Gold-Catalyzed Intramolecular Tandem Cyclization of Indole-Ynamides: Diastereoselective Synthesis of Spirocyclic Pyrrolidinoindolines

Article abstract of DOI:10.1002/asia.201500865

A gold-catalyzed intramolecular tandem cyclization of indole-ynamide affords tetracyclic spirocyclic pyrrolidinoindoline bearing an all-carbon quaternary stereocentre in a single step; however, when the reaction was carried out in the presence of BF₃·Et₂O, the corresponding tricyclic spirocyclic pyrrolidinoindoline-based enones are produced through a key 1,5-hydride shift. The developed chemistry provides a diastereoselective and straightforward entry to structurally diverse polycylic pyrrolidinoindolines from indole-ynamides in one-pot reactions under mild conditions.

Full text of DOI:10.1002/asia.201500865

DOI: 10.1002/asia.201500865
Communication
Gold Catalysis
Gold-Catalyzed Intramolecular Tandem Cyclization of Indole-
Ynamides: Diastereoselective Synthesis of Spirocyclic
Pyrrolidinoindolines
Nan Zheng,[a] Yuan-Yuan Chang,[a] Li-Jie Zhang,[a] Jian-Xian Gong,*[a] and Zhen Yang*[a, b, c]
Abstract: A gold-catalyzed intramolecular tandem cycliza-
tion of indole-ynamide affords tetracyclic spirocyclic pyrro-
lidinoindoline bearing an all-carbon quaternary stereocen-
tre in a single step; however, when the reaction was car-
ried out in the presence of BF3·Et2O, the corresponding tri-
cyclic spirocyclic pyrrolidinoindoline-based enones are
produced through a key 1,5-hydride shift. The developed
chemistry provides a diastereoselective and straightfor-
ward entry to structurally diverse polycylic pyrrolidinoin-
dolines from indole-ynamides in one-pot reactions under
mild conditions.
Figure 1. Spirocyclic pyrrolidinoindoline-based alkaloids.
Spirocyclic pyrrolidinoindolines are privileged scaffolds present
in a growing number of alkaloids, such as echitamidine 1,[1]
elacomine 2,[2] kopsifoline D 3,[3] aspidophytine 4,[4] and aspido-
During the past decade, ynamides have emerged as versatile
heterocyclic building blocks that have been widely used in the
spermine 5[5] (Figure 1). The biological importance and intrigu-
ing molecular architectures of these alkaloids render them
highly attractive targets in the synthetic community, and inten-
sive efforts have been concentrated on the development of
synthetic methods and strategies for their total synthesis.[6]
synthesis of structurally diverse heterocycles.[7] Brønsted acid-
catalyzed intermolecular vinylation of indoles with ynamides
(Scheme 1a),[8] and gold-catalyzed intermolecular alkylation of
indoles with ynamides (Scheme 1b)[9] have been successfully
applied to the synthesis of structurally diverse 3-substituted
indole derivatives.
Inspired by the above-mentioned chemistry, we intended to
explore the intramolecular consecutive annulation of indole-
ynamide A to generate pyrrolidinoindoline D, and ketenimini-
um ion B and iminium ion[10] were proposed as key intermedi-
ates in this transformation (Scheme 1c). However, the step
from A to C could alternatively go through a Au-catalyzed
direct auration step from the starting material A.[11]
[a] N. Zheng, Y.-Y. Chang, L.-J. Zhang, Prof. Dr. J.-X. Gong, Prof. Dr. Z. Yang
Laboratory of Chemical Genomics
School of Chemical Biology and Biotechnology
Peking University
Shenzhen Graduate School
Shenzhen 518055 (China)
E-mail: zyang@pku.edu.cn
gongjx@pku.edu.cn
[b] Prof. Dr. Z. Yang
Herein we report an unprecedented keteniminium ion-initi-
ated and iminium ion-mediated cascade annulation based on
Au-catalyzed reaction from functionalized indole-ynamides.
The chemistry developed allows the diastereoselective synthe-
sis of two types of spirocyclic pyrrolidinoindolines D and F
from common starting material A.
Ynamide 6 was prepared from the CuII-catalyzed cross-cou-
pling of the corresponding indole-sulfonamide and protected
alkynyl alcohol.[12] Initial reaction conditions tested for the
target reaction made use of the most commonly used Brønst-
ed acids (TsOH,[13] TfOH,[7] and Tf2NH[7,14]) and Lewis acids
(AgOTf[15] and AuCl3[16]) as catalysts, DCM and 1,2-dichloro-
ethane (DCE)> were chosen as solvents as they are commonly
used in this type of reaction. Only a small amount of desired
annulated product 7 was obtained when trifluoroacetic acid
Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Min-
istry of Education
and Beijing National Laboratory for Molecular Science (BNLMS)
Peking- Tsinghua Center for Life Sciences
Peking University
Beijing 100871 (China)
[c] Prof. Dr. Z. Yang
Key Laboratory of Marine Drugs
Chinese Ministry of Education
School of Medicine and Pharmacy
Ocean University of China
5 Yushan Road, Qingdao 266003 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500865.
This manuscript is part of a special issue on catalysis and transformation
of complex molecules. Click here to see the Table of Contents of the special
issue.
Chem. Asian J. 2016, 11, 371 – 375
371
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
better choice, and the reaction at 258C gave
a better and faster result than at 08C. Product 7 was
obtained in 94% when 5 mol% of [PPh3 AuCl]/
AgSbF6 was used at 258C in DCE.
To delineate the scope of this annulation, other
ynamides such as 8a–8q were synthesized as their
racemic forms, and 8s was prepared as an enantio-
meric pure substrate (see the Supporting Informa-
tion for details), and annulated under the optimized
reaction conditions, and the results are listed in
Table 2. All the tested substrates gave satisfactory
yields and the reaction went to completion at 258C
in less than 30 min using the conditions described.
From the results in Table 2, the following observa-
tions were made: (1) the use of a benzyl N-protect-
ing group (R1) on the indole ring gave better results
than para-methoxybenzyl (PMB) or allyl groups;
(2) varying the ynamide N-protecting groups (R2)
from 4-toluenesulfonyl (Ts) to 4-nitrobenzenesulfonyl
(Ns) or methanesulfonyl (Ms) (8d and 8e) did not
significantly influence the product yields; (3) the
presence of electron-donating groups on the indole
ring led to higher yields of annulation product than
electron-withdrawing groups (9j–9p), presumably
because electron-rich indoles favor the formation of
the keteniminium ion-induced spirocyclic pyrrolidi-
noindolines; (4) 2-methyl indole-based ynamides an-
Table 1. Screening of the reaction conditions.[a]
Scheme 1. Previous work and our synthetic analysis.
(TfOH) and triflimide (Tf2NH) were used (Table 1, entries 2 and
3), and extensive decomposition of ynamide 6 was observed
under the other reaction conditions tested (Table 1, entries 1–
5).
Entry
Catalyst
Additive T [o C] Solvent
t
Yield [%][b]
1[c]
2
3
4
5
6
7
8
TsOH
TfOH
Tf2NH
AgOTf
AuCl3
–
25
25
25
25
25
60
25
0
DCE
DCE
DCE
DCE
DCE
DCE
DCM
DCM
DCM
DCM
DCE
DCE
DCE
DCE
6 h
5 min
1 h
1 h
5 min
2 h
30 min 60
30 min 55
15 min 76
5 min
5 min
15 min 77
15 min 71
1 h
0
–
–
-
–
13
14
0
0
0
Recently, gold-catalyzed reactions of ynamides have
emerged as powerful methods for syntheses of structurally di-
verse scaffolds.[17] We therefore started to explore Au-catalyzed
annulation for the synthesis of product 7 from ynamide 6. We
first tested [(IPr)AuCl]/AgNTf2 (IPr=1,3-bis(diisopropylphenyl)i-
midazol-2-ylidene) as a catalyst,[7q] and to our delight, the de-
sired product 7 was obtained in 60% yield when the reaction
was carried out in DCM at 258C for 30 min. The structure of 7
was confirmed by 2D-NMR spectroscopy, and later supported
by an X-ray crystal structure of its derivative 9q (Table 2, see
the Experimental Section).
PtCl2
–
(IPr)AuCl
(IPr)AuCl
(IPr)AuCl
PPh3 AuCl AgSbF6
PPh3 AuCl AgSbF6
PPh3 AuCl AgNTf2
AgNTf2
AgNTf2
AgSbF6
9
25
25
25
25
25
25
10
11
12
13
14
89
94
LAuSbF6
–
–
AgNTf2
0
To find the optimal reaction conditions, we studied the
effect of various reaction parameters (catalyst, solvent, temper-
ature, etc.) on the outcome of the reaction. We found that
among the catalysts tested such as [(IPr)AuCl]/AgNTf2,
[(IPr)AuCl]/AgSbF6,[12]
[PPh3 AuCl]/AgNTf2,[12]
[PPh3 AuCl]/AgSbF6 was the most effi-
cient combination. Among the solvents used, DCE was the
[PPh3 AuCl]/
[a] Reaction conditions: 6=0.02m (0.2 mmol). [b] Isolated yield. [c] The
TsOH loading is 20 mol%.
[17d]
AgSbF6,[7q,17c] PtCl2,
Chem. Asian J. 2016, 11, 371 – 375
www.chemasianj.org
372
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
of R5 does not have any diastereoselective effect on
the spirocyclization of ynamides.
Table 2. Substrate scope for tetracyclic pyrrolidinoindolines 9.[a]
To achieve an asymmetric synthesis, we then tested
the gold-catalyzed annulation in the presence of chiral
ligands.[19] We expected that when an Au-catalyst bear-
ing chiral ligands approached the ynamide, the Au-as-
sociated keteniminium species formed could induce
the asymmetric nucleophilic addition of indole,[20] thus
affording chiral spirocyclic pyrrolidinoindoline. To ex-
plore this possibility, several commercially available li-
gands were tested (see the Supporting Information),
and we found that when ynamide 6 was annulated
under the optimized conditions in the presence of
chiral ligand (S)-L*, product 7 was obtained in 85%
yield with 60% ee (Scheme 2).
9a: R1 =Bn, 89%
9b: R1 =PMB,
86%
9d: R2 =Ns, 88%
9e: R2 =Ms, 90%
9c: R1 =Allyl,
74%
9 f: R3 =5-OMe,
9j: R3 =5-F, 72%
85%
To demonstrate the potential of our pyrrolidinoindo-
lines in the synthesis of complex molecules, we then
selected compound 9b as a model, and transformed it
into the useful synthetic intermediate 11,[21] which
could potentially be applied to the total synthesis of
spirocyclic pyrrolidinoindoline-based natural products,
such as those shown in Figure 1. In the event, com-
pound 9b was easily reduced with Na(BH3)CN, and the
resultant allylic alcohol 10 was then oxidized to alde-
hyde 11 through a Dess–Martin periodinane (DMP)-oxi-
dation in 71% overall yield over two steps (Scheme 3).
To study the substrate scope of the ynamide annula-
tion, we also interestingly found that the ynamides 12
bearing secondary alcohols could be directly converted
into their corresponding tricyclic products 13 in moder-
ate-to-good yields when the reactions were carried out
in the presence of additional BF3·Et2O (Table 3). It is
worthwhile to mention that when ynamide 12 f with
a substituent at the C2 position of the indole was used,
product 13 f was obtained as a single diastereomer.
Scheme 4 highlights our proposed reaction mecha-
nism to account for the observed Au-catalyzed intra-
molecular tandem cyclization of indole-ynamides and
9g: R3 =5-Me,
82%
9k: R3 =5-Cl, 75%
9l: R3 =5-Br, 70%
9m: R3 =5-CN, 52%
9h: R3 =6-Me,
77%
9i: R3 =7-OMe,
76%
9n: R3 =5-CO2 Me, 63%
9o: R3 =4-CO2 Me, 56%
9p: R3 =5-CF3, 68%
9q: R4 =Me, X=H, X-ray structure of 9s and 9s’:[b] R5 =Me, 69%, d.r.=1:1
89%
9q
9r: R4 =Me, X=F,
78%
[a] The yields of the products 9a–9q, which are isolated as pairs of enantiomers.
For products 9m, 9n, 9o and 9s, the reaction time was 30 min and for other sub-
strates, the reaction was completed within 5 min. [b] Substrate 8s was used as its
optical pure form.
nulated smoothly to afford the expected products 9q and 9r
in good yields, and again the electron-rich substrate 8q gave
better results than the electron-deficient substrate 8r; (5) in
contrast to a previous report,[18] when enantiomerically pure
substrate 8s underwent annulation under the typical reaction
conditions, products 9s and 9s’ were obtained as a pair of dia-
stereomers in a 1:1 ratio, indicating that the stereogenic center
1,5-hydride shift for the formation of the a,b-unsaturated car-
bonyl compounds. Accordingly, after coordination of gold cata-
lyst to ynamide A, the resultant keteniminium B might under-
go 5-endo-trig cyclization[22] to form intermediate C (through
an anti-addition format). Thus, if the reaction proceeded with-
out an additional Lewis acid, the intermediate C would under-
go a direct 6-exo-trig nucleophilic addition to afford a tetracy-
clic product D, and the regenerated Au-catalyst could enter
into another catalytic cycle. On the other hand, if the reaction
proceeded with an additional Lewis acid, such as BF3·Et2O,
product D would coordinate with the Lewis acid to form inter-
mediate E through a hemi-aminol ring opening reaction,
which in turn could undergo a 1,5-hydride shift to afford prod-
uct F in a diastereoselective manner.[23] It is noteworthy to
mention that other potentially competing reactions, such as
the Wagner–Meerwein rearrangement, was not observed in
this reaction.[9b,c]
To further support our proposed reaction mechanism, two
additional experiments were conducted. Our first experiment
Scheme 2. Application of the chiral dinuclear gold catalyst.
[a] Isolated yield. [b] Determined by HPLC analysis with a chiral column.
Chem. Asian J. 2016, 11, 371 – 375
www.chemasianj.org
373
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Table 3. One-pot synthesis of tricyclic pyrolidinoindolines 13.[a]
Scheme 3. Synthesis of aldehyde 11.
13a:[b] (R=Me,
62%)
13ea:[b] (R=H, 54%)
13 f:[b] (81%)
13b:[b] (R=Et,
70%)
was carried out by treating substrate 9b with BF3·Et2O
(1.0 equiv) in CH2Cl2 at À788C, as expected, product 11 was
13c:[b] (R=iPr,
75%)
13d:[b] (R=iBu,
68%)
13eb:[b] (R=4-Me, 60%)
obtained in 55% yield, indicating that intermediate
D
(Scheme 4) could be converted into F in the presence of Lewis
acid. Our second experiment deals with the proposed 1,5-hy-
dride shift in the catalytic cycle. To this end, we have synthe-
sized the deuterated ynamide 12g in a racemic form, and an-
nulated it under the optimized reaction conditions
(Table 2). As expected, the desired product 13g was
obtained in 77% yield as a pair of enantiomers. As
the deuterium in 12g was completely transferred to
the C2 position of the indole ring in a diastereoselec-
tive manner (see the Supporting Information for 2D-
NMR analysis), indicating that the proposed 1,5-hy-
dride shift proceeded through a tight six-membered
ring transition state (Scheme 5).
13ec:[b] (R=3,5-2OMe,
66%)
[a] Isolated yield. [b] all the products exist as a pair of enantiomers.
In summary, we have developed a one-pot intra-
molecular gold-catalyzed tandem cyclization reaction
of indole-ynamides for the stereoselective synthesis
of structurally complex tetracyclic and tricyclic spiro-
pyrrolidinoindolines with an all-carbon quaternary
center. A unique 1,5-hydride shift was observed
during the synthesis of the tricyclic pyrrolidinoindo-
line-based enones. Further investigations on the ap-
Scheme 4. Proposed reaction mechanism.
plication of this method in total synthesis of natural
products and additional mechanistic studies focused
on the 1,5-hydride shift are in progress.
Experimental Section
The synthetic procedures and characterization of the
compounds studied herein can be found in the Support-
ing Information. CCDC 1401360 (9q) contains the sup-
plementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge
Crystallographic Data Centre.
Acknowledgements
This work was supported by the NSF of China (Grant
Nos. 21372016, 21472006 and 21402002), 863 Pro-
gram (2013AA092903), Guangdong Natural Science
Scheme 5. Synthesis of compounds 11 and deuterated 13g.
Chem. Asian J. 2016, 11, 371 – 375
www.chemasianj.org
374
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
uthika, A. Nandakumar, P. T. Perumal, Org. Lett. 2014, 16, 4424; q) E. Re-
ttenmeier, A. M. Schuster, M. Rudolph, F. Rominger, C. A. Gade, A. S. K.
Hashmi, Angew. Chem. Int. Ed. 2013, 52, 5880; Angew. Chem. 2013, 125,
5993; r) M. C. B. Jaimes, V. Weingand, F. Rominger, A. S. K. Hashmi,
Chem. Eur. J. 2013, 19, 12504.
Foundation (2014A030312004), NSFC-Shandong Joint Fund for
Marine Science Research Centers (U1406402) and Shenzhen
Basic Research Program (Grant Nos. JCYJ20130329180217059,
ZDSYS20140509094114168 and JSGG20140717102922014).
[8] Y. S. Zhang, Tetrahedron 2006, 62, 3917.
[9] a) L. Li, C. Shu, B. Zhou, Y.-F. Yu, X.-Y. Xiao, L.-W. Ye, Chem. Sci. 2014, 5,
4057; b) A. S. K. Hashmi, W. Yang, F. Rominger, Adv. Synth. Catal. 2012,
354, 1273; c) A. S. K. Hashmi, W.-B. Yang, F. Rominger, Chem. Eur. J.
2012, 18, 6576.
Keywords: 1,5-hydride shif · cyclizationt · diastereoselectivity ·
gold · spirocyclic pyrrolidinoindoline
[10] B. E. Maryanoff, H. C. Zhang, J. H. Cohen, I. J. Turchi, C. A. Maryanoff,
Chem. Rev. 2004, 104, 1431.
[1] M. Zeches, T. Ravao, B. Richard, G. Massiot, L. LeMen-Olivier, J. Guilhem,
C. Pascard, Tetrahedron Lett. 1984, 25, 659.
[11] a) R. Dorel, A. M. Echavarren, J. Org. Chem. 2015, 80, 7321; b) R. Dorel,
A. M. Echavarren, Chem. Rev. 2015, 115, 9028.
[12] R. B. Dateer, B. S. Shaibu, R.-S. Liu, Angew. Chem. Int. Ed. 2012, 51, 113;
Angew. Chem. 2012, 124, 117.
[13] Y. Yamaoka, T. Yoshida, M. Shinozaki, K.-i. Yamada, K. Takasu, J. Org.
Chem. 2015, 80, 957.
[2] T. Kam, Y. Choo, Phytochemistry 2004, 65, 2119–2122.
[3] C. Pellegrin, M. Weber, H. J. Borschberg, Helv. Chim. Acta. 1996, 79, 151.
[4] I. D. Rae, M. Rosenberger, A. G. Szabo, C. R. Willis, P. Yates, D. E. Zacha-
rias, G. A. Jeffrey, B. Douglas, J. L. Kirkpatrick, J. A. Weisbach, J. Am.
Chem. Soc. 1967, 89, 3061.
[5] B. Witkop, J. Am. Chem. Soc. 1948, 70, 3712.
[14] Y. Zhang, R. P. Hsung, X. Zhang, J. Huang, B. W. Slafer, A. Davis, Org. Lett.
2005, 7, 1047.
[15] P. Garcia, Y. Harrak, L. Diab, P. Cordier, C. Ollivier, V. Gandon, M. Malacria,
L. Fensterbank, C. Aubert, Org. Lett. 2011, 13, 2952.
[16] A. S. K. Hashmi, L. Schwarz, J.-H. Choi, T. M. Frost, Angew. Chem. Int. Ed.
2000, 39, 2285; Angew. Chem. 2000, 112, 2382.
[17] a) R. B. Dateer, K. Pati, R.-S. Liu, Chem. Commun. 2012, 48, 7200; b) M.
Dos Santos, P. W. Davies, Chem. Commun. 2014, 50, 6001; c) Y. Tokimizu,
M. Wieteck, M. Rudolph, S. Oishi, N. Fujii, A. S. K. Hashmi, H. Ohno, Org.
Lett. 2015, 17, 604; d) A. H. Zhou, Q. He, C. Shu, Y.-F. Yu, S. Liu, T. Zhao,
W. Zhang, X. Lu, L.-W. Ye, Chem. Sci. 2015, 6, 1265; e) L. Zhu, Y. Yu, Z.
Mao, X. Huang, Org. Lett. 2015, 17, 30; f) S. J. Heffernan, J. M. Beddoes,
M. F. Mahon, A. J. Hennessy, D. R. Carbery, Chem. Commun. 2013, 49,
2314; g) J. Liu, M. Chen, L. W. Zhang, Y. H. Liu, Chem. Eur. J. 2015, 21,
1009.
[18] Y. X. Liu, W.-P. Xu, X. Wang, Org. Lett. 2010, 12, 1448.
[19] a) R. L. LaLonde, B. D. Sherry, E. J. Kang, F. D. Toste, J. Am. Chem. Soc.
2007, 129, 2452; b) W. W. Zi, F. D. Toste, J. Am. Chem. Soc. 2013, 135,
12600; c) M. Bandini, A. Eichholzer, Angew. Chem. Int. Ed. 2009, 48,
9533; Angew. Chem. 2009, 121, 9697.
[20] G. Cera, M. Chiarucci, A. Mazzanti, A. Mancinelli, M. Bandini, Org. Lett.
2012, 14, 1350.
[21] a) F. Y. Miyake, K. Yakushijin, D. A. Horne, Org. Lett. 2004, 6, 711; b) S. T.
Hilton, T. C. T. Ho, G. Pljevaljcic, K. Jones, Org. Lett. 2000, 2, 2639; c) H.
Liu, G. Dou, D. Shi, J. Comb. Chem. 2010, 12, 292.
[22] A. S. K. Hashmi, J. P. Weyrauch, W.-F. Frey, J. W. Bats, Org. Lett. 2004, 6,
4391.
[23] a) For a recent review see: M. C. Haibach, D. Seidel, Angew. Chem. Int.
Ed. 2014, 53, 5010; Angew. Chem. 2014, 126, 5110; b) A. S. K. Hashmi,
Angew. Chem. Int. Ed. 2010, 49, 5232; Angew. Chem. 2010, 122, 5360.
[6] a) For a review article see: G. S. Singh, Z. Y. Desta, Chem. Rev. 2012, 112,
6104–6155; for selected synthetic examples see: b) R. B. Woodward,
M. P. Cava, W. D. Ollis, A. Hunger, H. U. Daeniker, K. Schenker, J. Am.
Chem. Soc. 1954, 76, 4749; c) G. Buechi, K. E. Matsumoto, H. Nishimura,
J. Am. Chem. Soc. 1971, 93, 3299; d) K. Cardwell, B. Hewitt, M. Ladlow, P.
Magnus, J. Am. Chem. Soc. 1988, 110, 2242; e) S. Kobayashi, G. Peng, T.
Fukuyama, Tetrahedron Lett. 1999, 40, 1519; f) F. He, Y. Bo, J. D. Altom,
E. J. Corey, J. Am. Chem. Soc. 1999, 121, 6771; g) S. A. Kozmin, T. Iwama,
Y. Huang, V. H. Rawal, J. Am. Chem. Soc. 2002, 124, 4628; h) J. P. Marino,
M. B. Rubio, G. F. Cao, A. de Dios, J. Am. Chem. Soc. 2002, 124, 13398;
i) J. D. White, Y. Li, D. C. Ihle, J. Org. Chem. 2010, 75, 3569; j) B. B. Liau,
M. D. Shair, J. Am. Chem. Soc. 2010, 132, 9594; k) S. B. Jones, B. Sim-
mons, A. Mastracchio, D. W. C. MacMillan, Nature 2011, 475, 183; l) O.
Wagni›res, Z.-R. Xu, Q. Wang, J. Zhu, J. Am. Chem. Soc. 2014, 136,
15102; m) N. Sharma, Z. H. Li, U. K. Sharma, E. V. Van der Eycken, Org.
Lett. 2014, 16, 3884; n) M. Mewald, J. W. Medley, M. Movassaghi, Angew.
Chem. Int. Ed. 2014, 53, 11634; Angew. Chem. 2014, 126, 11818.
[7] For review articles see: a) K. A. DeKorver, H.-Y. Li, A. G. Lohse, R. Hayashi,
Z.-J. Lu, Y. Zhang, R. P. Hsung, Chem. Rev. 2010, 110, 5064; b) G. Evano,
A. Coste, K. Jouvin, Angew. Chem. Int. Ed. 2010, 49, 2840; Angew. Chem.
2010, 122, 2902; c) X.-N. Wang, H.-S. Yeom, L.-C. Fang, S.-Z. He, Z.-X. Ma,
B. L. Kedrowski, R. P. Hsung, Acc. Chem. Res. 2014, 47, 560;. for selected
syntheses see: d) S. Couty, C. Meyer, J. Cossy, Angew. Chem. Int. Ed.
2006, 45, 6726; Angew. Chem. 2006, 118, 6878; e) S. Couty, B. Liegault,
C. Meyer, J. Cossy, Tetrahedron 2009, 65, 1809; f) A. S. K. Hashmi, S. Pan-
kajakshan, M. Rudolph, E. Enns, T. Bander, F. Rominger, W. Frey, Adv.
Synth. Catal. 2009, 351, 2855; g) D. Vasu, H. H. Hung, S. Bhunia, S. A.
Gawade, A. Das, R. S. Liu, Angew. Chem. Int. Ed. 2011, 50, 6911; Angew.
Chem. 2011, 123, 7043; h) N. Saito, T. Ichimaru, Y. Sato, Org. Lett. 2012,
14, 1914; i) K. B. Wang, R. Q. Ran, S. D. Xiu, C. Y. Li, Org. Lett. 2013, 15,
2374; j) X. N. Wang, G. N. Winston-McPherson, M. C. Walton, Y. Zhang,
R. P. Hsung, K. A. DeKorver, J. Org. Chem. 2013, 78, 6233; k) N. Ghosh, S.
Nayak, A. K. Sahoo, Chem-Eur. J. 2013, 19, 9428; l) C. Theunissen, B.
MØtayer, N. Henry, G. Compain, J. Marrot, A. M. Mingot, S. Thibaudeau,
G. Evano, J. Am. Chem. Soc. 2014, 136, 12528; m) T. Okitsu, K. Nakata, K.
Nishigaki, N. Michioka, M. Karatani, A. Wada, J. Org. Chem. 2014, 79,
5914; n) Y. Tokimizu, S. Oishi, N. Fujii, H. Ohno, Org. Lett. 2014, 16, 3138;
o) S. N. Karad, R.-S. Liu, Angew. Chem. Int. Ed. 2014, 53, 9072; p) S. E. Kir-
Manuscript received: August 19, 2015
Accepted Article published: September 16, 2015
Final Article published: October 1, 2015
Chem. Asian J. 2016, 11, 371 – 375
www.chemasianj.org
375
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim