Integrated Synthesis of Thienyl Thioethers and Thieno[3,2-b]thiophenes via 1-Benzothiophen-3(2 H)-ones

Article abstract of DOI:10.1055/s-0040-1707280

A one-pot procedure for the synthesis of thienyl thioethers is described. Several thienyl thioethers were synthesized by a TfOH-promoted Friedel-Crafts-type cyclization, a subsequent nucleophilic attack by an arenethiol, and dehydration. This protocol was

Full text of DOI:10.1055/s-0040-1707280

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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2020, 31, A–F
letter
A
Integrated Synthesis Using Continuous-
K. Mitsudo et al.
Letter
Synlett
Integrated Synthesis of Thienyl Thioethers and Thieno[3,2-b]
thiophenes via 1-Benzothiophen-3(2H)-ones
Koichi Mitsudo*a
Nanae Habaraa
Yoshiaki Kobashia
Yuji Kurimotoa
Hiroki Mandaib
Seiji Suga*a
0
0
0
0
-
0
0
0
2
-
6
7
4
4-7
1
3
6
HS
O
Ar
2,6-lutidine
S
Ar
TfOH
Ar
OH
Ar
Ar
S
S
S
O
up to 76% yield
19 examples
Integrated Synthesis
one-pot
transition-metal-free
halide-free
a Division of Applied Chemistry, Graduate School of Natural Science and
Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama
700-8530, Japan
mitsudo@cc.okayama-u.ac.jp
suga@cc.okayama-u.ac.jp
b Department of Pharmacy, Faculty of Pharmacy, Gifu University of Medical
Science, 4-3-3 Nijigaoka, Kani, Gifu 509-0293, Japan
Corresponding Authors
Received: 20.01.2020
Accepted after revision: 05.08.2020
Published online: 21.09.2020
novel C–S coupling reactions have been explored to avoid
the use of these harsh and toxic reaction conditions.6–8 For
example, Glorius and co-workers reported a Co-catalyzed
dehydrogenative C–S coupling of indoles and thiols.6a Lei
and co-workers established an electrochemical dehydroge-
native C–S coupling reaction between indoles and thiols.7a
Light-driven C–S coupling reactions have also been de-
scribed;8 for example, the Miyake group reported a visible-
light-driven C–S coupling between aryl halides and arylthi-
ols.8a
DOI: 10.1055/s-0040-1707280; Art ID: st-2020-u0405-c
Abstract A one-pot procedure for the synthesis of thienyl thioethers
is described. Several thienyl thioethers were synthesized by a TfOH-pro-
moted Friedel–Crafts-type cyclization, a subsequent nucleophilic attack
by an arenethiol, and dehydration. This protocol was successfully ap-
plied to the synthesis of thienoacene derivatives by using a Pd-catalyzed
dehydrogenative cyclization.
Key words thienyl thioethers, thioetherification, one-pot synthesis,
metal-free, halide-free, thienoacenes
(a) Conventional synthesis of thienyl thioethers
X
S
HS
Cu or Pd
Ar1
Ar1
+
Ar2
Hetaryl thioethers are important motifs in the fields of
pharmaceuticals1 and organic materials.2 In particular, thie-
nyl thioethers are potent candidates for bioactive com-
pounds such as endothelin inhibitors3a and thrombin inhib-
itors3b (Figure 1). Hetaryl thioether moieties are also found
in -expanded thienoacene derivatives, such as [1]benzo-
thieno[3,2-b][1]benzothiophene (BTBT), which are used as
core units in high-performance semiconductors (Figure 1).4
Ar2
S
S
x transition-metal needed
x stoichiometric halogen waste
x strong base and high temperature
(b) Integrated synthesis via 3-benzo[b]thiophenone (this work)
O
Ar1
TfOH
Ar1
OH
S
cyclization
Reaction A
S
O
1
2
(modified reaction)
N
S
S
O
HS
Ar2
+ 2,6-lutidine
S
N
S
S
Ar1
O
Ar2
addition reaction
and dehydration
Reaction B
BTBT
semiconductor
S
thrombin inhibitor
3
(first example)
Figure 1 Thienyl thioether skeletons in a bioactive compound and an
further
cat. Pd transformation
to BTBT
one-pot
transition-metal-free
halide-free
organic material
S
Conventional syntheses of thioethers involve transition-
Ar1
Ar2
metal-catalyzed
cross-coupling
reactions
between
S
haloarenes and thiophenols, typically requiring the use of
4
strong bases and high temperatures (Scheme 1a).5 Several
Scheme 1 Representative synthesis of thienyl thioethers
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
K. Mitsudo et al.
Letter
Synlett
There have also been a few reports on halogen- and
transition-metal-free C–S bond-formation reactions for the
construction of thienyl thioethers.9 For example, Johnson
and co-workers reported a TsOH-promoted thioether syn-
thesis from 7-bromo-3-hydroxybenzo[b]thiophenes.9a
Procter and co-workers reported syntheses of thioethers,
including thienyl thioethers, by Tf2O-mediated C–H thiola-
tions of arenes by methyl sulfoxides.9b Yorimitsu and co-
workers developed acid-anhydride-promoted sulfanylation
reactions of aryl sulfoxides.9c However, methods for synthe-
sizing thienyl thioethers under halogen- and transition-
metal-free conditions remain limited, and a novel and gen-
eral method to access thienyl thioethers is attractive and in
demand.
To accomplish this, we focused on 1-benzothiophen-
3(2H)-ones 2, which are known to be readily available from
arylthioacetic acids 1 through intramolecular Friedel–Crafts
cyclization (Scheme 1b, Reaction A),10 and we designed a
novel integrated sequential approach.11 We expected that 2
could then be converted into 1-benzothien-3-yl thioethers
3 through Brønsted acid catalyzed addition of arylthiols and
subsequent dehydration (Scheme 1b, Reaction B). Here, we
report an integrated reaction system that combines Reac-
tions A and B for the synthesis of thienyl thioethers. The
products were successfully employed in Pd-catalyzed dehy-
drogenative cyclization reactions to give thienoacene deriv-
atives 4.
fluoroacetic acid was used (Table 1, entries 1–3). Although
thioetherification proceeded with H3PO4, the desired com-
pound 3a was obtained in only 6% yield (entry 4). Further
optimization revealed that sulfonic acids were suitable for
thioetherification and that MsOH, TfOH, and TsOH·H2O af-
forded 3a in yields of 65, 63, and 70%, respectively (entries
5–7).
Because 1-benzothiophen-3(2H)-one (2a) is relatively
unstable in air and gradually decomposes, we sought to
prepare the reactant in situ, and we developed a one-pot
reaction involving a Friedel–Crafts-type cyclization of 1a to
afford 2a (Reaction A), followed by its thioetherification to
give thioether 3a (Reaction B) (Table 2). Among the Brønsted
acids examined, only TfOH was effective for both Reaction A
and Reaction B [Table 1 and Supporting Information (SI), Ta-
ble S1]. Phenylthioacetic acid (1a) was treated with TfOH
(8.0 equiv) at 40 °C for three hours to give 1-benzothio-
phen-3(2H)-one (2a). The reaction mixture was then cooled
to 0 °C, 4-methylbenzenethiol and a base (7.6 equiv) were
added, and the mixture was heated at 80 °C for 18 h. A base
was essential for the formation of the desired product.
Without the addition of a base, Reaction B did not proceed,
and 3a was not obtained (Table 2, entry 1), probably be-
cause the interaction of 4-methylbenzenethiol and the ex-
cess TfOH decreased the nucleophilicity of the thiol. To neu-
tralize excess TfOH, we examined the addition of various
bases (entries 2–6).12 As expected, the addition of DIPEA
promoted the desired reaction (entries 2 and 3). Aniline
was not effective, probably because it was insufficiently ba-
sic (entry 4). The order of addition of the thiol and DIPEA
We first examined the thioetherification of 1-ben-
zothiophen-3(2H)-one (2a) with 4-methylbenzenethiol
(Reaction B) in the presence of various Brønsted acids, a key
step to complete our strategy (Table 1). The desired reaction
did not occur when acetic acid, trichloroacetic acid, or tri-
Table 2 One-Pot Synthesis of Thioether 3a via 1-Benzothiophen-
3(2H)-one (2a) with Various Basesa
Table 1 Optimization of Reaction B: Thioetherification of 1-Benzothio-
phen-3(2H)-one (2a) with Various Brønsted Acidsa
HS
TfOH
(8.0 equiv)
S
OH
[2a]
BA
(20 mol%)
S
O
S
base
DCE
40 °C, 3 h
HS
S
added at 0 °C
then 80 °C, 18 h
O
+
toluene
80 °C, 24 h
1a
3a
S
S
2a
Reaction A
Reaction B
3a
Entry
Base
Yieldb (%) of 3a
Entry
Brønsted acid
Conversionb (%)
Yieldb (%) of 3a
1
none
NDc
32
1
2
3
4
5
6
7
AcOH
9
19
NDc
ND
2d
3
i-Pr2NEt
i-Pr2NEt
aniline
CCl3CO2H
CF3CO2H
H3PO4
64
9
ND
4
32
17
6
5
piperidine
65
MsOH
>95
>95
>95
65
6e
2,6-lutidine
67 (63)f
TfOH
63
a Reaction conditions: Reaction A: 1a (0.20 mmol), TfOH (8.0 equiv), DCE
(0.66 M), 40 °C, 3 h. Reaction B: 4-methylbenzenethiol (1.0 equiv) and base
(7.6 equiv) added at 0 °C, then 80 °C, 18 h.
b Yield from 1a, determined by 1H NMR.
TsOH·H2O
70 (63)d
a Reaction conditions: 1a (0.20 mmol), Brønsted acid (20 mol%), toluene
(0.2 M), 80 °C, 24 h.
c ND = not detected.
b Determined by 1H NMR with 1,1,2,2-tetrachloroethane as an internal
d Reaction B; base added before 4-methylbenzenethiol.
e Performed with TfOH (7.7 equiv).
standard.
c ND = Not detected.
d Isolated yield.
f Isolated yield.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

C
K. Mitsudo et al.
Letter
Synlett
were also effective reactants: 4-(diphenylamino)- and 4-
methoxybenzenethiol gave the corresponding biaryl thio-
ethers 3i and 3j in yields of 52 and 76%, respectively. Thio-
etherification also proceeded successfully with naphtha-
lene-1-thiol (3k; 22% yield). In contrast, however, naphtha-
lene-2-thiol failed to yield the desired compound; although
the reason is unclear, nucleophilic attack by naphthalene-2-
thiol did not proceed. Hetaryl thiols also reacted successful-
ly. Thioetherification with thiophene-2-thiol and thio-
phene-3-thiol gave the corresponding dithienyl thioethers
3m and 3n in yields of 31 and 53%, respectively. One advan-
tage of this reaction is that it is easy to introduce a substitu-
ent onto the benzothiophene skeleton because substituted
precursors are readily available. Several substituted thienyl
thioethers 3o–s were obtained from the corresponding sub-
stituted precursors 1. Beneficially, this protocol provides
easy access to highly -expanded thioethers, such as 3t.
To clarify the mechanism of Reaction B, density func-
tional theory (DFT) calculations were performed. Based on
these calculations, a plausible mechanism is proposed
(Scheme 3).14 First, the carbonyl group of 1-benzothiophen-
3(2H)-one is protonated by TfOH while a second oxygen
atom of TfOH coordinates to the SH proton of benzenethiol
to form complex IM1. Next, the benzenethiol sulfur atom
attacks the carbonyl group to afford IM2 via an eight-mem-
bered cyclic concerted transition state TS1.15 TfOH-assisted
dehydration of IM3 proceeds via an eight-membered cyclic
transition state TS2 to afford the cationic intermediate IM4.
Finally, IM4 is deprotonated to form the desired thienyl
thioether via transition state TS3. The calculated activation
energy (Ea) of TS2 (Ea = 15.7 kcal mol–1) is higher than those
of TS1 (Ea = 10.5 kcal mol–1) and TS3 (Ea = 4.1 kcal mol–1),
suggesting that the C–O bond cleavage is the rate-determin-
ing step of this reaction.
Ar2
[1] HS
O
S
Ar1
Ar1
TfOH
[2] 2,6-lutidine
80 °C, 18 h
Ar1
OH
Ar2
S
DCE
40 °C, 3 h
S
S
O
1
2
3
S
S
S
S
S
S
3b, 54%a
3c, 38%
3d, 47%
3e (R = Cl), 40%b
3f (R = Br), 43%b
S
S
3g (R = NO2), 21%b
3h (R = NHAc), 45%
3i (R = NPh2), 52%
3j (R = OMe), 76%c
S
S
R
3k, 22%
S
S
S
S
S
S
S
S
3m, 31%d
3l, N.D.
3n, 53%
R
S
S
3o (R = Me), 72%a (77%)e
3p (R = Br), 22%f
S
3q (R = Cl), 14%f
S
3r, 26%
S
S
S
S
3s, 53%
3t, 64%
Scheme 2 One-pot syntheses of thienyl thioethers 3. Reagents and
conditions: Reaction A: 1 (0.20 mmol), TfOH (7.7 equiv), DCE (0.66 M),
40 °C, 3 h. Reaction B: arylthiol (1.0 equiv), 2,6-lutidine (7.4 equiv) add-
ed at 0 °C, then at 80 °C, 18 h. Yields are isolated yields based on 1. a
Thiol (1.2 equiv). b 2,6-lutidine (7.6 equiv). c 2,6-Lutidine was added be-
fore the thiol at –78 °C. d 2,6-Lutidine was added before the thiol at 0
°C. e 1.5 mmol scale. f 0.4 mmol scale.
affected the yield of the desired compound 3a (SI; Scheme
S1). When DIPEA was added first, 3a was obtained in only
32% yield, due to the competing aldol condensation of 2a to
form the dimer 2,3′-bi-1-benzothiophene-3-ol (entry 2).
When 4-methylbenzenethiol was added before DIPEA, the
side reaction was suppressed, and the yield of 3a increased
to 64% (entry 3). We next examined several bases, and we
found that 2,6-lutidine gave the best result (67% NMR yield
and 63% isolated yield; entry 6).13
Table 3 Effect of 2,6-Lutidine on the Pd-Catalyzed Dehydrogenative
Cyclization of 3oa
Pd(OPiv)2 (10 mol%)
AgOPiv (3.0 equiv)
S
S
2,6-lutidine
PivOH (0.1 M)
170 °C, 24 h
S
S
3o
4o
By using the optimized conditions, a series of thienyl
thioethers were synthesized (Scheme 2). Thioetherification
with phenylthiol gave thioether 3b in 54% yield, whereas 2-
and 3-methylbenzenethiol gave the corresponding thio-
ethers 3c and 3d in moderate yields. Next, several p-substi-
tuted benzenethiols were used in the reaction (3e–j). 4-
Chlorobenzenethiol and 4-bromobenzenethiol gave the ha-
logenated thioethers 3e and 3f in yields of 40 and 43%, re-
spectively. However, 4-nitrobenzenethiol, gave a low yield
of thioether 3g (21%), due to its low nucleophilicity. N-(4-
Sulfanylphenyl)acetamide gave aryl thioether 3h in 45%
yield. Benzenethiols containing electron-donating groups
Entry
2,6-Lutidine
(equiv)
Recoveryb (%) of 3o
Yieldb (%) of 4o
1
2
3
4
0
trace
NDc
ND
35
54
72
88
1.0
3.0
5.0
ND
a Reaction conditions: 3o (0.15 mmol), Pd(OPiv)2 (10 mol %), AgOPiv (3.0
equiv), 2,6-lutidine (0–5.0 equiv), PivOH (0.1 M), 170 °C, 24 h.
b Isolated yield.
c ND = not detected.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

D
K. Mitsudo et al.
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Synlett
S
S
+
+
S
S
O
H
O
O
S
O
O
O
H
O
H
O
S
S
Ea = 10.5
O
S
H
H
O
S
H
CF3
O
O
CF3
CF3
ΔG = (0.0)
IM1
(1.4)
TS1
(11.9)
/kcal mol–1
S
S
H
H
O
S
S
O
S
O
H
CF3
O
H
O
H
H
O
S
O
O
CF3
IM2
IM3
(8.6)
(11.2)
+
H
+
O
S
O
S
S
H
H
H
O
O
S
O
S
CF3
CF3
O
O
H
O
Ea = 15.7
H
S
H
H
TS2
IM4
(26.9)
(16.7)
+
+
H
H
O
O
S
S
O
H
O
S
S
CF3
CF3
H
O
H
O
H
O
O
S
Ea = 4.1
S
H
TS3
IM5
(20.8)
(7.7)
S
O
S
O
H
F3C
S
O
H
O
H
catalyst and product
(–3.9)
Scheme 3 A plausible mechanism for Reaction B. Gibbs free energies in kcal mol–1 are shown in parentheses.
We next focused on the transformation of thienyl thio-
Pd(OPiv)2 (10 mol%)
AgOPiv (3.0 equiv)
2,6-lutidine (5.0 equiv)
ethers into BTBT derivatives by Pd-catalyzed dehydrogena-
tive cyclization. Pd-catalyzed dehydrogenative coupling has
been established as a powerful method for the formation of
heteroacenes.16 However, to the best of our knowledge, this
method has not been used for the efficient dehydrogenative
construction of thiophene rings. Compound 3o was used as
a model to examine Pd-catalyzed dehydrogenative coupling
(Table 3). Benzothiophene 3o was heated at 170 °C for 24
hours in the presence of Pd(OPiv)2 (10 mol%) and AgOPiv
(3.0 equiv). We found that the addition of 2,6-lutidine was
essential for the reaction. In the absence of 2,6-lutidine, the
desired compound 4o was obtained in only 35% yield (Table
3, entry 1).17 The yield of 4o increased as the amount of 2,6-
lutidine increased. With 1.0 equivalents of 2,6-lutidine, the
yield of 4o was 54% yield (entry 2); this increased to 88%
with 5.0 equivalents of 2,6-lutidine (entry 4). Although the
role of 2,6-lutidine is not yet clear, it is likely to interact
with the Pd catalyst and suppress C–S bond fission.
S
S
Ar1
Ar1
Ar2
Ar2
PivOH (0.1 M), 170 °C, 24 h
S
S
4
3
S
S
S
S
4a
4b
65% (50%)a
65% (39%)a
S
S
S
S
S
Cl
4e
4m
30%
63% (39%)a
Scheme 4 Synthesis of several BTBT derivatives under the optimized
conditions. a Performed with 1.0 equiv of 2,6-lutidine.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

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K. Mitsudo et al.
Letter
Synlett
OH
Cl
O
[1] TfOH (7.7 equiv)
DCE (0.66 M)
40 °C, 5 h
(1.0 equiv)
S
OH
3 M aq NaOH
(2.2 equiv)
r.t., 10 h
S
SH
[2]
HS
(1.0 equiv)
S
O
5
1b
3o
(not isolated)
(not isolated)
[3] 2,6-lutidine (7.4 equiv)
0 °C to 80 °C, 18 h
S
Pd(OPiv)2 (7 mol%)
AgOPiv (2.4 equiv)
PivOH (0.1 M), 170 °C, 24 h
S
4o
46% (based on 5)
Scheme 5 Telescoped synthesis of 4o from 4-methylbenzenethiol (5)
By using the optimized conditions, several BTBT deriva-
tives were synthesized (Scheme 4). BTBT (4b) and substi-
tuted BTBTs 4a and 4e were readily obtained. The advan-
tages of this method are (i) a ready introduction of substitu-
ents and (ii) easy replacement of the benzene ring by
heterocycles such as thiophene (4m).
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1707280. Su
p
p
ortingI
n
formatio
n
S
u
p
p
ortingInformati
o
n
References and Notes
Finally, we examined a telescoped synthesis of 4o from
4-methylbenzenethiol (5) (Scheme 5). A solution of 5 in 3 M
aqueous NaOH was treated with chloroacetic acid to afford
1b. The reaction was quenched with aqueous HCl and ex-
tracted with CHCl3. After removal of the solvent, the crude
product was used in the one-pot procedure without further
purification to afford a crude solution of 3o, which was
quenched with saturated aqueous NaHCO3 and extracted
with CHCl3. After removal of the solvent, the crude mixture
was used in the Pd-catalyzed dehydrogenative reaction to
afford the desired BTBT derivative 4o in an 46% overall
yield.18 This result suggests that our protocol can be used to
prepare a variety of thienyl thioethers and BTBT derivatives
from easily accessible chloroacetic acid and the appropriate
arylthiol.
In conclusion, we have developed a transition-metal-
free and halide-free one-pot synthesis of thienyl thioethers.
Several novel thioethers were readily synthesized by using
the optimized conditions. An efficient conversion of the
thioethers into thienothiophenes was also established. We
also demonstrated a telescoped synthesis of a thienothio-
phene from an arylthiol. This strategy permits the efficient
and easy synthesis of 3-benzo[b]thienyl thioethers and
thienothiophenes. Further applications of this strategy are
currently being investigated in our laboratory.
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Huang, E.-L.; Yan, H.-T.; Zhu, X.-J.; Hao, X.-Q.; Song, M.-P. J. Org.
Chem. 2019, 84, 5213. (e) Nishino, K.; Tsukahara, S.; Ogiwara, Y.;
Sakai, N. Eur. J. Org. Chem. 2019, 1588. (f) Lu, S.; Zhu, Y.-S.; Yan,
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Funding Information
This work was supported in part by JSPS KAKENHI grants numbers
JP16K05695, JP16K05777, JP19K05477, JP19K05478, and JP18H04455
in Middle Molecular Strategy, and by the Okayama Foundation for Sci-
ence and Technology.
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© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

F
K. Mitsudo et al.
Letter
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TfOH (0.136 mL, 231 mg, 1.54 mmol) was added dropwise to a
solution of (phenylsulfanyl)acetic acid (1a; 33.6 mg, 0.20 mmol)
in anhyd DCE (0.3 mL), and the resulting mixture was stirred at
40 °C for 3 h then cooled to 0 °C. 4-Methylbenzenethiol (24.8
mg, 0.20 mmol) and 2,6-lutidine (0.18 mL, 1.5 mmol) were
added, and the mixture was stirred at 80 °C for 18 h then cooled
to r.t. The reaction was quenched with sat. aq NaHCO3 (3 mL),
and the mixture was extracted with CHCl3 (3 × 5 mL). The com-
bined organic phase was dried (MgSO4), filtered, and concen-
trated under reduced pressure. The residue was purified by
column chromatography (silica gel, hexane) to give a colorless
liquid; yield: 32.3 mg (0.13 mmol, 63%).
IR (neat): 3096, 3021, 1595, 1254, 1016 cm–1 1H NMR (400
.
MHz, CDCl3):  = 2.28 (s, 3 H), 7.03 (d, J = 8.4 Hz, 2 H), 7.11 (d, J =
8.4 Hz, 2 H), 7.35–7.40 (m, 2 H), 7.62 (s, 1 H), 7.78–7.83 (m, 1 H),
7.86–7.90 (m, 1 H). 13C NMR (100 MHz, CDCl3):  = 20.9, 122.9,
123.0, 124.7, 124.9, 125.0, 128.4, 129.8, 130.8, 132.5, 136.0,
138.8, 140.0.
(14) For details of the calculations, see SI.
(15) Li, X.; Ye, S.; He, C.; Yu, Z.-X. Eur. J. Org. Chem. 2008, 4296.
(16) (a) Saito, K.; Chikkade, P. K.; Kanai, M.; Kuninobu, Y. Chem. Eur. J.
2015, 21, 8365. (b) Kaida, H.; Satoh, T.; Hirano, K.; Miura, M.
Chem. Lett. 2015, 44, 1125. (c) Kurimoto, Y.; Mitsudo, K.;
Mandai, H.; Wakamiya, A.; Murata, Y.; Mori, H.; Nishihara, Y.;
Suga, S. Asian J. Org. Chem. 2018, 7, 1635. (d) Mitsudo, K.;
Kurimoto, Y.; Mandai, H.; Suga, S. Org. Lett. 2017, 19, 2821.
(17) The structure of 4o was confirmed by X-ray crystal structure
analysis. CCDC 1961314 contains the supplementary crystallo-
graphic data for compound 4o. The data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/getstructures.
(10) For representative examples, see: (a) Werner, L. H.; Schroeder,
D. C.; Ricca, S. Jr. J. Am. Chem. Soc. 1957, 79, 1675.
(b) Padmavathi, V.; Padmaja, A.; Reddy, D. B. Indian J. Chem.,
Sect. B: Org. Chem. Incl. Med. Chem. 1999, 38, 308. (c) Reddy, D.
(18) For details, see SI.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F