Merging gold and organocatalysis: A facile asymmetric synthesis of annulated pyrroles

Article abstract of DOI:10.1002/chem.201400407

The combination of cinchona-alkaloid-derived primary amine and Au ^I-phosphine catalysts allowed the selective C-H functionalization of two adjacent carbon atoms of pyrroles under mild reaction conditions. This sequential dual activation provides seven-membered-ring-annulated pyrrole derivatives in excellent yields and enantioselectivities. Outwitted: The combination of cinchona-alkaloid-derived primary amine and Au ^I-phosphine catalysts allowed the selective C-H functionalization of two adjacent carbon atoms of pyrroles under mild reaction conditions. This sequential dual activation provides seven-membered-ring-annulated pyrrole derivatives in excellent yields and enantioselectivities (see scheme).

Full text of DOI:10.1002/chem.201400407

DOI: 10.1002/chem.201400407
Communication
&
Multicatalysis
Merging Gold and Organocatalysis: A Facile Asymmetric Synthesis
of Annulated Pyrroles
Daniel Hack, Charles C. J. Loh, Jan M. Hartmann, Gerhard Raabe, and Dieter Enders*[a]
Therefore, we report a new asymmetric one-pot dual catalyt-
Abstract: The combination of cinchona-alkaloid-derived
ic protocol that uses primary amine and AuI catalysis to access
primary amine and AuI–phosphine catalysts allowed the
2,3-annulated pyrroles containing a seven-membered ring
selective CÀH functionalization of two adjacent carbon
(Scheme 1b). This method is intriguing, because medium-sized
atoms of pyrroles under mild reaction conditions. This se-
rings are difficult to synthesize by conventional organocatalytic
quential dual activation provides seven-membered-ring-
methods.[8] Moreover, a publication documenting a AuI-cata-
annulated pyrrole derivatives in excellent yields and enan-
lyzed 7-endo-dig cyclization mode on pyrrole substrates is not
tioselectivities.
known to date. Such cyclization modes have only been known
to occur when platinum or AuIII catalysis was utilized.[9] To the
best of our knowledge, the method described herein is the
Although gold catalysis and organocatalysis have rapidly
grown since the turn of the millennium and emerged as pow-
erful tools in the general field of catalysis, examples of the
combination of gold and organocatalysis in sequential and co-
operative tandem reactions exploiting complementary activa-
tion modes are still scarce.[1–3] Recently, we reported the asym-
metric synthesis of tetracyclic indole derivatives containing
seven-membered rings by the merger of a thioamide-based or-
ganocatalyst with a AuI catalyst to effect two consec-
utive Friedel–Crafts-type reactions on unsubstituted
indole substrates (Scheme 1a).[4]
first known example of an asymmetric one-pot operation, in
which pyrroles act as a double nucleophile, hence augmenting
the operational efficiency of this protocol.
To achieve the annulated pyrrole targets, we first focused on
the optimization of the Friedel–Crafts-Michael-type reaction.
For the 1,4-addition of pyrrole to enone 2a, primary amines 4–
6 derived from amino acids and cinchona-alkaloid-derived
amines 7–10 together with trifluoroacetic acid (TFA) as additive
Due to the immense importance of the indole
core, major emphasis has been given to the develop-
ment of asymmetric Friedel–Crafts reactions involving
indole derivatives. Pyrrole is another electron-rich
heteroaromatic compound, core of which is found in
many natural products.[5,6] One attractive aspect of
pyrrole chemistry that is unseen in indole substrates
is the inherent nucleophilicity on the C2 position,
which stands in contrast to indoles having a classical
C3 nucleophilic site. Because Michael-type reactions
of pyrroles usually gives 2,5-dialkylated products, it is
difficult to monofunctionalize pyrrole substrates
(Scheme 1c).[7] To avoid this problem, we wanted to
selectively functionalize two adjacent sites on the
pyrrolic heterocycle by using two different catalytic
modes of activation to generate new annulated pyr-
role derivatives in a one-pot reaction, which is quite
difficult to achieve by using conventional methods.
Scheme 1. Strategy comparison between current work and our recently reported annula-
tions of indoles.
[a] D. Hack, Dr. C. C. J. Loh, J. M. Hartmann, Prof. Dr. G. Raabe,
Prof. Dr. D. Enders
were employed (Scheme 2).[10,11] The primary amines 4–6
showed poor to good conversions with good enantioselectivity
values, whereas the reactions with the primary amines 7–10
were finished within one day and provided comparable or
better enantioselectivity values.
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax: (+49)241-809-2127
E-mail: enders@rwth-aachen.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400407.
Chem. Eur. J. 2014, 20, 3917 – 3921
3917
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Figure 1. AuI catalysts employed for the cyclization.
complexes (Figure 1). We observed that all gold catalysts pro-
moted the cyclization reaction of the Friedel–Crafts product 3
in toluene, generally within 30 min and in excellent yields
(Table 2). Only the triazole–gold complex 16 showed lower re-
activity due to its higher stability and the strong coordination
of the triazole ligand to the gold center (Table 2, entry 6).[13] Al-
though there was no huge difference in terms of yields, using
the Echavarren-type catalysts 12–14 resulted in a cleaner iso-
merization without the formation of unwanted by-products
(Table 2, Entry 2-4).[14]
Scheme 2. Catalyst screening for the Friedel–Crafts Michael-type reaction.
The catalyst 10 gave the highest enantioselectivity value,
and further optimization was carried out by screening different
solvents (Table 1). It turned out that the choice of the solvent
did not have any crucial influence on the yield or the observed
enantioselectivity values. However, we were able to obtain
better yields and slightly improved enantioselectivities at
higher dilution and lower temperature. Under these conditions,
the amount of dialkylated pyrrole was low, and no other by-
products could be observed. This fact is remarkable, because
most known methods on 1,4-additions of pyrroles mainly give
dialkylated products.[5] We did not investigate the influence of
different acids or different amounts of acid, because no benefi-
cial effect was observed in related pyrrole 1,4-additions.[12]
After having optimized the Michael addition, we directed
our focus on the cyclization step by screening various gold(I)
It is known from the literature that amines might deactivate
gold(I) complexes by coordination to the vacant binding si-
te.[3e–g,k,15] However, the active catalyst can be regenerated
upon addition of acidic additives. As was expected, we did not
observe any conversion of 3 under the reported conditions
when only organocatalyst 10 was present (Table 2, entry 12).
On the contrary, the reaction was completed within 30 min, if
30 mol% TFA was also present, and the product 17a was ob-
tained in excellent yields (Table 2, entry 13).
Table 2. Optimization studies for the gold-catalyzed cyclization.[a]
Table 1. Optimization of the reaction conditions for the Friedel–Crafts
Michael-type reaction.[a]
Entry
Catalyst
Additive
t [h]
0.5
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
11/AgNTf2
12/AgNTf2
13/AgNTf2
14
15/AgNTf2
16
AgNTf2
CuI
Cu(OTf)2
PtCl2
–
–
–
–
–
–
–
–
89
89
99
92
96
76
–
–
–
–
–
0.5
0.5
0.5
0.5
Entry
Solvent [mL]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7[d]
8[d]
CHCl3 (1.5)
CH2Cl2 (1.5)
toluene (1.5)
PhCl (1.5)
toluene (3.0)
CH2Cl2 (3.0)
CHCl3 (3.0)
toluene (3.0)
16
15
21
21
21
24
65
65
56
59
60
61
70
68
89
95
91
87
88
87
91
91
93
93
30
>24
>24
>24
>24
>24
>24
–
–
–
30 mol% TFA
20 mol% 10
20 mol% 10, 30 mol% TFA
13/AgNTf2
13/AgNTf2
–
96
[a] General reaction conditions: 2a (0.5 mmol), pyrrole 1a (1.0 mmol), 10
(20 mol%), TFA (30 mol%), rt. [b] Yield of isolated 3. [c] Determined by
HPLC analysis on a chiral stationary phase. [d] The reaction was per-
formed at 08C.
0.5
[a] General reaction conditions: 2 (0.3 mmol), AgNTf2 (10 mol%), toluene
(1.7 mL), rt. [b] Yield of isolated 17a.
Chem. Eur. J. 2014, 20, 3917 – 3921
www.chemeurj.org
3918
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
thylsilyl (TMS) protected alkynes. Although both substrates re-
acted smoothly in the Michael addition, no desired product
could be isolated after the gold-catalyzed cycloisomeriza-
tion.[16]
The absolute configuration was assigned by X-ray crystal-
structure analysis of (R)-17b (Figure 2).[17] The absolute configu-
ration of the other products was assigned assuming a uniform
reaction pathway. To demonstrate the practicability of this pro-
tocol, we conducted the asymmetric synthesis of 17b on
a larger scale with slightly lower yields, but improved enantio-
selectivity (Scheme 4). The product was converted to the corre-
sponding alcohol by reduction with sodium borohydride at
À788C to give a mixture of two diastereomers 18 in good
yield (Scheme 4).
Although we still lack further information on the mecha-
nism, a plausible reaction pathway is depicted in Scheme 5.
Figure 2. X-ray crystal structure of (R)-17b.
Thus, it was not necessary to add any further additives, be-
cause the same amount of TFA had to be already added in the
Michael addition. In an additional control experiment, we
could show that TFA does not catalyze the cycloisomerization,
because no product could be observed after 24 h (Table 2,
entry 11). In addition, other metal catalysts containing platinum
or copper also failed to promote this reaction, although those
metals are strongly associated with the activation of alkynes
(Table 2, entries 8–10).
With the optimized conditions in hand, a variety of substi-
tuted enones and pyrroles were used to demonstrate the flexi-
bility of the reported method (Scheme 3). To our delight, we
obtained good to excellent yields and enantioselectivity values
for all enones tested, tolerating electron-withdrawing, as well
as electron-donating, groups (EWG and EDG, respectively;
17a–f). Likewise, 2-aryl-pyrroles can also be used for this reac-
tion, albeit with slightly lower
Scheme 3. Scope of the sequential Michael addition/cyclization reaction.
yields and enantioselectivity
values (17g–l). Apparently, the
increased steric bulk introduced
by the additional aryl group on
pyrrole seems to hamper the
transition state in the enantiose-
lective step. In addition, we ob-
served that the products are less
stable to acid and heat than the
products, which are derived
from unsubstituted pyrrole, thus
leading to lower yields. Further,
we investigated if the method
could be extended to enones
with terminal alkynes and trime- Scheme 4. Large-scale synthesis of compound 17b followed by reduction to the alcohol 18.
Chem. Eur. J. 2014, 20, 3917 – 3921 www.chemeurj.org ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3919

Communication
Scheme 5. Plausible reaction mechanism.
action was monitored by TLC analysis. After completion, a suspen-
sion of AgNTf2 (10 mg, 0.10 mmol) and catalyst 13 (13 mg,
0.10 mmol) in toluene (1 mL) was added to the reaction mixture at
room temperature. After complete conversion, the crude product
was directly subjected to flash chromatography (silica, n-pentane/
diethyl ether).
The organocatalytic reaction is driven by the formation of the
iminium ion by condensation of the TFA salt of the primary
amine 10 and the enone 2a. This LUMO activation of the sub-
strate facilitates the nucleophilic attack of pyrrole. The ob-
served stereoselectivity can be attributed to the covalent
bonding between the enone and the primary amine, as well as
hydrogen bonding between pyrrole and the quinuclidine back-
bone of the catalyst with trifluoroacetate as mediator.[12]
After hydrolysis, the intermediate 3 can enter the gold-cata-
lyzed cycle. In consent with the reported literature, we believe
that the mechanism for the gold-catalyzed step can be ration-
alized by an initiating 6-endo-dig cyclization of the more nucle-
ophilic C2 position of pyrrole to the internal alkyne.[9,18] The
alkyne is activated by coordination via the p-acidic AuI com-
plex 20 to form a non-aromatic spirocyclic intermediate 21,
which undergoes fast rearrangement to the seven-membered
ring 22 followed by rearomatization and protodeauration.
Thus, the final products 17 seem to be derived from a 7-endo-
dig cyclization.
Acknowledgements
D.H. thanks the DFG (International Research Training Group
“Selectivity in Chemo- and Biocatalysis”—Seleca) and D.E.
thanks the European Research Council (ERC Advanced Grant
“DOMINOCAT”) for financial support.
Keywords: annulation · gold catalysis · organocatalysis ·
primary-amine catalysis · pyrroles
[1] For selected general reviews on gold catalysis, see: a) A. S. K. Hashmi,
G. J. Hutchings, Angew. Chem. 2006, 118, 8064; Angew. Chem. Int. Ed.
2006, 45, 7896; b) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; c) A.
Fꢁrstner, P. W. Davies, Angew. Chem. 2007, 119, 3478; Angew. Chem. Int.
Ed. 2007, 46, 3410; d) D. J. Gorin, F. D. Toste, Nature 2007, 446, 395; e) J.
Muzart, Tetrahedron 2008, 64, 5815; f) Z. Li, C. Brouwer, C. He, Chem.
Rev. 2008, 108, 3239; g) A. Arcadi, Chem. Rev. 2008, 108, 3266; h) E. Ji-
mꢂnez-Nffln?ez, A. M. Echavarren, Chem. Rev. 2008, 108, 3326; i) D. J.
Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351; j) R. Skouta,
C.-J. Li, Tetrahedron 2008, 64, 4917; k) R. A. Widenhoefer, Chem. Eur. J.
2008, 14, 5382; l) S. F. Kirsch, Synthesis 2008, 3183; m) N. Bongers, N.
Krause, Angew. Chem. 2008, 120, 2208; Angew. Chem. Int. Ed. 2008, 47,
2178; n) A. Fꢁrstner, Chem. Soc. Rev. 2009, 38, 3208; o) N. Shapiro, F. D.
Toste, Synlett 2010, 675; p) S. Sengupta, X. Shi, ChemCatChem 2010, 2,
609; q) A. S. K. Hashmi, Angew. Chem. 2010, 122, 5360; Angew. Chem.
Int. Ed. 2010, 49, 5232; r) M. Bandini, Chem. Soc. Rev. 2011, 40, 1358;
s) N. Krause, C. Winter, Chem. Rev. 2011, 111, 1994; t) A. Pradal, P. Y. Toul-
lec, V. Michelet, Synthesis 2011, 1501; u) A. Corma, A. Leyva-Pꢂrez, M. J.
Sabater, Chem. Rev. 2011, 111, 1657; v) M. Rudolph, A. S. K. Hashmi,
Chem. Commun. 2011, 47, 6536; w) M. Rudolph, A. S. K. Hashmi, Chem.
Soc. Rev. 2012, 41, 2448.
In summary, we have developed a convenient one-pot asym-
metric synthesis of annulated pyrroles based on a rare 7-endo-
dig cyclization, thus functionalizing two adjacent carbon atoms
on pyrrole by direct CÀH functionalization. The combination of
a cinchona-alkaloid-derived primary amine and a AuI–phos-
phine catalyst gave excellent yields and enantioselectivity
values, which are rarely achieved in pyrrole chemistry.
Experimental Section
Typical procedure
Freshly distilled pyrrole (69 mL, 1.00 mmol) was added to a solution
of 9-amino(9-deoxy)epi cinchonine (10; 29 mg, 0.20 mmol), TFA
(16 mL, 0.15 mmol), and enone (0.5 mmol) in toluene (3 mL) at 08C.
The reaction mixture was stirred at 08C, and the progress of the re-
Chem. Eur. J. 2014, 20, 3917 – 3921
www.chemeurj.org
3920
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[2] For selected reviews on organocatalysis, see: a) A. Berkessel, H. Grçger,
Asymmetric Organocatalysis, Wiley-VCH, Weinheim, 2005; b) Enantiose-
lective Organocatalysis (Ed.: P. I. Dalko), Wiley-VCH, Weinheim, 2007;
c) Special issue on organocatalysis: B. List, Chem. Rev. 2007, 107, 5413;
d) H. Pellissier, Tetrahedron 2007, 63, 9267; e) R. M. de Figueiredo, M.
Christmann, Eur. J. Org. Chem. 2007, 2575; f) D. Enders, C. Grondal,
M. R. M. Hꢁttl, Angew. Chem. 2007, 119, 1590; Angew. Chem. Int. Ed.
2007, 46, 1570; g) A. Dondoni, A. Massi, Angew. Chem. 2008, 120, 4716;
Angew. Chem. Int. Ed. 2008, 47, 4638; h) D. W. C. MacMillan, Nature
2008, 455, 304; i) C. F. Barbas, Angew. Chem. 2008, 120, 44; Angew.
Chem. Int. Ed. 2008, 47, 42; j) D. Enders, A. A. Narine, J. Org. Chem. 2008,
73, 7857; k) P. Melchiorre, M. Marigo, A. Carlone, G. Bartoli, Angew.
Chem. 2008, 120, 6232; Angew. Chem. Int. Ed. 2008, 47, 6138; l) K. A. Jør-
gensen, S. Bertelsen, Chem. Soc. Rev. 2009, 38, 2178; m) M. Bella, T. Gas-
peri, Synthesis 2009, 1583; n) H. Pellissier, Recent Developments in Asym-
metric Organocatalysis, RCS Publishing, Cambridge, 2010; o) K. Maruoka,
B. List, H. Yamamoto, L.-Z. Gong, Chem. Commun. 2012, 48, 10703; p) H.
Pellissier, Adv. Synth. Catal. 2012, 354, 237; q) B. List, K. Maruoka, Asym-
metric Organocatalysis (in Science of Synthesis), Thieme, Stuttgart, 2012;
r) Comprehensive Enantioselective Organocatalysis: Catalysts, Reactions
and Applications (Ed: P. I. Dalko), Wiley-VCH, Weinheim, 2013.
Wong, R. Wang, Chem. Eur. J. 2009, 15, 11105; g) P. K. Singh, V. K. Singh,
Org. Lett. 2010, 12, 80; h) Y. Huang, S. Suzuki, G. Liu, E. Tokunaga, M.
Shiro, N. Shibata, New J. Chem. 2011, 35, 2614; L. Liu, H. Ma, Y. Xiao, F.
Du, Z. Qin, N. Li, B. Fu, Chem. Commun. 2012, 48, 9281; i) P. Chauhan,
S. S. Chimni, RSC Adv. 2012, 2, 6117.
[6] a) A. Fꢁrstner, Angew. Chem. 2003, 115, 3706; Angew. Chem. Int. Ed.
2003, 42, 3582; b) H. Hoffmann, T. Lindl, Synthesis 2003, 1753; c) G.
Balme, Angew. Chem. 2004, 116, 6396; Angew. Chem. Int. Ed. 2004, 43,
6238; d) B. Jolicoeur, E. E. Chapman, A. Thompson, W. D. Lubell, Tetrahe-
dron 2006, 62, 11531; e) C. T. Walsh, S. Garneau-Tsodikova, A. R. Howard-
Jones, Nat. Prod. Rep. 2006, 23, 517; f) J. T. Gupton, Heterocyclic Antitu-
mor Antibiotics, Springer, Heidelberg, 2006, p.53.
[7] a) Y. R. Jorapur, C. H. Lee, D. Y. Chi, Org. Lett. 2005, 7, 1231; b) G. Blay, I.
Fernµndez, M. C. Mun?oz, J. R. Pedro, A. Recuenco, C. Vila, J. Org. Chem.
2011, 76, 6286.
[8] T. V. Nguyen, J. M. Hartmann, D. Enders, Synthesis 2012, 44, 845.
[9] a) E. Borsini, G. Broggini, A. Fasana, C. Baldassarri, A. M. Manzo, A. D.
Perboni, Beilstein J. Org. Chem. 2011, 7, 1468; b) S. G. Modha, A. Kumar,
D. D. Vachhani, S. K. Sharma, V. S. Parmar, E. V. Van der Eycken, Chem.
Commun. 2012, 48, 10916; c) A. Kumar, D. D. Vachhani, S. G. Modha,
S. K. Sharma, V. S. Parmar, E. V. Van der Eycken, Eur. J. Org. Chem. 2013,
2013, 2288.
[3] C. C. J. Loh, D. Enders, Chem. Eur. J. 2012, 18, 10212. For recent exam-
ples, see: a) J. T. Binder, B. Crone, T. T. Haug, H. Menz, S. F. Kirsch, Org.
Lett. 2008, 10, 1025; b) Z.-Y. Han, H. Xiao, X.-H. Chen, L.-Z. Gong, J. Am.
Chem. Soc. 2009, 131, 9182; c) X.-Y. Liu, C.-M. Che, Org. Lett. 2009, 11,
4204; d) M. E. Muratore, C. A. Holloway, A. W. Pilling, R. I. Storer, G. Tre-
vitt, D. J. Dixon, J. Am. Chem. Soc. 2009, 131, 10796; e) S. Belot, K. Vogt,
C. Besnard, N. Krause, A. Alexakis, Angew. Chem. 2009, 121, 9085; f) T.
Zweifel, D. Hollmann, B. Prꢁger, M. Nielsen, K. A. Jørgensen, Tetrahe-
dron: Asymmetry 2010, 21, 1624; g) K. L. Jensen, P. T. Franke, C. Arróniz,
S. Kobbelgaard, K. A. Jørgensen, Chem. Eur. J. 2010, 16, 1750; h) C.
Wang, Z.-Y. Han, H.-W. Luo, L.-Z. Gong, Org. Lett. 2010, 12, 2266; i) D.
Monge, K. L. Jensen, P. T. Franke, L. Lykke, K. A. Jørgensen, Chem. Eur. J.
2010, 16, 9478; j) D. M. Barber, H. J. Sanganee, D. J. Dixon, Org. Lett.
2012, 14, 5290; k) M. Chiarucci, M. di Lillo, A. Romaniello, P. G. Cozzi, G.
Cera, M. Bandini, Chem. Sci. 2012, 3, 2859; l) N. T. Patil, V. S. Raut, R. B.
Tella, Chem. Commun. 2013, 49, 570; m) H. Wu, Y.-P. He, L.-Z. Gong, Org.
Lett. 2013, 15, 460; n) A. W. Gregory, P. Jakubec, P. Turner, D. J. Dixon,
Org. Lett. 2013, 15, 4330.
[10] Y.-Q. Yang, G. Zhao, Chem. Eur. J. 2008, 14, 10888.
[11] For selected reviews and protocols on cinchona-derived amines, see:
a) L.-W. Xu, J. Luo, Y. Lu, Chem. Commun. 2009, 1807; b) T. Marcelli, H.
Hiemstra, Synthesis 2010, 1229; c) C. Cassani, R. Martín-Rapffln, E. Arceo,
F. Bravo, P. Melchiorre, Nat. Protoc. 2013, 8, 325; d) P. Melchiorre, Angew.
Chem. Int. Ed. 2012, 51, 9748; Angew. Chem. 2012, 124, 9889.
[12] D. Hack, D. Enders, Synthesis 2013, 45, 2904.
[13] H. Duan, S. Sengupta, J. L. Petersen, N. G. Akhmedov, X. Shi, J. Am.
Chem. Soc. 2009, 131, 12100.
[14] C. Nieto-Oberhuber, S. López, A. M. Echavarren, J. Am. Chem. Soc. 2005,
127, 6178.
[15] P. C. Young, S. L. J. Green, G. M. Rosair, A.-L. Lee, Dalton Trans. 2013, 42,
9645.
[16] From our observation, the TMS-protected product decomposed rapidly,
whereas the terminal alkyne underwent a complicated rearrangement
with loss of the stereocenter to an unidentified product.
[17] CCDC-977585 (17b) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[4] C. C. J. Loh, J. Badorrek, G. Raabe, D. Enders, Chem. Eur. J. 2011, 17,
13409.
[5] For selected asymmetric Michael additions of pyrroles, see: a) N. A.
Paras, D. W. C. MacMillan, J. Am. Chem. Soc. 2001, 123, 4370; b) C.-L.
Cao, Y.-Y. Zhou, X.-L. Sun, Y. Tang, Tetrahedron 2008, 64, 10676; c) B. M.
Trost, C. Mꢁller, J. Am. Chem. Soc. 2008, 130, 2438; d) Y.-F. Sheng, Q. Gu,
A.-J. Zhang, S.-L. You, J. Org. Chem. 2009, 74, 6899; e) N. Yokoyama, T.
Arai, Chem. Commun. 2009, 3285; f) L. Hong, W. Sun, C. Liu, L. Wang, K.
[18] D. B. England, A. Padwa, Org. Lett. 2008, 10, 3631.
Received: January 31, 2014
Published online on March 3, 2014
Chem. Eur. J. 2014, 20, 3917 – 3921
www.chemeurj.org
3921
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim