Lipase-catalysed N-acylation of β2-amino esters

Article abstract of DOI:10.1016/j.tetasy.2008.04.002

The Candida antarctica lipase A-catalysed N-acylation of ethyl 3-amino-2-ethylpropanoate rac-3 and methyl 3-amino-2-isopropylpropanoate rac-6 was performed with ethyl butanoate in tert-amyl alcohol at 4 °C. The resulting enantiomerically enriched derivati

Full text of DOI:10.1016/j.tetasy.2008.04.002

Available online at www.sciencedirect.com
Tetrahedron: Asymmetry 19 (2008) 1114–1119
Lipase-catalysed N-acylation of b2-amino esters
a
a
a
a
a,b,
*
}
´
´
´
´
´
´ ´
Monika Fitz, Eniko Forro, Edina Vigoczki, Laszlo Lazar and Ferenc Fulo¨p
¨
aInstitute of Pharmaceutical Chemistry, University of Szeged, H-6720 Szeged, Eo¨tvo¨s utca 6, Hungary
bResearch Group of Stereochemistry of the Hungarian Academy of Sciences, University of Szeged, H-6720 Szeged, Eo¨tvo¨s utca 6, Hungary
Received 19 March 2008; accepted 1 April 2008
´
´
Dedicated to Professor Gabor Bernath on the occasion of his 75th birthday
Abstract—The Candida antarctica lipase A-catalysed N-acylation of ethyl 3-amino-2-ethylpropanoate rac-3 and methyl 3-amino-2-iso-
propylpropanoate rac-6 was performed with ethyl butanoate in tert-amyl alcohol at 4 °C. The resulting enantiomerically enriched deriv-
atives were isolated as N-Boc-protected amino esters (R)-9 and (R)-10 and butyramides (S)-7 and (S)-8 (ee: 76–95%).
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
tion and interesterification) catalysed by CAL-A and CAL-
B, respectively.23 Various hydrolytic enzymes have been ap-
plied for the resolution of b2-amino acid esters, resulting in
enantiomerically enriched b2-amino acids and esters.24
Cyclic1 and acyclic2–5 b-amino acids are of key importance
in the organic chemistry. These are the constituents of both
naturally occurring and synthetic derivatives with valuable
physiological effects,6 for example, taxoids.4,7 b-Peptides
and a-peptides containing b-amino acids are not degrad-
able by proteases, and may lead to protease inhibitors,
which are currently important targets in pharmaceutical
chemistry.6 The folding properties and three-dimensional
structures of b-peptides have already been explored,3,8–10
while interest has recently turned to the preparation of
enantiomerically pure b-amino acids and their deriva-
tives;1,11 many different methods have been described for
the preparation of b2-amino acid enantiomers.12–16
Herein, we report an N-acylation-based enzymatic resolu-
tion method for the preparation of 2-ethyl- and 2-isopro-
pyl-3-aminopropanoic acid enantiomers.
2. Results and discussion
2.1. Syntheses of racemic b2-amino esters 3 and 6
Model racemic compounds were synthesised by a combina-
tion of literature procedures for the preparation of analo-
gous a-substituted b-amino esters.20,25,26 The addition of
benzylamine to ethyl ethacrylate27 1 furnished the corre-
sponding N-benzylamino ester 2, which was transformed
to ethyl 3-amino-2-ethylpropanoate rac-3 by catalytic
hydrogenolysis. The condensation of methyl cyanoacetate
4 and acetone under reductive conditions resulted in the
isopropyl-substituted malononitrile derivative 5,28 the cya-
no group of which was reduced by catalytic hydrogenation
to yield methyl 3-amino-2-isopropylpropanoate rac-6
(Scheme 1).
Lipase-catalysed kinetic resolution has proven to be an
excellent method for the preparation of b3- and b2,3-amino
acid enantiomers, for which, unlike the b2 derivatives, low
enantioselectivities were described.17 Enzymes can catalyse
the transformations on both functional groups of amino
esters: N-acylation18–20 of the amino group, and ester hydro-
lysis21 of the ester function. As far as we are aware, only three
enzymatic resolutions of b2-amino esters have been re-
ported.22–24 N-Phenylacetylated a-methyl-b-alanine has
been successfully hydrolysed with penicillin G amidase.22
The enantiomers of a-methyl-b-alanine ethyl ester have been
prepared through a two-step sequential resolution (N-acyla-
2.2. Lipase-catalysed enantioselective N-acylation of ( )-3
and ( )-6
*
Enzyme screening was first performed on model compound
Corresponding author. Tel.: +36 62 545564; fax: +36 62 545705; e-mail:
fulop@pharm.u-szeged.hu
( )-3. Ethyl 3-amino-2-ethylpropanoate was subjected to
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.04.002

M. Fitz et al. / Tetrahedron: Asymmetry 19 (2008) 1114–1119
1115
Et
COOEt
CH2
Et
COOEt
Et
COOEt
NH2
H2N
Ph
H2, Pd/C, EtOH
1 atm, 25 ºC
MeOH,
N
H
Ph
2
rac-3
1
O
Me
Me
iPr
COOMe
NH2
COOMe
iPr
COOMe
H2, Pd/C, MeOH
10 atm, 25 ºC
NH4OAc, AcOH
H2, Pd/C, EtOH
1 atm, 25 ºC
CN
CN
4
5
rac-6
Scheme 1.
Table 1. CAL-A-catalysed (25 mg/mL) N-acylation of ( )-3 (0.05 M)
with ethyl butanoate (0.55 equiv) in different solvents at 4 °C
an enzyme-catalysed reaction in iPr2O (0.05 M solution) by
the addition of vinyl acetate (2 equiv) in the presence of dif-
ferent lipase preparations (50 mg/mL) at 25 °C. The lipases
tested were CAL-A, CAL-B (Lipolase), and lipases AY,
AK, PS and PPL. CAL-A and CAL-B were previously
used in the resolution of a-methyl-b-alanine ethyl ester23
and b-substituted analogues;19,29 CAL-B also catalyses
N-acylation and interesterification. Our preliminary exper-
iments indicated that the CAL-B-catalysed reaction led
rapidly to multiple products, while the enantiomeric excess
of the substrate remained low. During the enzyme
screening, the enantioselectivities were under 2 in all the
cases (conversions between 43% and 58%), except for
CAL-A, which displayed a slightly higher enantioselectivity
(E = 4), and accordingly was chosen for further
optimisation.
b
Solvent
Time (day) eesa (%) eep (%) Conv. (%)
E
MeCN
tert-AmOH
DCM
THF
Et2O
25
25
25
25
3
45
68
4
8.7
14
52
>99
>99
40
45
24
26
40
9
16
37
47
>200
>200
2
2.9
2
iPr2O
10
58
6
a According to GC after derivatisation with trifluoroacetic anhydride in
the presence of 4-dimethylaminopyridine and pyridine.
b Calculated by using an internal standard (octadecane).
We next optimised the solvent, with reactions in MeCN,
tert-amyl alcohol (tert-AmOH), DCM, THF, Et2O and
iPr2O, the most commonly used organic solvents for
lipase-catalysed resolutions (Table 1). The reactions were
highly enantioselective (E > 200), but extremely slow in
polar solvents (MeCN and tert-amyl alcohol). The other
solvents proved to be useless due to their low selectivity.
In order to find a more appropriate acyl donor for the
CAL-A-catalysed (50 mg/mL) resolution, six achiral esters
(ethyl butanoate, butyl butanoate, vinyl butanoate, 2,2,2-
trifluoroethyl butanoate, vinyl acetate and 2,2,2-trifluoro-
ethyl chloroacetate) were tested. The reactions were started
in a 0.05 M iPr2O solution of ( )-3 at room temperature
(25 °C) by the addition of 0.55 or 1 equiv of an acyl donor.
The selectivities were low (E up to 6); the most promising E
values were obtained when using butanoates. As a result of
the low E values, new reactions were started at 4 °C (cold
room) using 0.55 equiv of ethyl butanoate and 2,2,2-trifluo-
roethyl butanoate. The results led us to continue our stud-
ies with ethyl butanoate.
In an attempt to shorten the reaction time, a higher reac-
tion temperature and an increased amount of enzyme were
applied. At 25 °C, the selectivity of the reaction was low-
ered dramatically, both in tert-amyl alcohol and in MeCN.
An increased amount of enzyme (50 mg/mL) accelerated
the reaction while E decreased (Table 2).
Table 2. Effects of temperature and enzyme amount on CAL-A-catalysed resolution of ( )-3a
c
Temperature
(°C)
Solvent
Enzyme amount
(mg/mL)
Time (day)
eesb (%)
eep (%)
Conv. (%)
E
4
MeCN
tert-Amyl alcohol
25
25
50
25
25
50
25
25
14
21
7
45
68
80
30
50
49
>99
>99
93
67
83
26
40
46
32
37
39
>200
>200
63
17
18
25
MeCN
tert-Amyl alcohol
3
77
12
a 0.05 M substrate, ethyl butanoate (0.55 equiv).
b According to GC after derivatisation with trifluoroacetic anhydride in the presence of 4-dimethylaminopyridine and pyridine.
c Calculated by using an internal standard (octadecane).

1116
M. Fitz et al. / Tetrahedron: Asymmetry 19 (2008) 1114–1119
Another possibility to enhance the reaction rate is the use
of various additives, which can also change the selectivity
of lipase-catalysed reactions, especially in the case of lipase
PS.30–32 Selected additives (CuCl2 0.1 equiv; TEA
0.1 equiv; and 15-Crown-5, 0.33 equiv) were tested with
lipase PS (25 mg/mL) under the above-described condi-
tions in tert-amyl alcohol at 4 °C. However, both the reac-
tion rate and the selectivity were low in all the cases
(31–61% conversion after 10–28 days; E: 2–4). The same
additives were next used in CAL-A-catalysed (25 mg/mL)
reactions, when these lowered not only the selectivity, but
also the reaction rate: the reactions reached 36–44%
conversion after 35 days, while E was between 20 and 74.
Unfortunately, LiCl (0.1 equiv) inactivated the CAL-A.
enriched substrate (ee = 58%) was then subjected to a sec-
ond enzymatic reaction under the above conditions. The
physical data on the enantiomers prepared are reported
in Table 3.
To determine the selectivity of the enzyme, N-Boc-pro-
tected amino ester 9 was hydrolysed with LiOHꢀH2O,33
resulting in 3-tert-butoxycarbonylamino-2-ethylpropanoic
25
acid 11. The specific rotation for 11 was ½aꢁD ¼ ꢂ13:2 (c
26
1.0, CH2Cl2); the literature values for (R)-11 {½aꢁD
>95%34} confirmed the (S)-selectivity.
¼
20
ꢂ17:3 (c 1.3, MeOH);33 ½aꢁD ¼ ꢂ18:2 (c 1.0, CH2Cl2), ee
3. Conclusion
Following the preliminary results, the gram-scale resolu-
tions of ( )-3 and ( )-6 were performed in tert-amyl alco-
hol (0.05 M) with 0.55 equiv of ethyl butanoate in the
presence of 50 mg/mL CAL-A preparation at 4 °C. The
less reactive enantiomers were transformed into their
N-Boc-protected derivatives (R)-9 and (R)-10, and sepa-
rated from butyramides (S)-7 and (S)-8 by column
chromatography.
In conclusion, ethyl 3-amino-2-ethylpropanoate ( )-3 and
methyl 3-amino-2-isopropylpropanoate ( )-6 were re-
solved via Candida antarctica lipase-A-catalysed (50 mg/
mL) N-acylation in tert-amyl alcohol (0.05 M) with
0.55 equiv of ethyl butanoate at 4 °C (Scheme 2). The
enzymatic reactions, followed by N-Boc protection of the
less reactive amino ester enantiomers, led to butyramides
(S)-7 and (S)-8 and N-Boc-protected amino esters (R)-9
and (R)-10, which were separated by column chromato-
graphy with moderate to good ee values (ee: 76–95%)
(Table 3).
Since the selectivity was relatively low (E = 9) in the case of
( )-6, the gram-scale reaction was stopped when the prod-
uct ee was 78%, and the separated enantiomerically
4. Experimental
Table 3. Physical data on enantiomers prepared
25
Compound
Yield (%)a
ee (%)
½aꢁD
4.1. Materials and methods
(S)-7
(R)-9
(S)-8
(R)-10
36
25
38
18
95b
85c
78c
76c
+26.0 (c 1.0, MeOH)
ꢂ20.6 (c 0.5, MeOH)
+26.4 (c 1.0, MeOH)
ꢂ13.4 (c 1.0, MeOH)
CAL-A (C. antarctica lipase A, Chirazyme L-5) was pur-
chased from Roche. Lipases AK (Burkholderia sp.) and
PS (Burkholderia cepacia) were from Aldrich. Lipase AY
(Candida rugosa) was from Amano Pharmaceuticals.
CAL-B (from C. antarctica, Chyrazime L-2) immobilised
on acrylic resin, known by the trade name Lipolase, was
from Sigma Aldrich. Before use, CAL-A and lipases PS,
a Maximum yield 50% at 50% conversion.
b From a comparison of the specific rotation with that of (R)-7, synthes-
ised from unreacted (R)-3 with ee = 88%.
c Determined by GC.
R1
R1
COOR2
NH2
COOR2
NH2
R1
COOR2
CAL-A
+
PrCOOEt
NHCOPr
tert-amyl alcohol
rac-3 R1 = R2 = Et
rac-6 R1 = iPr, R2 = Me
(S)-7 R1 = R2 = Et
(S)-8 R1 = iPr, R2 = Me
(Boc)2O
TEA
Et
COOH
NHBoc
R1
COOR2
NHBoc
LiOH, H2O
(R)-9 R1 = R2 = Et
(R)-10 R1 = iPr, R2 = Me
(R)-11
Scheme 2.

M. Fitz et al. / Tetrahedron: Asymmetry 19 (2008) 1114–1119
1117
AK and AY were adsorbed on Celite in the presence of
sucrose.35 2,2,2-Trifluoroethyl butanoate, butyl butanoate
and 2,2,2-trifluoroethyl chloroacetate were prepared from
the corresponding acid chlorides and alcohols. The solvents
were of the highest analytical grade. 1H NMR spectra were
recorded on a Bruker AM 400 spectrometer. Optical rota-
tions were measured with a Perkin Elmer 341 polarimeter.
with EtOAc (3 ꢃ 30 mL). The aqueous phase was made
alkaline with concd NH3 solution and extracted with
EtOAc (3 ꢃ 70 mL). The combined organic extracts were
dried over Na2SO4 and evaporated. The oily residue was
converted to the crystalline hydrochloride salt rac-6ꢀHCl
by treatment with 22% ethanolic HCl and Et2O. Recrystal-
lisation of the crude product from a mixture of MeOH and
Et2O resulted in rac-6ꢀHCl [1.34 g (7.4 mmol, 15%), mp:
144–145 °C] as colourless crystals. 1H NMR (400 MHz,
D2O): d (ppm) 0.96 (6H, d, J = 6.9 Hz, (CH3)2CH), 2.13
(1H, m, (CH3)2CH), 2.73 (1H, m, (CH3)2CHCH), 3.19
(1H, td, J = 3.5, 13.1, CH2NH), 3.34 (1H, ddd, J = 1.9,
10.3, 13.1 Hz, CH2NH), 3.79 (3H, s, OCH3).
4.2. Preparation of ethyl 3-amino-2-ethylpropanoate, ( )-3
A mixture of ethyl ethacrylate (1, 9.00 g, 70.2 mmol) and
benzylamine (5.00 g, 46.7 mmol) was refluxed in EtOH
(7 mL) for 4 h and then allowed to stand at room temper-
ature for 2 days. The solvent was evaporated off and the
residue was distilled to give 2 [7.36 g (31.3 mmol, 68%),
Amino ester base rac-6 was obtained from the above
hydrochloride salt by treatment with NH3 solution, extrac-
tion (EtOAc) and evaporation.
1
bp: 120–135 °C (6 mm Hg)] as a colourless oil. H NMR
(400 MHz, CDCl3): d (ppm) 0.91 (3H, t, J = 7.4 Hz,
CH3CH2CH), 1.26 (3H, t, J = 7.1 Hz, OCH2CH3), 1.51–
1.72 (2H, m, CH3CH2CH), 2.53–2.62 (1H, m,
CH3CH2CH), 2.72 (1H, dd, J = 4.9, 11.9 Hz, CH2NHCH2-
Ph), 2.90 (1H, dd, J = 8.9, 11.9 Hz, CH2NHCH2Ph), 3.79
(1H, d, J = 13.4 Hz, CH2Ph), 3.85 (1H, d, J = 13.4 Hz,
CH2Ph), 4.13–4.19 (2H, m, OCH2–CH3), 7.20–7.40 (5H,
m, Ar).
4.4. Small-scale resolutions
In a typical small-scale experiment, the acyl donor (0.1 M)
was added to a mixture of racemic amino ester (0.05 M)
and enzyme preparation (25 or 50 mg/mL) in an organic
solvent (1 mL), and the reaction mixture was then shaken
at 4 °C (cold room) or at 25 °C (room temperature). The
progress of the reaction and the ee values were determined
by taking samples (0.05 mL) at intervals, filtering off the
enzyme and analysing them by GC, on a Chrompack
CP-Chirasil-DEX CB column (25 m). For a good baseline
separation, the unreacted amino group was acylated with
acetic or trifluoroacetic anhydride in the presence of pyri-
dine containing 1% 4,4-dimethylaminopyridine.
A stirred mixture of ethyl 3-benzylamino-2-methylpropan-
oate (2, 11.00 g, 46.7 mmol), abs. EtOH (80 mL) and Pd
catalyst (10 wt % on activated charcoal, 1.00 g) was hydro-
genated at ambient temperature at atmospheric pressure
for 20 h. The catalyst was then filtered off and washed with
EtOH. The combined filtrates were evaporated and the res-
idue was purified by distillation to give rac-3 [3.60 g
(24.8 mmol, 53%), bp: 85–90 °C (28 mm Hg)] as a colour-
1
less oil. H NMR (400 MHz, CDCl3) d (ppm): 0.93 (3H,
Since the enantiomers of ( )-3 could be separated by GC
only as the trifluoroacetamide, but not as the N-acetyl,
N-chloroacetyl, N-propionyl or N-butyryl derivatives,
octadecane was used as the internal standard to follow
the N-acylation of ( )-3.
t, J = 7.5 Hz, CH3CH2CH), 1.27 (3H, t, J = 7.1 Hz,
OCH2CH3), 1.50–1.73 (2H, m, CH3CH2CH), 2.36–2.45
(1H, m, CH3CH2CH), 2.85 (1H, dd, J = 4.8, 12.9 Hz,
CH2NH), 2.96 (1H, dd, J = 8.4, 12.9 Hz, CH2NH), 4.13–
4.22 (2H, m, J = 7.1 Hz, OCH2CH3).
4.5. Preparative-scale resolution of ethyl 3-amino-2-ethyl-
propanoate, ( )-3
4.3. Preparation of methyl 3-amino-2-isopropylpropanoate,
( )-6
Racemic 3 (0.80 g, 6.88 mmol) was dissolved in tert-amyl
alcohol (70 mL). The CAL-A preparation (5.40 g) and
ethyl butanoate (0.50 mL, 3.78 mmol) were added in order
to start the reaction, with shaking at 4 °C. The reaction
mixture was stopped after 42 days, at 52% conversion, by
filtering off the enzyme. Evaporation of the solvent resulted
in a mixture of (R)-3 (ee = 88%) and (S)-7 (ee = 95%).
A stirred mixture of methyl cyanoacetate 4 (10.00 g,
100.9 mmol), abs. acetone (8.1 mL, 109.8 mmol), NH4OAc
(0.78 g, 10.0 mmol), glacial AcOH (1.15 mL, 19.7 mmol)
and EtOH (20 mL) was hydrogenated at ambient tempera-
ture and atmospheric pressure in the presence of Pd cata-
lyst (5 wt % on activated charcoal, 0.40 g) for 5 h. The
catalyst was then filtered off and the solvent evaporated
off. The crude oily product 5 was purified by distillation
[8.72 g (61.8 mmol, 61%), bp: 112–115 °C (28 mm Hg)].
The 1H NMR spectrum of 5 was in accordance with the lit-
erature36 data.
Half of the residue was dissolved in MeCN (15 mL). Next
TEA (0.14 g, 1.4 mmol, 0.19 mL) and di-tert-butoxycar-
bonyl (0.23 g, 1.05 mmol) were added and the reaction mix-
ture was stirred at room temperature overnight. After
evaporation and separation on silica [acetone/hexane
A stirred mixture of methyl 2-cyano-3-methylbutanoate 5
(8.70 g, 0.06 mol), MeOH (60 mL), 36% aqueous HCl
(3.15 mL) and Pd catalyst (5 wt % on activated charcoal,
0.50 g) was hydrogenated at ambient temperature at
10 atm for 20 h. The catalyst was then filtered off and
washed with MeOH and the filtrate was evaporated. The
oily residue was dissolved in water (150 mL) and washed
(1:19)], butyramide (S)-7 {260 mg (1.21 mmol, 36%);
25
½aꢁD ¼ þ26:0 (c 1.0, MeOH); ee = 95%} and the N-Boc-
protected amino ester (R)-9 {210 mg (0.86 mmol, 25%);
25
½aꢁD ¼ ꢂ20:6 (c 0.5, MeOH); ee = 85%} were obtained as
1
colourless oils. H NMR (400 MHz, CDCl3) d (ppm) for
(S)-7: 0.94 (6H, m, CH3CH2CH + COCH2CH2CH3), 1.26
(3H, t, J = 7.10 Hz, OCH2CH3), 1.55–1.70 (4H, m, CH3-

1118
M. Fitz et al. / Tetrahedron: Asymmetry 19 (2008) 1114–1119
CH2CH + COCH2CH2CH3), 2.13 (2H, t, J = 7.46 Hz,
COCH2CH2CH3), 2.55 (1H, m, CH3CH2CH), 3.35 (1H,
m, CH2NH), 3.51 (1H, m, CH2NH), 4.16 (2H, q, J =
7.20 Hz, OCH2CH3), 5.89 (1H, s, NH). 1H NMR
(400 MHz, CDCl3) d (ppm) for (R)-9: 0.94 (3H, t,
J = 7.46 Hz, CH3CH2CH), 1.26 (3H, t, J = 7.10 Hz,
OCH2CH3), 1.43 (9H, s, (CH3)3O), 1.53–1.69 (2H, m,
CH3CH2CH), 2.52 (1H, m, CH3CH2CH), 3.23–3.35 (2H,
m, CH2NH), 4.17 (2H, q, J = 7.13 Hz, OCH2CH3), 4.85
(1H, br s, NH).
tography [acetone/hexane (1:19)]. 1H NMR (400 MHz,
CDCl3) d (ppm) for (R)-10: 0.96 (6H, d, J = 6.88 Hz,
(CH3)2CH), 1.44 (9H, s, (CH3)3CO), 1.96 (1H, m,
(CH3)2CH), 2.43 (1H, m, (CH3)2CHCH), 3.24 (1H, m,
CH2NH), 3.41 (1H, m, CH2NH), 3.70 (3H, s, OCH3),
4.81 (1H, br s, NH).
Acknowledgements
The authors acknowledge the receipt of OTKA Grants K
71938 and T 049407.
The other half of the residue was dissolved in EtOAc
(30 mL) and extracted with 4% aqueous HCl (20 mL).
The aqueous phase was basified with aqueous KOH (8%)
and the free amino ester (R)-3 was extracted with EtOAc
(3 ꢃ 30 mL). The organic phase was dried over Na2SO4
and evaporated. This residue was used without further
purification to synthesise butyramide (R)-7, the antipode
of (S)-3 obtained from enzymatic resolution. The butyryla-
tion was carried out in pyridine (8 mL) with butanoic anhy-
dride (1.80 mL, 11.2 mmol; 2 equiv). The reaction mixture
was stirred overnight at room temperature. The next day,
20 mL of toluene was added to accelerate the evaporation.
References
1. Fulo¨p, F. Chem. Rev. 2001, 101, 2181–2204, and references
¨
cited therein.
2. Enantioselective Synthesis of b-Amino Acids; Juaristi, E.,
Soloshonok, V. A., Eds.; Wiley-VHC: New York, 2005.
3. Gademann, K.; Hintermann, T.; Schreiber, J. V. Curr. Med.
Chem. 1999, 6, 905–925.
4. Ojima, I.; Lin, S.; Wang, T. Curr. Med. Chem. 1999, 6, 927–
954.
5. Scarborough, R. M. Curr. Med. Chem. 1999, 6, 971–981.
6. Steer, D. L.; Lew, R. A.; Perlmutter, P.; Smith, A. I.; Aguilar,
M.-I. Curr. Med. Chem. 2002, 9, 811–822, and references
cited therein.
7. Denis, J.-N.; Greene, A. E. J. Am. Chem. Soc. 1988, 110,
5917–5919.
Purification by column chromatography [acetone/hexane
25
(2:8)] gave (R)-7 {260 mg (1.21 mmol, 89%); ½aꢁD ¼ ꢂ24:0
(c 1.0, MeOH); ee = 88%; ee did not decrease during the
reaction} as a colourless oil. The comparison of the specific
rotations of (R)-7 and (S)-7 confirmed the calculations
involving the use of an internal standard, which gave ee
1
(S)-7 = 95%. The H NMR (400 MHz, CDCl3) d (ppm)
8. Gellman, S. H. Acc. Chem. Res. 1998, 31, 173–180.
data for (R)-7 were similar to those for (S)-7.
´
9. Fulo¨p, F.; Martinek, T. A.; Toth, G. K. Chem. Soc. Rev.
¨
2006, 35, 323–334.
10. Martinek, T. A.; Fulo¨p, F. Eur. J. Biochem. 2003, 270, 3657–
¨
4.6. Preparative-scale resolution of methyl 3-amino-2-iso-
propylpropanoate, ( )-6
3666.
11. Liu, M.; Sibi, M. P. Tetrahedron 2002, 58, 7991–8035, and
references cited therein.
12. Della Rossa, C.; Gil, S.; Rodrıguez, P.; Parra, M. Synthesis
2006, 3092–3098.
13. Swiderska, M.; Stewart, J. D. Org. Lett. 2006, 8, 6131–
Racemic 6 (0.60 g, 4.8 mmol) was dissolved in tert-amyl
alcohol (80 mL). CAL-A preparation (4.80 g) and ethyl
butanoate (0.36 mL, 2.7 mmol) were added and the reac-
tion mixture was shaken at 4 °C. The reaction was stopped
after 26 days, at 40% conversion (ee (R)-6 = 52%, ee (S)-
8 = 78%), by filtering off the enzyme. After evaporation,
the residue was dissolved in 4% aqueous HCl and extracted
´
6133.
´
14. Moumne, R.; Denise, B.; Guitot, K.; Rudler, H.; Lavielle, S.;
Karoyan, P. Eur. J. Org. Chem. 2007, 1912–1920.
´
´
15. Dıaz-Sanchez, B. R.; Iglesias-Arteaga, M. A.; Melgar-Fern-
´
andez, R.; Juaristi, E. J. Org. Chem. 2007, 72, 4822–4825.
with EtOAc to obtain butyramide (S)-8 {390 mg
25
16. Chi, Y.; English, E. P.; Pomerantz, W. C.; Horne, W. S.;
Joyce, L. A.; Alexander, L. R.; Fleming, W. S.; Hopkins, E.
A.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129, 6050–
6055.
17. Liljeblad, A.; Kanerva, L. T. Tetrahedron 2006, 62, 5831–
5854.
18. Kanerva, L. T.; Csomos, P.; Sundholm, O.; Bernath, G.;
Fulo¨p, F. Tetrahedron: Asymmetry 1996, 7, 1705–1716.
19. Gedey, S.; Liljeblad, A.; Fulo¨p, F.; Kanerva, L. T. Tetrahe-
(1.8 mmol, 38%); ½aꢁD ¼ þ26:4 (c 1.0, MeOH); ee = 78%}
as a colourless oil. Aqueous KOH (8%) was added to the
aqueous phase and the free amino ester (R)-6 was extracted
with EtOAc. 1H NMR (400 MHz, CDCl3) d (ppm) for (S)-
8: 0.94 (9H, m, (CH3)2CH + COCH2CH2CH3), 1.64 (2H,
m, COCH2CH2CH3), 1.97 (1H, m, (CH3)2CH), 2.12
(2H, t, J = 7.42 Hz, COCH2CH2CH3), 2.47 (1H, m,
(CH3)2CHCH), 3.35 (1H, m, CH2NH), 3.59 (1H, m,
CH2NH), 3.70 (3H, s, OCH3), 5.87 (1H, br s, NH).
´
´
¨
¨
dron: Asymmetry 2001, 12, 105–110.
´
20. Solymar, M.; Fulo¨p, F.; Kanerva, L. T. Tetrahedron:
¨
Asymmetry 2002, 13, 2383–2388.
The enantiomerically enriched substrate (R)-6 (250 mg,
1.7 mmol) was subjected to enzymatic reaction (without
further purification) under the above conditions. After 24
days (ee (R)-6 = 76%, ee (S)-8 = 56%), the reaction was
stopped by filtering off the enzyme. After evaporation,
the free amino ester was transformed into the N-Boc-pro-
´
21. Forro, E.; Fulo¨p, F. Chem. Eur. J. 2007, 13, 6397–6401.
¨
22. Rossi, D.; Lucente, G.; Romeo, A. Experientia 1977, 33,
1557–1559.
´
´ ´
23. Solymar, M.; Liljeblad, A.; Lazar, L.; Fulo¨p, F.; Kanerva, L.
¨
T. Tetrahedron: Asymmetry 2002, 13, 1923–1928.
24. Kaptein, B.; Broxterman, Q. B.; Kierkels, J. G. T.; Milcent,
T. PCT Int. Appl., WO 2005/085462 A1, 2005.
25. Testa, E.; Fontanella, L.; Cristiani, G. F.; Mariani, L. Liebigs
Ann. Chem. 1961, 639, 166–180.
tected form by the method described above and (R)-10
25
{210 mg (0.86 mmol, 18%); ½aꢁD ¼ ꢂ13:4 (c1.0, MeOH);
ee = 76%} was separated from (S)-8 by column chroma-

M. Fitz et al. / Tetrahedron: Asymmetry 19 (2008) 1114–1119
1119
26. Leonard, F.; Wajngurt, A.; Klein, M.; Smith, C. M. J. Org.
Chem. 1961, 26, 4062–4065.
31. Im, D. S.; Cheong, C. S.; Lee, S. H.; Park, H.; Youn, B. H.
Tetrahedron: Asymmetry 1999, 10, 3759–3767.
27. Hewlins, S. A.; Murphy, J. A.; Lin, J.; Hibbs, D. E.;
Hursthouse, M. B. J. Chem. Soc., Perkin Trans. 1 1997, 1559–
1570.
32. Itoh, T.; Takagi, Y.; Murakami, T.; Hiyama, Y.; Tsukube, H.
J. Org. Chem. 1996, 61, 2158–2163.
33. Rimkus, A.; Sewald, N. Org. Lett. 2002, 5, 79–80.
28. Alexander, E. R.; Cope, A. C. J. Am. Chem. Soc. 1944, 66,
886–888.
34. Hubner, J.; Liebscher, J.; Pa¨tzel, M. Tetrahedron 2002, 58,
¨
10485–10500.
29. Gedey, S.; Liljeblad, A.; Fulo¨p, F.; Kanerva, L. T. Tetrahe-
35. Kanerva, L. T.; Sundholm, O. J. Chem. Soc., Perkin Trans. 1
1993, 2407–2410.
¨
dron: Asymmetry 1999, 10, 2573–2581.
30. Theil, F. Tetrahedron 2000, 56, 2905–2919.
36. Sammelson, R. E.; Allen, M. J. Synthesis 2005, 543–547.