Asymmetric synthesis of tetrahydropyran[3,2-c]quinolinones via an organocatalyzed formal [3 + 3] annulation of quinolinones and MBH 2-naphthoates of nitroolefin

Article abstract of DOI:10.1016/j.cclet.2019.08.040

An efficient asymmetric and enantio-swithchable organocatalytic [3 + 3] annulation reaction using MBH-2-naphthoates of nitroalkenes and 4-hydroxyquinolin-2(1H)-ones has been developed. Densely substituted tetrahydropyrano[3,2-c]quinolinones scaffolds with two adjacent stereogenic centers are obtained with high yield (up to 95% yield) and good stereoselectivities (up to >20:1 dr and 96% ee) in an enantio-switchable manner. Furthermore, gram scale synthesis was achieved and the nitro group could easily transform into an amino group without any appreciable loss in the diastereo- and enantioselectivity.

Full text of DOI:10.1016/j.cclet.2019.08.040

Journal Pre-proof
Asymmetric synthesis of tetrahydropyran[3,2-c]quinolinones via an
organocatalyzed formal [3+3] annulation of quinolinones and MBH
2-naphthoates of nitroolefin
Jian Li, Qi-Long Hu, Xue-Ping Chen, Ke-Qiang Hou, Albert S.C.
Chan, Xiao-Feng Xiong
PII:
S1001-8417(19)30513-3
https://doi.org/10.1016/j.cclet.2019.08.040
CCLET 5191
DOI:
Reference:
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Chinese Chemical Letters
Received Date:
Revised Date:
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8 August 2019
23 August 2019
Please cite this article as: Li J, Hu Q-Long, Chen X-Ping, Hou K-Qiang, Chan ASC, Xiong
X-Feng, Asymmetric synthesis of tetrahydropyran[3,2-c]quinolinones via an organocatalyzed
formal [3+3] annulation of quinolinones and MBH 2-naphthoates of nitroolefin, Chinese
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Communication
Asymmetric synthesis of tetrahydropyran[3,2-c]quinolinones via an organocatalyzed
formal [3+3] annulation of quinolinones and MBH 2-naphthoates of nitroolefin
Jian Li, Qi-Long Hu, Xue-Ping Chen, Ke-Qiang Hou, Albert S. C. Chan*, Xiao-Feng Xiong
Guangdong Key Laboratory of Chiral Molecule and Drug Discovery, School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, China
 Corresponding authors.
E-mail addresses: chenxz3@mail.sysu.edu.cn (A. S. C. Chan), xiongxf7@mail.sysu.edu.cn (X.-F. Xiong)
Graphical Abstract
An efficient asymmetric and enantio-swithchable organo-catalytic construction of tetrahydropyrano[3,2-c]quinolinones scaffolds
were developed.
ARTICLE INFO
ABSTRACT
Article history:
Received 5 July 2019
Received in revised form 8 August 2019
Accepted 22 August 2019
Available online
An efficient asymmetric and enantio-swithchable organocatalytic [3+3] annulation reaction using
MBH-2-naphthoates of nitroalkenes and 4-hydroxyquinolin-2(1H)-ones has been developed.
Densely substituted tetrahydropyrano[3,2-c]quinolinones scaffolds with two adjacent stereogenic
centers are obtained with high yield (up to 95% yield) and good stereoselectivities (up to >20:1 dr
and 96% ee) in an enantio-switchable manner. Furthermore, gram scale synthesis was achieved
and the nitro group could easily transform into an amino group without any appreciable loss in the
diastereo- and enantioselectivity.
Keywords:
Organocatalysis
Asymmetric synthesis
Enantioswitchable
Cinchona alkaloid
Tetrahydropyran[3,2-c]quinolines
Quinolinone fused tetrahydropyran skeletons have received enormous attention as a ubiquitous structural unit in drugs and biologically
active compounds [1]. Being an important constituent of quinolinones fused tetrahydropyrans family, tetrahydropyran[3,2-
c]quinolinones is also an important structural motif presented in plenty of natural products or synthetic compounds displaying a wide
range of biological activities (Fig. 1) [2]. For example, natural product euodenine A was identified as an agonist of the human TLR4
receptor [3]. Zanthodioline was found to have strong anti-platelet aggregation activity in vitro [4]. Toddacoumalone isolated from
Toddalia asiatica was reported as an effective phosphodiesterase-4 inhibitor with an IC50 around 140 nmol/L [5]. Despite the importance
of those bioactive tetrahydropyran[3,2-c]quinolinone molecules, the enantioselective synthesis of these kind of heterocycles are rarely
reported. Therefore, the diverse biological profiles of these tetrahydropyran[3,2-c]quinolinones prompted us to develop a novel and
efficient catalytic method for the enantioselective synthesis of structurally diverse tetrahydropyran fused quinolinones.
Construction of six-membered heterocycles with adjacent stereogenic centers has gain extensive research interests in recent years,
especially by using the cycloaddition strategies. Among all the methods that have been developed, [3+3] annulation strategy was much
less explored compared with [4+2] strategies [6]. Several asymmetric synthetic strategies to construct the chiral tetrahydropyran skeletons
have been developed successfully in recent years [7]. Among all the strategies that have been reported, the organocatalyzed [3+3]
annulation behaved as efficient methods [8]. For example, R. Gurubrahamam and co-workers successfully employed sequential
organocatalyzed and DABCO-catalyzed olefin isomerization for asymmetric nucleophilic conjugate addition of hydroxycoumarins and
pyranone to branched nitro enynes to afford pyrano-annulated scaffolds [9]. The similar strategy was used by V. Modrocká and J. Zhang
———

et al. to construct pyrano[3,2-c]chromen-5-on products [10]. Besides, Y. Zheng et al. developed a bifunctional squaramide catalyzed
enantioselective formal [3+3] annulation reaction with pyrazolin-5-ones and nitroallylic acetates for the synthesis of
tetrahydropyrano[2,3-c]pyrazoles [11]. Despite few heterocycle fused tetrahydropyrans have been reported, most of them are oxygen-
containing heterocycle and nitrogen-containing fused tetrahydropyrans are rarely. More importantly, nitrogen-containing heterocyclic
compounds are more abundant in drugs and with great potential [12]. Thus, the construction of nitrogen- containing heterocycles fused
tetrahydropyrans is of highly importance.
Fig. 1. Some bioactive pyrano[3,2-c]quinolinones.
Based on these research above and previous work of our group, we observed that quinolinone 1 is a highly reactive nucleophile and
could undergo nucleophilic addition twice (Scheme 1) [13]. Considering that Morita-Baylis-Hillman (MBH) esters of nitroalkenes 2,
which commonly behaved as Michael acceptor, followed by a substitution reaction owing to the good leaving ability of the ester groups
[14]. Thus, we hypothesized that chiral tetrahydropyrano[3,2-c]quinolinones 3 could be conveniently constructed through an asymmetric
Michael addition of 1,3-binucleophilic quinolinones 1 to MBH esters of nitroalkenes 2, followed by the in situ intramolecular annulation
reaction. The stereochemistry of this process could be controlled by chiral organo-catalyst. Herein, we reported a cascade two steps
asymmetric organocatalytic Michael reaction from inter- to intra- stepwise to access tetrahydropyrano[3,2-c]quinolinones in good yield,
high diastereo- and enantioselectivities.
Scheme 1. Construction of tetrahydropyrano[3,2-c]quinolinone scaffolds via [3+3] annulation strategy.
The research was initiated by testing the model reaction of easily prepared 4-hydroxy-1-methylquinolin-2(1H)-one (1a) [15] and (E)-
2-nitro-3-phenylallyl benzoate (2a) [14a] in the presence of catalyst 5a (quinine) in PhCF3 (1 mL) at room temperature. To our delight,
the reaction ran smoothly to afford the desired tetrahydropyran fused quinolinone derivative 3a in 88% yield with 8:1 dr and -80% ee
(Table 1, entry 1). Several catalysts (Fig. 2), including H-bond donating bifunctional catalysts 5b and 5c, primary amine catalyst 5d and
other natural cinchona alkaloids 5e, 5f and 5g were examined with the purpose of improving the stereoselectivity of this reaction (Table
1, entries 2-7). 5e (quinidine) turned out to be the best catalyst for this reaction, as it gave the reaction in 90% yield 7:1 dr and 84% ee
(Table 1, entry 5). Perhaps not surprised, catalysts 5a and 5e gave the product 3a in the opposite configuration. Subsequently, we screened
different protecting groups of MBH esters 2 under room temperature with the aim of further improving the stereoselectivity by using
catalyst 5e (Table 1, entries 8-12). The reaction has good diastereo- and enantioselectivity with p-nitrobenzoyl as protecting group (6:1
dr, 90% ee), however low yield was observed (30%) (Table 1, entry 9). To our delight, a considerable improvement in both the yield
(85%) and dr (10:1) was achieved by replacing the protecting group with 2-naphthoyl, despite the slightly diminished ee value (88%)
(Table 1, entry 12). Other protecting groups like acetyl or 2,4,6-trimethylbenzoyl were either gave the reaction in poor enantioselectivity,
or in low yield (Table S1 in Supporting information). Taken yield, dr and ee values into consideration, we choose 2-naphthoyl as the
protecting group for further optimization. Next, solvents, reaction concentration, temperatures and time were screened by using catalyst
5e (Table 1, entries 13-18). Finally, the best reaction condition was established as performing the reaction in dichloromethane at 0 °C for
72 h, with the concentration of 0.2 mol/L, and gave the reaction in 73% yield, 9:1 dr and 92% ee (Table 1, entry 16). The molecular
structure and absolute configuration of the two chiral centers within (3S, 4R)-(-)-3a were unambiguously determined by X-ray
crystallography analysis (Table 1, Fig. S3 in Supporting information), and the crystallographic data has been deposited in cambridge
centre with CCDC 1912415.
Table 1
Optimization of the reaction conditions.a

Entry Cat.
R
Solvent
Yield
(%)b
88
39
80
42
90
80
83
97
30
39
<5
85
76
79
89
73
drc
ee
(%)c
-80
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
Bz
Bz
Bz
Bz
Bz
Bz
Bz
Ac
pNBz
Mes
1-Nap
2-Nap
2-Nap
2-Nap
2-Nap
2-Nap
2-Nap
2-Nap
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCH3
MeCN
CH2Cl2
CH2Cl2
CHCl3
DCE
8:1
>20:1 43
15:1
11:1
7:1
13:1
12:1
6:1
6:1
9:1
1.2:1
10:1
9:1
8:1
10:1
9:1
10:1
6:1
11
27
84
70
-71
42
90
18
55
88
87
63
89
92
91
91
7
8
9
5g
5e
5e
5e
5e
5e
5e
5e
5e
5e
5e
5e
10
11
12
13
14
15
16d
17d
18d
67
57
pNBz: p-nitrobenzoyl, 1-Nap: 1-naphthoyl; 2-Nap: 2-naphthoyl.
a Unless otherwise specified, all reactions were carried out with 1a (0.10 mmol), 2 (0.20 mmol) in the presence of 10 mol% of catalyst in solvent
(1 mL) at room temperature.
b Isolated yield.
c Determined by HPLC analysis using a chiral stationary phase.
d The reaction was performed at 0 °C for 72 h and 0.5 mL of solvent was loaded.
Fig. 2. Catalysts investigated in the reaction.
After establishing the optimized reaction conditions, we set out to explore the substrate scope of this asymmetric organocatalyzed
reaction to provide the generality of this method by using catalyst 5e (Table 2). First, a variety of quinolinones 1 (Fig. S1 in Supporting
information) were tested. The reaction worked well with quinolinones bearing different N-substituent groups (3b, 3c), both phenyl and
benzyl substituted quinolinones gave the reaction in good results (Table 2, entries 1-2). When quinolinones have either electron-
withdrawing substituents, or electron-donating substituents, or substituents on different positions on the benzene ring, they were well
tolerated and delivered the corresponding cyclized products 3d-3f and 3g-3l in moderate to good yields (49%−95%) and high
stereoselectivities (6:1− >20:1 dr, 87−96% ee) (Table 2, entries 3-11). However, the reaction would not occur when the quinolinone
without an N-substituent group, which probably due to this structure prone to form an aromatic system and reduce the nucleophilicity.
We next investigated the versatility of MBH esters 2 (Fig. S2 in Supporting information) in this asymmetric reaction. We examined the
electronic effect of the substituents on the benzene ring of the MBH esters 2. Generally, aromatic rings bearing electron-withdrawing or
electron donating groups were turned out to be good candidates for the reaction, and gave the reaction in up to 95% yield and 95% ee
(3m-3p, 3r-3t) (Table 2, entries 12-15, 17-19).
Table 2
Substrate scope of the asymmetic [3+3] annulation reaction.a
Entry 3 (R1, R2, Ar)
Yield
(%)b
95
72
50
49
71
63
drc
ee
(%)c
94
92
87
92
94
93
1
3b (H, Ph, Ph)
12:1
9:1
8:1
10:1
>20:1
7:1
2
3c (H, Bn, Ph)
3d
4
3d (6-F, Bn, Ph)
3e (6-Cl, Bn, Ph)
3f (6-CF3, Bn, Ph)
3g (8-MeO, Me, Ph)
5
6

7
8
9
3h (6-Me, Bn, Ph)
3i (6-iPr, Bn, Ph)
3j (6-tBu, Bn, Ph)
3k (6-Ph, Bn, Ph)
3l (6,7-(MeO)2, Bn, Ph)
3m (H, Ph, 4-FPh)
3n (H, Ph, 4-ClPh)
3o (H, Ph, 4-MePh)
3p (H, Ph, 4-MeOPh)
3q (H, Ph, 2-Napth)
3r (6-tBu, Bn, 2-FPh)
3s (6-tBu, Bn, 2-MeOPh)
3t (6-tBu, Bn, 3-MePh)
3u (6-tBu, Bn, 2-Furyl)
3v (6-tBu, Bn, (E)-styryl)
56
89
88
95
65
95
74
93
82
95
69
57
86
69
33
11:1
6:1
>20:1
11:1
7:1
>20:1
>20:1
10:1
10:1
15:1
13:1
>20:1
16:1
11:1
7:1
94
95
96
94
89
93
91
91
91
91
92
92
95
69
57
10
11d
12
13
14
15
16
17
18
19
20
21
a Unless noted otherwise, the reactions were carried out with 1 (0.10 mmol), 2 (0.20 mmol) in the presence of catalyst 5e in DCM (0.5 mL) at
0 °C for 72 h.
b Isolated yield.
c Determined by HPLC analysis using a chiral stationary phase.
d Reaction temperature: room temperature.
We noticed that substrates 2 with either meta- or para-substituents on the benzene ring gave more satisfactory yields than those with
ortho-substituents, probably due to the higher steric hindrance arised from the ortho-substituents. MBH esters 2 derived from 2-naphthyl
nitroalkene turned out to be a suitable substrate and delivered the desired cyclic product 3q in high yield and good ee, whereas 2-furyl
nitroalkene derived MBH esters only gave moderate yield and ee (Table 2, entries 16 and 20). Interestingly, (E)-styryl substituted MBH
benzoate could react with quinolinone to give cyclic adduct 3v, albeit the low yield and moderate enantioselectivity (Table 2, entry 21).
However, the reaction would not occur when MBH 2-naphthoates of aliphatic nitroolefins was used, which may due to the low reactivity
of aliphatic nitroolefins. Moreover, other different heterocycles that contain the 1, 3-dicarbonyl structure are also applicable with similar
results, gave 3w and 3x in good yield and high ee. Bicyclic product 3y was obtained in low yield, which might be attributable to the low
nucleophilicity of the corresponding 1, 3-dicarbonyl compound 1o (Fig. S2).
As we have observed that catalyst 5a gave the product in opposite configuration compared with 5e, we also briefly examined the
substrate scope of the 5a catalyzed asymmetic [3+3] annulation reaction to further explore the generality of this asymmetric reaction. As
shown in Fig. 3, this reaction exhibited good substrate compatibility. Quinolinone analog 1m reacted with MBH ester 2b to give 4a in
good yield, excellent diastereoselectivity and enantioselectivitiy. Quinolinone bearing a trifluoromethyl or tert-butyl group on the
aromatic ring also participate in the reaction well, and gave the corresponding adducts 4b-4e up to 91% yield and 95% ee (Fig. 3). Thus,
by employing different catalysts, we could efficiently prepare a variety of chiral tetrahydropyrano[3,2-c]quinolinones and their
enantiomers.
Fig. 3. Asymmetic [3+3] annulation reaction catalyzed by 5a under the optimized conditions. The yield is isolated yield. The ee value and dr are
determined by HPLC analysis using a chiral stationary phase.
To illustrate the reaction mechanism conveniently, a plausible mechanism was proposed. As show in Scheme 2, intermediate A could
be formed via a hydrogen-bonding interaction between the catalyst quinidine and substrate 1a. Then intermediate B was formed in a
similar way and the reaction enantioselectivity was controlled in this process. Then inter-molecular Michael addition and the recovering
of the catalyst were occur, which resulted in the formation of intermediate C. Finally, the desired annulation product 3a was formed via
an intramolecular annulation reaction.

Scheme 2. Proposed mechanism.
Scheme 3. Scale up reaction and transformation study.
Finally, in order to demonstrate the potential practical utility of our method, we performed the reaction with 2.5 mmol of starting
material 1j and reduced the equivalent of 2b to 1.5 equiv. By simply extended the reaction time, the reaction ran smoothly and delivered
1.03 g of tetrahydropyran[3,2-c]quinolinone 3j with a 90% yield, >20:1 dr and 96% ee. The result was almost identical to the reaction
carried out in small scale, indicating that the reaction could be applied to prepare chiral building blocks in a large amount. In addition,
the nitro group can easily transform into amino group under the mild reaction condition without any appreciable loss in the diastereo-
and enantioselectivity, suggesting an additional potential utility of this chiral scaffold (Scheme 3).
In summary, we developed an organocatalyzed asymmetric formal [3+3] annulation of 4-hydroxyquinoline-2(1H)-one and MBH 2-
naphthoates of nitroolefins catalyzed by natural cinchona alkaloids. The reactions run smoothly to provide a variety of
tetrahydropyrano[3,2-c]quinolinones and analogs with good yields (up to 95%), high diastereo- and enantioselectivities (up to >20:1 dr,
96% ee) under the optimized reaction condition. By employing different catalysts, the reactions could be performed in an enantio-
switchable manner and gave both enantiomers in good yield and excellent enantioselectvities, respectively. In addition, this reaction
could be carried out in a gram scale, which has potential practical utility. Using this synthetic strategy in preparing bioactive compounds
are currently underway in our lab.
Acknowledgment
We are grateful for the financial support from the National Natural Science Foundation of China (No. 81602972), Guangdong Natural
Science Funds for Distinguished Young Scholar (No. 2018B030306017), Guangdong Province Universities and Colleges Pearl River
Scholar Funded Scheme (2018).
References
[1] (a) T. Shiro, T. Fukaya, M. Tobe, Eur. J. Med. Chem. 97 (2015) 397-408.
(b) B. Liu, F. Li, T. Zhou, X.Q. Tang, G.W. Hu, J. Heterocycl. Chem. 55 (2018) 1863-1873.
(c) T. Yasumoto, Chem. Rec. 1 (2001) 228-242.
(d) A. Bermejo, B. Figadère, M.C. Zafra-Polo, et al., Nat. Prod. Rep. 22 (2005) 269-303.
(e) M. Murata, T. Yasumoto, Nat. Prod. Rep. 17 (2000) 293-314.
[2] (a) V. Nadaraj, S. Thamarai Selvi, H. Pricilla Bai, S. Mohan, T. Daniel Thangadurai, Med. Chem. Res. 21 (2011) 2902-2910.

(b) Z.D. Yang, D.B. Zhang, J. Ren, M.J. Yang, Med. Chem. Res. 21 (2012) 722-725.
[3] J.E. Neve, H.P. Wijesekera, S. Duffy, et al., J. Med. Chem. 57 (2014) 1252-1275.
[4] I.S. Chen, I.W. Tsai, C.M. Teng, et al., Phytochemistry 46 (1997) 525-529.
[5] T.T. Lin, Y.Y. Huang, G.H. Tang, et al., J. Nat. Prod. 77 (2014) 955-962.
[6] (a) X.X. Sun, H.H. Zhang, G.H. Li, Y.Y. He, F. Shi. Chem. -Eur. J. 22 (2016) 17526-17532.
(b) Z.Q. Zhu, L. Yu, M. Sun, G.J. Mei, F. Shi, Adv. Synth. Cat. 360 (2018) 3109-3116.
(c) X.X. Sun, C. Li, Y.Y. He, et al., Adv. Synth. Cat. 359 (2017) 2660-2670.
(d) W. Dai, H. Lu, F. Shi, S.J. Tu, Chem. -Eur. J. 20 (2014) 11382-11389.
(e) X. Yang, Y.C. Zhang, Q.N. Zhu, M.S. Tu, F. Shi, J. Org. Chem. 81 (2016) 5056-5065.
(f) F. Shi, R.Y. Zhu, W. Dai, C.S. Wang, S.J. Tu, Chem. -Eur. J. 20 (2014) 2597-2604.
[7] (a) A. Temperini, A. Barattucci, P.M. Bonaccorsi, O. Rosati, L. Minuti, J. Org. Chem. 80 (2015) 8102-8112.
(b) C. Bosset, P. Angibaud, I. Stanfield, et al., J. Org. Chem. 80 (2015) 12509-12525.
(c) T.J. Potter, J.A. Ellman, Org. Lett. 18 (2016) 3838-3841.
(d) J. Scoccia, S.J. Pérez, V. Sinka, et al., Org. Lett. 19 (2017) 4834-4837.
(e) Y. Kurimoto, T. Nasu, Y. Fujii, K. Asano, S. Matsubara, Org. Lett. 21 (2019) 2156-2160.
(f) X. Xie, C. Peng, G. He, et al., Chem. Commun. 48 (2012) 10487-10489.
[8] (a) J.Y. Liu, X.C. Yang, H. Lu, Y.C. Gu, P.F. Xu, Org. Lett. 20 (2018) 2190-2194.
(b) D.K. Nair, R.F.S. Menna-Barreto, E.N. da Silva Júnior, S.M. Mobin, I.N.N. Namboothiri, Chem. Commun. 50 (2014) 6973-6976.
(c) R. Chen, X. Fan, J. Gong, Z. He, Asian J. Org. Chem. 3 (2014) 877-885.
(d) C.Y. Yu, M. Yaqub, Y.M. Jia, Z.T. Huang, Synlett 9 (2008) 1357-1360.
(e) W. Xiao, X. Yin, Z. Zhou, W. Du, Y.C. Chen, Org. Lett. 18 (2015) 116-119.
(f) S. Chandrasekhar, K. Mallikarjun, G. Pavankumarreddy, K.V. Rao, B. Jagadeesh, Chem. Commun. (2009) 4985-4987.
(g) B. Han, Y.C. Xiao, Z.Q. He, Y.C. Chen, Org. Lett. 11 (2009) 4660-4663.
(h) M.L. Shi, G. Zhan, S.L. Zhou, W. Du, Y.C. Chen, Org. Lett. 18 (2016) 6480-6483.
(i) L. Yang, W. Huang, X.H. He, et al., Adv. Synth. Cat. 358 (2016) 2970-2975.
[9] R. Gurubrahamam, B.F. Gao, Y.M. Chen, et al., Org. Lett. 18 (2016) 3098-3101.
[10] (a) V. Modrocká, E. Veverková, M. Mečiarová, R. Šebesta, J. Org. Chem. 83 (2018) 13111-13120.
(b) J. Zhang, G. Yin, Y. Du, et al., J. Org. Chem. 82 (2017) 13594-13601.
[11] Y. Zheng, L. Cui, Y. Wang, Z. Zhou, J. Org. Chem. 81 (2016) 4340-4346.
[12] E. Vitaku, D.T. Smith, J.T. Njardarson, J. Med. Chem. 57 (2014) 10257-10274
[13] (a) V. Nair, P.M. Treesa, C.N. Jayan, et al., Tetrahedron 57 (2001) 7711-7717.
(b) A. Klásek, O. Rudolf, M. Rouchal, A. Lyčka, A. Růžička, Tetrahedron 69 (2013) 492-499.
(c) Y. Gu, J. Barrault, F. Jérôme, Adv. Synth. Cat. 351 (2009) 3269-3278.
(d) M. Rueping, E. Merino, M. Bolte, Org. Biomol. Chem. 10 (2012) 6201-6210.
(e) Y.R. Lee, B.S. Kim, H.I. Kweon, Tetrahedron 56 (2000) 3867-3874.
(f) V. Nair, P.M. Treesa, C.N. Jayan, et al., Tetrahedron 57 (2001) 7711-7717.
(g) Y.R. Lee, H.I. Kweon, W.S. Koh, et al., Synthesis-Stuttgart (2001) 1851-1855.
(h) X. Zhu, A. Lin, Y. Shi, et al., Org. Lett. 13 (2011) 4382-4385.
[14] (a) W. Xiao, X. Yin, Z. Zhou, W. Du, Y.C. Chen, Org. Lett. 18 (2016) 116-119.
(b) L.F. Yeh, S. Anwar, K. Chen, Tetrahedron 68 (2012) 7317-7321.
(c) D.K. Nair, S.M. Mobin, I.N.N. Namboothiri, Tetrahedron Lett. 53 (2012) 3349-3352.
[15] A. Dey, A. Hajra, Org. Biomol. Chem. 15 (2017) 8084-8090.