Visible-light-initiated malic acid-promoted cascade coupling/cyclization of aromatic amines and KSCN to 2-aminobenzothiazoles without photocatalyst

Article abstract of DOI:10.1016/j.cclet.2020.02.011

By using ambient air as the oxidant and malic acid as the promoter, a practical method for the preparation of 2-aminobenzothiazoles through visible-light-initiated cascade reaction of aromatic amines and KSCN in eco-friendly bis(methoxypropy)ether under metal-, hazardous additive-, photocatalyst-free conditions was established.

Full text of DOI:10.1016/j.cclet.2020.02.011

Journal Pre-proof
Visible-light-initiated malic acid-promoted cascade coupling/cyclization of
aromatic amines and KSCN to 2-aminobenzothiazoles without
photocatalyst
Wei-Bao He, Lan-Qing Gao, Xin-Jie Chen, Zhi-Lin Wu, Ying Huang,
Zhong Cao, Xin-Hua Xu, Wei-Min He
PII:
S1001-8417(20)30071-1
https://doi.org/10.1016/j.cclet.2020.02.011
CCLET 5452
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
24 December 2019
21 January 2020
7 February 2020
Please cite this article as: He W-Bao, Gao L-Qing, Chen X-Jie, Wu Z-Lin, Huang Y, Cao Z, Xu
X-Hua, He W-Min, Visible-light-initiated malic acid-promoted cascade coupling/cyclization of
aromatic amines and KSCN to 2-aminobenzothiazoles without photocatalyst, Chinese
Chemical Letters (2020), doi: https://doi.org/10.1016/j.cclet.2020.02.011
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Chinese Chemical Letters
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Communication
Visible-light-initiated malic acid-promoted cascade coupling/cyclization of aromatic
amines and KSCN to 2-aminobenzothiazoles without photocatalyst
a
a
c
d
b
b
a
Wei-Bao He , Lan-Qing Gao , Xin-Jie Chen , Zhi-Lin Wu , Ying Huang , Zhong Cao , Xin-Hua Xu *, Wei-
a,b
Min He
a
Department of Chemistry, Hunan University, Changsha 410082, China
Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and Technology, Changsha
b
4
10114, China
c
School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China
d
Graphical Abstract
A simple and practical method for the synthesis of 2-aminobenzothiazoles through visible-light-initiated malic acid-
promoted cascade coupling/cyclization of aromatic amines and KSCN with ambient air as an oxidant in eco-friendly
bis(methoxypropy) ether at ambient temperature was developed.
ARTICLE INFO
ABSTRACT
Article history:
By using ambient air as the oxidant and malic acid as the promoter, a practical method for the
preparation of 2-aminobenzothiazoles through visible-light-initiated cascade reaction of
aromatic amines and KSCN in eco-friendly bis(methoxypropy)ether under metal-, hazardous
additive-, photocatalyst-free conditions was established.
Received 24 December 2019
Received in revised form 21 January 2020
Accepted 4 February 2020
Available online
Keywords:
Air
Photocatalysis
Cascade reaction
Photocatalyst-free
2
-Aminobenzothiazoles
Green and sustainable chemistry has put forward the requirement of replacing traditional non-renewable resources with clean,
inexpensive and abundant natural resources such as air [1] and natural plant extracts [2]. In response to this requirement, visible light
photocatalytic oxidation using dioxygen has become a hot spot in synthetic chemistry research in recent years [3]. However, noble metals
and/or ecologically unfriendly photocatalysts are needed in most photocatalytic oxidation processes, which not only increases production
costs, but also generates unnecessary chemical wastes, resulting in complex separation and purification steps and environmental pollution.
Compared with pure oxygen, ambient air is a better oxidant in that it is safer, cheaper, and more readily available [4]. Therefore,

Corresponding authors.
E-mail addresses: weiminhe2016@yeah.net (W.M. He); xinhuaxu2019@yeah.net (X.H. Xu)

developing the novel visible-light photocatalytic oxidation which could be conducted with ambient air as the terminal oxidant under
photocatalyst-free conditions is highly desirable[5], and great efforts have been made to develop various green processes to avoid the use
of hazardous chemicals and to use abundant natural raw materials for organic synthesis. Among them, the chemical process promoted by
natural acid has yielded fruitful results [6].
2
-Aminobenzothiazoles represent an important class of N-heterocycles [7] whose valuable pharmacological and biological activities
make them privileged scaffolds in research and drug developments [8]. A tremendous amount of effort has been paid towards developing
novel methods for their preparation [9]. Among the established synthetic approaches, the cascade oxidative coupling/cyclization of
aromatic amines with thiocyanate salts is an ideal choice for the construction of such compounds given the cheapness and readily
availability of raw materials. However, the traditional method generally requires the usage of excessive toxic molecular bromine (or other
halogenated oxidant) and corrosive solvent (such as hydrochloric acid, acetic acid), which not only leads to high manufacturing cost but
also causes series environmental problems (Scheme 1a). Recently, Wu and Jiang pioneered the copper-catalyzed oxidative thiocyanation
o
of anilines and KSCN with molecular oxygen as an oxidant and BF
3
·Et
2
O as the promoter in dimethyl sulfoxide (DMSO) at 100 C
(Scheme 1b) [10]. From a practical and safety point of view, the risk of employing pure O
2
and explosive DMSO at high temperature,
and the usage of hazardous synthetic acid and metal catalysts largely restrict its application for large-scale industrial manufactures.
Guided by the 12 principles of green and sustainable chemistry [11], herein we report a practical protocol for the preparation of 2-
aminobenzothiazoles through visible-light-initiated (Principle 6th, increase energy efficiency) malic acid-promoted (Principle 7th, use of
renewable feedstocks) cascade (Principle 8th, reduce derivatives) coupling and cyclization (Principle 2nd, atom economy) of aromatic
amines and KSCN by using ambient air as the sole oxidant (Principle 4th, designing safer chemicals) in eco-friendly bis(methoxypropy)
ether (Principle 5th, safer solvents) under metal-, hazardous additive-, photocatalyst-free (Principle 1st, prevent waste) conditions
(Scheme 1c).
Scheme 1. Preparation of 2-aminobenzothiazoles through cascade reaction.
Our preliminary exploration started with 4-toluidine (1a) as a template substrate. Treatment of 1a, NaSCN (2 equiv.), and HOAc (2
equiv.) in bis(methoxypropy)ether (BME, 1 mL) under ambient air by the irradiation of a 6 W blue LED lamp led to the production of
the expected 6-methylbenzo[d]thiazol-2-amine (2a) in 25% NMR yield (Table 1, entry 1). Next, the screening of thiocyanate salts (entries
1
-3) revealed that the KSCN was the optimal thiocyanation source (entry 2). A series of natural acids (entries 4-8), include glycolic acid,
lactic acid, malic acid, oxalic acid and citric acid were subjected to investigation, and malic acid (entry 6) was found to be the best choice
for this present reaction. An optimization of the solvents (entries 9-14) illustrated that none of the other solvents was superior to BME.
Finally, the impact of varying the concentration of 1a, the amount of KSCN and malic acid were investigated. Decreasing the
concentration of 1a resulted in no improvement in the reaction efficiency (entry 15), whereas a slightly lower yield of 2a was observed
when the concentration of 1a was increased to 0.4 mmol/mL (entry 16). Altering the amount of KSCN did not improve the yield of 2a
(entries 17 and 18). Increasing the loading of malic acid to 3 equiv. did not provide any significant changes in reaction outcome (entry
1
2
9). Reducing the amount of malic acid to 1 equiv. was feasible; however, further decreasing to 0.8 equiv. resulted in a lower yield (entries
0 and 21). No conversion of 4-toluidine was observed in the absence of a Brønsted acid (entry 22). No improvement in the photocatalytic
reaction was detected when pure oxygen gas was used instead of ambient air (entry 23). When the reaction was conducted under nitrogen
atmosphere, no reaction occurred (entry 24). Only a trace amount of 2a was formed while using 6 W green LED lamp as the light source
(
entry 25). Conducting the reaction under the radiation of sunlight led to much lower reaction efficiency (entry 26). The present
transformation did not proceed under the dark condition, indicating that this present reaction involves a visible-light-induced mechanism
entry 27).
(
Table 1
a
Optimization of reaction conditions.
[
SCN]
Natural Acid
(equiv.)
b
Entry
Solvent (mL)
Yield (%)
(equiv.)
1
2
3
4
5
6
NaSCN (2)
KSCN (2)
HOAc (2)
HOAc (2)
HOAc (2)
glycolic acid (2)
lactic acid (2)
malic acid (2)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
25
29
21
76
39
90
NH
4
SCN (2)
KSCN (2)
KSCN (2)
KSCN (2)

7
8
9
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (3)
KSCN (1.5)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
KSCN (2)
oxalic acid (2)
citric acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (2)
malic acid (3)
malic acid (1)
malic acid (0.8)
--
BME (1)
BME (1)
DME (1)
44
31
78
86
53
44
12
N.R.
90
81
90
78
90
90
76
N.R
90
N.R
trace
23
1
0
1,4-dioxane (1)
2-MeTHF (1)
MeCN (1)
DCE (1)
c
1
1
1
1
1
1
1
1
1
1
2
2
2
2
3
4
5
6
7
8
9
0
1
2
H
2
O (1)
BME (1.5)
BME (0.5)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
BME (1)
c
2
3
4
5
malic acid (1)
malic acid (1)
malic acid (1)
malic acid (1)
malic acid (1)
d
e
f
2
2
2
6
g
2
7
N.R
a
b
c
d
e
f
Conditions: 1a (0.2 mmol), KSCN, natural acid, solvent, 6 W blue LED lamp, r.t., air, 6 h.
Estimated by H NMR using 1,3,5-trimethoxybenzene as internal reference.
Pure oxygen gas was used.
Under nitrogen atmosphere.
Under the radiation of 6 W green LED lamp.
Under the radiation of sunlight.
Without visible-light irradiation. DME: 1,2-dimethoxyethane.
1
g
After establishing the optimized conditions (Table 1, entry 20) to access the 2-aminobenzothiazoles, the substrate scope was
investigated (Scheme 2). A series of anilines with electron-donating (2a-2f), -neutral (2g), and -withdrawing (2h-2j) groups were well-
tolerated and provided the expected products in good yields. Importantly, the substrates containing synthetic valuable functional-groups
such as chloro-, bromo-, and ester groups were also well compatible in the current reaction, which were useful for further
functionalizations of the 2-aminobenzothiazole moiety. Both di- and tri-substituted anilines are suitable substrates that underwent the
transformation smoothly to deliver the desired products (2k and 2l) in good yields. The cascade coupling/cyclization reactions of
polycyclic aromatic amines and KSCN were possible and proceeded with good yields (2m − 2o), thus enhancing the substrate scope of
the developed reaction. Impressively, our present protocol was further extended to the substrates of heteropolycyclic compounds. The
reaction of 5-aminoindole, 6-quinolylamine and 5-aminobenzodioxole afforded the aminothiazole products (2p–2r) in 72%-75% yields.
Scheme 2. Reaction Scope. Conditions: 1 (0.3 mmol), KSCN (0.6 mmol), malic acid (0.3 mmol), BME (1.5 mL), 6 W blue LED lamp, r.t. Isolated yields were
reported.
The synthetic applicability of the present reaction was further explored (Scheme 3). In the small-scale reaction, a concentration of 0.2
mol/L of the substrate 1 was required, and the increasing of substrate 1 in concentration would decrease the reaction efficiency. Much to
our delight, decreasing the amount of BME had virtually no effect on the yield. The cascade reaction of 4-toluidine (10 mmol) with a 5-

fold increase in the concentration of 1a provided the expected product 2a in 81% yield (1.29 g). In addition, the pure 2-aminobenzothiazole
a could be readily collected through extraction.
2
Scheme 3. Large-scale synthesis of 2a.
In order to understand the mechanism, several control experiments were performed (Scheme 4). Firstly, the photocatalytic reaction was
suppressed in the presence of a radical inhibitor (TEMPO or BHT) (Scheme 4a). The radical capture experiment with 1,1-diphenylethylene
provided the radical adduct 3a (Scheme 4b). These experimental results suggested that this photocatalytic reaction likely proceeded
through a free-radical pathway. While conducting the reaction with 2 equiv. of TBHP or H
2
2
O as an oxidant under nitrogen atmosphere,
a trace amount of 2a was observed (Scheme 4c). Under the standard conditions, 9,10-dimethylanthracene 4a was converted to the
endoperoxide product 4b through [4+2] cycloaddition reaction (Scheme 4d). When 2 equiv. of singlet oxygen scavenger (1,4-
diazabicyclo[2.2.2]octane, DABCO) was added to the reaction mixture, only a trace amount of 2a was formed (Scheme 4e). Taken
together, these results suggested that the singlet oxygen might be a reactive intermediate in the transformation. The lighting time profiles
of the cascade reaction showed that this reaction was fully suppressed in the absence of visible-light irradiation (Fig. 1). These results
suggested that the continuous visible-light irradiation was necessary for the transformation.
Scheme 4. Control experiments.
Fig. 1. On/off control experiments.
A plausible reaction mechanism for the light-initiated cascade reaction based on the above experimental results and related literature
1
was illustrated in Scheme 5 [12]. Initially, the singlet oxygen O
2
originated from molecular oxygen in air under the irradiation of visible-
light. Meanwhile, the thiocyanic acid was generated in situ by the reaction of thiocyanate salts with malic acid. Subsequently, the active
singlet oxygen abstracted one electron from thiocyanic acid to produce a thiocyanate radical and a hydroperoxyl radical. The thiocyanate
radical easily reacted with 1a to generate a free-radical intermediate IM1, which underwent dehydro-aromatization to form the 2-
thiocyanatoaniline IM2 along with the generation of hydrogen peroxide (detected by a KI-starch test paper) in the presence of

hydroperoxyl radicals. Finally, the nucleophilic amino group of IM2 intramolecular attacked the carbon atom of SCN group to generate
the target 2-aminobenzothiazole 3.
Scheme 5. Plausible mechanism.
In summary, we have developed a simple and practical method for the synthesis of 2-aminobenzothiazoles through visible-light-
initiated malic acid-promoted cascade coupling/cyclization of aromatic amines and KSCN with air ambient as an oxidant at room
temperature. Mechanistic investigation revealed that the singlet oxygen was produced from molecular oxygen (in air) under the visible-
light irradiation, which is pivotal for this cascade reaction to proceed. The protocol is operationally easy and practical with a simple
extraction necessary to obtain the target products in good yields and purity for large-scale preparations, thus circumventing the tedious
chromatographic purification procedures. Given the readily available materials, the clean conditions (metal-, halogen-, corrosive acid-
and photocatalyst-free), high scalability and operational simplicity, this protocol is highly attractive in the field of fine chemistry and
pharmaceutical industry.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
Acknowledgement
We are grateful for financial support from the Hunan Provincial Natural Science Foundation of China (No. 2019JJ20008).
References
[
1] (a) W.M. Zhu, W.H. Bao, W.W. Ying, et al., Asian J. Org. Chem. 7 (2018) 337-340;
(b) F.L. Zeng, X.L. Chen, S.Q. He, et al., Org. Chem. Front. 6 (2019) 1476-1480;
(c) R. Liu, M. Li, W. Xie, et al., J. Org. Chem. 84 (2019) 11763-11773;
(d) L.Y. Xie, Y.L. Chen, L. Qin, et al., Org. Chem. Front. 6 (2019) 3950-3955;
(e) Z. Gan, Q. Yan, G. Li, et al., Adv. Synth. Catal. 361 (2019) 4558-4567;
(f) B. Liu, L. Cheng, P. Hu, et al., Chem. Commun. 55 (2019) 4817-4820;
(g) K.J. Liu, J.H. Deng, J. Yang, et al. Green Chem. 22 (2020) 433-438
[
[
2] (a) Y. Gu, F. Jerome, Chem. Soc. Rev. 42 (2013) 9550-9570;
(b) B. Lai, R. Bai, Y. Gu, ACS Sustainable Chem. Eng. 6 (2018) 17076-17086;
(c) G.P. Yang, X. Wu, B. Yu, C. Hu, ACS Sustainable Chem. Eng. 7 (2019) 3727–3732;
(d) M. Li, X. Dong, N. Zhang, F. Jérôme, Y. Gu, Green Chem. 21 (2019) 4650-4655.
(e) W.H. Bao, Z. Wang, X. Tang, et al., Chin. Chem. Lett. 30 (2019) 2259-2262.
3] (a) D. Xia, Y. Li, T. Miao, P. Li, L. Wang, Green Chem. 19 (2017) 1732-1739;
(
(
(
(
(
(
(
(
(
(
(
(
(
b) R. Li, X. Chen, S. Wei, et al., Adv. Synth. Catal. 360 (2018) 4807-4813;
c) B.G. Cai, J. Xuan, W.J. Xiao, Sci. Bull. 64 (2019) 337-350;
d) Z. Wang, X. Ji, J. Zhao, H. Huang, Green Chem. 21 (2019) 5512-5516;
e) M. Zhang, M.N. Chen, J.M. Li, N. Liu, Z.H. Zhang, ACS Comb. Sci. 21 (2019) 685-691;
f) J. Wu, Y. Zhang, X. Gong, Y. Meng, C. Zhu, Org. Biomol. Chem. 17 (2019) 3507-3513;
g) L.Y. Xie, J.L. Hu, Y.X. Song, et al., ACS Sustainable Chem. Eng. 7 (2019) 19993-19999;
h) Y. Liu, Q.L. Wang, Z. Chen, et al., Chem. Commun. 55 (2019) 12212-12215;
i) Q. Liu, F. Liu, H. Yue, et al., Adv. Synth. Catal. 361 (2019) 5277-5282;
j) G. Li, Q. Yan, Z. Gan, et al., Org. Lett. 21 (2019) 7938-7942;
k) S. He, X. Chen, F. Zeng, et al., Chin. Chem. Lett. (2020) doi:http://dx.doi.org/10.1016/j.cclet.2019.12.031;
l) W. Ou, R. Zou, M. Han, L. Yu, C. Su, Chin. Chem. Lett. (2020) doi:http://dx.doi.org/10.1016/j.cclet.2019.12.017;
m) P. Bao, F. Liu, Y. Lv, et al., Org. Chem. Front. (2020) 10.1039/C9QO01334B;
n) L. Wang, M. Zhang, Y. Zhang, et al., Chin. Chem. Lett. 31 (2020) 67-70.
[
[
4] (a) K.J. Liu, J.H. Deng, T.Y. Zeng, et al., Chin. Chem. Lett. (2020) doi:http://dx.doi.org/10.1016/j.cclet.2020.01.036;
(
(
(
(
b) L.Y. Xie, Y.S. Lu, H.R. Ding, et al., Chin. J. Catal. (2020) S1872-2067(19)63526-6;
c) D.Q. Dong, L.X. Li, G.H. Li, et al., Chin. J. Catal. 40 (2019) 1494-1498;
d) D.Q. Dong, W.J. Chen, D.M. Chen, et al., Chin. J. Org. Chem. 39 (2019) 3190-3198.
e) L.Y. Xie, Y.S. Bai, X.Q. Xu, et al. Green Chem. (2020) doi:http://dx.doi.org/10.1039//C9GC03899J.
5] (a) M. Zhang, Q.Y. Fu, G. Gao, et al., ACS Sustainable Chem. Eng. 5 (2017) 6175-6182;
(b) L. Zhao, P. Li, X. Xie, L. Wang, Org. Chem. Front. 5 (2018) 1689-1697;
(c) L. Zhao, P. Li, H. Zhang, L. Wang, Org. Chem. Front. 6 (2019) 87-93;
(d) Y. Han, Y. Jin, M. Jiang, H. Yang, H. Fu, Org. Lett. 21 (2019) 1799-1803;

(e) Y.T. Xu, C.Y. Li, X.B. Huang, et al., Green Chem. 21 (2019) 4971-4975.
[
[
6] (a) J. Yang, J.N. Tan, Y. Gu, Green Chem. 14 (2012) 3304-3317;
(b) G. Gao, P. Wang, P. Liu, et al., Chin. J. Org. Chem. 38 (2018) 846-854;
(c) M. Sun, J. Jiang, J. Chen, Q. Yang, X. Yu, Tetrahedron 75 (2019) 130456;
(d) L. Peng, Z. Hu, Q. Lu, et al., Chin. Chem. Lett. 30 (2019) 2151-2156;
(e) Z. Cao, Q. Zhu, Y.W. Lin, W.M. He, Chin. Chem. Lett. 30 (2019) 2132-2138;
(f) L.H. Lu, Z. Wang, W. Xia, et al., Chin. Chem. Lett. 30 (2019) 1237-1240;
(g) X.M. Xu, D.M. Chen, Z.L. Wang, Chin. Chem. Lett. 31 (2020) 49-57.
7] (a) L. Wang, D. Xiong, L. Jie, C. Yu, X. Cui, Chin. Chem. Lett. 29 (2018) 907-910;
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
b) S. Du, C. Pi, T. Wan, Y. Wu, X. Cui, Adv. Synth. Catal. 361 (2019) 1766-1770;
c) K. Sun, X.L. Chen, Y.L. Zhang, et al., Chem. Commun. 55 (2019) 12615-12618;
d) L. Wang, Y. Zhang, M. Zhang, et al., Tetrahedron Lett. 60 (2019) 1845-1848;
e) Q. Huang, L. Zhu, D. Yi, X. Zhao, W. Wei, Chin. Chem. Lett. 31 (2020) 373-376.
f) X. Gong, G. Li, Z. Gan, et al., Asian J. Org. Chem. 8 (2019) 1472-1478;
g) Z. Yang, Z. Song, L. Jie, L. Wang, X. Cui, Chem. Commun. 55 (2019) 6094-6097;
h) Y. Zhang, K. Sun, Q. Lv, et al., Chin. Chem. Lett. 30 (2019) 1361-1368;
i) J. Sun, Y. Li, Y. Gui, et al., Chin. Chem. Lett. 30 (2019) 569-572;
j) Z. Shen, C. Pi, X. Cui, Y. Wu, Chin. Chem. Lett. 30 (2019) 1374-1378;
k) X. Mi, Y. Kong, J. Zhang, C. Pi, X. Cui, Chin. Chem. Lett. 30 (2019) 2295-2298;
l) X.X. Meng, Q.Q. Kang, J.Y. Zhang, et al. Green Chem. (2020) doi:http://dx.doi.org/10.1039/C9GC03769A;
m) M.Y. Min, R.J. Song, X.H. Ouyang, J.H. Li, Chem. Commun. 55 (2019) 3646-3649;
n) Y.C. Wu, S.S. Jiang, R.J. Song, J.H. Li, Chem. Commun. 55 (2019) 4371-4374;
o) X.P. Ma, C.M. Nong, J. Zhao, et al., Adv. Synth. Catal. (2020) doi:http://dx.doi.org/10.1002/adsc.201901206;
p) H.P. Zhao, G.C. Liang, S.M. Nie, et al., Green Chem. 22 (2020) 404-410;
q) L. Zhu, W. Liao, H. Chang, X. Liu, S. Miao, ChemistrySelect 5 (2020) 829-833;
r) Z. Wang, Q. Liu, X. Ji, G.J. Deng, H. Huang, ACS Catal. 10 (2020) 154-159.
s) Q.Q. Han, G.H. Li, Y.Y. Sun, et al. Tetrahedron Lett. 61 (2020) 151704.
[
[
8] A. Catalano, A. Carocci, I. Defrenza, et al., Eur. J. Med. Chem. 64 (2013) 357-364.
9] (a) Y. Xu, B. Li, X. Zhang, X. Fan, J. Org. Chem. 82 (2017) 9637-9646;
(b) M. Salah, M. Abdel-Halim, M. Engel, MedChemComm 9 (2018) 1045-1053;
(c) T.L. Dadmal, S.D. Katre, M.C. Mandewale, R.M. Kumbhare, New J. Chem. 42 (2018) 776-797.
[
[
10] H. Jiang, W. Yu, X. Tang, J. Li, W. Wu, J. Org. Chem. 82 (2017) 9312-9320.
11] (a) K.J. Liu, T.Y. Zeng, J.L. Zeng, et al., Chin. Chem. Lett. 30 (2019) 2304-2308;
(
(
(
(
(
(
(
(
b) S. Peng, D. Hu, J.L. Hu, et al., Adv. Synth. Catal. 361 (2019) 5721-5726;
c) S. Peng, Y.X. Song, J.Y. He, et al., Chin. Chem. Lett. 30 (2019) 2287-2290;
d) Z. Wang, W.M. He, Chin. J. Org. Chem. 39 (2019) 3594-3595;
e) L. Peng, Z. Hu, Z. Tang, Y. Jiao, X. Xu, Chin. Chem. Lett. 30 (2019) 1481-1487;
f) W.M. He, Y.W. Lin, D.H. Yu, Sci. China Chem. (2020) doi:http://dx.doi.org/10.1007/s11426-019-9675-5;
g) S. Peng, Y.W. Lin, W.M. He, Chin. J. Org. Chem. (2020) doi:http://dx.doi.org/10.6023/cjoc202000006.
h) Y.Y. Peng, Y. Wang, X.Y. Yu, et al. Chem. J. Chinese Univ. 41 (2020) 268-276.
i) Q.W. Gui, X. He, W. Wang, et al. Green Chem. 22 (2020) 118-122.
[
12] (a) X. Zhu, P. Li, Q. Shi, L. Wang, Green Chem. 18 (2016) 6373-6379;
(
(
(
(
b) Q. Shi, P. Li, X. Zhu, L. Wang, Green Chem. 18 (2016) 4916-4923;
c) Q. Liu, L. Wang, H. Yue, et al., Green Chem. 21 (2019) 1609-1603;
d) L. Zou, P. Li, B. Wang, L. Wang, Chem. Commun. 55 (2019) 3737-3740;
e) G. Li, Q. Yan, X. Gong, X. Dou, D. Yang, ACS Sustainable Chem. Eng. 7 (2019) 14009-14015.