Dynamic Kinetic resolution of α-amino acid esters in the presence of aldehydes

Article abstract of DOI:10.1002/ejoc.200800253

A convenient procedure for the racemization of α-amino acid esters in the presence of catalytic amounts of salicylaldehydes is described. The combination of this racemization protocol with lipase-catalyzed ester hydrolysis allows successful dynamic kinetic resolution of various α-amino acid esters. The corresponding α-amino acids are obtained in high yield and optical purity. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800253

FULL PAPER
DOI: 10.1002/ejoc.200800253
Dynamic Kinetic Resolution of α-Amino Acid Esters in the Presence of
Aldehydes
Daniel A. Schichl,[a] Stephan Enthaler,[a] Wolfgang Holla,[b] Thomas Riermeier,[c]
Udo Kragl,[a] and Matthias Beller*[a]
Keywords: Kinetic resolution / Biocatalysis / Enzymes / Racemization / Amino acids
A convenient procedure for the racemization of α-amino acid
esters in the presence of catalytic amounts of salicylalde-
hydes is described. The combination of this racemization pro-
tocol with lipase-catalyzed ester hydrolysis allows successful
dynamic kinetic resolution of various α-amino acid esters.
The corresponding α-amino acids are obtained in high yield
and optical purity.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
fermentation.[2,3] The synthesis of optically active nonnatu-
Optically pure amino acids and their derivatives are ral α-amino acids has been extensively studied for several
probably the most important chiral compounds due to their decades.[3] Despite various “classic” developments of, for
biochemical significance. In addition, α-amino acids are example, Evans,[4] Schöllkopf,[5] Seebach,[6] Corey,[7] and
used as pharmaceuticals, agrochemicals, artificial sweetener, many others,[8] there is still a demand for improved practical
food additives, chiral intermediates for fine chemicals and approaches to amino acids. With respect to kilogram-scale
pharmaceuticals, etc.[1] In Scheme 1, a few examples of in- synthesis, catalytic asymmetric hydrogenations,[9] and the
dustrially important amino acid derivatives are shown.
well-known Strecker synthesis in combination with resolu-
In general, naturally occurring α-amino acids are pro- tion techniques, are the industrial methods of choice. Ad-
duced on large scale either from the chiral pool or by ditionally, attention was directed to enzyme-catalyzed ki-
Scheme 1. Selection of industrially important α-amino acids.
netic resolution techniques to produce chiral amino acids
on industrial scale.[2b] Although a maximum yield of 50%
[a] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
in the resolution step is attainable, this approach is still at-
Fax: +49-381-1281-5000
tractive because of low-cost starting materials and simple
process technology.
Obviously, dynamic kinetic resolution (DKR) of abun-
dantly available and less costly amino acid precursors is an
E-mail: matthias.beller@catalysis.de
[b] Sanofi-Aventis,
65926 Frankfurt, Germany
[c] Evonik-Röhm GmbH,
Kirschenallee, 64293 Darmstadt, Germany
3506
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3506–3512

Dynamic Kinetic Resolution of α-Amino Acid Esters
efficient alternative to the classical resolution of N-acyl amide in 88%ee and 85% conversion. To ensure high selec-
amino acids, because complete conversion of the racemic tivity, the reaction was performed at low temperature
mixture to the desired enantiomer is feasible.[10] However, (–20 °C), which resulted in longer reaction times (66 h). On
certain requirements have to be fulfilled to gain the com- the basis of naturally occurring pyridoxal-5-phosphate,
plete set of advantages of DKR. First of all, the resolution Chen and Wang et al. developed an alcalase®-catalyzed
step must be irreversible and no product racemization DKR of amino acid esters.[22] Here, the free amino acids
should occur under the reaction conditions. In order to ob- were obtained in high yields (87–95%) and excellent optical
tain products with high optical purity, the selectivity E (kA/ purities (90–98%). However, the impact of this method is
kB) of the resolution step should be at least 20 (Figure 1). lowered, because the desired /-amino acid esters are often
Furthermore, the rate constant for the racemization process accessible only in moderate yield and because of the neces-
(kinv) should be faster than the rate constant of the resolu- sity of an expensive racemization catalyst.[23] More recently,
tion step (kA) otherwise a very high selectivity E has to be the group of Kanerva reported the DKR of N-heterocyclic
ensured.[11,12]
amino acid esters catalyzed by aldehydes, for example, acet-
aldehyde (racemization step) and Candida antarctica lipase
A (resolution step by N-acylation).[24] A combination of
two different enzymes (-aminopeptidase, α-amino-ε-cap-
rolactam racemase) for racemization and resolution of ala-
nine amide to yield -alanine was demonstrated to be useful
by Asano et al.[25]
Recently, we studied the kinetics of dynamic kinetic reso-
lutions in the presence of substituted salicylaldehydes.[26]
Herein, we describe a full account of our synthetic work
and demonstrate that catalytic amounts of various alde-
hydes allow for smooth racemization of α-amino acid esters
under mild conditions. Combination with enantioselective
ester cleavage by enzymes gives efficient access to diverse
enantiopure α-amino acids.
Figure 1. Comparison of classical resolution and dynamic kinetic
resolution.
Results and Discussion
So far, enzymes, as well as organo- or organometallic
catalysts, have been proven to be enantioselective catalysts
for this purpose.[11,13] Noteworthy, various nonenzymatic
Racemization of α-Amino Acids
In a first set of experiments, we searched for an efficient
efforts were reported on the resolution of racemic α-acet- amino acid ester racemization catalyst. More than 30 func-
amido β-keto esters by efficient hydrogenation in the pres- tionalized aldehydes and ketones were tested for the racemi-
ence of ruthenium–phosphane complexes by Noyori et al. zation of enantiomerically pure phenylalanine esters, which
and Genêt et al.[13a,14] Later on, this method was extended were generated from the corresponding HCl salts. Owing to
also by other groups to approach pharmaceutical interme- the easy determination of the corresponding enantiomers,
diates in excellent enantioselectivities.[15] More recently, and benzyl and ethyl esters were used as model substrates. No-
especially the group of Berkessel, described DKR methods tably, these amino acid esters are not activated towards ra-
catalyzed by organocatalysts.[16,17]
cemization relative to other amino acid esters, for example,
In case of enzymatic DKR reactions for the synthesis arylglycine derivatives. Selected results are shown in
of α-amino acids, the Hydantoinase-catalyzed resolution of Table 1. In preliminary experiments, the optical stability of
racemic hydantoins substituted with aryl groups in the 5- both esters was tested in the absence of catalyst under the
position to yield the corresponding -N-carbamoyl α- described reaction conditions, but no racemization was ob-
amino acids in slight alkaline media (pH: 7.5–10) is served after 24 h. Initial runs with benzaldehyde (3) as cata-
known.[18] However, this process is limited to aryl hydan- lyst showed complete stability of the model systems. How-
toin derivatives, as alkyl representatives required Hydantoin ever, the utilization of salicylaldehyde (4) as catalyst under
racemase for efficient resolution, which is relatively identical conditions showed some ability to racemize the
scarce.[18b,19] Similarly, oxazolones undergo fast base-cata- substrate. 2-Carboxybenzaldehyde (5) and 9-fluorenone-1-
lyzed racemization in alkaline media.[18b,20] Unfortunately, carboxylic acid (6) were also active for the racemization of
these compounds are highly sensitive towards unselective the model substrate.
chemical hydrolysis in aqueous solution.
Interestingly, the variation of pH demonstrated that race-
Notably, Sheldon and coworkers reported the successful mization at pH 7.0 (12%ee and 2%ee within 120 h) is sig-
dynamic kinetic resolution of racemic phenylglycine methyl nificantly improved to that obtained at pH 5.0 and 9.0.
ester by lipase-catalyzed ammonolysis in the presence of py- Nevertheless, the overall space–time yield is outside the
ridoxal, which mediates in situ racemization.[21] Careful ad- practical range. The addition of salts did not improve the
justment of the reaction conditions gave (R)-phenylglycine racemization activity in comparison to experiments without
Eur. J. Org. Chem. 2008, 3506–3512
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3507

D. A. Schichl, S. Enthaler, W. Holla, T. Riermeier, U. Kragl, M. Beller
FULL PAPER
Table 1. Racemization of phenylalanine esters in the presence of tained results were related to pyridoxal-5-phosphate (10),
different aldehydes.[a]
which has been proven to be an efficient racemization cata-
lyst.[22] All aldehydes racemized -2 fast and continuously
to lead to nearly complete racemization within 8 h. 5-Nitro-
salicylaldehyde (12; 2%ee in 4 h) was found to be a highly
active catalyst, and it was even more reactive than the natu-
rally occurring 10 (16%ee in 4 h). To control the selectivity
of the process, experiments with the free enantiopure amino
acid phenylalanine were carried out. To our delight, no ra-
cemization was observed under identical reaction condi-
tions, which indicates a high selectivity for amino acid ester
racemization.
[a] Reaction conditions: -Phe-OEt (10 mmol), catalyst (10 mol-%),
MeCN/H2O (9:1), 40 °C. [b] The enantiomeric excess was deter-
mined by chiral GC (25 m Chirasil-Val, Alltech, 120 °C, isotherm)
of the corresponding perfluoropropionyl O-methyl ester. [c] -Phe-
OBzl (0.7 mmol), catalyst (20 mol-%), dimethoxyethane (DME)/
H2O (4:1), 25 °C. [d] Additive: LiBr. [e] Additive: Sc(OTf)3.
Figure 2. Comparison of enantioselectivity–time dependency of
several salicylaldehydes and pyridoxal-5-phosphate (10).
Regarding the structure–activity relationship for effective
salts (Table 1, Entries 3, 4, 7, and 8). Noteworthy is that the racemization, it is important that electron-withdrawing
catalyst activity is related to the ortho substituent, and to groups in the 3-and 5-positions of the aromatic ring are
confirm this fact, 3-carboxy- and 4-carboxybenzaldehyde combined with a hydroxy group in the 2-position. Treat-
(7, 8) were tested as catalysts. Indeed, no racemization oc- ment of aldehydes, on the basis of these structural features,
curred under similar reaction conditions. Furthermore, 2- with phenylalanine esters results in the immediate forma-
hydroxy-1-naphthaldehyde, 1-carboxyl-8-naphthaldehyde, tion of the intermediate Schiff bases at room temperature.
2,2Ј-dihydroxybenzophenone, 9-fluorenone, 5-chloro-2-hy- On the basis of this fact, we propose the mechanism pre-
droxybenzophenone, 1H-indole-2,3-dione, and 4-nitrobenz- sented in Scheme 2 for the aldehyde-catalyzed racemiza-
aldehyde were tested under slightly basic conditions, but no tion.[21,27]
significant racemization was observed.
After condensation reaction of the amino acid ester with
A more promising result was obtained with 3-nitro-5- 3,5-dinitrosalicylaldehyde (11), the acidity of the α-hydro-
bromosalicylaldehyde (9) as catalyst. Complete racemiza- gen atom at the chiral center is remarkably increased.
tion of the model substrate -phenylalanine benzyl ester (2) Hence, rapid protonation–deprotonation at the α-carbon
was observed at pH 8.5 within 24 h at 25 °C. On the basis of atom takes place, which causes racemization. Notably, the
this result, the catalytic activity of several electron-deficient stabilization of the imine through hydrogen bonding en-
salicylaldehydes, for example, 3,5-dinitrosalicylaldehyde hances the opportunity for the protonation–deprotonation
(11), 5-nitrosalicylaldehyde (12), and 3,5-dichlorosalicylal- sequence. After hydrolysis of the intermediate Schiff base,
dehyde (13) was studied in more detail (Figure 2). The ob- the racemized amino ester and the aldehyde are liberated.
3508
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3506–3512

Dynamic Kinetic Resolution of α-Amino Acid Esters
Scheme 2. Proposed mechanism for the aldehyde-catalyzed racemization of α-amino acid esters.
Dynamic Kinetic Resolution of α-Amino Acids
pending on the racemization catalyst, -phenylalanine (15)
was isolated in 71 and 79% yield with an optical purity of
83 and 70%ee, respectively. Because the ee value of the
product was significantly higher by using 3,5-dichlorosali-
cylaldehyde (13), we chose this catalyst for our ongoing
purposes.
With a convenient racemization protocol under bio-
logical conditions in hand, we focused our attention on the
development of in situ coupling of the racemization step
with a resolution step. Thereby, the resolution step would
be based on enzyme-catalyzed ester cleavage to yield the
enantiopure amino acids. Here, commercially available alca-
lase®, an endoproteinase of the serine type, offers the op-
portunity for large-scale synthesis, as it is inexpensive and
has a high stability in organic solvents even at low water
concentrations.[28] The key element in alcalase®, which is
accessible from Bacillus licheniformis, is subtilisin Carlsberg
(E.C. 3.4.21.62, alkaline protease A).
Table 2. Dynamic kinetic resolution of several phenylalanine es-
ters.[a]
By using the crude enzyme extract in preliminary experi-
ments, we observed a crucial solvent–reactivity dependency;
hence, several solvent combinations were investigated. The
most efficient solvent mixture for the DKR of rac-phenylal-
anine benzyl ester (2) was found to be acetonitrile/water
(4:1).
In more detail, the reactions were carried out on a 20-
mmol scale by using 2.0 mL alcalase® (0.6 AU) at 35 °C
in acetonitrile/water (4:1). The pH value was kept constant
during the reaction by the addition of 2- solution of so-
dium hydroxide by an autotitrator. The end point of the
reaction was determined by HPLC and subsequent workup
by cooling to 0 °C and setting the pH to 5.5 within 60 min.
The addition of 1,2-dimethoxyethane led to nearly quanti-
tative precipitation of the free amino acid.
Entry
R
Aldehyde
pH
t [d] Yield [%] ee [%] ()
1
2
3
4
5
Bn
Bn
Bn
Et
20 mol-% 13
20 mol-% 11
10 mol-% 13
5 mol-% 13
5 mol-% 13
8.5
8.5
8.5
7.5
8.0
1
1
1
5
7
71
79
87
72
63
83
70
88
98
92
nBu
[a] Reaction conditions: alcalase® (2.0 mL, 0.6 AU), substrate
(20 mmol), acetonitrile/water (4:1, 50 mL), 35 °C; aldehyde 11: 3,5-
dinitrosalicylaldehyde; aldehyde 13: 3,5-dichlorosalicylaldehyde.
The enantiomeric excess was determined by chiral GC (25 m Chira-
sil-Val) of the corresponding perfluoropropionyl O-methyl ester.
Next, the reaction was carried out with a decreased cata-
The results presented in Table 2 underline the successful lyst loading (10 mol-%) in order to minimize the amount of
combination of alcalase®-mediated resolution of rac-phe- intermediate Schiff base products at the end of reaction and
nylalanine benzyl ester (2) with in situ racemization. De- make the process more economical. Indeed, the reduction
Eur. J. Org. Chem. 2008, 3506–3512
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3509

D. A. Schichl, S. Enthaler, W. Holla, T. Riermeier, U. Kragl, M. Beller
FULL PAPER
of catalyst led to an increased yield (87%) and optical pu- enantioselectivities. The main problem of these reactions is
rity (88%ee). After having demonstrated that dynamic ki- the solubility of the amino acid esters and the enzyme in
netic resolution of the model benzylic ester can be carried the reaction medium. Hence, we did some additional fine
out successfully, we focused our attention on phenylalanine tuning. After some experimentation, it turned out that a
derivatives with other ester functionalities. In general, ben- lower amount of acetonitrile led to improved DKR reac-
zyl esters are problematic to prepare in quantitative yields; tions. In general, a 1:1 mixture of acetonitrile/water fur-
thus, the DKR of aliphatic esters would constitute an im- nished excellent results with these aliphatic amino acid es-
portant improvement. In our test reaction, the catalytic ters. Here, only 2.5 mol-% of 3,5-dinitrosalicylaldehyde as
amount of aldehyde was reduced to 2.5 mol-% and all other catalyst at room temperature was sufficient to maintain
reaction conditions remained unchanged. Although the re- good racemization activity (Table 4).
action times increased for the ethyl and n-butyl ester of phe-
Table 4. Dynamic kinetic resolution of several amino acid ethyl es-
nylalanine (Table 2), which is in agreement with the litera-
ters.[a]
ture, as alcalase® hydrolyses benzyl esters about three times
faster than aliphatic counterparts,[29] the new substrates
were obtained in comparable yields and superior enantio-
selectivities up to 98%ee with respect to the benzyl ester!
As a result of the instability of the aliphatic esters towards
unselective chemical hydrolysis, the best results were ob-
tained at a nearly neutral pH value of 7.5.
Similar to phenylalanine, tyrosine esters can be efficiently
racemized in the presence of 3,5-dinitrosalicylaldehyde (11)
or 3,5-dichlorosalicylaldehyde (13) and resolved at very
mild conditions (25–35 °C; pH 7.5–8.5). Notably, even ali-
phatic esters give yields up to 90% and enantioselectivities
Ͼ98%. In the case of aliphatic esters, we recommend to not
run the DKR reaction above pH 8, because of the increased
chemical hydrolysis of the ester groups. Selected results with
the use of tyrosine esters are outlined in Table 3. In the case
of ,-TyrO-iPr (18; Table 3, Entry 6) a significantly longer
[a] Reaction conditions: alcalase® (1.0 mL, 0.6 AU; for tyrosine es-
ter 0.06 AU), substrate (20 mmol), acetonitrile/water (1:1, 50 mL),
35 °C, 3,5-dinitrosalicylaldehyde (11; 2.5 mol-%). The enantiomeric
excess was determined by chiral GC (25 m Chirasil-Val) of the cor-
responding perfluoropropionyl O-methyl ester. [b] Reaction was
continued for an additional 1 d to give 94% yield and 95%ee.
reaction time (5 d) was needed due to the slow enzymatic
hydrolysis of the sterically hindered isopropyl ester.
Table 3. Dynamic kinetic resolution of racemic tyrosine esters.[a]
Conclusions
We developed an easy and practical method to transform
racemic amino acid esters to the corresponding enantiomer-
ically pure amino acids. We showed that the salicylalde-
hyde-catalyzed racemization is applicable to a broad range
of aliphatic amino acid esters.
Entry
R
T [°C]
pH
t [h]
Yield [%] ee [%] ()
1[b]
2
3
4
5
Bn
Bn
Bn
Et
Et
iPr
35
35
35
25
35
35
8.5
8.5
8.5
8.0
7.5
8.0
24
24
48
20
72
95
87
92
85
92
92
99
97
97
98
98
99
Experimental Section
General: Alcalase® was purchased from Novo Nordisk (Denmark)
as a brownish crude extract with a specific activity of 2.5 and
0.6 Anson-UmL–1. Unless specified, all chemicals were commer-
cially available and used as received. The amino acid esters were
synthesized according to reported protocols by esterification of the
corresponding amino acid with freshly distilled thionyl chloride or
toluenesulfonic acids.[30] The optical purities of the α-amino acids
were determined by chiral HPLC [CSP Crownpak CR(+) 150ϫ4
DIACEL column, eluent: aqueous perchloric acid (1.63 gL–1)] or
chiral GC analysis (Alltech Chiralsil Val column). In the case of
chiral GC analysis, all amino acids were converted into the corre-
6
120
[a] Reaction conditions: alcalase® (2.0 mL, 0.06 AU), substrate
(20 mmol), acetonitrile/water (4:1, 50 mL), 3,5-dichlorosalicylalde-
hyde (13; 2.5 mol-%). The enantiomeric excess was determined by
chiral GC (25 m Chirasil-Val) of the corresponding perfluoropropi-
onyl O-methyl ester. [b] 3,5-Dinitrosalicylaldehyde (11) instead of
3,5-dichlorosalicylaldehyde (13).
Finally, the established method was used for a range of
aliphatic amino acid esters of alanine, leucine, norleucine,
norvaline, and methionine. However, the corresponding
amino acids were obtained only in moderate yields and sponding perfluoropropionate by reaction with perfluoropropionyl
3510 Eur. J. Org. Chem. 2008, 3506–3512
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Dynamic Kinetic Resolution of α-Amino Acid Esters
chloride before measurements were carried out. 1H NMR
(400 MHz) and 13C NMR (100.6 MHz) were recorded with Bruker
ARX 400 or Bruker DPX 250 instruments.
Methionine: H NMR (400 MHz, CF3CO2D): δ = 8.02 (br. s, 2 H,
1
NH2), 4.65 (dd, J = 4.7 Hz, J = 7.4 Hz, 1 H, 2-H), 2.93 (t, J =
6.4 Hz, 2 H, 4-H), 2.62 (m, 1 H, 3-H), 2.47 (m, 1 H, 3-H), 2.24 (s,
3 H, 5-H) ppm. 13C NMR (100.6 MHz, CF3CO2D): δ = 172.9 (C-
1), 53.9 (C-2), 29.1 (C-3), 27.1 (C-4), 13.0 (C-5) ppm. Retention
time based on the corresponding perfluoropropionate (GC, 25 m
Chirasil-Val, Alltech, 120 °C, isotherm): ()-enantiomer 7.57 min
and ()-enantiomer 8.06 min.
Racemization Reaction Mediated by Aldehydes: -Phenylalanine
benzyl ester hydrochloride (200 mg) was dissolved in DME/water
(8.0 mL/2.0 mL) or acetonitrile/water (8.0 mL/2.0 mL). The pH
value was set to 8.5 with aqueous solution of sodium hydroxide.
The reaction was initiated with the addition of aldehyde (20 mol-
%). If necessary, the pH value was corrected during the reaction.
Dynamic Kinetic Resolution of Phenylalanine and Tyrosine Esters:
The racemic amino acid ester hydrochloride (20 mmol) was dis-
solved in acetonitrile/water (40 mL/10 mL). The pH of the solution
was adjusted with a 2- sodium hydroxide solution; meanwhile,
phase separation occurred. The aqueous layer was removed, and
the remaining organic phase was transferred into a flask connected
with an autotitrator (aqueous 2  sodium hydroxide solution). Af-
ter heating to the required temperature, the reaction mixture was
charged with enzyme (0.6 AU for phenylalanine esters, 0.06 AU
for tyrosine esters) and aldehyde. The course of the reaction was
controlled by chiral HPLC. The addition of 1,2-dimeth-
oxyethane (100 mL) lead to nearly quantitative precipitation of the
free amino acid (pH 5.5; temperature: 0 °C), which was collected
by filtration.
Norvaline: 1H NMR (400 MHz, CF3CO2D): δ = 7.34 (br. s, 2 H,
NH2), 4.34 (dd ≈ t, J = 5.9 Hz, 1 H, 2-H), 2.09 (m, 2 H, 3-H), 1.54
(m, 2 H, 4-H), 1.02 (t, J = 7.4 Hz, 3 H, 5-H) ppm. 13C NMR
(100 MHz, CF3CO2D): δ = 175.6 (C-1), 56.3 (C-2), 33.8 (C-3), 19.7
(C-4), 13.5 (C-5) ppm. Retention time based on the corresponding
perfluoropropionate (GC, 25 m Chirasil-Val, Alltech, 90 °C, iso-
therm): ()-enantiomer 4.78 min and ()-enantiomer 5.25 min.
Phenylalanine: 1H NMR (400 MHz, CF3CO2D): δ = 7.70–7.45 (m,
5 H, 5-H, 6-H, 7-H, 8-H, 9-H), 4.88 (dd, J = 4.7 Hz, J = 8.7 Hz,
1 H, 2-H), 3.86 (dd, J = 4.7 Hz, J = 15.0 Hz, 1 H, 3-H), 3.58 (dd,
J = 8.7 Hz, J = 15.0 Hz, 1 H, 3-H) ppm. 13C NMR (100.6 MHz,
CF3CO2D): δ = 174.5 (C-1), 133.3 (C-4), 131.7 (C-6, C-8), 130.9
(C-5, C-9), 130.8 (C-7), 57.3 (C-2), 37.3 (C-3) ppm. Retention time
based on the corresponding perfluoropropionate (GC, 25 m Chira-
sil-Val, Alltech, 120 °C, isotherm): ()-enantiomer 11.03 min and
()-enantiomer 11.56 min.
1
Leucine: H NMR (400 MHz, CF3CO2D): δ = 4.34 (m, 1 H, 2-H),
1.99 (m, 1 H, 4-H), 1.86 (m, 2 H, 3-H), 1.02 (t, J = 5.6 Hz, 6 H,
5-H, 6-H) ppm. 13C NMR (100 MHz, CF3CO2D): δ = 173.7 (C-
1), 52.7 (C-2), 38.9 (C-3), 24.2 (C-4), 20.3 (C-6), 19.7 (C-5) ppm.
Retention time based on the corresponding perfluoropropionate
(GC, 25 m Chirasil-Val, Alltech, 90 °C, isotherm): ()-enantiomer
6.60 min and ()-enantiomer 7.69 min.
[1] A. Kleeman, W. Leuchtenberger, B. Hoppe, H. Tanner in
Ullmann’s Encyclopedia of Industrial Chemistry, VCH,
Weinheim, 1985, vol. A2, pp. 57–97.
[2] a) K. Yonoha, K. Soda, Adv. Biochem. Eng. 1986, 33, 96–130;
b) K. Drauz, H. Waldmann, Enzyme Catalysis in Organic
Chemistry, VCH, Weinheim, 2002; c) M. Breuer, K. Ditrich, T.
Habicher, B. Hauer, M. Keßeler, R. Stürmer, T. Zelinski, An-
gew. Chem. 2004, 116, 806–843.
[3] a) R. O. Duthaler, Tetrahedron 1994, 50, 1539–1650; b) R. M.
Williams, Synthesis of Optically Active α-Amino Acids, Perga-
mon, 1989; c) G. C. Barnett, D. T. Elmore, Amino Acids and
Peptides, Cambridge University Press, 1998; d) J. W. Scott in
Topics in Stereochemistry (Eds.: E. L. Eliel, S. H. Wilen), John
Wiley & Sons Inc. 2007, vol 19, pp. 209–226.
Tyrosine: 1H NMR (400 MHz, CF3CO2D): δ = 7.45 (d, J = 8.5 Hz,
2 H, 6-H, 8-H), 7.22 (d, J = 8.5 Hz, 2 H, 5-H, 9-H), 4.84 (dd, J =
4.8 Hz, J = 8.5 Hz, 1 H, 2-H), 3.72 (dd, J = 4.8 Hz, J = 15.1 Hz,
1 H, 3-H), 3.51 (dd, J = 8.7 Hz, J = 15.1 Hz, 1 H, 3-H) ppm. 13C
NMR (100.6 MHz, CF3CO2D): δ = 172.5 (C-1), 154.2 (C-7), 130.6
(C-4), 125.1 (C-5, C-9), 116.6 (C-6, C-8), 55.1 (C-2), 34.5 (C-3)
ppm. Retention time based on the corresponding perfluoropropi-
onate (GC, 25 m Chirasil-Val, Alltech, 130 °C, isotherm): ()-en-
antiomer 18.04 min and ()-enantiomer 19.08 min.
[4] D. A. Evans, T. C. Britton, J. A. Ellman, R. L. Dorow, J. Am.
Chem. Soc. 1990, 112, 4011–4030.
[5] U. Schöllkopf in Topics in Current Chemistry Vol. 109: Wittig
Chemistry, Springer, Berlin, 1983, pp. 65–84.
Dynamic Kinetic Resolution of Racemic Ethyl Ester of Phenylala-
nine, Tyrosine, Methionine, Norvaline, and Leucine: A mixture of
alcalase® (0.6 AU) and 3,5-dinitrosalicylaldehyde (2.5 mol-%) in
water (50 mL) were connected with an autotitrator and the pH
value was set to 7.5 with aqueous sodium hydroxide solution. Dur-
ing a period of 2 min, a solution of amino acid ester (20 mmol) in
acetonitrile (50 mL) was added. The course of the reaction was
monitored by chiral HPLC. The final reaction mixture was acidi-
fied (pH 5.5), and the solvent was removed. The residue was ex-
tracted with ethyl acetate.
[6] D. Seebach, A. R. Sting, M. Hoffmann, Angew. Chem. 1996,
108, 2880–2921.
[7] a) E. J. Corey, R. J. McCaully, H. S. Sachdev, J. Am. Chem.
Soc. 1970, 92, 2476–2488; b) E. J. Corey, H. S. Sachdev, J. Z.
Gougoutas, W. Saenger, J. Am. Chem. Soc. 1970, 92, 2488–
2501.
[8] Selected more recent examples: a) A. V. Lee, L. L. Schafer, Syn-
lett 2006, 2973–2976; b) K. J. M. Beresford, N. J. Church, D. W.
Young, Org. Biomol. Chem. 2006, 4, 2888–2897; c) A. S. Saghi-
yan, S. A. Dadayan, S. G. Petrosyan, L. L. Manasyan, A. V.
Eur. J. Org. Chem. 2008, 3506–3512
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3511

D. A. Schichl, S. Enthaler, W. Holla, T. Riermeier, U. Kragl, M. Beller
FULL PAPER
Geolchanyan, S. M. Djamgaryan, S. A. Andreasyan, V. I. Ma-
leev, V. N. Khrustalev, Tetrahedron: Asymmetry 2006, 17, 455–
467; d) A. Singh, R. A. Yoder, B. Shen, J. N. Johnston, J. Am.
Chem. Soc. 2007, 129, 3466–3467; e) R. Chinchilla, C. Najera,
F. J. Ortega, Tetrahedron: Asymmetry 2007, 17, 3423–3429; f)
S. Vassiliou, A. Yiotakis, P. A. Magriotis, Tetrahedron Lett.
2006, 47, 7339–7341; g) H. Miyabe, Y. Takemoto, Synlett 2005,
1641–1655; h) D. Gagnon, S. Lauzon, C. Godbout, C. Spino,
Org. Lett. 2005, 7, 4769–4771; i) S. Ogo, K. Uehara, T. Abura,
S. Fukuzumi, J. Am. Chem. Soc. 2004, 126, 3020–3021; j) S.
Shirakawa, Y. Tanaka, K. Maruoka, Org. Lett. 2004, 6, 1429–
1431; k) M. J. O’Donnell, Acc. Chem. Res. 2004, 37, 506–517;
l) H. Ueki, T. K. Ellis, C. H. Martin, T. U. Boettiger, S. B.
Bolene, V. A. Soloshonok, J. Org. Chem. 2003, 68, 7104–7107;
m) T. Ooi, M. Kameda, K. Maruoka, J. Am. Chem. Soc. 2003,
125, 5139–5151; n) R. Dhawan, R. D. Dghaym, B. A.
Arndtsen, J. Am. Chem. Soc. 2003, 125, 1474–1475; o) T. B.
Durham, M. J. Miller, J. Org. Chem. 2003, 68, 27–34; p) B.
Alcaide, P. Almendros, C. Aragoncillo, Chem. Eur. J. 2002, 8,
3646–3652; q) H.-g. Park, B.-S. Jeong, M.-S. Yoo, J.-H. Lee,
M.-k. Park, Y.-J. Lee, M.-J. Kim, S.-s. Jew, Angew. Chem. Int.
Ed. 2002, 41, 3036–3038; r) P.-F. Xu, Y.-S. Chen, S.-I. Lin, T.-
J. Lu, J. Org. Chem. 2002, 67, 2309–2314; s) Y. N. Belokon,
N. B. Bespalova, T. D. Churkina, I. Cisarova, M. G. Ezernits-
kaya, S. R. Harutyunyan, R. Hrdina, H. B. Kagan, P. Kocov-
sky, K. A. Kochetkov, O. V. Larionov, K. A. Lyssenko, M.
North, M. Polasek, A. S. Peregudov, V. V. Prisyazhnyuk, S. Vy-
skocil, J. Am. Chem. Soc. 2003, 125, 12860–12871.
Chem. Commun. 2005, 1898–1900; c) A. Berkessel, F. Cleem-
ann, S. Mukherjee, T. N. Müller, J. Lex, Angew. Chem. Int. Ed.
2005, 44, 807–811; d) A. Berkessel, S. Mukherjee, T. N. Müller,
F. Cleemann, K. Roland, M. Brandenburg, J.-M. Neudörfl, J.
Lex, Org. Biomol. Chem. 2006, 4, 4319–4330.
J. Liang, C. Ruble, G. C. Fu, J. Org. Chem. 1998, 63, 3154–
3155.
a) M. J. Garcia, R. Azerad, Tetrahedron: Asymmetry 1997, 8,
85–92; b) S. Servi, D. Tessaro, G. Pedrocchi-Fantoni, Coord.
Chem. Rev. 2008, 252, 715–726.
K. Drauz, M. Kottenhahn, K. Makryaleas, H. Klenk, M.
Bernd, Angew. Chem. Int. Ed. Engl. 1991, 30, 712–714.
a) H. S. Bevinakatti, R. V. Newadkar, A. A. Banerji, J. Chem.
Soc., Chem. Commun. 1990, 1091–1092; b) H. S. Bevinakatti,
A. A. Banerji, R. V. Newadkar, A. Mokashi, Tetrahedron:
Asymmetry 1992, 3, 1505–1508; c) R. Gu, I. Lee, C. J. Sih, Tet-
rahedron Lett. 1992, 33, 1953–1956; d) J. Crich, R. Bireva, P.
Marquart, R.-L. Gu, S. Flemming, C. J. Shi, J. Org. Chem.
1993, 58, 3252–3258; e) N. J. Turner, J. R. Winterman, Tetrahe-
dron Lett. 1995, 36, 1113–1116; f) S. A. Brown, M.-C. Parker,
N. J. Turner, Tetrahedron: Asymmetry 2000, 11, 1687–1690.
M. A. Wegman, M. A. P. J. Hacking, J. Rops, P. Pereira, F.
van Rantwijk, R. A. Sheldon, Tetrahedron: Asymmetry 1999,
10, 1739–1750.
S.-T. Chen, W.-H. Huang, K.-T. Wang, J. Org. Chem. 1994, 59,
7580–7581.
a) I. Dilek, M. Madrid, R. Singh, C. P. Urrea, B. A. Armitage,
J. Am. Chem. Soc. 2005, 127, 3339–3345; b) H. Bernard, G.
Bülow, U. E. W. Lange, H. Mack, T. Pfeiffer, B. Schaefer, W.
Seitz, T. Zierke, Synthesis 2004, 2367–2375; c) M. A. Brimble,
N. S. Trotter, P. W. R. Harris, F. Sieg, Bioorg. Med. Chem. 2004,
13, 519–532; d) P. Xu, W. Lin, X. Zou, Synthesis 2002, 1017–
1026; e) E. M. Stocking, J. F. Sanz-Cervera, C. J. Unkefer,
R. M. Williams, Tetrahedron 2001, 57, 5303–5320; f) D. L.
Hughes, J. J. Bergan, E. J. J. Grabowski, J. Org. Chem. 1986,
51, 2579–2585; g) R. P. Patel, S. Price, J. Org. Chem. 1965, 30,
3575–3576; h) H. K. Miller, H. Waelsch, J. Am. Chem. Soc.
1952, 74, 1092–1093.
A. Liljeblad, A. Kiviniemi, L. T. Kanerva, Tetrahedron 2004,
60, 671–677.
Y. Asano, S. Yamaguchi, J. Am. Chem. Soc. 2005, 127, 7696–
7697.
a) V. Zimmermann, D. A. Schichl, M. Beller, U. Kragl, Chem.
Ing. Technol. 2004, 76, 1256–1257; b) V. Zimmermann, M.
Beller, U. Kragl, Org. Process Res. Dev. 2006, 10, 622–627.
a) M. Pugniére, A. Commeyras, A. Previero, Biotechnol. Lett.
1983, 5, 447–452; b) R. Grigg, H. G. N. Gunaratne, Tetrahe-
dron Lett. 1983, 24, 4457–4460; c) V. S. Parmar, A. Singh, K. S.
Bisht, N. Kumar, Y. N. Belokon, K. A. Kochetkov, N. S. Ikon-
nikov, S. A. Orlava, V. I. Tararov, T. F. Saveleva, J. Org. Chem.
1996, 61, 1223–1227; d) A. Rios, J. Crugeiras, T. L. Amyes, J. P.
Richard, J. Am. Chem. Soc. 2001, 123, 7949–7950; e) M. D.
Toney, Archives Biochem. Biophysics 2005, 433, 279–287.
a) S.-T. Chen, S.-Y. Chen, S.-C. Hsiao, K.-T. Wang, Biotechnol.
Lett. 1991, 13, 773–778; b) S.-T. Chen, S.-C. Hisao, K.-T.
Wang, Bioorg. Med. Chem. Lett. 1991, 1, 445–450.
S.-T. Chen, K.-T. Wang, C.-H. Wong, J. Chem. Soc., Chem.
Commun. 1986, 1514–1516.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[9] a) H.-U. Blaser, B. Pugin, F. Spindler, J. Mol. Catal. A J. Mol.
Catal. 2005, 231, 1–20; b) M. van den Berg, B. L. Feringa, A. J.
Minnaard in Handbook of Homogeneous Hydrogenation (Eds.:
J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, 2007, pp.
995–1027.
[10] U. T. Strauss, U. Felfer, K. Faber, Tetrahedron: Asymmetry
1999, 10, 107–117.
[11] a) H. Pellissier, Tetrahedron 2003, 59, 8291–8327; b) H. Pelliss-
ier, Tetrahedron 2008, 64, 1563–1601.
[12] a) M. Kitamura, M. Tokunaga, R. Noyori, Tetrahedron 1993,
49, 1853–1860; b) A. J. J. Straathof, J. A. Jongejan, Enzyme
Microb. Technol. 1997, 21, 559–571.
[13] For reviews, see: a) R. Noyori, M. Tokunaga, M. Kitamura,
Bull. Chem. Soc. Jpn. 1995, 68, 36–56; b) R. S. Ward, Tetrahe-
dron: Asymmetry 1995, 6, 1475–1490; c) O. Pàmies, J.-E.
Bäckvall, Chem. Rev. 2003, 103, 3247–3261; d) V. Ratovelom-
anana-Vidal, J. P. Genêt, Can. J. Chem. 2000, 78, 846–851.
[14] a) R. Noyori, T. Ikeda, M. Ohkuma, M. Widhalm, H. Kita-
mura, S. Akutagawa, N. Sayo, T. Saito, T. Taketomi, H. Kumo-
bayashi, J. Am. Chem. Soc. 1989, 111, 9134–9135; b) J. P.
Genêt, S. Mallart, S. Jugé, French Patent, 8911159, 1989.
[15] a) A. Girard, C. Greck, D. Ferroud, J. P. Genêt, Tetrahedron
Lett. 1996, 37, 7967–7970; b) E. Coulon, M. C. Cano de And-
rade, V. Ratovelomanana-Vidal, J. P. Genêt, Tetrahedron Lett.
1998, 39, 6467–6470; c) B. Mohar, A. Valleix, J. R. Desmurs,
M. Felemez, A. Wagner, C. Mioskowski, Chem. Commun.
2001, 2572–2573; d) K. Makino, N. Okamoto, O. Hara, Y.
Hamada, Tetrahedron: Asymmetry 2001, 12, 1757–1762; e) K.
Makino, T. Goto, Y. Hiroki, Y. Hamada, Angew. Chem. Int.
Ed. 2004, 43, 882–884; f) K. Makino, Y. Hiroki, Y. Hamada,
J. Am. Chem. Soc. 2005, 127, 5784–5785; g) K. Makino, T.
Fujii, Y. Hamada, Tetrahedron: Asymmetry 2006, 17, 481–485;
h) K. Makino, M. Iwasaki, Y. Hamada, Org. Lett. 2006, 8,
4573–4576.
[24]
[25]
[26]
[27]
[28]
[29]
[30]
M. Bodanszky, A. Bodanszky, The Practice of Peptide Synthe-
sis, Springer, Berlin, 1984.
[16] a) A. Berkessel, Pure Appl. Chem. 2005, 77, 1277–1284; b) A.
Received: March 7, 2008
Published Online: May 27, 2008
Berkessel, S. Mukherjee, F. Cleemann, T. N. Müller, J. Lex,
3512
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3506–3512