Stereoselective Synthesis of cis-2,5-Disubstituted Pyrrolidines via Copper-Catalyzed Cyclization of Alkenes

Article abstract of DOI:10.1002/cjoc.201600380

An efficient methodology for the stereoselective synthesis of cis-2,5-disubstituted pyrrolidines using copper catalyst was developed. The corresponding cis-2,5-disubstituted pyrrolidines could be obtained in reasonable yields and with good stereoselectivities in the presence of 4,4′-di-tert-butyl-2,2′-bipyridine as ligand and 1-methyl-2-pyrrolidinone as solvent.

Full text of DOI:10.1002/cjoc.201600380

COMMUNICATION
DOI: 10.1002/cjoc.201600380
Stereoselective Synthesis of cis-2,5-Disubstituted Pyrrolidines
via Copper-Catalyzed Cyclization of Alkenes
Sai-Hu Cai,a Bing-Chao Da,a Jia-Hui Zhou,a Yun-He Xu,*,a and Teck-Peng Loh*,a,b
a Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China
b Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, 637371, Singapore
An efficient methodology for the stereoselective synthesis of cis-2,5-disubstituted pyrrolidines using copper
catalyst was developed. The corresponding cis-2,5-disubstituted pyrrolidines could be obtained in reasonable yields
and with good stereoselectivities in the presence of 4,4'-di-tert-butyl-2,2'-bipyridine as ligand and 1-methyl-2-pyrro-
lidinone as solvent.
Keywords stereoselectivity, pyrrolidines, copper-catalyzed, cyclization
Introduction
is highly desirable.
Nitrogen-containing heterocyclic motifs such as
pyrrolidines are widely observed as core fragments in
many natural products and pharmaceutical molecules.[1]
Furthermore, this type of compounds are used as im-
portant building blocks in alkaloid synthesis.[2] Accord-
ingly, the development of efficient methods for the con-
struction of these compounds has attracted much atten-
tion. Among the developed methods, both amination
reaction via utilization of nucleophilicity of amino
groups[3] as well as reaction between electrophilic ami-
no and C-nucleophiles[4] have been well explored. Nev-
ertheless, the development of rapid and efficient meth-
ods to access these compounds leaves room for more
exploration. During the past few decades, the employ-
ment of N-centered radicals (NCRs) as intermediate for
the synthesis of azaheterocycles has proven to be a very
useful method.[5] Narasaka and co-workers have succes-
sively developed elegant works on transition-metal-free
or transition-metal-catalyzed cyclization reactions of
oxime and its derivatives through which various
3,4-dihydro-2H-pyrrole and tetrahydroquinoline proucts
could be obtained.[6,7] In 2010, a copper-catalyzed cy-
clization reaction of N-alkenyl and alkynyl N-benzoyl
oxysulfonamides was also reported by the same group.[8]
The corresponding 2,5-disubstituted pyrrolidines could
be obtained in good yields albeit with poor stereoselec-
tivities in most cases. It is noteworthy that the
2,5-cis-disubstituted pyrrolidines are also important
components in organic synthesis.[9] Therefore, the de-
velopment of new method to access 2,5-cis-disubsti-
tuted pyrrolidines in a rapid and stereoselective fashion
Scheme 1 Synthesis of 2,5-disubstituted pyrrolidine products
Results and Discussion
Firstly, we chose N-4-alkenyl-N-pivalicoxysulfona-
mide (1a) as the model substrate to optimize the reac-
tion conditions. The reaction was examined under the
previously reported reaction conditions, the desired cy-
clization product could be obtained in 82% yield but
with poor stereoselectivity (cis∶trans＝40∶60).[8]
Therefore, further studies to improve the product’s ste-
reoselectivity were carried out. Different copper cata-
lysts were tested (Table 1, Entries 2 and 3) and it was
found that the copper(I) catalysts could slightly improve
the product’s stereoselectivity along with low to moder-
ate yield. Different copper(II) salts as catalyst precur-
sors were tested in this reaction. To our delights, the
product could be obtained with good stereoselectivity by
using CuCl2 as catalyst and NMP as solvent (Table 1,
Entries 7). In the previous work,[8] Narasaka proposed
the intermediacy of a carbo-copper species in the cy-
clization process of N-acyloxy N-alkenyl sulfonamide or
carboxamide derivatives. Therefore, different binitrogen
containing compounds were screened as ligands in this
reaction (Table 1, Entries 8－12). Delightfully, it was
*
E-mail: xyh0709@ustc.edu.cn; teckpeng@ntu.edu.sg; Tel.: 0086-0065-65138203; Fax: 0086-0065-67911961
Received June 20, 2016; accepted September 20, 2016; published online November 14, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600380 or from the author.
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Chin. J. Chem. 2016, 34, 1076—1080

Stereoselective Synthesis of cis-2,5-Disubstituted Pyrrolidines
Scheme 2 Optimization of nitrogen radical precursorsa
found that the desired product could be obtained in
moderate yield and with good stereoselectivity when
dtbbpy (4,4'-di-tert-butyl-2,2'-bipyridine) was used as
ligand. Using other Cu(II) salts led to slightly decreased
stereoselectivity of the product. Control experiment
showed that no desired product could be formed in the
absence of copper catalyst (Table 1, Entry 16).
Table 1 Optimization of reaction conditiona
a Reaction conditions: A mixture of 1 (0.4 mmol, 1 equiv.), CuCl2
Ms
Ms
OCO -Bu
t
(10 mol%), dtbbpy (20 mol%) in NMP (10 mL) was stirred for 4
N
N
[Cu]
b
c
h at 153 ℃ under N2 atmosphere. Isolated yield. Determined
by crude 1H NMR.
2a
1a
Therefore, different substrates protected with pivalic
group were subjected to this cyclization reaction. It was
found that various aliphatic and aryl substituents at the
R1 position could be well tolerated, and the correspond-
ing products could be obtained in moderate to good
yields (32%－76%) and with high stereoselectivity.
Moreover, it was noticed that the stereoselectivity of the
product was less affected by the R1 group regardless of
its steric and electronic effect. However, the yields of
the products were significantly affected by the R1 group.
Further screening found that the R2 group exerts critical
control over the product’s stereoselectivity. When R2
position was replaced by an ester group, the desired
product could be formed in moderate yield and with
reasonable stereoselectivity (Table 2, 2n). Unfortunately,
the corresponding product was obtained with loss of
stereoselectivity when R2 was a phenyl substituent (Ta-
ble 2, 2o). The relative configurations of cis-2k and
trans-2h were confirmed by single-crystal X-ray dif-
fraction analysis as shown in Figure 1.
Yieldb/%
(cis/transc)
Entry [Cu]/mol%
Ligand/mol%
1d,f
2d
3d
4e,f
5f
(CuOTf)2•C6H6
－
80 (40/60)
25 (59/41)
53 (59/41)
77 (69/31)
70 (79/21)
82 (78/22)
69 (86/14)
58 (86/14)
45 (89/11)
Trace
CuCl
－
CuBr
－
(CuOTf)2•C6H6
(CuOTf)2•C6H6
(CuOTf)2•C6H6
CuCl2
－
－
6
－
7
－
8
CuCl2
TMEDA
2,2'-Dipyridine
2,2'-Biquinoline
Dtbbpy
9
CuCl2
10
11
12
13
14
15
16
CuCl2
CuCl2
67 (92/8)
61 (88/12)
54 (88/12)
58 (87/13)
55 (85/15)
n.r.
CuCl2
Di(2-pyridyl)ketone
CuF2
Dtbbpy
Dtbbpy
Dtbbpy
－
CuBr2
Cu(OTf)2
－
a Reaction conditions: A mixture of 1a (0.4 mmol, 1 equiv.), [Cu]
(10 mol%), ligand (20 mol%) in NMP (10 mL) was stirred for 4 h
at 153 ℃ under N2 atmosphere. b Isolated yield. c Determined by
1
e
crude H NMR. d Run in DCE at 83 ℃. Run in DMF. f [Cu] (20
mol%). dtbbpy ＝ 4,4'-di-tert-butyl-2,2'-bipyridyl. TMEDA ＝
N,N,N',N'-tetra-methylethylenediamine.
Figure 1 X-ray structures of cis-2k and trans-2h with ellipsoids
Furthermore, different ester groups tethered on ni-
trogen atom were investigated. It was found that the
bulky pivalic group favored the formation of the desired
product in good yield and with high stereoselectivity.
at 50% probability.
To investigate the detailed effect of R2 substituent on
the product’s yield and stereoselectivity, the isomers of
Chin. J. Chem. 2016, 34, 1076—1080
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Cai et al.
Scheme 3 Cyclization of mono-substituted terminal olefins
COMMUNICATION
Table 2 Screening of different copper-catalyzed cyclization of
alkenesa,b,c
OPiv
Ms
Me
Ms
N
CuCl2 (10 mol %)
dtbbpy (20 mol %)
N
Me
(1)
NMP (10 mL)
153 °C, N2, 4 h
1p
2p 62 (50/50)
CuCl2 (10 mol %)
dtbbpy (20 mol %)
Ms
Ms
N
OPiv
Ms
N
Ms
N
Ms
N
C6H13
(2)
i-Pr
i-Pr
i-Pr
N
NMP (10 mL)
153 °C, N2, 4 h
C5H11
Me
Et
n
-Bu
1q
2q 22 (67/33 )
2c 76 (88/12)
2b 73 (94/6)
2a 67 (92/8)
CuCl2 (10 mol %)
dtbbpy (20 mol %)
C5H11
Ms
N
Ms
OPiv
N
Ms
N
Ms
N
Ms
N
C6H13
(3)
i-Pr
NMP (10 mL)
153 °C, N2, 4 h
Cy
i-Pr
1r
2r 20 (60/40 )
2d 48 (95/5)d
2e 54 (94/6)d
2f 77 (85/15)d
stituted pyrrolidine was used as the starting material to
couple with tert-butyl acrylate, the direct cross-coupling
product 3c could be obtained in 56% yield in the pres-
ence of palladium(II) acetate.[13]
Ms
N
Ms
N
Ms
N
Ph
t-Bu
i-Pr
2h 32 (97/3)d
2i 59 (83/17)d
2g 67 (96/4)d
Scheme 4 Transformation of cis-2a
Ms
Ms
Ms
N
MeO
1) O3, DCM, -78 °C
20 min
2) PPh3, 25 °C, 1 h
N
Ms
N
Ms
N
N
Ph(H2C)2
Bn
Me
Me
Me
(4)
(5)
2l 51 (89/11)d
O
2j 54 (84/16)d
2k 40 (85/15)d
96%
2a
3a
Ms
N
Ms
Ms
Ph
N
COOEt
Ms
Ms
N
1) H3B·SMe/THF
2) H2O2, NaOH
62%, dr = 55/45
N
Me
Me
N
Me
OH
O
2n 54 (72/28)d,e
2o 73 (52/48)d,f
2m 46 (86/14)d
3b
2a
a See the Supporting Information for the detailed reaction condi-
tion. b Isolated yield. c Determined by crude 1HNMR. d Run for 10
tert-butyl acrylate (5 equiv.)
Pd(OAc)2 (40 mol%)
AgOAc (2 equiv.)
Ms
N
f
h. e E-Configuration substrate was used. E/Z (5/1) mixture sub-
Me
strate was used.
DCE (2.5 mL)
110 °C, 24 h, Ar
56% (E/Z = 71/29)
2a
internal alkenes 1q and 1r were prepared and subjected
to this cyclization reaction. It was found that both E-
and Z-isomer afforded a same reductive cyclization
product 2q in low yield and with poor stereoselectivity.
Therefore, it could be concluded that the tuning of R2
substituent is important for improving the product’s
yield and stereoselectivity. On the other hand, R1 group
has less effect on the product’s yield. When the
mono-substituted terminal alkene 1p was subjected for
this cyclization reaction, the product 2p could be ob-
tained in 62% yield via allylic C－H bond functionali-
zation.[10]
Finally, the feasibility of the 2,5-cis-pyrrolidine
compounds for further derivation was tested. The
2-acetyl substituted cis-pyrrolidine product 3a could be
obtained in excellent yield via ozonolysis reaction.[11]
Additionally, the terminal double bond in the pyrrolidine
molecule could be transformed into the alcohol group
through hydroboration with BH3•SMe2 and subsequent
oxidation with H2O2.[12] When the terminal alkene sub-
Ms
N
Me
COOt-Bu (6)
3c
To gain mechanistic insights into this catalytic sys-
tem, radical trapping experiments were performed. It
was found that when the N-4-alkenyl-N-pivalicoxy sul-
fonamide 1a was subjected to this cyclization reaction
in the presence of TEMPO under optimized reaction
conditions (Scheme 5, Eq. 7), the yield of the desired
product was not affected but with complete loss of ste-
reoselectivity. Furthermore, 1,4-cyclohexadiene (CHD)
was introduced to trap the possible radicals generated in
situ. However, neither product 2a' or 2a'' was detected
after the reaction along with the isolation of 2a in 69%
yield (cis/trans＝86/14) (Scheme 5, Eq. 8). Finally, the
substrate 1s with a cyclopropyl substituent was prepared
and subjected to this reaction, the ring opened product
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© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, 34, 1076—1080

Stereoselective Synthesis of cis-2,5-Disubstituted Pyrrolidines
2s’ was detected only in 19% NMR yield (Scheme 5, Eq.
9). These results showed that the stereoselectivity-de-
termining step occurred mainly via aza-cupration of the
alkene.
state IV. After oxidation by Cu(II) or Cu(III), a carbo-
cation intermediate VII could be formed which leads to
the desired product after releasing a proton.
Conclusions
Scheme 5 Mechanism study
In conclusion, we have developed a copper catalytic
system that mediates the cyclization of N-4-alkenyl-N-
pivalicoxysulfonamide derivatives. Under the optimized
reaction conditions, the corresponding 2,5-cis-pyrroli-
dine products could be obtained in synthetically useful
yields and with high seteroselectivities. This methodol-
ogy provides a practical tool for the facile synthesis of
various 2,5-disubstituted pyrrolidine compounds with
cis-configuration. Currently, further studies on the ap-
plication of this protocol in natural product synthesis are
in progress in our laboratory.
CuCl2 (10 mol%)
dtbbpy (20 mol%)
Ms
N
OPiv
Ms
N
+
(7)
TEMPO
NMP (10 mL)
153 °C, N2, 4 h
2
1a
0.4 mmol, 1 equiv.
1.5 equiv. TEMPO 73%, cis/trans = 51/49 2a
3.0 equiv. TEMPO 77%, cis/trans = 50/50 2a
Ms
N
H
CuCl2 (10 mol%)
dtbbpy (20 mol%)
CHD (1.5 equiv.)
Ms
N
OPiv
Ms
2a'
not detected
N
+
(8)
2
NMP (10 mL)
1a
153 °C, N2, 4 h
Ms
2a
NH
0.4 mmol, 1 equiv.
69%, cis/trans = 86/14
Acknowledgement
2
2a''
not detected
We gratefully acknowledge the funding support of
the State Key Program of National Natural Science
Foundation of China (No. 21432009) and the National
Natural Science Foundation of China (No. 21372210).
The authors are grateful to Miss Jun Kee Cheng for her
careful proofreading of the final manuscript.
CuCl2 (10 mol%)
dtbbpy (20 mol%)
Ms
N
OPiv
Ms
Ms
N
N
(9)
+
NMP (10 mL)
2
153 °C, N2, 4 h
2s trace
no substrate recovered
1s (E/Z 70/30)
2s' 19%, cis/trans = 50/50
0.4 mmol, 1 equiv.
1H NMR yield
References
[1] For selected reviews, see: (a) Sardina, F. J.; Rapoport, H. Chem. Rev.
1996, 96, 1825; (b) O’Hagan, D. Nat. Prod. Rep. 2000, 17, 435; (c)
Liddell, J. R. Nat. Prod. Rep. 2002, 19, 773; (d) Han, M.-Y.; Jia,
J.-Y.; Wang, W. Tetrahedron 2014, 68, 784.
[2] (a) Tuo, S.; Liu, X.; Huang, P. Chin. J. Chem. 2013, 31, 55; (b)
Reddy, P. V.; Smith, J.; Kamath, A.; Jamet, H.; Veyron, A.; Koos, P.;
Philouze, C.; Greene, A. E.; Delair, P. J. Org. Chem. 2013, 78, 4840;
(c) Savaspun, K.; Au, C. W. G.; Pyne, S. G. J. Org. Chem. 2014, 79,
4569; (d) Aracil, J. M.; Nájera, C.; Castelló, L. M.; Sansano, J. M.;
Larrañaga, O.; Cózar, A. D.; Cossío, F. P. Tetrahedron 2015, 71,
9645.
Therefore, a plausible mechanism for this cop-
per-catalyzed cyclization reaction was proposed (Figure
2). Firstly, the copper(I) catalyst generated in situ[7d]
reacts with substrate II and forms an aza-cupration in-
termediate III in majority (A radical species III' can not
be completely excluded in this step). Subsequently, this
intermediate can react with the double bond to generate
two possible intermediates V and VI via a transition
[3] Reviews, see: (a) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37,
673; (b) Huang, L.-B.; Arndt, M.; Gooßen, K.; Heydt, H.; Gooßen, L.
J. Chem. Rev. 2015, 115, 2596. For selected articles, see: (c) Shao,
Z.-H.; Peng, F.-Z.; Zhu, B.-K.; Tu, Y.-Q.; Zhang, H.-B. Chin. J.
Chem. 2004, 22, 727; (d) Liu, G.-S.; Stahl, S. S. J. Am. Chem. Soc.
2007, 129, 6328; (e) Liu, Z.; Hartwig, J. F. J. Am. Chem. Soc. 2008,
130, 1570; (f) Weinstein, A. B.; Stahl, S. S. Angew. Chem., Int. Ed.
2012, 51, 11505.
[4] Selected recent examples: (a) Carson, C. A.; Kerr, M. A. J. Org.
Chem. 2005, 70, 8242; (b) Kang, Y.-B.; Tang, Y.; Sun, X.-L. Org.
Biomol. Chem. 2006, 4, 299; (c) Saito, S.; Tsubogo, T.; Kobayashi,
S. J. Am. Chem. Soc. 2007, 129, 5364; (d) Zeng, W.; Chen, G.-Y.;
Zhou, Y.-G.; Li, Y.-X. J. Am. Chem. Soc. 2007, 129, 750; (e)
Fukuzawa, S.-I.; Oki, H. Org. Lett. 2008, 10, 1747; (f) Oura, I.;
Shimizu, K.; Ogata, K.; Fukuzawa, S.-I. Org. Lett. 2010, 12, 1752;
(g) Trost, B. M.; Silverman, S. M. J. Am. Chem. Soc. 2012, 134,
4941.
[5] For reviews, see: (a) Neale, R. S. Synthesis 1971, 1; (b) Zard, S. Z.
Chem. Soc. Rev. 2008, 37, 1603; (c) Hofling, S. B.; Heinrich, M. R.
Synthesis 2011, 173; (d) Minozzi, M.; Nanni, D.; Spagnolo, P. Chem.
Eur. J. 2009, 15, 7830; (e) Chiba, S. Chimia 2012, 66, 377; (f) Sire,
B. Q.; Zard, S. Z. Beilstein J. Org. Chem. 2013, 9, 557. For selected
examples of NCRs in organic synthesis, see: (g) Cassayre, J.; Gag-
Figure 2 Plausible reaction mechanism.
Chin. J. Chem. 2016, 34, 1076—1080
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1079

Cai et al.
COMMUNICATION
osz, F.; Zard, S. Z. Angew. Chem., Int. Ed. 2002, 41, 1783; (h) Sharp,
L.; Zard, S. Z. Org. Lett. 2006, 8, 831; (i) Greulich, T. W.; Daniliuc,
C. G.; Studer, A. Org. Lett. 2015, 17, 254; (j) Qin, Q.; Yu, S.-Y. Org.
Lett. 2015, 17, 1894.
S. W.; Baeckvall, J. E. J. Org. Chem. 1995, 60, 6091; (d) Thanh, G.
V.; Célérier, J. P.; Fleurant, A.; Grandjean, C.; Rosset, S.; Lhommet,
G. Heterocycles 1996, 43, 1381; (e) Katritzky, A. R.; Cui, X.-L.;
Yang, B.-Z.; Steel, P. J. J. Org. Chem. 1999, 64, 1979; (f) Gribkov,
D. V.; Hultzsch, K. C. Angew. Chem., Int. Ed. 2004, 43, 5542.
[10] Selective recent examples on allylic amination reactions, please see:
(a) Fraunhoffer, K. J.; White, M. C. J. Am. Chem. Soc. 2007, 129,
7274; (b) Read, S. A.; White, M. C. J. Am. Chem. Soc. 2008, 130,
3316; (c) Rice, G. T.; White, M. C. J. Am. Chem. Soc. 2009, 131,
11707; (d) Qi, X.; Rice, G. T.; Lall, M. S.; Plummer, M. S.; White, M.
C. Tetrahedron 2010, 66, 4816; (e) Paradine, S. M.; White, M. C. J.
Am. Chem. Soc. 2012, 134, 2036; (f) Sun, J.; Wang, Y.; Pan, Y. J.
Org. Chem. 2015, 80, 8945; (g) Nishikawa, Y.; Kimura, S.; Kato, Y.;
Yamazaki, N.; Hara, O. Org. Lett. 2015, 17, 888; (h) Zhang, X.; Xu,
H.; Zhao, C. J. Org. Chem. 2014, 79, 9799; (i) Xie, Y.; Yu, K.; Gu, Z.
J. Org. Chem. 2014, 79, 1289.
[6] (a) Ono, A.; Uchiyama, K.; Hayashi, Y.; Narasaka, K. Chem. Lett.
1998, 27, 437; (b) Uchiyama, K.; Hayashi, Y.; Narasaka, K. Chem.
Lett. 1998, 27, 1261; (c) Uchiyama, K.; Hayashi, Y.; Narasaka, K.
Tetrahedron 1999, 55, 8915; (d) Misaim, T.; Narasaka, K. Chem.
Lett. 2000, 338; (e) Koganemaru, Y.; Kitamura, M.; Narasaka, K.
Chem. Lett. 2002, 784; (f) Yoshida, M.; Kitamura, M.; Narasaka, K.
Bull. Chem. Soc. Jpn. 2003, 76, 2003; (g) Kitamura, M.; Narasaka,
K. Eur. J. Org. Chem. 2005, 4505; (h) Kitamura, M.; Mori, Y.;
Narasaka, K. Tetrahedron Lett. 2005, 46, 2373; (i) Kitamura, M.;
Narasaka, K. Bull. Chem. Soc. Jpn. 2008, 81, 539.
[7] Recent studies on transition-metal catalyzed cyclization of oxime
esters, please see: (a) Faulkner, A.; Bower, J. F. Angew. Chem., Int.
Ed. 2012, 51, 1675; (b) Faulkner, A.; Scott, J. S.; Bower, J. F. Chem.
Commun. 2013, 49, 1521; (c) Race, N. J.; Bower, J. F. Org. Lett.
2013, 15, 4616; (d) Faulkner, A.; Race, N. J.; Scott, J. S.; Bower, J.
F. Chem. Sci. 2014, 5, 2416; (e) Faulner, A.; Scott, J. S.; Bower, J. F.
J. Am. Chem. Soc. 2015, 137, 7224; (f) Race, N. J.; Faulner, A.;
Shaw, M. H.; Bower, J. F. Chem. Sci. 2016, 7, 1508.
[11] Nocket, A. J.; Weinreb, S. M. Angew. Chem., Int. Ed. 2014, 53,
14162.
[12] Kong, C.; Jana, N.; Driver, T. G. Org. Lett. 2013, 15, 824.
[13] (a) Xu, Y.-H.; Lu, J.; Loh, T.-P. J. Am. Chem. Soc. 2009, 131, 1372;
(b) Wen, Z.-K.; Xu, Y.-H.; Loh, T.-P. Chem.-Eur. J. 2012, 18,
13284; (c) Zhang, J.; Loh, T.-P. Chem. Commun. 2012, 48, 11232;
(d) Wen, Z.-K.; Xu, Y.-H.; Loh, T.-P. Chem. Sci. 2013, 4, 4520; (e)
Zhang, X.; Wang, M.; Zhang, M.-X.; Xu, Y.-H.; Loh, T.-P. Org.
Lett. 2013, 15, 5531; (f) Hu, X.-H.; Zhang, J.; Yang, X.-F.; Xu,
Y.-H.; Loh, T.-P. J. Am. Chem. Soc. 2015, 137, 3169; (g) Hu, X.-H.;
Yang, X.-F.; Loh, T.-P. Angew. Chem., Int. Ed. 2015, 54, 15535.
[8] Liu, W.-M.; Liu, Z.-H.; Cheong, W.-W.; Priscilla, L. Y. T.; Li,
Y.-X.; Narasaka, K. Bull. Korean Chem. Soc. 2010, 31, 563.
[9] (a) Tokuda, M.; Yamada, Y.; Takagi, T.; Suginome, H.; Furusaki, A.
Tetrahedron Lett. 1985, 26, 6085; (b) Boga, C.; Manescalchi, F.;
Savoia, D. Tetrahedron 1994, 50, 4709; (c) Fasth, H. P.; Riesinger,
(Pan, B.; Qin, X.)
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Chin. J. Chem. 2016, 34, 1076—1080