N-Heterocyclic Carbene-Catalyzed Activation of Esters of N-Hydroxyphthalimide: A Highly Enantioselective Route to Chiral Dihydropyridinones Bearing an All Carbon Quaternary Stereogenic Center

Article abstract of DOI:10.1021/acs.orglett.5b02527

An N-heterocyclic carbene-catalyzed highly enantioselective [3 + 3] annulation reaction of N-hydroxyphthalimide (NHPI) 3,3-disubstituted acrylates and N-Ts ketimines was developed. In most cases, the desired chiral dihydropyridinone products bearing an all carbon quaternary stereogenic center could be obtained in good yields with excellent enantioselectivities (>99% ee's), which demonstrated the NHPI acrylates as a kind of excellent substrate in NHC-catalysis.

Full text of DOI:10.1021/acs.orglett.5b02527

Letter
pubs.acs.org/OrgLett
N‑Heterocyclic Carbene-Catalyzed Activation of Esters of
N‑Hydroxyphthalimide: A Highly Enantioselective Route to Chiral
Dihydropyridinones Bearing an All Carbon Quaternary Stereogenic
Center
Zhiming Zhang, Xiaofei Zeng,* Danbo Xie, Dongdong Chen, Liyuan Ding, Anna Wang, Limin Yang,*
and Guofu Zhong*
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
S
* Supporting Information
ABSTRACT: An N-heterocyclic carbene-catalyzed highly enantioselective [3 + 3] annulation reaction of N-hydroxyphthalimide
(NHPI) 3,3-disubstituted acrylates and N-Ts ketimines was developed. In most cases, the desired chiral dihydropyridinone
products bearing an all carbon quaternary stereogenic center could be obtained in good yields with excellent enantioselectivities
(>99% ee’s), which demonstrated the NHPI acrylates as a kind of excellent substrate in NHC-catalysis.
Recently, the use of para-nitro phenyl esters instead of the
compounds mentioned above for the generation of enolate and
homoenolate equivalent intermediates in NHC-catalyzed
reactions reported by the Chi group solved this problem
well.9 Alternately, the Scheidt10 and Ye11 groups reported the
formation of an α,β-unsaturated acyl azolium intermediate from
α,β-unsaturated carboxylic acid via in situ generation of cross
anhydride, while a similar strategy developed by the Chi group
through the use of anhydrides derived from the acids was also
reported.12 Although several examples of carboxylic acid and
their derivatives employed as substrates have been achieved, the
development of more successful examples in NHC-catalysis is
still sought-after.
ith the rapid development of organocatalysis, N-
Wheterocyclic carbenes (NHCs) have become one of
the most important kinds of catalysts for organic synthesis and
received intense attention in the past decade.1 In (oxidative2)
NHC-catalysis, reactions of enals,3 ynals,4 enones,5 α-
hydroxyenones,6 and α-bromoenals7 or α,β-unsaturated acyl
fluorides8 with imines, active carbonyl compounds, electron-
deficient carbon−carbon or carbon-hetereo double bonds have
been extensively studied, leading to various transformations
through the catalytical generation of enolate and homoenolate
equivalent intermediates (Figure 1). However, these kinds of
different substrates are usually expensive and unstable, and they
are often hard to prepare and handle.
N-Hydroxysuccinimide (NHSI)13 and N-hydroxyphthalimide
(NHPI)14 are widely seen in the preparation of amine-reactive
esters of carboxylate groups for chemical labeling, cross-linking,
and solid-phase immobilization applications. In these cases,
carboxylic acids (−COOH) react to NHSI or NHPI in the
presence of a carbodiimide such as DCC or EDC, resulting in a
semistable NHSI or NHPI ester, which then reacts with
primary amines to form amide cross-links, thus enhancing the
coupling efficiency greatly (Figure 2). Since either the NHSI or
NHPI unit acts as a good leaving group in such reactions, we
envisioned that this type of semistable ester units might be
applicable in the NHC-catalyzed activation of carbonyl
functionalities, where the α,β-unsaturated acyl azolium
intermediates could be easily generated. Herein, we report a
Figure 1. Formation of homoenolate or α,β-unsaturated acyl azolium
intermediate with NHC catalysts.
Received: September 2, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02527
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Organic Letters
Letter
Table 1. Condition Optimization of the Cyclization Reaction
Figure 2. Formation of amide cross-links through NHSI or NHPI
esters.
highly enantioselective [3 + 3] annulation reaction of 3,3-
disubstituted acrylates and N-Ts ketimines via the NHC-
catalyzed activation of the NHPI acrylates.
a
b
entry cat.
base
solvent
time [h] yield [%]
ee [%]
1
A
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
DBU
THF
THF
THF
THF
THF
THF
THF
CH3CN
EA
20
48
48
24
12
5
62
30
trace
67
77
55
36
13
18
78
53
−
98
Considering the wide application of the chiral dihydropyr-
idinone9a,15 scaffold in organic synthesis, we are very interested
in the formation of dihydropyridinone derivatives via the
reaction of N-Ts ketimines and the in situ generated β-
disubstituted α,β-unsaturated acyl azolium intermediate with
NHCs, affording the products with an all carbon quaternary
sterogenic center. In this approach, the NHSI and NHPI esters
used are readily available, cheap, and easy to handle, and a
series of optically pure dihydropyridinone derivatives were
conveniently obtained in good yields and excellent enantiose-
lectivities under mild conditions. Thus, we hope that this kind
of esters could be generally employed as an alternative strategy
to form activated Michael acceptors in NHC catalysis.
2
B
3
C
D
D
D
D
D
D
D
D
n.d.
>99
>99
>99
>99
98
4
5
6
7
C2H5ONa
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
1
8
48
48
24
24
9
>99
>99
>99
c
10
THF
d
11
THF
b
a
Yield of the isolated product. Determined by chiral HPLC (see the
c
d
Supporting Information). 10 mol % catalyst was used. 5 mol %
catalyst was used.
At the very onset, NHSI 3,3-disubstituted acrylates 1a and
the N-Ts ketimine 2a were tested as the electro- and
nucleophile precursors, respectively (Scheme 1). To our
Solvent screening revealed that solvents significantly
influenced the reaction yields; however, the enantioselectivities
were all excellent. Dramatically decreases in yields were
observed when polar solvents such as CH3CN and EtOAc
were used (Table 1, entries 8 and 9). And THF showed the
best result in yield. A comparable result (78% yield and >99%
ee) was achieved when half the amount of catalyst was used
with an extension of the reaction time to 24 h (Table 1, entry
10); further decreasing the catalyst loading to 5 mol % led to a
lower yield (53%, Table 1, entry 11). Thus, the best reaction
conditions were established when the reaction was performed
in the presence of 10 mol % of D and 1.0 equiv of CsCO3 in
THF at room temperature.
Scheme 1. Investigation of NHSI and NHPI Esters in the
Cyclization Reaction
delight, the reaction proceeded smoothly in the presence of
the achiral NHC catalyst A and provided the desired product
3a in 50% yield, which encouraged us in further investigation
with chiral NHC catalysts. When the reaction was catalyzed by
the Rovis triazolium catalyst D which is widely used in
enantioselective NHC catalysis, the product 3a was obtained in
45% yield, surprisingly, the enantiomeric excess (ee) was
excellent (99% ee). We next tried to use NHPI acrylate 1b
instead of 1a in the reaction, and it was found that the yield of
3a was improved to 67% with an even better ee value (>99%
ee) (Table 1, entry 4). Thus, the NHPI ester was selected in
the subsequent reaction condition optimization.
The screening of different chiral triazolium catalysts was
carried out, and when catalyst B was used the product 3a was
given in only 30% yield with 98% ee (Table 1, entry 2). Catalyst
C with a pentafluorophenyl group could not promote the
reaction efficiently; only a trace amount of product was
detected (Table 1, entry 3). Subsequent screening of bases
indicated that CsCO3 was the best due to the highest yield and
enantioselectivity. Other bases, such as K2CO3, DBU, and
C2H5ONa, provided lower yields, but ee values were still
excellent (Table 1, entries 4−7).
Having established the optimized reaction conditions, we
turned our attention to the exploration of the reaction scope
through variation of different NHPI acrylates and imines; the
results are listed in Scheme 2. From the scheme we observed
that the esters 1 with electron-donating and -withdrawing
groups on R1 were all well tolerated in the reaction. The desired
dihydropyridinone derivatives were isolated in moderate to
good yields (51−77%), and the enantioselectivities were all
excellent (more than 99% ee’s) (3a-3h). The hetereoaryl
acrylates 3o also reacted well providing the desired product
(3h) in 55% yield with >99% ee.
The influence of functionalities and substitutions on the N-
Ts ketimines 2 was also investigated. Both electron-with-
drawing and electron-rich substituents on the phenyl group of
the imine were also well tolerated, affording the desired
products 3i−3o in good yields with excellent enantioselectiv-
ities. The product from 3-fluorophenyl N-Ts ketimine 3k was
obtained in slightly lower ee; however, a noticeable electronic
effect of the imine substituents was not observed in the
reaction. Furthermore, the cyclic sulfonic imine 3i was also
tested and the desired tricyclic product was achieved in 63%
yield with 97% ee (Scheme 3). Unfortunately, only a trace
B
DOI: 10.1021/acs.orglett.5b02527
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
lactam formation transformation sequences, and the desired
product would be obtained with regeneration of the catalyst.
Because the six-membered lactams and derivatives are
important building blocks and scaffolds in organic synthesis
and pharmaceuticals, the derivation of cyclization products with
simple protocols was carried out. The optically enriched
product 3a was transformed to the phosphoric ester 4a with a
little erosion of the ee value in moderate yield (49% yield and
95% ee in Scheme 5), which could be a useful intermediate for
Scheme 2. Substrate Scope
Scheme 5. Derivation of the Dihydropyridine
the synthesis of chiral trisubstituted dihydropyridines bearing
an all carbon quaternary center. Some other transformations
could also be achieved by reduction, hydrogenation, and
deprotection, and different chiral intermediates could be
obtained conveniently.9a,16
In conclusion, an NHC-catalyzed highly enantioselective [3
+ 3] cyclization reaction of NHPI acrylates with N-Ts
ketimines was developed. In most cases, the desired products
were obtained in good yields with >99% ee values, which
illustrate that the NHPI esters could be another type of
excellent substrate in NHC catalysis. Given the advantages of
the NHPI esters, we expect it could be widely applied in more
organic reactions and methodologies.
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 3. Cyclization Reaction of 2i with NHPI Ester
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02527.
Experimental procedures and analytical data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
amount of the desired product was observed when the aliphatic
disubstituted acrylates were used. The absolute configuration of
the cyclization products was confirmed to be S by comparison
with the optical rotation data reported in the literature.9a
We thought that the reaction follows a similar reaction
pathway to the literatures.17 Due to the high leaving ability of
the NHPI group on the semi stable ester 1a, the key α,β-
unsaturated acyl azolium intermediate I could be easily
generated through the addition of the NHC catalyst to the
ester (Scheme 4), which would greatly benefit the following
steps of the reaction and the stereoselectivity, as proposed by
Bode18 and related literature.7h Then, a 1,2-addition of the
enamine isomerized from imine 2 under basic conditions would
occur, after further Claisen rearrangement, tautomerization, and
*E-mail: chemzxf@hznu.edu.cn.
*E-mail: myang@hznu.edu.cn.
*E-mail: zgf@hznu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Natural Science Foundation of
China (No. 21373073), the Young National Natural Science
Foundation of China (Nos. 21302033 and 21302032), the
PCSIRT (IRT 1231), and Hangzhou Normal University for
financial support. X.-F. acknowledges a Xihu Scholar award
from Hangzhou City, and G.Z. appreciated a Qianjiang
Scholarship from Zhejiang Province in China.
Scheme 4. Proposed Transition State
REFERENCES
■
(1) For selected reviews on NHC catalysis, see: (a) Enders, D.;
Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606. (b) Marion,
N.; Díez-Gonzalez, S.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46,
2988. (c) Moore, J. L.; Rovis, T. Top. Curr. Chem. 2009, 291, 77.
(d) Nair, V.; Menon, R. S.; Biju, A. T.; Sinu, C. R.; Paul, R. R.; Jose, A.;
C
DOI: 10.1021/acs.orglett.5b02527
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Sreekumar, V. Chem. Soc. Rev. 2011, 40, 5336. (e) Cohen, D. T.;
Scheidt, K. A. Chem. Sci. 2012, 3, 53. (f) Izquierdo, J.; Hutson, G. E.;
Cohen, D. T.; Scheidt, K. A. Angew. Chem., Int. Ed. 2012, 51, 11686.
(g) Vora, H. U.; Wheeler, P.; Rovis, T. Adv. Synth. Catal. 2012, 354,
1617. (h) Grossmann, A.; Enders, D. Angew. Chem., Int. Ed. 2012, 51,
314. (i) Douglas, J.; Churchill, G.; Smith, A. D. Synthesis 2012, 44,
2295. (j) Bugaut, X.; Glorius, F. Chem. Soc. Rev. 2012, 41, 3511.
(k) Izquierdo, J.; Hutson, G. E.; Cohen, D. T.; Scheidt, K. A. Angew.
Chem., Int. Ed. 2012, 51, 11686. (l) Ryan, S. J.; Candish, L.; Lupton, D.
W. Chem. Soc. Rev. 2013, 42, 4906. (m) Hopkinson, M. N.; Richter,
C.; Schedler, M.; Glorius, F. Nature 2014, 510, 485. (n) Chauhan, P.;
Enders, D. Angew. Chem., Int. Ed. 2014, 53, 1485.
(2) For selected examples on oxidative NHC catalysis, see: (a) Mo,
J.-M.; Chen, X.-K.; Chi, Y.-G. R. J. Am. Chem. Soc. 2012, 134, 8810.
(b) Mo, J.-M.; Yang, R.-J.; Chen, X.-K.; Tiwari, B.; Chi, Y.-G. R. Org.
Lett. 2013, 15, 50. (c) Mo, J.-M.; Shen, L.; Chi, Y.-G. R. Angew. Chem.,
Int. Ed. 2013, 52, 8588. (d) Wang, M.; Huang, Z.-J.; Xu, J.-F.; Chi, Y.-
G. R. J. Am. Chem. Soc. 2014, 136, 1214.
Wang, D.; Lu, J.; Li, T.; Jiao, W.; Yu, C. Chem. - Eur. J. 2012, 18, 1914.
(i) Yetra, S. R.; Bhunia, A.; Patra, A.; Mane, M. V.; Vanka, K.; Biju, A.
T. Adv. Synth. Catal. 2013, 355, 1089. (j) He, L.; Guo, H.; Li, Y.-Z.;
Du, G.-F.; Dai, B. Chem. Commun. 2014, 50, 3719. (k) Yetra, S. R.;
Roy, T.; Bhunia, A.; Porwal, D.; Biju, A. T. J. Org. Chem. 2014, 79,
4245.
(8) α,β-Unsaturated acyl fluorides: (a) Ryan, S. J.; Candish, L.;
Lupton, D. W. J. Am. Chem. Soc. 2009, 131, 14176. (b) Ryan, S. J.;
Candish, L.; Lupton, D. W. J. Am. Chem. Soc. 2011, 133, 4694.
(c) Ryan, S. J.; Stasch, A.; Paddon-Row, M. N.; Lupton, D. W. J. Org.
Chem. 2012, 77, 1113. (d) Candish, L.; Lupton, D. W. J. Am. Chem.
Soc. 2013, 135, 58.
(9) (a) Cheng, J.-J.; Huang, Z.-J.; Chi, Y.-G. R. Angew. Chem., Int. Ed.
2013, 52, 8592. (b) Chen, S.-J.; Hao, L.; Zhang, Y.-X.; Tiwari, B.; Chi,
Y.-G. R. Org. Lett. 2013, 15, 5822. (c) Hao, L.; Chen, S.-J.; Xu, J.-F.;
Tiwari, B.; Fu, Z.-Q.; Li, T.; Lim, J.-Y.; Chi, Y.-G. R. Org. Lett. 2013,
15, 4956. (d) Xu, J.-F.; Jin, Z.-C.; Chi, Y.-G. R. Org. Lett. 2013, 15,
5028. (e) Fu, Z.-Q.; Jiang, K.; Zhu, T.-S.; Torres, J.; Chi, Y.-G. R.
Angew. Chem., Int. Ed. 2014, 53, 6506.
(3) Enals: (a) Maki, B. E.; Chan, A.; Phillips, E. M.; Scheidt, K. A.
Org. Lett. 2007, 9, 371. (b) He, M.; Bode, J. W. J. Am. Chem. Soc. 2008,
130, 418. (c) De Sarkar, S.; Grimme, S.; Studer, A. J. Am. Chem. Soc.
2010, 132, 1190. (d) DeSarkar, S.; Studer, A. Angew. Chem., Int. Ed.
2010, 49, 9266. (e) Reddy, R. S.; Rosa, J. N.; Veiros, L. F.; Caddick, S.;
Gois, P. M. P. Org. Biomol. Chem. 2011, 9, 3126. (f) Rong, Z.-Q.; Jia,
M.-Q.; You, S.-L. Org. Lett. 2011, 13, 4080. (g) Jacobsen, C. B.; Jensen,
K. L.; Udmark, J.; Jørgensen, K. A. Org. Lett. 2011, 13, 4790.
(h) Biswas, A.; DeSarkar, S.; Tebben, L.; Studer, A. Chem. Commun.
2012, 48, 5190. (i) Lv, H.; Tiwari, B.; Mo, J.-M.; Xing, C.; Chi, Y.-G.
R. Org. Lett. 2012, 14, 5412. (j) Lu, Y.; Tang, W.; Zhang, Y.; Du, D.;
Lu, T. Adv. Synth. Catal. 2013, 355, 321. (k) Lu, S.; Poh, S. B.; Siau,
W.-Y.; Zhao, Y. Angew. Chem., Int. Ed. 2013, 52, 1731. (l) Izquierdo, J.;
Orue, A.; Scheidt, K. A. J. Am. Chem. Soc. 2013, 135, 10634. (m) Lv,
H.; Jia, W.-Q.; Sun, L.-H.; Ye, S. Angew. Chem., Int. Ed. 2013, 52, 8607.
(n) Sun, L.-H.; Liang, Z.-Q.; Jia, W.-Q.; Ye, S. Angew. Chem., Int. Ed.
2013, 52, 5803. (o) Jang, K. P.; Hutson, G. E.; Johnston, R. C.;
McCusker, E. O.; Cheong, P. H.-Y.; Scheidt, K. A. J. Am. Chem. Soc.
2014, 136, 76. (p) Guo, C.; Schedler, M.; Daniliuc, C. G.; Glorius, F.
Angew. Chem., Int. Ed. 2014, 53, 10232. (q) Dong, X.-Q.; Sun, J.-W.
Org. Lett. 2014, 16, 2450.
(10) Lee, A.; Younai, A.; Price, C. K.; Izquierdo, J.; Mishra, R. K.;
Scheidt, K. A. J. Am. Chem. Soc. 2014, 136, 10589.
(11) Chen, X.-Y.; Gao, Z.-H.; Song, C.-Y.; Zhang, C.-L.; Wang, Z.-X.;
Ye, S. Angew. Chem., Int. Ed. 2014, 53, 11611.
(12) Jin, Z.-C.; Chen, S.-J.; Wang, Y.-H.; Zheng, P.-C.; Yang, S.; Chi,
Y.-G. R. Angew. Chem., Int. Ed. 2014, 53, 13506.
(13) (a) Madler, S.; Bich, C.; Touboul, D.; Zenobi, R. J. Mass
̈
Spectrom. 2009, 44, 694. (b) Liu, Y.-X.; Liu, G.-Z.; Hnatowich, D. J.
Materials 2010, 3, 3204. (c) Samanta, D.; Sarkar, A. Chem. Soc. Rev.
2011, 40, 2567.
(14) (a) Tesser, G. I.; Schwyzer, R. Helv. Chim. Acta 1966, 49, 1013.
(b) Slabospitskaya, M. Y.; Vlakh, E. G.; Saprykina, N. N.; Tennikova,
T. B. J. Appl. Polym. Sci. 2009, 111, 692. (c) Sheikh, M. C.; Takagi, S.;
Sakai, M.; Abe, H.; Morita, H. Org. Biomol. Chem. 2008, 6, 4505.
(15) Zhang, H.-M.; Jia, W.-Q.; Liang, Z.-Q.; Ye, S. Asian J. Org. Chem.
2014, 3, 462.
(16) (a) Simal, C.; Lebl, T.; Slawin, A. M. Z.; Smith, A. D. Angew.
Chem., Int. Ed. 2012, 51, 3653. (b) Han, B.; Li, J.-L.; Ma, C.; Zhang, S.-
J.; Chen, Y.-C. Angew. Chem., Int. Ed. 2008, 47, 9971.
(17) Xie, D.; Yang, L.; Lin, Y.; Zhang, Z.; Chen, D.; Zeng, X.; Zhong,
G. Org. Lett. 2015, 17, 2318.
(4) Ynals: (a) Zeitler, K. Org. Lett. 2006, 8, 637. (b) Zhu, Z.-Q.; Xiao,
J.-C. Adv. Synth. Catal. 2010, 352, 2455. (c) Kaeobamrung, J.;
Mahatthananchai, J.; Zheng, P.-G.; Bode, J. W. J. Am. Chem. Soc. 2010,
132, 8810. (d) Mahatthananchai, J.; Zheng, P.-G.; Bode, J. W. Angew.
Chem., Int. Ed. 2011, 50, 1673. (e) Zhu, Z.-Q.; Zheng, X.-L.; Jiang, N.-
F.; Wan, X.; Xiao, J.-C. Chem. Commun. 2011, 47, 8670. (f) Du, D.;
Hu, Z.; Jin, J.; Lu, Y.; Tang, W.; Wang, B.; Lu, T. Org. Lett. 2012, 14,
1274. (g) Zhao, Y.-M.; Tam, Y.; Wang, Y.-J.; Li, Z.-G.; Sun, J.-W. Org.
Lett. 2012, 14, 1398. (h) Romanov-Michailidis, R.-M.; Besnard, C.;
Alexakis, A. Org. Lett. 2012, 14, 4906. (i) Qi, J.; Xie, X.-G.; Han, R.-F.;
Ma, D.-H.; Yang, J.; She, X.-G. Chem. - Eur. J. 2013, 19, 4146.
(j) Zhou, B.; Luo, Z.; Li, Y.-C. Chem. - Eur. J. 2013, 19, 4428. (k) Sun,
L.-H.; Liang, Z.-Q.; Jia, W.-Q.; Ye, S. Angew. Chem., Int. Ed. 2013, 52,
5803. (l) Lu, Y.-Y.; Tang, W.-F.; Zhang, Y.; Du, D.; Lu, T. Adv. Synth.
Catal. 2013, 355, 321.
(18) (a) Mahatthananchai, J.; Kaeobamrung, J.; Bode, J. W. ACS
Catal. 2012, 2, 494. (b) Lyngvi, E.; Bode, J. W.; Schoenebeck, F. Chem.
Sci. 2012, 3, 2346.
(5) Enones: (a) Jin, Z.-C.; Xu, J.-F.; Yang, S.; Song, B.-A.; Chi, Y.-G.
R. Angew. Chem., Int. Ed. 2013, 52, 12354. (b) Fu, Z.-Q.; Jiang, K.;
Zhu, T.-S.; Torres, J.; Chi, Y.-G. R. Angew. Chem., Int. Ed. 2014, 53,
6506.
(6) Wanner, B.; Mahatthananchai, J.; Bode, J. W. Org. Lett. 2011, 13,
5378.
(7) α-Bromoenals: (a) He, M.; Uc, G. J.; Bode, J. W. J. Am. Chem.
Soc. 2006, 128, 15088. (b) Sun, F.-G.; Sun, L.-H.; Ye, S. Adv. Synth.
Catal. 2011, 353, 3134. (c) Yang, L.; Wang, F.; Chua, P. J.; Lv, Y.;
Zhong, L.; Zhong, G. Org. Lett. 2012, 14, 2894. (d) Zhang, B.; Feng,
P.; Cui, Y.; Jiao, N. Chem. Commun. 2012, 48, 7280. (e) Yetra, S. R.;
Kaicharla, T.; Kunte, S. S.; Gonnade, R. G.; Biju, A. T. Org. Lett. 2013,
15, 5202. (f) Yetra, S. R.; Bhunia, A.; Patra, A.; Mane, M. V.; Vanka,
K.; Biju, A. T. Adv. Synth. Catal. 2013, 355, 1089. (g) Taylor, J. E.;
Daniels, D. S. B; Smith, A. D. Org. Lett. 2013, 15, 6058. (h) Yao, C.;
D
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