A fast catalytic asymmetric aza-Morita-Baylis-Hillman reaction of N-sulfonated imines with methyl vinyl ketone in the presence of chiral bifunctional phosphane Lewis bases

Article abstract of DOI:10.1002/ejoc.200800321

A series of novel bifunctional chiral phosphane Lewis bases having one phenyl group and an electron-donating alkyl group on the phosphorus atom was designed and successfully synthesized. The use of these bifunctional chiral phosphane Lewis bases in cataly

Full text of DOI:10.1002/ejoc.200800321

FULL PAPER
DOI: 10.1002/ejoc.200800321
A Fast Catalytic Asymmetric Aza-Morita–Baylis–Hillman Reaction of
N-Sulfonated Imines with Methyl Vinyl Ketone in the Presence of Chiral
Bifunctional Phosphane Lewis Bases
Zhi-Yu Lei,[a] Guang-Ning Ma,[b] and Min Shi*[a,b]
Keywords: Asymmetric catalysis / Aza-Morita–Baylis–Hillman reaction / Ketones / Lewis bases
A series of novel bifunctional chiral phosphane Lewis bases
having one phenyl group and an electron-donating alkyl
group on the phosphorus atom was designed and success-
fully synthesized. The use of these bifunctional chiral phos-
phane Lewis bases in catalytic asymmetric aza-Morita–
Baylis–Hillman reactions (aza-MBH reactions) of N-sul-
fonated imines with methyl vinyl ketone affords the corre-
sponding adducts in good-to-excellent yields and moderate-
to-good enantioselectivities within a few hours at room tem-
perature. To the best of our knowledge, this is the fastest
catalytic asymmetric MBH reaction reported thus far.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
required. Hence, the development of more efficient chiral
catalysts still remains a great challenge. For example, pre-
viously, we reported that the catalytic asymmetric aza-
MBH reaction of N-sulfonated imines with methyl vinyl
ketone (MVK) produced the adducts in 82–96% yield and
61–95%ee at –30 or –20 °C within 18–36 h in THF by using
(R)-2Ј-diphenylphosphanyl-1,1Ј-binaphthalen-2-ol as a chi-
ral bifunctional phosphane Lewis base catalyst (10 mol-%)
in the presence of 4 Å molecular sieves (Scheme 1).[3a,3c] On
the basis of this result, we envisaged that the reaction rate
of this aza-MBH reaction can be accelerated by replacing
one phenyl group with an electron-donating alkyl group to
enhance the nucleophilicity of the phosphorus atom.
Herein, we wish to report an efficient catalytic asymmetric
aza-MBH reaction of N-sulfonated imines with MVK by
using 2Ј-[alkyl(phenyl)phosphanyl]-1,1Ј-binaphthalen-2-ol
as the catalysts to give the corresponding adducts in good-
to-excellent yields and moderate-to-good enantioselectivit-
ies (up to 88%ee) within only 1–5 h at room temperature.
To the best our knowledge, this is the fastest asymmetric
MBH or aza-MBH reaction reported thus far.
Introduction
The asymmetric Morita–Baylis–Hillman reaction is one
of the most useful and attractive C–C bond-forming reac-
tions used to give enantiomerically enriched β-hydroxy car-
bonyl compounds or β-amino carbonyl compounds bearing
an α-alkylidene group, which are valuable building blocks
in the synthesis of medicinally relevant compounds.[1] Be-
cause of the great potential of the products and the superior
mild reaction conditions, the catalytic asymmetric version
of the aza-Morita–Baylis–Hillman (aza-MBH) reaction has
attracted increasing attention and has undergone remark-
able progress during the past decade.[2–4] Thus far, many
kinds of chiral bifunctional catalysts have been developed
and some excellent catalytic systems for the aza-MBH reac-
tion that are able to achieve high enantioselectivities have
been reported. Among these reported chiral catalysts, the
chiral BINOL-[3] and NOBIN-derived[4] bifunctional cata-
lysts developed by our group, Sasai, and others displayed
excellent catalytic activities with wide substrate scopes and
excellent enantioselectivities.
Nevertheless, the catalytic asymmetric aza-MBH reac-
tion is often hampered by a low reaction rate (Ͼ15 h) and a
great deal of chiral catalyst loading (Ͼ10 mol-%) is usually
[a] School of Chemistry & Molecular Engineering, East China
University of Science and Technology, Laboratory for Ad-
vanced Materials and Institute of Fine Chemicals,
130 Mei Long Road, Shanghai 200237, China
E-mail: Mshi@mail.sioc.ac.cn
[b] State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences,
354 Fenglin Road, Shanghai 200032, China
Fax: +86-21-64166128
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 1. Catalytic asymmetric aza-MBH reaction in the presence
of chiral bifunctional phosphane Lewis base.
Eur. J. Org. Chem. 2008, 3817–3820
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3817

Z.-Y. Lei, G.-N. Ma, M. Shi
FULL PAPER
Table 1. Optimization of the reaction conditions in the asymmetric
aza-MBH of N-[(4-chlorophenyl)methylene]-N-tosylamine (1a)
with MVK (2).
Results and Discussion
As show in Scheme 2, we first designed and synthesized
a series of chiral bifunctional phosphane Lewis bases L1–
L4 bearing an alkyl group on the phosphorus atom. These
phosphane compounds can be easily prepared by reduction
of the corresponding phosphane oxides[5] in the presence of
Et3N and HSiCl3.[6] The chirality at phosphorus atom is
lost during the reduction of the phosphane oxide. A pair
of inseparable diastereoisomers (diastereoisomeric ratio is
about 1:1) was acquired and used for the asymmetric aza-
MBH reaction. The detailed experimental procedures and
the spectroscopic data are summarized in the Supporting
Information.
Entry
L
Solvent
Time
[h]
Yield
[%][a]
ee/
Absolute
configuration
[%][b]
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L2
L3
L4
THF
CH2Cl2
MeCN
PhMe
Et2O
THF
THF
THF
4
5
7
8
9
3
3
4
99
48
98
62
76
99
97
94
60
52
52
27
37
43
68
34
S
S
S
S
S
S
S
S
[a] Isolated yields. [b] Determined by chiral HPLC.
significant improvement in yield or enantioselectivity could
be realized at 25, 0, or –10 °C, although the reaction rate
was accelerated significantly with the rising of the tempera-
ture (Table 2, Entries 1–3). At 25 °C, the reaction was com-
plete within 1 h to afford 3a in 99% yield and 68%ee
(Table 2, Entry 1). Having the partially optimized reaction
conditions in hand, the loading of phosphane Lewis base
L3 was also examined from the range of 1 to 10 mol-%
(Table 2, Entries 1, 4–6). It was found that the reaction rate
was accelerated significantly with an increase in catalyst
loading to afford 3a with similar ee values. The yield of 3a
could be maintained upon prolonging the reaction time in
the cases of less catalyst loading. In the presence of L3
(5 mol-%), corresponding adduct 3a was obtained in 96%
yield and 67%ee within 1.5 h (Table 2, Entry 4). Moreover,
adduct 3a could be attained in 98% yield and 66%ee after
a prolonged reaction time (5 h) with a catalyst loading of
1 mol-%. Overall, we established the optimized reaction
conditions: 5–10 mol-% of L3 as the catalyst and THF as
the solvent at 25 °C.
Scheme 2. Preparation of chiral bifunctional phosphane Lewis
bases.
Firstly, the catalytic activity of L1 was examined in the
reaction of N-[(4-chlorophenyl)methylene]-N-tosylamine
(1a) and MVK (2) as a model at room temperature (10 °C)
in a variety of solvents. The results of these experiments are
presented in Table 1, and THF was found to be the best
solvent in terms of both yield and ee of the corresponding
aza-MBH adduct 3a (Table 1, Entries 1–5). In dichloro-
methane, acetonitrile, toluene, or ether, 3a was formed in
48–98% yield and 27–52%ee under identical conditions
(Table 1, Entries 2–5). Next studies were aimed at determin-
ing the efficiency of other chiral bifunctional phosphane
Lewis bases L2–L4 in THF. As shown in Table 1, chiral
phosphane Lewis base L3 produced the corresponding aza-
MBH adduct 3a in 97% yield and 68%ee after 3 h (Table 1,
Entry 7). Under the same reaction conditions, chiral bifunc-
tional phosphane Lewis base L2 produced adduct 3a in
lower ee though high catalytic ability was also observed
(Table 1, Entry 6). In addition, more sterically encumbered
chiral bifunctional phosphane Lewis base L4 did not show
any improvement in enantioselectivity, affording 3a in only
34%ee (Table 1, Entry 8). These results indicate that the
steric bulkiness around the phosphorus atom in the cata-
lysts is crucial for this catalytic asymmetric reaction. More-
over, it should be also noted that although the chirality on
the phosphorus atom is lost during the reduction with
HSiCl3 and Et3N under reflux in toluene, the enantio-
selectivity of 3a is introduced by the axial chirality of the
BINOL scaffold.
Table 2. Optimization of the reaction conditions in the asymmetric
aza-MBH of N-[(4-chlorophenyl)methylene]-N-tosylamine (1a)
with MVK (2).
Entry
L3 [mol-%]
T [oC]
Time [h] Yield [%][a]
ee [%][b]
1
2
3
4
5
6
10
10
10
5
3
1
25
0
–10
25
25
25
1
6
10
1.5
2
99
98
98
96
97
98
68
67
67
67
67
66
By using L3 as the catalyst under the optimized reaction
conditions, we next carefully examined the temperature ef-
5
fect on this reaction. The results are outlined in Table 2. No [a] Isolated yields. [b] Determined by chiral HPLC.
3818 Eur. J. Org. Chem. 2008, 3817–3820
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A Fast Catalytic Asymmetric Aza-Morita–Baylis–Hillman Reaction
To investigate the scope and limitations of this highly
efficient catalytic asymmetric aza-MBH reaction of N-sul-
fonated imines with MVK, we examined several other im-
ines under the optimized conditions. The results of these
experiments are summarized in Table 3. When N-sulfonated
imines 1 contained an electron-donating group, such as a
methyl or methoxy group, on the aromatic ring, the corre-
sponding adducts 3c and 3d were obtained within 4–5 h in
the presence of 10 mol-% of L3 in nearly quantitative yield
with ee values of 51 and 44%, respectively (Table 3, En-
tries 2 and 3). As for N-sulfonated imines 1 bearing an elec-
tron-withdrawing group on the aromatic ring, the reaction
proceeded smoothly to afford the corresponding adducts in
excellent yields and moderate-to-good ee values within 1–
2 h in the presence of 5.0 mol-% of L3 (Table 3, Entries 4–
12). Substituents in the ortho, meta, or para position of the
aromatic ring in the employed imines did not affect the out-
come of the reactions, and higher enantioselectivities were
observed when ortho-substituted imines were treated with
MVK under the standard conditions (Table 3, Entries 6–9
and 12). In particular, N-sulfonated imines derived from 2-
chlorobenzaldehyde and 2,4-dichlorobenzaldehyde gave the
corresponding aza-MBH adducts in 88% ee within 2 h,
which is the highest enantioselectivity achieved in this inter-
esting catalytic system (Table 3, Entries 7 and 8). Naphtha-
lene-1-carbaldehyde-, heteroaromatic aldehyde-, and ali-
phatic aldehyde-derived imines gave the corresponding aza-
MBH adducts 3n–p in good-to-high yields and moderate
enantioselectivities within 3–5 h under identical conditions
(Table 3, Entries 13–15).
Conclusions
We designed and synthesized a series of novel chiral bi-
functional phosphane Lewis base catalysts having one
phenyl group and an electron-donating alkyl group on the
phosphorus atom for the catalytic asymmetric aza-MBH re-
action. We found that these chiral bifunctional phosphane
catalysts are very effective in this reaction under mild and
concise conditions to produce the corresponding adducts in
good-to-excellent yields and moderate-to-good enantio-
selectivities within 1–5 h. To the best of our knowledge, this
is the fastest catalytic asymmetric MBH reaction reported
thus far. Efforts are in progress to elucidate the mechanistic
details of this reaction and to study its scope and limita-
tions.
Experimental Section
General Remarks: 1H, 31P, and 13C NMR spectra were recorded
with a Varian Mercury vx-300 spectrometer for solution in DMSO
or CDCl3 with tetramethylsilane (TMS) as an internal standard.
Chiral HPLC was performed with a SHIMADZU SPD-10A series
with chiral columns. Elementary analyses (C, H, N) were per-
formed with a Carlo–Erba 1106 analyzer. Mass spectra were re-
corded by EI and HRMS was measured with a HP-5989 instru-
ment. Flash column chromatography was performed by using silica
gel (300–400 mesh). Melting points are uncorrected. All solvents
were purified by distillation under the standard conditions. Unless
otherwise noted, all commercially obtained reagents were used
without further purification. All reactions were carried out under
an argon atmosphere. Imines[2a,3a,3c] and phosphane oxides[5] were
synthesized according to literature procedures. Products 3a–f, 3h,
3i, 3k–p are known compounds.[2a,3a,3c,4a]
Table 3. Asymmetric aza-MBH reactions of N-tosylaldimines 1
with 2 catalyzed by L3.
General Procedure for the Reduction of Phosphane Oxides: At 0 °C,
HSiCl3 (8.0 mmol, 0.8 mL) was carefully added to a mixture of
phosphane oxide (2.0 mmol) and triethylamine (16 mmol, 2.1 mL)
in toluene (50 mL) in a three-necked round-bottom flask under an
argon atmosphere. The reaction mixture was heated to reflux for
16–24 h. After being cooled to room temperature, the mixture was
diluted with Et2O and quenched with a small amount of saturated
NaHCO3. The resulting suspension was filtered through Celite, and
the solid was washed with Et2O. The combined organic layer was
dried with anhydrous MgSO4 and concentrated under reduced
pressure. The crude product was purified by flash column
chromatography (SiO2; EtOAc/petroleum ether, 1:10) to give the
product as a colorless solid (a pair of diastereoisomers).
Entry Ar
L3
t
Product Yield
ee
Absolute
[mol-%] [h]
[%][a] [%][b] configuration
1
C6H5
10
10
10
5
5
5
5
5
5
5
3
4
5
2
2
2
2
2
2
1
1
1
5
3
4
3b
3c
3d
3e
3f
93
98
97
98
98
90
89
91
90
96
91
92
91
89
80
52
51
44
68
80
81
88
88
78
69
69
78
65
64
61
S
S
S
S
S
R
R
R
R
S
S
R
S
R
S
2
3
4
4-MeC6H4
4-MeOC6H4
4-BrC6H4
(R)-2Ј-[Ethyl(phenyl)phosphanyl]-1,1Ј-binaphthalen-2-ol (L1): White
solid. Yield: 520 mg (64%). M.p. 168–170 °C. [α]2D5 = –16.4 (c =
0.93, CH Cl ). IR (CH Cl ): ν = 3054, 2927, 1708, 1621, 1595,
˜
2
2
2
2
5
3-BrC6H4
1
1514, 1434, 1144, 815 cm–1. H NMR (300 MHz, CDCl3, TMS): δ
= 0.79–0.89 (m, 3 H, CH3), 1.01 (dt, J = 7.2, 17.4 Hz, 3 H, CH3),
1.92 (q, J = 7.5 Hz, 2 H, CH2), 2.06 (q, J = 7.2 Hz, 2 H, CH2),
4.50 (br., 1 H, OH), 4.84 (br., 1 H, OH), 6.54 (d, J = 8.4 Hz, 1 H,
ArH), 6.86–7.05 (m, 7 H, ArH), 7.14–7.38 (m, 14 H, ArH), 7.46–
7.51 (m, 2 H, ArH), 7.64–7.78 (m, 3 H, ArH), 7.86–8.01 (m, 7 H,
ArH) ppm. 31P NMR (121 MHz, CDCl3, 85% H3PO4): δ = –20.41,
–20.83 ppm. MS (EI): m/z = 406.1 (40.59) [M]+, 405.1 (26.48) [M –
1]+, 390.2 (25.11) [M – 16]+, 389.1 (100) [M – 17]+, 252.1 (13.55)
[M – 154]+. HRMS (EI): calcd. for C28H23OP 406.1487; found
406.1487.
6
2-BrC6H4
3g
3h
3i
7
2-ClC6H4
8
9
2,4-Cl2C6H3
2-FC6H4
3j
3k
3l
10
11
12
13
14
15
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
1-naphthyl
2-furyl
5
5
5
5
3m
3n
3o
3p
(E)-C6H5CH=CH
10
[a] Isolated yields. [b] Determined by chiral HPLC.
Eur. J. Org. Chem. 2008, 3817–3820
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3819

Z.-Y. Lei, G.-N. Ma, M. Shi
FULL PAPER
(R)-2Ј-[isopropyl(phenyl)phosphanyl]-1,1Ј-binaphthalen-2-ol
(L2):
Acknowledgments
White solid. Yield: 486 mg (58%). M.p. 149–150 °C. [α]2D5 = –13.0
(c = 0.58, CH Cl ). IR (CH Cl ): ν = 3055, 2944, 1714, 1621, 1595,
˜
2
2
2
1
2
We thank the Shanghai Municipal Committee of Science and Tech-
nology (04JC14083, 06XD14005) and the National Natural Science
Foundation of China for financial support (20472096, 203900502,
20672127, and 20732008).
1515, 1434, 1144, 815 cm–1. H NMR (300 MHz, CDCl3, TMS): δ
= 0.83–0.99 (m, 8 H, CH3), 1.12–1.19 (m, 4 H, CH3), 2.58–2.66
(m, 2 H, CH), 4.25 (br., 1 H, OH), 4.84 (br., 1 H, OH), 6.28 (d, J
= 9.0 Hz, 1 H, ArH), 6.72–6.78 (m, 1 H, ArH), 6.93–7.07 (m, 6 H,
ArH), 7.12–7.51 (m, 16 H, ArH), 7.78 (d, J = 8.1 Hz, 1 H, ArH),
7.88–7.96 (m, 7 H, ArH), 8.03 (t, J = 8.7 Hz, 2 H, ArH) ppm. 31P
NMR (121 MHz, CDCl3, 85% H3PO4): δ = –10.03, –12.40 ppm.
MS (EI): m/z = 420.2 (58.95) [M]+, 403.2 (100) [M – 17]+, 377.1
(80.13) [M – 43]+, 252.1 (25.60) [M – 168]+. HRMS (EI): calcd. for
C29H25OP 420.1643; found 420.1643.
[1]
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(R)-2Ј-[Butyl(phenyl)phosphanyl]-1,1Ј-binaphthalen-2-ol (L3): White
solid. Yield: 584 mg (67%). M.p. 68–70 °C. [α]2D5 = –14.3 (c = 0.83,
CH Cl ). IR (CH Cl ): ν = 3057, 2969, 1719, 1588, 1509, 1419,
˜
2
2
2
2
1
1140, 940 cm–1. H NMR (300 MHz, CDCl3, TMS): δ = 0.68 (t, J
= 6.9 Hz, 3 H, CH3), 0.84 (t, J = 6.9 Hz, 3 H, CH3), 1.11–1.15 (m,
4 H, CH2), 1.34–1.43 (m, 4 H, CH2), 1.88 (t, J = 7.5 Hz, 2 H,
CH2), 2.04 (t, J = 7.5 Hz, 2 H, CH2), 4.54 (br., 1 H, OH), 4.83 (br.,
1 H, OH), 6.54 (d, J = 8.7 Hz, 1 H, ArH), 6.87–7.06 (m, 7 H,
ArH), 7.15–7.38 (m, 14 H, ArH), 7.47–7.52 (m, 2 H, ArH), 7.60–
7.64 (m, 1 H, ArH), 7.71–7.79 (m, 2 H, ArH), 7.87–8.02 (m, 7
H, ArH) ppm. 31P NMR (121 MHz, CDCl3, 85% H3PO4): δ =
–24.68 ppm. MS (EI): m/z = 434.2 (53.07) [M]+, 417.2 (100) [M –
17]+, 377.1 (25.42) [M – 57]+, 268.1 (53.69) [M – 166]+. HRMS
(EI): calcd. for C30H27OP 434.1800; found 434.1780.
[2]
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4507–4510; b) S. Kawahara, A. Nakano, T. Esumi, Y. Iwabu-
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briglio, M. M. Vasbinder, S. J. Miller, Org. Lett. 2003, 5, 3741–
3743; d) D. Balan, H. Adolfsson, Tetrahedron Lett. 2003, 44,
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(R)-2Ј-[Cyclohexyl(phenyl)phosphanyl]-1,1Ј-binaphthalen-2-ol (L4):
White solid. Yield: 560 mg (61%). M.p. 74–76 °C. [α]2D5 = –18.8 (c
= 0.54, CH Cl ). IR (CH Cl ): ν = 3055, 2926, 1708, 1619, 1596,
˜
2
2
2
2
1
1513, 1433, 1144, 814 cm–1. H NMR (300 MHz, CDCl3, TMS): δ
= 0.92–1.43 (m, 8 H, CH2), 1.51–1.78 (m, 12 H, CH2), 2.25–2.44
(m, 2 H, CH), 4.24 (br., 1 H, OH), 4.81 (br., 1 H, OH), 6.31 (d, J
= 8.7 Hz, 1 H, ArH), 6.74–6.79 (m, 1 H, ArH), 6.95–6.97 (m, 5 H,
ArH), 7.03–7.53 (m, 17 H, ArH), 7.76–8.05 (m, 10 H, ArH) ppm.
31P NMR (121 MHz, CDCl3, 85% H3PO4):
δ = –15.35,
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–17.26 ppm. MS (EI): m/z = 460.2 (85.00) [M]+, 443.2 (100) [M –
17]+, 377.1 (89.54) [M – 83]+, 268.1 (55.87) [M – 192]+. HRMS
(EI): calcd. for C32H29OP 460.1956; found 460.1956.
Typical Reaction Procedure for L3-Catalyzed Aza-Morita–Baylis–
Hillman Reaction of N-Sulfonated Imine 1a with MVK: To a solu-
tion of imine 1a (58.6 mg, 0.2 mmol) and L3 (4.3 mg, 0.01 mmol)
in THF (1.0 mL) was added methyl vinyl ketone (34 µL, 0.4 mmol).
The reaction mixture was stirred at 25 °C. When the reaction was
completed, as monitored by TLC, the solvent was removed under
reduced pressure, and the residue was purified by a flash column
chromatography (SiO2; EtOAc/petroleum ether, 1:3) to afford 3a as
[4] a) Y.-L. Shi, M. Shi, Adv. Synth. Catal. 2007, 349, 2129–2135;
b) M.-J. Qi, T. Ai, M. Shi, G. Li, Tetrahedron 2008, 64, 1181–
1186.
[5] a) C. A. Busacca, J. C. Lorenz, N. Grinberg, N. Haddad, M.
Hrapchak, B. Latli, H. Lee, P. Sabila, A. Saha, M. Sarvestani,
S. Shen, R. Varsolona, X. Wei, C. H. Senanayake, Org. Lett.
2005, 7, 4277–4280; b) T. K. Olszewski, B. Boduszek, S. Sobek,
H. Kozłowski, Tetrahedron 2006, 62, 2183–2189; c) J. Bigeault,
L. Giordano, G. Buono, Angew. Chem. Int. Ed. Engl. 2005, 44,
4753–4757.
a white solid. Yield: 70.1 mg (96%). M.p. 91.2–91.8 °C. [α]2D5
=
+18.6 (c = 0.85, CH2Cl2) for 67%ee. 1H NMR (300 MHz, CDCl3,
TMS): δ = 2.14 (s, 3 H, Me), 2.41 (s, 3 H, Me), 5.26 (d, J = 9.0 Hz,
1 H), 6.02 (d, J = 9.0 Hz, 1 H), 6.07 (s, 1 H), 6.09 (s, 1 H), 7.04 (d,
J = 8.7 Hz, 2 H, Ar), 7.14 (d, J = 8.7 Hz, 2 H, Ar), 7.22 (d, J =
8.1 Hz, 2 H, Ar), 7.62 (d, J = 7.8 Hz, 2 H, Ar) ppm. HPLC (Chi-
ralcel AD; hexane/iPrOH, 70:30; 0.7 mL/min; 230 nm,): tmajor
13.61 min, tminor = 15.99 min.
=
[6] Y. Uozumi, A. Tanahashi, S.-Y. Lee, T. Hayashi, J. Org. Chem.
1993, 58, 1945–1948.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and spectroscopic and analytical
data for the compounds shown in Tables 1–3.
Received: March 27, 2008
Published Online: June 25, 2008
3820
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3817–3820