Triphosgene/Sodium Organosulfinate System: A General and Efficient Electrophilic Thiolation of Silylenol Ethers and Electron-Rich Heteroaromatics

Article abstract of DOI:10.1055/s-0040-1707299

An efficient and practical approach to electrophilic thiolation was developed by using commercially available triphosgene as a reductant and the appropriate alkyl- or arylsulfinates, which were transformed in situ into electrophilic RSCl intermediates in the presence of triphosgene. This procedure represents a general and powerful approach for the synthesis of α-(trifluoromethyl)thio-substituted ketones and thiolated electron-rich heteroaromatic compounds.

Full text of DOI:10.1055/s-0040-1707299

SYNLETT
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2020, 31, A–E
letter
A
en
H.-j. Gao et al.
Letter
Synlett
Triphosgene/Sodium Organosulfinate System: A General and Effi-
cient Electrophilic Thiolation of Silylenol Ethers and Electron-Rich
Heteroaromatics
Hai-jun Gao
OSiMe3
R2
O
Yu-xin Gu
SR
N
R
SCF3
R1
Zhe-fei Wang
Yan-qin Yuan*
Yan Wang
R'SO2Na
R2
R
Triphosgene
N
R1
R' = CF2H or Ar
R' = CF3
14 examples
up to 96% yield
18 examples
metal-free and mild conditions
broad functional group tolerance
novel reductive system
Sheng-rong Guo*
up to 89% yield
Department of Chemistry, Lishui University, Lishui, 323000,
P. R. of China
yuanyq5474@lsu.edu.cn
guosr9609@lsu.edu.cn
Corresponding Authors
Received: 30.07.2020
Accepted after revision: 31.08.2020
Published online: 08.10.2020
N-[(trifluoromethyl)thio]saccharin6 have been developed.
However, many of these strategies involve multiple steps
and require expensive substrates for the preparation of the
trifluoromethylthiolation reagents.
DOI: 10.1055/s-0040-1707299; Art ID: st-2020-v0417-l
Abstract An efficient and practical approach to electrophilic thiola-
tion was developed by using commercially available triphosgene as a re-
ductant and the appropriate alkyl- or arylsulfinates, which were trans-
formed in situ into electrophilic RSCl intermediates in the presence of
triphosgene. This procedure represents a general and powerful ap-
proach for the synthesis of -(trifluoromethyl)thio-substituted ketones
and thiolated electron-rich heteroaromatic compounds.
It is widely known that silyl enol ethers are advanta-
geous in terms of their electrophilic addition to electrophil-
ic reagents. In 2014, Shibata discovered a bistrifluorometh-
ylthiolation reaction of silyl enol ethers by using
TsN(Me)SCF3 as an electrophilic trifluoromethylthiolation
reagent that gave the corresponding products in moderate
to good yields.6b Shortly afterwards, the same group found
that a catalytic amount of CuF2 promoted the trifluoro-
methylthiolation of silyl enol ethers when a trifluorometh-
anesulfonyl hypervalent iodonium ylide was used as an
electrophilic reagent, giving the corresponding -CF3S-sub-
stituted ketones in good yields (Scheme 1a).7 In 2018, Sara-
vanan and Anbarasan developed an efficient and creative
approach to electrophilic trifluoromethylthiolation, in
which a combination of (diethylamino)sulfur trifluoride
(DAST) and (trifluoromethyl)trimethylsilane (F3CTMS)
were used as the source of CF3S groups to give -trifluoro-
methylthiolated carbonyl compounds (Scheme 1b).8 How-
ever those methods are limited by the nature of the CF3S
source, which are difficult to prepare, and by the high cost
of the substrates. Thus, the use of more readily available or
more easily handled sources of the CF3S group to promote
the trifluoromethylthiolation of silyl enol ethers is highly
desirable. We recently discovered a general trifluorometh-
ylthiolation method involving the treatment of F3CSO2Na
with triphosgene,9 Interestingly, by using triphosgene as a
reductant, sulfinates could be used as electrophilic re-
agents. In continuation of our interest in the potential uses
of the sulfinate/triphosgene system, we have expanded the
range of uses of sulfinates to include the thiolation of silyl
enol ethers or electron-rich aromatics under mild condi-
tions (Scheme 1c).
Key words drifluoromethylthiolation, difluoromethylthiolation, tri-
phosgene, silylenol ethers, indoles
Organosulfur and organofluorine compounds have re-
cently attracted considerable attention because of their in-
teresting applications in medicine, agrochemicals, and ma-
terials science.1 The incorporation of (trifluoromethyl)thio
and (difluoromethyl)thio groups into potential drug mole-
cules has been recognized as a useful improvement strategy
owing to various characteristics of these groups, such as
their strong electron-withdrawing effect, high lipophilicity,
and ideal bioavailability.2 Furthermore, compounds con-
taining a F3CS- or F2CHS- group are easily oxidized to form
sulfoxides, thereby altering their biological activity, which
is beneficial in the design of potential prodrugs.
In the last decade, there have been considerable efforts
to develop efficient and general approaches for selective in-
stallation of the F3CS– group onto the -position of ketones.
Early protocols for the synthesis of -SCF3-substituted ke-
tones normally involved the reactions of -bromo ketones
with various nucleophilic reagents.3 Recently, an electro-
philic strategy has attracted significant attention, and many
new electrophilic trifluoromethylthiolation reagents such
as Munavalli’s N-[(trifluoromethyl)thio]phthalimide,4
Billard’s (trifluoromethane)sulfanyl amides,5 and Shen’s
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
H.-j. Gao et al.
Letter
Synlett
Table 1 Optimization of the Reaction Conditionsa
1) previous work
O
SO2CF3
OSiMe3
O
OSiMe3
O
IPh
CuF2
Triphosgene
SCF3
H
SCF3
+
(a)
CF3SO2Na
Ar
Ar
conditions
1a
2a
OSiMe3
O
SCF3
DAST + CF3TMS
Entry
1a/F3CSO2Na/triphosgene Solvent
Temp (°C) Yieldb (%)
(b)
1
2
1:1:1
1:1.5:1.5
1:2:1.5
1:2:2
1:2:4
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
EtOAc
THF
25
25
56
65
75
78
70
82
85
91
93
85
88
86
0
2) this work
OSiMe3
O
Triphosgene
3
25
SCF3
(c)
CF3SO2Na
+
Ar
Ar
4
25
5
25
Scheme 1 Synthesis of -CF3S-substituted ketones from silyl enol
ethers
6
0
7
–20
–60
–78
–60
–60
–60
–60
8
We began our studies by exploring the reaction of 1-
phenyl-1-(trimethylsiloxy)ethene (1a) with the F3CSO2-
Na/triphosgene system as a model to optimize the reaction
conditions (Table 1). First, we examined the use of F3CSO2-
Na and triphosgene in various ratios. To our delight, at room
temperature in MeCN, without other additives, the -trifluo-
romethylthiolated product 2a was obtained in 56% isolated
yield (Table 1, entry 1) when the 1a/F3CSO2Na/triphosgene
mole ratio was 1:1:1. Experiments with various amounts of
F3CSO2Na and triphosgene revealed that the optimal mole
ratio of 1a to F3CSO2Na was 1:2 and the optimal mole ratio
of F3CSO2Na to triphosgene was 1:1 (Table 1, entries 1–6).
The reaction was next shown to be sensitive to tempera-
ture, and the optimal performance was achieved at –78 °C,
giving 93% of the -CF3S-substituted ketone 2a (entries 5–
9). Finally, replacement of MeCN by other solvents (CH2Cl2,
EtOAc, or THF) gave the trifluoromethylthiolated product
2a in good yields, whereas none of the target product was
obtained in DMSO.
Having established the optimal reaction conditions (Ta-
ble 1, entry 9), we then subjected silyl enol ethers with var-
ious substituents on the benzene ring to these conditions
(Table 2). Electron-neutral, electron-withdrawing, and elec-
tron-donating groups in the ortho-, meta- and para-posi-
tions were all tolerated, and the corresponding -trifluoro-
methylthiolated ketones were obtained in moderate to
good yields. With electron-donating methyl, methoxy, alk-
ylthio, or phenyl groups in the para-position of the aryl
ring, the corresponding products 2b–e were obtained in
yields of 88–96%. In comparison, substrates with electron-
withdrawing groups at the para-position on the aryl ring
were less reactive, giving the target products 2f–i in moder-
ate to good yields; for example, trimethyl({1-[4-(trifluoro-
methyl)phenyl]vinyl}oxy)silane, with a strongly electron-
withdrawing trifluoromethyl group on the benzene ring,
gave the corresponding -trifluoromethylthiolated ketone
2i in 82% yield. Products with halogen substituents on the
benzene ring 2f–n were obtained in yields of 81–85%, irre-
9
10
11
12
13
DMSO
a Reaction conditions: 1 (0.2 mmol), triphosgene, F3CSO2Na, solvent (3
mL), under N2, 1 h.
b Isolated yield.
spective of the halogen and its position on the aromatic
ring. Reactants with methyl or bromo ortho-substituents on
the aromatic ring were also converted effectively into the
corresponding trifluoromethylthio-substituted ketones 2m
and 2n in yields of 83 and 81%, respectively. The above re-
sults hint that steric hindrance by substituents on the aro-
matic rings has a limited influence on this transformation.
We next examined the use of the F2CHSO2Na/triphos-
gene system with electron-rich indoles to check the appli-
cability and generality of this approach (Table 3). Treatment
of indoles 3 with the F2CHSO2Na/triphosgene system in
MeCN at –78 °C gave the corresponding products 4a–n in
moderate to good yields. A wide range of N-protected or N-
unsubstituted indoles with electron-donating or electron-
withdrawing substituents were compatible with the reac-
tion conditions, and underwent direct C−H difluorometh-
ylthiolation to give the corresponding 3-difluoromethylthi-
olated indoles 4a–g in moderate to good yields. Notably,
substitution at both the C2 and C4 atoms was also well tol-
erated (4b–d, 4g), and even the substrate with a bulky 2-
phenyl group provided the expected product 4b in 65%
yield.
Indole and its derivatives can be directly functionalized
by various electrophiles due to the electron-rich nature of
the indole ring. Similarly, the ArSO2Na/triphosgene reagent
combination is also suitable for preparing 3-(arylsulfa-
nyl)indoles through an electrophilic substitution process
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

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H.-j. Gao et al.
Letter
Synlett
Table 2 Scope of the Silyl Enol Ethera
methylpyrrole to give the corresponding 2-sulfonyl pyrroles
4o–q in yields of up to 68%. Sodium (2-methylbenzene)sul-
finate gave product 4r in a low 37% yield, mainly because of
steric hindrance. Note that pyrrole without an N-protecting
group did not afford the expected product, owing to its de-
composition in the presence of triphosgene.
OSiMe3
O
Triphosgene
SCF3
(2.0 equiv)
R
CF3SO2Na
+
R
–76 °C–rt, 1 h,
CH3CN,
1a–o
2a–o
O
O
SCF3
SCF3
Table 3 Thiolation of Indoles and N-Methylpyrrole with RSO2Na in the
Presence of Triphosgenea
H3C
2a (93%)b
2b (92%)
SR
Triphosgene
R2
O
R2
O
+
RSO2Na
SCF3
N
R1
–78 °C–rt
CH3CN, 2 h
N
SCF3
R1
R = CF2H
or Ar
3
4a–r
H3CO
S
SCF2H
Ph
N
CH3
SCF2H
2c (96%)
2d (88%)
O
O
N
CH3
SCF3
SCF3
Ph
F
4a (89%)
SCF2H
4b (65%)
SCF2H
2e (89%)
2f (81%)
O
O
CH3
CH2CH3
CH3
SCF3
SCF3
N
N
H
Cl
Br
4c (84%)
4d (81%)
SCF2H
2g (82%)
2h (83%)
SCF2H
O
O
H3C
SCF3
H3C
SCF3
N
N
H
H
F3C
CH3
2i (82%)
2j (88%)
4e (85%)
4f (78%)
O
O
CO2Me
S
S
S
SCF2H
Br
SCF3
Cl
SCF3
N
N
H3C
H
2k (81%)
2l (85%)
4g (47%)
4h (73%)
CH3
O
Br
O
S
S
SCF3
SCF3
CH3
N
H
N
H3C
2m (83%)
2n (81%)
a Reaction condition: 1 (0.2 mmol), F3CSO2Na (0.4 mmol, 2 equiv), triphos-
gene (0.6 mmol, 3 equiv), MeCN (3 mL), –78 °C, N2, 1 h.
b Isolated yields are reported throughout.
4i (68%)
4g (54%)
CH3
Br
N
H
N
H
initiated by an ArS+ intermediate. Arylsulfinic acid sodium
salts containing alkyl or halogen substituents reacted
smoothly with indole under the optimized reaction condi-
tions to give the corresponding 3-(arylsulfanyl)indoles 4h–
n in moderate yields. However, we observed that electronic
effects of the substituents on the aryl ring exerted an obvi-
ous effect on the reaction reactivity, and sulfinates with
strongly electron-withdrawing groups, such as NO2 or CF3,
failed to give the corresponding C3-arylthioindole products.
Lastly, we turned our attention to the scope of another
N-heteroarene, N-methylpyrrole (4o–r). Several sulfinates
with methyl, bromo, or nitro substituents reacted with N-
4k (51%)
4l (66%)
Br
S
S
N
H3C
H
N
H
4m (68%)
4n (65%)
H3C
Br
N
CH3
N
S
S
CH3
4o (45%)
4p (57%)
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

D
H.-j. Gao et al.
Letter
Synlett
Table 3 (continued)
gene has general applicability owing to its strong electro-
philicity. It can therefore serve as an alternative and practi-
cal strategy for introducing sulfur-containing functional-
ities into organic molecules. Studies on the mechanism,
scope expansion, and limitations of this RSO2Na/triphos-
gene system are in progress and will be reported shortly.
SR
Triphosgene
R2
R2
+
RSO2Na
N
–78 °C–rt
CH3CN, 2 h
N
R1
R1
R = CF2H
or Ar
3
4a–r
O2N
CH3
S
N
CH3
S
N
CH3
Funding Information
This work was supported by the Natural Science Foundation of Zheji-
ang Province (No. LY19B020001, LY18B020004) and the Special Foun-
4q (68%)
4r (37%)
a Reaction conditions: Indole or N-methylpyrrole 3 (0.2 mmol), RSO2Na (2.0
equiv), triphosgene (2.0 equiv), MeCN (2.0 mL), under N2, –78 °C to rt, 2 h.
dation for Young Scientists of Lishui, Zhejiang (2018RC09).Natural
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cience
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datio
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f
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hejia
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cience
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Supporting Information
Although the exact reaction mechanism is still unclear,
on the basis of the above-mentioned results and reports in
the literature,5a,b,6a,10 we hypothesized that the generation
of highly reactive electrophilic CF3S+, CF2HS+, and ArS+ spe-
cies through deoxygenative reduction of RSO2Na with tri-
phosgene is the key step (Scheme 2). Initially, F3CSO2Na is
reduced by phosgene (formed in situ by decomposition of
triphosgene) to give the highly reactive electrophilic inter-
mediate F3CSCl (I), together with CO2, NaCl, and HClO,
among other products. The silyl enol ether then reacts with
the key intermediate F3CSCl (I) through electrophilic addi-
tion to give intermediate II. Finally, rapid elimination of the
Me3Si+ cation from intermediate II results in the generation
of the corresponding product.
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1707299. Su
p
p
ortingInformatio
n
Su
p
p
ortingInformatio
n
References and Notes
(1) (a) Uneyama, K. Organofluorine Chemistry; Blackwell: Oxford,
2006. (b) O’Hagan, D. Chem. Soc. Rev. 2008, 37, 308. (c) Wang, J.;
Sánchez-Roselló, M.; Aceña, J. L.; del Pozo, C.; Sorochinsky, A. E.;
Fustero, S.; Soloshonok, V. A.; Liu, H. Chem. Rev. 2014, 114, 2432.
(2) (a) Landelle, G.; Panossian, A.; Leroux, F. R. Curr. Top. Med. Chem.
(Sharjah, United Arab Emirates) 2014, 14, 941. (b) Lawton, G.;
Witty, D. R. Progress in Medicinal Chemistry; Elsevier Science
2015. (c) Xiong, H.-Y.; Pannecoucke, X.; Besset, T. Chem. Eur. J.
2016, 22, 16734. (d) Ni, C.; Hu, M.; Hu, J. Chem. Rev. 2015, 115,
765. (e) Diaferia, M.; Veronesi, F.; Morganti, G.; Nisoli, L.;
Fioretti, D. P. Parasitol. Res. 2013, 112, 163. (f) Xu, X.-H.;
Matsuzaki, K.; Shibata, N. Chem. Rev. 2015, 115, 731.
(3) (a) Zheng, H.; Huang, Y.; Weng, Z. Tetrahedron Lett. 2016, 57,
1397. (b) Shao, X.; Xu, C.; Lu, L.; Shen, Q. Acc. Chem. Res. 2015,
48, 1227. (c) Li, S.-G.; Zard, S. Z. Org. Lett. 2013, 15, 5898.
(d) Huang, Y.; He, X.; Li, H.; Weng, Z. Eur. J. Org. Chem. 2014,
7324. (e) Huang, Y.; He, X.; Lin, X.; Rong, M.; Weng, Z. Org. Lett.
2014, 16, 3284.
Triphosgene
Decomposition
O
Cl
Cl
O
O
fast
CF3SO2Na
(I)
CF3SCl
CF3S-Cl
Cl
Cl
–CO2
–CO2
–NaCl
(4) (a) Bootwicha, T.; Liu, X.; Pluta, R.; Atodiresei, I.; Rueping, M.
Angew. Chem. Int. Ed. 2013, 52, 12856. (b) Rueping, M.; Liu, X.;
Bootwicha, T.; Pluta, R.; Merkens, C. Chem. Commun. 2014, 50,
2508.
(5) (a) Alazet, S.; Ismalaj, E.; Glenadel, Q.; Le Bars, D.; Billard, T. Eur.
J. Org. Chem. 2015, 4607. (b) Wu, W.; Zhang, X.; Liang, F.; Cao, S.
Org. Biomol. Chem. 2015, 13, 6992. (c) Liu, Q.; Huang, T.; Zhao,
X.; Wu, J.; Cao, S. ACS Omega 2017, 2, 7755.
Me3SiO
Ar
F3CS
Ar
O
(II)
OSiMe3
SCF3
Ar
–Cl
–SiMe3
Scheme 2 Plausible reaction mechanism
(6) (a) Xu, C.; Ma, B.; Shen, Q. Angew. Chem. Int. Ed. 2014, 53, 9316.
(b) Alazet, S.; Zimmer, L.; Billard, T. Chem. Eur. J. 2014, 20, 8589.
(7) Arimori, S.; Takada, M.; Shibata, N. Org. Lett. 2015, 17, 1063.
(8) Saravanan, P.; Anbarasan, P. Adv. Synth. Catal. 2018, 360, 2894.
(9) He, X.-l.; Swarup, M.; Wu, J.; Jin, C.-d.; Guo, S.-r.; Guo, Z.-p.;
Yang, M. Org. Chem. Front. 2019, 6, 2435.
(10) (a) Liu, C.; Lu, Q.; Huang, Z.; Zhang, J.; Liao, F.; Peng, P.; Lei, A.
Org. Lett. 2015, 17, 6034. (b) Yang, Y.-D.; Iwamoto, K.; Tokunaga,
E.; Shibata, N. Chem. Commun. 2013, 49, 5510. (c) Hang, Z.; Li, Z.;
Liu, Z.-Q. Org. Lett. 2014, 16, 3648. (d) Zhu, L.; Wang, L.-S.; Li, B.;
Fu, B.; Zhang, C.-P.; Li, W. Chem. Commun. 2016, 52, 6371.
(e) Liu, Z.-Q.; Liu, D. J. Org. Chem. 2017, 82, 1649. (f) Jiang, L.;
Qian, J.; Yi, W.; Lu, G.; Cai, C.; Zhang, W. Angew. Chem. Int. Ed.
In summary, an efficient and practical approach has
been developed for the electrophilic trifluoromethylthiola-
tion of silyl enol ethers and for the difluoromethylthiolation
of indoles with various organosulfinates.11 This conversion
employs the appropriate sulfinate RSO2Na as a trifluoro-
methylthiolating, difluoromethylthiolating, or arylthiolat-
ing reagent, with the assistance of triphosgene as a reduc-
ing agent and activator to generate the key electrophilic RS+
intermediate in situ. Indeed, the use of F3CSO2Na, F2CHSO2-
Na, and ArSO2Na as sulfonyl precursors is well known, but
this combination of a sodium organosulfinate with triphos-
Thieme. All rights reserved. Synlett 2020, 31, A–E

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H.-j. Gao et al.
Letter
Synlett
2015, 54, 14965. (g) Bu, M.-j.; Lu, G.-p.; Cai, C. Org. Chem. Front.
2017, 4, 266. (h) Zhao, X.; Wei, A.; Yang, B.; Li, T.; Li, Q.; Qiu, D.;
Lu, K. J. Org. Chem. 2017, 82, 9175.
Electrophilic Thiolation of Indoles or N-Methylpyrrole;
General Procedure
A 10 mL Schlenk tube equipped with a magnetic stirring bar
was charged with RSO2Na (0.4 mmol) and the appropriate
indole or N-methylpyrrole (0.2 mmol). The tube was then evac-
uated and backfilled with dry N2 (this operation was repeated
three times), and the mixture was stirred at –78 °C for the
initial time. A solution of triphosgene (0.4 mmol) in MeCN (1
mL) was added slowly from a syringe, and the mixture was
stirred at –78 °C for 1 h. When the reaction was complete, the
mixture was warmed to rt and diluted with CH2Cl2 (20 mL). The
mixture was then washed with 5% aq NaHCO3 (10 mL), and the
combined organic phase was dried (Na2SO4) and concentrated
under reduced pressure. The residue was purified by column
chromatography (silica gel).
3-[(Difluoromethyl)sulfanyl]-1-methyl-1H-indole (4a)
Yellow oil; yield: 37.9 mg (89%). 1H NMR (300 MHz, CDCl3):  =
7.80 (d, J = 7.8 Hz, 1 H), 7.42–7.20 (m, 4 H), 6.67 (t, J = 57.7 Hz, 1
H), 3.83 (s, 3 H). 13C NMR (75 MHz, DMSO-d6):  = 137.31,
136.14, 130.50, 122.82, 121.15 (t, J = 275 Hz), 120.98, 119.47,
109.82, 94.28, 33.23. MS (EI): m/z = 213.1 [M]+. HRMS (EI-TOF):
m/z [M]+ calcd for C10H9F2NS: 213.0424; found: 213.042.
(11) Electrophilic Trifluoromethylthiolation of Silyl Enol Ethers;
General Procedure
A 10 mL Schlenk tube equipped with a magnetic stirring bar
was charged with F3CSO2Na (0.4 mmol), the appropriate silyl
enol ether (0.2 mmol), and MeCN (1 mL), and the mixture was
stirred at –78 °C for the initial time. A solution of triphosgene
(0.4 mmol) in MeCN (1 mL) was then added slowly from a
syringe. After 1 h at –78 °C, the mixture was warmed to rt over
1 h, then diluted with CH2Cl2 (20 mL). The mixture was washed
5% aq NaHCO3 (10 mL), and the combined organic phase was
dried (Na2SO4) and concentrated under reduced pressure. The
residue was purified by column chromatography (silica gel).
1-Phenyl-2-[(trifluoromethyl)sulfanyl]ethanone (2a)
Yellow oil; yield: 40.9 mg (93%). 1H NMR (300 MHz, CDCl3):  =
7.96 (d, J = 6.8 Hz, 2 H), 7.65 (t, J = 6.8 Hz, 1 H), 7.54 (t, J = 6.8 Hz,
2 H), 4.53 (s, 2 H). 13C NMR (75 MHz, CDCl3):  = 192.1 (s), 134.7
(s), 134.3 (d, J = 1.5 Hz), 130.7 (q, J = 306.4 Hz), 129.0 (s), 128.5
(s), 38.5 (q, J = 1.8 Hz). HRMS (EI-TOF): m/z [M]+ calcd for
C9H7F3OS: 220.0170; found: 220.0162.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E