Highly enantioselective friedel-crafts reaction of indole with alkylidenemalonates catalyzed by heteroarylidene malonate-derived bis(oxazoline) copper(II) complexes

Article abstract of DOI:10.1002/adsc.200900669

A series of cheap and easily accessible heteroarylidenemalonate-derived bis(oxazoline) ligands 1 and 2 were synthesized and their copper(II) complexes were applied to the catalytic Friedel-Crafts reaction between indoles and diethyl alkylidenemalonates, E

Full text of DOI:10.1002/adsc.200900669

COMMUNICATIONS
DOI: 10.1002/adsc.200900669
Highly Enantioselective Friedel–Crafts Reaction of Indole with
Alkylidenemalonates Catalyzed by Heteroarylidene Malonate-
Derived Bis(oxazoline) Copper(II) Complexes
Yan-Jin Sun,a Nan Li,a Zhong-Bo Zheng,a Lei Liu,a Yan-Bo Yu,a Zhao-Hai Qin,a
and Bin Fua,*
a
Department of Applied Chemistry, China Agricultural University, Beijing 100193, Peopleꢀs Republic of China
Fax : (+86)-10-62732-873; e-mail: fubinchem@cau.edu.cn
Received: September 27, 2009; Published online: December 8, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900669.
Abstract: A series of cheap and easily accessible
heteroarylidenemalonate-derived bis(oxazoline) li-
gands 1 and 2 were synthesized and their copper(II)
complexes were applied to the catalytic Friedel–
Crafts reaction between indoles and diethyl alkyli-
denemalonates, Excellent asymmetric enantioselec-
tivities were afforded for the S-enantiomer (up to
>99% ee) in isobutyl alcohol, and the R-enantio-
mer (up to 96.5% ee) in dichloromethane.
while in 1,1,2,2-tetrachloroethane the opposite enan-
tiomer of the product was formed with up to 89%
ee.[5] The enantioselectivity of the model reaction of
indole with diethyl benzylidenemalonate was further
improved to >99% ee by Reiser et al. by using aza-
BOX-Cu(II) complexes. The fine control of the ratio
of chiral ligand to copper proved to be a critical
factor.[6] Recently, Feng et al. reported Friedel–Crafts
alkylation of different indole derivatives with alkyl-
AHCTUNGTRENNUNG
Keywords: asymmetric catalysis; bis(oxazolines);
Friedel–Crafts reaction; heteroarylidenemalonates;
indoles
ACHTUNGTRENNUNG
impressive contributions, the highly efficient chiral
catalyst for this type of reaction is still limited. The
development of cheap, easily accessible and highly ef-
fecient chiral ligands is still one great challenge in
asymmetric catalysis. Herein we would like to docu-
ment a series of heteroarylidenemalonate-derived bi-
The Friedel–Crafts reaction is one of the most power- s(oxazoline) ligands 1 and 2 and their Cu(II) com-
ful carbon-carbon bond-forming reactions in synthetic plexes in the highly enantioselective Friedel–Crafts al-
organic chemistry.[1] During the past decade the cata- kylation of indole with alkylidenemalonates.
lytic asymmetric Friedel–Crafts alkylation reaction of
For bis(oxazoline) (BOX) ligands derived from
indole derivatives with a variety of conjugated a,b-un- malonate and its analogues, the bridge angle F, corre-
saturated carbonyl compounds has been extensively lating with the bite angle V of the BOX-metal com-
developed,[2] owing to the importance of indolea as plex, is regarded as an important structural factor in-
building blocks in a variety of interesting natural fluencing the enantioselectivity (Scheme 1). 1n 1996,
products and potential medicinal agents.[3] Among the Davis reported the copper(II) complex of BOX
publications, the Friedel–Crafts alkylation between ligand I containing different spirorings catalyzed
indole and alkylidenemalonate has intrigued some Diels–Alder reactions.[8] A conclusive trend has been
chemistsꢀ interest. In 2001, Jørgensen et al. reported obtained: the larger the value of F (hence the ligand
the first example of this reaction. The addition pro- bite angle V), the higher the observed enantioselec-
ceeded smoothly under the catalysis of t-Bu-BOX- tivity. As the spiroring size decreased, the bridge
Cu(II) catalyst with excellent yields and moderate angle F correspondingly increased, then the highest
enantioselectivities (up to 69% ee).[4] Subsequently, ee value (96%) was afforded by the copper complex
Tang et al. explored the pseudo-C3-symmetrical tris- of BOX ligand I bearing a cyclopropyl group at the
ACHTUNGTRENNUNG(oxazoline)-Cu(II) complex to catalyze the the same bridge carbon. On the contrary, in 2000 Denmark
reaction. The desired adducts were obtained in excel- et al. reported the tuning of the chelation bridge
lent yields with up to 93% ee in isobutyl alcohol, angle F in bis(oxazoline) ligands II (Scheme 1)
Adv. Synth. Catal. 2009, 351, 3113 – 3117
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3113

Yan-Jin Sun et al.
COMMUNICATIONS
Scheme 1. Rational design of new bis(oxazolines) 1 and 2.
through variation of the spiroring size,[9] the worst heteroarylidenemalonate-derived bis(oxazoline) li-
enantioselectivity was afforded for the BOX ligand gands 1 and 2 in good yields (70–78%).
bearing a cyclopropyl group in the asymmetric addi-
With the new ligands 1a–d and 2a–d in hand, we
tion of methyllithium to imines. How is the better first optimized the ligand structure for the model
enantioselective correlated with the larger or smaller asymmetric Friedel–Crafts alkylation of indole with
bridge angle in the catalytic asymmetric reaction? diethyl benzylidenemalonate as summarized in
Much more experimental proof will be needed. Trig- Table 1. The reaction proceeded well in isobutyl alco-
gered by the above-described work, a straight strategy hol. Full conversion and excellent yields (95–99%)
to tune the bridge angle was introduced as illustrated can be achieved within 12 h under the catalysis of
in the type of BOX ligand III, in which two oxazoline 10 mol% ligand-CuACTHUNRGTNEUNG(OTf)2 complexes for all ligands
rings are attached to an sp2 hybridized carbon and tested. However, the enantiomeric excesses of the
then generally provide a larger bridge angle than indole adducts were significantly affected by the
those with sp3 hybridized bridge carbon. So far very ligand structure. Ligand 1a and 2a with an isopropyl
few reports involving this type of BOX ligand have group on the oxazoline ring gave 98.3% and 97.5%
appeared in the literature.[10] For convenience, ee, respectively (entries 1 and 5). Such a phenomenon
through further rationally modification we designed was basically in agreement with reports on Tangꢀs tris-
the heteroarylidenemalonate-derived bis(oxazoline)
ACHTUNGTREN(NUNG oxazoline) and Reiserꢀs aza-BOX that the Cu(II)
ligands 1 and 2 in which an approximately 1208 bridge complexes of oxazoline with an isopropyl substituent
angle would be formed between the two oxazoline usually result in the highest ee values.[5,6] Surprisingly,
ring.
under the same conditions 1c and 2c with a benzyl
The requisite chiral bis(oxazoline) 1 and 2 were group on the oxazoline ring also afforded excellent ee
conveniently synthesized from commercially available values (98.7 and 99.3%, entries 3 and 7). To the best
material diethyl 2-thienyldicarboxylate 5 and 2-furyl- of our knowledge, this is the first report to find that
dicarboxylate 6 in 4 steps sequence as illustrated in the Cu(II) complex of an oxazoline ligand bearing a
Scheme 2.[11] Hydrolysis of diethyl dicarboxylates 5 benzyl substituent gives such excellent results in this
and 6 by the solution of NaOH in a mixture of water type of reaction. On the contrary, 1b and 2b with an
and ethanol gave the corresponding dicarboxylic acid, isobutyl group on the oxazoline ring gave almost race-
which reacted with oxalyl chloride in the presence of mic products (3.0 and 5.5% ee respectively, entries 2
DMF to afford the diacyl chloride. The diacyl chlo- and 6). For ligands 1d and 2d, 73.0% and 11.0% ee
ride condensed with chiral b-amino alcohols in the were obtained (entries 4 and 8). The enantioselectivi-
presence of Et3N to give the corresponding chiral in- ty can be enhanced to >99.0% when the temperature
termediate dihydroxy diamides 3 and 4 in 74–88% was lowered to 08C (entries 9 and 10). As comparison
yields, which were treated with methanesulfonyl chlo- the known ligand benzylidene-BOX 9 was synthesized
ride and excess Et3N in dichloromethane to afford and tested in the same reaction.[10] Although a high
Scheme 2. Synthesis of new chiral bis(oxazoline) ligands.
3114
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 3113 – 3117

Highly Enantioselective Friedel–Crafts Reaction of Indole with Alkylidenemalonates
COMMUNICATIONS
Table 1. Effect of ligands in Cu
(OTf)2-catalyzed Friedel–
Table 2. Further optimization of ligands, Cu(II) salts, and
Crafts alkylation reaction.[a]
solvents.[a]
Entry Ligand Cu(II) salt
Solvent
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
1a
2a
1c
2c
1a
Cu
U
98.5
97.7
25.0
53.0
23.0
Cu
Cu
Cu
Cu
acetone/ 40
ether
6
7
8
9
10
11
12
13
14
15
1a
1a
1a
1a
1a
1c
1a
2a
2a
2c
Cu
N
DCE
54
99
99
95
98
97
98
96
99
99
À14.7
À16.0
À96.5
83.0
Cu
Cu
Cu
N
TTCE
DCM
MeOH
EtOH
DCM
Cu
Cu
98.0
Entry
Ligands
Temp. [oC]
Yield [%][b]
ee [%][c]
À5.0
1
2
3
4
5
6
7
8
1a
1b
1c
1d
2a
2b
2c
2d
1a
2a
9
15
15
15
15
15
15
15
15
0
99
99
96
96
98
96
97
96
99
98
95
98.3
3.0
98.7
73.0
97.5
5.5
99.3
11.0
99.3
99.0
65.0
Cu
A
À29.5
À33.0
À48.2
À66.1
Cu
Cu
ACHTUNGTRENNUNG
A
DCM
DCM
CuACHTUGNTRENNNUG
[a]
Reactions were run with 10 mol% chiral catalyst at 158C
for 12 h.
Isolated yield.
Determined by chiral HPLC.
[b]
[c]
9
10
11
0
15
[a]
this is the best case for inversion of enantioselectivity
in this type of reaction just through changing the sol-
vent. Methanol and ethanol as solvent also gave good
results (83% and 98% ee, respectively, entries 9 and
10). Finally, the copper(II) complexes of 1a, 2a, 1c
and 2c as catalysts were further investigated using di-
chloromethane as solvent. Although high reactivity
Reactions in isobutyl alcohol were carried out under N2
for 12 h using 11 mol% of ligand 1 and 10 mol% of
CuACHTUNGTRENNUNG(OTf)2.
Isolated yield.
[b]
[c]
Determined by chiral HPLC.
yield was obtained, the ee was only moderate (65.0%, was observed, the ee values decreased drastically.
entry 11). Only the 2a- and 2c-Cu(OTf)2 complexes furnished
AHCTUNGTRENNUNG
Encouraged by preliminary results, other reaction moderate ee (À48.2 and À66.1%, respectively, en-
conditions such as Cu(II) salts and solvents were fur- tries 14 and 15).
ther optimized for the four selected ligands (1a, 2a,
From the above experimental results, 1a is the most
1c, 2c), as summarized in Table 2. Initially, the Frie- effective ligand which exhibited more broad generali-
del–Crafts alkylation was conducted at 158C in isobu- ty. Excellent results can be achieved with different
tyl alcohol using Cu
yields and enantioselectivities were obtained for li- ent solvents (isobutyl alcohol and DCM). However,
gands 1a and 2a (98.5% and 97.7% ee, entries 1 and Cu(OTf)2 seems to be the superior copper source.
2), while inferior ee values were obtained for 1c and The scope of the 1a-Cu(OTf)2 complex in the Frie-
ACTHUGNNERTU(GNN ClO4)4·6H2O as Lewis acid. High Cu(II) salts [CuACHUTNTREGG(NNUN OTf)2 and CuACHTNUGRTEN(NNGU ClO4)2·6H2O] in differ-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
2c under the same conditions (entries 3 and 4). Subse- del–Crafts alkylation was tested using a variety of
quently, the effect of solvents was tested using the 1a- structurally different indole derivatives and arylidene-
CuACHTUNGTRENNUNG(OTf)2 complex as catalyst. In acetone-ether (1:1, malonates (Table 3). In most cases the reaction
v/v) or 1,2-dichloroethane (DCE), only moderate worked well to afford the desired products in good to
yields and very low ee values were obtained after 12 h excellent yields. When a Cl atom is located at the
(entries 5 and 6). In 1,1,2,2-tetrachloroethane ortho-position of the phenyl group, the reaction
(TTCE), the reaction worked well with up to 98% became very sluggishly so that a prolonged reaction
yield, while the ee value was only À16% (entry 7). Di- time was essential for obtaining good yield (entry 4).
chloromethane (DCM) was the best solvent for ach- Enantiomeric excesses ranging from 97.5–99.5% were
ieving the opposite enantiomer, whereby high yields obtained for various arylidenemalonates (entries 1–5),
and ee values were achieved (À96.5% ee, entry 8). except for diethyl ethylidenemalonate (33.7% ee,
This phenomenon was also found in the same and entry 6). The diethyl benzylidene-substituted malo-
other reactions.[12,5b,c] To the best of our knowledge, nate reacted with 5-methoxyindole, 5-methyindole, 5-
Adv. Synth. Catal. 2009, 351, 3113 – 3117
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3115

Yan-Jin Sun et al.
COMMUNICATIONS
Table 3. 1a-Cu
ACHTUNGTRENNUNG(OTf)2-catalyzed Friedel–Crafts reaction of
fact that the trifÀlate anion can tune the chiral space
better than ClO4 . Further investigations of the mech-
anism are underway in our laboratory.
indole derivatives with alkylidenemalonates.[a]
In conclusion, we demonstrated that the simple he-
tetoarylidenemalonate-derived bis(oxazoline) Cu(II)
complexes with a larger bridge angle than the 2,2-di-
AHCTUNGTREGaNNNU lkylmalonate-derived ligands have exciting asymmet-
ric catalytic properties in the Friedel–Crafts alkylation
of indoles with arylidenemalonates. In isobutyl alco-
hol, the Cu
2c gave the same high enantioselectivity (97.5–99.3%
ee), while the complexes of Cu(ClO4)2·6H2O with li-
gands 1a, 2a were also very efficient (>97% ee). Very
interestingly, when complex 1a-Cu(OTf)2 was used,
changing the solvent from isobutyl alcohol to di-
chloromethane resulted in the inversion of the prod-
uctꢀs absolute configuration with up to 96.5% ee. In
the same reaction for structurally different indoles
and diethyl arylidenemalonates, our new chiral cata-
ACHTUNGRTEN(NUNG OTf)2 complexes of ligands 1a, 2a, 1c and
Entry R1
R2
Time Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
9
10
H
H
H
H
H
H
4-MeO-C6H4 12
99
99
96
70
98
95
99
98
99
98
97.5
98.3
98.6
99.5
99.3
33.7
99.3
97.3
95.2
97.9
AHCTUNGTRENNUNG
4-Me-C6H4
3-Br-C6H4
2-Cl-C6H4
2-thienyl
Me
12
12
72
12
12
12
12
12
12
AHCTUNGTRENNUNG
5-MeO Ph
5-Me
5-Cl
6-Cl
Ph
Ph
ph
lysts 1a-CuACTHUNRGTNEUNG(OTf)2 gave higher enantioselectivity than
the previously reported methods in most cases,[4,5,6,7]
which indicated the good potential for wide applica-
tion. All the catalytic reactions proceeded well under
mild condition (0–158C). Our results on the tuning of
the bridge angle for the malonate-derived bis(oxazo-
line) ligand may shed light on the design of novel and
[a]
Reactions were run in isobutyl alcohol under N2 with
10 mol% chiral catalyst at 08C.
Isolated yield.
[b]
[c]
Determined by chiral HPLC
chloroindole and 6-chloroindole to afford the adducts efficient chiral catalystic systems. Further investiga-
in excellent yields and ee values (95.2–99.3% ee, en- tions on the catalytic asymmetric Friedel–Crafts reac-
tries 7–10), indicating that the substituents on indole tions of other substances and other asymmetric reac-
ring had little effect on the enantioselectivity.
tions using our heteroarylidenemalonate-derived
Regarding the reaction mechanism, according to bis(oxazoline) system are in progress in our group.
several catalytic processes of BOX-Cu(II) complex al-
ready disclosed,[13] and considering the change trend
in the catalytic results and dramatic solvent effect are
all consistent with Tangꢀs report, the two catalytic sys-
Experimental Section
tems may have similar mechanism. In Tangꢀs system,
the three oxazoline rings coordinate simultaneously
to the Cu(II) center in dichloromethane, while one
oxazoline ring is replaced by one molecule of alcohol
when isobutyl alcohol is used as solvent. In our
system, the thiophene or furan moiety directs away
from the two oxazoline rings and the copper(II)
center, however, the heterocycle moiety may partici-
pate in intermolecular coordination in solution as a
weak ligand, the direct evidence was observed for the
crucial role of the heterocycles on the high enantiose-
Representative Example for Asymmetric F-C
Alkylation Reaction of Indoles with
Alkylidenemalonates
To a Schlenk tube CuACHTNUGTRNEUNG(OTf)2 (0.025 mmol) was added, fol-
lowed by ligand 1a (0.0275 mmol) in the solvent isobutyl al-
cohol (1.0 mL) under N2, the solution was stirred for 1.5 h at
room temperature, a mixture of diethyl benzylidenemalo-
nate (0.25 mmol) in the above solvent (1.0 mL) was added.
The resulting mixture was kept stirring for 15 min, cooled to
08C and stirred for another 15 min before the indole
(0.25 mmol) was added. After stirring for 12 h at 08C, the
lectivity in comparison to inferior result obtained by solution was concentrated under reduced pressure, The
crude product was purified by flash column chromatography
on silica gel [eluted with ethyl acetate/petroleum ether (1/5,
v/v)] to afford the (S)-ethyl 2-ethoxycarbonyl3-(3-indolyl)-3-
phenylpropanoate as a white solid in near quantitative yield
(99%); mp 179–1808C; [a]2D5: +45.6 (10 mg/2 mL CH2Cl2);
1H NMR (300 MHz, CDCl3): d=0.96–1.03 (m, 6H), 3.93–
4.04 (m, 4H), 4.28 (d, J=11.7 Hz, 1H), 5.07 (d, J=11.7 Hz,
1H), 7.00–7.07 (m, 1H), 7.09–7.37 (m, 8H), 7.55 (d, J=
8.0 Hz, 1H), 8.04 (brs, 1H); HPLC analysis (Chiralcel OD-
H, n-hexane/2-PrOH, 88:12, 0.8 mLminÀ1, 254 nm): tr
Burkeꢀs benzylidene-bis(oxazoline) ligand 9 in the
same reaction.[10] However, the role of the different
electronic and steric natures of heterocycles and
phenyl group cannot be excluded at this stage. The in-
version of the absolute configutation is mainly depen-
dent on the change of solvent, In the catalytic process,
À
the counterion OTfÀ or ClO4 can coordinate with
Cu(II) as a weak bonding interaction, however, the
better enantioselectivity obtained by CuACTHNUGTRENUNG(OTf)2-BOX
than CuACHTUNGTRENNUNG(ClO4)·6H2O-BOX is probably due to the (minor)=10.03 min, tr (major)=12.03 min; 99.3% ee.
3116
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 3113 – 3117

Highly Enantioselective Friedel–Crafts Reaction of Indole with Alkylidenemalonates
COMMUNICATIONS
nor; J. J. Maresh, Nat. Prod. Rep. 2006, 23, 532; d) M.
Amat, N. Llor, J. Bosch, X. Solans, Tetrahedron 1997,
53, 719; e) A. Kleeman, J. Engel, B. Kutscher, D.
Reichert, Pharmaceutical Substances, 4th edn., Georg
Thieme Verlag, New York, 2001.
Acknowledgements
We are grateful to the National Natural Sciences Foundation
of China (No. 20772151) and the Ministry of Science and
Technology of China (No. 2006BAE01A01) for financial
support. We deeply thank Professor Da-Ming Du and Dr.
Han Liu for their valuable discussions.
[4] a) W. Zhuang, T. Hansen, K. A. Jørgensen, Chem.
Commun. 2001, 347; b) T. B. Poulsen, K. A. Jørgensen,
Chem. Rev. 2008, 108. 2903.
[5] a) J. Zhou, Y. Tang, J. Am. Chem. Soc. 2002, 124, 9030;
b) J. Zhou, M.-C. Ye, Z.-Z. Huang, Y. Tang, J. Org.
Chem. 2004, 69, 1309; c) J. Zhou, Y. Tang, Chem.
Commun. 2004, 432; d) J. Zhou, M.-C. Ye, Y. Tang, J.
Comb. Chem. 2004, 6, 301; e) M.-C. Ye, B. Li, J. Zhou,
X.-L Sun, Y. Tang, J. Org. Chem. 2005, 70, 6108.
[6] a) R. Rasappan, M. Hager, A. Gissibl, O. Reiser, Org.
Lett. 2006, 8, 6099; b) A. Schatz, R. Rasappan, M.
Hager, A. Gissibl, O. Reiser, Chem. Eur. J. 2008, 14,
7259.
References
[1] For reviews of Friedel–Crafts alkylation reactions see
a) G. A. Olah, R. Krishnamurit, G. K. S. Prakash, Frie-
del–Crafts Alkylation, in: Comprehensive Organic Syn-
thesics, (Eds.: B. M. Trost, I. Flemming, Pergamon
Press, Oxford, 1st edn, 1991, Vol. III, p 293; b) R. M.
Roberts, A. A. Khalaf, Friedel–Crafts Alkylation
Chemistry – A Century of Discovery, Dekker, New
York, 1984; c) G. A. Olah, Friedel–Crafts and Related
Reactions, Wiley-Interscience, New York, 1964, Vol. II,
part 1; d) M. Bandini, A. Melloni, S. Tommasi, A.
Umani- Ronchi, Synlett 2005, 1199.
[7] Y.-l. Liu, D.-J. Shang, X. Zhou, X. H. Liu, X.-M. Feng,
Chem. Eur. J. 2009, 15, 2055.
[8] a) I. W. Davies, L. Gerena, L. Castonguay, C. H. Sena-
nayake, R. D. Larsen, T. R. Verhoeven, P. J Reider,
Chem. Commun. 1996, 1753; b) I. W. Davies, R. J.
Deeth, R. D. Larsen, P. J. Reider, Tetrahedron Lett.
1999, 40, 1233.
[2] For selected examples, see: a) J. F. Austin, D. W. C.
MacMillan, J. Am. Chem. Soc. 2002, 124, 1172; b) R. P.
Herrera, V. Sgarzani, L. Bernardi, A. Ricci, Angew.
Chem. Angew.Chem. 2005, 117, 6734; Angew. Chem.
2005, 117, 6734; Angew. Chem. Int. Ed. 2005, 44, 6576;
c) K. B. Jensen, J. Thorhauge, R. G. Hazell, K. A. Jø-
gensen, Angew. Chem. 2001, 113, 164; Angew. Chem.
Int. Ed. 2001, 40, 160; d) H. Li, Y.-Q. Wang, L. Deng,
Org. Lett. 2006, 8, 4063; e) Y. Q. Wang, J. Song, R.
Hong, H. m. Li, L. Deng, J. Am. Chem. Soc.2006, 128,
8156; f) D. A. Evans, K. R. Fandrick, H.-J. Song, J. Am.
Chem. Soc. 2005, 127, 8942; g) Y.-X. Jia, S.-F. Zhu, Y.
Yang, Q.-L. Zhou, J. Org. Chem. 2006, 71, 75; h) S.-F.
Lu, D.-M. Du, J. Xu, Org. Lett. 2006, 8, 2115; i) H. Liu,
S. F. Lu, J. X. Xu, D. M. Du, Chem. Asian J. 2008, 3,
1111; j) Q. Kang, Z.-A. Zhao, S.-L. You, J. Am. Chem.
Soc. 2007, 129, 1484; k) Y.-X. Jia, J. Zhong, S.-F. Zhu,
C.-M. Zhang, Q.-L. Zhou, Angew. Chem. 2007, 119,
5661; Angew. Chem. Int. Ed. 2007, 46, 5565; l) A. J.
Boersma, B. L. Feringa, G. Roelfes, Angew. Chem.
2009, 121, 3396; Angew. Chem. Int. Ed. 2009, 48, 3346;
m) D. A. Evans, K. R. Fandrick, Org. Lett. 2006, 8,
2249; n) D. A. Evans, K. R. Fandrick, H.-J. Song, K. A.
Scheidt, R. Xu, J. Am. Chem. Soc. 2007, 129, 10029;
o) Q. Kang, X.-J. Zheng, S.-L. You, Chem. Eur. J. 2008,
14, 3539; p) Y.-F. Sheng, G.-Q. Li, Q. Kang, A.-J.
Zhang, S.-L. You, Chem. Eur. J. 2009, 15, 3351; q) H. L.
Cui, X. Feng, J. Peng, J. Lei, K. Jiang, Y. C. Chen,
Angew. Chem. 2009, 121, 5847; Angew. Chem. Int. Ed.
2009, 48, 5737.
[9] S. E Denmark, C. M. Stiff, J. Org. Chem. 2000, 65,
5875.
[10] a) E. P. Carreiro, S. Chercheja; N. Moura, S. C Ger-
trudes, A. J. Burke, . Inorg. Chem. Commun. 2006, 9,
823; b) A. J. Burke, E. P. Carreiro, S. Chercheja,
N. M. M. Moura, J. P. Ramalho, A. I. Rorigues, C. I. M.
Santos, J. Organomet. Chem. 2007, 692, 4863.
[11] a) D.-M. Du, B. Fu, W.-T. Hua, Tetrahedron 2003, 59,
1933; b) S. E Denmark, N. Nakajimn, O. J. Nicoise,
A. M. Paucher, J. P. Edwards, J. Org. Chem. 1995, 60,
4884.
[12] For the reversal of enantioselectivity just by changing
solvents in catalysis, please see: a) J. Thorhauge, M.
Roberson, R. G. Hazell, K. A. Jørgensen, Chem. Eur. J,
2002, 8, 1888; b) M. P. Sibi, M. Liu, Curr. Org. Chem.
2001, 5, 719; c) G. Zanoni, F. Castronovo, M. Franzini,
G. Vidari, E. Giannini, Chem. Soc. Rev. 2003, 32, 115.
[13] a) J. S. Johnson, D. A. Evans, Acc. Chem. Res. 2000, 33,
325; b) R. Rasappan, D. Laventine, O. Reiser, Coord.
Chem. Rev. 2008, 252, 702; c) D. A. Evans, S. J. Miller,
T. Lectka, P. von Matt, J. Am. Chem. Soc. 1999, 121,
7559; d) D. A. Evans, T. Rovis, M. C. Kozlowski, C. W.
Downey, J. S. Tedrow, J. Am. Chem. Soc. 2000, 122,
9134; e) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S.
Tedrow, J. Am. Chem. Soc. 1999, 121 1994; f) G. Desi-
moni, G. Faita, K. A. Jørgensen, Chem. Rev. 2006, 106,
3561; g) T. B. Poulsen, K. A. Jøgensen, Chem. Rev.
2008, 108, 2903.
[3] a) J. Bosch, M.-L. Bennasar, Synlett 1995, 587; b) D. J.
Faulkner, Nat. Prod. Rep. 2002, 19, 1; c) S. E. OꢀCon-
Adv. Synth. Catal. 2009, 351, 3113 – 3117
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3117