Vanadium(V) salen complex catalyzed highly enantioselective cyanoformylation of aldehydes in the presence of imidazole as a cocatalyst

Article abstract of DOI:10.1002/ejoc.200800476

Highly enantioselective cyanoformylation of aldehydes catalyzed by V ^V chiral salen complex by using ethyl cyanoformate as a source of cyanide was accomplished in the presence of several cocatalysts. Excellent yields and ee values for the resul

Full text of DOI:10.1002/ejoc.200800476

FULL PAPER
DOI: 10.1002/ejoc.200800476
Vanadium(V) Salen Complex Catalyzed Highly Enantioselective
Cyanoformylation of Aldehydes in the Presence of Imidazole as a Cocatalyst
Noor-ul H. Khan,*[a] Santosh Agrawal,[a] Rukhsana I. Kureshy,[a] Sayed H. R. Abdi,[a]
K. Jeya Prathap,[a] and Raksh V. Jasra[a]
Keywords: Cyanides / Vanadium / Aldehydes / Nitrogen heterocycles / Asymmetric catalysis
Highly enantioselective cyanoformylation of aldehydes cata-
lyzed by VV chiral salen complex by using ethyl cyanofor-
mate as a source of cyanide was accomplished in the pres-
ence of several cocatalysts. Excellent yields and ee values for
the resulting cyanohydrin carbonates were achieved when
imidazole was used as a cocatalyst at –20 °C. The ee value of
the cyanohydrin carbonate of 2-naphthaldehyde was further
improved to Ͼ99% by recrystallization.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
similar enantioselectivities. Feng et al.[10c] also tried the
same reaction but in the presence of 2-propanol/chloroform
mixture as a solvent at –20 °C; however, the reaction took
10–92 h and only moderate to high conversions and ee val-
ues were obtained. With our interest in enantioselective cy-
anation of aldehydes and ketones, we previously reported
TiIV, VV, and MnIII metal complexes with dimeric and poly-
meric salen ligands as catalysts with different cyanide
sources.[11] In the present manuscript we explored the cata-
lytic efficacy of VV salen complex 1 (Figure 1)[3f] in the
enantioselective cyanation of aldehydes by using ethyl cya-
noformate as a source of cyanide in combination with vari-
ous cocatalysts. Excellent yields and ee values for the cya-
nohydrin carbonate of hydrocinnamaldehyde were achieved
with this catalyst in the presence of imidazole as a cocata-
lyst. Further, in the case of solid products such as the cya-
nohydrin carbonate of 2-naphthaldehyde, the ee was im-
proved to Ͼ99% upon recrystallization. Notably, the car-
bonate products are more stable towards unwanted hydroly-
sis that takes place in the case of cyanohydrin trimethylsilyl
ethers.
Introduction
The enantioselective synthesis of cyanohydrins through
carbonyl compounds is a topic of current interest because
the resulting optically pure cyanohydrins can be trans-
formed into versatile synthetic building blocks with no loss
in optical purity.[1] They also play an important role in the
preparation of a wide range of pharmaceuticals, agrochem-
icals, and insecticides.[2] To achieve high chiral induction in
cyanohydrins various catalytic systems for the enantio-
selective cyanation of aldehydes and ketones were used
with KCN, NaCN, trimethylsilyl cyanide (TMSCN), and
HCN as sources for cyanide.[3,4] Among these catalytic sys-
tems, a VV salen complex showed impressive enantioinduc-
tion in the product O-trimethylsilyl cyanohydrins with
TMSCN[3e–3h] as a cyanide source; however, O-trimethyl-
silyl cyanohydrins are not so stable and readily undergo hy-
drolysis to form cyanohydrins that are prone to racemiza-
tion. Recently, attempts were made to utilize various other
sources of cyanide, for example, cyanoformate esters
(ROCOCN), acetyl cyanide, diethyl cyanophosphonate, and
benzoyl cyanide to synthesize optically pure cyanohydrins
with different catalytic systems.[5–10] Belokon et al.[6b] re-
ported a TiIV salen complex as an efficient catalyst (5 mol-
%) for cyanoformylation of aldehydes at –40 °C in 6–48 h
with high conversions and ee values. Later, Moberg et al.[8a]
reported an improvement over this catalytic system by con-
ducting the cyanoformylation of aldehydes in the presence
of various Lewis bases. The presence of triethylamine as a
Lewis base significantly cut the reaction time (4–12 h) with
Figure 1. Structure of complexes 1 and 2.
[a] Discipline of Inorganic Materials and Catalysis, Central Salt
and Marine Chemicals Research Institute (CSMCRI, CSIR),
Bhavnagar- 364 002, Gujarat, India
Results and Discussion
Fax: +91-0278-2566970
E-mail: khan2593@yahoo.co.in
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Our systematic study started with the asymmetric cyano-
formylation of benzaldehyde (2a) by using chiral VV salen
Eur. J. Org. Chem. 2008, 4511–4515
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4511

N.-u. H. Khan et al.
FULL PAPER
complexes 1 and 2 as catalysts at –20 °C in dichlorometh- Table 1. Effect of cocatalysts on the asymmetric addition of ethyl
cyanoformate to benzaldehyde at –20 °C.[a]
ane. However, even after 48 h there was no product forma-
tion (Table 1, Entries 1 and 2). When the same reaction was
conducted in the presence of imidazole as a cocatalyst
(Table 1, Entries 3 and 4), excellent conversion (96%) into
the cyanohydrin carbonate in 93%ee was achieved with cat-
alyst 1. We further explored the role of other cocatalysts,
viz., KCN, LiOH, 2,6-lutidine, pyridine, 2-methylimidazole,
and triethylamine with catalyst 1 (Table 1, Entries 5–10) in
the cyanoformylation of benzaldehyde as a model substrate.
Very good to excellent yield of cyanohydrin carbonate was
achieved except for pyridine where there was no product
formation in 24 h (Table 1, Entry 8). The use of LiOH and
triethylamine as cocatalysts in the cyanoformylation reac-
tion severely affected the enantioselectivity, though the lat-
ter cocatalyst tremendously accelerated the reaction
(Table 1, Entry 10). Among all the cocatalysts used in the
present study, imidazole was found to be the most efficient
in terms of high chiral induction and product yield (Table 1,
Entry 3). Therefore, our subsequent studies for asymmetric
cyanoformylation were carried out with chiral VV salen
complex 1 as a catalyst and imidazole as a cocatalyst. In
order to get the optimal reaction conditions we carried out
asymmetric cyanoformylation of benzaldehyde (2a) for 18 h
at various temperatures and loadings of the catalyst, cocata-
lyst, and ethyl cyanoformate. The results are summarized in
Table 2. At first the catalyst loading was varied over a range
of 1 to 7.5 mol-% keeping the cocatalyst loading at 10 mol-
% at –20 °C (Table 2, Entries 1–4). It is evident from the
results that 2.5 mol-% catalyst loading is optimum (Table 2,
Entry 2) at –20 °C. At –40 °C, there was an improvement in
the enantioselectivity, although marginal, but there was a
concomitant decrease in the product yield (Table 2, En-
try 5). By raising the temperature from –20 to 0 °C and to
room temperature, the yields of the cyanohydrin carbonates
were increased, but at the expense of the enantioselectivities
(Table 2, Entries 6 and 7). Therefore, for the rest of the cata-
Entry Complex
Cocatalyst
Time [h] Yield [%] ee [%][c]
[b]
1
2
3
4
5
6
7
8
9
10
VV EtSO4
VV Cl
–
–
48
48
18
18
18
18
36
24
12
08
–
–
–
–
VV EtSO4 imidazole
96
92
91
93
89
–
93
90
92
03
93
–
VV Cl
imidazole
VV EtSO4 KCN
VV EtSO4 LiOH
VV EtSO4 2,6-lutidine
VV EtSO4 pyridine
VV EtSO4 2-methylimidazole
VV EtSO4 triethylamine
95
95
90
09
[a] All reactions were carried out at –20 °C by using catalyst
(0.031 mmol), benzaldehyde (0.62 mmol), ethyl cyanoformate
(1.24 mmol), cocatalyst (0.062 mmol) in dry DCM (0.8 mL). [b]
Isolated yield. [c] The ee values were determined by using a chiracel
OD column.
Table 2. Effect of catalyst loading, cocatalyst loading, temperature,
and solvent variation for the synthesis of the cyanohydrin carbon-
ate of benzaldehyde.[a]
Entry Solvent
Catalyst Cocatalyst loading
[mol-%]
T
[°C]
Yield[b] ee[c]
[mol-%]
[%]
[%]
1
2
3
4
5
6
7
8
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
CHCl3
1,2-DCE
THF
1
2.5
5
10
10
10
10
10
10
10
5
15
10
10
10
10
10
10
10
10
–20
–20
–20
–20
–40
0
92
96
96
96
85
91
93
86
96
89
96
96
90
87
68
79
70
86
93
93
95
96
84
80
88
93
90
93
93
89
82
70
72
73
7.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
r.t.
–20
–20
–20
–20
–20
–20
–20
–20
–20
–20
9
10[d]
lytic experiments –20 °C was taken as the optimum tem- 11[e]
12[f]
perature (Table 2, Entry 2). We next optimize the loadings
of the cocatalyst and ethyl cyanoformate by keeping the
other parameters constant (Table 2, Entries 8–12). It is
emerged that 10 mol-% cocatalyst loading with 200 mol-%
ethyl cyanoformate is optimum (Table 2, Entry 2).
13
14
15
16
17
CH3CN
toluene
We also evaluated the catalytic activity of VV salen com-
plex 1 for the cyanoformylation of benzaldehyde (2a) under
the above-optimized reaction conditions in various solvents,
for example, 1,2-dichloroethane (1,2-DCE), dichlorometh-
ane (DCM), chloroform (CHCl3), tetrahydrofuran (THF),
toluene, and acetonitrile (CH3CN) (Table 2, Entries 2, 13–
17). Out of all the solvents used, DCM was found to be the
best solvent for this system (Table 2, Entry 2).
[a] All reactions were carried out by using catalyst 1, aldehyde
(0.62 mmol), ethyl cyanoformate (1.24 mmol). [b] Isolated yield. [c]
The ee values were determined by using a chiracel OD column. [d]
By using 160 mol-% of ethyl cyanoformate. [e] By using 240 mol-
% of ethyl cyanoformate. [f] By using 320 mol-% of ethyl cyanofor-
mate.
tronic factors of different substituents on the substrate did
Under the above-optimized reaction conditions the scope not have much effect on the yield and selectivity of the
of this protocol for the cyanoformylation reaction was fur- product. However, reactions of benzaldehyde and aromatic
ther extended to a variety of aromatic and aliphatic alde- aldehydes with smaller substituents at the 2- and 3-positions
hydes. Data in Table 3 is indicative of applicability of this were fast (Table 3, Entries 1–4) relative to the aromatic alde-
protocol over a range of substrates where good to excellent hydes having bulkier groups in the same positions (24–48 h;
isolated yields (80–97%) and ee values (76–97%) for the Table 3, Entries 6, 7, 9, 10). In general, 4-substituted aro-
products were achieved in 18–48 h. Surprisingly, the elec- matic aldehydes reacted slowly (24–60 h; Table 3, Entries 5,
4512
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4511–4515

Vanadium(V) Salen Complex Catalyzed Cyanoformylation
1
8,11–13). In the case of α, β-unsaturated aldehydes high by UV/Vis spectrophotometry and H and 13C NMR spec-
yields and ee values were achieved in 24–48 h (Table 3, En- troscopy. During the catalytic run D first reacts with imid-
tries 14 and 15). Aliphatic aldehydes bearing an aromatic azole to form an intermediate species with a probable struc-
ring in the alkyl chain, such as hydrocinnamaldehyde, gave ture F (Scheme 1), as evidenced by the emergence of new
the highest ee value of 97% with 92% isolated yield within sets of peaks at δ = 8.125, 7.438, 7.038 ppm in the 1H NMR
24 h (Table 3, Entry 16), whereas aliphatic aldehydes with spectrum (Figure 2). Similar changes were also observed in
no such functionality gave moderate yields and ee values the 13C NMR (Figure 3) and UV/Vis spectra (spectrum is
(Table 3, Entries 17 and 18). For all catalytic runs in which given in the Supporting Information). Species F is likely to
the VV salen complex possessed (R) stereochemistry, the cy- react with intermediate B [formed by the interaction of S
anohydrin carbonate product was obtained as the (S) en- with 1(A)] to form species C, which eventually gives product
antiomer. Further, the cyanohydrin carbonate of 2-naph-
thaldehyde was obtained as a white solid, which upon
recrystallization (hexane/DCM, 60:40) gave a product with
99%ee. The high ee value was probably due the preferential
homochiral aggregation of the product, which crystallized
out first to leave behind racemic product with less pro-
nounced heterochiral aggregation in the solution.[12]
Table 3. Synthesis of cyanohydrin carbonates of various aldehydes
by using the VV salen complex under optimum reaction condi-
tions.[a]
Entry
Substrate
Time
[h]
Yield[b]
[%]
ee[c]
[%]
1
2
3
4
5
6
7
8
benzaldehyde (2a)
2-naphthaldehyde (2b)
18
18
18
18
24
24
48
60
48
48
48
48
48
24
48
24
24
24
95
93
96
97
91
90
89
90
92
93
95
80
90
91
91
92
89
82
93
95(Ͼ99)[d]
91
2-methoxybenzaldehyde (2c)
3-methylbenzaldehyde (2d)
4-methylbenzaldehyde (2e)
2-methoxybenzaldehyde (2f)
3-methoxybenzaldehyde (2g)
4-methoxybenzaldehyde (2h)
2-ethoxybenzaldehyde (2i)
2-benzyloxybenzaldehyde (2j)
4-fluorobenzaldehyde (2k)
4-chlorobenzaldehyde (2l)
4-bromobenzaldehyde (2m)
(E)-cinnamaldehyde (2n)
crotonaldehyde (2o)
94
90
96
89
93
93
96
93
9
10
11
12
13
14
15
16
17
18
Scheme 1. Probable mechanism for cyanoformylation of aldehydes
(X) and proposed working model for enantioselection (Y).
85
91
92
90[e]
97
hydrocinnamaldehyde (2p)
isovalerladehyde (2q)
hexanal (2r)
76[f]
83[f]
[a] All reactions were carried out by using catalyst 1 (2.5 mol-%),
aldehyde (0.62 mmol), ethyl cyanoformate (1.24 mmol), imidazole
(10 mol-%) as cocatalyst at –20 °C. [b] Isolated Yield. [c] The ee
values were determined by using OD and OD-H chiral columns.
[d] The ee value was obtained by recrystallization. [e] The ee value
was determined by using an AD chiral column. [f] The ee value
was determined by using a GC chiral G-TA column.
Mechanism
To understand the probable mechanism of the catalytic
reaction (Scheme 1, X) we carried out a series of experi-
ments with catalyst 1 with benzaldehyde as the substrate (S)
and imidazole (E) as a cocatalyst by using ethyl cyanofor-
Figure 2. 1H NMR spectrum of a mixture of ethyl cyanoformate
mate (D) as a cyanide source. The reaction was monitored and imidazole in CDCl3.
Eur. J. Org. Chem. 2008, 4511–4515
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4513

N.-u. H. Khan et al.
FULL PAPER
a Perkin–Elmer Spectrum GX spectrophotometer in KBr window.
High-resolution mass spectra were obtained with LC–MS (Q-
TOFF), LC (Waters), MS (Micromass) instruments. For the prod-
uct purification flash chromatography was performed by using sil-
ica gel 60–200 mesh purchased from s. d. Fine-Chem. Limited
Mumbai (India). Enantiomeric excesses (ee) of the products were
determined by HPLC (Shimadzu SCL-10AVP) by using Daicel
Chiralpak AD and OD chiral columns with 2-propanol/hexane as
the eluent. Optical rotations were measured with a Digipol 781
Automatic Polarimeter Rudolph Instruments.
P. On the basis of the X-ray structure of the vanadium salen
complex reported by Belokon et al.,[3f] the potential transi-
tion state was generated where the attack of cyanide
through the less hindered Re face is favored to form the (S)
enantiomer in excess (Scheme 1, Y).
Complex 1: Complex 1 was synthesized by the reported pro-
cedure.[3f] A solution of (1R,2R)-N,NЈ-bis[3,5-bis(tert-butyl)salicyl-
idene]cyclohexane-1,2-diamine (2.7 mmol, 1.5 g) in THF (20 mL)
and vanadyl sulfate hydrate (2.7 mmol, 0.69 g) in hot ethanol
(30 mL) were mixed. The resulting solution was heated at reflux
for 2 h under an inert atmosphere and then cooled to room tem-
perature. The reaction mass was further allowed to stir for an ad-
ditional 12 h while opening the side arm of the reaction flask for
autooxidation. The solvent was completely evaporated, and the res-
idue was dissolved in DCM (15 mL) and washed with water
(3ϫ5 mL) and brine. The organic layer was dried with anhydrous
Na2SO4, and the complex was purified by column chromatography
to afford a dark-green solid.
Complex 2: Complex 2 was synthesized by the reported pro-
cedure.[3c] A solution of (1R,2R)-N,NЈ-bis[3,5-bis(tert-butyl)salicyl-
idene]cyclohexane-1,2-diamine (0.91 mmol, 0.5 g) in THF (20 mL)
and VOCl3 (1.38 mmol, 0.13 mL) were mixed. The resulting solu-
tion was stirred at room temperature for 30 min, and the solvent
was then evaporated. The product was purified by column
chromatography to afford a dark-green solid.
Figure 3. 13C NMR spectrum of a mixture of imidazole and ethyl
cyanoformate in CDCl3.
Conclusions
Typical Experimental Procedure for the Enantioselective Cyanofor-
mylation of Aldehydes: A solution of VV salen complexes 1 and 2
(0.015 mmol) and the appropriate aldehyde (0.62 mmol) in dry
CH2Cl2 (0.8 mL) was stirred for 10 min at room temperature under
a N2 atmosphere. To this solution was added imidazole (4 mg,
0.062 mmol), and the solution was cooled to –20 °C. To this cooled
solution was dropwise added ethyl cyanoformate (77 µL,
0.78 mmol) over a period of 5 min. The reaction was monitored on
TLC. After completion of the reaction the product was purified by
flash column chromatography on a silica gel column (hexane/ethyl
acetate, 90:10). The purified products were characterized by 1H and
13C NMR spectroscopy, and the spectra were in agreement with
the reported values.[6d,10d]
We carried out an efficient enantioselective cyanofor-
mylation of various aromatic and aliphatic aldehydes by
using VV chiral salen complex 1 as a catalyst with ethyl
cyanoformate as a source of cyanide in the presence of
imidazole as a cocatalyst. Excellent yields and enantio-
selectivities up to 97% for the product cyanohydrin carbon-
ate were achieved at –20 °C. The chiral purity of the cya-
nohydrin carbonate of naphthaldehyde was further im-
proved to Ͼ99% by recrystallization.
Experimental Section
Supporting Information (see footnote on the first page of this
article): Characterization data and HPLC and GC profiles of the
cyanohydrin carbonate products.
General: Vanadyl sulfate hydrate (Loba Chemie, India), imidazole,
LiOH, triethylamine (s. d. Fine-Chem. India), KCN (Merck), 2,6
lutidine, 2-methylimidazole, ethyl cyanoformate, benzaldehyde (2a),
2-naphthaldehyde (2b), 2-methoxybenzaldehyde (2f), 3-methoxy-
benzaldehyde (2g), 4-methoxybenzaldehyde (2h), 2-ethoxybenzal-
dehyde (2i), 2-benzyloxybenzaldehyde (2j), 4-fluorobenzaldehyde
(2k), 4-chlorobenzaldehyde (2l), 4-bromobenzaldehyde (2m), (E)-
cinnamaldehyde (2n), crotonaldehyde (2o), hydrocinnamaldehyde
(2p), isovaleraldehyde (2q), and hexanal (2r) were purchased from
Aldrich Chemicals and 2-methylbenzaldehyde (2c), 3-methylbenz-
aldehyde (2d), and 4-methylbenzaldehyde (2e) were purchased from
Merck chemicals and used as received. All the solvents were dried
by standard procedures,[13] distilled, and stored under nitrogen.
Chiral salen ligand viz., (1R,2R)-N,NЈ-bis[3,5-bis(tert-butyl)salicyl-
idene]cyclohexane-1,2-diamine was synthesized by the reported
method.[14] NMR spectra were obtained with a Bruker F113V spec-
Acknowledgments
N. H. K. is thankful to DST and CSIR Network project on cataly-
sis for financial assistance, S. A. is thankful to CSIR for providing
a senior research fellowship and also Dr. P. K. Ghosh, the Director
of the Institute for providing instrumentation facilities.
[1] For reviews on the synthesis and applications of cyanohydrins,
see: a) M. North in Science of Synthesis (Ed.: S.-I. Murahashi),
Thieme, Stuttgart, 2004, vol. 19, pp. 235–284; b) M. Shibasaki,
M. Kanai, K. Funabashi, Chem. Commun. 2002, 1989–1999; c)
M. B. Smith, J. March, March’s Advanced Organic Chemistry,
5th ed., John Wiley & Sons, New York, 2001, pp. 1239–1240;
d) I. Ojima, Catalytic Asymmetric Synthesis, Wiley, New York,
2000, pp. 235–284; e) A. Mori, S. Inoue, “Cyanation of Car-
trometer (500 and 125 MHz for H and 13C, respectively) and are
referenced internally with TMS. FTIR spectra were recorded with
1
4514
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4511–4515

Vanadium(V) Salen Complex Catalyzed Cyanoformylation
bonyl and Amino Groups” in Comprehensive Asymmetric Syn-
thesis (Eds.: E. N. Jacobsen, A. Pfaltz, H. Yamamoto),
Springer, Heidelberg, 1999, pp. 983–992; f) F. Effenberger, An-
gew. Chem. 1994, 106, 1609–1619; Angew. Chem. Int. Ed. Engl.
1994, 33, 1555–1564.
Tian, N. Yamagiwa, S. Matsunaga, M. Shibasaki, Angew.
Chem. Int. Ed. 2002, 41, 3636–3638.
[6]
[7]
a) Y. N. Belokon, A. J. Blacker, L. A. Clutterbuck, N. Michael,
Tetrahedron 2004, 60, 10433–10447; b) Y. N. Belokon, A. J.
Blacker, L. A. Clutterbuck, M. North, Org. Lett. 2003, 5, 4505–
4508; c) Y. N. Belokon, E. Ishibashi, H. Nomura, M. North,
Chem. Commun. 2006, 1775–1777; d) Y. N. Belokon, W. Clegg,
R. W. Harrington, C. Young, M. North, Tetrahedron 2007, 63,
5287–5299; e) Y. N. Belokon, W. Clegg, R. W. Harrington, E.
Ishibashi, H. Nomura, M. North, Tetrahedron 2007, 63, 9724–
9740.
a) S.-K. Tian, L. Deng, J. Am. Chem. Soc. 2001, 123, 6195–
6196; b) H. Li, J. Song, X. Liu, L. Deng, J. Am. Chem. Soc.
2005, 127, 8948–8949; c) S.-K. Tian, L. Deng, Tetrahedron
2006, 62, 11320–11330; d) S.-K. Tian, Y. Chen, J. Hang, L.
Tang, P. MaDaid, L. Deng, Acc. Chem. Res. 2004, 37, 621–
631.
a) S. Lundgren, E. Wingstrand, M. Penhoat, C. Moberg, J.
Am. Chem. Soc. 2005, 127, 11592–11593; b) S. Lundgren, E.
Wingstrand, C. Moberg, Adv. Synth. Catal. 2007, 349, 364–
372.
a) A. Baeza, J. Casas, C. Nájera, J. Sansano, J. M. Saá, Eur. J.
Org. Chem. 2006, 1949–1958; b) A. Baeza, C. Nájera, J. San-
sano, J. M. Saá, Tetrahedron: Asymmetry 2005, 16, 2385–2389;
c) J. Casas, A. Baeza, J. M. Sansano, C. Nájera, J. M. Saá, Tet-
rahedron: Asymmetry 2003, 14, 197–200; d) A. Baeza, J. Casas,
C. Nájera, J. Sansano, J. M. Saá, Angew. Chem. Int. Ed. 2003,
42, 3143–3146.
[2] a) T. Kusumoto, T. Hanamoto, T. Hiyama, S. Takehera, T.
Shoji, M. Osawa, T. Kuriyama, K. Nakamura, T. Fujisawa,
Chem. Lett. 1990, 19, 1615–1618; b) R. J. A. Thierry, A. C.
Lisa, N. Michael, Synlett 2005, 125, 1828–1847; c) M. Kanai,
N. Kato, E. Ichikawa, M. Shibasaki, Synlett 2005, 125, 1491–
1508; d) F.-X. Chen, X.-M. Feng, Synlett 2005, 125, 892–899;
e) J. M. Brunel, I. P. Holmes, Angew. Chem. Int. Ed. 2004, 43,
2752–2778; f) N. Michael, Tetrahedron: Asymmetry 2003, 14,
147–176; g) R. J. H. Gregory, Chem. Rev. 1999, 99, 3649–3682.
[3] a) Y. N. Belokon, A. V. Gutnov, M. A. Moskalenko, L. V.
Yashkina, D. E. Lesovoy, N. S. Ikonnikov, V. S. Larichev, M.
North, Chem. Commun. 2002, 244–245; b) Y. N. Belokon, S. C.
Cepas, B. Green, N. S. Ikonnikov, V. N. Khrustalev, V. S. Lar-
ichev, M. A. Moscalenko, M. North, C. Orizu, V. I. Tararov,
M. Tasinazzo, G. I. Timofeeva, L. V. Yashkina, J. Am. Chem.
Soc. 1999, 121, 3968–3973; c) Y. N. Belokon, V. I. Maleev, M.
North, D. L. Usanov, Chem. Commun. 2006, 4614–4616; d) W.
Huang, Y. Song, J. Wang, G. Cao, Z. Zheng, Tetrahedron 2004,
60, 10469–10477; e) Y. N. Belokon, M. North, V. I. Maleev,
N. V. Voskoboev, M. A. Moskalenko, A. S. Peregudov, A. V.
Dmitriev, N. S. Ikonnikov, H. B. Kagan, Angew. Chem. Int. Ed.
2004, 43, 4085–4089; f) Y. N. Belokon, P. Carta, A. V. Gutnov,
V. Maleeev, M. A. Moskalenko, L. V. Yashkina, N. S. Ikonni-
kov, N. V. Voskoboev, V. N. Khrustalev, M. North, Helevetica.
Chim. Acta 2002, 85, 3301–3312; g) Y. N. Belokon, M. North,
T. Parsons, Org. Lett. 2000, 2, 1617–1619; h) Y. N. Belokon, B.
Green, N. S. Ikonnikov, M. North, T. Parsons, V. I. Taravov,
Tetrahedron 2001, 57, 771–779.
[4] a) Y. Xiong, X. Huang, S.-H. Gou, J.-L. Huang, Y.-H. Wen,
X.-M. Feng, Adv. Synth. Catal. 2006, 348, 538–544; b) X.-H.
Liu, B. Qin, X. Zhou, B. He, X.-M. Feng, J. Am. Chem. Soc.
2005, 127, 12224–12225; c) H.-D. Ryu, E. J. Corey, J. Am.
Chem. Soc. 2005, 127, 5384–5387; d) D. E. Fuerst, E. N. Ja-
cobsen, J. Am. Chem. Soc. 2005, 127, 8964–8965; e) Y.-H. Wen,
X. Huang, J.-L. Hang, Y. Xiong, B. Qin, X.-M. Feng, Synlett
2005, 125, 2445–2447; f) Y. C. Qin, L. Liu, L. Pu, Org. Lett.
2005, 7, 2381–2383; g) Y. Li, B. He, B. Qin, X.-M. Feng, G.-
L. Zhang, J. Org. Chem. 2004, 69, 7910–7913; h) S.-K. Tian,
R. Hong, L. Deng, J. Am. Chem. Soc. 2003, 125, 9901–9901;
i) H. Griengl, A. Hickel, D. V. Johnson, C. Kratky, M.
Schmidt, H. Schwab, Chem. Commun. 1997, 1933–1940; j) F.
Effenberger, Chimia 1999, 53, 3–10; k) M. Schmidt, H. Griengl,
Top. Curr. Chem. 1999, 200, 193–226; l) G. Seoane, Curr. Org.
Chem. 2000, 4, 283–304.
[8]
[9]
[10]
[11]
a) Q. Li, L. Chang, X. Liu, X. Feng, Synlett 2006, 1675–1678;
b) S. Gou, X. H. Chen, Y. Xiong, X. Feng, J. Org. Chem. 2006,
71, 5732–5736; c) S. K. Chen, D. Peng, H. Zhou, L. W. Wang,
F. X. Chen, X. Feng, Eur. J. Org. Chem. 2007, 639–644; d) S.
Gou, J. Wang, X. Liu, W. Wang, F. X. Chen, X. Feng, Adv.
Synth. Catal. 2007, 349, 343–349.
a) N. H. Khan, S. Agrawal, R. I. Kureshy, S. H. R. Abdi, V. J.
Mayani, R. V. Jasra, Tetrahedron: Asymmetry 2006, 17, 2659–
2666; b) N. H. Khan, S. Agrawal, R. I. Kureshy, S. H. R. Abdi,
V. J. Mayani, R. V. Jasra, J. Mol. Catal. A: Chem. 2007, 264,
140–145; c) N. H. Khan, S. Agrawal, R. I. Kureshy, S. H. R.
Abdi, V. J. Mayani, R. V. Jasra, Eur. J. Org. Chem. 2006, 3175–
3180; d) N. H. Khan, R. I. Kureshy, S. H. R. Abdi, S. Agrawal,
R. V. Jasra, Coord. Chem. Rev. 2008, 252, 593–623; e) N. H.
Khan, S. Agrawal, R. I. Kureshy, S. H. R. Abdi, K. J. Prathap,
R. V. Jasra, Chirality 2008 (DOI: 10.1002/chir. 20534; in press).
V. A. Soloshonk, Angew. Chem. Int. Ed. 2006, 45, 766–769.
D. D. Perrin, W. L. F. Armarego, D. R. Perrin, Purification of
Laboratory Chemicals, Pergamon, New York, 1981.
J. F. Larrow, E. N. Jacobsen, Y. Gao, Y. Hong, X. Nie, C. M.
Zepp, J. Org. Chem. 1994, 59, 1939–1942.
[12]
[13]
[14]
[5] a) N. Yamagiwa, J. Tian, S. Matsunaga, M. Shibasaki, J. Am.
Chem. Soc. 2005, 127, 3413–3422; b) J. Tian, N. Yamagiwa, S.
Matsunaga, M. Shibasaki, Org. Lett. 2003, 5, 3021–3024; c) J.
Received: May 15, 2008
Published Online: July 23, 2008
Eur. J. Org. Chem. 2008, 4511–4515
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4515