Electrocatalytic Synthesis of gem -Bisarylthio Enamines and α-Phenylthio Ketones via a Radical Process under Mild Conditions

Article abstract of DOI:10.1055/a-1335-7902

The novel method for the synthesis of gem -bisarylthio enamines and α-phenylthio ketones was developed via the coupling of α-substituted vinyl azides with thiols in the presence of tetrabutylammonium iodide (TBAI) as a redox catalyst and electrolyte at ro

Full text of DOI:10.1055/a-1335-7902

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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2020, 31, 593–600
letter
593
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Y.-Z. Pan et al.
Letter
Synlett
Electrocatalytic Synthesis of gem-Bisarylthio Enamines and
-Phenylthio Ketones via a Radical Process under Mild Conditions
Yong-Zhou Pan
Ar
Shi-Yan Cheng
NH2
1,3-H transfer
S
Qian-Yu Li
R
R = strong EWG
S
Hai-Tao Tang
Ying-Ming Pan
Xiu-Jin Meng*
Zu-Yu Mo*
0
0
0
0
-0
0
0
1
-
7
5
3
1
-
0
4
5
8
Ar
N3
N
SH
C
C
gem-bisphenythio enamines
S
R
Ar
R
Ar
constant current
O
S
hydrolysis
Metal- and oxidant-free
Environment-friendly
Room-temperature
Convenient operation
R
Ar
State Key Laboratory for Chemistry and Molecular Engineering
of Medicinal Resources, School of Chemistry and Pharmaceuti-
cal Sciences, Guangxi Normal University, Guilin 541004, P. R. of
China
R = not a strong
EWG
a-phenylthio ketones
18877380547@163.com
mengxiujin@163.com
Corresponding Authors
Received: 16.10.2020
Accepted after revision: 11.12.2020
Published online: 11.12.2020
treated sulfenimine with lithium diisopropylamide (LDA).
Recently, Zhang et al.6 reported a copper-catalyzed oxida-
tive cross-coupling of oxime acetates with diaryl disulfides
leading to a unique class of gem-bisarylthio enamines.
However, these methods involved a strong base and multi-
step reaction, long reaction time, inert atmosphere, and
metal catalysts. Therefore, it is still a challenge to develop a
mild method to construct gem-bisphenylthio enamines -
substituted vinyl azides,7 a special subclass of vinyl azides
that are active and outstanding synthons, which display
DOI: 10.1055/a-1335-7902; Art ID: st-2020-r0554-l
Abstract The novel method for the synthesis of gem-bisarylthio
enamines and -phenylthio ketones was developed via the coupling of
-substituted vinyl azides with thiols in the presence of tetrabutylam-
monium iodide (TBAI) as a redox catalyst and electrolyte at room tem-
perature. Electronic properties were crucial in the generated products.
This protocol features metal- and oxidant-free materials, broad toler-
ance of substrates, and mild reaction conditions.
Key words electrocatalytic, vinyl azides, thiophenols, gem-bisarylthio
enamines, metal-free
A. Reported for the preparation of functionalized gem-alkene
R
Ar
R
cat. [NiII-H]
(i)
F
(Norton's work)
PhSiH3
Ar
R
CF3
A multisubstituted alkene is an important structural
motif frequently found in functional materials and bioac-
tive compounds. To efficiently prepare multisubstituted
alkenes, various stereoselective synthetic methods have
been developed. gem-Dialkenes are important and useful
building blocks for chemo- and regioselective synthesis of
multisubstituted alkenes.1 Therefore, much attention has
been paid to the preparation of functionalized gem-di-
alkenes. For example, Norton2 reported a method for the
synthesis of gem-difluoroalkenes via the nickel hydride cat-
alyzed hydrodefluorination of CF3 alkenes (Scheme 1A, path
i). The rhodium-catalyzed desulfanylative gem-diborylation
of 2-arylvinyl sulfides for the synthesis of (2,2-diborylvi-
nyl)arenes was also reported (Scheme 1A, path ii).3 In addi-
tion, Engle et al.4 recently performed a copper-catalyzed di-
boration of terminal alkynes for the preparation of E-selec-
tive 1,1-diborylalkenes(Scheme 1A, path iii). However, the
gem-bisphenylthio enamine has been reported rarely over
the past decades. Up to now, only two reports have dis-
cussed the synthesis of this kind of gem-bisphenylthio
enamine (Scheme 1B). In 1980, Davis et al.5 first disclosed
this gem-bisphenylthio enamine as a byproduct when they
F
cat. [Rh(OH)(cod)]2
cat. P(n-Bu)3
(Bpin)2, K2CO3
SMe
(ii)
Bpin
Bpin
R
(Hosoya's work)
n-hexane
r.t. or 80 °C
Bdan
Bpin
H
cat. Cu
HBdan
cat. Cu
HBpin
R
(iii)
(Engle's work)
R
Bdan
(E)
R
B. Reported synthesis of gem-bisarylthio enamines
NH2
SAr
SAr′
N
Ar'SSAr'
LDA
X
R
SAr′
N
R
SAr′
R
NH2
CuI (20 mol%), NaI (1 equiv)
MeCN, N2, 90 °C, 16 h
SAr
SAr
R
X = SAr, OAc
C. Electrocatalytic synthesis of gem-bisarylthio enamines (this work)
C-C, 3 mA
R2
SH
TBAI (10 mol%)
DBU (2.0 equiv), MeOH, r.t.
R1
N3
NH2
S
R1
R2
S
R2
Scheme 1 Reactions of organic azides
© 2020. Thieme. All rights reserved. Synlett 2021, 32, 593–600
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

594
Y.-Z. Pan et al.
Letter
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distinct and unique chemical reactivities from other organ-
ic azides, such as alkyl azides and aryl azides, and can act as
radical acceptors,8 nucleophile acceptors,9 electrophile ac-
ceptors,10 2H-azirine, vinylnitrene precursors,11 enamine-
type nucleophiles,12 and 1,3-dipoles.13 Generally, -substi-
tuted vinyl azides can be attacked by other radicals to gen-
erate imine radical,8,14 which likely undergo hydrolysis and
afford ketones,15 or 1,3-H transfer and deliver enamines16 or
intramolecular cyclization and furnish N-heterocycles.17
Organic electrochemistry, which employs electrons as
reagents to achieve redox reactions, is a green and benign
tool for generating radicals.18 In 2019, Xuʼs group19 dis-
closed an efficient strategy for the synthesis of enaminones
via an imine radical process which was generated from the
addition of a carbonyl radical to vinyl azide under electro-
chemical conditions. Since the introduction of an electron-
withdrawing radical, the imine radical then underwent 1,3-
H migration and gave the enamine compounds. In addition,
Terent’ev and co-workers20 also described a similar work.
Very recently, our group21 reported a radical cyclization
cascade for the synthesis of sulfide imidazopyridines via an
imine radical intermediate under electrochemical condi-
tions. However, employing vinyl azide and mercaptan as
raw materials for the synthesis of gem-bisphenylthio enam-
ine through the electrochemical conditions has not been re-
ported. We herein describe an efficient electrochemical
method of achieving gem-bisphenylthio enamine from vi-
nyl azides and thiophenols (Scheme 1C).
In our initial studies, 1-(1-azidovinyl)-4-nitrobenzene
(1a) and 4-methylbenzenethiol (2a) were selected as model
substrates to optimize the reaction conditions in an undi-
vided cell equipped with a graphite rod cathode and a plati-
num anode under the constant current of 3 mA with a
mixed solvent of MeCN/MeOH (v/v = 1:1) and 1,8-diazabi-
cyclo[5.4.0]undec-7-ene (DBU) as a base. The target prod-
uct 1-(4-nitrophenyl)-2,2-bis(p-tolylthio)ethenamine (3aa)
was obtained in 24% yield (Table 1, entry 1). Subsequently,
we investigated the effect of electrolytes, solvents, and elec-
trodes in this reaction (Table 1). When another iodine salt,
TBAI, was used as electrolyte, the desired product 3aa was
produced in a much-improved yield of 62% (entry 2). Two
noniodized salts were investigated. Only the trace target
product 3aa was detected when n-Bu4NBF4 was used as an
electrolyte (entry 3). The desired product 3aa was generat-
ed in 50% yield when n-Bu4NPF6 was employed as an elec-
trolyte (entry 4). After screening the different solvents,
namely, MeCN/MeOH, MeOH/H2O (v/v = 10:1), and MeOH,
we found that MeOH was the best choice, yielding 75% (en-
Table 1 Optimization of Reaction Conditionsa,b
NH2
S
N3
SH
+
O2N
S
solvent (6 mL), r.t.
electrolyte (10 mol%)
DBU (2 equiv)
O2N
1a
2a
3aa
Entry
Electrolyte
Solvent
Electrode
Yield (%)b
1
2
NH4I
MeCN/MeOHc
MeCN/MeOH
MeCN/MeOH
MeCN/MeOH
MeOH/H2Od
MeOH
Pt (+)|C (–)
Pt (+)|C (–)
Pt (+)|C (–)
Pt (+)|C (–)
Pt (+)|C (–)
Pt (+)|C (–)
Pt (+)|Pt (–)
C (+)|C (–)
C (+)|C (–)
C (+)|C (–)
C (+)|C (–)
24
62
TBAI
3
n-Bu4NBF4
n-Bu4NPF6
TBAI
trace
50
4
5
52
6
TBAI
75
7
TBAI
MeOH
60
8
TBAI
MeOH
88
9e
10f
11g
TBAI
MeOH
48
TBAI
MeOH
NR
87
TBAI
MeOH
a Reaction conditions: undivided cell with a Pt plate anode (1 cm × 1 cm) and graphite rod cathode, constant current = 3 mA, 1a (0.3 mmol, 1 equiv), 2a (0.72
mmol, 2.4 equiv), DBU (0.6 mmol, 2.0 equiv), TBAI (10 mol %), and solvent (6 mL) under air atmosphere at room temperature for 1 h.
b Isolated yields; NR = no reaction.
c MeCN/MeOH (v/v = 1:1).
d MeOH/H2O (v/v = 10:1).
e Constant current = 5 mA.
f Without electricity.
g Under Ar atmosphere.
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Y.-Z. Pan et al.
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try 6). Different electrodes as an anode or a cathode were
also tested. When platinum was used as an anode and cath-
ode, the target product 3aa was given in a reduced yield of
60% (entry 7). When graphite rod was employed as both an
anode and a cathode, the target product 3aa was afforded in
the highest yield of 88% (entry 8). We also investigated the
effect of current on the yield of the product. When we in-
creased the current to 5 mA, the yield of the target product
3aa was markedly decreased (entry 9). The target product
3aa was not observed in the absence of electricity (entry
10). When this reaction was conducted in an inert atmo-
sphere (Ar), the yield of the target product 3aa was not
markedly improved (entry 11). Various bases, including in-
organic bases, such as K2CO3, 2,6-lutidine, and 4-dimethyl-
amino- pyridine, were examined. DBU was found as the op-
timal base (Table S1, entries 12–14). In summary, we found
that an 88% isolated yield of the target product 3aa could be
furnished at a constant current of 3.0 mA in MeOH with
10% TBAI as an electrolyte, DBU as a base, and graphite rod
as an anode and cathode at room temperature (entry 8).
Under optimal electrolytic conditions, we then screened
the scope of various thiophenols 2 under the standard con-
ditions (Scheme 2). Most of the thiophenols could react
with 1-(1-azidovinyl)-4-nitrobenzene (1a) and produce the
desired products containing the enamine skeleton 3aa–ak.
We found that substituents at the para position with elec-
tron-withdrawing groups (F, Cl, and Br) and electron-donat-
ing groups (t-Bu, OMe, Me) could be well tolerated in this
process and provided the corresponding products in mod-
erate to good yields (3aa–ai). The para, ortho, and meta sub-
stituents were proceeded smoothly to yield the desired
products in good yields (3ab,ac). However, the para substit-
uents delivered the highest yield (3aa, 88%), the meta sub-
stituents afforded the second-highest yield (3ab, 76%), and
the ortho substituents produced a much lower yield (3ac,
60%), which might be due to the steric properties of the
substrate. A dimethyl substrate was compatible with this
reaction and generated the expected product with a yield of
66% (3af). 1-Naphthyl mercaptan (2-naphthalenethiol) and
heterocyclic thiophenols were well tolerated under the
standard conditions and delivered the final products with
yields of 62% and 65%, respectively (3aj and 3ak).
Subsequently, the scope of -substituted vinyl azides
was comprehensively explored (Scheme 3). Conventionally,
a diverse range of -substituted vinyl azides with different
substituents (R = strong electron-withdrawing groups) on
R
Ar
S
NH2
S
C-C, 3 mA
TBAI (10 mol%)
SH
N3
Ar
R
+
O2N
DBU (2.0 equiv), MeOH, r.t.
O2N
R
Ar
1a
2a–k
3aa–ak
OMe
S
NH2
S
NH2
S
NH2
S
S
S
NH2
S
S
O2N
O2N
O2N
O2N
3aa 88%
3ac 60%
3ab 76%
3ad 77%
OMe
t-Bu
F
Cl
NH2
NH2
S
NH2
NH2
S
S
S
S
O2N
S
O2N
S
O2N
S
O2N
t-Bu
3ag 72%
3af 66%
3ae 80%
F
Cl
3ah 75%
Br
S
S
NH2
NH2
S
S
NH2
S
S
S
S
O2N
O2N
O2N
Br
3ai 70%
3aj 62%
3ak 65%
Scheme 2 Substrate scope of thiophenols 2. Reagents and conditions: graphite rod as an anode and cathode, an undivided cell, a constant current of
3.0 mA, 1a (0.3 mmol, 1 equiv), 2 (0.72 mmol, 2.4 equiv), TBAI (10 mol %), DBU (0.6 mmol, 2.0 equiv), and solvent (6 mL) under air atmosphere at
room temperature for 1 h. Isolated yields are given.
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Y.-Z. Pan et al.
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C-C, 3 mA
TBAI (10 mol%)
NH2
S
SH
N3
S
R
+
R
DBU (2.0 equiv), MeOH, r.t.
1b–g
2a
3ba–bg
NH2
S
NH2
S
NH2
S
NH2
S
S
O
S
S
S
F3C
OHC
NC
O
3da 78%
3ea 76%
3ba 80%
3ca 72%
NH2
NH2
S
S
S
O
S
NO2
3fa 79%
3ga 0%
Scheme 3 Scope for vinyl azides 1 with a strong electron-withdrawing group. Reagents and conditions: graphite rod as an anode and cathode, an
undivided cell, a constant current of 3.0 mA, 1 (0.3 mmol, 1 equiv), 2a (0.72 mmol, 2.4 equiv), TBAI (10 mol %), DBU (0.6 mmol, 2.0 equiv), and solvent
(6 mL) under air atmosphere at room temperature for 1 h. Isolated yields are given.
the benzene ring were found to be well tolerated in this re-
action and yielded the desired products. For example, elec-
tron-withdrawing groups, such as CHO, CN, CF3, and CO2Me,
on the benzene ring at the para position were well tolerated
under the standard conditions and generated the expected
products (3ba–ea) in good yields (72–80%). When the elec-
tron-withdrawing group was at the meta position, this pro-
cess could undergo smoothly and furnish the desired prod-
uct 3fa in satisfactory yields of 79%. However, 2-azido-1-
phenylprop-2-en-1-one (1g) was found to be incompatible
under these optimized electrolytic conditions, and the ex-
pected product was not detected.
When the substituent was no strong electron-with-
drawing group (R = MeO, Me, t-Bu, Et, Cl) on the benzene
ring of different -substituted vinyl azides, a series of -
phenylthio ketones (4aa–ag) were generated in moderate
to excellent yields under the optimized electrolytic condi-
tions (Scheme 4). The main reason may be that dithiolated
enamine is a product of high electron-cloud density, so it
needs an electron-withdrawing group to stabilize it. When
no substituent was present on the benzene ring of the -
substituted vinyl azide, the corresponding -phenylthio ke-
tone (4ah) was obtained in 86% yield. Polycyclic vinyl azide
could also react with p-methylphenylene mercaptan to
form the corresponding ketone with 83% yield (4ai). In ad-
dition, an -substituted vinyl azide bearing a heterocyclic
ring formed smoothly and gave the desired product (4aj) in
88% yield. Finally, alkyl azides such as (2-azidoallyl)benzene
were compatible with the standard conditions and afforded
the -phenylthio ketone (4ak) in 80% yield. Unfortunately,
this method cannot be applied to aliphatic vinyl azides and
thiols.
To gain further understanding of the reaction mecha-
nism, we performed some control experiments. First, the
reaction of 1-(1-azidovinyl)-4-nitrobenzene (1a) and 4-
methylbenzenethiol (2a) was conducted under the opti-
mized reaction conditions without a constant current. The
corresponding gem-bisphenylthio enamine product 3aa
also failed to be detected (Scheme 5a). In addition, a reac-
tion was conducted in the presence of molecular iodine, in
which the enamine 3aa was observed only in trace amount
(Scheme 5b). At last, we also did free-radical-trapping ex-
periments, the TEMPO-trapped radicals I, II, and III were
detected with an HRMS spectrometer (Scheme 5c). The re-
sults indicated that a radical process might be involved in
this transformation.
To confirm this hypothesis, we subsequently carried out
a
radical-trapping experiment, which was monitored
through EPR spectroscopy. In Figure 1, when 3.0 equivalents
5,5-dimethyl-1-pyrroline N-oxide (DMPO) were used as
radical-spin-trapping agents under standard reaction con-
ditions, a strong radical signal was detected (red line in Fig-
ure 1). The radical signal belonged to the thiyl radical (g =
2.0067, AN = 14.06, AH = 12.92) by comparing with the sim-
ulated EPR spectrum (black line in Figure 1). Consequently,
the above EPR results verified that a radical process was in-
volved in this transformation.
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597
Y.-Z. Pan et al.
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A cyclic voltammetry experiment was also conducted to
gain further insights into this transformation. As shown in
the left of Figure 2, 1-(1-azidovinyl)-4-nitrobenzene (1a)
showed an oxidation peak at 1.68 V (curve b), whereas the
first oxidation peak of 4-methylbenzenethiol (2a) was at
1.40 V (curve c). These experimental data indicated that 4-
methylbenzenethiol (2a) was earlier oxidated than 1-(1-az-
idovinyl)-4-nitrobenzene (1a). As shown in the right of Fig-
ure 2, when 4-methylbenzenethiol (2a) was added to the
mixture of LiClO4 and TBAI, the current increased distinctly
(curve e), indicating that TBAI played a catalyst role in this
reaction.
Subsequently, the thiyl radical A attacked the C=C bonds
of the vinyl azides 1 and provided the intermediate B and
the imine radical intermediate C furnished with the rapid
exit of the molecular N2 (Scheme 6). The imine radical in-
termediate C might undergo two completely different reac-
tion paths (paths a and b). When R1 was a strong electron-
Figure 2 Cyclic voltammograms of reactants and their mixtures in 0.1
M LiClO4/CH3OH by using a glassy carbon disk (3 mm diameter) as a
working counter and a Pt disk and Ag/AgCl (in saturated KCl solution) as
reference electrodes at a scan rate of 100 mV/s: (a) background, (b) LiC-
lO4 (0.1 mmol/L) + 1a (10 mmol/L), and (c) LiClO4 (0.1 mmol/L) + 2a
(10 mmol/L), (d) LiClO4 (0.1 mmol/L) + TBAI (10 mmol/L), and (e) LiClO4
(0.1 mmol/L) + TBAI (10 mmol/L) + 2a (10 mmol/L).
Figure 1 Simulated (black line) and experimental (red line) EPR spectra
of the trapped radical intermediates
O
C-C, 3 mA
S
SH
N3
TBAI (10 mol%)
R1
R1
R2
+
R2
DBU (2.0 equiv), MeOH, r.t.
1
2
4aa–ak
O
O
O
O
S
S
S
S
O
O
O
4ad 85%
4ac 78%
4ab 92%
4aa 90%
O
S
O
S
O
O
S
S
Cl
Cl
Cl
Cl
4af 70%
4ag 80%
4ah 86%
4ae 75%
O
S
O
O
S
S
S
4ai 83%
4aj 88%
4ak 80%
Scheme 4 Scope for vinyl azides 1 with no strong electron-withdrawing group. Reagents and conditions: graphite rod as an anode and cathode, an
undivided cell, a constant current of 3.0 mA, 1 (0.3 mmol, 1 equiv), 2 (0.36 mmol, 1.2 equiv), TBAI (10 mol %), DBU (0.6 mmol, 2.0 equiv), and solvent
(6 mL) under air atmosphere at room temperature for 1 h. Isolated yields are given.
© 2020. Thieme. All rights reserved. Synlett 2021, 32, 593–600

598
Y.-Z. Pan et al.
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NH2
S
no current
TBAI (10 mol%)
SH
S
N3
+
(a)
DBU (2.0 equiv), MeOH, r.t.
O2N
O2N
1a
2a
3aa, not observed
NH2
I2 (2.0 equiv)
DBU (2.0 equiv), MeOH, r.t.
SH
S
N3
(b)
+
S
O2N
O2N
1a
2a
3aa, trace
N
O
NH2
standard conditions
TEMPO (4.0 equiv)
S
N
N
N
O
S
+
+
+
1a
2a
(c)
S
O
O2N
O2N
I
II
III
[M+H]+ 442.2161
[M+H]+ 280.1735
[M+H]+ 442.2161
detected by HRMS
Scheme 5 Control experiments
e–
e–
1
I
R2
N3
Ar
2
– e–
S
R2
S
I
Ar
R1
A
1/2 H2
Ar
N2
B
+ e–
3
H
R2
Ar
S
R2
Ar
N
NH2
Path a
Path b
hydrolysis
4
1,3-H transfer
R1
S
R1
Ar
Ar
D
R1 = not a strong electron-
withdrawing group
C
R1 = strong electron-withdrawing group
Scheme 6 Plausible mechanism
withdrawing group, the imine radical intermediate C might
undergo 1,3-H transfer and generate a D radical. Generally
speaking, direct 1,3-H transfer is forbidden. This process re-
quires the help of water in the air.22 It is worth mentioning
that if too much water is added, it would promote the hy-
drolysis process and reduce the yield of 3aa. Finally, anoth-
er thiyl radical coupled with the intermediate D offered the
target products 3. By contrast, when R1 was not a strong
electron-withdrawing group, the imine radical intermedi-
ate C might experience hydrolysis and deliver the -phen-
ylthio ketone 4.23
In conclusion, we developed an efficient and convenient
strategy for the straightforward construction of a series of
gem-bisarylthio enamines and -phenylthio ketones.24,25
The notable features of this electrochemical approach are
wide substrate scope, convenient operation, and a catalytic
© 2020. Thieme. All rights reserved. Synlett 2021, 32, 593–600

599
Y.-Z. Pan et al.
Letter
Synlett
amount of inexpensive electrolyte requirement. In compar-
ison with the reported metal catalysts or strong base condi-
tions for the synthesis of gem-bisarylthio enamines, our
present reaction conditions are mild, green, and environ-
mentally friendly without the use of metals, oxidants, or
harsh reaction conditions. Further investigations on their
applications will be carried out by our group in the future.
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Funding Information
We thank the National Natural Science Foundation of China
(22061003), the Natural Science Foundation of Guangxi Province
(2019GXNSFAA245027), Bagui Scholars Program of Guangxi Zhuang
Autonomous Region (2016A13), and the Special Fund for Distin-
guished Experts in Guangxi of China for financial support.
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Supporting Information
Supporting information for this article is available online at
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References and Notes
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(24) General Procedure for the Synthesis of gem-Bisphenylthio
Enamine 3aa
The azide 1a (0.3 mmol), thiophenol 2a (0.75 mmol), DBU (0.6
mmol), and TBAI (0.03 mmol) were placed in a 10 mL three-
necked round-bottomed flask. The flask was equipped with a
© 2020. Thieme. All rights reserved. Synlett 2021, 32, 593–600

600
Y.-Z. Pan et al.
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Synlett
condenser and graphite rod as cathode and anode. MeOH (6.0
mL) was added. The electrolysis was carried out under air atmo-
sphere at room temperature using a constant current of 3 mA
until complete consumption of the substrate (monitored by TLC,
about 1 h). The reaction mixture was concentrated, and the
residue was chromatographed through silica gel eluting with
ethyl acetate/petroleum ether to give the product 3aa.
(25) Analytical Data for Compound 3aa
88% yield; yellow solid; mp 94.1–95.4 °C. Analytical TLC on
silica gel. Rf = 0.48 (hexane/ether; 6.1). 1H NMR (400 MHz,
CDCl3):  = 8.19 (d, J = 8.4 Hz, 2 H), 7.66 (d, J = 8.4 Hz, 2 H), 7.22–
7.17 (m, 2 H), 7.14–7.04 (m, 6 H), 5.04 (s, 2 H), 2.33 (s, 3 H), 2.31
(s, 3 H). 13C NMR (100 MHz, CDCl3):  = 158.0, 148.1, 143.8,
136.0, 135.3, 134.7, 131.8, 129.9, 129.7, 129.1, 127.8, 126.5,
123.5, 91.0, 21.0, 21.0. HRMS (ESI): m/z calcd for C22H21N2O2S2
[M + H]+: 409.1039; found: 409.1036.
© 2020. Thieme. All rights reserved. Synlett 2021, 32, 593–600