4-Dimethylaminopyridine-catalyzed synthesis of isothiocyanates from amines and carbon disulfide

Article abstract of DOI:10.1016/j.tetlet.2021.152868

Isothiocyanates were synthesized by reactions between primary amines and CS₂ in the presence of 4-dimethylaminopyridine as a catalyst and tert-butyl hydroperoxide as an oxidant. Various aryl, benzyl, alkyl, and hydroxyl amines were transformed into the corresponding isothiocyanates in 41–82% yields.

Full text of DOI:10.1016/j.tetlet.2021.152868

Tetrahedron Letters 68 (2021) 152868
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
4-Dimethylaminopyridine-catalyzed synthesis of isothiocyanates
from amines and carbon disulfide
Hao-Jie Rong a,b, Tao Chen , Ze-Gang Xu , Tian-Duo Su , Yu Shang , Yong-Qiang Wang , Cui-Feng Yang
b
a
c
a
b
a,d,
⇑
a
Modern Chemistry Research Institute of Xi’an, Xi’an 710065, China
Department of Chemistry & Materials Science, Northwest University, Xi’an 710069, China
Shaanxi Environmental Monitoring Center, Xi’an 710054, China
State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an 710065, China
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Isothiocyanates were synthesized by reactions between primary amines and CS2 in the presence of 4-
dimethylaminopyridine as a catalyst and tert-butyl hydroperoxide as an oxidant. Various aryl, benzyl,
alkyl, and hydroxyl amines were transformed into the corresponding isothiocyanates in 41–82% yields.
Ó 2021 Elsevier Ltd. All rights reserved.
Received 30 November 2020
Revised 14 January 2021
Accepted 21 January 2021
Available online 4 February 2021
Keywords:
Primary amines
Carbon disulfide
Catalytic synthesis
Isothiocyanates
Introduction
3 3
fluoride [17] derived from CF SiMe , elemental sulfur, and KF has
also been reported for the synthesis of isothiocyanates under air.
In the second strategy (Scheme 1), the primary amine is first con-
verted to the corresponding dithiocarbamate salt by reaction with
Isothiocyanates are an important class of heteroallenic com-
pounds because they have a broad range of bioactivities (Figure 1).
For example phenyl, allyl, benzyl, and phenethyl isothiocyanates
isolated from Brassicales show antibiotic and antitumor activities
CS
salt undergoes desulfurization to afford the isothiocyanate.
Na [18], H [19], I [20–21], Fe(NO [22], and various
2
in the presence of excess base, and then the dithiocarbamate
[
1–3]. Sulforaphane [4], which can be isolated from cruciferous
2
2
S O
8
2
O
2
2
3 3
)
vegetables, exhibits anticancer activity. In addition, isothio-
cyanates are versatile building blocks for the synthesis of chiral
thiourea catalysts [5–8] and various heterocycles [9–10]. There-
fore, the development of methods for the synthesis of isothio-
cyanates has attracted considerable attention.
Isothiocyanates have traditionally been synthesized from ami-
nes, tertiary alcohols, thioureas, amides, nitrile oxides, and aldox-
imines [11]. Of these precursors, amines are used the most
frequently, owing to their versatility and ready accessibility. There
are two general strategies for the synthesis of isothiocyanates from
amines. The first involves direct thiocarbonylation of amines with
a thiocarbonyl transfer reagent [12–16] such as thiophosgene, di
other reagents [23–31] have been used for this purpose. However,
these two strategies require excess base, and therefore the devel-
opment of new methods would be desirable.
Results and discussion
Recently, we reported a catalytic desulfurization strategy for
the synthesis of guanidines [32–33], and we envisioned that a sim-
ilar strategy could be used for the synthesis of isothiocyanates with
catalytic base. In fact, in 1997 Tajima and Li reported the synthesis
of hydroxyl isothiocyanates from hydroxy primary amines and CS
in the presence of catalytic NEt [34], but similar transformations
of other amines require excess base [19].
We began our work on the development of a catalytic method
for the synthesis of isothiocyanates by exploring reactions of CS
and 4-methoxyaniline (1a) as a model substrate (Table 1). Initially,
we used I as an additive and 70% aqueous tert-butyl hydroperox-
2
(
2-pyridyl) thiocarbamate, or (Me
4
N)SCF
3
. Although this strategy
3
can provide isothiocyanates in a single step, the transfer reagents
are either toxic or not commercially available, and the transforma-
tion usually requires anhydrous conditions. Recently, thiocarbonyl
2
2
⇑
Corresponding author at: Modern Chemistry Research Institute of Xi’an, Xi’an
10065, China.
ide (TBHP) as an oxidant at rt in methanol and found that only 25%
of the desired isothiocyanate (2a) was produced (entry 1).
7
https://doi.org/10.1016/j.tetlet.2021.152868
040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
0

Hao-Jie Rong, T. Chen, Ze-Gang Xu et al.
Tetrahedron Letters 68 (2021) 152868
Table 2
Catalytic synthesis of isothiocyanates from various primary amines.
Fig. 1. Representative isothiocyanates and related compounds.
a
Reaction conditions: to a solution of amine (0.5 mmol) in MeOH (2.5 mL) at 0 °C
Scheme 1. Synthesis of isothiocyanates from amines and CS2.
4 2
was added Bu NI (16 mg), 4-dimethylaminopyridine (DMAP, 6 mg), CS (133 mg),
and 70% aqueous tert-butyl hydroperoxide (TBHP, 96 mg) in that order.
Table 1
Optimization of reaction conditions.
a
To elucidate the reaction mechanism, we conducted a series of
control experiments (Scheme 2). We started with reactions of 1a
and CS
2
in MeOH in the presence of Bu
4
NI, DMAP, or TBHP alone.
The reaction in the presence of Bu
isothiocyanate 2a (Scheme 2a), and reactions in the presence of
4
NI alone gave only a trace of
Entry
Solvent
t (°C)
Additive
Base
Oxidant
Yield
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
DMF
MeOH
MeOH
MeOH
MeOH
MeOH
25
25
25
25
25
25
25
25
0
I
2
None
None
DMAP
TBHP
TBHP
TBHP
TBHP
TBHP
25
27
65
60
58
45
56
48
73
78
DMAP or TBHP alone gave the isothiocyanate in 25% or 30% yield,
respectively, after 6 h (Scheme 2b,c). When a combination of Bu -
4
NI and TBHP or a combination of DMAP and TBHP was used, 2a was
isolated in 27% or 60% yield, respectively (Scheme 2d,e). These
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
4
NI
4
NI
4
NI
4
NI
4
NI
4
NI
4
NI
4
NI
4
NI
b
NEt
3
DIPEA
DMAP
DMAP
DMAP
DMAP
DMAP
2
H O
2
4
experiments demonstrate that Bu NI, DMAP, and TBHP were
2 2 8
Na S O
essential for the transformation. In a radical capture experiment
conducted with 2.0 equiv of TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy), none of
NaCIO
TBHP
TBHP
0c
1
0
the TEMPO capture product was detected, and the yield of the
isothiocyanate was 73% (results not shown). This finding suggests
that the reaction did not proceed via a radical pathway [35].
On the basis of our control experiments and previously reported
results for desulfurization of dithiocarbamate salts [19,36], we pro-
pose the reaction pathway outlined in Scheme 3. First,
a
Reaction conditions: the catalyst and then the oxidant were added to a solution
of the amine (0.5 mmol) in the solvent (5 mL.
b
DMAP = 4-dimethylaminopyridine, DIPEA = N,N-diisopropylamine.
The amount of MeOH was decreased to 2.5 mL.
c
4
Changing the additive to Bu NI (entry 2) or an iodine reagent such
as N-iodosuccinimide or phenyliodine(III) diacetate (not shown)
had no obvious effect on the yield. However, when 0.1 equiv of
N,N-dimethylaminopyridine (DMAP) was present in a reaction
2
addition of the amine to CS gives dithiocarbamate acid A,
which reacts with the base to give dithiocarbamate salt B. Salt B
either can be oxidized by TBHP [37] to give the isothiocyanate
via intermediate C (path b) or can undergo elimination of S2 to
form the isothiocyanate directly [36] (path c). Intermediate C can
also be obtained by direct oxidation of intermediate A (path a).
À
4
mixture containing Bu NI, the desired isothiocyanate was obtained
in 65% yield (entry 3). Careful screening of several other bases, sol-
vents and oxidants (entries 4–8) revealed that the combination of
DMAP and TBHP was optimal. In addition, the yield of the isothio-
cyanate could be improved to 78% by decreasing the reaction tem-
perature to 0 °C and increasing the concentration of the reactants
(
entries 9 and 10).
With the optimal conditions in hand (Table 1, entry 10), we car-
ried out reactions of CS with various primary anilines, benzyl ami-
2
nes, hydroxyl amines, and alkyl amines (Table 2). Most of the
tested substrates afforded the corresponding isothiocyanates, and
the yields were moderate to good (41–82%). The reaction worked
well with electron-rich anilines (2a and 2b) and sterically hindered
anilines (2c and 2d). In contrast, electron-deficient amines gave
thioureas as the major products, and only traces of the desired
isothiocyanates were detected (not shown). We surmised that this
outcome was probably due to reaction of the product isothio-
cyanates with the amine substrates rather than with CS
the unchanged starting amines reacted preferentially with the gen-
erated product isothiocyanates but not with CS
2
; that is,
2
.
Scheme 2. Control experiments.
2

Hao-Jie Rong, T. Chen, Ze-Gang Xu et al.
Tetrahedron Letters 68 (2021) 152868
1
White solid; 66 mg, 74% yield; H NMR (500 MHz, CDCl
3
) d 6.84
1
3
(
s, 2H), 2.32 (s, 6H), 2.26 (s, 3H). C NMR (126 MHz, CDCl ) d
3
1
36.96, 135.19, 134.79, 128.69, 126.93, 21.01, 18.51.
-isothiocyanato-1,3-dimethylbenzene (2d) [39].
2
1
Colorless oil; 62 mg, 75% yield; H NMR (500 MHz, CDCl
3
) d
) d
1
3
7
.16–6.88 (m, 3H), 2.37 (s, 6H). C NMR (126 MHz, CDCl
35.68, 135.06, 129.63, 127.97, 126.88, 18.62.
3
1
4
-isothiocyanato-1H-indole (2e) [18].
1
Yellow oil; 52 mg, 60% yield; H NMR (500 MHz, Chloroform-d)
d 8.33 (brs, 1H), 7.32 (d, J = 8.1 Hz, 1H), 7.28–7.23 (m, 1H), 7.11 (t,
J = 7.9 Hz, 1H), 7.04–6.90 (m, 1H), 6.75–6.66 (m, 1H). 13C NMR
Scheme 3. Proposed mechanism.
(
1
126 MHz, CDCl
22.19, 116.61, 110.81, 100.30.
isothiocyanatomethyl)benzene(2f) [18].
3
) d 136.65, 136.15, 125.37, 125.09, 122.90,
(
Meanwhile, TBHP may oxidize S2À to elemental sulfur [32] or [SO
H] [37]. Considering that Bu NI was involved in the reaction and
that in the absence of Bu NI, the reaction afforded the desired
isothiocyanate in only 60% yield (Scheme 2e), we surmised that
Bu NI facilitated addition of the amine to CS
1
x
-
Colorless oil; 46 mg, 61% yield; H NMR (500 MHz, CDCl
3
) d
1
3
7.46–7.28 (m, 5H), 4.69 (s, 2H). C NMR (126 MHz, CDCl
134.27, 132.67, 128.92, 128.32, 126.80, 48.68.
3
) d
4
4
1-isothiocyanato-4-(methoxymethyl)benzene(2g) [18].
1
Colorless oil; 72 mg, 80% yield; H NMR (500 MHz, CDCl
3
) d 7.16
4
2
.
(
3
d, J = 8.9 Hz, 2H), 6.85 (d, J = 8.9 Hz, 2H), 4.61 (s, 2H), 3.80 (s,
1
3
3
H). C NMR (126 MHz, CDCl ) d 159.63, 132.28, 128.31, 126.34,
Conclusion
1
14.31, 55.29, 48.26.
1
-(isothiocyanatomethyl)-4-methoxybenzene(2h) [40].
In conclusion, we have developed a protocol for DMAP-cat-
alyzed synthesis of isothiocyanates from amines and CS . Various
1
Colorless oil; 65 mg, 72% yield; H NMR (500 MHz, CDCl
3
) d
2
7
3
1
.39–7.21 (m, 1H), 7.02–6.75 (m, 3H), 4.66 (s, 2H), 3.81 (s,
aryl, benzyl, alkyl, and hydroxyl primary amines were transformed
into the corresponding isothiocyanates in 41–82% yields, and the
mechanism was explored by means of control experiments.
Although the yields achieved with this protocol were moderate,
we believe our findings will lead to the development of more-effi-
cient methods for catalytic synthesis of isothiocyanates.
13
H). C NMR (126 MHz, CDCl
19.01, 113.88, 112.50, 55.33, 48.64.
-fluoro-4-(isothiocyanatomethyl)benzene(2i) [41].
3
) d 160.11, 135.80, 132.80, 130.03,
1
1
Colorless oil; 44 mg, 53% yield; H NMR (500 MHz, CDCl
3
) d
.35–7.27 (m, 2H), 7.12–7.00 (m, 2H), 4.68 (s, 2H). C NMR
) d162.63 (d, J = 247.5 Hz), 133.28, 130.17 (d,
J = 3.3 Hz), 128.72 (d, J = 8.3 Hz), 115.93 (d, J = 22.0 Hz), 48.11.
-(isothiocyanatomethyl)furan(2j) [18].
13
7
(
126 MHz, CDCl
3
2
Experimental section
1
Colorless oil; 29 mg, 41% yield; H NMR (500 MHz, CDCl
dd, J = 1.8, 0.9 Hz, 1H), 6.35 (dd, J = 9.0, 1.4 Hz, 2H), 4.64 (s, 2H).
NMR (126 MHz, CDCl ) d 147.38, 143.18, 135.31, 110.60, 108.65,
3
3
) d 7.42
13
(
C
General information
4
1.92.
-(2-isothiocyanatoethyl)benzene-1,2-diol(2k) [42].
Colorless oil; 49 mg, 50% yield; H NMR (500 MHz, CDCl ) d 6.82
3
Unless otherwise noted, all reagents were used without further
purification. Flash column chromatographies were performed on
4
1
1
13
Qingdao silica gel (200–300 mesh). H, C spectra were measured
1
13
(d, J = 8.1 Hz, 1H), 6.74 (d, J = 2.1 Hz, 1H), 6.64 (dd, J = 8.1, 2.1 Hz,
1H), 5.84–5.67 (brs, 2H), 3.65 (t, J = 6.8 Hz, 2H), 2.85 (t, J = 6.8 Hz,
on a NMR instrument (500 MHz for H NMR; 126 MHz for
C
NMR). Chemical shifts of 1H NMR spectra were recorded relative
to internal standard (TMS d 0.00). Chemical shifts of C NMR spec-
tra were recorded relative to solvent resonance (CDCl : d 77.0).
1
3
13
2H). C NMR (126 MHz, CDCl
21.38, 115.99, 115.75, 46.55, 35.81.
-isothiocyanatophenol(2l) [18].
3
) d 143.77, 142.69, 130.69, 130.10,
1
3
4
1
3
Colorless oil; 59 mg, 78% yield; H NMR (500 MHz, CDCl ) d 7.10
(d, J = 8.8 Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H), 6.00 (s, 1H).1 C NMR
3
General procedure for the synthesis of isothiocyanates
(
126 MHz, CDCl ) d 154.76, 134.00, 127.18, 123.75, 116.40.
3
To a solution of amine (0.5 mmol) in MeOH (2.5 mL) at 0 °C was
4-isothiocyanatobutan-1-ol (2m) [43].
Colorless oil; 51 mg, 78% yield; H NMR (500 MHz, CDCl
1
added Bu
sequentially. The solution was stirred at 0 °C for 10 min. After that,
0% aq. TBHP (96 mg, 0.75 mmol), CS (133 mg, 1.75 mmol) was
4
NI (18 mg, 0.05 mmol), DMAP (6 mg, 0.05 mmol)
3
) d 3.69
(t, J = 5.8 Hz, 2H), 3.58 (t, J = 6.5 Hz, 2H), 1.91–1.76 (m, 2H), 1.74–
1
3
7
2
1.64 (m, 2H). C NMR (126 MHz, CDCl
29.51, 26.65.
3
) d 130.30, 61.79, 44.99,
added. The solution was stirred at 0 °C for 6–18 h. Upon consump-
tion of the amine, 50 mL EtOAc was added, the organic phase was
5-isothiocyanatopentan-1-ol (2n) [19].
Colorless solid; 60 mg, 82% yield; 1H NMR (500 MHz, CDCl
) d
3
washed with H
2
O (10 mL) and dried with Na
2
SO
4
. After concentra-
tion of the organic phase under reduced pressure, the residue was
purified by column chromatography on silica gel using EtOAc/
PE = 1:20 to give the desired product.
3.66 (t, J = 6.4 Hz, 2H), 3.54 (t, J = 6.6 Hz, 2H), 1.85 (brs, 1H),
1.79–1.70 (m, 2H), 1.68–1.57 (m, 2H), 1.56–1.44 (m, 2H).13C NMR
3
(126 MHz, CDCl ) d 130.01, 62.29, 44.97, 31.70, 29.69, 22.87.
1
-isothiocyanato-4-methoxybenzene (2a) [18].
Isothiocyanatocyclohexane (2o) [18].
1
Colorless oil; 35 mg, 50% yield; 1H NMR (500 MHz, CDCl
Colorless oil; 64 mg, 78% yield; H NMR (500 MHz, CDCl
3
) d 7.16
3
) d
1
3
(
(
d, J = 8.9 Hz, 2H), 6.85 (d, J = 8.9 Hz, 2H), 3.80 (s, 3H). C NMR
126 MHz, CDCl ) d 158.62, 134.29, 126.90, 123.76, 114.84, 55.54.
-isothiocyanato-1,2,3-trimethoxybenzene(2b) [38].
3.87–3.49 (m, 1H), 2.04–1.86 (m, 2H), 1.77–1.61 (m, 4H), 1.54–
1
3
3
1.45 (m, 1H), 1.44–1.32 (m, 3H). C NMR (126 MHz, CDCl
130.18, 55.46, 33.24, 25.07, 23.24.
3
) d
5
1
Brown solid; 90 mg, 80% yield; H NMR (500 MHz, CDCl
s, 2H), 3.84 (s, 6H), 3.83 (s, 3H).13C NMR (126 MHz, CDCl
53.66, 137.89, 135.04, 126.60, 103.40, 60.99, 56.30.
-isothiocyanato-1,3,5-trimethylbenzene(2c) [18].
3
) d 6.46
isothiocyanatocyclopentane (2p) [18].
(
1
3
) d
Colorless oil; 26 mg, 41% yield; 1 1H NMR (500 MHz, CDCl
4.60–3.68 (m, 1H), 2.07–1.74 (m, 6H), 1.73–1.58 (m, 2H) [13].
3
) d
2
3
C NMR (126 MHz, CDCl ) d 129.80, 57.70, 34.35, 23.21.
3

Hao-Jie Rong, T. Chen, Ze-Gang Xu et al.
Tetrahedron Letters 68 (2021) 152868
19] G. Li, H. Tajima, T. Ohtani, J. Org. Chem. 62 (1997) 4539–4540.
20] J. Nath, H. Ghosh, R. Yella, B.K. Patel, Eur. J. Org. Chem. (2009) 1849–1851.
21] J. Nath, L. Jamir, B.K. Patel, Green Chem. Lett. Rev. 4 (2011) 1–34.
[
[
[
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[22] S. Pinapati, U. Mandapati, R.R. Rudraraju, Chemistryselect 2 (2017) 295–299.
[23] J.E. Hodgkins, W.P. Reeves, J. Org. Chem. 29 (1964) 3098–3099.
[
24] U. Boas, H. Gertz, J.B. Christensen, P.M.H. Heegaard, Tetrahedron Lett. 45
2004) 269–272.
25] G.F. Bian, W.G. Shan, W.K. Su, J. Chem. Res. (2005) 585–586.
(
[
Acknowledgments
[26] R. Wong, S.J. Dolman, J. Org. Chem. 72 (2007) 3969–3971.
[
[
[
27] H. Munch, J.S. Hansen, M. Pittelkow, J.B. Christensen, U. Boas, Tetrahedron Lett.
9 (2008) 3117–3119.
28] L. Jamir, B. Alimenla, A. Kumar, D. Sinha, U.B. Sinha, Synth. Commun. 41 (2010)
147–155.
4
This project was funded by the China Postdoctoral Science
Foundation (2019M653782).
29] R. Yella, H. Ghosh, S. Murru, S.K. Sahoo, B.K. Patel, Synth. Commun. 40 (2010)
2
083–2096.
References
[
[
30] M. Seelam, B. Shaik, P.R. Kammela, Synth. Commun. 46 (2016) 1759–1765.
31] U. Mandapati, S. Pinapati, R. Rudraraju, Tetrahedron Lett. 58 (2017) 125–128.
[
[
1] E.S. Hwang, E.H. Jeffery, Food Chem. Toxicol. 41 (2003) 1817–1825.
2] A.P. Lawson, M.J.C. Long, R.T. Coffey, Y. Qian, E. Weerapana, F. El Oualid, L.
Hedstrom, Cancer Res. 75 (2015) 5130–5142.
[32] H.J. Rong, C.F. Yang, T. Chen, Z.G. Xu, T.D. Su, Y.Q. Wang, B.K. Ning, Org. Biomol.
Chem. 17 (2019) 9280–9283.
[33] H.-J. Rong, C.-F. Yang, T. Chen, Y.-Q. Wang, B.-K. Ning, Tetrahedron Lett. 60
(2019) 150970.
[34] H. Tajima, G. Li, Synlett 1997 (1997) 773–774.
[35] R.-L. Guo, X.-Q. Zhu, X.-L. Zhang, Y.-Q. Wang, Chem. Commun. 56 (2020) 8976–
8979.
[36] A.D. Jangale, P.P. Kumavat, Y.B. Wagh, Y.A. Tayade, P.P. Mahulikar, D.S. Dalal,
Synth. Commun. 45 (2014) 376–385.
[
[
[
3] D. Li, Y. Shu, P. Li, W. Zhang, H. Ni, Y. Cao, Med. Chem. Res. 22 (2013) 3119–
3
125.
4] A. Yao, Y. Shen, A. Wang, S. Chen, H. Zhang, F. Chen, Z. Chen, H. Wei, Z. Zou, Y.
Shan, X. Zhang, Biochem. Biophys. Res. Commun. 465 (2015) 696–701.
5] I.-J. Kang, L.-W. Wang, T.-K. Yeh, C.-C. Lee, Y.-C. Lee, S.-J. Hsu, Y.-S. Wu, J.-C.
Wang, Y.-S. Chao, A. Yueh, J.-H. Chern, Biorg, Med. Chem. 18 (2010) 6414–
6
421.
[37] H. Esteves, Â. De Fátima, R.D.P. Castro, J.R. Sabino, F. MacEdo, T.O. Brito,
Tetrahedron Lett. 56 (2015) 6872–6874.
[38] S. Bouayad-Gervais, T. Scattolin, F. Schoenebeck, Angew. Chem. Int. Ed. 59
(2020) 11908–11912.
[
6] J. Li, Z. Tan, S. Tang, I. Hewlett, R. Pang, M. He, S. He, B. Tian, K. Chen, M. Yang,
Biorg. Med. Chem. 17 (2009) 3177–3188.
7] H. Peng, Y. Liang, L. Chen, L. Fu, H. Wang, H. He, Bioorg. Med. Chem. Lett. 21
[
(
2011) 1102–1104.
[39] Ł. Janczewski, A. Gajda, T. Gajda, Eur. J. Org. Chem. 2019 (2019) 2528–2532.
[40] N.S. Radulovi c´ , M.S. Deki c´ , Z.Z. Stojanovi c´ -Radi c´ , Phytochem Lett 5 (2012) 351–
357.
[
[
8] A.G. Doyle, E.N. Jacobsen, Chem. Rev. 107 (2007) 5713–5743.
9] A.K. Mukerjee, R. Ashare, Chem. Rev. 91 (1991) 1–24.
[
[
[
[
[
[
[
[
[
10] G. Sommen, Synlett (2004) 1323–1324.
11] K. Eschliman, S.H. Bossmann, Synthesis 51 (2019) 1746–1752.
12] R. Gottfried, Angew. Chem. Int. Ed. 5 (1966) 963–964.
13] H.A. Staab, G. Walther, Justus Liebigs Ann. Chem. 657 (1962) 104–107.
14] C. Larsen, K. Steliou, D.N. Harpp, J. Org. Chem. 43 (1978) 337–339.
15] E. Dyer, T.B. Johnson, J. Am. Chem. Soc. 54 (1932) 777–787.
16] T. Scattolin, A. Klein, F. Schoenebeck, Org. Lett. 19 (2017) 1831–1833.
17] L. Zhen, H. Fan, X. Wang, L. Jiang, Org. Lett. 21 (2019) 2106–2110.
18] Z. Fu, W. Yuan, N. Chen, Z. Yang, J. Xu, Green Chem. 20 (2018) 4484–4491.
[41] T. Kim, Y.-J. Kim, I.-H. Han, D. Lee, J. Ham, K.S. Kang, J.W. Lee, Bioorg. Med.
Chem. Lett. 25 (2015) 62–66.
[42] E.S. Spencer, E.J. Dale, A.L. Gommans, M.T. Rutledge, C.T. Vo, Y. Nakatani, A.B.
Gamble, R.A.J. Smith, S.M. Wilbanks, M.B. Hampton, J.D.A. Tyndall, Eur. J. Med.
Chem. 93 (2015) 501–510.
[43] X. Chen, Z. Li, X. Sun, H. Ma, X. Chen, J. Ren, K. Hu, Synthesis 2011 (2011) 3991–
3996.
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