Catalytic enantioselective reactions. Part 16. Oxazaborolidine-catalyzed asymmetric borane reduction of α-keto acetals

Article abstract of DOI:10.1039/a903335a

Asymmetric reductions of α-keto acetals using various oxazaborolidines and borane reagents as catalyst and the hydride source, respectively, were compared. The reduction catalyzed by Corey's CBS reagents with N-phenylamine-borane reagents provided α-hydro

Full text of DOI:10.1039/a903335a

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Catalytic enantioselective reactions. Part 16.1 Oxazaborolidine-
catalyzed asymmetric borane reduction of ꢀ-keto acetals
Byung Tae Cho* and Yu Sung Chun
Department of Chemistry, Hallym University, Chunchon, Kangwon Do 200-702,
Republic of Korea
Received (in Cambridge) 27th April 1999, Accepted 7th June 1999
Asymmetric reductions of α-keto acetals using various oxazaborolidines and borane reagents as catalyst and the
hydride source, respectively, were compared. The reduction catalyzed by Corey’s CBS reagents with N-phenylamine–
borane reagents provided α-hydroxy acetals with very high enantioselectivities for most aromatic analogues.
Introduction
Optically active α-hydroxy aldehydes are not only useful chiral
building blocks for synthesis of natural products, such as
rhodianose,2 rocellaric acid,3 lipoxine A,4 endo-brevicomine,5
grayanotoxins6 and amino sugars,7 but are also important
substrates for diastereofacial selective reactions of the carbonyl
groups, e.g. nucleophilic 1,2-addition or aldol reactions, and
cycloadditions.8 Accordingly, many synthetic methods,
including transformation of chiral precursors such as α-
hydroxy acids9 and α-amino acids,10 biocatalytic reduction11 or
catalytic asymmetric hydrogenation of α-keto (thio)acetals,12
and asymmetric synthesis from achiral aldehydes5b,13 have been
developed. Asymmetric reduction of α-keto acetals offers a
promising route to chiral α-hydroxy acetals. Recently we
reported asymmetric reduction of α-keto acetals with a chiral
borohydride, K xylide (potassium 9-O-(1,2-isopropylidene-5-
deoxy-α--xylofuranosyl)-9-boratabicyclo[3.3.1]nonane), in a
stoichiometric manner to afford α-hydroxy acetals with high
enantioselectivities.14 However, limitations to the use of this
stoichiometric reagent include its availability, cost, ease of
product purification and chiral auxiliary recovery on a large
scale. On the other hand, the pioneering works of Itsuno15 and
Corey16 have resulted in oxazaborolidines which catalytically
provide alcohols of predictable absolute stereochemistry and
high enantiomeric excess (ee). Accordingly, a number of
oxazaborolidine-catalyzed asymmetric reductions of prochiral
ketones have been reported.17 In connection with our continu-
ing efforts toward asymmetric reduction of functionalized
ketones,14,18 we undertook the study of oxazaborolidine-
catalyzed asymmetric reduction of α-keto acetals using various
borane reagents. During this study, we found a practically
useful method for such a reduction using N,N-diethylaniline–
borane complex (DEANB) as a borane source.1 In this paper,
we describe details of such an oxazaborolidine-catalyzed asym-
metric reduction and the scope and limits of these reactions.
Scheme 1
lidine 622 using borane–tetrahydrofuran (BH3ؒTHF, 9) (Scheme
1). Thus, slow addition of 7b over a period of 1 h to a solution
of 0.6 mol equiv. of borane–THF in the presence of 10 mol%
Results and discussion
Effect of oxazaborolidines and boranes on asymmetric induction
Our first effort was to compare the asymmetric reduction of
2,2-diethoxy-1-phenylethanone 7b catalyzed by structurally
diverse oxazaborolidines such as proline-based oxazaboro-
lidines (Corey’s CBS reagents, 116), valine-based oxazaboro-
lidine (Itsuno’s reagent, 215), aminodiphenylethanol-based
oxazaborolidine (Pfizer’s reagent, 319), phenylglycine-based
oxazaborolidine (420), aminoindanol-based oxazaborolidine
(Sepracor’s reagent, 521) and aminoaltritol-based oxazaboro-
of one of the oxazaborolidines in THF at 25 ЊC afforded 2,2-
diethoxy-1-phenylethanol 8b within 10 min in 92–97% yield.
The enantiomeric excess (ee) of the α-hydroxy acetal 8b product
was determined by HPLC analysis using a Chiralcel OD col-
umn (eluent: hexane–PriOH = 40:1). As shown in Table 1, of
the oxazaborolidines examined, Corey’s CBS reagents 1a and
1b provided the best enantioselectivities such as 92% ee with 1a,
91% ee with 1b, 65% ee with 2, 71% ee with 3, 59% ee with 4,
J. Chem. Soc., Perkin Trans. 1, 1999, 2095–2100
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Table 1 Asymmetric borane reduction of 2,2-diethoxy-1-phenyl-
ethanone 7b in the presence of 10 mol% of each of various oxazaborol-
idines in THF at 25 ЊCa
Table 2 Solvent and the reaction temperature effect on enantio-
selectivity for oxazaborolidine-catalyzed asymmetric reduction of 7b
using various borane reagentsa
8b
8b
Borane Temp
Borane reagent
(equiv.)
Entry Cat. Solvent
reagent (T/Њ) Yieldb % eec Config. d
Entry
Cat.
Yieldb
% eec
Config.d
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
THF
THF
THF
THF
toluene
CH2Cl2
toluene
CH2Cl2
toluene
CH2Cl2
CH2Cl2
11
11
12
12
10
10
11
11
12
10
12
0
Ϫ25
0
Ϫ25
25
25
25
25
25
25
25
93
86e
85
65e
96
94
91
89
90
94
90
90
79
62
8
90
81
51
24
77
81
58
S
S
S
S
S
S
S
S
S
S
S
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
9 (0.6)
9 (0.6)
1a
1b
1c
1d
2
96
97
95
92
93
95
93
95
96
94
97
99
96
97
96
94
48e
47e
92
91
83
84
65
71
59
71
57
91
95
96
96
94
95
2
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
9 (0.6)
9 (0.6)
9 (0.6)
9 (0.6)
3
9 (0.6)
4
9 (0.6)
5
9 (0.6)
6
10
11
10 (0.6)
11 (1.1)
12 (1.0)
12 (1.0)
13 (1.0)
14 (1.0)
15 (1.0)
17 (1.0)
18 (1.0)
1b
1b
1a
1b
1b
1b
1b
1b
1b
a–d See the corresponding footnotes in Table 1. e In 6 h.
instead of THF at 25 ЊC. The reduction in these solvents
provided 8b in much lower enantioselectivity than those
obtained at 25 ЊC in all the cases using the borane reagents
10–12 (entries 5–10 in Table 2).
16
56
a [7b] = 0.3 M in the reaction with 9; [7b] = 0.6 M in the reactions with
10–18. b Isolated yield. c By HPLC analysis with a Chiralcel OD column
using hexane–propan-2-ol (40:1) as eluent. d Based on (S)-(ϩ)-2,2-
dimethoxy-1-phenylethanol and (S)-(ϩ)-1-phenylethane-1,2-diol: ref.
11a and 32. e In 24 h.
Asymmetric reduction of ꢀ-keto acetals 7 catalyzed by 1b with 12
The above results revealed that the reduction of compound 7b
in the presence of 10 mol% of 1b with 1 mol equiv. of N-
phenylamine–borane reagents 12–14 in THF at 25 ЊC was the
optimum reaction with conditions giving high yields and
enantioselectivities of product alcohol 8b. It is noteworthy
that amine–borane reagents as the borane carrier, are very
effective for asymmetric reduction resulting in a practically
useful synthesis of optically active α-hydroxy acetals on a large
scale. This is because such borane reagents are not only less
sensitive to air and moisture, but are also soluble in most
solvents with high concentration compared with borane
hydrides 9–11. Thus we carried out the catalytic reduction of
some other aromatic and aliphatic α-keto acetals using 12 as
the hydride source under these optimum reaction conditions
(Scheme 2). The results illustrated in Table 3 indicate that the
71% ee with 5, and 57% ee with 6 (entries 1–9). B-Bun (1c, 83%
ee) and B-Ph (1d, 84% ee) groups of the catalyst 1 afforded
somewhat lower enantioselectivities than those obtained by
B-H and B-Me groups. Such steric effects of B-substituents are
a common phenomenon in oxazaborolidine-catalyzed reduc-
tions.17a,23 Next, we examined the effect of the borane carriers
for their enantioselectivity by comparing the reduction for 7b in
the presence of 10 mol% of the CBS reagent 1b using various
borane reagents, such as borane–dimethyl sulfide (BMS, 10),
catecholborane 11, and amine–borane complexes, namely, N,N-
diethylaniline–borane 12, N-ethyl-N-isopropylaniline–borane
13, N-phenylmorpholine–borane 14, morpholine–borane 15,
pyridine–borane 16, triethylamine–borane 17 and diisopropyl-
ethylamine–borane 18. The reductions were carried out with
0.6 mol equiv. of 10, 1.1 mol equiv. of 11 and 1.0 mol equiv. of
each of the amine–borane reagents 12–18 in THF at 25 ЊC.
With the exception of the reactions with 16–18, all the reduc-
tions were complete within 10 min to produce 8b in 92–99%
yield. The reductions with 17 and 18 provided 48% and 47%
yield after 24 h, respectively. No reaction with 16 was observed
in 24 h. With respect to enantioselectivity, 10 and 11 afforded
91% ee and 95% ee, respectively (entries 10 and 11). Among
the amine–borane reagents examined, N-phenylamine–borane
complexes 12–14 emerge as the most valuable borane sources to
give 94–96% ee (entries 12–15). The reduction with 15, 17 and
18 provided low to moderate enantioselectivities (entries 16–18).
Scheme 2
Effect of reaction temperature and solvent on asymmetric
induction
reduction of aromatic analogues such as 7e and 7g provided the
corresponding α-hydroxy acetals (8e and 8g) with very high
optical purity in 99% ee. Interestingly, the reduction of 1-(2-
chlorophenyl)-2,2-dimethoxyethanone 7f provided 8f in 30% ee
in contrast to >99% ee for 1-(4-chlorophenyl)-2,2-dimethoxy-
ethanone 7e (entries 5 and 6). Similarly, we observed a
remarkable decrease in enantioselectivity for reduction of 7d
possessing a diisopropyl group in PhCOCH(OR)2, compared
with those having the dimethoxy, diethoxy and propane-1,3-
dioxy groups [95% ee for 7a, 96% ee for 7b, 93% ee for 7c and
33% ee for 7d (entries 1–4)]. These results indicate that the
asymmetric induction is sensitive to steric effects of the sub-
It has been reported that reaction temperature24 and solvents25
in CBS reduction of prochiral ketones influence significantly
the enantioselectivity. To examine the reaction temperature
effect for the asymmetric reduction, 7b was treated with the
borane reagents 11 and 12 in the presence of 10 mol% of 1b in
THF at Ϫ25 and 0 ЊC. At these temperatures, we obtained 8b in
lower yields and enantioselectivities in comparison with those
at 25 ЊC (entries 11 and 13 in Table 1; entries 1–4 in Table 2).
Such phenomena appeared more significant in the case of
12. We also examined solvent effects on such reactions by carry-
ing out the same reduction in toluene and dichloromethane
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J. Chem. Soc., Perkin Trans. 1, 1999, 2095–2100

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Table 3 Asymmetric reduction of α-keto acetals 7 in the presence of
10 mol% of 1b with 1 mol equiv. of 12 in THF at 25 ЊCa
catalytic asymmetric reduction. This asymmetric process is
based on slow reduction of ketones with amine–borane adducts
themselves and easier dissociation of N-phenylamine–borane
complexes to the free BH3 compared with other amine–borane
adducts.28
8
Entry
7
Yield (%)b
[α]D23/10Ϫ1 deg cm2 gϪ1
% ee Config.i
95c
96c
93c
33c
S
S
S
S
S
S
S
S
S
S
S
S
Sj
Conclusion
1
2
3
4
5
6
7
8
9
10
11
12
13
7a
7b
7c
7d
7e
7f
93
96
94
97
91
97
97
85
65
73
76
98
93
ϩ14.03 (c 5.02, CHCl3)
ϩ19.22 (c 5.11, CHCl3)
ϩ3.13 (c 5.10, CHCl3)
4.11 (c 5.02, CHCl3)
We have developed a practical, useful method for the synthesis
of chiral α-hydroxy acetals by oxazaborolidine-catalyzed asym-
metric reduction of α-keto acetals using N-phenylamine–
borane complexes as the hydride source. This is the first such
example to obtain aromatic α-hydroxy acetals with high enantio-
selectivity approaching 100% ee. Further applications using this
methodology are now under investigation.
ϩ11.14 (c 5.41, CHCl3) >99d
ϩ7.11 (c 5.41, CHCl3)
ϩ2.64 (c 5.11, CHCl3)
ϩ3.68 (c 4.80, CHCl3)
Ϫ8.38 (c 3.11, MeOH)
Ϫ20.51 (c 1.15, CH2Cl2)
Ϫ18.71 (c 1.82, CH2Cl2)
Ϫ28.42 (c 1.83, CH2Cl2)
Ϫ10.98 (c 4.11, CH2Cl2)
30d
99e
71f
60g
42f
47f
62h
66h
7g
7h
7i
7j
7k
7l
Experimental
7m
General
a [7] = 0.6 M. b Isolated yield. c By HPLC analysis with a Chiralcel OD
column using hexane–propan-2-ol (40:1) as eluent. d By HPLC analysis
with a Chiralcel OD column using hexane–propan-2-ol (9:1) as eluent.
e By HPLC analysis with a Chiralcel OT column using hexane–propan-
2-ol (9:1) as eluent. f By GLC analysis of its trifluoroacetate using a
20 m Chiraldex GTA column. g By GLC analysis of its (Ϫ)-menthyl
carbonate using a 25 m SupelcowaxTM 10 capillary column. h By GLC
analysis of its (R)-MPTA ester using a 25 m Supelcowax 10 capillary
column. i By comparison of the absolute configurations of the known
compounds, the corresponding diols and/or the elution orders of peaks
in GLC or HPLC analyses. j Probably S based on comparison of the
order of elution of GLC analysis and the sign of the optical rotation
with those of aliphatic analogues.
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.29 The
reactions were monitored by TLC using silica gel plates and the
products were purified by flash column chromatography on
silica gel (Merck; 230–400 mesh). NMR spectra were recorded
at 300 or 400 MHz for H and 75 or 100 MHz for 13C using
1
Me4Si as the internal standard in CDCl3. J-Values are given in
Hz. Optical rotations were measured with a high-resolution
digital polarimeter, with [α]D-values given in units of 10Ϫ1 deg
cm2 gϪ1. Mps were measured on a capillary tube apparatus and
are uncorrected. Enantiomeric excesses (% ees) of the product
α-hydroxy acetals were determined by capillary GLC analyses
of their (R)-MTPA [methoxy(trifluoro)phenylacetate] esters or
(Ϫ)-menthyl carbonates using a 25 m SpelcowaxTM column
or a 20 m Chiraldex G-TA chiral column or with an HPLC
apparatus fitted with a 25 cm Chiralcel OB or OD column. Hex
implies hexane.
stituent proximal to the carbonyl group. This is a common
phenomenon in oxazaborolidine-catalyzed reductions.18a,26
A
heterocyclic α-keto acetal, 1-(2-furyl)-2,2-dimethoxyethanone
7h, produced the α-hydroxy acetal 8h with a moderate optical
purity. In the case of aliphatic analogues, the asymmetric reduc-
tion afforded somewhat lower enantioselectivities than those
obtained from aromatic analogues (entries 9–13). In all the
cases examined, the product α-hydroxy acetals 8 obtained are
consistently enriched in the S-enantiomer. The stereochemical
course of the asymmetric reduction can be explained by the
proposed mechanism involving a transition state 19, where
the α-keto acetals are attacked by hydride on their re faces to
provide (S)-α-hydroxy acetals (Scheme 3).18a,26,27 Although the
Materials
Most of the organic compounds utilized in this study were
commercial products of the highest purity. They were further
purified by distillation when necessary. THF was distilled
over sodium benzophenone ketyl and stored in ampoules
under a nitrogen atmosphere. The chiral oxazaborolidines 1–6
except for 1b were prepared from treatment of the corre-
sponding amino alcohols with BH3–THF according to the
known procedure.15,16,19–22 (S)-α,α-Diphenylpyrrolidine-2-meth-
anol, (1S,2R)-cis-1-aminoindan-2-ol, (1R,2S)-2-amino-1,2-
diphenylethanol, a CBS oxazaborolidine 1b, 2,2-diethoxy-1-
phenylethanone 7b, pyruvic aldehyde dimethyl acetal 7i,
borane–tetrahydrofuran 9, borane–dimethyl sulfide 10, N,N-
diethylaniline–borane 12, N-ethyl-N-isopropylaniline–borane
13, N-phenylmorpholine–borane 14, morpholine–borane 15,
pyridine–borane 16, triethylamine–borane 17 and diisoprop-
ylethylamine–borane 18 were purchased from the Aldrich
Chemical Company. Catecholborane 11 was prepared from
reaction of catechol with 10 according to the literature.29
Preparation of ꢀ-keto acetals
The α-keto acetals 7 used as substrates were prepared by
addition of Grignard reagents to dialkoxyacetopiperidides30 or
the reaction of methyl ketones with catalytic amounts of
diphenyl diselenide and an excess of ammonium peroxydi-
sulfate in methanol.31 2-Acyl-1,3-dioxanes 7c and 7l were
prepared by transacetalization of 7b and 7k, respectively, with 1
mol equiv. of propane-1,3-diol in the presence of a catalytic
amount of toluene-p-sulfonic acid, (PTSA). The products 7
were isolated by distillation or further purified by flash column
chromatography on silica gel when necessary.
Scheme 3
reason why different reactivities and enantioselectivities of
amine–borane reagents are observed for reduction of α-keto
acetals is so far unclear, it might be attributable to differential
dissociation of amine–borane adducts leading to liberation of
free BH3, which coordinates with oxazaborolidines to initiate
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2,2-Dimethoxy-1-phenylethanone 7a.31 86% yield; bp 130–
132 ЊC/40 mmHg (Found: C, 66.62; H, 6.87. C10H12O3 requires
C, 66.65; H, 6.71%); νmax(film)/cmϪ1 2998, 2837, 1705, 1508,
1463, 1285, 1127, 868; δH(400 MHz) 3.48 (6 H, s, 2 × OCH3),
5.25 [1 H, s, CH(OMe)2], 7.25–7.61 (3 H, m, ArH), 8.00–8.15
(2 H, m, ArH); δC(100 MHz) 193.43 (CO), 133.73, 129.51,
128.49 and 127.51 (arom. C) 103.08 [CH(OMe)2], 54.51
(OCH3).
H, 10.07%); νmax(film)/cmϪ1 2961, 1730, 1466, 1193, 1079, 998,
735; δH(400 MHz) 0.91 (3 H, t, J 7.5, CH3), 1.11–1.85 (4 H, m,
2 × CH2), 2.55 (2 H, t, J 7.5, CH2CO), 3.45 (6 H, s, 2 × OCH3),
4.45 [1 H, s, CH(OMe)2]; δC(100 MHz) 205.82 (CO), 104.04
[CH(OMe)2], 54.63 (OCH3), 37.01 (CH2CO), 24.98 and 22.25
(CH2), 13.78 (CH3).
1,1-Diethoxyoctan-2-one 7k. 78% yield; Rf 0.54 (AcOEt–
Hex = 1:4), oil (Found: C, 66.53; H, 11.15. C12H24O3 requires
C, 66.63; H, 11.18%); νmax(film)/cmϪ1 2862, 1730, 1445, 1163,
1063; δH(400 MHz) 0.88 (3 H, t, J 7.02, CH3), 1.25 (6 H, t,
J 7.02, 2 × OCH2CH3), 1.28–1.37 (6 H, m, 3 × CH2), 1.54–1.61
(2 H, m, CH2), 2.58 (2 H, t, J 7.43, CH2CO), 3.60 (2 H, m,
OCH2), 3.71 (2 H, m, OCH2), 4.55 [1 H, s, CH(OMe)2]; δC(100
MHz) 205.92 (CO), 102.14 [CH(OMe)2], 62.71 (OCH2), 36.24
(CH2CO), 30.99, 28.28, 22.40 and 21.93 (CH2), 14.58 and 13.45
(CH3).
2-Benzoyl-1,3-dioxane 7c. 68% yield; 128–131 ЊC/0.25 mmHg;
Rf 0.58 (AcOEt–Hex = 1:1), oil (Found: C, 68.80; H, 6.18.
C11H12O3 requires C, 68.74; H, 6.29%); νmax(film)/cmϪ1 3056,
2924, 1703, 1596, 1457, 1288, 1114, 1015, 928, 695; δH(300
MHz) 1.45–1.53 (1 H, m, OCH2CHaHbCH2O), 2.27–2.32 (1 H,
m, OCH2CHaHbCH2O), 3.94–4.03 (2 H, m, OCH2), 4.28–4.33
(2 H, m, OCH2), 5.50 [1 H, s, CHO2(CH2)3], 7.43–7.48 (2 H,
m, ArH), 7.54–7.59 (1 H, m, ArH), 8.10–8.13 (2 H, m, ArH);
δC(75 MHz) 192.02 (CO), 134.23, 134.17, 130.30 and 128.83
(arom. C), 100.63 [CHO2(CH2)3], 67.59 (OCH2), 25.73 (CH2).
2-Octanoyl-1,3-dioxane 7l. 81% yield; Rf 0.58 (AcOEt–
Hex = 1:2), oil (Found: C, 66.01; H, 10.15. C11H20O3 requires C,
65.97; H, 10.07%); νmax(film)/cmϪ1 2959, 1737, 1467, 1377, 1150,
1115, 1021, 934; δH(400 MHz) 0.88 (3 H, t, J 7.0, CH3), 1.28–
1.33 (6 H, m, 3 × CH2), 1.40–1.44 (1 H, m, OCH2CHa-
HbCH2O), 1.54–1.61 (2 H, m, CH2), 2.10–2.22 (1 H, m,
OCH2CHaHbCH2O), 2.59 (2 H, t, J 7.42, CH2CO), 3.14–3.88
(2 H, m, OCH2), 4.19–4.13 (2 H, m, OCH2), 4.77 [1 H, s, CHO2-
(CH2)3]; δC(100 MHz) 204.38 (CO), 100.88 [CHO2(CH2)3],
67.45 (OCH2), 37.65 (CH2CO), 31.92, 29.16, 26.01, 23.10 and
22.86 (CH2), 14.39 (CH3).
2,2-Diisopropoxy-1-phenylethanone 7d. Rf 0.72 (AcOEt–
Hex = 1:4); 72% yield, oil (Found: C, 71.23: H, 8.45. C14H20O3
requires C, 71.16; H, 8.53%); νmax(film)/cmϪ1 3031, 2887, 1706,
1599, 1466, 1284, 1120, 947, 787; δH(400 MHz) 1.14 (6 H, d,
J 6.12, 2 × CH3), 1.27 (6 H, d, J 6.2, 2 × CH3), 3.90–3.99 (2 H,
m, 2 × OCHMe2), 5.19 [1 H, s, CH(OPri)2], 7.42–7.57 (3 H, m,
ArH), 8.20–8.22 (2 H, m, ArH); δC(100 MHz) 195.09 (CO),
133.50, 133.21, 130.28 and 128.13 (arom. C), 101.96 [CH(O-
Pri)2], 70.40 (OCHMe2), 22.97 (CH3), 22.32 (CH3).
1-(4-Chlorophenyl)-2,2-dimethoxyethanone 7e. 88% yield; bp
140–143 ЊC/10 mmHg; Rf 0.65 (AcOEt–Hex = 1:2), oil (Found:
C, 55.98; H, 5.17; Cl, 16.62. C10H11ClO3 requires C, 55.96; H,
5.17; Cl, 16.52%); νmax(film)/cmϪ1 3000, 2837, 1702, 1589, 1489,
1286, 1092, 870; δH(400 MHz) 3.48 (6 H, s, 2 × OCH3), 5.12
[1 H, s, CH(OMe)2], 7.43 (2 H, d, J 8.7, ArH), 8.08 (2 H, d,
J 8.6, ArH); δC(100 MHz) 192.43 (CO), 140.15, 131.11 and
128.80 (arom. C), 104.04 [CH(OMe)2], 54.88 (OCH3).
1-Cyclohexyl-2,2-diethoxyethanone 7m. 84% yield; Rf 0.67
(AcOEt–Hex = 1:4), oil (Found: C, 67.28; H, 10.40. C12H22O3
requires C, 67.26; H, 10.35%); νmax(film)/cmϪ1 2934, 1724, 1449,
1133, 1099, 1064; δH(400 MHz) 1.24 (6 H, t, J 7.12, 2 × CH3),
1.31–1.37 (4 H, m, 2 × CH2), 1.75–1.83 (6 H, m, 3 × CH2), 2.82
(1 H, m, CHCO), 3.59 (2 H, m, OCH2), 3.68 (2 H, m, OCH2),
4.66 [1 H, s, CH(OMe)2]; δC(100 MHz) 208.72 (CO), 102.10
[CHO(CH2)3], 63.10 (OCH2), 45.33 (CHCO), 28.57, 25.84 and
25.62 (CH2), 15.18 (CH3).
1-(2-Chlorophenyl)-2,2-dimethoxyethanone 7f. 79% yield; bp
130–132 ЊC/10 mmHg; Rf 0.62 (AcOEt–Hex = 1:2), oil (Found:
C, 56.02; H, 5.13; Cl, 16.45%. νmax(film)/cmϪ1 2944, 2838, 1713,
1591, 1436, 1288, 1069, 764; δH(400 MHz) 3.46 (6 H, s, 2 ×
OCH3), 5.25 [1 H, s, CH(OMe)2], 7.31–7.41 (3 H, m, ArH),
7.60–7.63 (1 H, m, ArH); δC(100 MHz) 190.60 (CO), 136.39,
132.19, 131.75, 130.41, 129.91 and 126.67 (arom. C), 103.05
[CH(OMe)2], 56.66 (OCH3).
Oxazaborolidine-catalyzed reduction of ꢀ-keto acetals 7
The asymmetric reduction of 7e is representative. To a stirred
solution of 1b (0.2 M; 0.1 mmol, 0.5 cm3) in dry THF was
added neat DEANB 12 (1 mmol, 163 mg, 0.18 cm3) in an
atmosphere of nitrogen. To this was added slowly 1.1 cm3 of a
THF solution of 7d (1 mmol, 0.215 g) over a period of 1 h using
a syringe pump at 25 ЊC. The reaction mixture was stirred for 10
min at the same temperature and then quenched cautiously
with methanol (0.5 cm3). Solvent was evaporated off under
reduced pressure to leave an oil, which was purified by flash-
column chromatography on silica gel (230–400 mesh) using
ethyl acetate–hexane (1:2) as eluent to give (S)-chlorophenyl-
2,2-dimethoxyethanol 8e (197 mg, 91%); Rf 0.43 (AcOEt–
Hex = 1:2), oil (Found: C, 55.41; H, 6.10; Cl, 16.37. C10H13ClO3
requires C, 55.44; H, 6.05; Cl, 16.36%); [α]D23 ϩ11.14 (c 5.41,
CHCl3); νmax(film)/cmϪ1 3436, 2930, 1597, 1490, 1187, 1120,
1076, 1012, 972, 823; δH(400 MHz) 2.72 (1 H, br s, OH), 3.29 (3
H, s, OCH3), 3.42 (3 H, s, OCH3), 4.23 [1 H, d, J 6.46,
CH(OMe)2], 4.59 (1 H, d, J 6.44, CHOH), 7.31–7.37 (4 H,
m, ArH); δC(100 MHz) 137.87, 133.63, 128.47 and 128.37
(arom. C), 107.54 [CH(OMe)2], 73.32 (CHOH), 56.23 and
55.00 (OCH3). HPLC analysis of the product 8e with a
Chiralcel OD column using hexane–propan-2-ol (9:1) as eluent
showed a composition 99.9% S-isomer and 0.1% R-isomer
(i.e., 99.8% ee).
2,2-Dimethoxy-1-(2-naphthyl)ethanone 7g.31 89% yield; Rf
0.65 (AcOEt–Hex = 1:2), oil (Found: C, 73.09; H, 6.04.
C14H14O3 requires C, 73.03; H, 6.13%); νmax(film)/cmϪ1 2999,
2836, 1695, 1466, 1294, 1069, 980, 817, 772; δH(400 MHz) 3.50
(6 H, s, 2 × OCH3), 5.35 [1 H, s, CH(OMe)2], 7.45–7.65 (2 H,
m, ArH), 7.80–8.15 (4 H, m, ArH), 8.6–8.8 (1 H, m, ArH);
δC(100 MHz) 193.46 (CO), 135.87, 132.45, 131.84, 131.05,
129.92, 128.77, 128.29, 127.44, 126.71 and 124.65 (arom. C),
103.34 [CH(OMe)2], 54.55 (OCH3).
1-(2-Furyl)-2,2-dimethoxyethanone 7h.31 69% yield; bp 120–
122/12 mmHg; Rf 0.5 (AcOEt–Hex = 1:2), oil (Found: C, 56.43;
H, 5.96. C8H10O4 requires C, 56.47; H, 5.92%); νmax(film)/cmϪ1
3137, 2998, 2839, 1739, 1692, 1467, 1395, 1271, 1197, 1081,
1015, 769; δH(400 MHz) 3.48 (6 H, s, 2 × OCH3), 5.10 [1 H, s,
CH(OMe)2], 6.57 (1 H, dd, J 1.5 and 3.5, furan H), 7.46 (1 H, d,
J 3.41, furan H), 7.67 (1 H, d, J 1.09, furan H); δC(100 MHz)
182.31 (CO), 150.05, 147.60, 121.08 and 112.30 (furan C),
102.25 [CH(OMe)2], 54.46 (OCH3).
(S)-2,2-Dimethoxy-1-phenylethanol 8a.11a 93% yield; bp 132–
134 ЊC/40 mmHg; Rf 0.37 (AcOEt–Hex = 1:2), oil (Found: C,
65.91; H, 7.75. C10H14O3 requires C, 65.87; H, 7.83%); [α]D23
1,1-Dimethoxy-2-hexanone 7j.31 72% yield; bp 70–73 ЊC/10
mmHg); (Found: C, 59.87; H, 9.95. C8H16O3 requires C, 59.97;
2098
J. Chem. Soc., Perkin Trans. 1, 1999, 2095–2100

View Article Online
ϩ14.03 (c 5.02, CHCl3), which shows 95% ee by HPLC analysis
with a Chiralcel OD using hexane–propan-2-ol (40:1) as
eluent; νmax(film)/cmϪ1 3435, 2929, 1493, 1451, 1187, 1121,
1080, 971; δH(300 MHz) 2.64–2.65 (1 H, br s, OH), 3.28 (3 H, s,
OCH3), 3.48 (3 H, s, OCH3), 4.30 [1 H, d, J 6.44, CH(OMe)2],
4.62 (1 H, d, J 6.32, CHOH), 7.27–7.44 (5 H, m, ArH);
δC(75 MHz) 140.01, 128.42, 127.81 and 127.62 (arom. C),
108.01 [CH(OMe)2], 74.28 (CHOH), 58.16 and 57.60 (OCH3).
0.32 (AcOEt–Hex = 1:2), thick oil (Found: C, 72.30; H, 6.98.
C14H16O3 requires C, 72.34; H, 6.94%); [α]D23 ϩ2.64 (c 5.11,
CHCl3), 99% ee by HPLC analysis with a Chiralcel OT using
hexane–propan-2-ol (9:1) as eluent; νmax(film)/cmϪ1 3433, 2920,
1600, 1507, 1463, 1361, 1189, 1119, 1073, 972, 746; δH(300
MHz) 2.86 (1 H, br s, OH), 3.26 (3 H, s, OCH3), 3.48 (3 H, s,
OCH3), 4.37 [1 H, d, J 6.4, CH(OMe)2], 4.78 (1 H, d, J 6.3,
CHOH), 7.46–7.55 (3 H, m, ArH), 7.81–7.89 (4 H, m, ArH);
δC(75 MHz) 137.48, 133.71, 128.56, 128.34, 128.13, 126.63,
126.46, 126.37 and 125.49 (arom. C), 108.12 [CH(OMe)2], 74.41
(CHOH), 56.31 and 55.16 (OCH3).
(S)-2,2-Diethoxy-1-phenylethanol 8b.14 96% yield; bp 146–
149 ЊC/40 mmHg; Rf 0.37 (AcOEt–Hex = 1:4), oil (Found: C,
73.02; H, 6.13. C12H18O3 requires C, 73.01; H, 6.04%); [α]D23
ϩ19.22 (c 5.11, CHCl3), which corresponds to an optical purity
of 96% ee by HPLC analysis with a Chiralcel OD using
hexane–propan-2-ol (40:1) as eluent; νmax(film)/cmϪ1 3447,
3056, 2921, 1452, 1116, 758, 697; δH(300 MHz) 1.02 (3 H, t,
J 7.02, CH2CH3), 1.24 (3 H, t, J 7.02, CH2CH3), 2.90 (1 H, br s,
OH), 3.22 (1 H, dq, J 7.01 and 4.68, OCHaHbCH3), 3.55 (2 H,
m, OCH2CH3), 3.79 (1 H, dq, J 7.02 and 4.82, OCHaHbCH3),
4.37 [1 H, d, J 6.44, CH(OEt)2], 4.58 (d, 1 H, d, J 6.43, CHOH),
7.24–7.44 (m, 5 H, m, ArH); δC(75 MHz) 139.96, 128.34, 128.03
and 127.48 (arom. C), 106.26 [CH(OMe)2], 74.84 (CHOH),
64.67 and 63.70 (OCH2), 15.35 and 15.11 (CH3).
(S)-1-(2-Furyl)-2,2-dimethoxyethanol 8h. 85% yield; Rf 0.27
(AcOEt–Hex = 1:2), oil (Found: C, 55.65; H, 6.92. C8H12O4
requires C, 55.81; H, 7.02%); [α]D23 ϩ3.68 (c 4.80, CHCl3), which
corresponds to an optical purity of 71% ee by GLC analysis
using a 20 m Chiraldex GTA column and the (S)-isomer on the
basis of (S)-(ϩ)-1-(2-furyl)ethane-1,2-diol;34 νmax(film)/cmϪ1
3445, 2943, 1504, 1466, 1369, 1193, 1149, 1120, 1081, 976, 787,
761; δH(300 MHz) 2.72 (1 H, br s, OH), 3.34 (3 H, s, OCH3),
3.48 (3 H, s, OCH3), 4.57 [1 H, d, J 6.42, CH(OMe)2], 4.66 [1 H,
d, J 6.44, CHOH], 6.35–6.37 (2 H, m, furan H), 7.40–7.41 (1 H,
m, furan H); δC(75 MHz) 153.23, 142.89, 110.77 and 108.58
(furan C), 105.64 [CH(OMe)2], 68.27 (CHOH), 55.68 and 57.36
(OCH3).
(S)-1-(1,3-Dioxan-2-yl)-1-phenylmethanol 8c.14 94% yield; mp
80–82 ЊC; Rf 0.42 (AcOEt–Hex = 1:1), white solid (Found: C,
68.02; H, 7.35. C11H14O3 requires C, 68.02; H, 7.26%); [α]D23
ϩ3.13 (c 5.10, CHCl3), which shows 93% ee by HPLC analysis
with a Chiralcel OD using hexane–propan-2-ol (40:1) as
eluent; νmax(KBr)/cmϪ1 3490, 2855, 1493, 1460, 1236, 1137,
1085, 971, 766; δH(300 MHz) 1.26–1.40 (1 H, m, OCH2CHaHb-
CH2O), 2.01–2.25 (1 H, m, OCH2CHaHbCH2O), 2.80 (1 H, br s,
OH), 3.66–3.87 (2 H, m, OCH2), 4.07–4.24 (2 H, m, OCH2)
4.61 [2 H, s, CHOH and CHO2(CH2)3], 7.26–7.44 (5 H, m,
ArH); δC(75 MHz) 140.00, 128.45, 128.21 and 127.40 (arom.
C), 103.36 [CHO2(CH2)3], 75.20 (CHOH), 67.13 (OCH2), 25.79
(OCH2CH2CH2O).
(S)-1,1-Dimethoxypropan-2-ol 8i.12 65% yield; bp 62–64 ЊC/
40 mmHg; [α]D23 Ϫ8.38 (c 3.11, MeOH), which shows 60% ee by
GLC analysis of its (Ϫ)-menthyl carbonate using a 25 m Supel-
cowaxTM 10 capillary column; νmax(film)/cmϪ1 3440, 2930, 1459,
1379, 1112, 975; δH(300 MHz) 1.19 (3 H, d, J 6.4, CH3), 2.05 (1
H, br s, OH), 3.43 and 3.46 (each 3 H, s, OCH3), 3.77 (1 H,
quintet, J 6.3, CHOH), 4.08 [1 H, d, J 6.3, CH(OMe)2]; δC(75
MHz) 107.93 [CH(OMe)2], 67.25 (CHOH), 54.84 (OCH3),
17.11 (CH3).
(S)-1,1-Dimethoxyhexan-2-ol 8j.11a 73% yield; bp 73–75Њ/10
mmHg; [α]D23 –20.51 (c 1.15, CH2Cl2), which corresponds to an
optical purity of 42% ee by GLC analysis of the trifluoroacetate
using a 20 m Chiraldex GTA column; νmax(film)/cmϪ1 3468,
2956, 1467, 1372, 1195, 1118, 1079, 975; δH(300 MHz) 0.91 (3
H, t, J 7.3, CH3), 1.11–1.87 (4 H, m, 2 × CH2), 2.04 (1 H, br s,
OH), 2.54 (2 H, m, CH2CH2CHOH), 3.41 (6 H, s, OCH3), 3.81
(1 H, m), 4.23 [1 H, d, J 6.3, CH(OMe)2]; δC(75 MHz) 108.93
[CH(OMe)2], 66.35 (CHOH), 53.45 (OCH3), 29.15, 25.48 and
22.59 (CH2), 15.38 (CH3).
(S)-2,2-Diisopropoxy-1-phenylethanol 8d. 97% yield; Rf 0.53
(AcOEt–Hex = 1:4), oil (Found: C, 70.63; H, 9.18. C14H22O3
requires C, 70.56; H, 9.30%); [α]D23 ϩ4.11 (c 5.02, CHCl3), which
corresponds to an optical purity of 33% ee by HPLC analysis
with a Chiralcel OD using hexane–propan-2-ol (40:1) as eluent
and the (S)-isomer on the basis of (S)-(ϩ)-1-phenylethane-1,2-
diol;32 νmax(film)/cmϪ1 3558, 2932, 1466, 1432, 1320, 1381, 1125,
1044, 760, 699; δH(400 MHz) 0.74 (3 H, d, J 6.14, CH3), 1.13
(3 H, d, J 6.15, CH3), 1.16 (3 H, d, J 6.07, CH3), 1.26 (3 H, d,
J 6.16, CH3), 2.81 (1 H, s, OH), 3.51 (1 H, heptet, J 6.1,
CHMe2), 3.92 (1 H, heptet, J 6.11, CHMe2), 4.50 [2 H, m,
CHOH and CH(OPri)2], 7.25–7.35 (3 H, m, ArH), 7.41–7.44
(2 H, m, ArH); δC(100 MHz) 139.58, 127.97, 127.77 and 127.46
(arom. C), 102.82 [CH(OPri)2], 75.59 (CHOH), 70.69 and 69.54
(OCHMe2), 23.51, 22.96, 22.32 and 21.93 (CH3).
(S)-1,1-Diethoxyoctan-2-ol 8k. 76% yield; Rf 0.30 (AcOEt–
Hex = 1:4), oil (Found: C, 66.05; H, 12.08. C12H26O3 requires C,
66.01; H, 12.00%); [α]D23 ϩ18.71 (c 1.82, CH2Cl2), which shows
47% ee by GLC analysis of its trifluoroacetate using a 20 m
Chiraldex GTA column and the (S)-isomer on the basis of
(S)-(ϩ)-octane-1,2-diol;35 νmax(film)/cmϪ1 3455, 2916, 1467,
1368, 1190, 1109, 1074, 977; δH(400 MHz) 0.87 (3 H, t, J
6.5, CH3), 1.23 (6 H, t, J 6.72, 2 × OCH2CH3), 1.25–1.42 (8 H,
m, 4 × CH2), 1.51–1.61 (2 H, m, CH2), 2.07 (1 H, br s, OH),
3.52–3.59 (2 H, m, OCH2CH3), 3.63–3.73 (2 H, m, OCH2CH3),
3.77 (1 H, m, CHOH), 4.24 [1 H, d, J 5.8, CH(OEt)2]; δC(100
MHz) 105.14 [CH(OMe)2], 71.77 (CHOH), 63.36 and 63.27
(OCH2), 29.35, 28.83, 25.48, 22.97 and 22.59 (CH2), 15.38,
15.12 and 14.04 (CH3).
(S)-1-(2-Chlorophenyl)-2,2-dimethoxyethanol 8f. 97% yield;
Rf 0.35 (AcOEt–Hex = 1:2), oil (Found: C, 55.51; H, 5.98, Cl,
16.56. C10H13ClO3 requires C, 55.44; H, 6.05, Cl, 16.36%);
[α]D23 ϩ7.11 (c 5.41, CHCl3), which corresponds to an optical
purity of 30% ee by HPLC analysis with a Chiralcel OD using
hexane–propan-2-ol (9:1) as eluent and the (S)-isomer on the
basis of (S)-(ϩ)-1-(2-chlorophenyl)ethane-1,2-diol;33 νmax(film)/
cmϪ1 3444, 2941, 1597, 1477, 1442, 1194, 1122, 1080, 979, 757;
δH(400 MHz) 2.84 (1 H, d, J 3.95, OH), 3.39 (3 H, s, OCH3),
3.40 (3 H, s, OCH3), 4.47 [1 H, d, J 4.36, CH(OMe)2], 5.20 (1 H,
d, J 4.01, CHOH), 7.23–7.36 (3 H, m, ArH), 7.56–7.59 (1 H, m,
ArH); δC(100 MHz) 137.03, 132.82, 129.31, 128.92, 128.58 and
126.92 (arom. C), 106.04 [CH(OMe)2], 70.54 (CHOH), 55.87
and 55.59 (OCH3).
(S)-(1,3-Dioxan-2-yl)heptan-1-ol 8l. 98% yield; Rf 0.38
(AcOEt–Hex = 1:2), oil (Found: C, 65.23; H, 10.89. C11H22O3
requires C, 65.31; H, 10.96%); [α]D23 Ϫ28.42 (c 1.83, CH2Cl2),
which corresponds to an optical purity of 62% ee by GLC
analysis of its (R)-MTPA ester using a 25 m SupelcowaxTM 10
capillary column and to the (S)-isomer on the basis of (R)-(ϩ)-
octane-1,2-diol;35 νmax(film)/cmϪ1 3400, 2957, 2951, 1467, 1378,
(S)-2,2-Dimethoxy-1-(2-naphthyl)ethanol 8g.14 97% yield; Rf
J. Chem. Soc., Perkin Trans. 1, 1999, 2095–2100
2099

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C. Scolastico, J. Org. Chem., 1984, 49, 3784; (c) K. Takai,
1242, 1148, 1018; δH(400 MHz) 0.86 (3 H, t, J 6.84, CH3), 1.27–
1.58 (11 H, m, 5 × CH2CH2CH2 and OCH2CHaHbCH2O),
2.04–2.14 (1 H, m, OCH2CHaHbCH2O), 2.23 (1 H, br s,
OH), 3.51 (1 H, m, CHOH), 3.77 (2 H, dt, J 11.47 and
2.44, OCH2CH2), 4.11–4.16 (2 H, m, OCH2CH2), 4.37 [1 H, d,
J 4.50, CHO2(CH2)3]; δC(100 MHz) 102.74 [CHO2(CH2)3], 72.54
(CHOH), 66.81 and 66.77 (OCH2), 31.72, 31.36, 29.27, 25.79,
25.31 and 22.55 (CH2), 14.00 (CH3).
M. Larcheveque and Y. Petit, Bull. Soc. Chim. Fr., 1989, 130; (d)
F. Effenberger, M. Hopf, T. Ziegler and J. Hudelmayer, Chem. Ber.,
1991, 124, 1651.
10 B. Zagalak, P. A. Frey, G. L. Karabatsos and R. H. Abels, J. Biol.
Chem., 1966, 241, 3028.
11 (a) P. Ferraboschi, E. Santaniello, M. Tingoli, F. Aragozzini and
F. Molinari, Tetrahedron: Asymmetry, 1993, 4, 1931; (b) D. Bianchi,
P. Cesti and P. Golini, Tetrahedron, 1989, 45, 869; (c) G. Guanti,
L. Banfi and E. Narisano, Tetrahedron Lett., 1986, 27, 3547; (d)
T. Fujisawa, E. Kojima, T. Itoh and T. Sato, Chem. Lett., 1985, 1784.
12 H. Takahashi, T. Morimoto and K. Achiwa, Chem. Lett., 1987, 855.
13 R. Enders and U. Reinhold, Liebigs. Ann., 1996, 11.
14 B. T. Cho and Y. S. Chun, Tetrahedron: Asymmetry, 1994, 5, 1147.
15 (a) S. Itsuno, M. Nakano, K. Miyazaki, H. Masuda, K. Ito,
A. Hirao and S. Nakahama, J. Chem. Soc., Perkin Trans. 1, 1985,
2039; (b) S. Itsuno, K. Ito, A. Hirao and S. Nakahama, Bull. Chem.
Soc. Jpn., 1987, 60, 395.
16 (a) E. J. Corey, R. K. Bakshi and S. Shibata, J. Am. Chem. Soc.,
1987, 109, 5551; (b) E. J. Corey, R. K. Bakshi, S. Shibata, C.-P. Chen
and V. K. Singh, J. Am. Chem. Soc., 1987, 109, 7925; (c) E. J. Corey
and J. O. Link, Tetrahedron Lett., 1990, 31, 601; (d) D. J. Mathre,
T. K. Jones, L. C. Xavier, T. J. Blacklock, R. A. Reamer, J. J. Mohan,
E. T. Turner Jones, K. Hoogsteen, M. W. Baum and E. J. J.
Grabowski, J. Org. Chem., 1991, 56, 761.
(S)-1-Cyclohexyl-2,2-diethoxyethanol 8m. 93% yield; Rf 0.43
(AcOEt–Hex = 1:2.4), oil (Found: C, 66.54; H, 11.23. C12H24O3
requires C, 66.63; H, 11.18%); [α]D23 Ϫ10.98 (c 4.11, CH2Cl2),
which shows 66% ee by GLC analysis of its (R)-MTPA ester
using a 25 m SupelcowaxTM 10 capillary column and probably
the (S)-isomer on the basis of comparison of the order of
elution of GLC analysis and the sign of the optical rotation
with those of aliphatic analogues; νmax(film)/cmϪ1 3491, 2913,
2857, 1450, 1346, 1161, 1068, 995; δH(400 MHz) 1.16–1.28
(10 H, m, 2 × CH3 and 2 × CH2), 1.53–1.74 (7 H, m, 3 × CH2
and CH2CHCH2), 2.16 (1 H, br s, OH), 3.35 (1 H, m, CHOH),
3.53–3.60 (2 H, m, OCH2), 3.70 (1 H, dq, J 7.60 and 6.84,
OCHaHb), 3.78 (1 H, dq, J 7.80 and 6.84, OCHaHb), 4.41 [1 H,
d, J 6.35, CH(OEt)2]; δC(100 MHz) 102.85 [CH(OEt)2], 75.32
(CHOH), 63.19 and 62.95 (OCH2), 39.15, 29.87, 26.92, 26.49,
26.47 and 26.19 (CH2), 15.39 (CH3).
17 For general reviews on the oxazaborolidine-catalyzed reductions,
see: (a) E. J. Corey and C. J. Helal, Angew. Chem., Int. Ed., 1998, 37,
1986; (b) V. K. Sing, Synthesis, 1992, 605; (c) S. Wallbaum and
J. Martens, Tetrahedron: Asymmetry, 1992, 3, 1475.
18 B. T. Cho and Y. S. Chun, (a) J. Org. Chem., 1998, 63, 5280; (b)
Tetrahedron: Asymmetry, 1992, 3, 341.
19 G. J. Quallich and T. M. Woodall, Tetrahedron Lett., 1993, 34, 4145.
20 R. Bergenguer, J. Garcia and Vilarrasa, Tetrahedron: Asymmetry,
1994, 5, 165.
21 H. Yapping, Y. Gao, X. Nie and C. M. Zepp, Tetrahedron Lett.,
1994, 35, 6631.
Acknowledgements
This research was financially supported by the Hallym
Academy of Sciences, Hallym University.
22 B. T. Cho and Y. S. Chun, Bull. Korean Chem. Soc., 1996, 17, 1098.
23 B. T. Cho and M. H. Ryu, Bull. Korean Chem. Soc., 1994, 15, 1080.
24 G. B. Stone, Tetrahedron: Asymmetry, 1993, 5, 465.
25 B. T. Cho and M. H. Ryu, Bull. Korean Chem. Soc., 1994, 15, 1027.
26 E. J. Corey and C. J. Helal, Tetrahedron Lett., 1996, 37, 5675
27 D. K. Jones, D. C. Liotta, I. Shinkai and D. J. Mathre, J. Org. Chem.,
1993, 58, 799.
28 H. C. Brown and L. T. Murray, Inorg. Chem., 1984, 23, 2746.
29 H. C. Brown, G. W. Kramer, A. B. Levi and M. M. Midland,
Organic Synthesis via Boranes, John Wiley & Sons, New York, 1975,
p. 63.
30 A. Wohl and M. Lange, Ber. Dtsch. Chem. Ges., 1908, 41, 3612.
31 M. Tiecco, M. Testaferri, M. Tingoli and D. Bartoli, J. Org. Chem.,
1990, 55, 4523.
32 H. Becker, S. B. King, M. Taniguchi, K. P. M. Vanhessche and K. B.
Sharpless, J. Org. Chem., 1995, 60, 3940.
References
1 Part 15. B. T. Cho and Y. S. Chun, Bull. Korean Chem. Soc., 1999,
20, 397.
2 T. R. Kelly and P. L. Kauel, J. Org. Chem., 1983, 48, 2775.
3 J. Mulzer, N. Salimi and H. Hartl, Tetrahedron: Asymmetry, 1993, 4,
457.
4 S. Okamoto, T. Shimazaki, Y. Kitano, Y. Kobayashi and F. Sato,
J. Chem. Soc., Chem. Commun., 1986, 1352.
5 (a) Y. Noda and M. Kikuchi, Chem. Lett., 1989, 1755; (b) S. Vettel,
C. Lutz and P. Knochel, Synlett, 1996, 731.
6 T. Kan, S. Oikawa, S. Hosokawa, M. Yanagiya, F. Matsuda and
H. Shirahama, Synlett, 1994, 801 and 805.
7 (a) S. Hanessian and J. Kloss, Tetrahedron Lett., 1985, 26, 1261; (b)
B. Brandänge and B. Lindvquist, Acta Chem. Scand., Sect. B, 1985,
39, 589.
8 For some leading references, see: (a) D. A. Evans, J. V. Nelson and
T. R. Taber, Top. Stereochem., 1983, 13, 1; (b) C. H. Heathcock,
in Asymmetric Synthesis, ed. J. D. Morrison, Academic Press,
Orlando, 1984, vol. 3B, p. 111; (c) M. T. Reetz, Angew. Chem., Int.
Ed. Engl., 1984, 23, 556; (d) J. Jurczak, S. Pikul and T. Bauer,
Tetrahedron, 1986, 42, 488.
33 T. Hudlicky, E. E. Boros and C. H. Boros, Tetrahedron: Asymmetry,
1993, 4, 1365.
34 F. Gonzalez, S. Lesage and A. S. Perlin, Carbohydr. Res., 1975, 42,
267.
35 H. Becker and K. B. Sharpless, Angew. Chem., Int. Ed. Engl., 1996,
35, 448.
9 (a) S. K. Massad, L. D. Hawkins and D. C. Baker, J. Org. Chem.,
1983, 48, 3784; (b) L. Benfi, A. Bernadi, C. Gennari and
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