Amino alcohols with the bicyclo[3.3.0]octane scaffold as ligands for the catalytic enantioselective addition of diethylzinc to aldehydes

Article abstract of DOI:10.1016/S0957-4166(02)00584-0

Amino alcohols with the bicyclo[3.3.0]octane framework were synthesized, characterized and used as chiral ligands for the addition of diethylzinc to aldehydes. Quantitative yields and enantiomeric excesses of up to 92% were obtained in the ethylation reac

Full text of DOI:10.1016/S0957-4166(02)00584-0

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 2251–2255
Amino alcohols with the bicyclo[3.3.0]octane scaffold as ligands
for the catalytic enantioselective addition of diethylzinc to
aldehydes
Yu-wu Zhong, Xin-sheng Lei and Guo-qiang Lin*
Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 354 Fenglin Lu, Shanghai 200032, PR China
Received 21 August 2002; accepted 20 September 2002
Abstract—Amino alcohols with the bicyclo[3.3.0]octane framework were synthesized, characterized and used as chiral ligands for
the addition of diethylzinc to aldehydes. Quantitative yields and enantiomeric excesses of up to 92% were obtained in the
ethylation reactions. © 2002 Elsevier Science Ltd. All rights reserved.
1
. Introduction
Herein, we report studies into the synthesis of chiral
amino alcohols with a bicyclo[3.3.0]octane framework
and their use as ligands for the enantioselective addi-
tion of diethylzinc to aldehydes. To the best of our
knowledge, there is no report on the synthesis of chiral
ligands or auxiliaries with the bicyclo[3.3.0]octane
framework and their application in asymmetric
reactions.
The design and synthesis of chiral ligands for catalytic
asymmetric synthesis continues to attract great interest.
Of the chiral ligands reported, amino alcohols have
proven to be highly efficient chiral ligands for a variety
of asymmetric processes, particularly, the enantioselec-
tive addition of diethylzinc to benzaldehyde discov-
ered by Oguni and Omi.
1,2
3
A convenient method for the preparation of chiral
amino alcohols is the nucleophilic addition of N-func-
tionalized organolithium compounds to readily avail-
able chiral ketones, such as (+)-camphor, (−)-fenchone,
and (−)-menthone. Amino alcohols of this type were
found to be very effective ligands in the enantioselective
addition of diethylzinc to aryl aldehydes. Obviously,
the origin of the stereoselectivity in this reaction derives
from the rigidity of the chiral ketone framework.
2. Results and discussion
Firstly, we required a bicyclo[3.3.0]octane bearing a
ketone moiety, such as compound 3 (Scheme 1), which
could be used as a starting material in the synthesis of
the target amino alcohols. As shown in Scheme 1,
enantiomerically pure diketone 3 was prepared from
1,5-cyclooctadiene in three steps, involving palladium
4
5
chloride-mediated transannular cyclization, enzymatic
Scheme 1. Synthesis of ligands 4 and 5.
*
0
Corresponding author. Tel.: 86-21-64163300; fax: 86-21-64166128; e-mail: lingq@pub.sioc.ac.cn
957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00584-0

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Y. Zhong et al. / Tetrahedron: Asymmetry 13 (2002) 2251–2255
6
resolution and PCC oxidation according to minor
modifications of our previous report and improvement
of the e.e. of diol 2. Thus, 2-methylquinoline was first
lithiated with equimolar n-BuLi, followed by trapping
with diketone 3 to produce the bis-amino alcohol
(
(
(1S,5S,6S)-5 are different which results in the drastic
difference in enantioselectivity. Detailed studies to
address this problem will be conducted in due course.
We then turned our attention to other mono-amino
alcohols with the bicyclo[3.3.0]octane skeleton. As
shown in Scheme 3, diketone (1R,5R)-3 was first mono-
protected as mono-ketal (1R,5R)-6, then treated with
1S,2S,5S,6S)-4 and the mono-amino alcohol
1S,5S,6S)-5. Their relative configurations were con-
1
firmed by H NMR analyses and observed NOE effects
(
see Section 4).
2
,6-lutidine lithium and 2-methylquinoline lithium
respectively to provide ligand (1R,5R,6R)-7 and
Ligands (1S,2S,5S,6S)-4 and (1S,5S,6S)-5 were then
used as catalysts in the addition of diethylzinc to benz-
aldehyde under the standard conditions (0°C to rt; 10%
mol catalyst; toluene/n-hexane mixture (1:1) as the
solvent) (Scheme 2). The results are shown in Table 1.
(
1R,5R,6R)-8 which were deprotected to afford
1R,5R,6R)-9 and (1R,5R,6R)-5.
(
The effects of ligands 5, 7, 8 and 9 in the addition of
diethylzinc to aryl aldehydes were then tested under the
reaction conditions depicted in Scheme 2. The results
are summarized in Table 2.
The ketal-protected ligands (1R,5R,6R)-7 and
(
1R,5R,6R)-8 induced moderate enantioselectivities (78
and 63% e.e., entries 1 and 2) in the addition of
diethylzinc to benzaldehyde, while the amino alcohols
Scheme 2.
(
1R,5R,6R)-9 and (1R,5R,6R)-5, with an unmasked
Table 1. Enantioselective addition of diethylzinc to benz-
aldehyde catalyzed by 4 and 5
carbonyl function, proved to be more effective. The
best result was obtained when (1R,5R,6R)-9 was used
as catalyst (92% e.e., entry 3). Thus, (1R,5R,6R)-9 was
used to catalyse the addition of diethylzinc to a number
of structurally different aldehydes. As far as aromatic
aldehydes are concerned, it can be seen from entries
Yield (%)a
E.e. (%)b
Conf.c
Entry
Ligand
1
2
(1S,2S,5S,6S)-4
(1S,5S,6S)-5
74
100
7
76
R
R
5
–11 that the enantiomeric excesses remained at the
0% level irrespective of the presence of electron-donat-
a
Isolated yield.
Determined by chiral HPLC analyses.
Determined by the sign of the specific rotation.
9
b
c
ing and electron-withdrawing groups. However, with
aliphatic aldehydes, the enantioselectivities were moder-
ate (entry 12), or worse (entries 13 and 14). One point
worthy of mention is that when the catalyst was
Surprisingly, unlike many C -symmetric ligands which
proved effective in asymmetric reactions, bis-amino
2
7
(
1R,5R,6R)-5 (Table 2, entry 4) instead of its enan-
alcohol (1S,2S,5S,6S)-4 only produced low enantiose-
lectivity (7% e.e., entry 1). In contrast, mono-amino
alcohol (1S,5S,6S)-5 was more efficient to catalyze this
reaction (entry 2), providing the product with 76% e.e.
in quantitative yield. At the time of writing we are
unsure why the e.e. was so low when (1S,2S,5S,6S)-4
was used as ligand in this reaction. We speculate that
the stereostructure and conformation of the bicy-
clo[3.3.0]octane framework of (1S,2S,5S,6S)-4 and
tiomer (1S,5S,6S)-5 (Table 1, entry 2), the configura-
tion of the phenylpropan-1-ol product switched from R
to S. It is well known that obtaining chiral materials in
both enantiomeric forms from common starting materi-
als is one of the key requirements in asymmetric reac-
8
tions. Thus, it is of interest to note that with both
(1R,5R,6R)-5 and (1S,5S,6S)-5 in hand, it is possible to
obtain both enantiomers of the chiral products.
Scheme 3. Syntheses of amino alcohols 5, 7–9.

Y. Zhong et al. / Tetrahedron: Asymmetry 13 (2002) 2251–2255
2253
Table 2. Enantioselective addition of diethylzinc to aldehydes catalyzed by 5, 7–9
Entry
RCHO
Catalyst
Yield (%)a
E.e. (%)b
Conf.c
1
2
3
4
5
6
7
8
9
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
p-Tolualdehyde
p-Chlorobenzaldehyde
p-Anisaldehyde
(1R,5R,6R)-7
(1R,5R,6R)-8
(1R,5R,6R)-9
(1R,5R,6R)-5
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
(1R,5R,6R)-9
96
92
100
100
100
84
100
95
88
100
100
80
100
60
78
63
92
83
91
91
91
90
92
89
91
S
S
S
S
S
S
S
S
S
S
S
S
S
S
o-Anisaldehyde
3,4-Dimethoxybenzaldehyde
1-Naphthaldehyde
2-Naphthaldehyde
Cyclohexanecarboxaldehyde
trans-Cinnamaldehyde
Heptanal
10
11
12
13
14
d
70
51
14
e
a
b
c
Isolated yield.
Determined by chiral HPLC analysis.
Determined by the sign of the specific rotation.
Determined by comparison of the specific roation with that reported in the literature, see Ref. 9.
Determined by comparison of the specific roation with that reported in the literature, see Ref. 10.
d
e
3. Conclusion
argonatmospherewereadded2-methylquinoline(0.4mL,
mmol) and anhydrous ether (10 mL). This solution was
3
In summary, a new category of chiral amino alcohol, with
the bicyclo[3.3.0]octane framework has been synthesized,
and used as ligands for the enantioselective addition of
diethylzinc to aldehydes. The bis-amino alcohol ligand
cooled to 0°C and 1.6 M n-BuLi solution in n-hexane (1.9
mL) was added dropwise. After 15 min, the ice bath was
removed and stirring was continued for 1 h at rt. Then,
the system was cooled to 0°C again and a solution of
diketone (1S,5S)-3 (138 mg, 1 mmol) in 10 mL anhydrous
ether was added dropwise. The mixture was stirred for
(
1S,2S,5S,6S)-4 only provided the product with low e.e.
On the other hand, the mono-amino alcohol ligands 5–9
proved to be more efficient as catalysts for this reaction.
Studies to elaborate the effect of the unmasked carbonyl
function of ligands 5 and 9 will be published in due course.
Further work to extend the use of these amino alcohol
ligands to other asymmetric reactions is now in progress.
1
0 h and 10 mL saturated aqueous NH Cl solution was
4
added to quench the reaction. The ether layer was
separated, and the water layer was extracted with ethyl
acetate (3×20 mL). The combined organic layers were
washed with brine and dried over with anhydrous
Na SO . The solvent was evaporated under reduced
2
4
pressure and the crude product was purified by flash
chromatography to give (1S,2S,5S,6S)-4 (211 mg, 49.7%)
and (1S,5S,6S)-5 (52 mg, 19.6%).
4. Experimental
4.1. General
2
0
(
1S,2S,5S,6S)-4: mp 150°C; [h] =+53.1 (c 1.40 CHCl );
D
3
Melting points were uncorrected. Optical rotations were
1
H NMR (CDCl , l ppm): 1.19 (m, 2H), 1.50 (m, 2H),
1
3
measured on a Perkin–Elmer 241MC polarimeter. H and
1
.66–1.85 (m, 4H), 2.65 (m, 2H), 3.18 (s, 4H), 6.00–6.20
1
3
C NMR spectra were taken in CDCl on 300 and 75
3
(
br, 2H), 7.44 (d, J=8.55 Hz, 2H), 7.53 (t, J=7.33 Hz,
MHz FT-spectrometers, respectively, using SiMe as the
4
2
H), 7.72 (m, 2H), 7.81 (d, J=8.56 Hz, 2H), 8.12 (m, 4H).
internal reference. IR spectra were recorded on a Bio-Rad
FTS-185 IR spectrometer. Mass spectra were recorded
by the EI method, and HRMS were measured on a
Finnigan MAT-8430 mass spectrometer. Elemental
analyses were performed on Heraeus Rapid-CHNO.
Enantiomeric excess (e.e.) determination was carried out
using HPLC with Chiralcel OD, AS, AD, OJ columns.
The silica gel used for flash chromatography was 300–400
mesh. All solvents were dried by standard methods.
Unless otherwise noted, commercially available reagents
were used without further purification.
13
C NMR (CDCl , l ppm): 20.52, 43.45, 46.10, 53.24,
3
8
1
1
2
1
0.20, 123.11, 126.13, 126.77, 127.53, 128.14, 129.74,
36.59, 146.39, 160.58. FT-IR (KBr): 3403, 3062, 2967,
577, 758. EIMS (m/z, %): 424 (M , 1.42), 406 (M −H O,
+
+
2
.59), 170 (10.13), 144 (16.11), 143 (100.00), 142 (14.73),
28(9.58), 116(8.97), 115(14.46), 43(16.26). HRMScalcd
+
for C H N O (M ): 424.2157. Found: 424.2188. Anal.
28 28
2
2
calcd for C H N O : C, 79.22; H, 6.65; N, 6.60. Found:
C, 78.83; H, 6.57; N, 6.47.
28 28
2
2
2
0
(
1S,5S,6S)-5: mp 102°C; [h] =+147.6 (c 0.65 CHCl );
H NMR (CDCl , l ppm): 1.50 (m, 1H), 1.65 (m, 1H),
D
3
1
4
.2. (1S,2S,5S,6S)-2,6-Di-(quinolin-2-ylmethyl)-bicyclo-
3.3.0]octan-2,6-diol, 4 and (1S,5S,6S)-6-hydroxy-6-
quinolin-2-ylmethyl)-bicyclo[3.3.0]octan-2-one, 5
3
[
1.90 (m, 4H), 2.20 (m, 1H), 2.46 (m, 2H), 2.61 (m, 1H),
2.95 (d, J=14.97 Hz, 1H), 3.29 (d, J=14.97 Hz, 1H), 6.00
(
(
br, 1H), 7.20 (d, J=8.37 Hz, 1H), 7.45 (m, 1H), 7.60 (m,
To a 100 mL flame-dried three-necked flask under an
1H), 7.70 (d, J=8.13 Hz, 1H), 7.90 (d, J=8.42

2
254
Y. Zhong et al. / Tetrahedron: Asymmetry 13 (2002) 2251–2255
Hz, 1H), 8.10 (d, J=8.38 Hz, 1H). FT-IR (KBr): 3294,
2.40 (m, 1H), 2.50 (s, 3H), 2.90 (s, 2H), 3.90 (m, 4H),
6.95 (d, J=7.60 Hz, 1H), 7.00 (d, J=7.70 Hz, 1H), 7.50
(t, J=7.70 Hz, 1H). FT-IR (film): 3338, 3065, 2955,
+
2
942, 1733, 1139, 832, 753. EIMS (m/z, %): 281 (M ,
+
1
.80), 263 (M −H O, 1.21), 199 (8.10), 198 (6.98), 185
2
+
(
10.27), 170 (14.45), 144 (7.15), 143 (100.00), 128 (7.60),
1595, 1579, 1108, 796. EIMS (m/z, %): 272 (M −OH,
1
2
15 (10.71). HRMS m/z calcd for C H NO :
81.1416. Found: 281.1404.
15.44), 244 (10.14), 186 (8.19), 172 (11.08), 107 (56.42),
99 (9.89), 55 (6.98), 41 (7.87). HRMS m/z calcd for
C H NO : 289.1678. Found: 289.1698. Anal. calcd for
18
19
2
17
23
3
The relative configurations of 4 and 5 are shown in Fig.
C H NO : C, 70.56; H, 8.01; N, 4.84. Found: C,
70.30; H, 7.73; N, 4.83%.
17 23 3
1
and were confirmed by NOE effects. As far as 4 is
1
concerned, it should be C -symmetric according to H
2
NMR analyses. The relative configuration is that
shown in Fig. 1 confirmed by the existence of NOE
effects between the benzylic hydrogen H1 and the
4
.5. (1R,5R,6R)-6-Hydroxy-6-(quinolin-2-ylmethyl)-
bicyclo[3.3.0]octan-2-one ethylene ketal, 8
bridgehead hydrogen H . The relative configuration of
2
Prepared from (1R,5R)-6 (546 mg, 3.0 mmol), 2-
methylquinoline (0.43 mL, 4.5 mmol), and 1.6 M n-
BuLi solution in n-hexane (2.8 mL, 4.5 mmol) in a
similar way as described in Section 4.2 to give 8 as a
5
can also been confirmed by the existence of NOE
effects between the benzylic hydrogen H4 and the
bridgehead hydrogen H3.
20
pale yellow solid (796 mg, 88%); [h] =−14.6 (c 0.85
D
4
.3. (1R,5R)-Bicyclo[3.3.0]octan-2,6-dione monoethylene
1
11
CHCl3); H NMR (CDCl , l ppm): 1.38 (m, 1H),
3
ketal, 6
1
4
.40–1.85 (m, 7H), 2.38 (m, 2H), 3.05 (s, 2H), 3.80 (m,
H), 5.80 (br, 1H), 7.25 (t, J=7.60 Hz, 1H), 7.50 (t,
(
1R,5R)-Bicyclo[3.3.0]octan-2,6-dione 3 (1.9 g, 13.7
J=8.0 Hz, 1H), 7.70 (m, 1H), 7.80 (d, J=8.1 Hz, 1H),
.06 (d, J=8.4 Hz, 1H), 8.12 (d, J=8.3 Hz, 1H). FT-IR
film): 3345, 3061, 2957, 1600, 1111, 828. EIMS (m/z,
mmol), ethylene glycol (0.84 mL, 15.1 mmol) and p-
toluenesulfonic acid monohydrate (261 mg, 1.37 mmol)
were added to toluene (120 mL). The solution was
heated under reflux for 8 h while removing the water
8
(
%
+
): 325 (M , 2.05), 143 (100.00), 115 (17.65), 100
(
(
19.17), 99 (33.79), 86 (15.83), 55 (17.42), 42 (19.62), 41
azeotropically. Then, saturated aqueous NaHCO solu-
3
18.64). HRMS m/z calcd for C H NO : 325.1678.
20
23
3
tion was added to stop the reaction after the mixture
was cooled to rt. The mixture was extracted with ethyl
acetate (3×100 mL). The combined organic layers were
washed with brine and dried over with anhydrous
Na SO . The solvent was evaporated under reduced
Found: 325.1688. Anal. calcd for C H NO : C, 73.82;
20
23
3
H, 7.12; N, 4.30. Found: C, 73.77; H, 7.01; N, 4.25%.
2
4
4
.6. (1R,5R,6R)-6-Hydroxy-6-(6-methylpyridin-2-
pressure and purification by flash chromatography gave
ylmethyl)-bicyclo[3.3.0]octan-2-one, 9
2
0
(
1R,5R)-6 (1.57 g, 80.2%); [h] =−160.0 (c 1.35
D
1
CHCl ); H NMR (CDCl , l ppm): 1.75 (m, 4H), 2.00
3
3
Compound (1R,5R,6R)-7 (583 mg, 2.02 mmol), 5%
HCl (4.0 mL) and acetone (2.0 mL) were added into 25
mL THF. The solution was stirred at rt for 5 h. THF
was evaporated under reduced pressure. Then, satu-
(
m, 2H), 2.28 (m, 2H), 2.73 (m, 2H), 3.97 (m, 4H).
FT-IR (film): 2964, 1737, 1162, 1118. EIMS (m/z, %):
00 (61.82), 99 (100.00), 86 (35.37), 55 (16.04), 53
12.81), 43 (10.26), 42 (21.78), 41 (16.24).
1
(
rated aqueous NaHCO solution was added to neutral-
3
ize the mixture. The mixture was extracted with ethyl
acetate. The combined organic layer was washed with
water and brine and dried over with anhydrous
Na SO . The solvent was evaporated under reduced
4
.4. (1R,5R,6R)-6-Hydroxy-6-(6-methylpyridin-2-
ylmethyl)-bicyclo[3.3.0]octan-2-one ethylene ketal, 7
2
4
Prepared from (1R,5R)-6 (728 mg, 4.0 mmol), 2,6-
lutidine (0.7 mL, 6.0 mmol), and 1.6 M n-BuLi solution
in n-hexane (3.75 mL, 6.0 mmol) in a similar way as
described in Section 4.2 to give 7 as an oil (972 mg,
pressure and purification by flash chromatography gave
20
(
(
1R,5R,6R)-9 as a white solid; mp 69°C; [h] =−145.7
D
1
c 2.45 CHCl ); H NMR (CDCl , l ppm): 1.50 (m,
3
3
2
3
H), 1.90 (m, 4H), 2.18 (m, 1H), 2.35 (m, 2H), 2.45 (s,
H), 2.55 (m, 1H), 2.70 (d, J=14.6 Hz, 1H), 3.00 (d,
2
0
1
8
4%); [h] =−26.2 (c 1.10 CHCl ); H NMR (CDCl , l
D 3 3
ppm): 1.45 (m, 2H), 1.50–1.90 (m, 6H), 2.30 (m, 1H),
J=14.5 Hz, 1H), 6.80 (d, J=7.6 Hz, 1H), 6.90 (d,
J=7.7 Hz, 1H), 7.45 (t, J=7.7 Hz, 1H). FT-IR (KBr):
3
268, 2946, 1732, 1594, 1154, 749. EIMS (m/z, %): 228
+
(M −OH, 3.53), 163 (9.47), 149 (8.82), 134 (11.76), 107
(100.00), 79 (8.40), 77 (8.74), 65 (9.24), 41 (12.06).
HRMS m/z calcd for C H NO : 245.1416. Found:
15
19
2
2
45.1413.
4
.7. (1R,5R,6R)-6-Hydroxy-6-(quinolin-2-ylmethyl)-
bicyclo[3.3.0]octan-2-one, 5
Prepared from (1R,5R,6R)-8 in a similar manner to
2
0
that described in Section 4.6; [h] =−145.6 (c 2.45
CHCl3).
D
Figure 1.

Y. Zhong et al. / Tetrahedron: Asymmetry 13 (2002) 2251–2255
2255
4
.8. General procedure for the asymmetric addition of
C.-S.; Xin, Z.-Q.; Wang, R.; Choi, M. C. K.; Chan, A. S.
C. Org. Lett. 2001, 3, 2733–2735; (c) Xu, Q.-Y.; Wang,
H.; Pan, X.-F.; Chan, A. S. C.; Yang, T. K. Tetrahedron
Lett. 2001, 42, 6171–6173; (d) Dahmen, S.; Brase, S.
Chem. Commun. 2002, 26–27; (e) Vilaplana, M. J.;
Molina, P.; Arques, A.; Andres, C.; Pedrosa, R. Tetra-
hedron: Asymmetry 2002, 13, 5–8; (f) Goanvic, D. L.;
Holler, M.; Pale, P. Tetrahedron: Asymmetry 2002, 13,
119–121.
diethylzinc to arylaldehydes
To a solution of ligand (1R,5R,6R)-9 (0.10 mmol) in
toluene (2 mL) and hexane (2 mL) at 0°C was added
15% w/w solution of diethylzinc in hexane (2.3 mL, 2.0
mmol). After stirring for 30 min at 0°C, freshly distilled
benzaldehyde (1.0 mmol) was added. The reaction mix-
ture was stirred for 4 h at 0°C, then allowed to warm to
room temperature gradually with stirring for 24 h.
After the addition of 1N HCl (10 mL), the phases were
separated. The aqueous layer was extracted with ethyl
acetate (3×5 mL). The combined organic layer was
washed with brine and dried over with anhydrous
Na SO . After purification by flash chromatography,
3. Oguni, N.; Omi, T. Tetrahedron Lett. 1984, 25, 2823–
2824.
4. (a) Zhou, Y.-G.; Dai, L.-X.; Hou, X.-L. Chin. J. Chem.
2000, 18, 121–123; (b) Xu, Q.-Y.; Wang, G.-X.; Pan,
X.-F.; Chan, A. S. C. Tetrahedron: Asymmetry 2001, 12,
381–385; (c) Genov, M.; Kostova, K.; Dimitrov, V. Tet-
rahedron: Asymmetry 1997, 8, 1869–1876; (d) Genov, M.;
Dimitrov, V; Ivanova, V. Tetrahedron: Asymmetry 1997,
2
4
the enantiomeric excess of the product was determined
by chiral HPLC analysis.
8
, 3703–3706.
5
6
. Henry, P. M.; Davies, M.; Ferguson, G.; Phillips, S.;
Restivo, R. Chem. Commun. 1974, 112–113.
. Lemke, K.; Ballschuh, S.; Kunath, A.; Theil, F. Tetra-
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Acknowledgements
We thank for the financial support by the NSFC
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8. Kim, Y. H. Acc. Chem. Res. 2001, 34, 955–962.
(
20172063), the Major State Basic Research Develop-
ment Program 973 (6200077506) and Chinese Academy
of Sciences.
20
20
9. [h]
1.09, CHCl
Paquette, L. A.; Zhou, R. J. Org. Chem. 1999, 64,
=−4.0 (c 2.50, CHCl
). Literature: [h] =+5.6 (c
3 D
D
) for R enantiomer with 96% e.e. See:
3
7
929–7934.
2
0
20
D 3 D
References
10. [h] =+1.4 (c 2.00, CHCl ). Literature: [h] =+9.1 (c 7.2,
CHCl ) for the S enantiomer with 88% e.e. See: Soai, K.;
3
1
2
. For reviews, see: (a) Pu, L.; Yu, H.-B. Chem. Rev. 2001,
Yokoyama, S.; Hayasaka, T. J. Org. Chem. 1991, 56,
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11. (a) Shigeyashu, K.; Syuzi, H.; Hiroyuki, I.; Mariko, I.;
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. For some recent references, see: (a) Reddy, K. S.; Sola,
L.; Moyano, A.; Pericas, M. A.; Riera, A. Synthesis 2000,
1
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