Organocatalyzed highly enantioselective and anti-selective construction of γ-butenolides through vinylogous Mukaiyama aldol reaction

Article abstract of DOI:10.1002/adsc.201000099

The formation of chiral γ-butenolides has been achieved with good yields (up to 90%), high enantioselectivity (up to 91%) and diastereoselectivity (up to 9/1, anti-selective) through an organocatalyzed vinylogous Mukaiyama aldol reaction of 2-(trimethylsilyloxy)furan and aldehydes. A wide range of chiral γ-butenolides was obtained under mild conditions by this methodology.

Full text of DOI:10.1002/adsc.201000099

COMMUNICATIONS
DOI: 10.1002/adsc.201000099
Organocatalyzed Highly Enantioselective and anti-Selective
Construction of g-Butenolides through Vinylogous Mukaiyama
Aldol Reaction
Ning Zhu,a Bao-Chun Ma,a,* Yong Zhang,a and Wei Wanga,*
a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou, Gansu 730000, Peopleꢀs Republic of China
Fax : (+86)-931-891-5557; e-mail: wang_wei@lzu.edu.cn or mabaochun@lzu.edu.cn
Received: February 6, 2010; Published online: May 7, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000099.
Abstract: The formation of chiral g-butenolides has
been achieved with good yields (up to 90%), high
enantioselectivity (up to 91%) and diastereoselec-
tivity (up to 9/1, anti-selective) through an organo-
catalyzed vinylogous Mukaiyama aldol reaction of
2-(trimethylsilyloxy)furan and aldehydes. A wide
range of chiral g-butenolides was obtained under
mild conditions by this methodology.
biological activities of these natural products and
their analogues depend on the absolute configuration
of the stereogenic centers. Thus, many efforts have
been directed towards the enantio- and diastereose-
lective synthesis of the building blocks, i.e., chiral g-
butenolides.[3] With respect to this issue, the asymmet-
ric vinylogous Mukaiyama aldol (VMA) reaction[4,5]
of 2-(trimethylsilyloxy)furan (TMSOF) and aldehydes
is a very effective strategy. Accordingly, several chiral
Lewis acids have been developed as effective catalysts
for the synthesis of g-chiral butenolides via VMA re-
actions.[6–11] For example, Evans et al. reported C2-
symmetrical copper(II) complexes serving as catalysts
for asymmetric aldol addition of TMSOF to benzyl-
Keywords: butenolides; diastereoselectivity; enan-
tioselectivity; organocatalysis; vinylogous Mukaiya-
ma aldol reaction
AHCTUNGTRENNUNG
The butenolide ring system represents one of the complex which could catalyze the VMA reaction of
most ubiquitous structural motifs found in diverse TMSOF and aldehydes with high enantioselectivity
natural products.[1] Consequently, g-substituted bute- and moderate diastereoselectivity.[7,8] Katsuki and co-
nolides are useful building blocks for the synthesis of workers exploited a CrACTHUNRTGNEUNG(salen) complex as VMA cata-
biologically active products, such as the metabolite lyst and investigated the effect of water and alcohol
from Acremonium sp, (À)-muricatacin, nakiterpiosin, on the enantioselectivity of the reaction products.[9]
(À)-iso-cladospolide B, and so on (Figure 1).[2] The Scettri and co-workers applied SiCl4 and a chiral
Lewis base system to promote a VMA reaction in
which high enantioselectivity and moderate diastereo-
selectivity could be achieved.[10] Recently, Frings et al.
developed a chiral copper complex which could cata-
lyze the VMA reaction with controlled enantioselec-
tivity and diastereoselectivity.[11]
On the other hand, organocatalysis is one of the
most rapidly growing and fruitful research areas in
synthetic organic chemistry during the last decade.[12]
Complementing biocatalysis and metal catalysis,
asymmetric organocatalysis covers a wide range of or-
ganic processes and methodologies, providing efficient
and environmentally friendly access to enantiomeri-
cally pure compounds including many drugs and bio-
Figure 1. Typical butenolide-containing natural products.
active natural products. In terms of VMA reactions
Adv. Synth. Catal. 2010, 352, 1291 – 1295
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1291

Ning Zhu et al.
COMMUNICATIONS
catalyzed by organocatalysts, however, only very few TMSOF (2a) as substrates in the presence of chiral
reports could be found in the literature. Utilizing the hydrogen-bonding-donor catalysts (C1–C5). The ob-
concept of hydrogen-bonding catalysis, Rawal and co- tained results are summarized in Table 1. Toluene was
workers were the first who applied metal-free hydro- initially chosen as the solvent since most of the reac-
gen-bonding catalysts, e.g., 1-naphthyl-TADDOL, in tions mediated by hydrogen-bonding donors proceed
asymmetric VMA reactions.[13] Villano et al. realized smoothly in toluene. Different reactivities were ob-
the enantioselective VMA reaction of Chanꢀs diene tained in toluene for the C1–C5 catalysts (20 mol%)
catalyzed by hydrogen-bonding catalysts.[14] Soriente in terms of conversion and selectivity (Table 1, en-
and co-workers identified that hydrogen-bonding tries 1–5). The quinine-thiourea catalyst (C4) proved
donors are efficient catalysts for the diastereoselective to be the most efficient catalyst that affords the VMA
organocatalytic addition of TMSOF to carbonyl com- product (3a) with good yield (76%), diastereoselectiv-
pounds.[15] Besides, Mukaiyama and co-workers re- ity (4/1 dr), and enantioselectivity (77% ee) after the
ported the asymmetric and syn-selective synthesis of reaction had been conducted for 18 h (Table 1,
butenolide derivatives via VMA reactions of alde- entry 4). The quinidine-thiourea catalyst (C3) shows
hydes and 4-methyl-2-(trimethylsilyloxy)furan by the same diastereoselectivity (4/1 dr) as C4, but with
using a cinchonidine-derived quanternary ammonium the reverse enantioselectivity (À77% ee, Table 1,
phenoxide.[16] When the non-substituted TMSOF, i.e., entry 3).
2-(trimethylsilyloxy)furan, was examined, however,
the enantioselectivity of the butenolide product was vent effect was investigated with the use of catalyst
not satisfactory. C4 (Table 1, entries 6–10). CHCl3 was confirmed as
To optimize the reaction conditions further, the sol-
Herein, we present our contribution to the efficient the best solvent for this asymmetric VMA reaction
synthesis of chiral g-butenolides via asymmetric orga- (Table 1, entry 8). When the reaction was performed
nocatalysis and report the first examples of bifunc- in THF, the enantioselectivity of 3a (73% ee) was
tional alkaloid thiourea catalysts[17] as hydrogen-bond- good, but a lower yield (47%) was obtained (Table 1,
ing donors for highly enantio- and diastereoselective entry 6). A similar result was also observed in CCl4
VMA reactions of TMSOF and aldehydes.
(Table 1, entry 9). In comparison with the case in
As shown in Figure 2, our initial examination was CHCl3, a lower diastereoselectivity of 3a (4/1 dr) was
carried out by using p-nitrobenzaldehyde (1a) and obtained in CH2Cl2 (Table 1, entry 7). When 2-propa-
nol (i-PrOH) was applied as the solvent, a better
yield (79%) and higher ee (93%) could be reached,
but the diastereoselectivity (3/2 dr) was poor (Table 1,
Table 1. Screening of catalysts (C1–C5) and solvents for the
asymmetric vinylogous Mukaiyama aldol (VMA) reactions
by using p-nitrobenzaldehyde (1a) and TMSOF (2a) as sub-
strates.
Entry[a] Catalyst Solvent Yield [%][b] anti/syn[c] ee [%][c]
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C4
C4
C4
C4
C4
C4
PhMe 50
PhMe 60
PhMe 75
PhMe 76
PhMe 65
80/20
78/22
80/20
80/20
25/75
80/20
80/20
88/12
80/20
60/40
88/12
À73
74
À77
77
57
73
75
79
THF
47
CH2Cl2 75
CHCl3 77
9
10
11
CCl4
69
70
i-PrOH 79
CHCl3 78
93
91[d]
[a]
Reaction conditions: 1a (0.30 mmol) and 2a (0.15 mmol)
in solvent at room temperature for 18 h.
Isolated yields after silica gel column chromatography.
Determined by HPLC (Daicel chiral AD-H column).
The diastereoisomeric ratio was determined according to
refs.[7,8]
Reaction conditions: 1a (0.30 mmol) and 2a (0.15 mmol)
in CHCl3 at À208C for 6 h and then at 08C for 60 h.
[b]
[c]
Figure 2. Chiral hydrogen-bonding donors (C1–C5) as asym-
metric organocatalysts for the asymmetric vinylogous Mu-
kaiyama aldol (VMA) reactions of p-nitrobenzaldehyde (1a)
and TMSOF (2a).
[d]
1292
asc.wiley-vch.de
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 1291 – 1295

Organocatalyzed Highly Enantioselective and anti-Selective Construction of g-Butenolides
entry 10). Eventually, we found that the best result Schneider and co-workers have observed the quanti-
(78% yield, 91% ee, and 88/12 dr) was produced after tative formation of silanol in organocatalyzed vinylo-
the reaction was conducted in CHCl3 at À208C for gous Mukaiyama–Mannich reactions in aqueous sol-
6 h and then at 08C for 60 h (Table 1, entry 11). A vent.[19] The effects of water or alcohol on the organo-
lower yield (51%) was, however, obtained when catalyzed VMA reaction in our study may be similar
À208C was applied throughout the reaction.
to those[19] that Schneider and co-workers proposed.
It is well known that water is an efficient additive A small amount of water or alcohol in the reaction
in Lewis acid-catalyzed VMA reactions.[9,18] We, there- would trap the TMS species to silanol or silyl ether,
fore, investigated the influence of water and other ad- which could regenerate the organocatalyst and, there-
ditives on the asymmetric VMA reactions catalyzed fore, improve the yield. On the contrary, more water
by the above-mentioned organocatalysts. The investi- or alcohol (ꢀ50 mol%) would deactivate the double
gation was first carried out by using p-methylbenzal- hydrogen-bonding catalyst. The addition of 4 ꢂ MS
dehyde (1b) and TMSOF (2a) as substrates in the (30 mg) gave a lower ee (67%, Table 2, entry 7), while
presence of catalyst C4. The obtained results are sum- 10 mol% of DIPEA gave the racemic products
marized in Table 2.
(Table 2, entry 8).
In the absence of water, the VMA product 3b was
On the basis of the optimization efforts presented
obtained with 83% yield, 89/11 dr, and 88% ee above, the substrate scope of asymmetric VMA reac-
(Table 2, entry 1). With the addition of 10 mol% tions catalyzed by organocatalyst C4 was further in-
water, a higher yield (90%) but slightly lower enantio- vestigated (Table 3). In general, all the examined sub-
and diastereoselectivities (87/13 dr and 87% ee) were strates could furnish the desired products in good
obtained (Table 2, entry 2). Increasing the amount of yields and high selectivities. A variety of aromatic al-
water to 30 mol% did not cause any additional im- dehydes bearing either electron-donating or electron-
provement (83% yield, 86/14 dr, and 87% ee, Table 2, withdraw groups underwent the asymmetric VMA re-
entry 3). When more water (50 mol%) was added, a action to afford high enantioselectivities ranging from
lower yield of 30%, dr of 80/20 and ee of 74% were 82 to 91% ee (Table 3, entries 1–5). Both the ortho-
obtained (Table 2, entry 4). Similar results, i.e., higher
conversion and lower stereoselectivity, were observed
when 2-propanol was added (Table 2, entries 5 and 6).
Table 3. Asymmetric vinylogous Mukaiyama aldol (VMA)
reactions catalyzed by the organocatalyst C4.
Table 2. Effect of water and other additives on the asymmet-
ric vinylogous Mukaiyama aldol (VMA) reactions by using
p-methyl-benzaldehyde (1b) and TMSOF (2a) as substrates.
Entry[a]
Electrophile
R
Yield
[%][b]
anti/
ee
syn[b]
[%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
4-NO2
4-Me
4-Cl
4-OMe
H
2-NO2
3-Me
4-CN
2,4-Cl2
3,4-Me2
4-F
78
83
80
72
78
75
80
77
82
82
77
75
90
88/12
89/11
90/10
60/40
89/11
90/10
87/13
85/15
82/18
86/14
89/11
88/12
90/10
91
88
87
82
86
85
85
86
85
86
87
84
89
Entry[a] Additive (mol%) Yield [%][b] anti/syn[b] ee [%][c]
1
2
3
4
5
6
7
8
None
83
90
83
30
87
79
85
78
89/11
87/13
86/14
80/20
84/16
80/20
75/25
90/10
88
87
87
74
84
84
67
racemic
H2O (10)
H2O (30)
H2O (50)
i-PrOH (10)
i-PrOH (50)
4 ꢂ MS[d]
DIPEA (10)
9
10
11
12
13
1j
1k
1l
4-Br
2-naph
1m
[a]
Reaction conditions: 1b (0.30 mmol) and 2a (0.15 mmol)
in CHCl3 at À208C for 6 h and then at 08C for 60 h.
Isolated yields after silica gel column chromatography.
Determined by HPLC (Daicel chiral AD-H column).
The diastereoisomeric ratio was determined according to
refs.[7,8]
[a]
Reaction conditions: 1 (0.30 mmol) and 2a (0.15 mmol) in
CHCl3 at À208C for 6 h and then at 08C for 60 h.
Isolated yields after silica gel column chromatography.
Determined by HPLC (Daicel chiral AD-H column).
The diastereoisomeric ratio was determined according to
refs.[7,8]
[b]
[c]
[b]
[c]
[d]
30 mg of 4 ꢂ MS.
Adv. Synth. Catal. 2010, 352, 1291 – 1295
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1293

Ning Zhu et al.
COMMUNICATIONS
References
and meta-substituted benzaldehydes resulted in high
enantioselectivities (85% ee, Table 3, entries 6–7).
More strikingly, disubstituted benzaldehydes also
gave satisfactory results (Table 3, entries 9–10). 2-
Naphthaldehyde resulted in high yield (90%), enan-
tioselectivity (89%ee) and diastereoselectivity (9/1) as
well (Table 3, entry 13).
[1] For reviews on butenolide-containing natural products,
see, for example: a) A. D. Rodriguez, Tetrahedron
1995, 51, 4571–4618; b) F. W. Alali, X. X. Liu, J. L.
McLaughlin, J. Nat. Prod. 1999, 62, 504–540; c) R. S.
Ward, in: Recent Advances in the Chemistry of Lignans,
Vol. 24, Part E, (Ed.: Atta-ur-Rahman), Elsevier, Am-
sterdam, 2000, pp 739–798; d) J. R. Hanson, Nat. Prod.
Rep. 2002, 19, 381–389.
[2] a) S. Hanessian, J. P. Murray, P. S. Sahoo, Tetrahedron
Lett. 1985, 26, 5627–5630; b) S. Hanessian, J. P. Munay,
Can. J. Chem. 1986, 64, 2231–2234; c) S. Hanessian,
P. S. Sahoo, M. Botta, Tetrahedron Lett. 1987, 28, 1147–
1150; d) H. Mattes, K. Hamada, C. Benezra, J. Med.
Chem. 1987, 30, 1948–1951; e) L. Ferriꢃ, S. Reymond,
P. Capdevielle, J. Cossy, Synlett 2007, 18, 2891–2893;
f) K. D. Sarma, J. Zhang, T. T. Curran, J. Org. Chem.
2007, 72, 3311–3318.
[3] For reviews and selected examples on synthetic ap-
proaches to butenolides, see: a) Y. S. Rao, Chem. Rev.
1976, 76, 625–694; b) P. A. Jacobi, Adv. Heterocycl.
Nat. Prod. Synth. 1992, 2, 251–298; c) D. W. Knight,
Contemp. Org. Synth. 1994, 1, 287–315; d) H. Kitajima,
K. Ito, T. Katsuki, Tetrahedron 1997, 53, 17015–17028;
e) R. BrJckner, Curr. Org. Chem. 2001, 5, 679–718;
f) S. P. Brown, N. C. Goodwin, D. W. C. MacMillan, J.
Am. Chem. Soc. 2003, 125, 1192–1194; g) C. W. Cho,
M. J. Krische, Angew. Chem. 2004, 116, 6857–6859;
Angew. Chem. Int. Ed. 2004, 43, 6689–6691; h) K.
Kong, D. Romo, Org. Lett. 2006, 8, 2909–2912; i) Y. Q.
Jiang, Y. L. Shi, M. Shi, J. Am. Chem. Soc. 2008, 130,
7202–7203; j) J. S. Yadav, B. V. S. Reddy, G. Narasim-
hulu, G. Satheesh, Tetrahedron Lett. 2008, 49, 5683–
5686; k) H. Ube, N. Shimada, M. Terada, Angew.
Chem. 2010, 122, 1902–1905; Angew. Chem. Int. Ed.
2010, 49, 1858–1861.
[4] For recent reviews on vinylogous Mukaiyama aldol re-
actions, see: a) G. Casiraghi, F. Zanardi, G. Appendino,
G. Rassu, Chem. Rev. 2000, 100, 1929–1972; b) S. E.
Denmark, J. R. Heemstra, Jr., G. L. Beutner, Angew.
Chem. 2005, 117, 4760–4777; Angew. Chem. Int. Ed.
2005, 44, 4682–4698; c) M. Kalesse, Top. Curr. Chem.
2005, 244, 43–76; d) S. Hosokawa, K. Tatsuta, Mini-
Rev. Org. Chem. 2008, 5, 1–18.
In conclusion, we have developed an asymmetric,
organocatalyzed vinylogous Mukaiyama aldol (VMA)
reaction of 2-(trimethylsilyloxy)furan (TMSOF) and
aldehydes, which gives the desired products with high
enantioselectivities and anti-selectivities. This is the
first example of asymmetric VMA reactions of
TMSOF and aldehydes catalyzed by bifunctional alka-
loid thiourea organocatalysts. Accordingly, a wide
range of chiral g-butenolides could be obtained in
good yields and high enantioselectivities. The catalytic
system we have developed might be an efficient meth-
odology for the construction of the enantioselective g-
butenolides that exist in many biologically active nat-
ural products. For example, Gao et al. achieved very
recently the total synthesis of nakiterpiosin, in which
the construction of butenolides was indeed realized
by vinylogous Mukaiyama aldol reactions.[20] Our fur-
ther studies will focus on the development of new
methodologies for the synthesis of different kinds of
chiral g-butenolides via organocatalysis.
Experimental Section
Typical Procedure for the Asymmetric Vinylogous
Mukaiyama Aldol (VMA) Reactions Catalyzed by
Organocatalysts (C1–C5) as Hydrogen-Bonding
Donors
A solution of aldehyde 1 (0.3 mmol) and organocatalyst C4
(0.03 mmol) in CHCl3 (0.2 mL) was stirred at À208C for
10 min, then 2a (0.15 mmol) was added. The reaction mix-
ture was stirred at À208C for 6 h and then at 08C for 60 h.
TFA (0.5 mL) was added to the reaction mixture thereafter.
The solution was then warmed to room temperature and
stirred for 1 h after the desilylation was completed. The so-
lution was diluted with ethyl acetate and a saturated aque-
ous solution of NaHCO3 was added dropwise until the evo-
lution of gas ceased. The organic layer was separated,
washed with brine, dried over Na2SO4, and concentrated
under vacuum. The crude mixture was purified by silica gel
column chromatography to afford product 3.
[5] For recent contributions on asymmetric vinylogous Mu-
kaiyama aldol reactions, see, for example: a) S. E. Den-
mark, G. L. Beutner, J. Am. Chem. Soc. 2003, 125,
7800–7801; b) B. Bazan-Tejeda, G. Bluet, G. Broustal,
J. M. Campagne, Chem. Eur. J. 2006, 12, 8358–8366;
c) S. E. Denmark, J. R. Heemstra, Jr., J. Am. Chem.
Soc. 2006, 128, 1038–1039; d) P. Rꢃmy, M. Langner, C.
Bolm, Org. Lett. 2006, 8, 1209–1211; e) S. E. Denmark,
J. R. Heemstra, Jr., J. Org. Chem. 2007, 72, 5668–5688;
f) S. Simsek, M. Horzella, M. Kalesse, Org. Lett. 2007,
9, 5637–5639; g) L. V. Heumann, G. E. Keck, Org.
Lett. 2007, 9, 4275–4278; h) G. Broustal, X. Ariza,
J. M. Campagne, J. Garcia, Y. Georges, A. Marinetti,
R. Robiette, Eur. J. Org. Chem. 2007, 4293–4297.
[6] a) D. A. Evans, C. S. Burgey, M. C. Kozlowski, S. W.
Tregay, J. Am. Chem. Soc. 1999, 121, 686–699; b) D. A.
Evans, M. C. Kozlowski, J. A. Murry, C. S. Burgey,
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 20972064), the 111 project,
and the Program for New Century Excellent Talents in Uni-
versity (NCET-06-0904).
1294
asc.wiley-vch.de
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 1291 – 1295

Organocatalyzed Highly Enantioselective and anti-Selective Construction of g-Butenolides
K. R. Campos, B. T. Connell, R. J. Staples, J. Am.
Chem. Soc. 1999, 121, 669–685.
[7] M. Szlosek, X. Franck, B. Figadꢃre, A. Caveꢃ, J. Org.
[17] For asymmetric reactions catalyzed by alkaloid thiour-
ea catalysts, see, for example: Michael reactions, a) B.
Vakulya, S. Varga, A. Csꢅmpai, T. Soꢆs, Org. Lett.
2005, 7, 1967–1969; b) S. H. McCooey, S. J. Connon,
Angew. Chem. 2005, 117, 6525–6528; Angew. Chem.
Int. Ed. 2005, 44, 6367–6370; c) J. Ye, D. J. Dixon, P. S.
Hynes, Chem. Commun. 2005, 4481–4483; Henry reac-
tions, d) T. Marcelli, R. N. S. van der Haas, J. H. van
Maarseveen, H. Hiemstra, Angew. Chem. 2006, 118,
943–945; Angew. Chem. Int. Ed. 2006, 45, 929–931;
e) H. Li, B. Wang, L. Deng, J. Am. Chem. Soc. 2006,
128, 732–733; Mannich reactions, f) J. Song, Y. Wang,
L. Deng, J. Am. Chem. Soc. 2006, 128, 6048–6049;
g) A. L. Tillman, J. Ye, D. J. Dixon, Chem. Commun.
2006, 1191–1193; Friedel–Crafts reactions, h) Y.-Q.
Wang, J. Song, R. Hong, H. Li, L. Deng, J. Am. Chem.
Soc. 2006, 128, 8156–8157; reviews on other examples,
i) P. R. Schreiner, Chem. Soc. Rev. 2003, 22, 289–296;
j) T. Akiyama, J. Itoh, K. Fuchibe, Adv. Synth. Catal.
2006, 348, 999–1010; k) A. G. Doyle, E. N. Jacobsen,
Chem. Rev. 2007, 107, 5713–5743.
Chem. 1998, 63, 5169–5172.
[8] M. Szlosek, B. Figadꢃre, Angew. Chem. 2000, 112,
1869–1871; Angew. Chem. Int. Ed. 2000, 39,1799–1800.
[9] a) Y. Matsuoka, R. Irie, T. Katsuki, Chem. Lett. 2003,
32, 584–585; b) S. Onitsuka, Y. Matsuoka, R. Irie, T.
Katsuki, Chem. Lett. 2003, 32, 974–975.
[10] a) M. De Rosa, M. R. Acocella, M. F. Rega, A. Scettri,
Tetrahedron: Asymmetry 2004, 15, 3029–3033; b) R.
Villano, M. R. Acocella, M. De Rosa, A. Scettri, Tetra-
hedron: Asymmetry 2004, 15, 2421–2424; d) L. Palom-
bi, M. R. Acocella, N. Celenta, A. Massa, R. Villano,
A. Scettri, Tetrahedron: Asymmetry 2006, 17, 3300–
3303.
[11] M. Frings, I. Atodiresei, J. Runsink, G. Raabe, C.
Bolm, Chem. Eur. J. 2009, 15, 1566–1569.
[12] See, for example: a) P. I. Dalko, Enantioselective Orga-
nocatalysis: Reactions and Experimental Procedures,
Wiley-VCH, Weinheim, 2007; b) S. Mukherjee, J. W.
Yang, S. Hoffmann, B. List, Chem. Rev. 2007, 107,
5471–5569; c) A. Erkkilꢄ, I. Majander, P. M. Pihko,
Chem. Rev. 2007, 107, 5416–5470; d) D. W. C. MacMil-
lan, Nature 2008, 455, 304–308.
[13] See, for example: a) V. B. Gondi, M. Gravel, V. H.
Rawal, Org. Lett. 2005, 7, 5657–5660; b) V. B. Gondi,
K. Hagihara, V. H. Rawal, Chem. Commun. 2010, 46,
904–906.
[14] a) R. Villano, M. R. Acocella, A. Massa, L. Palombi,
A. Scettri, Tetrahedron Lett. 2007, 48, 891–895; b) R.
Villano, M. R. Acocella, A. Massa, L. Palombi, A.
Scettri, Tetrahedron Lett. 2009, 65, 5571–5576.
[15] M. De Rosa, L. Citro, A. Soriente, Tetrahedron Lett.
2006, 47, 8507–8510.
[18] a) M. Sodeoka, K. Ohrai, M. Shibasaki, J. Org. Chem.
1995, 60, 2648–2649; b) O. Fujimura, J. Am. Chem.
Soc. 1998, 120, 10032–10039; c) S. Kobayashi, S. Na-
gayama, T. Busujima, Tetrahedron 1999, 55, 8739–
8746; d) Y. Yamashita, H. Ishitani, H. Shimizu, S. Ko-
bayashi, J. Am. Chem. Soc. 2002, 124, 3292–3302;
e) M. Nakalima, Y. Orito, T. Ishizuka, S. Hashimoto,
Org. Lett. 2004, 6, 3763–3765.
[19] a) M. Sickert, C. Schneider, Angew. Chem. 2008, 120,
3687–3690; Angew. Chem. Int. Ed. 2008, 47, 3631–
3634; b) D. S. Giera, M. Sickert, C. Schneider, Org.
Lett. 2008, 10, 4259–4262.
[20] S. Gao, Q. L. Wang, C. Chen, J. Am. Chem. Soc. 2009,
131, 1410–1412.
[16] H. Nagao, Y. Yamane, T. Mukaiyama, Chem. Lett.
2007, 36, 8–9.
Adv. Synth. Catal. 2010, 352, 1291 – 1295
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1295