Salicylic Acid-Promoted Three-Component Annulation of Benzimidazoles, Aryl Nitroalkenes and Elemental Sulfur

Article abstract of DOI:10.1002/adsc.202001564

Herein, a three-component cyclization reaction of benzimidazoles, aryl nitroalkenes and elemental sulfur has been developed. Cheap and easily available salicylic acid found to be an efficient mediator for the present transformation. This protocol provides

Full text of DOI:10.1002/adsc.202001564

COMMUNICATIONS
DOI: 10.1002/adsc.202001564
Salicylic Acid-Promoted Three-Component Annulation of
Benzimidazoles, Aryl Nitroalkenes and Elemental Sulfur
Ruhuai Mei,a Feng Xiong,a Chenrui Yang,a and Jinwu Zhaob,*
a
Key Laboratory of Coarse Cereal Processing,
Ministry of Agriculture and Rural Affairs,
Chengdu University,
Chengdu 610106, People’s Republic of China
School of Pharmacy,
b
Guangdong Medical University,
Dongguan 523808, People’s Republic of China
E-mail: jwzhao@gdmu.edu.cn
Manuscript received: December 17, 2020; Revised manuscript received: January 11, 2021;
Version of record online: Februar 5, 2021
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001564
On the other hand, sulfur-containing frameworks
widely exist in pharmaceuticals. Among the molecules
Abstract: Herein, a three-component cyclization
reaction of benzimidazoles, aryl nitroalkenes and
in the table of top 200 pharmaceuticals by US
elemental sulfur has been developed. Cheap and
prescription in 2012, for examples, more than 20% of
easily available salicylic acid found to be an
them contain at least one sulfur atom.[3] Considering
efficient mediator for the present transformation.
the pharmaceutical importance of sulfur compounds,[4]
This protocol provides a facile access to structurally
significant imidazo[2,1-b]thiazole skeleton from
efficient methods for their construction has been
intensively studied.[5] Elemental sulfur is a cheap,
simple raw materials with a range of compatible
bench-stable, user-friendly sulfur reagent that was
synthetically useful functionalities. Furthermore,
recently exploited to produce sulfur compounds includ-
gram-scale preparation of this method is effective,
ing some valuable sulfur heterocycles such as
which enables potential applications of it in broader
thiophenes,[6] thiazoles,[7] benzothiazoles,[8] and among
fields of molecule synthesis. Mechanistically, a
others.[9] Remarkably, the multi-component reactions
reaction cascade involving sequential aza-Michael
addition, nucleophilic sulfuration, and deaminative
through direct CÀ H sulfuration with elemental sulfur
found to be versatile to give atom- and step-economic
aromatization was proposed.
entries to complex sulfur-containing molecules. For
example, Deng and coworkers have developed three-
component heterocyclization of indoles, ketones, and
Keywords: multi-component reaction; elemental sul-
fur; benzimidazoles; nitroalkenes; imidazo[2,1-b]
elemental sulfur, which affords a novel access to
indole-fused (benzo)thiophenes.[10] On the basis of this
thiazole
result, the same group utilized alkenes or alkynes as
the coupling partner instead of ketones to construct the
same heterocycle skeleton under metal-free
One-pot multi-component cascade assembly represents conditions.[11] Complementarily, Fu et al. discovered
an advanced technique in modern synthesis that that acrylic acids were also capable to engage into this
provides an efficient platform for rapid preparation of three-component assembly and this reaction proceeded
complex molecules from simple raw materials.[1] This smoothly under additive-free conditions because of the
strategy has also been extensively exploited in medic- acidity of reactant (Figure 1, a).[12]
inal chemistry, total synthesis of naturally occurring
Given the structurally nucleophilic resemblance
products, and materials science.[2] In spite of the great between indoles and benzimidazoles, we speculated
progress, novel multi-component cascade reactions benzimidazoles could be used to replace indoles in the
especially under facile reaction conditions remain three-component assembly with elemental sulfur.
desirable for method development in synthetic Hence, a range of coupling partners such as acetopho-
chemistry.
none, phenylacetylene, styrene, cinnamic acid, and β-
nitrostyrene were initially screened as the third
Adv. Synth. Catal. 2021, 363, 1861–1866
1861
© 2021 Wiley-VCH GmbH

COMMUNICATIONS
asc.wiley-vch.de
Table 1. Optimization of Reaction Conditions.[a]
Entry
Catalyst
Additive
Solvent
Yield[b]
1
2
3
4
5
6
7
8
Cu(OAc)2
CuCl2
CuCl
KI
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
30
trace
trace
25
–
TFA
TfOH
MsOH
HOAc
BzOH
salicylic acid
2-NBA
4-NBA
BF3·OEt2
Zn(OTf)2
salicylic acid
salicylic acid
salicylic acid
salicylic acid
salicylic acid
salicylic acid
salicylic acid
salicylic acid
24
trace
trace
trace
36
48
75
50
35
48
30
34
31
60
trace
72
56
41
73
9
10
11
12
13
14
15
16
17
18
19
20[c]
Figure 1. Three-component reactions with elemental sulfur for
tricyclic heteroaromatics and valuable imidazo[2,1-b]thiazole
compounds.
DMA
NMP
–
–
–
–
–
–
toluene
DMSO
DMSO
DMSO
DMSO
component. Among them, only β-nitrostyrene could
deliver the target benzoimidazothiazole product (Fig- 21[d]
22[e]
ure 1, b). Notably, high levels of regioselectivity were
observed in this reaction, in which C3-substituted
23[f]
heterocycle was exclusively formed, on the contrary to
the result of indole assembly. More importantly, many
imidazo[2,1-b]thiazole compounds are drugs or drug
candidates.[13] For example, pifithrin-β (Figure 1c, I) is
a highly efficient p53 inhibitor (IC50 <23 nM); the 11C-
labeled imidazo[2,1-b] thiazole II could serve as a
fluorescent probe in PET analysis of Alzheimer’s
disease; and the patented imidazo[2,1-b]thiazole com-
pound III was developed as sirtuin modulator. Accord-
ingly, it is highly attractive to develop facile methods
for the construction of imidazo[2,1-b]thiazole motif.
With the primary results, we next conducted the
[a] Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), S8
(0.6 mmol, the mole number of sulfur based on S: 32 g/
mol), catalyst (0.01 mmol, 5.0 mol%), additive (1.0 equiv.),
°
DMSO (0.6 mL), at 100 C under air for 28 h. NBA=
nitrobenzoic acid.
[b] Isolated yield.
[c]
°
At 110 C.
[d]
°
At 90 C.
[e] Salicylic acid (0.5 equiv.) was used.
[f] Under Ar.
optimization studies with benzimidazole 1a, β-nitro- productivity of 3a. Thus, substituted benzoic acids
styrene 2a, and elemental sulfur (S8) as the model were further tested (entries 11–13). To our delight,
substrates under air (Table 1). Given previous reports salicylic acid found superior to others that generated
that copper catalysts may play an essential role in CÀ S 3a in 75% yield (entry 11). Lewis acids were found
bond formation,[14] therefore, some copper salts were also to be certain effective for this reaction, where
employed (entries 1–3). Of them, while Cu(OAc)2 BF3·OEt2 (entry 14) and Zn(OTf)2 (entry 15) afforded
afforded the desired product 3a in 30% yield, CuCl2 3a in 48% and 30% yield, respectively. The solvent
and CuCl quenched the three-component assembly. effect was then studied, in which those of high polarity
These results indicate copper catalyst may not function such as DMSO, DMF, DMA, and NMP worked with
in the current CÀ H sulfuration. The reaction performed certain productivities (entries 16–18). Low-polar media
with or without additional KI gave similar results such as toluene quenched the transformation (en-
(entries 4 and 5). Subsequently, an array of Brønsted try 19), which could be attributed to the low solubility
acids were screened to disclose that while strong acids of elemental sulfur in low-polar solvent. The reaction
°
almost completely prohibited the desired transforma- performed at elevated reaction temperature to 110 C
°
tion (entries 6–8), weak acids such as acetic acid gave a similar result with that at 100 C (entry 20);
(entry 9) and benzoic acid (entry 10) enhanced the however, dramatically reduced reaction yield was
Adv. Synth. Catal. 2021, 363, 1861–1866
1862
© 2021 Wiley-VCH GmbH

COMMUNICATIONS
asc.wiley-vch.de
°
observed at 90 C (entry 21), in line with the result thiazoles have been accessed in moderate yields. In the
obtained by reducing the amount of acid additive case of halogen functionalities, fluoro, chloro, and
(entry 22). Finally, the control experiment under inner bromo were all tolerated to produce the halo-function-
atmosphere suggests that oxygen did not participate in alized benzoimidazothiazole products (3e–g), which
the present heterocyclization (entry 23).
provide ready opportunities for further manipulation
With the optimized reaction conditions in hand, the and late stage diversification. Then, the β-nitrostyrenes
substrate scope and generality of the Brønsted acid- with ortho- or meta-functionalities were employed
promoted three-component sulfur-heterocyclization (3h–l). These substrates afforded the desired products
was probed (Scheme 1). First, a pad of β-nitrostyrenes with similar efficiency to that of para-variants, ruling
bearing para-substituents were subjected to the acidic out the steric hindrance effect of this component. Other
system. Those with electron-donating functional decorated β-nitrostyrenes with dimethyl (3m), difluoro
groups such as methoxy (3b), methyl (3c), and tert- (3n), and dichloro (3o) were compatible in the present
butyl (3d) slightly decreased the reaction efficiency. system, furnishing the corresponding benzoimidazo-
The corresponding 3-phenylbenzo[4,5]imidazo[2,1-b] thiazoles in generally modest yields. That bearing
trimethoxy function groups proceeded to afford 3p in
22% yield under the standard conditions while
evaluated reaction temperature proved helpful. Hence,
°
the reaction at 120 C produced 3p in 45% yield.
Nitroethylenes with naphthyl and thienyl moieties
worked to give products 3q and 3r, respectively, with
moderate efficiency. Gratefully, β-methyl-β-nitrostyr-
enes were also accommodated with the present system
that afford 2,3-disubstituted benzo[4,5]imidazo[2,1-b]
thiazoles, albeit in lower efficiency (3s and 3t).
Unfortunately, aliphatic nitroalkenes did not work in
the present acid system.
While mono-substituted benzoimidazoles gave a
mixture of two isomer products with poor selectivity,
5,6-dimethyl-1H-benzo[d]imidazole afforded the corre-
sponding product in good yield (3u). Generally, the
imidazole-type substrates featured low reactivity. For
example, 3,4-diphenyl-1H-imidazole produced the tar-
get 3,5,6-triphenylimidazo[2,1-b]thiazole (3v) in 20%
°
yield even at 120 C. But unsubstituted imidazole gave
only trace amounts of corresponding product (3w).
However, the present system still provides a promising
approach for the synthesis of structurally significant
imidazo[2,1-b]thiazole motif.
The gram-scale reaction in the facile Brønsted acid
system proved to be effective, where 10 mmol of β-
nitrostyrene could afford 4.5 mmol of product 3a
(Scheme 2, a). The product 3a bearing an undecorated
C2 CÀ H, which could be readily transferred to CÀ C
and CÀ Br bonds via palladium-catalyzed arylation
with aryl iodide (Scheme 2, b) and bromination with
the treatment of NBS (Scheme 2, c), respectively.
However, because of the immature direct CÀ H
arylation of this kind of heterocycles, the two step
manipulation via sequential bromination and Suzuki-
Miyaura coupling may feature advantage in productiv-
ity.
With the established reactivity of our Brønsted acid
system for the three-component assembly, we were
attracted by depicting the reaction model of the
heterocyclization. To this end, some control experi-
ments were carried out (Scheme 3). First, the treatment
of benzimidazole 1a with elemental sulfur in the
Scheme 1. Three-component reaction of benzimidazoles, β-
a
°
nitrostyrene, and elemental sulfur. Yield of reaction at 120 C.
Adv. Synth. Catal. 2021, 363, 1861–1866
1863
© 2021 Wiley-VCH GmbH

COMMUNICATIONS
asc.wiley-vch.de
in this three-component heterocyclizaiton. Finally, the
addition of radical scavenger such as TEMPO, buty-
lated hydroxytoluene (BHT), and 1,1-diphenylethylene
(DPE) has a slight effect on the reaction yield
(Scheme 3, d). Hence, a radical pathway would not be
mechanistically involved.
On the basis of the results from above experiments
and previous reports,[10–11,15] a tentative reaction mech-
anism was proposed (Scheme 4). With the assistance
of Brønsted acid, aza-Michael addition of benzimida-
zole 1a to β-nitrostyrene 2a proceeds to afford the
intermediate B bearing an imine cation moiety. Then,
nucleophilic attack of elemental sulfur to the inter-
mediate B generates polysulfur intermediate C or D,
with subsequent proton transfer of D to afford
intermediate E. Then, intramolecular CÀ H sulfuration
gives the annulation intermediate F, along with the
elimination of hydrogen sulfide and elemental sulfur
Sn-1. Finally, the thermal-driven elimination of nitrous
acid moiety and deprotonation lead to the final benzo
[4,5]imidazo[2,1-b]thiazole product 3a.
Scheme 2. Synthetic applications.
In summary, we have developed a facile Brønsted
acid-promoted three-component reaction that give a
platform for the synthesis of benzo[4,5]imidazo[2,1-b]
thiazoles from readily available benzimidazoles, nitro-
alkenes, and elemental sulfur. This metal-free protocol
provides a straightforward entry to complex S,N-
heterocycles from simple raw materials with a range of
compatible functionalities. Mechanistic studies reveal
a cascade reaction pathway involving sequential aza-
Michael addition, nucleophilic sulfuration, and deami-
native aromatization.
Scheme 3. Control experiments.
absence of β-nitrostyrene did not afford 1H-benzo[d]
imidazole-2-thiol (Scheme 3, a). Further, the direct
cyclization of 1H-benzo[d]imidazole-2-thiol and β-
nitrostyrene could proceed under copper catalysis,
which was discovered by Hajra and coworkers.[14]
However, this reaction did not occur in our copper-free
system (Scheme 3, b). These results suggest that the
CÀ H sulfuration of 1H-benzo[d]imidazole with ele-
mental sulfur would not occur in the initial step. Given
the strong nitrogen nucleophilicity of benzo[d]
imidazole, the aza-Michael addition of it to β-nitro-
styrene may be more operative. Then, we found that
oxygen atmosphere for the reaction significantly
diminished the reaction productivity (Scheme 3, c),
suggesting that oxidative annulation was not involved
Scheme 4. Possible reaction mechanism.
Adv. Synth. Catal. 2021, 363, 1861–1866
1864
© 2021 Wiley-VCH GmbH

COMMUNICATIONS
asc.wiley-vch.de
2015, 357, 4050–4054; d) L. Meng, T. Fujikawa, M.
Experimental Section
Kuwayama, Y. Segawa, K. Itami, J. Am. Chem. Soc.
2016, 138, 10351–10355; e) Z. Wang, Z. Qu, F. Xiao, H.
Huang, G. Deng, Adv. Synth. Catal. 2018, 360, 796–800.
[7] a) K. K. Childers, A. M. Haidle, M. R. Machacek, J. P.
Rogers, E. Romeo, Tetrahedron Lett. 2013, 54, 2506–
2510; b) P. Jiang, X. Che, Y. Liao, H. Huang, G.-J.
Deng, RSC Adv. 2016, 6, 41751–41754; c) X. Wang, X.
Qiu, J. Wei, J. Liu, S. Song, W. Wang, N. Jiao, Org. Lett.
2018, 20, 2632–2636; d) M. Wu, Y. Jiang, Z. An, Z. Qi,
R. Yan, Adv. Synth. Catal. 2018, 360, 4236–4240; e) J.
Jiang, H. Huang, G.-J. Deng, Green Chem. 2019, 21,
986–990.
[8] a) T. B. Nguyen, L. Ermolenko, A. Al-Mourabit, Org.
Lett. 2013, 15, 4218–4221; b) T. B. Nguyen, L. Ermo-
lenko, P. Retailleau, A. Al-Mourabit, Angew. Chem. Int.
Ed. 2014, 53, 13808–13812; Angew. Chem. 2014, 126,
14028–14032; c) T. B. Nguyen, K. Pasturaud, L. Ermo-
lenko, A. Al-Mourabit, Org. Lett. 2015, 17, 2562–2565;
d) H. Huang, J. Cai, X. Ji, F. Xiao, Y. Chen, G. J. Deng,
Angew. Chem. Int. Ed. 2016, 55, 307–311; Angew. Chem.
2016, 128, 315–319; e) X. Che, J. Jiang, F. Xiao, H.
Huang, G.-J. Deng, Org. Lett. 2017, 19, 4576–4579;
f) G. Li, H. Xie, J. Chen, Y. Guo, G.-J. Deng, Green
Chem. 2017, 19, 4043–4046; g) J. Ying, H. Wang, X. Qi,
J.-B. Peng, X.-F. Wu, Eur. J. Org. Chem. 2017, 2018,
688–692; h) Y. Hu, Z. Yin, T. Werner, A. Spannenberg,
X.-F. Wu, Eur. J. Org. Chem. 2018, 2018, 1274–1276;
i) Q. Xing, Y. Ma, H. Xie, F. Xiao, F. Zhang, G. J. Deng,
The J. Org. Chem. 2019, 84, 1238–1246; j) Z. Xu, H.
Huang, H. Chen, G.-J. Deng, Org. Chem. Front. 2019, 6,
3060–3064; k) X. Zhu, Y. Yang, G. Xiao, J. Song, Y.
Liang, G. Deng, Chem. Commun. 2017, 53, 11917–
11920; l) J. Jiang, G. Li, F. Zhang, H. Xie, G.-J. Deng,
Adv. Synth. Catal. 2018, 360, 1622–1627.
[9] a) H. Huang, Q. Wang, Z. Xu, G.-J. Deng, Adv. Synth.
Catal. 2018, 360, 591–596; b) H. Huang, Z. Xu, X. Ji, B.
Li, G.-J. Deng, Org. Lett. 2018, 20, 4917–4920; c) H.
Huang, Z. Qu, X. Ji, G.-J. Deng, Org. Chem. Front.
2019, 6, 1146–1150; d) Z. Xu, G.-J. Deng, F. Zhang, H.
Chen, H. Huang, Org. Lett. 2019, 21, 8630–8634; e) J.-
R. Zhang, L.-Z. Zhan, L. Wei, Y.-Y. Ning, X.-L. Zhong,
J.-X. Lai, L. Xu, R.-Y. Tang, Adv. Synth. Catal. 2018,
360, 533–543; f) R. Mei, W. Ma, Y. Zhang, X. Guo, L.
Ackermann, Org. Lett. 2019, 21, 6534–6538; g) M.
Zhang, M. N. Chen, Z. H. Zhang, Adv. Synth. Catal.
2019, 361, 5182–5190; h) W. Chen, X. Zhu, F. Wang, Y.
Yang, G. Deng, Y. Liang, J. Org. Chem. 2020, 85, 3349–
3357; i) S. Moon, M. Kato, Y. Nishii, M. Miura, Adv.
Synth. Catal. 2020, 362, 1669–1673; j) X. Zhu, F. Zhou,
Y. Yang, G. Deng, Y. Liang, ACS Omega 2020, 5,
13136–13147; k) W. Chen, X. Zhu, F. Wang, Y. Yang, G.
Deng, Y. Liang, J. Org. Chem. 2020, 85, 3349–3357.
[10] a) C. Lu, H. Huang, X. Tuo, P. Jiang, F. Zhang, G.-J.
Deng, Org. Chem. Front. 2019, 6, 2738–2743; b) P. Ni,
B. Li, H. Huang, F. Xiao, G.-J. Deng, Green Chem.
2017, 19, 5553–5558; c) Y. Liao, Y. Peng, H. Qi, G.-J.
General Procedure for the Three-Component Reac-
tion
Benzimidazole 1 (0.3 mmol), nitroalkene 2 (0.2 mmol), S8
(19.2 mg, 0.6 mmol), salicylic acid (27.6 mg, 0.2 mmol,
1.0 equiv.), and DMSO (0.6 mL) were added successfully to a
10 mL oven-dried reaction vessel. The sealed reaction vessel
°
was stirred at 100 C for 28 h. After cooling to room temper-
ature, the reaction was diluted with ethyl acetate (10 mL) and
washed with saturated sodium chloride solution. The organic
layer was separated, and the aqueous layer was extracted with
ethyl acetate (10 mL) for three times. The combined organic
layer was dried over magnesium sulfate and the volatiles were
removed under reduced pressure. The residue was purified by
column chromatography on silica gel (petroleum ether/ethyl
acetate) to yield the desired product 3.
Acknowledgements
Support by “Thousand Talents Program” of Sichuan Province
(R. M.) is gratefully acknowledged.
References
[1] a) D. M. D’Souza, T. J. Muller, Chem. Soc. Rev. 2007,
36, 1095–1108; b) V. Estevez, M. Villacampa, J. C.
Menendez, Chem. Soc. Rev. 2010, 39, 4402–4421;
c) C. M. Volla, I. Atodiresei, M. Rueping, Chem. Rev.
2014, 114, 2390–2431; d) R. C. Cioc, E. Ruijter, R. V. A.
Orru, Green Chem. 2014, 16, 2958–2975.
[2] a) T. Zarganes-Tzitzikas, A. Domling, Org. Chem. Front.
2014, 1, 834–837; b) B. O. Okesola, A. Mata, Chem.
Soc. Rev. 2018, 47, 3721–3736; c) S. Saxer, C. Marestin,
R. Merciera, J. Dupuy, Polym. Chem. 2018, 9, 1927–
1933.
[3] a) Z. Jin, Nat. Prod. Rep. 2003, 20, 584–605; b) D.
Davyt, G. Serra, Mar. Drugs 2010, 8, 2755–2780.
[4] a) T. Suzuki, S. Hisakawa, Y. Itoh, N. Suzuki, K.
Takahashi, M. Kawahata, K. Yamaguchi, H. Nakagawa,
N. Miyata, Bioorg. Med. Chem. Lett. 2007, 17, 4208–
4212; b) H. Mohammad, A. S. Mayhoub, A. Ghafoor, M.
Soofi, R. A. Alajlouni, M. Cushman, M. N. Seleem, J.
Med. Chem. 2014, 57, 1609–1615; c) T. Tomašič, S.
Katsamakas, Ž. Hodnik, J. Ilaš, M. Brvar, T. Solmajer, S.
Montalvão, P. Tammela, M. Banjanac, G. Ergović, M.
Anderluh, L. P. Mašič, D. Kikelj, J. Med. Chem. 2015,
58, 5501–5521.
[5] a) H. Liu, X. Jiang, Chem. Asian J. 2013, 8, 2546–2563;
b) T. B. Nguyen, Adv. Synth. Catal. 2017, 359, 1066–
1130; c) S. Liu, G.-J. Deng, H. Huang, Synlett 2020,
DOI: 10.1055/s-0040–1707217; d) T. B. Nguyen, Adv.
Synth. Catal. 2020, 362, 3448; e) T. B. Nguyen, Asian J.
Org. Chem. 2017, 6, 477.
[6] a) G. Revelant, S. Dunand, S. Hesse, G. Kirsch, Syn-
thesis 2011, 18, 2935–2940; b) G. Zhang, H. Yi, H.
Chen, C. Bian, C. Liu, A. Lei, Org. Lett. 2014, 16, 6156–
6159; c) W. Liu, C. Chen, H. Liu, Adv. Synth. Catal.
Adv. Synth. Catal. 2021, 363, 1861–1866
1865
© 2021 Wiley-VCH GmbH

COMMUNICATIONS
asc.wiley-vch.de
Deng, H. Gong, C.-J. Li, Chem. Commun. 2015, 51,
1031–1034.
[11] B. Li, P. Ni, H. Huang, F. Xiao, G.-J. Deng, Adv. Synth.
Catal. 2017, 359, 4300–4304.
J. Med. Chem. 2006, 49, 3645–3652; b) B. H. Yousefi,
A. Manook, A. Drzezga, B. von Reutern, M. Schwaiger,
H. J. Wester, G. Henriksen, J. Med. Chem. 2011, 54,
949–956.
[12] H.-L. Zhang, F. Wen, W.-B. Sheng, P. Yin, C.-T. Zhang, [14] S. Jana, A. Chakraborty, V. Z. Shirinian, A. Hajra, Adv.
C.-Y. Peng, D.-M. Peng, D.-F. Liao, R.-G. Fu, Tetrahe-
dron Lett. 2019, 60, 80–83.
[13] a) N. Pietrancosta, A. Moumen, R. Dono, P. Lingor, V.
Planchamp, F. Lamballe, M. Bahr, J.-L. Kraus, F. Maina,
Synth. Catal. 2018, 360, 2402–2408.
[15] S. Chen, Y. Li, P. Ni, H. Huang, G. J. Deng, Org. Lett.
2016, 18, 5384–5387.
Adv. Synth. Catal. 2021, 363, 1861–1866
1866
© 2021 Wiley-VCH GmbH