Tuning electronic and steric effects: Highly enantioselective [4+1] pyrroline annulation of sulfur ylides with α,β-unsaturated imines

Article abstract of DOI:10.1002/anie.201000755

(Chemical equation presented) A change in tune: A novel [4+1] annulation between atropisomeric sulfur ylides 1 and unsaturated imines 2 was developed. The method allows the synthesis of enantioenriched pyrroline products 3 in high yield and with excellent stereocontrol. The origin of stereoinduction was primarily rationalized by a conformational analysis of the ylides.

Full text of DOI:10.1002/anie.201000755

Angewandte
Chemie
DOI: 10.1002/anie.201000755
Cycloadditions
Tuning Electronic and Steric Effects: Highly Enantioselective [4+1]
Pyrroline Annulation of Sulfur Ylides with a,b-Unsaturated Imines**
Liang-Qiu Lu, Ji-Ji Zhang, Fang Li, Ying Cheng, Jing An, Jia-Rong Chen,* and Wen-Jing Xiao*
The [4+1] annulation of 1,3-conjugated systems with two-
electron, one-carbon fragments presents a particularly attrac-
tive but underexploited strategy to construct naturally occur-
ring and biologically active carbo- and heterocyclopentenes,
especially asymmetrically.[1] In this context, besides carbon
monoxide,[2] isocyanides,[3] nucleophilic carbenes,[4] and diazo
reagents[5] used as typical one-carbon units, ylides[6] have also
been gradually developed as functional units and reacted with
various electron-deficient conjugated components to provide
such scaffolds.[7] For example, Danheiser and co-workers have
shown that sulfur ylides can be used in the [4+1] annulation of
moieties with high chemo- and stereoselectivity (83–99%
yields, up to > 95:5 d.r., and 98% ee).
In spite of the remarkable significance of pyrrolines and
their derivatives in enantioselective catalysis,[8] natural prod-
ucts, and pharmaceuticals,[9] the alternative [4+1] annulation
of azadienes has been little used for their synthesis,[10]
compared with the popular [3+2] approach.[11] To our knowl-
edge, almost all previous endeavors with unstable or semi-
stabilized ylides have suffered from the kinetic preference to
form aziridines or cyclopropanes (Scheme 2a).[12,13] Therefore
(trialkylsilyl)vinylketenes
to
yield
cyclopentenones
(Scheme 1, top).[7a] Recently, by employing a camphor-
Scheme 1. Previous [4+1] annulations of sulfur ylides largely relied
upon steric hindrance. Dashed arrows indicate the [2+1] pathway.
derived sulfur ylide, Tangꢀs group developed a highly efficient
method to synthesize enantiopure dihydrofuran derivatives
from substituted methene-2,4-pentanediones (Scheme 1, bot-
tom).[7c] However, as analyzed by Sun and Tang,[6c] the success
of these [4+1] routes over competitive [2 + 1] routes is largely
attributed to the steric hindrance at the reactive centers.
Herein, independent of this effect, we describe an unprece-
dented [4+1] annulation of sulfur ylides with a,b-unsaturated
imines, which allows efficient construction of pyrroline
Scheme 2. a) Previously reported methods for cyclopropanes and azir-
idines. b) Realizing the [4+1] pyrroline annulation of sulfur ylides and
a,b-unsaturated imines by tuning electronic effects. PMP=para-
methoxyphenyl, Ts=4-toluenesulfonyl.
the [4+1] annulation between sulfur ylides and a,b-unsatu-
rated imines to synthesize pyrrolines remains a stimulating
challenge.
On the basis of our previous study on the chemistry of
stable sulfur ylides,[14] we speculated that the electronic effect
could be another potential factor by which to alter the kinetic
preference. If we could extend the lifetime of the key
zwitterionic intermediates, generated from the Michael
addition of sulfur ylides to a,b-unsaturated imines, this
would allow formation of the thermodynamic products
chemoselectively. Indeed, treatment of phenylacyl sulfur
ylide 1 with ester-bearing Ts-protected a,b-unsaturated
imine[15] in CH2Cl2 at room temperature (Scheme 2b) pro-
duced the desired pyrroline-2-carboxylate in 83% yield
within 0.5 hours. In contrast, no reaction occurred when the
PMP-protected imine was used. According to these results,
we can conclude that the introduction of electron-withdraw-
[*] L.-Q. Lu, J.-J. Zhang, Dr. F. Li, Y. Cheng, J. An, Dr. J.-R. Chen,
Prof. Dr. W.-J. Xiao
Key Laboratory of Pesticide & Chemical Biology, Ministry of
Education, College of Chemistry, Central China Normal University
152 Luoyu Road, Wuhan, Hubei 430079 (China)
Fax: (+86)27-6786-2041
E-mail: wxiao@mail.ccnu.edu.cn
Homepage: http://chem-xiao.ccnu.edu.cn/
[**] We are grateful to the National Science Foundation of China
(20872043) and the Program for Academic Leader in Wuhan
Municipality (200851430486) for support of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000755.
Angew. Chem. Int. Ed. 2010, 49, 4495 –4498
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4495

Communications
Table 2: Investigating the scope of sulfur ylides in asymmetric [4+1]
ing groups such as tosyl and ester groups is helpful in
annulation.[a]
1) stabilizing the anionic enamine intermediates and there-
fore leading to chemoselective pyrroline formation by attack
of the nitrogen atom, and 2) it also helps to promote the
Michael addition of ylides to unsaturated imines (Ts vs.
PMP).
Next, we focused our attention on the asymmetric version
of this novel [4+1] annulation. Atropisomeric sulfide 5 was
chosen as the element of stereocontrol because of its ease of
preparation from cheap and readily available starting materi-
als (both (S)- and (R)-binol).[16] Treating sulfur ylide 1a with
imine 2 (PG = Ts) at ꢀ808C for 48 hours and subsequently
warming to room temperature provided the desired enan-
tioenriched [4+1] cycloadduct with an encouraging result of
93% yield and 81% ee (Table 1, entry 1). Optimization of the
reaction conditions led to improved enantioselectivity.
Enhancing the size of the ester (Table 1, entry 1 vs. 2),
electron deficiency of the protecting group (Table 1, entry 1
vs. 3), solvent polarity (Table 1, entries 4–7), or concentration
(Table 1, entry 7 vs. 8) offered no advantage. However, we
found that increasing the steric bulk of the sulfonyl group
substantially improved the enantioselectivity (Table 1,
entries 1 and 4 vs. 9). Owing to the greater repulsive ability
of the TIPBS group, a satisfactory result was finally achieved
with 95% yield, a d.r. value greater than 95:5, and 98% ee
under optimal reaction conditions (Table 1, entry 9).[17]
Experiments that probed the generality of this asymmet-
ric [4+1] annulation were performed. As summarized in
Table 2, the reaction displays a broad scope for sulfur ylides
and excellent levels of chemo- and stereoselectivity are
achieved. Sulfur ylides having electron-rich (Table 2, entries 2
and 3), electron-neutral (Table 2, entry 1), and electron-
deficient substitutents (Table 2, entries 4–6) on the aryl
rings all perform well, furnishing products with 88–98%
yields, d.r. values greater than 95:5, and 95–98% ee. Sub-
Entry
1, R1
3
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
1a, Ph
3a[e]
3b
3c
3d
3e
3 f
3g
3h
3i
95
88
98
90
95
93
85
91
96
99
83
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
98
97
96
98
95
97
97
90
98
82
88
1b, p-MeOC6H4
1c, p-MeC6H4
1d, p-FC6H4
1e, p-ClC6H4
1 f, p-BrC6H4
1g, o-BrC6H4
1h, m-BrC6H4
1i, thiophen-2-yl
9
10
11
=
1j, Ph-CH CH
1k, PhCH2CH2
3j
3k
[a] Conditions: 1 (0.22 mmol), 2 (0.20 mmol), and toluene/CH2Cl2 (9:1,
20 mL), 48 h. [b] Yield of isolated product. [c] Determined by H NMR
methods. [d] Determined by HPLC analysis using a chiral stationary
phase. [e] The absolute configuration of product 3a was established by
X-ray analysis.
1
stituents at the ortho, meta, and para positions on the aryl
rings are tolerated (Table 2, entries 6–8). In the case of the
meta-bromo-substituted ylide 1h, the enantiomeric excess
was at a good level although slightly decreased (Table 2,
entry 8). Notably, the feasibility of this reaction could also be
realized effectively for other stable sulfur ylides, such as
heteroaryl- (Table 2, entry 9), alkenyl- (Table 2, entry 10),
and alkylacyl-substituted (Table 2, entry 11) substrates.
Additionally, a series of ester-bearing unsaturated imines
proved to be suitable for this reaction as highlighted in
Table 3. The electronic nature of aromatic substrates (Table 3,
entries 1–6) has little effect on the yield and generally,
Table 3: Investigating the scope of unsaturated imines in asymmetric
[4+1] annulation.[a]
Table 1: Optimization of reaction conditions for asymmetric [4+1]
annulation.[a]
Entry
R’
PG
Solvent
Yield [%][b]
ee [%][c]
Entry
2, R2
3
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
Me
iPr
Ts
Ts
Ns
TMBS
TMBS
TMBS
TMBS
TMBS
TIPBS
toluene/CH2Cl2 (4:1)
toluene/CH2Cl2 (4:1)
toluene/CH2Cl2 (4:1)
toluene/CH2Cl2 (4:1)
toluene
93
70
82
88
86
87
95
85
95
81
81
83
85
86
33
87
82
98
1
2
3
4
5
6
7
8
2a, Ph
3a
3l
95
91
92
99
96
91
94
89
95
96
93
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
98
97
98
95
95
94
86
90
95
93
96
2b, p-MeOC6H4
2c, p-MeC6H4
2d, p-FC6H4
2e, p-ClC6H4
2 f, p-BrC6H4
2g, o-FC6H4
2h, m-BrC6H4
2i, furan-2-yl
Me
Me
Me
Me
Me
Me
Me
3m
3n
3o
3p
3q
3r
3s
3t
3a
CH2Cl2
toluene/CH2Cl2 (9:1)
toluene/CH2Cl2 (9:1)
toluene/CH2Cl2 (9:1)
8[d]
9[e]
9
=
10
2j, Ph-CH CH
2a, Ph
[a] Conditions: 1a (0.25 mmol), 2 (0.20 mmol), and solvent (20 mL),
48 h. [b] Yield of isolated product. [c] Determined by HPLC analysis using
a chiral stationary phase. [d] 4 mL solvent. [e] Ylide 1a was reduced to
1.1 equiv of imine 2 and d.r. >95:5, determined by H NMR methods.
EWG=electron-withdrawing
11[e]
[a] Conditions: 1a (0.22 mmol), 2 (0.20 mmol), and toluene/CH2Cl2 (9:1,
20 mL), 48 h; entries 1–8 and entry 11: R3 =Me; entries 9 and 10: R3 =
Et. [b] Yield of isolated product. [c] Determined by 1H NMR methods.
[d] Determined by chiral HPLC analysis. [e] Gram scale: 1a (2.5 mmol,
1.08 g), 2a (2.5 mmol, 1.14 g).
1
group,
Ns=4-nitrobenzenesulfonyl,
TMBS=2,4,6-trimethylbenzenesulfonyl, TIPBS=2,4,6-triisopropylben-
zenesulfonyl.
4496 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 4495 –4498

Angewandte
Chemie
excellent stereoselectivity was achieved (91–99% yields,
> 95:5 d.r., 94–98% ee). In exploring another dimension by
varying the position of the substitution on the aryls rings
appended to the imines (Table 3, entries 7 and 8), good results
were still achieved (89% yield, > 95:5 d.r., ꢁ 86% ee).
Furthermore, the heteroaryl- and alkenyl-substituted unsatu-
rated imines can also be employed in this annulation, with
excellent yields and high levels of stereoselectivity being
achieved (Table 3, entries 9 and 10).[18] Importantly, the
reaction can be carried out on the gram scale without obvious
loss in the reaction efficiency (Table 3, entry 11), and with
essentially quantitatively recovered sulfide 5 (95% yield).
In addition, the resultant [4+1] cycloadducts could be
applied to the synthesis of important building blocks. For
instance, two-step deprotection/reduction operations for
optically pure 3a delivered the 4,5-substituted proline ester
6 as a single isomer [Eq. (1); MSA = methanesulfonic acid,
TFA = trifluoroacetic acid]. Moreover, the functional group
introduced together with the one-carbon unit could be
effectively transformed into other moieties. As illustrated in
Equation (2), the carbonyl group in 3a was reduced to a
hydroxy group through the acid-assisted Pd-catalyzed hydro-
genation with excellent yield and significant diastereoselec-
tivity (> 99% yield and 92:8 d.r.).
Scheme 3. Rationalizing the origin of sterꢀeoselectivity based on con-
formational analysis of chiral ylides. ClO4 counterion omitted for
clarity in X-ray crystallographic structure. Thermal ellipsoids are drawn
at 30% probability. q=dihedral angle (q=C37-C38-S2-C50, q was differ-
ent in dimer).
sulfide lead to an asymmetric process that conveniently
provides chiral pyrroline-2-carboxylates with good to excel-
lent results (83–99% yields, up to 98% ee, > 95:5 d.r.).
Although detailed investigations of the reaction mechanism
and stereochemistry with Gaussian calculations are ongoing,
we have rationalized the origin of stereoinduction based on a
conformational analysis of chiral ylides; the high stereoselec-
tivity is attributed to steric effects and possible Coulombic
interactions. What is more promising is that this work could
open up new opportunities for selectively constructing other
carbon- and heterocycles beyond traditional small rings.
To better understand the stereoinduction in this asym-
metric [4+1] annulation, we additionally collected and
analyzed data on the conformations of these ylides in
solution. In view of the open character (Scheme 3, top:
arylacyl group away from atropisomeric backbone) and near
coplanarity (q = 177.38 or 167.48) in the single crystal of 1e’,[19]
we supposed a comformer of ylide 1e as shown in Scheme 3
(top, right). The NOE effects and the presence of an
intramolecular hydrogen bond for the ylide 1e in solution
strongly supports the above proposal. On the basis of this
fundamental assumption, we rationalized the important
effects responsible for the stereochemical course as follows
(Scheme 3, bottom): 1) the unsaturated imines attack the
exposed Si face of chiral ylides to avoid the steric repulsion
between the TIPBS group in the imine and the naphthalenyl
ring in the chiral backbone; 2) this process possibly adopted
the cisoid/quasi [4+2]-addition conformation because of
favorable Coulombic interactions between the negatively
charged N atoms of the unsaturated imines and positively
charged S atoms of the stable ylides.[20] The configuration of
cycloadduct 3a was confirmed by X-ray diffraction analysis.
In summary, we have successfully developed a novel [4+1]
annulation of stable sulfur ylides with ester-bearing unsatu-
rated imines by manipulating the electronic effects. The low
cost, ready availability, and recoverability of the binol-derived
Experimental Section
Representative procedure: Unsaturated imine 2a (0.2 mmol) and
toluene/CH2Cl2 (9:1, 20 mL) were added to a 50 mL flask equipped
with a magnetic stir bar. After the solution had been stirred at ꢀ808C
for 0.5 h, chiral sulfur ylide 1a (0.22 mmol) was introduced and the
mixture stirred for 48 h at the same temperature. The reaction
mixture was then slowly warmed to room temperature. Upon
completion of the reaction, as monitored by TLC, the solvent was
removed under reduced pressure. The crude product was purified by
flash chromatography on silica gel [silica: 200–300; eluant: petroleum
ether/ethyl acetate (5:1!3:1)] to provide pure product 3a in 95%
yield. Diastereomeric ratio: > 95:5 as determined by 1H NMR
analysis of the reaction mixture. Enantiomeric excess: 98% as
determined by HPLC analysis (Daicel Chirapak OD-H, n-hexane/
isopropyl alcohol = 95:5, flow rate 0.7 mLminꢀ1, T= 258C, 254 nm):
tR = 7.51 min
(major),
tR = 9.26 min
(minor).
33
½aꢂD ¼ꢀ93 degcm3 gꢀ1 dmꢀ1 (c = 1.2 gcmꢀ3, CHCl3).
Received: February 8, 2010
Published online: May 10, 2010
Angew. Chem. Int. Ed. 2010, 49, 4495 –4498
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4497

Communications
using a,b-unsaturated imines. Please see T. Morimoto, N.
Chatani, S. Murai, J. Am. Chem. Soc. 1999, 121, 1758.
[11] For selected reviews, see: a) I. Coldham, R. Hufton, Chem. Rev.
2005, 105, 2765; b) G. Pandey, P. Banerjee, S. R. Gadre, Chem.
Rev. 2006, 106, 4484.
Keywords: [4+1] annulation · nitrogen heterocycles · pyrroline ·
sulfur · ylides
.
[1] a) R. W. Coscia, T. H. Lambert, J. Am. Chem. Soc. 2009, 131,
2496; b) R. A. Widenhoefer, Angew. Chem. 2009, 121, 7084;
Angew. Chem. Int. Ed. 2009, 48, 6950.
[12] Unstable sulfur ylides: a) D. Pearson, R. A. Field, R. A. Stock-
man, Synlett 2003, 1985. Semi-stablized benzylic sulfonium ylide:
b) V. K. Aggarwal, M. Ferrara, C. J. OꢀBrien, A. Thompson,
R. V. H. Jones, R Fieldhouse, J. Chem. Soc. Perkin Trans. 1 2001,
1635. Semi-stablized allylic sulfonium ylides: c) A.-H. Li, L.-X.
Dai, X.-L. Hou, M.-B. Chen, J. Org. Chem. 1996, 61, 4641; d) A.-
H. Li, L.-X. Dai, X.-L. Hou, J. Chem. Soc. Perkin Trans. 1 1996,
867; e) D. Morton, D. Pearson, R. A. Field, R. A. Stockman,
Org. Lett. 2004, 6, 2377. Semi-stablized allylic tellurium ylides:
f) J.-C. Zheng, W.-W. Liao, Y. Tang, X.-L. Sun, L.-X. Dai, J. Am.
Chem. Soc. 2005, 127, 12222. Semi-stablized propargylic sulfo-
nium ylides: g) A.-H. Li, Y.-G. Zhou, L.-X. Dai, X.-L. Hou, L.-J.
Xia, L. Lin, J. Org. Chem. 1998, 63, 4338.
[13] Synthesis of pyrrolidines from sulfur ylides other than by [4+1]
annulations: a) S. Koep, H.-J. Gais, G. Raabe, J. Am. Chem. Soc.
2003, 125, 13243; b) H.-J. Gais, L. R. Reddy, G. S. Babu, G.
Raabe, J. Am. Chem. Soc. 2004, 126, 4859; c) S. K. Tiwari, H.-J.
Gais, A. Lindenmaier, G. S. Babu, G. Raabe, L. R Reddy, F.
Kꢄhler, M. Gꢅnter, S. Koep, V. B. R. Iska, J. Am. Chem. Soc.
2006, 128, 7360; d) C. G. Kokotos, V. K. Aggarwal, Chem.
Commun. 2006, 2156; e) M. G. Unthank, N. Hussain, V. K.
Aggarwal, Angew. Chem. 2006, 118, 7224; Angew. Chem. Int. Ed.
2006, 45, 7066; f) C. G. Kokotos, V. K. Aggarwal, Org. Lett. 2007,
9, 2099; g) J. M. Schomaker, S. Bhattacharjee, J. Yan, B. Borhan,
J. Am. Chem. Soc. 2007, 129, 1996; h) M. G. Unthank, B.
Tavassoli, V. K. Aggarwal, Org. Lett. 2008, 10, 1501.
[14] a) L.-Q. Lu, Y.-J. Cao, X-P. Liu, J. An, C.-J. Yao, Z.-H. Ming, W.-
J. Xiao, J. Am. Chem. Soc. 2008, 130, 6946; b) L.-Q. Lu, F. Li, J.
An, J.-J. Zhang, X.-L. An, Q.-L. Hua, W.-J. Xiao, Angew. Chem.
2009, 121, 9706; Angew. Chem. Int. Ed. 2009, 48, 9542.
[15] B. Han, Z.-Q. He, J.-L. Li, R. Li, K. Jiang, T.-Y. Liu, Y.-C. Chen,
Angew. Chem. 2009, 121, 5582; Angew. Chem. Int. Ed. 2009, 48,
5474, and references therein.
[16] Y. Miyake, A. Oyamada, Y. Nishibayashi, S. Uemura, Heteroat.
Chem. 2002, 13, 270.
[17] See the Supporting Information for more details.
[18] Attempts to carry out the reaction of aliphatic imines were
unsuccessful. The reaction gave a complex reaction mixture
under the current reaction conditions. Additional studies are
ongoing in our laboratory.
[19] CCDC 764746 (3a) and 764745 (1e’) contain 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.
[20] a) V. K. Aggarwal, J. N. Harvey, J. Richardson, J. Am. Chem.
Soc. 2002, 124, 5747; b) V. K. Aggarwal, E. Alonso, I. Bae, G.
Hynd, K. M. Lydon, M. J. Palmer, M. Patel, M. Porcelloni, J.
Richardson, R. A. Stenson, J. R. Studley, J. Vasse, C. L. Winn, J.
Am. Chem. Soc. 2003, 125, 10926; c) V. K. Aggarwal, J. P. H.
Charmant, D. Fuentes, J. N. Harvey, G. Hynd, D. Ohara, W.
Picoul, R. Robiette, C. Smith, J. Vasse, C. L. Winn, J. Am. Chem.
Soc. 2006, 128, 2105.
[2] For selected examples with carbon monoxide as a one-carbon
unit in [4+1] annulations, see: a) B. E. Eaton, B. Rollman, J. A.
Kaduk, J. Am. Chem. Soc. 1992, 114, 6245; b) T. Mandai, J. Tsuji,
Y. Tsujiguchi, S. Saito, J. Am. Chem. Soc. 1993, 115, 5865;
c) M. S. Sigman, C. E. Kerr, B. E. Eaton, J. Am. Chem. Soc. 1993,
115, 7545; d) M. S. Sigman, B. E. Eaton, J. Org. Chem. 1994, 59,
7488; e) M. Murakami, K. Itami, Y. Ito, Angew. Chem. 1995, 107,
2943; Angew. Chem. Int. Ed. Engl. 1995, 34, 2691; f) M. S.
Sigman, B. E. Eaton, J. D. Heise, C. P. Kubiak, Organometallics
1996, 15, 2829; g) M. S. Sigman, B. E. Eaton, J. Am. Chem. Soc.
1996, 118, 11783.
[3] For a representative example with isocyanides as one-carbon
units in [4+1] annulations, see: M. Oshita, K. Yamashita, M.
Tobisu, N. Chatani, J. Am. Chem. Soc. 2005, 127, 761.
[4] For selected examples with nucleophilic carbenes as one-carbon
units in [4+1] annulations, see: a) J. H. Rigby, W. Dong, Org.
Lett. 2002, 2, 1673; b) C. Spino, H. Rezaei, K. Dupont-Gaudet, F.
Bꢁlanger, J. Am. Chem. Soc. 2004, 126, 9926; c) L. Boisvert, F.
Beaumier, C. Spino, Org. Lett. 2007, 9, 5361.
[5] For selected examples with diazo reagents as one-carbon units in
[4+1] annulations, see: a) A. M. Dalton, Y. Zhang, C. P. Davie,
R. L. Danheiser, Org. Lett. 2002, 4, 2465; b) L.-B. Zhao, Z.-H.
Guan, Y. Han, Y.-X. Xie, S. He, Y.-M. Liang, J. Org. Chem. 2007,
72, 10276; c) M. W. Giese, W. H. Moser, Org. Lett. 2008, 10, 4215.
Some other metal reagents were also used as one-carbon units in
[4+1] annulations, such as Fischer alkoxy(alkenyl)carbene and
a-benzotriazolyl organolithium compounds: d) J. Barluenga, S.
Lꢂpez, J. Flꢂrez, Angew. Chem. 2003, 115, 241; Angew. Chem.
Int. Ed. 2003, 42, 231; e) C. P. Davie, R. L. Danheiser, Angew.
Chem. 2005, 117, 6017; Angew. Chem. Int. Ed. 2005, 44, 5867.
[6] For recent reviews, see: a) V. K. Aggarwal, C. L. Winn, Acc.
Chem. Res. 2004, 37, 611; b) E. M. McGarrigle, E. L. Myers, O.
Illa, M. A. Shaw, S. L. Riches, V. K. Aggarwal, Chem. Rev. 2007,
107, 5841; c) X.-L. Sun, Y. Tang, Acc. Chem. Res. 2008, 41, 937.
[7] a) J. L. Loebach, D. M. Bennett, R. L. Danheiser, J. Am. Chem.
Soc. 1998, 120, 9690; b) Z. Yang, M. Fan, R. Mu, W. Liu. Y.
Liang, Tetrahedron 2005, 61, 9140; c) J.-C. Zheng, C.-Y. Zhu, X.-
L. Sun, Y. Tang, L.-X. Dai, J. Org. Chem. 2008, 73, 6909; d) C.-Y.
Zhu, X.-M. Deng, X.-L. Sun, J.-C. Zheng, Y. Tang, Chem.
Commun. 2008, 738; e) C.-Y. Zhu, X.-L. Sun, X.-M. Deng, J.-C.
Zheng, Y. Tang, Tetrahedron 2008, 64, 5583, and references
therein.
[8] For selected reviews, see: a) A. Erkkilꢃ, I. Majander, P. M.
Phihko, Chem. Rev. 2007, 107, 5416; b) S. Mukherjee, J. W. Yang,
S. Hoffmann, B. List, Chem. Rev. 2007, 107, 5471.
[9] a) D. OꢀHagan, Nat. Prod. Rep. 2000, 17, 435; b) D. Enders, C.
Thiebes, Pure Appl. Chem. 2001, 73, 573.
[10] Only one example was reported by Murai and co-workers in
1999 regarding a Ru-catalyzed carbonylative [4+1] annulations
4498 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 4495 –4498