Highly enantioselective synthesis of syn- and anti-propionate aldols without diastereoselection in the chiral oxazaborolidinone-promoted aldol reaction with a silyl ketene acetal derived from ethyl 2- (methylthio)propionate

Article abstract of DOI:10.1016/S0957-4166(98)00159-1

A mixture of syn- and anti-aldol products containing an α-methylthio group were obtained in good yields with high enantioselectivities in the chiral oxazaborolidinone-promoted aldol reactions of a novel silyl ketene acetal, derived from ethyl 2-(methylthio)propionate, with aldehydes. Subsequent desulfurization resulted in an effective preparation of essentially enantiopure syn- and anti-propionate aldols which were separable.

Full text of DOI:10.1016/S0957-4166(98)00159-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 1883–1890
Highly enantioselective synthesis of syn- and anti-propionate
aldols without diastereoselection in the chiral oxazaborolidinone-
promoted aldol reaction with a silyl ketene acetal derived from
ethyl 2-(methylthio)propionate
Mostofa Abu Hena,a Saori Terauchi,b Chul-Sa Kim,a Michio Horiike a and
Syun-ichi Kiyooka b,∗
aDepartment of Biosources Science, Kochi University, Nankoku, Kochi 783-0093, Japan
bDepartment of Chemistry, Kochi University, Akebono-cho, Kochi 780-8072, Japan
Received 12 March 1998; accepted 20 April 1998
Abstract
A mixture of syn- and anti-aldol products containing an α-methylthio group were obtained in good yields with
high enantioselectivities in the chiral oxazaborolidinone-promoted aldol reactions of a novel silyl ketene acetal,
derived from ethyl 2-(methylthio)propionate, with aldehydes. Subsequent desulfurization resulted in an effective
preparation of essentially enantiopure syn- and anti-propionate aldols which were separable. © 1998 Elsevier
Science Ltd. All rights reserved.
1. Introduction
Enantioselective synthesis of syn- and anti-propionate aldols is quite important for the asymmetric
synthesis of natural products bearing 1,3-polyol moieties. Considerable success has been reported in the
field of asymmetric aldol reactions with chiral nucleophiles related to propionate synthones bound to a
variety of chiral auxiliaries.1 In spite of the development of chiral Lewis acid mediated aldol reactions,
the enantioselective synthesis of propionate aldols with high diastereoselectivity is still far from attaining
a practical level. For chiral borane-catalyzed (promoted) aldol reactions with silyl ketene acetals, with
regard to propionate synthones, Yamamoto reported a highly enantioselective synthesis of syn-propionate
aldols with chiral (acyloxy)borane (CAB) reagents, derived from L- or D-tartaric acid, in which the
syn predominance is obtained independent of the geometry of the silyl ketene acetals.2 However, the
reaction using chiral oxazaborolidinones resulted in the preparation of a slight excess of anti-propionate
∗
Corresponding author. E-mail: kiyo@cc.kochi-u.ac.jp
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00159-1

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aldols with moderate enantioselectivity, but the minor syn-isomer showed quite high enantioselectivity.3
In any event, to date, it seems to be difficult to produce each isomer separately with a level of high
enantioselectivity by means of chiral borane-mediated aldol reactions.
2. Results and discussion
We planned the practical enantioselective synthesis of syn- and anti-propionate aldols. We have already
reported an excellent enantioselective synthesis of acetate aldols using the oxazaborolidinone 1 promoted
aldol reaction with silyl ketene acetal 2, bearing a dithiolane, in which the corresponding acetate aldol
was obtained in a good yield with almost complete enantioselectivity after desulfurization with nickel
boride, as shown in Eq. 1.4 The steric bulkiness at the α-position of the nucleophiles resulted in a high
enantioselectivity in the aldol reaction. The success in the preparation of enantiopure acetate aldols with
2 encouraged us to apply a silyl ketene acetal such as 3 to the enantioselective synthesis of propionate
aldols.
(1)
Ethyl 2-(methylthio)propionate 4 was prepared by treatment of the dianion from ethyl 2-
mercaptoacetate with methyl iodide in 82% yield. The corresponding silyl ketene acetal 3 was
obtained according to the usual method in 86% yield which was isolated as a mixture of geometrical
isomers (the E:Z ratio was 1:3 assigned by a NOESY experiment), as shown in Eq. 2.
(2)
Reaction of benzaldehyde with silyl nucleophile 3 in the presence of a stoichiometric amount of 1
smoothly proceeded to give a mixture of syn- and anti-5 in 85% yield, as shown in Eq. 3.

M. A. Hena et al. / Tetrahedron: Asymmetry 9 (1998) 1883–1890
1885
(3)
The isomers were easily separated by silica gel flash column chromatography (7% AcOEt in hexanes).
The structure of syn- and anti-isomers was assigned by using the shielding effects of the phenyl ring to the
methyl chemical shifts of the methylthio and methyl groups at C-2 (the methyl signals at the equatorial
position resonate up-field), as depicted in Fig. 1. The enantioselectivity of syn-5 was determined to
be 94% ee by HPLC with Chiralcel AD (Daicel) (hexanes:2-propanol=95:5) while that of anti-5 was
determined to be 95% ee by using Chiralcel OD (hexanes:2-propanol=95:5). Desulfurization of 5 led to
a mixture of syn- and anti-propionate aldols 6a and 6b in good yields by the treatment with nickel boride
prepared in situ from large excesses of nickel chloride, which was finely ground to a powder, and sodium
borohydride under a hydrogen atmosphere, as shown in Eqs 4 and 5.
(4)
(5)
Assignment of the diastereomers was performed by comparing the carbinol resonances.5 Scrambling
of the stereochemistry at C-2 of 6 during desulfurization was observed, presumably owing to a radical
process, so that similar results on the enantio- and diastereoselectivity were obtained starting from syn-
and anti-5, however, no loss of stereochemistry at C-3 was found during the desulfurization. Direct
desulfurization of a mixture of syn- and anti-5 obtained directly from the reaction resulted in similar
selectivities (Eq. 6).

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Fig. 1.
(6)
(7)
Thus, from the stereochemical point of view, a successive sequence of the aldol reaction followed
by desulfurization is recommended without the isolation of the sulfur-containing intermediates from
the practical point of view. The successive procedure was applied to a variety of aldehydes (Eq. 7)
and the results are summarized in Table 1. The enantiomeric excess of all products was determined
by using Chiralcel AD and OD columns. Each reaction proceeded smoothly for the enantioselective
preparation of propionate aldols. syn- and anti-Isomers of 6a, 6b, and 6c could be separated by silica gel
flash column chromatography, whereas separation of the isomers of 6d was performed by preparative
HPLC (YMC-Pack SIL SH-043-5 with hexanes:2-propanol=99:1). The isomers of phenyl esters of
propionate aldols were found to be more easily separated by simple silica gel chromatography, but the
oxazaborolodinone-promoted aldol reaction did not work with a silyl nucleophile derived from phenyl
2-(methylthio)propionate, presumably because of the steric bulkiness of the nucleophile.
In conclusion, our chiral oxazaborolidinone-promoted asymmetric aldol reaction with 3 achieved a
very highly enantioselective synthesis of propionate aldols without diastereoselection. This is a quite
versatile method to give both enantiomers simultaneously, for example, when both product enantiomers
are required in the course of the synthesis of medicines having a complex stereochemistry. We are
now engaged in a study of the enantioselective acyclic stereoselection with catalyst (promoter) based

M. A. Hena et al. / Tetrahedron: Asymmetry 9 (1998) 1883–1890
1887
Table 1
Enantioselective synthesis of syn- and anti-propionate aldols (Eq. 7)
stereocontrol using this aldol reaction with silyl nucleophile 3.6 Futhermore, we are trying to trap
stereoselectively radical intermediates arising from the desulfurization process.
3. Experimental
3.1. General
Unless otherwise noted, materials were obtained from commercial suppliers and used without further
purification. Diisopropylamine was distilled from CaH2. Dichloromethane was distilled from P2O5.
Tetrahydrofuran (THF) was distilled from sodium–benzophenone immediately prior to use. All reactions
involving organometallic reagents were conducted under an argon atmosphere. IR spectra were deter-
1
mined with a JASCO FT-IR 5300 spectrometer. H-NMR spectra were determined with a Hitachi R-90H
(90 MHz) NMR spectrometer. 13C-NMR spectra were determined with a Hitachi R-90H (22.6 MHz)
NMR spectrometer. Chemical shifts are expressed in ppm downfield from internal tetramethylsilane.
1
Significant H-NMR data are tabulated in order: multiplicity (s, singlet; d, doublet; t, triplet; q, quartet;
m, multiplet), number of protons, coupling constant(s) in hertz. Optical rotations were determined with
a JASCO DIP-370 digital polarimeter. High performance liquid column chromatography (HPLC) was
performed with a JASCO 875-UV/880-PU and a JASCO UV-970/PC-350/PM-350 using Daicel Chiralcel
OD and AD, and YMC-A012 columns. Merck silica gel 60 (230–400 mesh) was used for flash column
chromatography. N-p-Toluenesulfonyl-(S)-valine was prepared by the Schotten–Baumann procedure.
BH3·THF (1.0 M solution in THF) was obtained from Aldrich.
3.2. Ethyl 2-(methylthio)propanoate 4
To a solution of diisopropylamine (5.6 mL, 44 mmol) in THF (40 mL) and TMEDA (6.64 mL, 44
mmol) at 0°C was added dropwise a 1.6 M solution of n-BuLi (27.4 mL, 44 mmol) in hexanes. After
stirring for 30 min, the solution was cooled to −78°C. Ethyl 2-mercaptoacetate (2.2 mL, 20 mmol) was
added dropwise to this solution over 15 min. After 1 h, CH3I (8.0 mL, 120 mmol) was added. After
stirring for 3 h at that temperature, the solution was allowed to warm to room temperature. After 1 h, the
reaction mixture was quenched with a saturated NH4Cl solution and poured into 10% HCl solution. The

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organic layer was extracted with CH2Cl2 and dried over anhydrous MgSO4. The solvent was removed
with a rotary evaporator to give a crude product. The crude product was distilled under reduced pressure
1
(75°C/25 mmHg) (82% yield). IR (neat): 1732 cm−1; H-NMR (CDCl3): δ 1.29 (t, 3H, J=7.04, 7.25),
1.44 (d, 3H, J=7.25), 2.16 (s, 3H), 3.33 (q, 1H, J=7.03, 7.25), 4.20 (q, 2H, J=7.04, 7.25).
3.3. 1-Trimethylsiloxy-1-ethoxy-2-methyl-2-methylthioethene 3
To a solution of diisopropylamine (2.12 mL, 16.2 mmol) in THF (45 mL) at 0°C was added dropwise
a 1.6 M solution of n-BuLi (10.13 mL, 16.2 mmol) in hexanes. After stirring for 30 min, the solution was
cooled to −78°C. To this solution was added ethyl 2-(methylthio)propanoate (2.0 g, 13.5 mmol) dropwise
over 10 min. After 30 min, TMSCl (5.14 mL, 40.5 mmol) was added and the solution was stirred for 15
h at room temperature. After the solvent was removed with a rotary evaporator, the resulting mixture was
filtered with suction through a Celite pad and washed with hexanes. The hexane extract was removed
and the residue was distilled under reduced pressure (110°C–120°C/30 mmHg) to give a mixture of pure
isomers (82% yield). IR (neat): 1651 cm−1; 1H-NMR (CDCl3) of major isomer: δ 0.20 (s, 9H), 1.16 (t,
3H, J=7.03, 7.04), 1.80 (s, 3H), 2.05 (s, 3H), 3.77 (q, 2H, J=7.03, 7.04); minor isomer: δ 1.23 (t, 3H,
J=7.03), 1.76 (s, 3H), 2.08 (s, 3H), 4.13 (q, 2H, J=7.2).
3.4. General procedure for the chiral oxazaborolidinone-promoted aldol reaction with 3. A mixture of
ethyl (2S,3R)- and (2R,3R)-3-hydroxy-2-methyl-2-methylthio-3-phenylpropanoate syn-5 and anti-5
To a solution of N-p-toluenesulfonyl-(S)-valine (542 mg, 2.2 mmol) in 10 mL of CH2Cl2 at 0°C was
added BH3·THF (2.0 mmol, 2.0 mL of a 1.0 M solution in THF) over 10 min and the solution was stirred
for 30 min. After stirring for an additional 30 min at room tenperature, the solution was cooled to −78°C.
To this solution was added benzaldehyde (212 mg, 2.0 mmol) in CH2Cl2 over 5 min. After stirring for
10 min, 1-trimethylsiloxy-1-ethoxy-2-methyl-2-methylthioethene (484 mg, 2.2 mmol) was added. After
stirring for 3 h, the reaction mixture was quenched with a buffer solution (pH 6.8) at that temperature and
extracted with CH2Cl2. The organic layer was washed with a saturated NaHCO3 solution. The extract was
dried over anhydrous MgSO4 and the solvent was removed to give a crude product. The crude product
was purified by flash column chromatography (7.5% AcOEt in hexanes) (85% yield). The enantiomeric
excess of each isomer was determined by HPLC. IR (neat): 3489, 1714 cm−1; syn-5: [α]D24 36.4 (c=1.24,
CHCl3); 1H-NMR (CDCl3): δ 1.25 (t, 3H, J=7.04, 7.25), 1.24 (s, 3H), 2.23 (s, 3H), 3.25 (d, 1H, J=1.54),
4.17 (q, 2H, J=7.15), 5.19 (d, 1H, J=1.31), 7.23–7.42 (m, 5H); 13C-NMR (CDCl3): δ 12.7, 14.0, 16.4,
53.4, 56.5, 61.1, 72.5, 127.7, 138.2, 171.4; anti-5: [α]D −18.7 (c=0.79, CHCl3); 1H-NMR (CDCl3): δ
24
1.32 (t, 3H, J=7.03, 7.25), 1.35 (s, 3H), 2.08 (s, 3H), 3.14 (d, 1H, J=4.83), 4.27 (q, 2H, J=7.14), 5.12 (d,
1H, J=4.62), 7.25–7.44 (m, 5H); 13C-NMR (CDCl3): δ 13.4, 15.0, 17.6, 54.2, 56.2, 62.4, 77.0, 128.3,
128.7, 139.5, 173.6. Anal. calcd for C13H18O3S: C 61.39, H 7.13. Found: C 61.30, H 7.14.
3.5. General procedure for desulfurization of aldol products involving a dithiuolane. A mixture of ethyl
(2S,3S)- and (2R,3S)-3-hydroxy-2-methyl-3-phenylpropanoate syn-6a and anti-6a
A solution of a mixture of syn- and anti-ethyl 3-hydroxy-2-methyl-2-methylthio-3-phenylpropanoate
(99 mg, 0.39 mmol), obtained from the reaction (Eq. 3), in ethanol (15 mL) was kept under H2 pressure.
Anhydrous NiCl2 (1.01 g, 20 equiv.) was introduced into this solution and then NaBH4 (148 mg, 10
equiv.) was added rapidly at 0°C. The mixture was stirred vigorously for the next 15 h. The reaction
mixture was filtered, evaporated, extracted with CH2Cl2, and dried over anhydrous MgSO4. After

M. A. Hena et al. / Tetrahedron: Asymmetry 9 (1998) 1883–1890
1889
evaporation of the solvent, the crude product was purified by flash column chromatography (10% AcOEt
in hexanes) to give a mixture of pure isomers (86% yield). IR (neat): 3454, 1716 cm−1; syn-6a: [α]D
17
−22.0 (c=0.87, CHCl3); 1H-NMR (CDCl3): δ 1.14 (d, 3H, J=7.26), 1.19 (t, 3H, J=7.04, 7.25), 2.63–2.91
(dq, 1H, J=3.96, 7.26), 2.97 (d, 1H, J=3.08), 4.11 (q, 2H, J=7.03, 7.25), 5.02–5.10 (dd, 1H, J=3.08, 3.96),
7.31 (br.s, 5H); 13C-NMR (CDCl3): δ 11.0, 14.1, 53.4, 60.6, 73.7, 126.0, 127.3, 128.1, 141.5, 175.5; anti-
6a: [α]D −15.3 (c=1.11, CHCl3); 1H-NMR (CDCl3): δ 1.02 (d, 3H, J=7.25), 1.20 (t, 3H, J=7.03, 7.25),
17
2.71–2.88 (dq, 1H, J=7.25, 8.24), 3.03 (d, 1H, J=4.61), 4.17 (q, 2H, J=7.03, 7.25), 4.67–4.81 (dd, 1H,
J=4.61, 8.24), 7.32 (br.s, 5H). 13C-NMR (CDCl3): δ 14.9, 15.2, 47.9, 61.4, 77.0, 127.3, 128.6, 129.0,
142.3, 176.3. Anal. calcd for C12H16O3: C 69.21, H 7.74. Found: C 70.02, H 7.77.
3.6. Ethyl (2S,3R)- and (2R,3R)-3-hydroxy-2-methyl-5-phenylpentanoate 6b
25
1
IR (neat): 3447, 1730 cm−1; syn-6b: [α]D 22.3 (c=2.20, CHCl3); H-NMR (CDCl3): δ 1.18 (d,
3H, J=7.5), 1.25 (t, 3H, J=7.0), 1.6–2.1 (m, 2H), 2.52 (dq, 1H, J=3.8, 7.0), 2.5–3.1 (m, 2H), 2.65 (d, 2H,
J=4.6), 3.87 (dt, 1H, J=3.95, 7.0), 4.14 (q, 2H, J=7.0), 7.21 (s, 5H); anti-6b: [α]D25 9.2 (c=1.97, CHCl3);
1H-NMR (CDCl3): δ 1.20 (d, 3H, J=7.5), 1.24 (t, 3H, J=7.0), 1.6–2.0 (m, 3H), 1.7–2.0 (m, 2H), 2.55
(quint, 1H, J=7.3), 2.7–3.1 (m, 2H), 2.76 (d, 2H, J=5.05), 3.66 (dt, 1H, J=6.5, 7.0), 4.16 (q, 2H, J=7.0),
7.22 (s, 5H).
3.7. Ethyl (2S,3R)- and (2R,3R)-2,4-dimethyl-3-hydroxypentanate 6c
IR (neat): 3497, 1718 cm−1; syn-6c: [α]D25 5.16 (c=0.775, CHCl3); 1H-NMR (CDCl3): δ 0.87 (d, 3H,
J=6.6), 1.00 (d, 3H, J=6.6), 1.17 (d, 3H, J=7.0), 1.26 (t, 3H, J=7.2), 1.68 (2×q, 1H, J=6.6), 2.50 (d,
25
1H, J=3.0), 2.64 (dq, J=3.5, 7.0), 3.55 (dd, 1H, J=3.5, 7.5), 4.15 (q, 2H, J=7.2); anti-6c: [α]D −6.0
1
(c=1.66, CHCl3); H-NMR (CDCl3): δ 0.92 (d, 3H, J=6.6), 0.96 (d, 3H, J=6.6), 1.21 (d, 3H, J=7.0),
1.27 (t, 3H, J=7.2), 1.5–2.0 (m, 1H), 2.5–2.9 (m, 1H), 2.60 (d, 1H, J=7.0), 3.36 (dd, 1H, J=5.7, 7.0), 4.16
(q, 2H, J=7.2).
3.8. Ethyl (2S,3R)- and (2R,3R)-3-hydroxy-2-methylhexanate 6d
IR (neat): 3447, 1732 cm−1; syn-6d: [α]D 5.4 (c=1.48, CHCl3); H-NMR (CDCl3): δ 0.93 (t, 3H,
J=6.5), 1.17 (d, 3H, J=7.2), 1.27 (t, 3H, J=7.0), 1.4–1.7 (m, 4H), 3.4–3.8 (m, 1H), 4.08 (q, 2H, J=7.0);
25
1
25
anti-6d: [α]D −55.5 (c=0.59, CHCl3); 1H-NMR (CDCl3): δ 0.93 (t, 3H, J=6.5), 1.20 (d, 3H, J=7.2),
1.27 (t, 3H, J=7.2), 1.3–1.6 (m, 4H), 2.4–2.8 (m, 2H), 3.6–3.9 (m, 1H), 4.17 (q, 2H, J=7.0).
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (nos. 9231236 and 9554046) from
the Ministry of Education.
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