Facile synthesis of chiral isopropyl carbinols with high enantiomeric excess via catalytic enantioselective addition of diisopropylzinc to aldehydes

Article abstract of DOI:10.1016/S0957-4166(00)00253-6

Highly effective syntheses of chiral alkyl and aryl isopropyl carbinols with high enantiomeric excess (94-98% ee) via catalytic enantioselective addition of diisopropylzinc to aldehydes have been developed.

Full text of DOI:10.1016/S0957-4166(00)00253-6

Tetrahedron: Asymmetry 11 (2000) 2947±2953
Facile synthesis of chiral isopropyl carbinols with high
enantiomeric excess via catalytic enantioselective addition of
diisopropylzinc to aldehydesy
Weon Ki Yang and Byung Tae Cho*
Department of Chemistry, Hallym University, Chunchon, Kangwondo 200-702, South Korea
Received 18 May 2000; accepted 27 June 2000
Abstract
Highly e€ective syntheses of chiral alkyl and aryl isopropyl carbinols with high enantiomeric excess
(94±98% ee) via catalytic enantioselective addition of diisopropylzinc to aldehydes have been developed.
# 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The introduction of stereogenic isopropyl carbinol groups is a key step in preparing biologically
active substances such as cerebresterol,1 1a,24(R)-dihydroxy-vitamin D32 and squalamine.3 Also,
the stereogenic isopropyl carbinol groups play important roles in asymmetric autocatalytic
reactions.4 One of the most convenient methods for introducing such groups is probably the
enantioselective isopropylation to aldehydes.5 Although a number of the catalytic enantio-
selective additions of dialkylzincs to aldehydes to give secondary alcohols with high enantiomeric
excess have been extensively studied,7 among dialkylzinc reagents employed, diethylzinc has been
the most often utilized in the reaction. Only a few examples have been reported on the enantio-
selective addition of diisopropylzinc.4,8 Recently, we reported the enantioselective addition of
diethylzinc to aldehydes catalyzed by a g-dialkylamino alcohol, 1,2-O-isopropylidene-5-deoxy-5-
morpholino-a-d-xylofuranose 1 derived from a-d-xylose.9 In the course of developing the syn-
thetic usefulness of this catalyst for such reactions, we found that the catalyst is highly e€ective
for the enantioselective isopropylation to both aliphatic and aromatic aldehydes: we describe here
these results.
* Corresponding author. E-mail: btcho@sun.hallym.ac.kr
y
Catalytic enantioselective reactions. Part 19.
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00253-6

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W. K. Yang, B. T. Cho / Tetrahedron: Asymmetry 11 (2000) 2947±2953
2. Results and discussion
To ®nd the optimum conditions for the catalytic isopropylation to aldehydes, we initially
investigated the catalytic, temperature and solvent e€ects on reactivity and enantioselectivity for
the reaction to benzaldehyde. First, we examined the e€ect of the amount of catalyst. Thus, the
reaction was carried out by the addition of 2 equiv. of diisopropylzinc to benzaldehyde in the
presence of varying amounts of 1 in toluene at 0ꢀC. As shown in Table 1, the use of 10 mol% of 1
provided the best results, giving 2-methyl-1-phenylpropanol in 96% ee. Increasing the amount of
1 from 2 to 25 mol% did not signi®cantly a€ect the enantioselectivity, which was 90% ee with 2
mol%, 92% ee with 5 mol%, 96% ee with 10 mol% and 25 mol% of 1, although the reaction
using both 2 and 5 mol% of 1 proceeded more slowly (entries 1±4). Next, we examined
temperature e€ects by carrying out the same reaction with 10 mol% of 1 at various temperatures.
When the reaction was performed at 0ꢀC or ^25ꢀC, the best enantioselectivity (96% ee) was
observed. However, the reaction at ^25ꢀC was very slow to give the product alcohol in 64% yield
after 35 h (entry 6). Among the solvents examined, the reaction in toluene provided the best
result. The same reaction in dichloromethane a€orded lower enantioselectivity (entry 7). The
catalytic isopropylation of the aldehyde in THF did not occur. This may be attributable to
instability of diisopropylzinc reagent in THF. Based on the optimum conditions obtained from
Table 1, we carried out the enantioselective addition of diisopropylzinc to other aldehydes 2 in
the presence of 10 mol% of 1 in toluene at 0ꢀC (Scheme 1). As shown in Table 2, all the reactions
proceeded smoothly to give the corresponding isopropyl carbinols 3 in good yields. The catalyst
is found to be highly e€ective for the isopropylation to both aliphatic and aromatic aldehydes
examined with one exception, leading to the corresponding alcohols with very high enantiomeric
excesses in the range of 94±98% ee. In the case of an a,b-unsaturated aldehyde, trans-cinnamaldehyde,
the reaction provided low enantioselectivity (entry 6, Table 2). The steric e€ect of the ortho-
substituent of the aromatic ring on asymmetric induction is not signi®cant (entries 8 versus 9). In
particular, it is noteworthy that optically active (1-hydroxy-2-methylpropyl)ferrocene 3l
approaching almost 100% ee has been achieved in the reaction of ferrocenecarboxaldehyde
Table 1
Catalytic, temperature and solvent e€ects on asymmetric induction for enantioselective addition of
diisopropylzinc to benzaldehyde using 1 as the catalysta

W. K. Yang, B. T. Cho / Tetrahedron: Asymmetry 11 (2000) 2947±2953
2949
(entry 12), since this chiral alcohol may be used as a synthetic intermediate for the preparation of
chiral ligands bearing a ferrocenyl ring.10 Moreover, the absolute con®gurations of the product
alcohols 3 obtained showed that aldehydes 2 were consistently attacked by diisopropylzinc on
their re sides in their transition states.9
Scheme 1.
Table 2
Enantioselective addition of diisopropylzinc to aldehydes catalyzed by 10 mol% of 1 in toluene at 0ꢀCa

2950
W. K. Yang, B. T. Cho / Tetrahedron: Asymmetry 11 (2000) 2947±2953
3. Conclusion
We have established a highly e€ective synthesis of optically active alkyl and aryl isopropyl
carbinols with 94±98% ee via enantioselective addition of diisopropylzinc to aldehydes using a
g-amino alcohol 1 derived from a-d-xylose as the catalyst. The excellent enantiomeric excess for
secondary alcohols bearing similar steric bulk of two groups adjacent to the carbinol group is
especially noteworthy, because it is well known that asymmetric reductions of simple ketones
with small di€erence of steric size between two groups attached to the carbonyl using biological
or chemical manners generally provide low enantioselectivity.5,12
4. Experimental
4.1. General
All operations with air-sensitive materials were carried out under a nitrogen atmosphere with
oven-dried glassware. Liquid materials were transferred with a double-ended needle. The reactions
were monitored by TLC using silica gel plates and the products were puri®ed by ¯ash column
chromatography on silica gel (Merck; 230±400 mesh). NMR spectra were recorded at 400 MHz
1
for H and 100 MHz for 13C using Me4Si as the internal standard in CDCl3, and J values are
given in hertz. Optical rotations were measured with a high resolution digital polarimeter. Melting
points were uncorrected. Enantiomeric excesses (% ees) of the product alcohols were determined
by capillary GC analyses using a 20 m Chiraldex G-TA, B-PH and b-Dex 120 chiral column or by
an HPLC analysis using a 25 cm Chiralcel OD.
4.2. Materials
Most of the organic compounds utilized in this study were commercial products of the highest
purity. They were further puri®ed by distillation when necessary. Toluene was distilled over
sodium and stored in an ampoule under nitrogen atmosphere. The chiral ligand 1 was synthesized
from a-d-xylose by our previous procedure.9 Diisopropylzinc was prepared according to the
literature procedure.13
4.3. Catalytic enantioselective addition of diisopropylzinc to aldehydes. General procedure
Under a nitrogen atmosphere, a toluene solution (2 ml) of diisopropylzinc (2 mmol) was added
to 1 (0.1 mmol) in toluene (1 ml) and stirred at 0ꢀC for 30 min. After aldehyde 2 (1 mmol) was
added to this, the mixture was stirred at the same temperature for the appropriate time and then
diluted with ether (10 ml). The excess diisopropylzinc was destroyed by addition of 1N HCl and
extracted with ether (3Â10 ml). The ether extract was dried over anhydrous magnesium sulfate and
concentrated in vacuo. The product alcohol 3 was further puri®ed by ¯ash column chromatography
on silica gel using an appropriate solvent as eluent.

W. K. Yang, B. T. Cho / Tetrahedron: Asymmetry 11 (2000) 2947±2953
2951
4.3.1. (S)-2-Methyl-3-nonanol 3a
Yield, 88%; Rf 0.59 (EtOAc:hexane=1:4); oil; IR (neat, cm^1) 3399, 2930, 1467, 1382, 1030; 1H
NMR ꢀ 0.87±0.93 (m, 9H), 1.26±1.36 (m, 9H), 1.45±1.47 (m, 2H), 1.65 (m, 1H), 3.36 (m, 1H); 13C
21
D
NMR ꢀ 14.11, 17.06, 18.89, 22.66, 26.03, 29.45, 31.90, 33.47, 34.19, 76.73; ‰ꢁŠ ^14.1 (0.7,
CHCl3); GC analysis using a 30 m b-Dex 120 chiral column showed it to be 96% ee [95ꢀC
(isothermal) tR(S) 50.66 min and tR(R) 53.14 min].
4.3.2. (S)-2-Methyl-3-tridecanol 3b
1
Yield, 84%; Rf 0.72 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3365, 2957, 1460; H NMR ꢀ
0.86±0.93 (m, 9H), 1.26±1.35 (m, 17H), 1.44±1.47 (m, 2H), 1.63 (m, 1H), 3.36 (m, 1H); 13C NMR
ꢀ 14.13, 17.07, 18.89, 22.70, 26.06, 29.35, 29.64, 29.65, 29.67, 29.78, 31.92, 33.45, 34.19, 76.70;
‰ꢁŠ21 ^15.1 (1.07, CHCl3); GC analysis using a 30 m b-Dex 120 chiral column showed it to be 96%
D
ee [130ꢀC (isothermal) tR(S) 80.71 min and tR(R) 83.09 min].
4.3.3. (S)-2,5-Dimethyl-3-hexanol 3c
1
Yield, 62%; Rf 0.67 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3365, 2959, 1468, 1368; H
NMR ꢀ 0.89±0.94 (m, 13H), 1.18±1.24 (m, 1H), 1.31±1.37 (m, 2H), 1.63 (m, 1H), 3.45 (m, 1H);
21
D
13C NMR ꢀ 17.38, 19.14, 22.15, 24.19, 25.07, 34.28, 43.71, 74.87; ‰ꢁŠ ^18.8 (0.84, CHCl3); GC
analysis using a 30 m b-Dex 120 chiral column showed it to be 96% ee [80ꢀC (isothermal) tR(S)
23.04 min and tR(R) 25.19 min].
4.3.4. (S)-2-Methyl-5-phenyl-3-pentanol 3d
Yield, 90%; Rf 0.62 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3390, 3062, 2872, 1485, 1467,
1
1038, 1011, 745, 697; H NMR ꢀ 0.92 (d, 6H, J=6.93), 1.40 (br s, 1H), 1.65±1.72 (m, 2H), 1.78±
1.80 (m, 1H), 2.65 (m, 1H), 2.84 (m, 1H), 3.40 (m, 1H), 7.16±7.22 (m, 3H), 7.27±7.30 (m, 2H); 13C
21
D
NMR ꢀ 17.16, 18.77, 32.48, 33.69, 35.96, 76.14, 125.78, 128.39, 128.44, 142.36; ‰ꢁŠ ^38.2 (3.02,
26
EtOH) {lit.8 ‰ꢁŠ +39.0 (c 3.08, EtOH), 96.1% ee R}; HPLC analysis using a 25 cm Chiralcel OD
D
column showed it to be 96% ee [i-PrOH:hexane=1:9, ¯ow rate 0.7 mL/min, tR(S) 9.42 min and
tR(R) 13.98 min].
4.3.5. (R)-1-Cyclohexyl-2-methylpropanol 3e
1
Yield, 82%; Rf 0.68 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3411, 2829, 1467; H NMR ꢀ
0.90 (d, 3H, J=6.70), 0.92 (d, 3H, J=6.80), 0.99±1.28 (m, 6H), 1.40 (m, 1H), 1.57±1.60 (m, 1H),
1.65±1.67 (m, 1H), 1.72±1.87 (m, 3H), 1.85±1.88 (m, 1H), 3.04 (m, 1H); 13C NMR ꢀ 16.53, 19.89,
21
D
25
D
26.16, 16.44, 26.55, 27.66, 29.76, 29.91, 40.61, 81.06; ‰ꢁŠ +4.24 (0.38, CHCl3) {lit.14 ‰ꢁŠ ^3.4
(neat), S}; GC analysis of its tri¯uoroacetate using a 20 m Chiraldex G-TA chiral column showed
it to be 98% ee [65ꢀC (isothermal) tR(S) 20.86 min and tR(R) 22.19 min].
4.3.6. trans-(R)-4-Methyl-1-phenyl-1-penten-3-ol 3f
Yield, 80%; Rf 0.58 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3403, 3077, 2962, 1493, 1466,
1
1023, 988, 746, 691; H NMR ꢀ 0.95 (d, 3H, J=6.77), 0.99 (d, 3H, J=6.79), 1.83 (m, 1H), 4.03
(m, 1H), 6.23 (dd, 1H, J=6.96, 15.92), 6.57 (d, 1H, J=15.91), 7.24±7.40 (m, 5H); 13C NMR ꢀ
21
D
18.06, 18.31, 34.11, 78.16, 126.45, 127.60, 128.57, 130.87, 131.20, 136.82; ‰ꢁŠ ^8.0 (1.02, EtOH)
{lit.8 ‰ꢁŠ25 +18.1 (c 1.07, EtOH), 93.4% ee, S}; HPLC analysis using a 25 cm Chiralcel OD column
D
showed it to be 69% ee [i-PrOH:hexane=1:9, ¯ow rate 0.9 mL/min, tR(R) 8.83 min and tR(S)
12.83 min].

2952
W. K. Yang, B. T. Cho / Tetrahedron: Asymmetry 11 (2000) 2947±2953
4.3.7. (R)-2-Methyl-1±phenylpropanol 3g
Yield, 95%; Rf 0.60 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3424, 3063, 2934, 1467, 1366,
1021, 757, 699; 1H NMR ꢀ 0.80 (d, 3H, J=6.90), 1.00 (d, 3H, J=6.66), 1.85 (d, 1H, J=2.35), 1.96
(m, 1H), 4.36 (dd, 1H, J=2.09, 6.83), 7.25±7.36 (m, 5H); 13C NMR ꢀ 18.23, 19.01, 35.27, 80.05,
21
D
20
D
126.56, 127.42, 128.19, 143.65; ‰ꢁŠ +52.1 (1.06, Et2O) {lit.11 ‰ꢁŠ +47.7 (c 6.8, Et2O), S}; GC
analysis using a 30 m b-Dex 120 chiral column showed it to be 96% ee [115ꢀC (isothermal) tR(R)
56.79 min and tR(S) 59.07 min].
4.3.8. (R)-2-Methyl-1-(o-tolyl)propanol 3h
Yield, 67%; Rf 0.63 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3414, 2932, 1486, 1381, 1031,
1
726; H NMR ꢀ 0.84 (d, 3H, J=6.88), 1.03 (d, 3H, J=6.61), 1.72 (br s, 1H), 1.97 (m, 1H), 2.33
(s, 3H), 4.63 (d, 1H, J=6.89), 7.12±7.85 (m, 3H), 7.41 (d, 1H, J=7.48); 13C NMR ꢀ 17.87, 19.40,
21
D
19.44, 34.57, 75.77, 126.06, 127.04, 130.31, 134.96, 142.17; ‰ꢁŠ +37.9 (1.12, CHCl3); GC analysis
using a 30 m b-Dex 120 chiral column showed it to be 94% ee [130ꢀC (isothermal) tR(R) 42.33
min and tR(S) 44.14 min].
4.3.9. (R)-2-Methyl-1-(p-tolyl)propanol 3i
Yield, 92%; Rf 0.63 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3392, 2960, 1514, 1467, 1397,
1030, 826, 762; 1H NMR ꢀ 0.78 (d, 3H, J=6.83), 1.00 (d, 3H, J=6.64), 1.80 (d, 1H, J=3.23), 1.94
(m, 1H), 2.34 (s, 3H), 4.31 (dd, 1H, J=3.05, 6.97), 7.14 (d, 2H, J=8.02), 7.20 (d, 2H, J=8.02);
21
13C NMR ꢀ 18.36, 19.01, 21.11, 35.21, 79.96, 126.50, 128.88, 137.04, 140.70; ‰ꢁŠ +44.8 (1.08,
D
CHCl3); GC analysis using a 20 m Chiraldex B-PH chiral column showed it to be 94% ee [110ꢀC
(isothermal) tR(R) 36.04 min and tR(S) 38.76 min].
4.3.10. (R)-1-(p-Chlorophenyl)-2-methylpropanol 3j
Yield, 92%; Rf 0.67 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3405, 2985, 1491, 1460, 1083,
1032, 1011, 829, 783; 1H NMR ꢀ 0.79 (d, 3H, J=6.81), 0.97 (d, 3H, J=6.74), 1.87±1.94 (m, 2H),
2.34 (s, 3H), 4.35 (dd, 1H, J=2.47, 6.56), 7.24 (d, 2H, J=8.44), 7.30 (d, 2H, J=8.47); 13C NMR ꢀ
21
D
18.01, 18.84, 35.28, 79.23, 127.91, 128.30, 133.01, 142.03; ‰ꢁŠ +38.9 (1.15, CHCl3); HPLC
analysis using a 25 cm Chiralcel OD column showed it to be 95% ee [i-PrOH:hexane=1:9, ¯ow
rate 0.7 mL/min, tR(S) 28.23 min and tR(R) 30.70 min].
4.3.11. (R)-2-Methyl-1-(10-naphthyl)propanol 3k
Yield, 81%; Rf 0.65 (EtOAc:hexane=1:2); oil; IR (neat, cm^1) 3437, 3055, 2932, 1566, 1511,
1
1382, 1169, 1035, 995, 793; H NMR ꢀ 0.92 (d, 3H, J=6.83), 1.02 (d, 3H, J=6.66), 1.96 (br s,
1H), 2.25 (m, 1H), 5.18 (d, 1H, J=6.16), 7.44±7.50 (m, 3H), 7.59 (d, 1H, J=7.01), 7.77 (d, 1H,
J=8.17), 7.86 (m, 1H), 7.99 (m, 1H); 13C NMR ꢀ 17.50, 20.09, 34.46, 76.51, 123.51, 123.92,
21
D
125.25, 125.40, 125.78, 127.83, 128.88, 130.75, 133.83, 139.69; ‰ꢁŠ +23.1 (1.00, CHCl3); GC
analysis using a 30 m b-Dex 120 chiral column showed it to be 95% ee [175ꢀC (isothermal) tR(R)
55.58 min and tR(S) 57.06 min].
4.3.12. (R)-(1-Hydroxy-2-methylpropyl)ferrocene 3l
Yield, 93%; Rf 0.84 (Et2O:CHCl3=1:2); mp 32±33ꢀC (lit.8 mp 33.3±33.5ꢀC); IR (KBr, cm^1)
1
3567, 2963, 1468, 1385, 1042, 1016, 815; H NMR (400 MHz, CDCl3) ꢀ 0.77 (d, 3H, J=6.86),
0.93 (d, 3H, J=6.67), 1.73 (m, 1H), 2.12 (d, 1H, J=1.68), 4.05 (m, 1H), 4.24±4.25 (m, 9H); 13C
21
D
NMR (100 MHz, CDCl3), ꢀ 18.47, 18.69, 34.77, 64.74, 67.50, 67.76, 68.20, 75.11, 93.18; ‰ꢁŠ

W. K. Yang, B. T. Cho / Tetrahedron: Asymmetry 11 (2000) 2947±2953
2953
^91.7 (1.01, PhH) {lit.8 [ꢁ]D24.5 +86.1 (c 0.99, PhH), 97.7% ee, S}; HPLC analysis using a 25 cm
Chiralcel OD column showed it to be 98% ee [i-PrOH:hexane=3:97, ¯ow rate 0.35 mL/min,
tR(R) 22.61 min and tR(S) 24.63 min].
Acknowledgements
We are grateful to the Hallym Academy of Sciences, Hallym University (2000) for ®nancial
support.
References
1. Koch, P.; Nakatani, Y.; Luu, B.; Ourisson, G. Bull. Soc. Chim. Fr. 1983, 7/8 (Part II), 189±194.
2. Okamoto, M.; Tabe, M.; Fujii, T.; Tanaka, T. Tetrahedron: Asymmetry 1995, 6, 767±778.
3. (a) Moore, K. S.; Wehrli, S.; Roder, H.; Rogers, M.; Forrest Jr., J. N.; McCrimmon, D.; Zaslo€, M. Proc. Natl.
Acad. Sci. USA 1993, 90, 1354±1358; (b) Moriarty, R. M.; Enache, L. A.; Kinney, W. A.; Allen, C. S.; Canary,
J. W.; Tuladhar, S. M.; Guo, L. Tetrahedron Lett. 1995, 36, 5139±5142; (c) Rao, M. N.; McGuigan, M. A.; Zhang,
X.; Shaked, Z.; Kinney, W. A.; Bulliard, M.; Laboue, B.; Lee, N. J. Org. Chem. 1997, 62, 4541±4545.
4. (a) Soai, K.; Shibata, T. J. Synth. Org. Chem. Jpn. 1997, 55, 994±1005, and references cited therein; (b) Shibata, T.;
Yonekudo, S.; Soai, K. Angew. Chem., Int. Ed. 1999, 38, 659±661.
5. In a separate experiment, we carried out the CBS oxazaborolidine-catalyzed borane reduction6 of n-hexyl
isopropyl ketone and isobutyrophenone. From the reaction, we obtained the corresponding isopropyl carbinols
having 6% ee and 21% ee, respectively.
6. For a review, see: (a) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986±2012; (b) Itsuno, S. In
Comprehensive Asymmetric Catalysis; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer: Berlin, 1999;
Vol. 1, Chapter 6.4, pp. 289±315.
7. For a recent review, see: Soai, K. Shibata, T. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N.; Pfaltz, A.;
Yamamoto, H., Eds.; Springer: Berlin, 1999; Vol. 2, Chapter 26.1, pp. 911±922; For earlier reviews, see: (a) Soai, K.;
Niwa, S. Chem. Rev. 1992, 92, 833±856, and references cited therein; (b) Noyori, R.; Kitamura, M. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 49±69; (c) Noyori, R.; Kitamura, M. In Modern Synthetic Methods; Sche€old, R., Ed.;
Springer-Verlag, 1989; pp. 115±198; (d) Knochel, P. In Comprehensive Organic Synthesis; Trost, B. M., Ed.;
Pergamon Press, 1991; pp. 211±229; For recent works, see: (e) Reddy, K. S.; Sola, L.; Moyano, A.; Pericas, M. A.;
Riera, A. J. Org. Chem. 1999, 64, 3969±3974; (f) Ding, K.; Ishii, A.; Mikami, K. Angew. Chem., Int. Ed. 1999, 38,
497±501; (g) Lawrence, C. F.; Nayak, S. K.; Thijs, L.; Zwanenburg, B. Synlett 1999, 1571±1572; (h) Bolm, C.;
Muniz-Fernandez, K.; Seger, A.; Raabe, G.; Gunther, K. J. Org. Chem. 1998, 63, 7860±7867; (i) Cho, B. T.; Chun,
Y. S. Tetrahedron: Asymmetry 1998, 9, 1489±1492; (j) Huang, W.-S.; Hu, Q.-S.; Pu, L. J. Org. Chem. 1998, 63,
1364±1365.
8. Soai, K.; Hayase, T.; Takai, K.; Sugiyama, T. J. Org. Chem. 1994, 59, 7908±7909.
9. Cho, B. T.; Kim, N. J. Chem. Soc., Perkin Trans 1. 1996, 2901±2907.
10. (a) Matsumoto, Y.; Ohno, A.; Lu, S.; Hayashi, T.; Oguni, N.; Hayashi, M. Tetrahedron: Asymmetry 1993, 4,
1763±1766. (b) Sawamura, M.; Ito, Y. Chem. Rev. 1992, 92, 857±871.
11. MacLeod, R.; Welch, F. J.; Mosher, M. S. J. Am. Chem. Soc. 1960, 82, 876±880.
12. (a) Sih, C. J.; Chen, C.-S. Angew. Chem., Int. Ed. Engl. 1984, 23, 570±578. (b) Brown, H. C.; Park, W. S.; Cho,
B. T.; Ramachandran, P. V. J. Org. Chem. 1987, 52, 5406±5412; Erratum: J. Org. Chem. 1988, 53, 3396, and
references cited therein.
13. Rathke, M. W.; Yu, H. J. Org. Chem. 1972, 37, 1732.
14. Beilsteins Handbuch Organischen Chemie 6, EIII, 125.