. In the reactions of
1,2-dithiete 1 with acetylenes, two mechanisms are
also considered just like mentioned by Nakayama et al.7
If the reaction proceeded via ionic pathway, dimethyl
acetylenedicarboxylate may be a better reactant than
diphenylacetylene. However, the result was the contrary.
The FMO calculations of ethane-1,2-dithione 13 and the
acetylenes, as shown in Figure 3, also indicate the higher
reactivity of diphenylacetylene as the reverse electron
demand hetero Diels-Alder reaction.
ever, even if the ionic reaction occurred initially, ethane-
1,2-dithione 13 is formed as a side product of the 1,4-
dithiin derivative. Thermal reactions of 2 and 3 in
refluxing p-xylene also yielded thiophene 10 in similar
yields with that by the thermal reaction of 1,2-dithiete
1, indicating the reaction is also proceeding via 1,2-
dithiete 1.
Con clu sion
Reactions of 3,4-bis(methoxycarbonyl)-1,2-dithiete (1)
with various alkenes yielded 2,3-dihydro-1,2-dithiins,
stereospecifically. It was clarified the reactions proceed
between ethane-1,2-dithione, the valence isomer of the
1,2-dithiete, and alkenes via reverse electron demand
hetero Diels-Alder reaction by experimental and theo-
retical study. The 1,2-dithiete was also found to react
with alkynes to give thiophene derivatives. Reactions of
1,2,5,6-tetrathiocin 2 and 16-membered cyclic compound
3 with alkenes and alkynes were found to give the same
products as those obtained from the reactions of the 1,2-
dithiete with alkenes and alkynes.
1,2,5,6-Tetrathiocin 2 and 16-membered cyclic com-
pound 3 also reacted with ethyl vinyl ether at room
temperature to give 2,3-dihydro-1,4-dithiin derivative 4
in 64 and 63% yields, respectively, as shown in Scheme
4. In these reactions, 1,2-dithiete 1 is considered to be
formed initially by nucleophilic reaction of ethyl vinyl
ether to 2 or 3,9 and then the valence isomer 13 reacted
Exp er im en ta l Section
Gen er a l Meth od s. Benzene and p-xylene were distilled
from sodium metal before use. All reactions were performed
under nitrogen. Gel permeation chromatography (GPC) was

Reaction of 1,2-Dithiete with Alkenes and Alkynes
J . Org. Chem., Vol. 64, No. 23, 1999 8493
performed using a J AI LC-08 and a LC-908 liquid chromato-
graph with two J AIGEL-1H columns (20 mm × 600 mm), and
the products were eluted with chloroform.
8 h. After removal of the solvent in vacuo, thiophene 107,12
was purified by gel permeation chromatography and obtained
in 31% yield (3.8 mg).
Rea ction of 1,2-Dith iete 1 w ith Acetylen es. A p-xylene
solution (2 mL) of 1,2-dithiete 1 (20.6 mg, 100 µmol) and
dimethyl acetylenedicarboxylate (71.2 mg, 500 µmol) or diphe-
nylacetylene (89.0 mg, 500 µmol) was refluxed for 2 h. After
removal of the solvent in vacuo, purification by gel permeation
chromatography gave thiophene 10 (25%, 7.2 mg) or thiophene
1511 (44%, 14 mg), respectively.
Rea ction of Tetr a th iocin 2 or 16-Mem ber ed Cyclic
Com p ou n d 3 w ith Eth yl Vin yl Eth er . A benzene solution
(1 mL) of tetrathiocin 29 (20.6 mg, 50 µmol) or 16-membered
cyclic compound 39 (20.6 mg, 25 µmol) and ethyl vinyl ether
(0.48 mL, 5.0 mmol) was stirred for 40 h at room temperature.
After removal of the solvent in vacuo, purification by gel
permeation chromatography gave 2,3-dihydro-1,4-dithiin 4 in
64% (16 mg) yield from 2 or in 63% (15 mg) yield from 3,
respectively.
Rea ction of Tetr a th iocin 2 or 16-Mem ber ed Cyclic
Com p ou n d 3 w ith Dip h en yla cetylen e. A p-xylene solution
(2 mL) of tetrachiocin 2 (20.6 mg, 50 µmol) or 16-membered
cyclic compound 3 (20.6 mg, 25 µmol) and diphenylacetylene
(89.1 mg, 500 µmol) was refluxed for 2 h. After removal of the
solvent in vacuo, purification by gel permeation chromatog-
raphy gave thiophene 1511 in 49% (16 mg) yield from 2 or in
41% (13 mg) yield from 3, respectively.
Th er m a l Rea ction of Tetr a th iocin 2 or 16-Mem ber ed
Cyclic Com p ou n d 3. A p-xylene solution (2 mL) of tetrathio-
cin 2 (20.6 mg, 50 µmol) or 16-membered cyclic compound 3
(20.6 mg, 25 µmol) was refluxed for 8 h. After removal of the
solvent in vacuo, purification by gel permeation chromatog-
raphy gave thiophene 10 in 30% (4.1 mg) yield from 2 or in
32% (4.3 mg) yield from 3, respectively.
X-r a y Cr ysta llogr a p h ic An a lysis of 5. A crystal was
mounted on a glass fiber in a random orientation. Preliminary
examination and data collection were performed with Mo KR
radiation (λ ) 0.710 69 Å) on a Mac Science MXC18 diffrac-
tometer equipped with a graphite crystal, incident beam
monochromator. Cell constants and an orientation matrix for
data collection were obtained from least-squares refinement.
The data were collected at a temperature of 23 ( 1 °C using
the ω-2θ scan technique. The scan rate varied from 0 to 5°/
min (in ω). Lorentz and polarization corrections were applied
to the data. No absorption correction was made. An extinction
correction was not necessary. Hydrogen atoms were included
in the refinement but restrained to ride on the atom to which
they are bonded. The structure was refined in full-matrix least-
squares where the function minimized was ∑w(|Fo| - |Fc|)2 and
the weight w is defined as 1.0 for all observed reflections. A
pale yellow prism of C22H22O4S4 having approximate dimen-
sions of 0.30 × 0.20 × 0.15 mm3 was found to have triclinic
space group P1h with a ) 9.762(5) Å, b ) 11.586(5) Å, c ) 12.69-
(1) Å, R ) 110.54(5)°, â ) 101.08(5)°, γ ) 110.11(4)°, V )
1179.86(1) Å3, Z ) 2, and F(calcd) ) 1.347 g cm-3. Of the 5008
unique data, the 3827 with I > 3σ(I) were used in the least-
squares refinement to yield R ) 0.041 and Rw ) 0.041.
Th eor etica l Stu d y. MO calculations of 1,2-dithiete 1,
ethane-1,2-dithione 13, alkenes, and alkynes were performed
by the Hartree-Fock method with a 6-31G(d) basis set. The
structural optimization and energy calculation of 1, 13, and
the transition state for the tautomerization were carried out
by the second-order Møller-Plesset perturbation method with
a 6-31G(d) basis set. All calculations were performed by using
the GAUSSIAN94 program on an IBM RS/6000 computer.
Gen er a l P r oced u r e for Rea ction of 1,2-Dith iete 1 w ith
Alk en es. A benzene solution (4 mL) of 1,2-dithiete 19 (41.2
mg, 200 µmol) and alkene (1.00 mmol) was stirred for desired
period at desired temperature. After removal of the solvent
in vacuo, products were purified by gel permeation chroma-
tography: 4, 65%, 35 mg (room temperature, 40 h); 5, 72%,
64 mg (reflux, 2 h); 6, 60%, 52 mg (reflux, 2 h); 7, 59%, 40 mg
(reflux, 30 h); 8, 22%, 15 mg (reflux, 45 h); 9, 11%, 8.0 mg
(reflux, 45 h).
2,3-Bis(m et h oxyca r b on yl)-5-et h oxy-5,6-d ih yd r o-1,4-
d ith iin (4): pale yellow oil; 1H NMR (500 MHz, CDCl3) δ 1.27
(t, 3H, J ) 7.0 Hz), 3.16 (dd, 1H, J ) 2.0, 13.3 Hz), 3.24 (dd,
1H, J ) 5.2, 13.3 Hz), 3.61 (qd, 1H, J ) 7.0, 9.3 Hz), 3.806 (s,
3H), 3.812 (s, 3H), 3.98 (qd, 1H, J ) 7.0, 9.3 Hz), 5.30 (dd, 1H,
J ) 2.0, 5.2 Hz); 13C NMR (125 MHz, CDCl3) δ 14.7, 33.0, 53.0,
53.2, 64.9, 77.6, 126.9, 128.6, 164.2, 164.9; IR (neat) νmax 2970,
2900, 1730, 1550, 1430, 1250, 1100, 1030, 990 cm-1; MS (EI,
70 eV) m/z 278 (M+), 246, 232, 218, 72; HRMS calcd for
C
10H14O5S2 278.0282, found 278.0270.
cis-2,3-Bis(b en zylt h io)-5,6-b is(m et h oxyca r b on yl)-2,3-
d ih yd r o-1,4-d ith iin (5): pale yellow prisms; mp 107.5-108.0
1
°C (dichloromethane-hexane); H NMR (500 MHz, CDCl3) δ
3.79 (s, 6H), 3.86 (d, 2H, J ) 13.6 Hz), 3.94 (d, 2H, J ) 13.6
Hz), 4.25 (s, 2H), 7.23-7.33 (m, 10H); 13C NMR (125 MHz,
CDCl3) δ 36.1, 50.1, 53.2, 127.2, 127.5, 128.7, 129.0, 136.4,
164.0; IR (KBr) νmax 3040, 2950, 1740, 1560, 1500, 1460, 1440,
1420, 1250, 1080, 1020 cm-1; MS (EI, 70 eV) m/z 478 (M+),
357, 272, 232, 91. Anal. Calcd for C22H22O4S4: C, 55.20; H,
4.63. Found: C, 55.11; H, 4.66.
tr a n s-2,3-Bis(b en zylt h io)-5,6-b is(m et h oxyca r b on yl)-
2,3-d ih yd r o-1,4-d ith iin (6): colorless oil; 1H NMR (500 MHz,
CDCl3) δ 3.79 (d, 2H, J ) 13.6 Hz), 3.81 (s, 6H), 3.95 (d, 2H,
J ) 13.6 Hz), 4.08 (s, 2H), 7.20-7.31 (m, 10H); 13C NMR (125
MHz, CDCl3) δ 35.0, 48.7, 53.2, 126.0, 127.5, 128.7, 129.0,
136.4, 164.0; IR (neat) νmax 3040, 2950, 1740, 1550, 1490, 1460,
1440, 1430, 1240, 1070, 1010 cm-1; MS (EI, 70 eV) m/z 478
(M+), 357, 272, 232, 91; HRMS calcd for C22H22O4S4 478.0401,
found 478.0419.
tr a n s-2,3-Bis(m eth oxyca r bon yl)-5-(4′-m eth oxyp h en yl)-
6-m eth yl-5,6-d ih yd r o-1,4-d ith iin (7): pale yellow oil; 1H
NMR (500 MHz, CDCl3) δ 1.18 (d, 3H, J ) 6.8 Hz), 3.54 (qd,
1H, J ) 6.8, 8.7 Hz), 3.80 (s, 3H), 3.81 (s, 3H), 3.82 (s, 3H),
4.07 (d, 1H, J ) 8.7 Hz), 6.89 (d, 2H, J ) 8.7 Hz), 7.17 (d, 2H,
J ) 8.7 Hz); 13C NMR (125 MHz, CDCl3) δ 18.7, 41.4, 51.2,
53.0, 53.1, 55.3, 114.5, 126.8, 127.5, 128.8, 129.3, 159.7, 164.3,
164.4; IR (neat) νmax 3060, 2970, 2850, 1730, 1610, 1550, 1510,
1440, 1260, 1080, 1030 cm-1; MS (EI, 70 eV) m/z 354 (M+),
323, 263, 233, 148, 117; HRMS calcd for C16H18O5S2 354.0596,
found 354.0609.
tr a n s-2,3-Bis(m et h oxyca r b on yl)-5-m et h oxym et h yl-6-
p h en yl-5,6-d ih yd r o-1,4-d ith iin (8): pale yellow oil; 1H NMR
(500 MHz, CDCl3) δ 3.32 (s, 3H), 3.16 (dd, 1H, J ) 6.1, 10.1
Hz), 3.52 (dd, 1H, J ) 6.1, 10.1 Hz), 3.59 (td, 1H, J ) 6.1, 6.4
Hz), 3.81 (s, 3H), 3.82 (s, 3H), 4.51 (d, 1H, J ) 6.4 Hz), 7.28-
7.39 (m, 5H); 13C NMR (125 MHz, CDCl3) δ 45.7, 46.5, 53.12,
53.16, 59.2, 73.0, 126.5, 127.9, 128.35, 128.39, 128.9, 138.6,
164.36, 164.43; IR (neat) νmax 2970, 1740, 1560, 1440, 1260,
1130, 1100, 1030 cm-1; MS (EI, 70 eV) m/z 354 (M+), 309, 245,
147, 115, 91; HRMS calcd for C16H18O5S2 354.0596, found
354.0577.
ci s-3,4-B is (m e t h o x y c a r b o n y l)-8-p h e n y l-8-a za -2,5-
d ith ia bicyclo[4.3.0]n on -3-en -7,9-d ion e (9): pale yellow oil;
1H NMR (500 MHz, CDCl3) δ 3.85 (s, 6H), 4.78 (s, 2H), 7.25-
7.28 (m, 2H), 7.41-7.50 (m, 3H); 13C NMR (125 MHz, CDCl3)
δ 51.9, 53.6, 126.5, 129.36, 129.40, 131.0, 138.8, 163.3, 171.0;
IR (neat) νmax 2950, 1780, 1720, 1550, 1490, 1430, 1380, 1260,
1200, 1080, 1010 cm-1; MS (EI, 70 eV) m/z 379 (M+), 348, 259,
232, 201, 173; HRMS calcd for C16H13NO6S2 379.0184, found
379.0167.
Ack n ow led gm en t. The authors are grateful to Prof.
J . Nakayama (Saitama University) for the gift of a
preprint of their review (ref 1a) and Prof. S. Ikuta
(Tokyo Metropolitan University) for discussion about the
theoretical study. This work was financially supported
Th er m a l Rea ction of 1,2-Dith iete 1. A p-xylene solution
(2 mL) of 1,2-dithiete 1 (20.6 mg, 100 µmol) was refluxed for
(12) (a) Michael, A. Ber. Dtsch. Chem. Ges. 1895, 28, 1633. (b) Hopff,
H.; von der Crone, J . Chimia 1959, 13, 107.

8494 J . Org. Chem., Vol. 64, No. 23, 1999
Shimizu et al.
and thermal parameters, bond lengths and angles, and
torsional angles, Cartesian coordinates of 1,2-dithiete 1,
ethane-1,2-dithione 13, and the geometry at the transition
state for the tautomerization between 1 and 13 calculated by
the second-order Møller-Plesset perturbation method with the
6-31G(d) basis set, and NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
in part by a Grant-in-Aid for Scientific Research on
Priority Area and General Scientific Research from the
Ministry of Education, Science, Sports, and Culture of
J apan.
Su p p or tin g In for m a tion Ava ila ble: Detailed informa-
tion of the X-ray crystallographic analysis of 5, including
structure diagrams, details of data collection and reduction
and structure solution and refinement, and tables of positional
J O9908086