Process for the Preparation of Enantiomeric ally Enriched
Amino Acids
The present invention is concerned with a process for the
resolution of a mixture of enantiomers of N-protected amino
acids, in particular α-amino acids. Especially, this
process claims a crystallisation of a diastereomeric salt
pair consisting of one enantiomer of an N-protected amino
acid and an enantiomerically pure N-unprotected β-amino
acid derivative.
Enantiomerically enriched amino acids, especially α-amino
acids, are versatile tools for synthesising
enantiomerically pure bio-actives, intermediates for
pharmaceuticals or for infusion liquors for parenteral
nutrition. A large amount of these amino acids, especially
L-lysine, is produced for feed additive compositions.
The resolution of mixtures of enantiomers of amino acids,
e.g. racemates, via classical crystallisation techniques is
a rather old and common method for the generation of
enantiomerically enriched amino acids. Although there have
been great efforts to create new methodologies which allow
to synthesise the amino acids in pure form via e.g.
biotechnological means, the classical pathway is still an
option for special amino acids which are not or only poorly
generated by these methods (Izumi et al. Angew. chem. 1978,
90, 187; B. Hoppe et al. Chemie in unserer Zeit, 1984, 18,
73) .
The object of the present invention was, therefore, to
create a technical process for the resolution of
enantiomeric mixtures of such amino acids. In particular
the process should allow to gain especially pure forms of
one enantiomer of these amino acids in a robust and easy to
handle mode. From the perspective of an economical and
ecological viewpoint the present process shall be
advantageous over those described in the prior art, in

particular for the generation of those amino acids which
are disadvantageously produced via known methods.
Mentioned objective is addressed properly by applying a
process according to features of present claim 1 or 7.
Independent claims 2 to 6 or 8 to lldepict preferred
embodiments of the process according to the invention.
By performing a process for the preparation of
enantiomerically enriched N-protected amino acids, wherein
a mixture of both enantiomers of an N-protected amino acid
is resolved through crystallisation of the salt of one
enantiomer of the N-protected amino acid and an
enantiomerically pure N-unprotected β-amino acid
derivative, the solution to the mentioned object is
attained, in particular in a manner that is surprising, by
no means foreseeable and, according to the invention,
particularly advantageous. The instant process is very
feasibly performed even on large scale and allows to
produce enantiomerically enriched amino acids in an
extremely easy and robust fashion. Nonetheless, the results
achieved in view of enantiomeric purity and yield are
outstandingly superb in the light of the easiness of the
process of the invention.
In view of the N-protected amino acids used for instant
process, these can be chosen by the skilled man. Amino
acids preferably resolved are, however, α- and β-amino
acids. Taking these N-protected α-amino acids are most
preferred.
The process of the present invention is advantageously
performed in an organic solvent which can contain water to
an extent that it does not adversely affect the
crystallisation, e. g. through phase separation. In
particular the solvent used is a polar one, preferably
selected from the group consisting of esters, ethers,
ketones, alkohols, aromatic hydrocarbons or mixtures

thereof. Especially preferred is a process in which the
solvent is selected from the group consisting of ethyl
acetate, ethanol, methyl-tert.-butyl ether, toluene.
The choice of an enantiomerically pure N-unprotected β-
amino acid derivative as resolving agent is up to the
skilled man's discretion. The β-amino acid derivative used
may be chosen by performing routine testing in present
process and comparing the results. Enaniomerically pure β-
amino acids are commercially available or can
advantageously be made according to processes known in the
art (DE10220740 or DE10320211 and cited literature). Also
the generation of the respective derivatives, like esters
or amides, is known to the skilled worker (Chemistry of
Amino Acids, Wiley&Sons 1961, J.P. Greenstein, M Winitz).
Preferably an enantiomerically pure N-unprotected β-amino
acid derivative according to formula (I) or (II) is used,

wherein
R1, R2 independently of one another denote H or R,
R, R' independently of one another denote (C1-C8) -alkyl,
(C2-C8)-alkoxyalkyl, (C6-C18)-aryl, (C7-C19) -aralkyl,
(C3-C18) -heteroaryl, (C4-C19) -heteroaralkyl, (C1-C8) -alkyl-
(C6-C18)-aryl, (C1-C8) -alkyl- (C3-C18) -heteroaryl, (C3-C8)-
cycloalkyl, (C1-C8) -alkyl- (C3-C8) -cycloalkyl, (C3-C8)-
cycloalkyl- (C1-C8) -alkyl,
or R and R' ' or R and R'/R1 or R' ' and R'/R1 represent a
(C3-C5)-alkylene bridge mono- or polysubstituted with
(C1-C8)-alkyl, HO-(C1-C8)-alkyl, (C1-C8)-alkoxy,
(C2-C8)-alkoxyalkyl, (C5-C18)-aryl, (C7-C19) -aralkyl,

(C1-C8)-alkyl-(C6-C18)-aryl, (C3-C8) -cycloalkyl,
(C1-C8) -alkyl-(C3-C8) -cycloalkyl,
(C3-C8) -cycloalkyl- (C1-C8) -alkyl,
R" being HO-, (C1-C8)-alkyl, (C2-C8) -alkoxyalkyl, (C6-C18)-
aryl, (C7-C19)-aralkyl, (C3-C18)-heteroaryl, (C4-C19)-
heteroaralkyl, (C1-C8) -alkyl- (C6-C18) -aryl, (C1-C8)-
alkyl- (C3-Cx8) -heteroaryl, (C3-C8) -cycioalkyl, (C1-C8) -
alkyl- (C3-C8) -cycloalkyl, (C3-C8) -cycloalkyl- (C1-C8) -alkyl.
Especially, suitable for present process are esters of β-
amino acids. Preferably aromatic ester like 3-amino-3-
phenylpropionic acid ethyl ester or the respective amide is
used.
The temperature applied during the crystallisation process
shall be as high or as low to allow a maximum
crystallisation yield with an optimum of enantiomeric
excess in view of the N-protected amino acids. The
temperature is, therefore, preferably held within a range
of -30° to 100°C. More preferably the temperature is set to
-25 to 50°C, and most preferred the temperature varies
between -20°C and 30°C.
After crystallisation the mixture is worked-up according to
the knowledge of a skilled worker. Most convenient seems to
be to separate the solid material from the mixture via
filtration. Subsequently the already highly enriched
diastereomeric salt pair can be recrystallised to maximise
diastereomeric purity. Afterwards the enantiomerically
enriched N-protected amino acid can be liberated from this
salt by measures known to the man in the art, e.g. ion-
exchange or classical acidification and extraction
techniques.
It is possible in view of the instant invention to exchange
substrates and crystallising agent for each other to make
the process one, which allows to obtain enantiomerically
enriched N-unprotected β-amino aicds. Hence, the present

invention also concerns a process for the preparation of
enantiomerically enriched N-unprotected β-amino acid
derivates, wherein a mixture of both enantimers of an N-
unprotected β-amino acid derivative is resolved through
crystallisation of the salt of one enantiomer of the N-
unprotected β-amino acid derivative and an enantiomerically
pure N-protected α-amino acid. Preferred embodiments of
this process are in line with those mentioned for
performing the process for the preparation of N-protected
amino acids.
Thus the present invention can easily be performed e.g. by
dissolving the racemic N-protected amino acid in an
appropriate solvent. Upon total dissolution the
enantiomerically pure N-unprotected β-amino acid
derivative, especially the ester, is added in an amount
sufficient to obtain optimal crystallisation results in
view of yield and diastereomeric purity. The molar ratio of
added enantiomerically pure N-unprotected β-amino acid
derivative to the antipode of N-protected amino acid to be
crystallised can vary from 1:0,1 to 1:2, preferably from
1:0,5 to 1:1,8 or most preferably from 1:0,7 to 1:1,5.
Preferably after cooling and completion of crystallisation
the solid material is - as already stated - separated via
filtration from the mixture and optionally recrystallised.
Liberation of the desired N-protected amino acid takes
place preferably through acidification and extraction.
In view of the production of β-amino acid derivates above
mentioned procedure can be applied like with the exception
that N-protected amino acids have to be substituted for N-
unprotected β-amino acid derivates and vice versa.
N-Protected amino acid mean any carboxylic acid having both
a carboxyl-function and an protected amino-function within
one molecule. Preferably an α-amino acid being protected
by a common protective group at the α-nitrogen atom of the
amino acid is used. Suitable protective groups are known to

the skilled worker (Green et al. Protective Groups in
Organic Chemistry, Wiley&Sons, 1981). Preferably residues
selected from the groups consisting of Z-, Boc-, Moc-, Eoc-
, Fmoc-, formyl-, acetyl-, phthaloyl-radicals are chosen.
oc-Amino acid denotes a naturally or unnaturally occurring
amino acid like depicted in Bayer-Walter, Lehrbuch der
organischen Chemie, S. Hirzel Verlag, Stuttgart, 22.
Auflage, S. 822 et seq. Unnatural amino acids are those
mentioned for example in DE19903268.8. Most preferred in
this regard is the resolution of N-protected tert.-leucine.
Methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,
sec-butyl, tert-butyl, pentyl, hexyl, heptyl or octyl,
together with all their bond isomers, can be considered as
(C1-C8)-alkyl radicals.
The (C1-C8) -alkoxy radical corresponds to the (C1-C8) -alkyl
radical, with the proviso that this is bonded to the
molecule via an oxygen atom.
As (C2-C8) -alkoxyalkyl, radicals in which the alkyl chain

is interrupted by at least one oxygen function are meant,
wherein two oxygen atoms cannot be joined to one another.
The number of carbon atoms gives the total number of carbon
atoms contained in the radical.
A (C3-C5)-alkylene bridge is a carbon chain with three to
five C atoms, this chain being bonded to the molecule in
question via two different C atoms. The bridge optionally
can be unsaturated and or can contain one or more
heteroatoms like N, 0, S within the chain.
The radicals just described can be mono- or polysubstituted
with (C1-Cs) -alkoxy, (C1-C8) -alkyl, halogens and/or radicals
containing N, 0, P, S or Si atoms. These are particularly
alkyl radicals of the type mentioned above having one or
more of these heteroatoms in their chain or being bonded to
the molecule via one of these heteroatoms.
(C3-C8)-Cycloalkyl means cyclopropyl, cyclobutyl,
cyclopentyl, cyclohexyl or cycloheptyl radicals etc. These

can be substituted with one or more halogens and/or
radicals containing N, O, P, S or Si atoms and/or can have
N, O, P or S atoms in the ring, such as e.g. 1-, 2-, 3-, 4-
piperidyl, 1-, 2-, 3-pyrrolidinyl, 2-, 3-tetrahydrofuryl,
2~, 3-, 4-morpholinyl.
A (C3-C8) -Cycloalkyl- (C1-C8)-alkyl radical refers to a
cycloalkyl radical as set out above, which is bonded to the
molecule via an alkyl radical as stated above.
(C1-C8) -Acyloxy within the framework of the invention means
an alkyl radical as defined above with a maximum of 8 C
atoms, which is bonded to the molecule via a COO- function.
(C1-C8)-Acyl within the framework of the invention means an
alkyl radical as defined above with a maximum of 8 C atoms,
which is bonded to the molecule via a CO- function.
A (C6-C18)-aryl radical is understood to mean an aromatic
radical with 6 to 18 C atoms. These include in particular
compounds such as phenyl, naphthyl, anthryl, phenanthryl or
biphenyl radicals, or systems of the type described above
annelated to the molecule in question, such as e.g. indenyl
systems, which can optionally be substituted with (C1-C8)-
alkyl, (C1-C8)-alkoxy, (C1-C8)-acyl or (C1-C8)-acyloxy.
A (C7-C19)-aralkyl radical is a (C6-C18)-aryl radical bonded
to the molecule via a (C1-C8) -alkyl radical.
A (C3-C18)-heteroaryl radical within the framework of the
invention refers to a five-, six- or seven-membered
aromatic ring system of 3 to 18 C atoms, which contains
heteroatoms such as e.g. nitrogen, oxygen or sulfur in the
ring. In particular, radicals such as 1-, 2-, 3-furyl, such
as 1-, 2-, 3-pyrrolyl, 1-, 2-, 3-thienyl, 2-, 3-,
4-pyridyl, 2-, 3-, 4-, 5-, 6-, 7-indolyl, 3-, 4-,
5-pyrazolyl, 2-, 4-, 5-imidazolyl, acridinyl, quinolinyl,
phenanthridinyl and 2-, 4-, 5-, 6-pyrimidinyl are
considered as such heteroaromatics.

A (C4-C19)-heteroaralkyl means a heteroaromatic system
corresponding to the (C7-C19) -aralkyl radical.
Fluorine, chlorine, bromine and iodine are suitable as
halogens (Hal).
The term enantiomerically enriched within the framework of
the invention means the proportion of an enantiomer in a
mixture with its optical antipode in a range of >50 % and
<100 %. The ee (enantiomeric excess) value is calculated as
follows:
([Enantiomerl]- [Enantiomer2]} / ( [Enantiomerl] + [Enantiomer2]) =ee value
The term diastereomerically enriched or diastereomeric
purity is meant as to define the proportion of one
diastereomer in a mixture with its other diastereomers in a
range of >50 % and <100 %.
The naming of the molecules used according to the invention
contains, within the framework of the invention, all
possible diastereomers, the two optical antipodes of any
diastereomer also being included therein.
The literature references cited in this document are deemed
to be contained in the disclosure.

Examples:
Example 1:
A solution of 59,9 g Benzyloxycarbonylphenylalanine in ethyl acetate was treated with 60 g (R)-3-Amino-3-
phenylpropionic acid ethyl ester in ethyl acetate at 35°C,
cooled to 0°C and stirred for 40 h. After filtration and
drying 30,5 g of a 1:1 salt of R-
Benzyloxycarbonylphenylalanine x (R) -3-Amino-3-
phenylpropionic acid ethyl ester (86:14 optical purity) was
obtained.
Example 2:
A solution of 50,2 g Benzyloxycarbonylvaline in methyl-
tert.-butyl ether was treated with 60 g (S)-3-Amino-3-
phenylpropionic acid ethyl ester in methyl-tert.-butyl
ether at 30°C, cooled to 0°C and stirred for 30 h. After
filtration and drying 27,1 g of a 1:1 salt of (S)-
Benzyloxycarbonylvaline x (S).-3-Amino-3-phenylpropionic
acid ethyl ester (97:3 optical purity) was obtained.
Example 3:
A solution of 47,6 g Benzyloxycarbonylamino-butyric acid in
methyl-tert.-butyl ether was treated with 21,3 g (S)-3-
Amino-3-phenylpropionic acid ethyl ester in methyl-tert.-
butyl ether at 30°C, cooled to 10°C and stirred for 10 h.
After filtration and drying 45,1 g of a 1:1 salt of (S)-
Benzyloxycarbonylamino-butyric acid x (S) -3-Amino-3-
phenylpropionic acid ethyl ester (92:8 optical purity) was
obtained.

Example 4:
A solution of 106,1 g Benzyloxycarbonylamino-tert.leucine
in methyl-tert.-butyl ether was treated with 42,56 g (R)-3-
Amino-3-phenylpropionic acid ethyl ester in methyl-tert.-
butyl ether at 35°C, cooled to 0°C and stirred for 15 h.
After filtration and drying 71,8 g of a 1:1 salt of (R)-
Benzyloxycarbonylamino-butyric acid x (R)-3-Amino-3-
phenylpropionic acid ethyl ester (97:3 optical purity) was obtained.
Example 5:
A solution of 59,9 g Benzyloxycarbonylamino-tert. -leucine
in ethyl acetate was treated with 60 g (R)-3-Amino-3-
phenylpropionic acid ethyl ester in ethyl acetate at 42°C,
cooled to -10°C and stirred for 10 h. After filtration and
drying 38,5 g of a 1:1 salt of (R) -Benzyloxycarbonylamino-
tert.-leucine x (R)-3-Amino-3-phenylpropionic acid ethyl
ester (98,8:1,2 optical purity) was obtained.
After recristallisation all salt pairs were obtained in an
optical purity of >99:1.

WE CLAIM:
1. Process for the preparation of enantiomerically enriched N-protected
amino acids, wherein a mixture of both enantiomers of an N-protected amino
acid is resolved through crystallisation of the salt of one enantiomer of the N-
protected amino acid and an enantiomerically pure N-unprotected β-amino
acid derivative.
2. Process as claimed in claim 1, wherein the N-protected amino acid is an N-
protected α-amino acid.
3. Process as claimed in claim 1 and/or 2, wherein the crystallisation is
performed in a polar solvent.
4. Process as claimed in one or more of the preceding claims, wherein the
enantiomerically pure N- unprotected β-amino acid derivative is an ester or an
amide.
5. Process as claimed in one or more of the preceding claims, wherein the
temperature during crystallisation is between -20°C and 30°C.

6. Process as claimed in one or more of the preceding claims, wherein the
crystallised salt is separated from the mixture via filtration.
7. Process for the preparation of enantiomerically enriched N-unprotected β-
amino acid derivates, wherein a mixture of both enantiomers of an N-unprotected
β-amino acid derivate is resolved through crystallisation of the salt of one
enantiomer of the
N-unprotected β-amino acid derivative and an enantiomerically pure N-protected α-amino acid.
8. Process as claimed in claim 7, wherein the crystallisation is performed in a
polar solvent.
9. Process as claimed in claims 7 to 8, wherein the enantiomerically pure N-
unprotected β-amino acid derivative is an ester or an amide.
10. Process as claimed in claims 7 to 9, wherein the temperature during
crystallisation is between -20°C and 30°C.

11. Process as claimed in claims 7 to 10, wherein the crystallised salt is
separated from the mixture via filtration.

ABSTRACT

Title: Process for the preparation of enantiomerically enriched amino acids
Process for the preparation of enantiomerically enriched N-protected amino
acids, wherein a mixture of both enantiomers of an N-protected amino acid is
resolved through crystallisation of the salt of one enantiomer of the N-protected
amino acid and an enantiomerically pure N-unprotected β-amino acid derivative.