Regio- and stereo-chemistry of addition of molecular bromine to s-oxidized derivatives of 3,4-di-t-butylthiophene: Exclusive 1,4-cis-additions

Article abstract of DOI:10.1246/bcsj.76.619

Bromine added to the 1-oxide, 1,1-dioxide, and sulfoximide derivatives of 3,4-di-t-butylthiophene exclusively in a 1,4-cis-addition mode. Thus, bromine added to 3,4-di-t-butylthiophene 1-oxide at room temperature in CH₂Cl₂ to give the 1,4-cis-adducts exclusively. The adducts are composed of two compounds in the ratio 57:43, where the major one is the anti-adduct respective to the S=O bond and the minor one is the syn-adduct. Addition of bromine to 3,4-di-t-butylthiophene 1,1-dioxide gave the 1,4-cis-adduct as the sole product. Addition of bromine to the N-tosyl substituted sulfoximide derivative of 3,4-di-t-butylthiophene afforded the single 1,4-cis-adduct in which two bromine atoms are anti to the N-tosyl group. Bromine added to the N-unsubstituted sulfoximide derivative of 3,4-di-t-butylthiophene to produce the 1,4-cis-addition products exclusively, which are a 67:33 mixture of the anti- and syn-adducts to the S=O bond.

Full text of DOI:10.1246/bcsj.76.619

c. Jpn.
© 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 619–625 (2003)
619
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Regio- and Stereo-Chemistry of Addition of Molecular Bromine to
S-Oxidized Derivatives of 3,4-Di-t-butylthiophene: Exclusive
1,4-Cis-Additions
1,4-Cis-Additions of Bromine to Cyclic Dienes
J. Nakayama et al.
03
9
Juzo Nakayama,* Tomohiro Furuya, Akihiko Ishii, Akira Sakamoto, Takashi Otani, and
Yoshiaki Sugihara
5
SJA8
09-2673
278
Department of Chemistry, Faculty of Science, Saitama University, Saitama, Saitama 338-8570
(Received September 25, 2002)
02
02
Bromine added to the 1-oxide, 1,1-dioxide, and sulfoximide derivatives of 3,4-di-t-butylthiophene exclusively in a
1,4-cis-addition mode. Thus, bromine added to 3,4-di-t-butylthiophene 1-oxide at room temperature in CH2Cl2 to give
the 1,4-cis-adducts exclusively. The adducts are composed of two compounds in the ratio 57:43, where the major one is
the anti-adduct respective to the SwO bond and the minor one is the syn-adduct. Addition of bromine to 3,4-di-t-butyl-
thiophene 1,1-dioxide gave the 1,4-cis-adduct as the sole product. Addition of bromine to the N-tosyl substituted sulfox-
imide derivative of 3,4-di-t-butylthiophene afforded the single 1,4-cis-adduct in which two bromine atoms are anti to the
N-tosyl group. Bromine added to the N-unsubstituted sulfoximide derivative of 3,4-di-t-butylthiophene to produce the
1,4-cis-addition products exclusively, which are a 67:33 mixture of the anti- and syn-adducts to the SwO bond.
Recently, considerable interest has been paid to the chemis-
try of thiophene 1-oxides from many viewpoints such as syn-
theses, structures, reactivities, applications to organic synthe-
ses, and intermediates in the metabolism of thiophenes.1
Thiophene 1-oxides act both as extremely reactive cyclic
dienes and dienophiles, and are transient intermediates unless
Scheme 1. π-Face-selective Diels–Alder reaction (syn to the
SwO bond).
their double bonds are suitably deactivated either electronical-
ly or sterically. These compounds, which have the general
structure shown in Fig. 1,2,3 possess two π-faces for Diels–Al-
der reactions, i.e., syn-and anti-faces to the SwO bond. There-
fore, one of the most interesting features in the chemistry of
these compounds is the stereochemical course in Diels–Alder
reactions. Recent reports described that the Diels–Alder reac-
tions of thiophene 1-oxides take place exclusively at the syn-π-
face to the SwO bond in an endo-mode (Scheme 1).2e,4,5 It was
also reported that even bromine addition to the thiophene 1-ox-
ide 1 took place at the syn-face to the SwO bond and in a cis-
1,4-addition mode to give the adduct 2 as the sole product.6 A
concerted mechanism was proposed to explain the observed
stereochemistry (Scheme 2). Ab initio calculations also sup-
port the above mechanism. An important mechanistic problem
Scheme 2. π-Face-selective 1,4-cis-addition of bromine.
which remains to be fully investigated is that of the stereo-
chemistry of the 1,4-addition to 1,3-dienes. In this respect, the
stereochemistry of bromine additions to cyclopentadiene and
1,3-cyclohexadiene has been investigated previously in some
detail.7
Previously, we reported the synthesis of a series of S-oxi-
dized species of 3,4-di-t-butylthiophene, such as 1-oxide 3,2d
1,1-dioxide 4,8 1-imide 5,9 and sulfoximides 6 and 7 (Chart
1).10 Our recent study has also revealed that the Diels–Alder
reactions of 3 and 5 also take place at the syn-π-face to the
SwX (X = O or NTs) bond in an endo-mode.11 Keeping these
Fig. 1. General structure and two π-faces of thiophene 1-ox-
ides.

620 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
1,4-Cis-Additions of Bromine to Cyclic Dienes
1,4-trans-adduct 8c, the 1,2-cis-adducts 8d and 8e, and the 1,2-
trans-adducts 8f and 8g, and shows unambiguously that the
two compounds formed possess the structures described
above.
1
Discrimination of 8a and 8b was made based on the H
NMR chemical shift data (Table 1). The methine hydrogens of
8a (δ 5.62) resonated at 0.27 ppm lower than those of 8b
(δ 5.35) in CDCl3 as the solvent. The difference would be best
explained as a result of anisotropic effect by the SwO group.12
In order to obtain further support for the above assignment,
DFT calculations were carried out.13,14 The optimized struc-
tures of dibromides 8a and 8b are shown in Fig. 2. The dibro-
mide 8b is thermodynamically more stable by 1.66 kcal mol−1
than the other dibromide 8a. As shown in Fig. 2, the methine
protons of 8b are located on the side of the SwO group so that
they experience a high-field shift due to the anisotropic effect
of the SwO group.12 Calculations of the NMR shielding con-
stants by the GIAO method15 predicted that the methine pro-
tons of 8b are more shielded than those of 8a. Thus, the aver-
aged shielding constants of those of 8b are larger by 0.33 ppm
than those of 8a, and this value is in good agreement with the
observed chemical shift difference of 0.27 ppm (δ 5.62 for 8a
and δ 5.35 for 8b) and is no in contradiction with the structural
assignment.
Chart 1. Structural formulas 3–7.
results in mind, we have now investigated the stereochemistry
of the additions of bromine to 3–7. Reactions of S-oxidized
derivatives of thiophenes with bromine have hitherto not been
reported except for the bromine addition to 1.
Results
Product Analysis and Regio- and Stereo-Chemistry of
Bromine Additions. Bromine added to 3 at room tempera-
ture to give the two 1,4-cis-adducts 8a and 8b in the ratio
43:57 in 87% combined yield (Scheme 3). Isomerization be-
tween 8a and 8b did not take place even when each of them
was heated to its melting point. The observed ratio is therefore
the kinetically controlled one. The two methine protons of
both 8a and 8b are equivalent in the 1H NMR spectra. Similar-
ly, the two methine, vinyl, and t-butyl carbons of 8a and 8b are
equivalent in the 13C NMR spectra. These data rule out the
Furthermore, aromatic solvent-induced and [Eu(thd)3]-in-
duced shifts also support the above assignment (Table 1). In
C6D6, the positive end (sulfur atom) of the SwO group of 8a is
expected to coordinate to the solvent molecule C6D6 to result
in the aromatic solvent-induced shift.16 Indeed, a larger down-
Scheme 3. Stereochemistry of bromine addition to 3.
Table 1. Chemical Shift Data of the Methine Hydrogen of 8a and 8b: Aromatic Solvent- and [Eu(thd)3]-
Induced Shiftsa)
δH
CDCl3
5.62
δH
∆δH
CDCl3 − C6D6
0.64
δH
CDCl3 + [Eu(thd)3]
5.64b)
∆δH
C6D6
4.98
5.00
CDCl3 − [CDCl3 + [Eu(thd)3]]
8a
8b
−0.02
−0.23
5.35
0.35
5.58b)
a) [Eu(thd)3] = Tris(2,2,6,6-tetramethyl-3,5-heptanedionate)europium(Ⅲ). b) 1.0 molar amount of
[Eu(thd)3] was used; δ 5.64 for 8a and δ 5.47 for 8b by use of 0.5 molar amount of Eu(thd)3.

J. Nakayama et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 621
days at room temperature to give 9a in 98% yield, whereas that
of 8b was not completed even after a prolonged reaction of
eight days and gave 9a in 90% yield with 9% recovery of 8b.
The slower oxidation of 8b, compared to 8a, is ascribable to
the steric hindrance due to the two bromine atoms which are
syn to the lone pair of the sulfur atom. In other words, these
findings support the structural assignment of 8a and 8b. 1,2-
1
Adduct structures were ruled out unambiguously by H and
13C NMR spectra, which indicated a symmetrical structure of
the product.
Addition of bromine to 6, which was much slower than
those to 3 and 4 probably because of steric and/or electronic
effects, afforded the 1,4-adduct 10a exclusively in 84% yield
(Scheme 5). The diastereomeric adduct 10b was not formed.
The structure of 10a was determined by an independent syn-
thesis. Thus, treatment of 8a with phenyl[(p-tolylsulfonyl)-
imino]-λ3-iodane (TsNwIPh)18 in the presence of
[Cu(MeCN)4]PF6 produced 10a in 72% yield.9c,10 The configu-
ration at the sulfur atom of 8a would be retained during this re-
action since it is the mere addition of the tosyl nitrene or its
equivalent to the lone pair of the sulfur atom.19 When the
crude mixture of the above reaction was washed with aqueous
NaOH to remove p-toluenesulfonamide, formed from TsN-
wIPh, the dehydrobromination of 10a took place to give a con-
siderable amount of the sulfoximide 11. Indeed, stirring a so-
lution of 10a with 1 M NaOH for 1 h gave 11 in 95% yield.
The exclusive formation of 10a suggests that bromine added to
Fig. 2. Predicted optimized structures of 8a and 8b.
field shift of the methine hydrogen was observed for 8a
(0.64 ppm) than for 8b (0.35 ppm). On the other hand,
[Eu(thd)3] coordinates to the negative end (oxygen atom) of
the SwO group.17 Thus the chemical shift value of the methine
hydrogen of 8b would be more influenced by [Eu(thd)3] than
that of 8a. Actually, the methine proton of 8b shifted up-field
by 0.23 ppm in the presence of [Eu(thd)3], whereas the up-field
shift of 8a is negligibly small (0.02 ppm).
Bromine added to the 1,1-dioxide 4 to furnish 9a as a single
diastereomer in 85% yield (Scheme 4). The cis-structure of 9a
was differentiated from the trans-structure 9b by the experi-
mental fact that 9a was obtained both from 8a and 8b on oxi-
dation. Thus, the oxidation of 8a with 1.1 molar amounts of
m-chloroperbenzoic acid (MCPBA) was completed in four
Scheme 4. Stereochemistry of bromine addition to 4.
Scheme 5. Stereochemistry of bromine addition to 6.

622 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
1,4-Cis-Additions of Bromine to Cyclic Dienes
1
6 from the less hindered anti-side respective to the more bulky
TsN group. Attempted preparation of 10b by reaction of 8b
with TsNwIPh failed. No reaction took place even by use of
excess TsNwIPh on a prolonged reaction time because of the
steric hindrance of the two bromine atoms which are syn to the
sulfur lone pair. In other words, these findings support again
the validity of the structural assignment of 8a and 8b.
Addition of bromine to the N-unsubstituted sulfoximide 7
showed a small π-face selectivity (Scheme 6). Thus, bromine
added to 7 from the NH side with a slight preference to pro-
duce 12a and 12b in the ratio 1:2 in a good combined yield.
The reaction is again an exclusive 1,4-cis-addition. The dis-
crimination of the structures 12a and 12b was made by the fact
that 12a was derived by treatment of 10a with concentrated
sulfuric acid.9c,10,20
(Scheme 7). The product ratio was determined by H NMR
spectrum analysis. Structures of 13 and 14 were determined
by comparison of spectral data with those of authentic sam-
ples, whereas the exact positions of the t-butyl group and the
bromine atoms in 15 and 16 remained undetermined. The ex-
pected bromine adduct such as 17 might be formed in a small
amount. Treatment of the reaction mixture by silica-gel col-
umn chromatography followed by aqueous NaOH resulted in
the isolation of a compound whose structure is tentatively as-
signed as 18, the dehydrobromination product of 17.
The nitrogen atom of the imide 5 is expected to be more nu-
cleophilic than is the oxygen atom of 3 and 4 or the nitrogen
atom of 6 and 7. Thus, the initial reaction would take place at
the nitrogen atom to produce a sulfonium bromide 19, al-
though bromine addition may occur competitively as a minor
path. The addition of bromide ion to 19 gives a sulfonium
ylide 20. Elimination of TsNHBr from 20 would produce 13
as one of the final products. The TsNHBr, thus formed, serves
Interestingly, the reaction of the 1-imide 5 with 1.1 molar
amounts of bromine afforded 5, 13, 14, 15, and 16 in the ratio
26:17:7:25:24, in addition to p-toluenesulfonamide (TsNH2)
Scheme 6. Stereochemistry of bromine addition to 7.
Scheme 7. Reaction of 5 with bromine.

J. Nakayama et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 623
as a brominating agent of thiophenes, as does N-bromosuccin-
imide (NBS),8a and reacts with 13 to produce the dibro-
mothiophene 14 (Br2 used for the reaction also might be in-
volved in this bromination, if it still exists). Further reactions
of 14 with TsNHBr (Br2) would result in the formation of 15
and 16 as observed when 3,4-di-t-butylthiophene was treated
with excess NBS.8a,21
Discussion
The experimental findings, described above, raise two ques-
tions. The first question involves why bromine additions to 3,
4, 6, and 7 take place exclusively in a 1,4-mode, and the sec-
ond one involves why only cis-addition takes place. The sim-
plest answer is to presume a concerted mechanism (Fig. 3) as
was proposed for the bromine addition to 1.6
Here, the third question arises why the bromine addition to
3 is not π-face selective, though that to 1 is π-face selective.6
In other words, which is more general for bromine additions to
thiophene 1-oxides? In order to answer this question, we need
more experimental findings, although the study is hampered by
the fact that only a few thiophene 1-oxides are stable enough to
be handled under ordinary conditions.1
DFT MO calculations (B3LYP/6-31G* level) predicted that
the lobe size of the π-HOMO at the α-carbon atoms of 3 is
slightly greater at the syn-side than the anti-side respective to
the SwO bond (Fig. 4).14 Thus, the experimental findings sug-
gest that nonequivalent orbital extension is not always a crucial
factor to determine the π-face selectivity in additions that in-
volve strong electrophiles such as bromine, even if the addition
proceeds in a concerted mechanism.
In order to examine a possibility of mechanisms involving
ionic intermediates in the bromine addition to the 1,1-dioxide
4, calculation study (B3LYP/6-31G* level) was carried out.14
The calculations were started by placing Br+ above the center
of the C2–C3 bond (21) and also above the center of the non-
Chart 2. Intermediates of bromine addition to 4.
bonded C2–C5 bond (22) of the optimized structure of 4
(Chart 2). The calculations in both cases converged at the state
of the open-chain carbonium intermediate 25, thus predicting
that 25 is more stable than are the 1,2-bridged bromonium ion
intermediate 23 and 1,4-bridged intermediate 24. This will ex-
plain why the 1,4-addition takes place in preference to the 1,2-
addition; Br− would add to the less crowded C5 of 25, and not
the sterically hindered C3, due to the contribution of the ca-
nonical structure 25b. The question still remains, however, as
to why cis-addition takes place exclusively.
In conclusion, we have presented interesting bromine addi-
tions to cyclic dienes which took place exclusively in a 1,4-cis-
addition mode. Seemingly, the concerted mechanism would
be best open to the above experimental findings, at least at the
present stage of the investigation. However, we cannot present
any direct evidence that rules out the ionic mechanism. Al-
though bromine additions to cyclopentadiene and 1,3-cyclodi-
ene were previously reported,7 such exclusive 1,4-cis-additions
are rather exceptional and would raise a new question for
mechanistic organic chemistry.
Experimental
Solvents were purified and dried in the usual manner. All of the
reactions were carried out under argon. Silica-gel column chro-
matography was performed on silica gel 7734 (Merck, 70–230
mesh). Melting points were determined on a Mel-Temp capillary
tube apparatus and are uncorrected. 1H and 13C NMR spectra
were recorded on a Bruker ARX400, a Bruker AM400, a Bruker
AC300P, or a Bruker AC200 spectrometer using CDCl3 as the sol-
vent with TMS as the internal standard. IR spectra were taken on
a Hitachi 270-50 or a Perkin Elmer System 2000 FT-IR spectrom-
eter. UV spectrum was determined on a JASCO V-560 spectro-
photometer. Elemental analyses were performed by the Chemical
Analysis Center of Saitama University.
Fig. 3. Concerted bromine addition.
Addition of Bromine to Thiophene 1-Oxide 3.
A
solution
of 352 mg (2.2 mmol) of bromine in 8 mL of CH2Cl2 was added
slowly to a stirred solution of 426 mg (2.0 mmol) of 3 in 5 mL of
CH2Cl2 at room temperature. The mixture was stirred for 1 h and
then evaporated under reduced pressure. The resulting crystalline
residue was chromatographed on a column of silica gel. The elu-
tion of the column by hexane/ether (3:2) as the eluent afforded
369 mg (50%) of 8b and 273 mg (37%) of 8a in this order. 8a: mp
174–177 °C; colorless crystals (from hexane); 1H NMR
Fig. 4. Nonequivalent orbital extension of π-HOMO of 3.

624 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
1,4-Cis-Additions of Bromine to Cyclic Dienes
(400 MHz) δ 1.48 (18H, s), 5.35 (2H, s); 13C NMR (100.6 MHz) δ
32.0, 35.7, 66.3, 147.3; IR (KBr) 1115 cm−1 (SwO). Anal. Calcd
for C12H20Br2OS: C, 38.73; H, 5.42%. Found: C, 38.99; H, 5.43%.
8b: mp 142.0–143.5 °C; colorless needles (from hexane); 1H
NMR (400 MHz) δ 1.45 (18H, s), 5.62 (2H, s); 13C NMR
(100.6 MHz) δ 32.3, 36.1, 68.2, 146.0; IR (KBr) 1060 cm−1
(SwO). Anal. Calcd for C12H20Br2OS: C, 38.73; H, 5.42%. Found:
C, 38.80; H, 5.41%.
Addition of Bromine to Thiophene 1,1-Dioxide 4. A mix-
ture of 176 mg (1.1 mmol) of bromine and 228 mg (1.0 mmol) of
4 in 10 mL of CH2Cl2 was stirred at room temperature for 40 min;
1H NMR analysis of an aliquot of the mixture showed that 9a had
been formed as the sole product with complete consumption of 4.
The reaction mixture was evaporated under reduced pressure. The
resulting crystalline residue was crystallized from CHCl3/hexane
to give 328 mg (85%) of the adduct 9a in analytically pure form:
mp 179–181 °C; colorless crystals (from hexane); 1H NMR
(400 MHz) δ 1.48 (18H, s), 5.41 (2H, s); 13C NMR (100.6 MHz) δ
32.1, 36.6, 59.0, 148.0. Anal. Calcd for C12H20Br2O2S: C, 37.13;
H, 5.19%. Found: C, 36.85; H, 5.13%.
(12 molar amounts) for 10 h at room temperature.
Addition of Bromine to N-Unsubstituted Sulfoximide 7.
A
mixture of 35 mg (0.22 mmol) of bromine and 50 mg (0.22 mmol)
of 7 in 5 mL of CH2Cl2 was stirred at room temperature for 1 h.
The reaction mixture was evaporated under reduced pressure. The
resulting crystalline residue was purified by silica-gel column
chromatography with hexane/AcOEt (3:1) as the eluent to give
51 mg (60%) of 12b and 25 mg (29%) of 12a in this order. 12a:
mp 151–152 °C (dec); colorless crystals (from hexane); 1H NMR
(400 MHz) δ 1.48 (18H, s), 2.33 (1H, broad s), 5.41 (2H, s); 13C
NMR (100.6 MHz) δ 32.1, 36.4, 63.5, 148.2; IR (KBr) 3272 cm−1
(NH). Anal. Calcd for C12H21Br2NOS: C, 37.23; H, 5.47; N,
3.62%. Found: C, 37.25; H, 5.45; N, 3.56%. 12b: mp 160–162 °C
1
(dec); colorless crystals (from hexane); H NMR (400 MHz) δ
1.47 (18H, s), 4.14 (1H, broad s), 5.34 (2H, s); 13C NMR
(100.6 MHz) δ 32.1, 36.5, 64.7, 147.9; IR (KBr) 3284 cm−1 (NH).
Anal. Calcd for C12H21Br2NOS: C, 37.23; H, 5.47; N, 3.62%.
Found: C, 37.18; H, 5.39; N, 3.45%.
Conversion of 10a to 12a. A mixture of 100 mg (0.18 mmol)
of 10a in 1 mL of concentrated H2SO4 was stirred at room temper-
ature for 4 h. The resulting mixture was poured into an aqueous
NaOH containing ice and was extracted with CH2Cl2. The CH2Cl2
extracts were washed with water, dried over MgSO4, and evapo-
rated. The residue was purified by silica-gel column chromatogra-
phy with hexane/ether (1:2) as the eluent to give 36 mg (50%) of
12a.
Oxidations of 8a and 8b to 9a.
A mixture of 50 mg
(0.13 mmol) of 8a and 25 mg (0.14 mmol) of MCPBA in 5 mL of
CH2Cl2 was stirred at room temperature for 4 days. The mixture
was treated in a usual way and the resulting crude product was
chromatographed on a column of silica gel to give 51 mg (98%) of
9a. The oxidation of 8b in the same scale was carried out at room
temperature for 8 days to give 47 mg (90%) of 9a with recovery of
5 mg (9%) of 8b.
Reaction of Imide 5 with Bromine. The imide 5 (100 mg,
0.27 mmol) and bromine (48 mg, 0.30 mmol) were dissolved in
10 mL of CH2Cl2 and the solution was stirred for 48 h at room
temperature. 1H NMR analysis of the resulting reaction mixture
showed that 5, 13, 14, 15, and 16 were formed in the ratio
26:17:7:25:24. The resulting crude product was chromato-
graphed on a column of silica gel. Elution of the column with
hexane gave a mixture of 13–16. Further elution of the column
with ether gave a mixture of p-toluenesulfonamide and 17 (18).
Removal of the p-toluenesulfonamide by washing the mixture
with aqueous NaOH allowed the isolation of 5 mg (4%) of 18.
Purification of a mixture of 13–16 by silica-gel column chroma-
tography and GPC allowed only isolation of 14 in pure form; the
Addition of Bromine to Sulfoximide 6. A mixture of 46 mg
(0.29 mmol) of bromine and 100 mg (0.26 mmol) of 10 in 10 mL
of CH2Cl2 was stirred at room temperature. 1H NMR analysis af-
ter 15 h showed the exclusive formation of 10a with complete
consumption of 6. The mixture was evaporated under reduced
pressure. The resulting crystalline residue was crystallized from
ether/hexane to give 105 mg (74%) of 10a in analytically pure
form. The mother liquor of the crystallization was evaporated and
the residue was purified by silica-gel column chromatography to
give a further amount of 10a (14 mg, 10%). 10a: mp 154.0–
157.5 °C; colorless crystals; 1H NMR (400 MHz) δ 1.49 (18H, s),
2.41 (3H, s), 6.14 (2H, s), 7.28 (2H, J = 7.9 Hz, d); 7.83 (2H, J =
7.9 Hz, d); 13C NMR (100.6 MHz) δ 21.5, 32.0, 36.7, 61.4, 126.7,
129.4, 139.6, 143.4, 147.5. Anal. Calcd for C19H27Br2NO3S2: C,
42.15; H, 5.03; N, 2.59%. Found: C, 42.26; H, 4.95; N, 2.53%.
Stirring a solution of 10a (25 mg) in ether (10 mL) with 1 M
NaOH (10 mL) for 1 h at room temperature gave 20 mg (95%) of
1
others were not isolated in pure form. 13: H NMR (200 MHz) δ
1.46 (s, 9H), 1.64 (s, 9H), 7.09 (s, 1H); 13C NMR (50 MHz) δ
33.3, 33.7, 35.9, 36.3, 109.2, 122.0, 146.5, 151.6. 14: mp 75.5–
76.5 °C; 1H NMR (200 MHz) δ 1.60 (s, 18H); 13C NMR (50 MHz)
δ 33.6, 37.9, 107.0, 149.2. 15: 1H NMR (200 MHz) δ 1.41 (s, 9H),
7.03 (s, 1H); 13C NMR (50 MHz) δ 29.1, 34.9, 113.2, 114.3,
1
1
11: mp 154.0–155 °C; colorless crystals; H NMR (300 MHz) δ
120.0, 149.2. 16: H NMR (200 MHz) δ 1.57 (s, 9H); 13C NMR
1
1.46 (9H, s), 1.57 (9H, s), 2.42 (3H, s), 7.29 (2H, d, J = 8.3 Hz),
7.40 (1H, s), 7.89 (2H, d, J = 8.3 Hz); 13C NMR (75.5 MHz) δ
21.5, 31.6, 32.0, 37.1, 37.3, 120.7, 125.2, 126.7, 129.3, 140.4,
143.0, 152.0, 160.8. Anal. Calcd for C19H26BrNO3S2: C, 49.56; H,
5.69; N, 3.04%. Found: C, 49.41; H, 5.60; N, 2.96%.
(50 MHz,) δ 30.6, 37.6, 104.7, 112.8, 114.3, 143.5. 18: H NMR
(200 MHz) δ 1.42 (s, 9H), 1.56 (s, 9H), 2.40 (s, 3H), 6.87 (s, 1H),
7.24 (d, J = 8.3 Hz, 2H), 7.80 (d, J = 8.3 Hz, 2H).
References
Conversion of 8a to 10a. A mixture of 50 mg (0.13 mmol)
of 8a, 150 mg (0.40 mmol) of TsNwIPh, and 7 mg of
[Cu(MeCN)4]PF6 in 5 mL of MeCN was stirred at room tempera-
ture for 4 h. After the reaction had been quenched by addition of
iced-water, the mixture was extracted with ether. The ether ex-
tracts were washed with water, dried over MgSO4, and evaporated.
The resulting crystalline residue was purified by silica-gel column
chromatography with hexane/ether (3:1) as the eluent to give
53 mg (72%) of 10a. Attempted conversion of 8b to 10b failed;
no reaction took place on treatment of 8b with excess TsNwIPh
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