A short and efficient stereoselective synthesis of all four diastereomers of sphingosine

Article abstract of DOI:10.1016/S0957-4166(02)00118-0

Practical syntheses of all four stereomers of sphingosine from serine have been achieved through highly diastereoselective reduction of the N-trityl protected α′-amino enone derivative 5 with NaBH₄ and reduction of the free α′-amino enone derivative 7 with Zn(BH₄)₂.

Full text of DOI:10.1016/S0957-4166(02)00118-0

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 343–347
A short and efficient stereoselective synthesis of all four
diastereomers of sphingosine
Jae-Mok Lee, Hyun-Suk Lim and Sung-Kee Chung*
Department of Chemistry, Division of Molecular and Life Sciences, Pohang University of Science and Technology,
Pohang 790-784, South Korea
Received 27 December 2001; accepted 7 March 2002
Abstract—Practical syntheses of all four stereomers of sphingosine from serine have been achieved through highly diastereoselec-
tive reduction of the N-trityl protected a%-amino enone derivative 5 with NaBH4 and reduction of the free a%-amino enone
derivative 7 with Zn(BH ) . © 2002 Elsevier Science Ltd. All rights reserved.
4
2
1
. Introduction
thetic methods for sphingosine has been a long standing
target, and a great deal of effort has been expended to
this end. Recently we reported a stereodivergent syn-
5
Sphingolipids are distributed ubiquitously in the mem-
branes of eukaryotic cells and in all plasma mem-
Their metabolites such as ceramide,
sphingosine, sphingosine-1-phosphate and glycosphin-
golipids are involved in diverse cellular processes
thesis of all four stereoisomers of sphingosine via syn/
anti diastereoselective reduction of Boc-N-PMB and
N-PMB protected a%-amino enones that were derived
1
branes.
6
from serine. However, this approach has some draw-
including cell growth, survival, differentiation and
backs due to the relatively lengthy procedure and prob-
lematic deprotection of the PMB group in the last step.
For these reasons, we have now devised a more conve-
nient, stereoselective synthesis of all four diastereomers
of sphingosine using the N-trityl amine-protecting
group to induce a strong directing effect through its
2
adhesion, etc.
D
-erythro-Sphingosine 1a, which is the
backbone of various sphingolipids, has itself been
found to be a potent and specific inhibitor of protein
kinase C and plays crucial roles in cellular signal trans-
3
duction. And the other diastereomers 1b–d also show a
variety of bioactivities
ical importance the development of asymmetric syn-
3
a,4
(Fig. 1). Because of its biolog-
6
bulk. Based on our previous reasoning and experience,
employment of the N-trityl protecting group was found
to provide
a
more convenient access to the
diastereomers: generation of the syn and anti products
via non-chelation controlled reduction (open Felkin–
Anh model) of tritylated amino enone 5 and chelation
controlled reduction (cyclic Felkin–Anh model) of free
OH
OH
HO
C13H27
HO
C13H27
NH2
NH2
7
amino enone 7, respectively (Scheme 1).
(
2S,3R,4E)-D-erythro 1a
(2S,3S,4E)-L-threo 1b
OH
OH
2
. Results and discussion
HO
C13H27
HO
C13H27
NH2
NH2
As shown in Scheme 2, our synthesis started with the
fully protected serine methyl ester 3, which was readily
obtained from the commercially available serine deriva-
tive 2. The protected serine ester 3 was quantitatively
converted to the b-ketophosphonate 4 by treatment
with excess lithium dimethyl methylphosphonate in
THF at −78°C. The Horner–Wadsworth–Emmons
olefination of the phosphonate 4 with tetradecyl alde-
(
2R,3S,4E)-L-erythro 1c
(2R,3R,4E)-D-threo 1d
Figure 1. Structure of diastereomers of sphingosine.
*
Corresponding author. Tel.: +82-54-279-2103; fax: +82-54-279-3399;
e-mail: skchung@postech.ac.kr
0
957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00118-0

3
44
J.-M. Lee et al. / Tetrahedron: Asymmetry 13 (2002) 343–347
1
0
O
identical to the literature data. By employing the same
procedures on -serine esters, -erythro- and -threo-
D
L
D
R1O
C13H27
sphingosines were also synthesized in comparable yields
and stereoselectivities.
NHR2
5
7
: R1 = TBS, R2 = Trityl
: R1 = R2 = H
open Felkin-Anh
cyclic Felkin-Anh
3. Conclusion
In conclusion, we have developed short (six steps from
commercially available serine derivatives) and practical
OH
OH
(
51–69% overall yields) synthetic routes to the four
TBSO
C13H27
HO
C13H27
stereoisomers of sphingosine using highly efficient syn/
anti diastereoselective reduction of 3-ketosphingosine
derivatives 5 and 7, which are complementary to our
NHTr
syn
NH2
anti
6
Scheme 1.
previous report.
hyde under Masamune conditions provided the corre-
8
sponding enone 5 in good yield. Reduction of enone 5
4. Experimental
with NaBH in the presence of CeCl ·7H O afforded
4
3
2
the N,O-diprotected
L
-threo-sphingosine 6 in 92% d.e.,
4.1. General procedure
presumably via an open Felkin–Anh transition state
non-chelation control) in accordance with the litera-
(
Melting points were determined on a Thomas–Hoover
9
ture precedents. Removal of the two protecting groups
N-trityl and O-TBDMS) of 6 with hot aqueous HCl in
MeOH–THF yielded -threo-sphingosine 1b and in
5% yield. On the other hand, in order to obtain
apparatus and were uncorrected. IR spectra were
1
(
obtained on a BOMEM DA8 FT-IR Spectrometer. H
13
L
and C NMR spectra were recorded on a Bruker AM
300 (300 MHz) Spectrometer. Mass spectra (EI or
FAB) were determined on a Micro Mass Platform II
8410E Spectrometer. Optical rotations were measured
with a JASCO-DIP-360 digital polarimeter.
8
D
-erythro-sphingosine 1a, enone 5 were first depro-
tected with 2N HCl in MeOH–THF under reflux to
give 3-ketosphingosine 7. Reduction of 3-ketosphin-
gosine 7 with Zn(BH ) in THF at −78°C produced
4
2
D
-erythro-sphingosine 1a in 90% d.e., presumably via a
4.2. L-(N-Trityl-O-tert-butyldimethylsilyl)serine methyl
ester 3
cyclic Felkin–Anh transition state, probably due to the
high chelating ability of the zinc ion. The anti stereo-
chemistry of the reduction is in accord with our previ-
To a solution of -serine methyl ester HCl 2 (0.5 g, 3.2
L
6
ous observations. Acetylation of
-threo-sphingosine with acetic anhydride afforded the
D
-erythro- and
mmol) and Et N (1 mL, 7.3 mmol) in CH Cl (30 mL)
3 2 2
L
at −20°C under an N2 atmosphere was added tert-
butyldimethylsilyl chloride (0.85 g, 3.85 mmol). The
resulting solution was stirred for 48 h at room tempera-
triacetates 8a,b, which were easier to purify and charac-
terize. Their spectroscopic and physical properties were
O
O
b
c
CO2CH3
.
a
CO2CH3
NHTr
P(OMe)2
HO
TBSO
TBSO
NH2 HCl
NHTr
2
3 (95%)
4 (quant.)
O
OH
OR
d
e
TBSO
C13H27
TBSO
C13H27
RO
C13H27
NHTr
(95%)
NHTr
NHR
1b : R=H (85%)
b : R=Ac (95%)
5
6 (90%, 92%de)
g
8
e
O
OR
f
HO
C13H27
RO
C13H27
.
NH2 HCl
(75%)
NHR
1
8
a : R=H (75%, 90%de)
a : R=Ac (97%)
7
g
Scheme 2. Reagents and conditions: (a) (i) TBSCl, Et N, CH Cl , rt, (ii) TrCl, Et N, CH Cl , rt; (b) LiCH PO(OMe) , THF,
3
2
2
3
2
2
2
2
−
78°C; (c) C H CHO, DBU, LiCl, THF, rt; (d) NaBH , CeCl ·7H O, MeOH, 0°C; (e) 2N HCl, THF–MeOH, reflux; (f)
13 27 4 3 2
Zn(BH ) , THF, −78°C; (g) Ac O, pyridine, 0°C.
4
2
2

J.-M. Lee et al. / Tetrahedron: Asymmetry 13 (2002) 343–347
345
ture. To the solution were added trityl chloride (0.98 g,
.52 mmol) and then Et N (0.6 mL, 4.2 mmol) and the
4.5. (2S,3S,4E)-2-[N-(Trityl)amino]-1-O-t-butyldimethyl-
silyl-4-octadecen-1,3-diol 6
3
3
resulting mixture was heated under reflux for 2 h. After
cooling to room temperature, water (15 mL) was added
to quench the reaction. The mixture was extracted with
CH Cl (3×30 mL) and the extract was washed with
To a stirred solution of 5 (2.4 g, 3.67 mmol) in MeOH
(100 mL) at 0°C were added CeCl ·7H O (2.73 g, 7.34
3
2
mmol) and then NaBH (0.56 g, 14.68 mmol). After 3
2
2
4
brine, dried (MgSO ) and passed through a short pad
h, water was slowly added to quench the reaction. The
4
of silica gel. After concentration, the pale yellow
mixture was extracted with EtOAc (3×100 mL) and the
residue was crystallized from EtOH/H O (4:1) to give 3
extract was washed with brine, dried (MgSO ) and
2
4
as a white solid (1.53, 95%). Compound 3: mp 88–89°C.
concentrated under reduced pressure. The resulting oil
was purified by flash chromatography using n-hexane/
EtOAc (30:1) to give 6 as a colorless oil (2.17 g, 90%,
2
5
[
2
h] =+45.8 (c 1.0, CHCl ). IR (NaCl film): w=3443,
D
3
−
1 1
951, 1734, 1113 cm . H NMR (CDCl ) l −0.04 (3H,
3
1
25
s), −0.01 (3H, s), 0.82 (9H, s), 2.68 (1H, brs), 3.14 (3H,
s), 3.44 (1H, brs), 3.60 (1H, dd, J=9.6, 7.2 Hz), 3.87
92% d.e. based on H NMR). Compound 6: [h] =
D
−10.9 (c 3.0, CHCl ), IR (NaCl film): w=3479, 2926,
3
13
−1
1
(
1H, dd, J=9.6, 7.2 Hz), 7.10–7.48 (15H, m). C NMR
2854, 1673, 1464 cm . H NMR (CDCl ) l −0.13 (3H,
3
(
CDCl ) l −5.5, −5.4, 18.2, 25.7, 51.4, 58.3, 66.1, 70.6,
s), −0.11 (3H, s), 0.82 (9H, s), 0.88 (3H, t, J=6.3 Hz),
1.26–1.35 (22H, m), 2.01 (2H, m), 2.64 (1H, dd, J=9.8,
6.2 Hz), 2.79 (1H, m), 3.03 (1H, dd, J=9.8, 2.3 Hz),
3.80 (1H, dd, J=6.8, 6.7 Hz), 5.39 (1H, dd, J=15.3, 7.5
Hz), 5.64 (1H, dt, J=15.3, 6.7 Hz), 7.16–7.55 (15H, m).
3
1
26.4, 127.8, 128.8, 146.0, 174.4. EIMS: m/z=973
+
(
7
2M+Na) . Anal. calcd for C H NO Si: C, 73.22; H,
29 37 3
.84; N, 2.94. Found: C, 73.53; H, 7.74; N, 2.97%.
4
.3. [4-(tert-Butyldimethylsilanyloxy)-3-tritylamino-2-
1
3
C NMR (CDCl ) l −5.2, −5.3, 14.5, 18.5, 23.1, 26.2,
3
oxo-butyl]phosphonic acid dimethyl ester 4
2
7
9.7, 29.8, 29.9, 30.0, 30.1, 32.3, 32.9, 56.7, 62.3, 71.3,
4.2, 127.0, 128.3, 129.2, 130.3, 135.1, 147.1, 157.7.
To a stirred solution of dimethyl methylphosphonate
+
FABMS: m/z=656 (M+H) . HRMS (EI, m/z): calcd
(
11.8 mL, 105.1 mmol) in dry THF (300 mL) at −78°C
for C H NO Si 655.4785, found 655.4785.
43
65
2
under an N atmosphere was added n-BuLi (1.6 M in
2
n-hexane, 92 mL, 147.1 mmol) over 0.5 h. After stirring
at the same temperature for 0.5 h, a solution of 3 (10 g,
4
.6. L-threo-Sphingosine 1b
2
1.0 mmol) in dry THF (30 mL) was added. The
To a stirred solution of 6 (350 mg, 0.53 mmol) in THF
3 mL) and MeOH (10 mL) was added 2N aqueous
resulting mixture was allowed to slowly warm to −20°C
(
and then quenched with saturated aq. NH Cl (200 mL).
4
HCl solution (1 mL). The resulting mixture was stirred
at 40°C for 5 h and then cooled to room temperature.
The mixture was washed with n-hexane (2×20 mL) and
evaporated under reduced pressure. The resulting
The mixture was extracted with EtOAc (3×200 mL),
and the extract was washed with brine, dried (MgSO4)
and passed through a short pad of silica gel. After
concentration, 4 was obtained quantitatively as a yel-
low oil (12 g) and used in the next step without further
purification.
residue was dissolved in H O (20 mL) and adjusted to
2
pHꢀ10 with 1N NaOH. The mixture was extracted
with chloroform (3×20 mL) and the extract was washed
with brine, dried (Na SO ) and concentrated to provide
4
.4. (2S,4E)-2-[N-(Trityl)amino]-1-O-tert-butyldimethyl-
2
4
silyl-3-oxo-4-octadecene 5
1b as a white solid (136 mg, 85%). L-threo-Sphingosine
2
5
10c
1
b: mp 87–88°C. [h] =+1.5 (c 0.5, CHCl ). {lit. mp
24 1
D
3
A solution of 4 (11 g, 19.4 mmol), tetradecyl aldehyde
86–87°C. [h] =+2.7 (c 1.0, CHCl )}. H NMR
D 3
(
8.24 g, 38.8 mmol), DBU (2.94 mL, 19.2 mmol) and
(CDCl ) l 0.88 (3H, t, J=6.4 Hz), 1.26–1.37 (22H, m),
3
LiCl (1.65 g, 38.8 mmol) in dry THF (170 mL) was
2.02–2.14 (6H, m), 2.81 (1H, m), 3.55 (1H, dd, J=10.8,
4.2 Hz), 3.68 (1H, dd, J=10.8, 4.2 Hz), 4.01 (1H, dd,
J=6.0, 5.9 Hz), 5.45 (1H, dd, J=15.4, 6.7 Hz), 5.75
stirred at room temperature under an N atmosphere.
2
After 5 h, 1 M citric acid (50 mL) was added to quench
the reaction. The mixture was extracted with EtOAc
13
(1H, dt, J=15.4, 6.7 Hz). C NMR (CDCl ) l 14.5,
3
(
3×150 mL) and the extract was washed with brine,
23.1, 29.6, 29.7, 29.8, 29.9, 30.0, 30.1, 32.3, 32.7, 56.9,
65.2, 74.3, 130.2, 134.7. EIMS: m/z=300 (M+H) .
+
dried (MgSO ) and concentrated under reduced pres-
sure. The residue was chromatographed on silica gel
4
(
(
n-hexane/EtOAc=30:1) to yield 5 as a pale yellow oil
4.7. L-threo-Sphingosine-N,O,O-triacetate 8b
2
5
12 g, 95%). Compound 5: [h] =+59.6 (c 1.19, CHCl ).
D
3
IR (NaCl film): w=3445, 2926, 2854, 1692, 1627, 1464,
1
(
To a stirred solution of 1b (24 mg, 0.08 mmol) in
−
1
1
105 cm . H NMR (CDCl ) l −0.04 (3H, s), −0.01
pyridine (0.5 mL) at 0°C under an N atmosphere were
3
2
3H, s), 0.82 (9H, s), 0.88 (3H, t, J=6.3 Hz), 1.26–1.39
added consecutively acetic anhydride (39 mg, 0.38
mmol) and DMAP (cat.). The mixture was stirred at
room temperature for 1 h, and then poured into water
and extracted with EtOAc (3×10 mL). The combined
(
22H, m), 1.98 (2H, m), 3.25 (1H, brs), 3.42 (1H, dd,
J=9.5, 7.5 Hz), 3.68 (1H, m), 3.84 (1H, dd, J=9.5, 4.4
Hz), 5.80 (1H, d, J=15.7 Hz), 6.35 (1H, dt, J=15.7,
1
3
6
−
.8 Hz), 7.11–7.49 (15H, m). C NMR (CDCl ) l −5.5,
extract was washed with brine, dried (MgSO ) and
3
4
5.6, 14.1, 18.2, 22.7, 25.8, 28.0, 29.3, 29.4, 29.5, 29.7,
concentrated under reduced pressure. Recrystallization
from n-hexane afforded 8b as a white solid (32 mg,
95%) which was identical in all spectroscopic detail to
3
1
1.9, 32.2, 61.3, 66.7, 70.9, 126.3, 127.8, 128.5, 129.0,
+
46.0, 146.5, 203.0. FABMS: m/z 654 (M+H) . HRMS
1
0c
(
EI, m/z): calcd for C H NO Si 653.4628, found
the literature data.
L-threo-Sphingosine-N,O,O-tri-
4
3
63
2
25
6
53.4637.
acetate 8b: mp 42–44°C. [h] =+7.65 (c 0.56, CHCl ).
D
3

3
46
J.-M. Lee et al. / Tetrahedron: Asymmetry 13 (2002) 343–347
1
0c
25
1
{
lit. mp 42–44°C. [h] =+7.0 (c 2.05, CHCl )}. H
Hz), 4.40 (1H, m), 5.25 (1H, pseudo t, J=6.1 Hz),
13
D
3
NMR (CDCl ) l 0.87 (3H, t, J=6.7 Hz), 1.17–1.40
5.36 (1H, dd, J=15.2, 7.4 Hz), 5.62–5.80 (2H, m).
C
3
(
(
5
2
6
22H, m), 1.99–2.07 (11H, m), 4.00–4.12 (2H, m), 4.39
1H, m), 5.33–5.42 (2H, m), 5.66 (1H, d, J=9.3 Hz).
NMR (CDCl ) l 14.5, 21.2, 21.5, 23.0, 23.7, 29.2,
29.6, 29.7, 29.8, 30.0, 30.1, 32.3, 32.7, 51.0, 63.0,
124.5, 13 7.8, 170.1, 170.4, 171.4.
3
1
3
.74 (2H, m). C NMR (CDCl ) l 14.5, 21.5, 23.1,
3
3.7, 29.2, 29.5, 29.7, 29.8, 30.0, 30.1, 32.3, 32.7, 51.3,
3.5, 124.4, 137.8, 170.3, 170.5, 171.1.
4
.11. L-erythro-Sphingosine 1c
4
.8. (2S,4E)-2-Amino-3-oxo-4-octadecen-1-ol·HCl 7
In the same manner as described for 1a, 1c was pre-
pared from commercially available -serine methyl
ester HCl. -erythro-Sphingosine 1c: mp 80–82°C.
D
To a stirred solution of 5 (0.6 g, 0.92 mmol) in MeOH
L
(
(
10 mL) and THF (2 mL) was added 2N HCl solution
1 mL). This reaction mixture was heated under reflux
25
10d
[
h] =+2.1 (c 0.53, CHCl ). {lit.
mp 81–82°C.
D
3
24
[
h] =+2.8 (c 0.6, CHCl )}. Its spectroscopic data
for 1 h. After cooling, the reaction mixture was
washed with n-hexane (3×20 mL) and then evaporated
under reduced pressure. The resulting residue was
D
3
10b
were identical to the literature data.
recrystallized from i-PrOH/Et O (1:3) to afford 7 as a
2
4
.12. -threo-Sphingosine 1d
D
white solid (0.23 g, 75%). (2S,4E)-2-Amino-3-oxo-4-
2
5
octadecen-1-ol·HCl 7: mp 148–150°C. [h] =+24.4 (c
D
In the same manner as described for 1b, 1d was pre-
pared from commercially available -serine methyl
ester HCl. -threo-Sphingosine, 1d: mp 85–87°C.
0
1
.95, MeOH). IR (NaCl film): w=3444, 2921, 2851,
D
−1 1
673, 1635 cm . H NMR (CD OD) l 0.90 (3H, t,
3
D
J=6.9 Hz), 1.29–1.55 (22H, m), 2.31 (2H, m), 3.99
2H, m), 4.43 (1H, dd, J=4.7, 3.9 Hz), 6.42 (1H, d,
2
5
10c
[
[
h] =−2.15 (c 0.48, CHCl ). {lit.
mp 85–87°C.
D
3
(
25
h] =−2.65 (c 1.13, CHCl )}. Its spectroscopic data
13
D
3
J=15.8 Hz), 7.16 (1H, dt, J=15.8, 6.8 Hz). C NMR
CD OD) l 13.4, 22.7, 28.0, 29.3, 29.4, 29.5, 29.6,
10c
were identical to the literature data.
(
3
2
9.7, 32.0, 32.8, 59.7, 59.8, 75.8, 125.6, 152.3, 192.8.
+
FABMS: m/z=298 (M+H) . Anal. calcd for
C H ClNO : C, 64.74; H, 10.87; N, 4.19. Found: C,
18
36
2
Acknowledgements
64.65; H, 10.84; N, 4.30%.
4
.9. -erythro-Sphingosine 1a
D
Financial support from the Korea Science and Engi-
neering Foundation and the Ministry of Education/
BSRI Fund is gratefully acknowledged.
To a stirred solution of 7 (0.3 g, 0.9 mmol) in dry
THF (30 mL) at −78°C under an N atmosphere was
2
added Zn(BH ) (0.1M in THF, 1.8 mL, 1.8 mmol)
4
2
dropwise. After 5 h, the reaction mixture was cau-
tiously quenched with water (30 mL), adjusted to
pHꢀ1 with 1N aqueous HCl, washed with n-hexane
and then adjusted to pHꢀ10 with 1N aqueous NaOH.
The mixture was extracted with chloroform (3×30
mL), and the extract was washed with brine, dried
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(
Na SO ) and concentrated in vacuo. Recrystallization
2
4
from CHCl /Et O/n-hexane (1:1:4) gave 1a as a white
solid (0.2 g, 75%, 90% d.e. based on H NMR).
thro-Sphingosine 1a: mp 79–82°C. [h] =−1.5 (c 0.52,
CHCl ). {lit.
3
2
1
D
-ery-
2
5
D
1
0d
25
mp 81–82°C. [h] =−2.8 (c 1.0,
D
3
1
CHCl )}. H NMR (CDCl ) l 0.86 (3H, t, J=6.9 Hz),
3
3
1
.20–1.37 (22H, m), 2.05 (1H, q, J=6.7 Hz), 2.87 (1H,
q, J=5.5 Hz), 3.66 (2H, m), 4.05 (1H, t, J=6.2 Hz),
5
6
2
1
.47 (1H, dd, J=15.4, 7.1 Hz), 5.75 (1H, dt, J=15.4,
1
3
.7 Hz). C NMR (CDCl ) l 14.5, 23.1, 29.6, 29.7,
3
9.8, 29.9, 30.0, 30.1, 32.3, 32.8, 56.5, 64.5, 75.9,
29.7, 135.2.
4
4.10.
D
-erythro-Sphingosine-N,O,O-triacetate 8a
In the same manner as described for 8b, compound 8a
was prepared in 97% yield. -erythro-Sphingosine-
N,O,O-triacetate 8a: mp 101–102°C. [h] =−13.2 (c
5. For reviews, see: (a) Merrill, A. H., Jr.; Hannun, Y. A.
Methods Enzymol. 2000, 311, 391–479; (b) Koskinen, P.
M.; Koskinen, A. M. P. Synthesis 1998, 1075–1091.
6. Chung, S. K.; Lee, J. M. Tetrahedron: Asymmetry 1999,
10, 1441–1444.
D
2
5
D
10c
25
1
1
6
.04, CHCl ). {lit. mp 102.5–103°C, [h] =−13.0 (c
3 D
1
.08, CHCl )}. H NMR (CDCl ) l 0.85 (3H, t, J=
.4 Hz), 1.20–1.35 (22H, m), 1.95–2.04 (11H, m), 4.02
1H, dd, J=11.5, 3.9 Hz), 4.26 (1H, dd, J=11.5, 6.1
3
3
7. Eliel, E. L. Asymmetric Synthesis; Academic Press: New
York, 1983; Vol. 2, pp. 125–155.
(

J.-M. Lee et al. / Tetrahedron: Asymmetry 13 (2002) 343–347
347
8
. (a) Chung, S. K.; Kang, D. H. Tetrahedron: Asymmetry
997, 8, 3027–3030; (b) Blanchette, M. A.; Choy, W.;
3985.
1
10. (a) Johnson, D. V.; Felfer, U.; Griengl, H. Tetrahedron
2000, 56, 781–790; (b) Enders, D.; Whitehouse, D. L.;
Runsink, J. Chem. Eur. J. 1995, 1, 382–388; (c) Garner,
P.; Park, J. M.; Malechi, E. J. Org. Chem. 1988, 53,
4395–4398; (d) Shibuya, H.; Kawashima, K.; Ikeda, M.;
Kitagawa, I. Tetrahedron Lett. 1989, 30, 7205–7208.
Davis, J. T.; Essinfeld, A. P.; Masamune, S; Roush, W.
R.; Sakai, T. Tetrahedron Lett. 1984, 25, 2183–2186; (c)
Abiko, A.; Masamune, S. Tetrahedron Lett. 1996, 37,
1
077–1080.
9
. Hoffman, R. V.; Tao, J. J. Org. Chem. 1998, 63, 3979–