Synthesis of aliphatic α-ketoamides from α-substituted methyl ketones: Via a Cu-catalyzed aerobic oxidative amidation

Article abstract of DOI:10.1039/d1ob00129a

α-Ketoamides are an important key functional group and have been used as versatile and valuable intermediates and synthons in a variety of functional group transformations. Synthetic methods for making aryl α-ketoamides as drug candidates have been greatly improved through metal-catalyzed aerobic oxidative amidations. However, the preparation of alkyl α-ketoamides through metal-catalyzed aerobic oxidative amidations has not been reported because generating α-ketoamides from aliphatic ketones with two α-carbons theoretically provides two distinct α-ketoamides. Our strategy is to activate the α-carbon by introducing an N-substituent at one of the two α-positions. The key to this strategy is how heterocyclic compounds such as triazoles and imidazoles affect the selectivity of the synthesis of the alkyl α-ketoamides. From this basic concept, and by optimizing the reaction and elucidating the mechanism of the synthesis of aryl α-ketoamides via a copper-catalyzed aerobic oxidative amidation, we prepared fourteen aliphatic α-ketoamides in high yields (48-84%). This journal is

Full text of DOI:10.1039/d1ob00129a

Organic &
Biomolecular Chemistry
View Article Online
View Journal
PAPER
Synthesis of aliphatic α-ketoamides from
α-substituted methyl ketones via a Cu-catalyzed
aerobic oxidative amidation†
Cite this: DOI: 10.1039/d1ob00129a
Hyojin Cha,
Jin Young Chai,
Hyeong Baik Kim
and Dae Yoon Chi
*
α-Ketoamides are an important key functional group and have been used as versatile and valuable inter-
mediates and synthons in a variety of functional group transformations. Synthetic methods for making
aryl α-ketoamides as drug candidates have been greatly improved through metal-catalyzed aerobic oxi-
dative amidations. However, the preparation of alkyl α-ketoamides through metal-catalyzed aerobic oxi-
dative amidations has not been reported because generating α-ketoamides from aliphatic ketones with
two α-carbons theoretically provides two distinct α-ketoamides. Our strategy is to activate the α-carbon
by introducing an N-substituent at one of the two α-positions. The key to this strategy is how heterocyclic
compounds such as triazoles and imidazoles affect the selectivity of the synthesis of the alkyl
α-ketoamides. From this basic concept, and by optimizing the reaction and elucidating the mechanism of
the synthesis of aryl α-ketoamides via a copper-catalyzed aerobic oxidative amidation, we prepared four-
teen aliphatic α-ketoamides in high yields (48–84%).
Received 23rd January 2021,
Accepted 15th April 2021
DOI: 10.1039/d1ob00129a
rsc.li/obc
α-ketoesters, as shown in Scheme 1, the mechanisms of the
aerobic oxidative amidation and esterification are different
Introduction
α-Ketoamides, founded in natural products and biologically according to the kind of aryl ketone used as the starting
relevant molecules such as a cytokine inhibitor,1 an RARγ material. In the case of aryl methyl ketones without substitu-
agonist,1 an epoxide hydrolase inhibitor,2 a PLA2 inhibitor,3 a ents on their α-carbons, the common major intermediate in
histone deacetylase inhibitor,4 a coronavirus inhibitor,5 and Scheme 1(a) is phenylglyoxal, and no cleavage of the C(CO)–C
an HCV protease inhibitor,6 are an important key functional (CO) bonds was observed in this process (Scheme 1(a)).11,14 On
group, as shown in Fig. 1. These α-ketoamides have been used the other hand, aerobic oxidative C–C σ-bond cleavage of sym-
as versatile and valuable intermediates and synthons in a metrical aryl-substituted-1,3-diones can smoothly convert
variety of functional group transformations and in total synth- them into α-ketoesters.15 The C(CO)–C(alkyl) bond cleavage of
eses.7 In the last decade, synthetic methods8–13 for making aryl–alkyl–ketones as unsymmetrical ketones for the synthesis
aryl α-ketoamides in pharmaceutical research have been of aryl long-chain-alkyl esters via an aerobic oxidation and oxy-
greatly advanced by the development of metal-catalyzed genation process with molecular oxygen was reported by the
aerobic oxidative amidation reactions from phenylacetylene,9 Jiao group.16 And when there are substituents on the α-carbon
phenylacetaldehyde,10 acetophenone,11 propiophenone,12 and of the ketone, the desired oxygenation pathway competes with
2-phenylpropanoic acid.13 However, the preparation of alkyl a C(CO)–C(CO) bond cleavage pathway in the aerobic oxidation
α-ketoamides via a metal-catalyzed aerobic oxidative amidation mechanism. This competition is related with hydroperoxide
has not been reported.1
formation and dioxetane formation. The Jiao group reported
Aerobic oxidative amidation or esterification reactions are the synthesis of α-ketoamide from α-carbonyl aldehyde, which
the most attractive and powerful strategy for oxidizing the is in a higher oxidation state of the α-carbon via aerobic oxi-
α-carbons of ketones with molecular oxygen. According to pre- dation (Scheme 1(b)).17
vious works on the preparation of aryl α-ketoamides or
Interestingly, the Schoenebeck group reported how to avoid
the two competing pathways: the desired oxygenation (C–H
α-hydroxylation) and C(CO)–C(CO) cleavage in the copper-cata-
lyzed selective aerobic oxidation of the α-carbon of a 2-methyl
cyclic ketone.18 The Zhang group was also able to obtain dia-
rylmethanones and arylmethanoic acid by controlling the oxy-
genation and C(CO)–C(CO) bond cleavage pathways, respect-
ively, in the aerobic oxidation of 2-phenylacetophenone.19
Department of Chemistry, Sogang University, 35 Baekbeomro Mapogu, Seoul 04107,
Korea. E-mail: dychi@sogang.ac.kr
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, and characterization for desired products (α-ketoamides), starting
materials (α-N-heteroaryl ketones), and compounds trapped by TEMPO. See DOI:
10.1039/d1ob00129a
This journal is © The Royal Society of Chemistry 2021
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
Scheme 1 Cu-Catalyzed aerobic oxidative coupling to prepare
α-ketoamides from ketones.
Fig. 1 Various α-ketoamides.1–6
Scheme 2 Model reaction of the Cu-catalyzed aerobic oxidation.
Despite the significance of these reports, there are two challen-
ging tasks for new synthesis of aliphatic α-ketoamides: (1) con-
trolling hydroperoxide formation and dioxentane formation in
the oxygenation of unsymmetrical 2-substituted-ketone is still acid was also formed (Scheme 2(a)), as shown in the plausible
challenging and (2) while aryl ketones have one reaction site, mechanism. We also proposed that phenylglyoxylic acid is
alkyl ketones have two possible reaction sites, making their formed when hydroxide attacks the key triazol-1-yl-1,2-diketo
reactions more complicated.
intermediate. If an amine or alcohol is present in that reaction,
Our work applies imidazole and triazole substituted at the α-ketoamides or α-ketoesters would be formed, respectively.
α-carbon of the ketone as an unsymmetrical ketone to solve According to this hypothesis, α-triazolyl acetophenones are
the challengeable tasks in the Cu-catalyzed aerobic oxidative preferred and, as a result, aryl α-ketoamides could be obtained
amidation. Initially, factors of inducing major hydroperoxide in a high yield (Scheme 2(b)). Thus, these results are also
formation were found, depending on whether the synthesis of applicable to the preparation of alkyl α-ketoamides. Herein,
aryl α-ketoamides was improved or not. Then, the optimized this paper reports a new chemoselective, Cu-catalyzed aerobic
conditions were applied to synthesize the alkyl α-ketoamide. oxidative amidation to synthesize aliphatic α-ketoamides from
Finally, the predominant regioselectivity of introducing mole- α-substituted methyl ketones using amines and molecular
cular oxygen at the doubly activated carbon of aliphatic ketone oxygen as the oxidant. As ketones have two α-positions, the
was proved with a TEMPO trapping experiment.
synthesis of aliphatic α-ketoamides from ketones selectively by
Recently, we reported a synthetic method to prepare aerobic oxidation is challenging; however, it has been achieved
4-substituted-NH-1,2,3-triazoles via an aerobic oxidative herein (Scheme 1, our work). We optimized the aerobic oxi-
N-dealkylation using copper(II) acetate from 7.20 The evidence dation reaction selectively using an α-substituted acetophe-
for the formation of triazole 9 indicates that phenylglyoxylic none 7, which has one α-position (see ESI, Tables S1 and S2†).
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2021

View Article Online
Organic & Biomolecular Chemistry
Paper
Then, based on the optimized conditions, we searched the (k2) than that of I8 (k4) because the carbonyl carbon of I3 will
optimized reaction conditions for aliphatic α-ketoamide.
be activated by the copper(II) ion, resulting in a greater partial
positive charge on the carbon compared to that of I8. In com-
paring the reaction rates (k1 and k2), products 10a and 12 were
formed in a ratio of 67% and 32%, respectively. In the case of
I8, the formation of 12 via I9 was completely suppressed, while
the desired product 10a was obtained selectively in a high
yield. To understand what the new function of imidazole
under the optimized conditions for selective aerobic oxidation
is, further investigation is needed in comparison to pyridine
as the base.
Results and discussion
Preliminary investigation for the synthesis of α-ketoamide by
using isotope 18O2, and a plausible mechanism
To prove the mechanism of incorporation of O2 in the hypoth-
esis (Scheme 2(b)), previously reported reaction conditions20
with the addition of p-anisidine as a probe and bpy as a ligand
were tested in the presence of 18O2, instead of molecular
oxygen 16O2. This result is 16O/18O-10a (73%) and 18O/18O-10a
(20%) in the isotopic ratio of 10a (GC-MS spectrum, see ESI†).
Synthesis of aliphatic α-ketoamides from aliphatic ketones
under optimization of the conditions
Furthermore, investigation that 10a was left under previously Theoretically, preparing α-ketoamides from aliphatic ketones
reported reaction conditions with the addition of H218O (2 with two α-carbons can afford two distinct α-ketoamides. Our
equiv.) and bpy under an argon atmosphere resulted in strategy is to activate the α-carbon by introducing an
18O/16O-10a (58%) and 16O/16O-10a (42%) in the isotopic ratio N-substituent at one of the two α-positions. As we have devel-
of 10a (GC-MS spectrum, see ESI†). This result means that the oped a copper-catalyzed aerobic oxidative C–N bond cleavage
reason for 18O/18O-10a formation is isotope scrambling of the reaction, a triazole group would be a good substituent to acti-
16O/18O-10a product with the 18O-water that is generated after vate one of the α-carbons. The key to this strategy is determin-
the formation of 16O/18O-10a. The evidence that only a one ing how heterocyclic compounds such as triazoles and imid-
oxygen atom from molecular oxygen O2 is incorporated into azoles affect selectivity in the synthesis of aliphatic compounds.
α-carbon of ketone is related to selective aerobic oxidative ami- When discussing the two α-positions of the ketone, we consider
dation as the ultimate goal. Based on the experimental results one carbon to be singly activated and the other carbon bearing
(see ESI, Table S1†) and the literature,20 a plausible mecha- the heterocycle to be doubly activated.21 In a similar manner to
nism that explains the different outcomes when using triazolyl the results (see ESI, Table S1†) for preparing aryl α-ketoamides,
and imidazolyl groups is proposed, as shown in Scheme 3. The the key question is whether the carbon of the aliphatic ketone
mechanism of the formation of Intermediate 1 (I1), as pro- doubly activated by the heterocyclic moiety could be regioselec-
posed in a previous report,20 was determined by trapping with tively oxidized. To the best of our knowledge, this is the first
TEMPO. TEMPO-trapped α-triazolyl-acetopheone was proved finding that aliphatic-α-ketoamides are prepared regioselectively
by single-crystal X-ray diffraction studies.20 From I1, two key on doubly activated carbon, when triazole or imidazole is
intermediates, I3 and I8, can be generated from 7 (triazole attached into the α-carbon of the aliphatic ketone to conduct a
group) and 11 (imidazole group), respectively. I3 can chelate Cu-catalyzed aerobic oxidative amidation reaction.
with Cu(II) ions between the oxygen of the ketone and the N2-
To prepare aliphatic α-ketoamides, α-heteroaryl acetones 13
nitrogen of the triazole, while I8 cannot. Consequently, the and 14 were subjected to the optimal reaction conditions
dioxygen radical in I3 will attack the carbonyl carbon faster for preparing aryl α-ketoamides (see ESI, Table S1†). From
Scheme 3 Plausible mechanism and difference between the triazolyl and imidazolyl group.
This journal is © The Royal Society of Chemistry 2021
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
Table 1 Optimization studies for the synthesis of pyruvic amidea
Table 2 The scope of amines in the copper-catalyzed aerobic oxidative
amidation to prepare aliphatic α-ketoamidesa,b
Variation from the
Entry sm standard conditions
15ab
(%)
12b
(%)
1
13 None
14 None
14 Imidazole (0 equiv.)
14 Pyridine (1 equiv.) instead of
imidazole (3 equiv.)
68
32
11
0
0
38
43
59
2
3c
4
a Reaction conditions: ketone (1.0 mmol scale), O2 balloon, 0.5 N HCl
(aq) work-up. b Isolated yield. c Reaction time = 24 h.
a Reaction conditions: ketone (1.0 mmol scale), O2 balloon, 0.5 N HCl
(aq) work-up. b Isolated yields.
α-imidazol-1-yl acetone 13, the desired product, pyruvic amide
15a, was isolated in a moderate yield (68%, Table 1, entry 1),
while the undesired side product, formamide 12, was not iso-
lated (0%). In the case of α-triazolyl acetone 14, the side
product was isolated in 38% yield, which was due to the clea-
vage of a C–C bonding in the starting material (Table 1, entry
2). Without imidazole as a base, the reaction time increased to
24 h (full consumption of the starting material), and the
selectivity for 15a decreased (entry 3). When we used pyridine
as the base instead of imidazole, the yield of the side product
12 increased and that of the desired product 15a decreased
(entry 4). We considered the heteroaryl group substituted on
the α-carbon of acetone to be a key factor in determining the
regioselectivity of the incorporation of the oxygen. Among the
heterocycles tested, the imidazole group induced a better
regioselectivity than was achieved with the triazole group.
The scope of various amines, including anilines, in the
copper-catalyzed aerobic oxidative amidation was investigated
as shown in Table 2. Products 15a–d from anilines, 15e–h
from primary amines and 15i from secondary amines were
obtained in quite good yields. In the case of ortho-anisidine,
which could have steric hindrance, the desired compound 15c
was obtained in a slightly lower yield (48%). Secondary amines
can easily react with α-imidazol-1-yl acetone to afford the
corresponding α-ketoamide product 15i (74%).
Table 3 Preparation of various aliphatic α-ketoamides via copper-cata-
lyzed aerobic oxidative amidationa,b
a Reaction conditions: ketone (1.0 mmol scale), O2 balloon, 0.5 N HCl
(aq) work-up. b Isolated yields.
Long-chain alkyl ketones containing various substituents
provided the corresponding α-ketoamides. Unfortunately, acyl
amide could also be isolated as a side product. This side
product was not observed with the previously tested substitu- chain α-ketoamides 22 (62%) and 23 (68%) in moderate yields.
ents, such as aryl and methyl groups. Control reactions for the Although the long chains contained ester or olefin moieties,
synthesis of long-chain α-ketoamides were conducted which could be converted into carboxylic acids and epoxides
(Table 3). Intriguingly, α-imidazol-1-yl ketones 16 and 17 with as side products, these reactions proceeded smoothly in mod-
simple, long-chain aliphatic substituents afforded simple long- erate yields to afford 25 (58%) and 26 (72%). However, 21 did
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2021

View Article Online
Organic & Biomolecular Chemistry
Paper
To understand the reaction mechanism, we performed the
reactions shown in Scheme 4. When aryl α-ketoamide 10a, the
product of the aerobic oxidative amidation, was subjected to
the optimized conditions, no reaction occurred. When ali-
phatic α-ketoamide 28, the product of the aerobic oxidative
amidation, was left under the optimized conditions, but in the
presence of 1-phenylpiperazine instead of p-anisidine, 1-(4-
phenylpiperazin-1-yl)nonan-1-one (29) was obtained in 62%
yield. On the other hand, when aliphatic α-ketoamide 28, the
product of the aerobic oxidative amidation, was left under the
optimized conditions, N-(4-methoxyphenyl)nonanamide (30)
was obtained in 42% yield. In these two reactions, the aliphatic
α-ketoamide could be transformed to 29 or 30. The mecha-
nism of this reaction should be studied further. The Baeyer–
Villiger oxidation did not occur from the aryl α-ketoamide.
In the case of the aryl ketones in Scheme 3, there is only
one radical formation at the one α-carbon, such as
Intermediate 1 (I1). Clarifying the selectivity of the radical for-
mation at the two α-carbons in the aerobic oxidation of ali-
phatic ketones would be important for understanding the reac-
tion mechanism. TEMPO has been widely used to determine if
a radical pathway is active during a reaction.22 It has also been
Scheme 4 Three additional experiments to elucidate the reaction
mechanism.
not afford the desired product because both α-carbons of the reported that when TEMPO is introduced, an α-hydroxylated
substrate were activated as benzyl groups, even though starting product can be obtained by an additional reduction.23 Fig. 2
1
material 21 was completely consumed.
shows H NMR spectra indicating the selectivity of the radical
Fig. 2 1H NMR studies to determine the selectivity of the radical formation at the two α-carbons of three aliphatic ketones as starting materials by
scavenging with TEMPO.
This journal is © The Royal Society of Chemistry 2021
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
formation at the two α-carbons of three aliphatic ketone start- the selective aerobic oxidation of long-chain lipid ketones.26 In
ing materials in the presence of TEMPO as a scavenger. From addition, scavenging doubly activated carbon with TEMPO in
imidazole 16, the ratio of product 35 to 37 was 94 : 6. From the unsymmetrical ketone showed the selectivity of the radical
another imidazole, 13, products 36 and 37 were produced at a formation at the two α-carbons of three aliphatic ketones as
ratio of 79 : 21. Because C3 in compound 16 is not only more starting materials. Further studies to understand the mecha-
sterically hindered but also bears a less acidic hydrogen due to nism of the selective oxidative amidation and esterification,
the heptyl substituent compared with the C3 position of com- and the synthetic application, are in progress.
pound 13, the generation of the C3 radical of compound 16
was not as favorable.
In the case of 14, the product ratio of 38 to 39 was 53 : 47. Conflicts of interest
The C1/C3 selectivity with triazolyl-substituted starting
There are no conflicts to declare.
material 14 was lower than that of the previous two reactions,
which started from imidazole-substituted compounds 16 and
13. These results can be explained by the fact that imidazole
13 provided a better selectivity for the synthesis of the desired
Acknowledgements
product than 14, which has a triazolyl-substituent at the
α-carbon, as shown in Table 1.
This work was supported by the Radiation Technology R&D
program through the National Research Foundation of Korea
funded by the Ministry of Science, ICT & Future Planning
(2016M2A2A7A03913537).
The chelation of the N2-nitrogen of the triazole and the
oxygen of the ketone to the Lewis acidic copper ions24,25
will activate the H3 hydrogen, providing the C3 radical inter-
mediate. This chelate structure can be found in the literature
in the analogous reaction of acylpyrazole with TEMPO.25
Notes and references
In our triazole reaction, the chelation in 14 decreased the
electron density on the C2 carbonyl carbon, which favored the
C–C bond cleavage reaction, decreasing the selectivity of the
aerobic oxidation.
1 D. Kumar, S. R. Vemula and G. R. Cook, ACS Catal., 2016,
6, 4920–4945.
2 D. V. Patel, R. D. Gless Jr., H. K. Webb Hsu, S. K. Anandan
and B. R. Aavula, PCT Int. Appl WO2008073623A2, 2008.
3 G. Kokotos, S. Kotsovolou, D. A. Six, V. Constantinou-
Kokotou, C. C. Beltzner and E. A. Dennis, J. Med. Chem.,
2002, 45, 2891–2893.
Conclusions
In summary, we have developed a new copper-catalyzed
aerobic oxidative amidation of aliphatic ketones for the syn-
thesis of aliphatic α-ketoamides via the introduction of imid-
azole at the α-carbon of the aliphatic ketone and using extra
imidazole as the base. First of all, aerobic oxidative amidation
4 C. K. Wada, R. R. Frey, Z. Ji, M. L. Curtin, R. B. Garland,
J. H. Holms, J. Li, L. J. Pease, J. Guo, K. B. Glaser,
P. A. Marcotte, P. L. Richardson, S. S. Murphy, J. J. Bouska,
P. Tapang, T. J. Magoc, D. H. Albert, S. K. Davidsen and
M. R. Michaelides, Bioorg. Med. Chem. Lett., 2003, 13,
3331–3335.
of
α-(1,2,3-triazol-1-yl)acetophenone
resulted
in
aryl
α-ketoamides and formamides through formation of peroxide
and C(CO)–C(CO) cleavage of the ketone, respectively. 18O
labeling experiments demonstrate that carbonyl oxygen atom
of amide in the aryl α-ketoamide originated from molecular
oxygen. This means that inhibiting the C(CO)–C(CO) cleavage
pathway in the aerobic oxidative amidation of α-(1,2,3-triazol-1-
yl)acetophenone is the major issue for the selective aerobic
amidation. As a result, applying an α-imidazolyl acetophenone
instead of α-(1,2,3-triazol-1-yl) acetophenone as the substrate,
and imidazole as the base allowed the selective synthesis of
aryl α-ketoamides. Based on these results, using α-imidazolyl-
aliphatic ketones as the substrates gives rise to alkyl
α-ketoamides, highlighting the versatility of this protocol.
Moreover, the observation of acyl amides as side products in
the synthesis of alkyl α-ketoamides provides insight into the
stepwise pathway involving the imine from the air overoxida-
tion of the alkyl α-ketoamide. However, the acyl amide was not
observed in the synthesis of aryl α-ketoamides. The discovery
of a stepwise pathway through the aerobic oxidative amidation
of alkyl α-ketoamides provides a new platform for achieving
5 (a) L. Zhang, D. Lin, Y. Kusov, Y. Nian, Q. Ma, J. Wang,
A. von Brunn, P. Leyssen, K. Lanko, J. Neyts, A. de Wilde,
E. J. Snijder, H. Liu and R. Hilgenfeld, J. Med. Chem., 2020,
63, 4562–4578; (b) L. Zhang, D. Lin, X. Sun, U. Curth,
C. Drosten, L. Sauerhering, S. Becker, K. Rox and
R. Hilgenfeld, Science, 2020, 368, 409–412; (c) V. Frecer and
S. Miertus, RSC Adv., 2020, 10, 40244–40263.
6 (a) F. Sutanto, M. Konstantinidou and A. Dömling, RSC
Med. Chem., 2020, 11, 876–884; (b) M. Wang, Z. Dai and
X. Jiang, Nat. Commun., 2019, 10, 2661; (c) L. Kaysser, Nat.
Prod. Rep., 2019, 36, 1654–1686.
7 (a) C. De Risi, G. P. Pollini and V. Zanirato, Chem. Rev.,
2016, 116, 3241–3305; (b) A. Muthukumar, S. Sangeetha
and G. Sekar, Org. Biomol. Chem., 2018, 16, 7068–7083.
8 (a) J. Zhang, C. Liu, M. Yang, Z. Fang and K. Guo,
Tetrahedron Lett., 2020, 61, 152555; (b) A. K. Sharma,
A. Jaiswal and K. N. Singh, Org. Biomol. Chem., 2019, 17,
9348–9351; (c) A. Ragupathi, V. P. Charpe, A. Sagadevan
and K. C. Hwang, Adv. Synth. Catal., 2017, 359, 1138–1143;
(d) F. Liu, K. Zhang, Y. Liu, S. Chen, Y. Chen, D. Zhang,
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2021

View Article Online
Organic & Biomolecular Chemistry
Paper
C. Lin and B. Wang, RSC Adv., 2017, 7, 7158–7162; 19 X. Wang, R.-X. Chen, Z.-F. Wei, C.-Y. Zhang, H.-Y. Tu and
(e) Y. Kumar, M. Shaw, R. Thakur and A. Kumar, J. Org. A.-D. Zhang, J. Org. Chem., 2016, 81, 238–249.
Chem., 2016, 81, 6617–6625; (f) S. Guo, Z. Fang, Z. Yang, 20 H. Cha, K. Lee and D. Y. Chi, Tetrahedron, 2017, 73, 2878–
C. Liu, Z. Dai, L. Zhao and K. Guo, RSC Adv., 2016, 6, 1503– 2885.
1507; (g) W. Fan, D. Shi and B. Feng, Tetrahedron Lett., 21 H. Y. Choi and D. Y. Chi, Org. Lett., 2003, 5, 411–
2015, 56, 4638–4641; (h) S. S. Kotha, S. Chandrasekar, 414.
S. Sahu and G. Sekar, Eur. J. Org. Chem., 2014, 7451–7457; 22 (a) M. P. Sibi and M. Hasegawa, J. Am. Chem. Soc., 2007,
(i) J. K. Laha, K. V. Patel, K. S. S. Tummalapalli and
M. K. Hunjan, ACS Omega, 2018, 3, 8787–8793.
9 (a) C. Zhang and N. Jiao, J. Am. Chem. Soc., 2010, 132, 28–
29; (b) Y. Xiao, Z. Yi, X. Yu and F. Xiao, RSC Adv., 2020, 10,
29114–29118.
10 C. Zhang, Z. Xu, L. Zhang and N. Jiao, Angew. Chem., Int.
Ed., 2011, 50, 11088–11092.
11 F.-T. Du and J.-X. Ji, Chem. Sci., 2012, 3, 460–465.
12 C. Liu, Z. Yang, Y. Zeng, Z. Fang and K. Guo, Org. Chem.
Front., 2017, 4, 2375–2379.
129, 4124–4125; (b) N.-N. Bui, X.-H. Ho, S.-I. Mho and
H.-Y. Jang, Eur. J. Org. Chem., 2009, 5309–5312; (c) J. F. Van
Humbeck, S. P. Simonovich, R. R. Knowles and
D. W. C. MacMillan, J. Am. Chem. Soc., 2010, 132, 10012–
10014; (d) T. Kano, H. Mii and K. Maruoka, Angew. Chem.,
Int. Ed., 2010, 49, 6638–6641; (e) K. Akagawa, T. Fujiwara,
S. Sakamoto and K. Kudo, Org. Lett., 2010, 12, 1804–1807;
(f) K. Akagawa and K. Kudo, Org. Lett., 2011, 13, 3498–
3501; (g) S. P. Simonovich, J. F. Van Humbeck and
D. W. C. MacMillan, Chem. Sci., 2012, 3, 58–61;
(h) G. A. Abeykoon, S. Chatterjee and J. S. Chen, Org. Lett.,
2014, 16, 3248–3251.
13 P.-H. Li, Z. Wang, H. Fu, Q.-P. Dai and C.-W. Hu, Chem.
Commun., 2018, 54, 12471–12474.
14 X. Xu, W. Ding, Y. Lin and Q. Song, Org. Lett., 2015, 17, 23 A. de la Torre, D. Kaiser and N. Maulide, J. Am. Chem. Soc.,
516–519. 2017, 139, 6578–6581.
15 C. Zhang, P. Feng and N. Jiao, J. Am. Chem. Soc., 2013, 135, 24 C. Menendez, S. Gau, S. Ladeira, C. Lherbet and M. Baltas,
15257–15262. Eur. J. Org. Chem., 2012, 409–416.
16 X. Huang, X. Li, M. Zou, S. Song, C. Tang, Y. Yuan and 25 (a) K. Tokumasu, R. Yazaki and T. Ohshima, J. Am.
N. Jiao, J. Am. Chem. Soc., 2014, 136, 14858–14865.
17 C. Zhang, X. Zong, L. Zhang and N. Jiao, Org. Lett., 2012,
14, 3280–3283.
Chem. Soc., 2016, 138, 2664–2669; (b) S. Taninokuchi,
R. Yazaki and T. Ohshima, Org. Lett., 2017, 19, 3187–
3190.
18 A. S.-K. Tsang, A. Kapat and F. Schoenebeck, J. Am. Chem. 26 M. K. Foret, R. Lincoln, S. Do Carmo, A. C. Cuello and
Soc., 2016, 138, 518–526.
G. Cosa, Chem. Rev., 2020, 120, 12757–12787.
This journal is © The Royal Society of Chemistry 2021
Org. Biomol. Chem.