Decarboxylation of indole-3-carboxylic acids under metal-free conditions

Article abstract of DOI:10.1080/00397911.2019.1703137

Two reaction systems have been developed for the decarboxylation of indole-3-carboxylic acids. The decarboxylation can be achieved smoothly under K₂CO₃-catalyzed or acetonitrile-promoted basic conditions. It provided an efficient and simple method for the transformation of indole-3-carboxylic acids and the corresponding indoles were isolated with good to excellent yields. From the experimental facts, we put forward the possible reaction mechanism.

Full text of DOI:10.1080/00397911.2019.1703137

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Decarboxylation of indole-3-carboxylic acids under
metal-free conditions
Xia Chen & Xiao-Yu Zhou
To cite this article: Xia Chen & Xiao-Yu Zhou (2020): Decarboxylation of indole-3-carboxylic acids
under metal-free conditions, Synthetic Communications, DOI: 10.1080/00397911.2019.1703137
To link to this article: https://doi.org/10.1080/00397911.2019.1703137
View supplementary material
Published online: 14 Feb 2020.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=lsyc20

R
SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1703137
Decarboxylation of indole-3-carboxylic acids under
metal-free conditions
Xia Chen and Xiao-Yu Zhou
School of Chemistry and Materials Engineering, Liupanshui Normal University, Liupanshui, China
ABSTRACT
ARTICLE HISTORY
Received 26 October 2019
Two reaction systems have been developed for the decarboxylation
of indole-3-carboxylic acids. The decarboxylation can be achieved
smoothly under K2CO3-catalyzed or acetonitrile-promoted basic con-
ditions. It provided an efficient and simple method for the transform-
ation of indole-3-carboxylic acids and the corresponding indoles
were isolated with good to excellent yields. From the experimental
facts, we put forward the possible reaction mechanism.
KEYWORDS
Decarboxylation; indole-3-
carboxylic acids; indoles;
metal-free
GRAPHICAL ABSTRACT
COOH
: K2CO3 (20 mol%), EtOH, 140 oC
A
R'
R
R'
R
: CH3CN, 140 oC
N
H
B
N
H
@ Metal-Free
Yield: 89%->99%
@ Without Oxidants
@ Simple Reaction System
@ Excellent Yields
Decarboxylation is an available method in organic chemistry and bioscience[1] and
many efforts have been devoted to the decarboxylative functionalization of carboxylic
acids and carboxylates.[2] Enzyme or transition-metal catalyzed oxidative decarboxyl-
ation and decarboxylative functionalization are primary means. Among these transfor-
mations, the main areas were focused on the decarboxylation of amino acids,[3]
decarboxylative functionalization of aromatic[4] and aliphatic[5] carboxylic acids.
Especially, the development of decarboxylative functionalization had been promoted by
photocatalytic technique.[6] Many works also focused on the research of decarboxylative
mechanism and quantum chemistry calculation,[7] such as the mechanism of protonic
acid-catalyzed decarboxylative protonation of pyrrole-2-carboxylic acid,[8] p-chloranil
catalyzed decarboxylative alkoxylation[9] and decarboxylative arylation[10] of 2-picolinic
acid in the presence of tert-butyl hypochlorite (tBuOCl). The decarboxylative transform-
ation of hetero-aromatic carboxylic acids is also an important field for the synthesis of
hetero-cyclic compounds.
CONTACT Xiao-Yu Zhou
Zhouxiaoyu20062006@126.com
School of Chemistry and Materials Engineering,
Liupanshui Normal University, Liupanshui 553004, China.
Supplemental data for this article is available online at https://doi.org/10.1080/00397911.2019.1703137
ß 2020 Taylor & Francis Group, LLC

2
X. CHEN AND X.-Y. ZHOU
Scheme 1. Decarboxylation of indolecarboxylic acids and its derivatives.
Scheme 2. Decarboxylation of indole carboxylic acids.
Indole-3-carboxylic acids can be prepared[11] and used as starting materials in organic
synthesis. In the previous works, there were some findings on the decarboxylative func-
tionalization of indolyl carboxylic acids.[12] And, a few works had been focused on the
decarboxylation of indole-3-carboxylic acids (Scheme 1). In 2012, the mechanism for
the decarboxylation of indole-2-carboxylic acid and indole-3-carboxylic acid in acidic
conditions had been revealed by Kluger and his coworkers.[13] And, kinetic analysis
over a wide range of solution acidities showed the importance of protonated carbonic
acid as a reaction intermediate. On the other hand, the selective decarbonylative reduc-
tion of aryl and heteroaryl esters and amides were developed in Rueping’s group.[14]
The ester and amide groups from arenes and heteroarenes can be removed using
Ni(OAc)2ꢀ4H2O as a catalyst and commercially available hydrosilane as a reagent. Most
recently, Szostak et al.[15] reported a palladium-catalyzed reduction of carboxylic acids
to arenes in the presence of Et3SiH. It provided a practical method for the construction
of arenes and the DFT study was conducted to demonstrate the mechanism.
Through our study, decarboxylation of indole-3-carboxylic acid can be achieved
under inorganic base-catalyzed or acetonitrile-promoted conditions (Scheme 2). These
new findings are described herein.
Our study began by evaluating the decarboxylation of indole-3-carboxylic acid (1a) in
1,4-dioxane at 140 ꢁC and trace amount 1 H-indole (2a) was detected (Table 1, Entry 1).
After some experimentation, we found that inorganic base-catalyzed protocol consisting
of Na2CO3 (100 mol%) in 1,4-dioxane afforded 2a in 24% isolated yield at 140 ꢁC (Entry
2). Subsequently, the effects of base, solvent and temperature were screened. The results
indicated that K2CO3 (100 mol%) had more positive effect on the decarboxylation of 1a
in 1,4-dioxane with 37% yield (Entry 3) and the yield increased to 98% in ethanol
(EtOH, Entry 4). The solubility of base in EtOH is higher than 1,4-dioxane, which may
be the predominant factor. In order to simplify the reaction system, the dosage of base
was tested (Entries 5–7), and it was confirmed that K2CO3 (20 mol%) was essential to
afford the target product with >99% yield (Entry 5). Moreover, 1,4-dioxane, toluene,
water and n-hexanol as a solvent in the presence of K2CO3 (20 mol%) led to lower
yields (42–76%, Entries 8–11). Surprisingly, >99% yields can be obtained with or with-
out K2CO3 in acetonitrile (CH3CN, Entries 12 and 13). Besides, CH3CN showed no
catalytic roles in the transformation due to almost no target product detected in the

R
SYNTHETIC COMMUNICATIONSV
3
Table 1. Conditions optimization.a
Entry
Cat. (x)
Solvent (T)
T (oC)
Yield (%)b
1
2
3
4
5
6
7
8
None
1,4-dioxane
1,4-dioxane
1,4-dioxane
EtOH
EtOH
EtOH
140
140
140
140
140
140
140
140
140
140
140
140
140
140
120
100
90
Trace
24
37
98
> 99
87
Trace
60
42
76
53
>99
>99
N/A
79
56
45
71
49
36
Na2CO3 (100 mol%)
K2CO3 (100 mol%)
K2CO3 (100 mol%)
K2CO3 (20 mol%)
K2CO3 (10 mol%)
None
K2CO3 (20 mol%)
K2CO3 (20 mol%)
K2CO3 (20 mol%)
K2CO3 (20 mol%)
K2CO3 (20 mol%)
None
CH3CN (200 mol%)
K2CO3 (20 mol%)
K2CO3 (20 mol%)
K2CO3 (20 mol%)
None
EtOH
1,4-dioxane
Toluene
water
n-hexanol
CH3CN
CH3CN
EtOH
EtOH
EtOH
EtOH
9
10
11
12
13
14
15
16
17
18
19
20
CH3CN
CH3CN
CH3CN
120
100
90
None
None
aUnless other stated, all reactions are carried out with 1a (81 mg, 0.5 mmol), catalyst and solvent (3.0 mL) under air at
T
oC.
bIsolated yields.
presence of 200 mol% CH3CN in EtOH (Entry 14). The strong solvent dependence of
acetonitrile was noticed that may be due to the solvation effect to help stabilize ionic
intermediates. Lastly, the temperature was tested in the above two reaction systems con-
sisting of K2CO3 (20 mol%)/EtOH or CH3CN, respectively. And, the results showed that
140 ꢁC was a better choice for all these two reaction conditions (Entries 15–20).
With these conditions in hand, we next turned our attention to evaluating the scope
of this reaction under optimal conditions. As evident from the results compiled in
Table 2, Entries 1–12, the decarboxylation can be applied to a wide number of indole-3-
carboxylic acids. Indoles 2a–2l were obtained in good to excellent yields (89%ꢂ>99%)
with indole-3-carboxylic acids possessing alkyl, chloro, bromo, alkoxy, cyano or nitro
substituents at either C2, C4, C5, C6 and/or C7 (1a–1l) as starting materials. However,
there was an unexpected fact that these two optimal conditions failed to conduct
decarboxylation of N-methyl-1H-indole-3-carboxylic acid 1m and N-benzyl-1H-indole-
3-carboxylic acid 1n (Entries 13 and 14). It is suggested that a procedure involving
N–H may have been undergone in the decarboxylation of indole-3-carboxylic acids. In
order to further expand the scope of the decarboxylation reaction, ethyl 1 H-indole-3-
carboxylate 1o, indole-3-carboxylic acid 1p, indole-4-carboxylic acid 1q and 2-naphthoic
acid 1r were tested in these two optimal conditions (Entries 15–18), and the substrates
were recovered almost completely. Therefore, the optimal conditions are limited to be
applied to the decarboxylation of 3-carboxyl indoles.

4
X. CHEN AND X.-Y. ZHOU
Table 2. The screening on the scope of substrate.a
Entry
Substrate 1
Product 2
Yield (%)b
A: > 99%
B: > 99%
1
2a
1a
A: 95%
B: 89%
2
2b
2c
1b
1c
A: 91%
B: 98%
3
A: > 99%
B: 97%
4
1d
1e
1f
2d
2e
A: 97%
B: > 99%
5
A: > 99%
B: > 99%
6
2f
A: 96%
B: > 99%
7
2g
2h
2i
1g
1h
1i
A: > 99%
B: 99%
8
A: > 99%
B: 93%
9
A: 91%
B: > 99%
10
11
2j
1j
A: > 99%
B: > 99%
2k
1k
A: 96%
B: 98%
12
2l
1l
(continued)

R
SYNTHETIC COMMUNICATIONSV
5
13
14
NA
NA
2m
1m
1n
2n
2a
15
16
17
18
NA
NA
NA
NA
1o
1p
2a
2a
1q
1r
2r
aReaction conditions A: 1 (0.50 mmol), K2CO3 (13.8 mg, 0.10 mmol, 20 mol%), EtOH (3.0 mL), 140 ꢁC, 24 h; Reaction condi-
tions B: 1 (0.50 mmol), CH3CN (3.0 mL), 140 ꢁC, 24 h.
bIsolated yield.
Scheme 3. Proposed mechanism for decarboxylation of indole-3-carboxylic acid.
Based on the reaction results and the known mechanism of protonic acid-catalyzed
decarboxylation of pyrrole-2-carboxylic acid,[8] the possible mechanism for decarboxyl-
ation of indole-3-carboxylic acids is proposed in Scheme 3. The 1 H-indole-3-carboxyl-
ate anion 3 is produced under the standard conditions from 1a. In the proposed path
A, the decarboxylation of 3 may occur directly to form a carbon anion 4 and be proto-
nated to produce the product 2a. In the proposed path B, the formation of intermediate
cyclic-imine 5 from the isomerization of 4 under the optimal conditions is favored.
Then, the decarboxylation process may occur to produce 6 and be protonated to pro-
duce the product 2a. In consistent with the decarboxylation of N-alkyl-1H-indole-3-car-
boxylic acids 1m and 1n, it is firmly held that the proposed path B is reasonable than
path A.

6
X. CHEN AND X.-Y. ZHOU
In summary, K2CO3-catalyzed and acetonitrile-promoted decarboxylation of indole-3-
carboxylic acids are achieved, respectively, and the approaches allow for a wide range of 3-
carboxyl indoles with good to excellent yields. More applications of base-catalyzed decar-
boxylative functionalization of indole-2-carboxylic acids and indole-3-carboxylic acids are
under active investigation in our laboratory and will be disclosed in due course.
Experimental section
General methods
All commercially available reagents were used without further purification. Column
chromatography was performed on silica gel (200–300 mesh). The chemical shifts
1
(d) of the H NMR (600 MHz) signals are reported in ppm relative to tetramethylsi-
lane (TMS) and by employing the residual solvent resonance as the internal standard.
Data are reported in the following order: chemical shift, multiplicity [s (singlet), d
(doublet), dd (doublet of doublets), t (triplet), q (quartet), and m (multiplet)], coupling-
constants (Hz), and integration. The chemical shifts
(d) of the 13CNMR (151 MHz) signals are reported in ppm relative tetramethylsilane
(TMS) and by employing the solvent resonance as the internal standard.
Procedure A: A mixture of indole-3-carboxylic acid 1 (0.50 mmol) and K2CO3
(13.8 mg, 0.10 mmol, 20 mol%) in EtOH (3 mL) was added into a Schlenk flask (25 mL)
and stirred at 140 ꢁC. The reaction was monitored using thin layer chromatography
until the disappearance of starting material 1. Then, the solvent was evaporated under
reduced pressure and the residue was purified using column chromatography (petrol-
eum ether/ethyl acetate 20:1 to 10:1).
Procedure B: A mixture of indole-3-carboxylic acid 1 (0.50 mmol) in CH3CN (3 mL)
was added into a Schlenk flask (25 mL) and stirred at 140 ꢁC. The reaction was moni-
tored by thin layer chromatography until the disappearance of starting material 1. Then
the solvent was evaporated under reduced pressure and the residue was purified by col-
umn chromatography (petroleum ether/ethyl acetate 20:1 to 10:1).
1 H-indole (2a)
Yield A: > 99%, 59.0 mg; Yield B: > 99%, 59.0 mg. White solid, mp 50–52 ꢁC, Rf 0.51
(Hexane/EtOAc, 10:1). 1H NMR (600 MHz, CDCl3) d 8.08 (s, 1 H), 7.74 (d, J ¼ 7.9 Hz, 1 H),
7.43 (dd, J ¼ 8.1, 0.6 Hz, 1 H), 7.31–7.26 (m, 1 H), 7.24–7.19 (m, 2 H), 6.64 (t, J ¼ 2.1 Hz,
1 H); 13C NMR (151 MHz, CDCl3) d 135.8, 127.9, 124.2, 122.0, 120.8, 119.9, 111.1, 102.6.
1
Full experimental detail, H and 13C NMR spectra. This material can be found via
the “Supplementary Content” section of this article’s webpage.
Funding
The work was supported by the Natural Science Foundation of Guizhou Province [Grant No.
qiankehejichu [2018] number 1141], the Science and Technology Foundation Project of
Liupanshui [Grant No. 52020-2018-04-10] and National Natural Science Foundation of China
[No. 21763017].

R
SYNTHETIC COMMUNICATIONSV
7
References
[1] (a) Lichtenberger, L. M.; Delansorne, R.; Graziani, L. A. Nature 1982, 295, 698. DOI: 10.
1038/295698a0. (b) Hilpert, W.; Dimroth, T. Nature 1982, 296, 584. DOI: 10.1038/
296584a0.
[2] (a) Weaver, J. D.; Recio III., A.; Grenning, A. J.; Tunge, J. A. Chem. Rev. 2011, 111, 1846.
DOI: 10.1021/cr1002744. (b) Xuan, J.; Zhang, Z.-G.; Xiao, W.-J. Angew. Chem. Int. Ed.
Engl. 2015, 54, 15632. DOI: 10.1002/anie.201505731. (c) Wei, Y.; Hu, P.; Zhang, M.; Su,
W. Chem. Rev. 2017, 117, 8864. DOI: 10.1021/acs.chemrev.6b00516.
[3] (a) Bi, H.-P.; Zhao, L.; Liang, Y.-M.; Li, C.-J. Angew. Chem. Int. Ed. Engl. 2009, 48, 792.
DOI: 10.1002/anie.200805122. (b) Kalutharage, N.; Yi, C. S. Angew. Chem. Int. Ed. Engl.
2013, 52, 13651. DOI: 10.1002/anie.201307766. (c) Noble, A.; Mega, R. S.; Pfl-Sterer, D.;
Myers, E. J.; Aggarwal, V. K. Angew. Chem. Int. Ed. Engl. 2018, 57, 2155. DOI: 10.1002/
anie.201712186.
[4] (a) Gooßen, L. J.; Deng, G.; Levy, L. M. Science 2006, 313, 662. DOI: 10.1126/science.
1128684. (b) Goossen, L. J.; Zimmermann, B.; Knauber, T. Angew. Chem. Int. Ed. Engl.
2008, 47, 7103. DOI: 10.1002/anie.200800728. (c) Hu, P.; Shang, Y.; Su, W. Angew. Chem.
Int. Ed. Engl. 2012, 51, 5945. DOI: 10.1002/anie.201200153. (d) Song, B.; Knauber, T.;
Gooßen, L. J. Angew. Chem. Int. Ed. Engl. 2013, 52, 2954. DOI: 10.1002/anie.201208025.
(e) Howe, G. W.; Vandersteen, A. A.; Kluger, R. J. Am. Chem. Soc. 2016, 138, 7568. DOI:
10.1021/jacs.6b01770. (f) Perry, G. J. P.; Quibell, J. M.; Panigrahi, A.; Larrosa, I. J. Am.
Chem. Soc. 2017, 139, 11527. DOI: 10.1021/jacs.7b05155. (g) Drapeau, M. P.; Bahri, J.;
Lichte, D.; Gooßen, L. J. Angew. Chem. Int. Ed. Engl. 2019, 58, 892. DOI: 10.1002/anie.
201812068.
ꢀ
[5] (a) Goossen, L. J.; Rudolphi, F.; Oppel, C.; Rodrıguez, N. Angew. Chem. Int. Ed. Engl.
2008, 47, 3043. DOI: 10.1002/anie.200705127. (b) Hu, F.; Shao, X.; Zhu, D.; Lu, L.; Shen,
Q. Angew. Chem. Int. Ed. Engl. 2014, 53, 6105. DOI: 10.1002/anie.201402573. (c) Chu, L.;
Lipshultz, J. M.; MacMillan, D. W. C. Angew. Chem. Int. Ed. Engl. 2015, 54, 7929. DOI:
10.1002/anie.201501908. (d) Huang, H.; Zhang, G.; Chen, Y. Angew. Chem. Int. Ed. Engl.
2015, 54, 7872. DOI: 10.1002/anie.201502369. (e) Tan, H.; Li, H.; Ji, W.; Wang, L. Angew.
Chem. Int. Ed. Engl. 2015, 54, 8374. DOI: 10.1002/anie.201503479. (f) Jian, W.; Ge, L.;
Jiao, Y.; Qian, B.; Bao, H. Angew. Chem. Int. Ed. Engl. 2017, 56, 3650. DOI: 10.1002/anie.
201612365. (g) Till, N. A.; Smith, R. T.; MacMillan, D. W. C. J. Am. Chem. Soc. 2018,
140, 5701. DOI: 10.1021/jacs.8b02834.
[6] (a) Blake, J. A.; Lukeman, M.; Scaiano, J. C. J. Am. Chem. Soc. 2009, 131, 4127. DOI: 10.
1021/ja8094872. (b) Liu, J.; Liu, Q.; Yi, H.; Qin, C.; Bai, R.; Qi, X.; Lan, Y.; Lei, A. Angew.
Chem. Int. Ed. Engl. 2014, 53, 502. DOI: 10.1002/anie.201308614. (c) Zuo, Z.; MacMillan,
D. W. C. J. Am. Chem. Soc. 2014, 136, 5257. DOI: 10.1021/ja501621q. (d) Chu, L.;
Lipshultz, J. M.; MacMillan, D. W. C. Angew. Chem. Int. Ed. Engl. 2015, 54, 7929. DOI:
10.1002/anie.201501908. (e) Huang, H.; Jia, K.; Chen, Y. Angew. Chem. Int. Ed. Engl.
2015, 54, 1881. DOI: 10.1002/anie.201410176. (f) Huang, X.; Liu, W.; Hooker, J. M.;
Groves, J. T. Angew. Chem. Int. Ed. Engl. 2015, 54, 5241. DOI: 10.1002/anie.201500399.
(g) Noble, A.; McCarver, S. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2015, 137, 624.
DOI: 10.1021/ja511913h. (h) Vaillant, F. L.; Courant, T.; Waser, J. Angew. Chem. Int. Ed.
Engl. 2015, 54, 11200. DOI: 10.1002/anie.201505111. (i) Ventre, S.; Petronijevic, F. R.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2015, 137, 5654. DOI: 10.1021/jacs.5b02244. (j)
Zhou, Q.-Q.; Guo, W.; Ding, W.; Wu, X.; Chen, X.; Lu, L.-Q.; Xiao, W.-J. Angew. Chem.
Int. Ed. Engl. 2015, 54, 11196. DOI: 10.1002/anie.201504559. (k) Zuo, Z.; Cong, H.; Li, W.
; Choi, J.; Fu, G. C.; MacMillan, D. W. C. J. Am. Chem. Soc. 2016, 138, 1832. DOI: 10.
1021/jacs.5b13211. (l) Cao, H.; Jiang, H.; Feng, H.; Kwan, J. M. C.; Liu, X.; Wu, J. J. Am.
Chem. Soc. 2018, 140, 16360. DOI: 10.1021/jacs.8b11218. (m) Kautzky, J. A.; Wang, T.;
Evans, R. W.; MacMillan, D. W. C. J. Am. Chem. Soc. 2018, 140, 6522. DOI: 10.1021/jacs.
8b02650.

8
X. CHEN AND X.-Y. ZHOU
[7] (a) Cleland, W. W. Acc. Chem. Res. 1999, 32, 862. DOI: 10.1021/ar990065a. (b) Tanaka,
D.; Romeril, S. P.; Myers, A. G. J. Am. Chem. Soc. 2005, 127, 10323. DOI: 10.1021/
ja052099l. (c) Zhang, S.-L.; Fu, Y.; Shang, R.; Guo, Q.-X.; Liu, L. J. Am. Chem. Soc. 2010,
132, 638. DOI: 10.1021/ja907448t. (d) Fromm, A.; van Wu€llen, C.; Hackenberger, D.;
Gooßen, L. J. J. Am. Chem. Soc. 2014, 136, 10007. DOI: 10.1021/ja503295x. (e) Kluger, R.
Acc. Chem. Res. 2015, 48, 2843. DOI: 10.1021/acs.accounts.5b00306. (f) Samala, S.; Singh,
G.; Kumar, R.; Ampapathi, R. S.; Kundu, B. Angew. Chem. Int. Ed. Engl. 2015, 54, 9564.
DOI: 10.1002/anie.201504429. (g) Vandersteen, A. A.; Howe, G. W.; Lollar, B. S.; Kluger,
R. J. Am. Chem. Soc. 2017, 139, 15049. DOI: 10.1021/jacs.7b07504. (h) Yu, J.-L.; Zhang,
S.-Q.; Hong, X. J. Am. Chem. Soc. 2017, 139, 7224. DOI: 10.1021/jacs.7b00714.
[8] (a) Mundle, S. O. C.; Kluger, R. J. Am. Chem. Soc. 2009, 131, 11674. DOI: 10.1021/
ja905196n. (b) Mundle, S. O. C.; Lacrampe-Couloume, G.; Lollar, B. S.; Kluger, R. J. Am.
Chem. Soc. 2010, 132, 2430. DOI: 10.1021/ja910608m.
[9] Yu, X.; He, M.; Wu, J.; Zhou, C.; Feng, X.; Yamamoto, Y.; Bao, M. Org. Lett. 2018, 20,
6780. DOI: 10.1021/acs.orglett.8b02896.
[10] Zhang, X.; Feng, X.; Zhou, C.; Yu, X.; Yamamoto, Y.; Bao, M. Org. Lett. 2018, 20, 7095.
DOI: 10.1021/acs.orglett.8b03043.
[11] (a) Allen, G. R. Jr. Org. React. 1973, 20, 337. (b) Baiocchi, L.; Giannangeli, M.
J. Heterocyclic Chem. 1988, 25, 1905. DOI: 10.1002/jhet.5570250654. (c) Sakamoto, T.;
Nagano, T.; Kondo, Y.; Yamanaka, H. Synthesis 1990, 1990, 215. DOI: 10.1055/s-1990-
26835. (d) Kondo, Y.; Shiga, F.; Murata, N.; Sakamoto, T.; Yamanaka, H. Tetrahedron
1994, 50, 11803. DOI: 10.1016/S0040-4020(01)89295-4. (e) Yamazaki, K.; Nakamura, Y.;
Kondo, Y. J. Org. Chem. 2003, 68, 6011. DOI: 10.1021/jo0340307. (f) Islam, M. S.;
Brennan, C.; Wang, Q.; Hossain, M. M. J. Org. Chem. 2006, 71, 4675. DOI: 10.1021/
€
jo0601821. (g) Soderberg, B. C. G.; Banini, S. R.; Turner, M. R.; Minter, A. R.; Arrington,
A. K. Synthesis 2008, 2008, 903. DOI: 10.1055/s-2008-1032208.
[12] (a) Walser, A.; Blount, J. F.; Fryer, R. I. J. Org. Chem. 1973, 38, 3077. DOI: 10.1021/
jo00958a001. (b) Maehara, A.; Tsurugi, H.; Satoh, T.; Miura, M. Org. Lett. 2008, 10, 1159.
DOI: 10.1021/ol8000602. (c) Yamashita, M.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett.
2009, 11, 2337. DOI: 10.1021/ol900736s. (d) Hikawa, H.; Kotaki, F.; Kikkawa, S.;
Azumaya, I. J. Org. Chem. 2019, 84, 1972. DOI: 10.1021/acs.joc.8b02947. (e) Zhang, Y.;
Hu, Z.-Y.; Li, X.-C.; Guo, X.-X. Synthesis 2019, 51, 1803. DOI: 10.1055/s-0037-1611946.
[13] Vandersteen, A. A.; Mundle, S. O. C.; Kluger, R. J. Org. Chem. 2012, 77, 6505. DOI: 10.
1021/jo301032f.
[14] Yue, H.; Guo, L.; Lee, S.-C.; Liu, X.; Rueping, M. Angew. Chem. Int. Ed. Engl. 2017, 56,
3972. DOI: 10.1002/anie.201612624.
[15] Liu, C.; Qin, Z.-X.; Ji, C.-L.; Hong, X.; Szostak, M. Chem. Sci. 2019, 10, 5736. DOI: 10.
1039/c9sc00892f.