Stereoselective reduction of β-hydroxy α-ketoesters: A concise synthesis of anti-α,β-dihydroxy esters

Article abstract of DOI:10.1055/s-2004-831210

A new synthetic route to anti-α,β-dihydroxy esters has been developed. The new method consists of three steps starting from an aldehyde: the nucleophilic condensation with ethyl diazoacetate, oxidation with dimethyldioxirane, and stereoselective reduction with NaBH₄.

Full text of DOI:10.1055/s-2004-831210

PAPER
2633
Stereoselective Reduction of b-Hydroxy a-Ketoesters: A Concise Synthesis of
anti-a,b-Dihydroxy Esters
Stereoselective
R
educt
i
ionof
n
-Hydroxy
a
g
-Ketoesters yi Liao, Wengang Yao, Jianbo Wang*
Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Department of Chemical Biology, College
of Chemistry, Peking University, Beijing 100871, P. R. of China
E-mail: wangjb@pku.edu.cn
Received 25 May 2004; revised 12 July 2004
The b-hydroxy a-ketoesters 2a–k were prepared by the
Abstract: A new synthetic route to anti-a,b-dihydroxy esters has
oxidation of the b-hydroxy a-diazoesters 1a–k with di-
been developed. The new method consists of three steps starting
from an aldehyde: the nucleophilic condensation with ethyl diazo-
methyldioxirane (DMD).6 The b-hydroxy a-diazoesters
1a–k were easily obtained by the nucleophilic condensa-
tion of aldehydes with ethyl diazoacetate.7 The oxidation
of 1a–k with dimethyldioxirane (DMD) in acetone is
highly efficient (–35 °C, 15–30 min, Scheme 1). The b-
hydroxy a-ketoesters 2a–k thus obtained were found to be
unstable in silica gel column. Consequently, they were
used in the subsequent reduction without further purifica-
tion.
acetate, oxidation with dimethyldioxirane, and stereoselective re-
duction with NaBH4.
Key words: diazo compounds, diols, nucleophilic addition, oxida-
tion, reduction, diastereoselectivity
The reduction of a-ketols is an important approach lead-
ing to 1,2-diol compounds. The stereoselectivity of the re-
duction has been extensively studied over the past
decades, and it has been well known that the stereochem-
ical process is controlled by either the Cram model or by
the chelation model.1,2 However, to our knowledge, there
has been no report on the reduction of b-hydroxy a-ke-
toesters. The reaction will give a,b-dihydroxy carbonyl
compounds, which are important as natural product frag-
ments and also as building blocks in organic synthesis. Al-
though the structure is quite simple, the stereoselective
synthesis of this type of compounds is limited. The most
common approach is the dihydroxylation of the a,b-unsat-
urated carbonyl compounds.3 Since the easily available
a,b-unsaturated carbonyl compounds usually have trans
configuration, the dihydroxylation method therefore in
most cases gave only syn-a,b-dihydroxy carbonyl com-
pounds. To the best of our knowledge, the only general
method to prepare anti a,b-dihydroxy carbonyl com-
pounds with high diastereoselectivity is the aldol conden-
sation of glycolate enolate with aldehydes.4,5 Here we
report a concise and highly diastereoselective approach to
this type of compounds through the reduction of easily
available b-hydroxy a-ketoesters.
Table 1 Reduction of a-Ketoesters 2a
OH
O
OH
O
[H]
Ph
OEt
Ph
OEt
O
OH
( )-3a
2a
Entry
Hydride
NaBH4
NaBH4
NaBH4
Solvent
EtOH
EtOH
EtOH
EtOH
EtOH
THF
Temp (°C)
dra
88:12
1
2
–35
–78
89:11
90:10
76:24
73:27
87:13
91:9
3
–98
4
NaBH(OAc)3
NaBH3(CN)
Zn(BH4)2
NaBH4
–78
5
–78
6
–35
7
THF
–78
8
NaBH4
CH2Cl2
CH2Cl2
CH2Cl2
–78 to r.t.b
–41
>95:5
90:10
>95:5
9
NaBH4
10
NaBH4
–63 to r.t.
O
OH
O
OH O
a The diastereomeric ratio was determined by the 1H NMR (300 MHz)
of the crude product.
O
R
OEt
R
OEt
Acetone, –35 °C
15–30 min
b The reduction at –78 °C was very slow.
N2
1a–k
O
2a–k
First, a-ketoesters 2a (R = Ph) was used as the substrate to
optimize the reduction condition. When sodium borohy-
dride was used as the reducing agent in EtOH, 2a was re-
duced efficiently to give a,b-dihydroxy ester, but the
diastereoselectivity is moderate (88:12) (Table 1, entry 1).
The reduction also proceeded well at low temperature;
however, the stereoselectivity almost remained the same.
Moreover, it was found that diastereoselectivity could not
Scheme 1
SYNTHESIS 2004, No. 16, pp 2633–2636
Advanced online publication: 16.09.2004
DOI: 10.1055/s-2004-831210; Art ID: F07304SS
x
x.
x
x
.
2
0
0
4
© Georg Thieme Verlag Stuttgart · New York

2634
M. Liao et al.
PAPER
O
be improved by changing the reducing agent to
NaBH(OAc)3, NaBH3(CN) or Zn(BH4)2. The effect of the
solvent was then examined. We were delighted to find that
with CH2Cl2 as the solvent the stereoselectivity could be
O
HO
H
HO
Ph
CO2Et
Felkin's model
H
H
EtO2CPh
H
1
dramatically improved. H NMR inspection of the crude
Cram's model
product suggested that only one diastereoisomer was
formed. The configuration of the main product was deter-
mined to be anti by comparing with the 1H NMR spectrum
of the syn product, which was prepared by the dihydroxyl-
ation of ethyl cinnamate.
Figure 1
for the observed stereoselectivity, b-methoxy a-ketoester
was prepared and its reduction was investigated
(Scheme 2). Under the identical reduction condition, the
product was found to have a diastereomeric ratio of 88:12.
This result is in favor of simple Cram’s or Felkin’s model
in the stereochemical process, although it does not conclu-
sively remove the possible role of hydrogen bonding in
the stereochemical process.
The optimized reduction condition was then applied to the
b-hydroxy a-ketoesters 2a–k. As shown by the data in
Table 2, all the a-ketoesters with b-aryl substituent gave
excellent diastereoselectivities, regardless of the position
of the substituent in the phenyl ring. When the b substitu-
ent is an alkyl group, the diastereoselectivity decreases to
some extent (Table 2, entries 8–11).
OMe O
OMe O
NaBH4
Table 2 Diastereoselective Reduction of a-Ketoesters 2a–k with
Ph
OEt
Ph
OEt
NaBH4 in CH2Cl2
CH2Cl2
–63 °C to r.t.
O
OH
OH
O
OH
O
NaBH4
( )-5
4
90%; dr = 88:12
R
OEt
R
OEt
CH2Cl2
–63 °C to r.t.
8 h
O
OH
( )-3a–k
Scheme 2
2a–k
In summary, it is found that the reduction of b-hydroxy a-
ketoesters with NaBH4 in CH2Cl2 can give good to excel-
lent diastereoselectivity. Thus, the anti a,b-dihydroxy es-
ters become easily available from aldehydes and ethyl
diazoacetate in three steps.
Entry
a-Ketoester 2 (R = )
2a, C6H5
dra
Yield (%)b
1
2
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
93:7
75
86
78
71
75
63
62
60
80
69
74
2b, o-CH3C6H4
2c, m-BrC6H4
2d, m-CF3C6H4
2e, p-FC6H4
3
All reactions were performed under a nitrogen atmosphere in a
flame-dried reaction flask, and the components were added via sy-
ringe. All solvents were distilled prior to use. CH2Cl2 was freshly
distilled from CaH2 before use. For chromatography, 100–200 mesh
silica gel (Qindao, China) was employed. 1H and 13C NMR spectra
were recorded at 300 MHz and 75 MHz, respectively, on a Varian
Mercury 300 spectrometer. Chemical shifts are reported in ppm us-
ing TMS as internal standard. IR spectra were recorded on a Nicolet
5MX-S infrared spectrometer. Mass spectra were obtained on a VG
ZAB-HS mass spectrometer.
4
5
6
2f, p-MeOC6H4
2g, p-PhC6H4
2h, CH3CH2
7
8
9
2i, CH3(CH2)5
2j, (CH3)2CHCH2
2k, Cyclopentyl
92:8
10
11
85:15
85:15
Oxidation of b-Hydroxy a-Diazoesters 1a–k with DMD and the
Subsequent Reduction of the b-Hydroxy a-Ketoesters 2a–k with
NaBH4; General Procedure
b-Hydroxy a-diazoesters 1a–k (0.25 mmol) was dissolved in ace-
tone (2 mL) and a solution of dimethyldioxirane in acetone (5 mL)
was added to the solution at –35 °C. The mixture was allowed to stir
for 15–30 min until the starting diazo compound was no longer
present when monitored by TLC. Then the solvent was removed
with rotary evaporator, and the residue was dissolved in CH2Cl2 (4
mL) and dried over anhyd Na2SO4. The drying agent was filtered off
and washed with CH2Cl2. The solvent was removed by rotary evap-
oration to give crude b-hydroxy a-ketoesters 2a–k, which were
used in the following reaction without further purification.
a The diastereomeric ratio was determined by the 1H NMR (300 MHz)
of the crude product.
b Isolated yields for 2 steps.
The anti selectivity of the reduction can be rationalized by
either Cram’s rule8 or Felkin’s rule,9 as shown in Figure 1.
In Cram’s model, the attacking hydride approaches from
the small group (the hydrogen) side in the conformation,
while in Felkin’s model, the hydride attacks the carbonyl
group from the direction between hydroxyl group and the
hydrogen. Both models predict the anti isomer to be pre-
dominant.
The crude product was dissolved in CH2Cl2 (4 mL), then the solu-
tion was cooled to –63 °C and NaBH4 (10 mg, 0.25 mmol) was add-
ed. The mixture was stirred for about 8 h, during which the reaction
temperature rose to r.t. Then sat. solution of NH4Cl was added, and
the reaction mixture was stirred for an additional 30 min and was
extracted with CH2Cl2 (4 × 10 mL). The combined organic layer
was dried over anhyd Na2SO4 and concentrated under reduced pres-
Another possibility in the stereochemical process is that
the intramolecular hydrogen bonding may play a role. To
determine whether the hydrogen bonding is responsible
Synthesis 2004, No. 16, 2633–2636 © Thieme Stuttgart · New York

PAPER
Stereoselective Reduction of b-Hydroxy a-Ketoesters
2635
sure with rotary evaporator. The residue was subjected to silica gel
chromatography (petroleum ether–CHCl3–CH3OH, 30:4:1) to af-
ford the pure product.
5.00 (dd, J = 4.2, 4.2 Hz, 1 H), 6.99–7.05 (m, 2 H), 7.27–7.33 (m, 2
H).
13C NMR (75 MHz, CDCl3): d = 13.93, 61.89, 74.28, 74.66, 114.90,
115.19, 128.07, 128.18, 134.38, 134.43, 160.81, 164.07, 171.84.
EI–MS: m/z (relative intensity) = 228 (5) [M+], 209 (27), 155 (5),
137 (61), 125 (100), 109 (100), 104 (92), 97 (100), 95 (85), 77 (98),
76 (93), 29 (97).
( )-anti-Ethyl 2,3-Dihydroxy-3-phenylpropanoate (3a)4a,10
IR (film): 3399, 1731, 1208, 1100 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.14 (t, J = 7.2 Hz, 3 H), 3.30 (br
s, 2 H), 4.10 (q, J = 7.2 Hz, 2 H), 4.48 (dd, J = 6.6, 4.2 Hz, 1 H),
5.02 (dd, J = 5.1, 4.2 Hz, 1 H), 7.26–7.36 (m, 5 H).
13C NMR (75 MHz, CDCl3): d = 13.86, 61.70, 74.80, 74.91, 126.35,
128.00, 128.14, 138.58, 171.92.
EI–MS: m/z (relative intensity) = 210 (6) [M+], 119 (10), 107 (78),
104 (100), 91 (23), 76 (99), 29 (21).
Anal. Calcd for C11H13O4F: C, 57.89; H, 5.74. Found: C, 58.05; H,
5.76.
( )-anti-Ethyl 2,3-Dihydroxy-3-(p-methoxyphenyl)propanoate
(3f)
IR (film): 3426, 1732, 1248, 1107, 1028 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.18 (t, J = 7.2 Hz, 3 H), 3.20 (br
s, 2 H), 3.78 (s, 3 H), 4.13 (q, J = 7.2 Hz, 2 H), 4.45 (d, J = 4.2 Hz,
1 H), 4.96 (d, J = 4.2 Hz, 1 H), 6.85 (d, J = 9.0 Hz, 2 H), 7.24 (d,
J = 9.0 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 13.97, 55.17, 61.73, 74.49, 74.70,
113.54, 127.67, 130.67, 159.31, 171.99.
( )-anti-Ethyl 2,3-Dihydroxy-3-(o-methylphenyl)propanoate
(3b)
IR (film): 3438, 1732, 1210, 1090 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.08 (t, J = 7.2 Hz, 3 H), 2.35 (s,
3 H), 3.23 (br s, 1 H), 3.51 (br s, 1 H), 4.02–4.13 (m, 2 H), 4.40 (dd,
J = 6.3, 4.5 Hz, 1 H), 5.22 (dd, J = 4.2, 4.5 Hz, 1 H), 7.12–7.22 (m,
3 H), 7.46–7.49 (m, 1 H).
13C NMR (75 MHz, CDCl3): d = 13.75, 19.14, 61.65, 71.87, 73.71,
125.85, 126.02, 127.72, 130.24, 134.82, 137.06, 172.33.
EI–MS: m/z (relative intensity) = 240 (7) [M+], 137 (100), 109 (22),
77 (17), 29 (10).
Anal. Calcd for C12H16O5: C, 59.99; H, 6.71. Found: C, 59.76; H,
6.70.
EI–MS: m/z (relative intensity) = 224 (12) [M+], 133 (9), 121 (100),
104 (60), 93 (53), 91 (36), 77 (35), 76 (56), 29 (12).
( )-anti-Ethyl 2,3-Dihydroxy-3-(p-phenylphenyl)propanoate
Anal. Calcd for C12H16O4: C, 64.27; H, 7.19. Found: C, 64.12; H,
7.06.
(3g)
IR (film): 3349, 1727, 1211, 1099 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.15 (t, J = 7.2 Hz, 3 H), 3.39 (br
s, 2 H), 4.12 (q, J = 7.2 Hz, 2 H), 4.51 (d, J = 3.6 Hz, 1 H), 5.07 (d,
J = 3.6 Hz, 1 H), 7.33–7.58 (m, 9 H).
13C NMR (75 MHz, CDCl3): d = 13.92, 61.80, 74.75, 74.79, 126.86,
126.99, 127.30, 128.72, 137.63, 140.58, 140.84, 171.95.
EI–MS: m/z (relative intensity) = 286 (6) [M+], 183 (100), 155 (29),
115 (4), 102 (13), 76 (16), 29 (7).
( )-anti-Ethyl 2,3-Dihydroxy-3-(m-bromophenyl)propanoate
(3c)
IR (film): 3395, 1732, 1209, 1101 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.16 (t, J = 7.2 Hz, 3 H), 3.46 (br
s, 2 H), 4.07–4.18 (m, 2 H), 4.45 (br s, 1 H), 5.01 (br s, 1 H), 7.17–
7.51 (m, 4 H).
13C NMR (75 MHz, CDCl3): d = 13.93, 62.00, 74.34, 74.76, 122.31,
125.02, 129.56, 129.70, 130.98, 141.06, 171.75.
Anal. Calcd for C17H18O4: C, 71.31; H, 6.34. Found: C, 71.29; H,
6.32.
EI–MS: m/z (relative intensity) = 288 (4) [M+], 259 (4), 215 (19),
185 (100), 104 (100), 76 (100), 29 (98).
( )-anti-Ethyl 2,3-Dihydroxypentanoate (3h)
Anal. Calcd for C11H13O4Br: C, 45.70; H, 4.53. Found: C, 45.88; H,
4.60.
IR (film): 3397, 1735, 1209, 1125 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.01 (t, J = 7.2 Hz, 3 H), 1.32 (t,
J = 7.2 Hz, 3 H), 1.48–1.61 (m, 2 H), 2.35 (d, J = 6.6 Hz, 1 H), 3.17
(d, J = 5.7 Hz, 1 H), 3.74–3.82 (m, 1 H), 4.21–4.32 (m, 3 H).
13C NMR (75 MHz, CDCl3): d =10.24, 14.14, 24.78, 61.96, 73.62,
74.69, 172.83.
EI–MS: m/z (relative intensity) = 163 (0.3) [M + 1]+, 133 (4), 104
(78), 89 (16), 76 (100), 59 (35), 29 (39).
( )-anti-Ethyl 2,3-Dihydroxy-3-(m-trifluoromethylphenyl)pro-
panoate (3d)
IR (film): 3405, 1734, 1328, 1170, 1123 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.11 (t, J = 7.2 Hz, 3 H), 3.57 (br
s, 2 H), 4.10 (q, J = 7.2 Hz, 2 H), 4.49 (d, J = 3.7 Hz, 1 H), 5.11 (d,
J = 3.7 Hz, 1 H), 7.43–7.62 (m, 4 H).
13C NMR (75MHz, CDCl3): d = 13.70, 62.03, 74.43, 74.73, 122.23,
123.17, 123.22, 123.27, 123.32, 124.61, 124.66, 124.71, 124.76,
125.84, 128.57, 129.76, 130.23, 130.66, 139.79, 171.73.
EI–MS: m/z (relative intensity) = 259 (8) [M – 19]+, 187 (14), 175
(41), 127 (43), 104 (99), 76 (100).
Anal. Calcd for C7H14O4: C, 51.84; H, 8.70. Found: C, 51.59; H,
8.95.
( )-anti-Ethyl 2,3-Dihydroxynonanoate (3i)
IR (film): 3403, 1736, 1208, 1084 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.88 (t, J = 6.6 Hz, 3 H), 1.28–1.60
(m, 13 H), 2.58 (br s, 1 H), 3.30 (br s, 1 H), 3.83–3.88 (m, 1 H),
4.20–4.33 (m, 3 H).
Anal. Calcd for C12H13O4F3: C, 51.80; H, 4.71. Found: C, 51.66; H,
4.75.
13C NMR (75 MHz, CDCl3): d = 14.02, 14.13, 22.54, 25.65, 29.10,
( )-anti-Ethyl 2,3-Dihydroxy-3-(p-fluorophenyl)propanoate
31.65, 61.86, 73.22, 73.96, 172.80.
(3e)
IR (film): 3417, 1733, 1221, 1106 cm–1.
EI–MS: m/z (relative intensity) = 219 (3) [M + 1]+, 145 (9), 133 (3),
1H NMR (300 MHz, CDCl3): d = 1.16 (t, J = 7.2 Hz, 3 H), 3.33 (br
s, 2 H), 4.12 (q, J = 7.2 Hz, 2 H), 4.45 (dd, J = 6.3, 4.2 Hz, 1 H),
104 (100), 76 (99), 55 (38), 29 (34).
Anal. Calcd for C11H22O4: C, 60.52; H, 10.16. Found: C, 60.47; H,
9.67.
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M. Liao et al.
PAPER
( )-anti-Ethyl 2,3-Dihydroxy-5-methylhexanoate (3j)
References
IR (film): 3377, 1734, 1211, 1079 cm–1.
(1) For a review, see: Oishi, T.; Nakata, T. Acc. Chem. Res.
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1H NMR (300 MHz, CDCl3): d = 0.93 (dd, J = 10.8, 6.6 Hz, 6 H),
1.08–1.17 (m, 1 H), 1.32 (t, J = 7.2 Hz, 3 H), 1.48–1.58 (m, 1 H),
1.78–1.87 (m, 1 H), 2.60 (br s, 1 H), 3.42 (br s, 1 H), 3.96 (d, J = 4.5
Hz, 1 H), 4.19–4.37 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.13, 21.57, 23.51, 24.32, 40.45,
61.81, 71.24, 74.23, 172.72.
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EI–MS: m/z (relative intensity) = 191 (0.2) [M + 1]+, 117 (7), 104
(100), 87 (6), 76 (99), 43 (33), 29 (26).
Anal. Calcd for C9H18O4: C, 56.82; H, 9.54. Found: C, 56.59; H,
9.35.
(3) For reviews, see: (a) Schroder, M. Chem. Rev. 1980, 80,
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(4) (a) Pearson, W. H.; Cheng, M.-C. J. Org. Chem. 1987, 52,
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( )-anti-Ethyl 3-Cyclopentyl-2,3-dihydroxy propanoate (3k)
IR (film): 3403, 1734, 1207, 1090 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.17–1.92 (m, 11 H), 1.99–2.10
(m, 1 H), 2.57 (br s, 1 H), 3.36 (dd, J = 6.0, 6.0 Hz, 1 H), 3.61–3.67
(m, 1 H), 4.22–4.31 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.12, 25.13, 25.52, 28.91, 29.84,
41.85, 61.80, 73.35, 78.16, 172.86.
EI–MS: m/z (relative intensity) = 133 (3), 104 (100), 81 (34), 76
(81), 41 (37), 29 (31).
(5) anti-a,b-Dihydroxy esters were also synthesized through
epoxidation of trans-a,b-unsaturated esters followed by
hydrolysis and esterification. See: English, J.; Heywood, D.
L. J. Am. Chem. Soc. 1955, 77, 4661.
(6) Ihmels, H.; Maggini, M.; Prato, M.; Scorrano, G.
Tetrahedron Lett. 1991, 32, 6215.
Anal. Calcd for C10H18O4: C, 59.39; H, 8.97. Found: C, 59.48; H,
8.73.
(7) Jiang, N.; Wang, J. Tetrahedron Lett. 2002, 43, 1285; and
Acknowledgment
references cited therein.
The project is generously supported by Natural Science Foundation
of China (Grant No. 20225205, 20172002, 20390050).
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(10) Touzin, A. M. Tetrahedron Lett. 1975, 16, 1477.
Synthesis 2004, No. 16, 2633–2636 © Thieme Stuttgart · New York