A process for the production of carnitine
The invention relates to methods for the production of L-carnitine.
Background of the invention
Carnitine (vitamin Bt; 3-hydroxy-4-trimethylammonio-butanoate) is a quaternary
ammonium compound biosynthesized from the amino acids lysine and methionine.
In living cells, it is required for the transport of fatty acids from the cytosol into the
mitochondria during the breakdown of lipids for the generation of metabolic
energy. It is used as a nutritional supplement. Carnitine exists in two
stereoisomers. The biologically active form is L-carnitine, whilst its enantiomer, Dcarnitine,
is biologically inactive. When producing L-carnitine in an industrial
process, it is desirable to produce the biologically active L-form in high purity.
Various methods were described for the industrial production of L-carnitine.
Microbiological processes are known, in which L-carnitine is produced directly by
bacteria. In other processes, a racemate is produced by organic synthesis and
separated subsequently into enantiomers.
Methods have been described for producing L-carnitine by chirai synthesis in the
presence of asymmetric catalysts. In this respect, Santaniello et al. (1984) disclose
the direct production of chirai L-carnitine from various precursors. In a specific
approach, a ketoester corresponding to carnitine is reduced in the presence of an
asymmetric catalyst, followed by the reaction of the product with trimethylamine in
order to obtain L-carnitine. The authors also suggest producing L-carnitine from a
b-lactam cyclic precursor.
Attempts have been made to synthesize L-carnitine from b-lactone intermediates.
In principle, b-lactones are available by [2+2] cycloadditions of ketenes and
aldehydes. Chiral catalysts for obtaining chiral b-lactones were originally described
by Wynberg et al. (1982). The authors found that a cycloaddition reaction could be
carried out in the presence of a chiral quinidine catalyst. However, the availability
of b-lactones suitable for carnitine synthesis is severely limited, because the
reactions usually require activated ketones or aldehydes.
Based on the findings of Wynberg et al., Song et al. (1995) developed a full
synthesis of L-carnitine starting from chloral (trichloroethanal) and ketene. The
overall reaction is shown in scheme 1 below.
H
Scheme 1: Synthesis of L-carnitine according to Song et al., 1995.
After the ring opening reaction, two chlorine atoms have to be eliminated from the
carnitine precursor. Several steps are necessary to convert the trichloral ester into
the corresponding monochlorinated equivalent before final conversion into
carnitine. This renders the overall process time consuming and complicated.
Further, the method requires chloral and tin organic reactants, which are toxic.
Since carnitine is usually required for food or feed applications, it is desirable to
avoid such toxic substances. Further, n-Bu3SnH has to be produced in situ, which
is relatively complicated. In view of these drawbacks, the industrial applicability of
this pathway is severely limited.
Since chiral L-carnitine is an important industrial product, it would be desirable to
provide alternative efficient processes for its production. Specifically, it would be
desirable to provide processes which allow the production of L-carnitine in a
relatively simple manner and at a high yield.
Problem underlying the invention
The problem underlying the invention is to provide a method for producing Lcarnitine,
which overcomes the above-mentioned drawbacks. Specifically, the
problem is to provide an efficient and simple process for the production of Lcarnitine.
The total yield as well as the chiral yield shall be high. Further, the necessary
chemicals shall be readily available and should not be too expensive. Specifically,
the use of catalysts comprising precious metals, such as platinum, shall be
avoided.
The number of process steps shall be relatively low and the process shall not
require complicated apparatuses. Overall, the process shall be cost and labour
efficient.
Disclosure of the invention
Surprisingly, the problem underlying the invention is solved by the process
according to the claims. Further inventive embodiments are disclosed throughout
the description.
Subject of the invention is a process for the production of L-carnitine, wherein a
chiral b-lactone carnitine precursor is obtained by a [2+2] cycloaddition of ketene
with an aldehyde X-CH2-CHO, wherein X is selected from CI, Br, I and
trimethylammonium, in the presence of a chiral catalyst.
Ketene (ethenone, formula C2H2O) is a colorless gas, which is highly reactive due
to two adjacent double bonds in the molecule.
According to the invention, a direct and simple synthesis of carnitine is carried out
starting from monohalogenated acetaldehyde or from trimethylammonium
acetaldehyde in a [2+2] cycloaddition. The chiral b-lactone obtained thereby can
be converted directly into L-carnitine.
Surprisingly it was found that chiral b-lactones, which can be simply converted into
L-carnitine, can be obtained in the presence of chiral catalysts. Chiral catalysts
usually comprise at least one asymmetric atom. However, other chiral catalysts
are known, which are chiral although not comprising a chiral C-atom, for example
BINAP. They interact with the reactants in a manner such that chiral products are
obtained instead of a racemate. When the ketene reacts with the aldehyde in a
[2+2] cycloaddition in the presence of a chiral catalyst, a chiral b-lactone is
obtained. The chiral b-lactone is a substituted 4-methyloxetane-2-one, which has
the residue R attached to the methyl group. Thus, the reaction product, depending
on residue R, is 4-(chloromethyl)oxetane-2-one, 4-(bromomethyl)oxetane-2-one,
4-(iodomethyl)oxetane-2-one or 4-[(trimethylammonium)methyl]oxetane-2-one.
In a preferred embodiment of the invention, the chiral catalyst is selected from
Lewis acid-Lewis base bifunctional metal catalysts and phosphine catalysts.
Preferably, the chiral catalyst is a Lewis acid-Lewis base bifunctional metal
catalyst. The Lewis acid and Lewis base can either be separate compounds or can
be associated with each other by ionic, covalent or other interactions, for example
in a metal complex. When being separate components, the Lewis acid and Lewis
base are associated with each other at least in the catalytic state in order to
catalyze the enantioselective reaction. The Lewis acids are preferably metal
atoms, metal ions or metal salts and the Lewis bases are chiral organic ligands,
usually comprising amine, phosphine, alcohol and/or amide groups. The catalysts
are bifunctional, because the chirality is a property of the ligands and thus
independent from the Lewis base. Therefore, the bifunctional catalysts are distinct
from chiral metal complex catalysts such as Wilkinson catalyst, in which only the
overall complex, but not the ligands themselves, are chiral.
In a preferred embodiment of the invention, the Lewis acid/Lewis base bifunctional
catalyst comprises a metal atom as the Lewis acid. The Lewis acid may be
provided in the form of an ion, a salt or a metal complex. One, two or more ligands
may be attached to the metal to form a metal complex. In a preferred embodiment
of the invention, the metal is selected from those of groups (I) and (II) of the
periodic table, preferably lithium, sodium, potassium, magnesium and calcium.
Further preferred are silver, gold, cobalt, aluminum, copper, nickel, chromium,
iron, tin, zinc, manganese, scandium, titanium and boron. In a more preferred
embodiment, the metal is lithium, aluminum or cobalt, especially cobalt (II) or (III).
Preferably, the central cobalt ion has two ligands attached thereto, which may be
covalently linked to each other.
The Lewis acid-Lewis base bifunctional metal catalyst may be a metal chelate
complex, in which the metal is bound in a fixed position. Alternatively, the Lewis
acid and the Lewis base may be separate entities, which can be added separately
to the reactants. In this embodiment, a metal complex is formed in situ at least
transiently.
When the Lewis acid is an ion or salt of silver or metals of groups (I) or (II) of the
periodic table, preferably lithium, it is preferably provided in the form of a salt,
especially lithium perchlorate, to the reaction mixture. Preferably, the anion of the
salt is chosen such that it does not interfere with the reaction. It should not
become a coordinating entity in the Lewis acid/Lewis base complex. This means,
that it is not become part of or bind to the catalytic complex and should not
contribute to or inhibit the catalytic reaction. Preferred such anions are perchlorate,
mesylate, PF6 or BF4 . Suitable anions are also those used in ionic liquids.
Preferably, the chiral catalyst comprises a Lewis base selected from chiral amines,
chiral phosphines, chiral alcohols and chiral amides.
The chiral amine is preferably an alkaloid, preferably quinine or quinidine, a di -
triamine or salen. The chiral phosphine is preferably SEGPHOS or TUNEPHOS.
The chiral phosphine is preferably BINAP. The chiral amide is preferably a
bissulfonamide. The chiral catalyst may also be a derivative of any of the above.
Quinine (CAS 130-95-0) and its stereoisomer quinidine (CAS 56-54-2) are
heterocyclic aromatic compounds of the formula C20H24N2O2. Quinine is a natural
alkaloid having pharmaceutical properties.
Salen (CAS 94-93-9) is a chelating ligand used in coordination chemistry and
homogeneous catalysis. The name salen is a contraction for salicylic aldehyde
and ethylenediamine.
The Lewis base may be a derivative of the above mentioned ligands. The term
"derivative" relates to ligands, which are chemically altered, but still function as
ligands in the complex and provide catalytic activity. For example, derivatives may
be those in which have additional residues, such as alkyl, halogen or arylic groups,
attached to the basic molecular structure. Further, portions of the molecules may
be replaced by other groups. For example, arylic rings may be replaced by
heteroarylic rings. However, the derivatives or catalysts comprising the derivatives
should have the same or essentially the same catalytic activity as the above
mentioned compounds. Preferably, the catalytic activity of the derivative should be
at least 50% or at least 100% of the original compound. Derivatives may be
designed to improve the stability or the catalytic activity of the chiral catalyst.
In a specific embodiment of the invention, a Lewis acid-Lewis base bifunctional
metal catalyst is used having two ligands, which are covalently linked to each
other. For example, a salen molecule is covalently linked to a quinine or quinidine
molecule, specifically by an ester bond.
In a highly preferred embodiment of the invention, the Lewis acid/Lewis base
bifunctional catalyst is a chiral alkaloid in combination with a lithium salt. Preferred
respective Lewis acid/Lewis base catalyst systems are disclosed by Calter (1996),
Zhu et al. (2004), and Shen et al. (2006). The catalysts comprise cinchona alkaloid
Lewis bases and derivatives thereof in combination with lithium perchlorate as a
Lewis acid. Usually, the Lewis base and the salt are added separately into the
reaction mixture. Thus the catalyst is formed in situ. According to the invention, the
alkaloid is preferably a derivative of quinine or quinidine, which is substituted at the
chiral 9-position with a bulky substituents. Preferably, the bulky substituents
comprises between 3 and 15, more preferably between 4 and 8 carbon and /or
silicium atoms. In preferred embodiments, it is selected from branched alkyl
groups, such as /so-butyl and e -buty , and branched silyl group with alkyl and/or
aryl substituents, preferably triarylsilyl groups and trialkylsilyl groups. Especially
preferred is (trimethylsilyl)quinine in combination with lithium perchlorate.
The alkaloids are preferably quinine, quinidine or derivatives thereof. Preferred
derivatives are propionylquinidine and propionylquinine, and especially preferred
are (trimethylsilyl)quinine (TMSQ, shown in scheme 2a) below) and
(trimethylsilyl)quinidine (TMSq). The catalysts disclosed in these documents are
incorporated herein by reference. Specifically, the catalysts of Calter (1996) in
table 1, of Zhu et al. (2004) in Fig. 1 and of Shen et al. (2006) in Fig. 1 are
incorporated by reference. Further, the methods and conditions of their use are
preferably applied according to the invention.
A preferred group of catalysts comprises a central Al(lll) atom, to which two
sulfonamide groups and one additional residue are attached. The additional
residue is preferably an organic residue, preferably methyl, ethyl or phenyl, or
inorganic, such as halide, especially CI, or CN. Thereby, the Al(lll) is coordinated
by the respective N-atoms of the sulfonamide groups. The sulfonamide groups
may be substituted, preferably with aryl or alkyl groups. Preferably, they are linked
to each other through a bridging group. Chirality is conferred to the catalyst either
by one or more stereocenters adjacent to nitrogen atom(s) of the sulfonamide
groups or by the bridging group. Preferably, such catalysts have the following
formula:
wherein
R 1 is alkyl having 1 to 10 carbon atoms, preferably methyl, ethyl, propyl or butyl;
or aryl, preferably phenyl; or a halide, such as CI, or CN
R2 and R3 are independently from each other defined as R 1 above, or
halogenated alkyl, such as -CF3,
X is an organic bridging group comprising 1 to 20, preferably 1 to 6 atoms, which
can be substituted, preferably by aryl, especially phenyl, or alkyl;
and/or wherein X optionally comprises at least one heteroatom, such as N, O or S;
and/or wherein X optionally is a cyclic group, such as a cycyoalkyi, preferably
cyclohexyl.
In another embodiment of the invention, the bifunctional catalyst comprises a
Lewis acid, which is a central aluminum (III), which is attached to Lewis base. In a
preferred embodiment, the Lewis base is a chiral triamine compound, optionally in
combination with an additional non-chiral ligand. Such catalysts are disclosed by
Nelson et al. (1999), Nelson and Spencer (2000) and Nelson et al. (2002).
Specifically, aluminum (III) catalysts useful according to the invention are
disclosed in Nelson et al., 1999, Fig. (Ill) on page 9742, and incorporated herein
by reference. A catalyst of Nelson et al., 1999, is shown in scheme 2c) below.
Further, the specific catalyst disclosed in Nelson and Spencer (2000) in scheme 2
is applicable according to the invention and incorporated by reference. Catalysts
from the same class are disclosed in Nelson et al., 2002, Fig. 2 , and also
incorporated by reference. Further, the methods and conditions of the use of the
catalysts according to Nelson and co-workers are preferably applied according to
the invention.
In another preferred embodiment, the central aluminum (III) is coordinated with a
chiral alcohol, or with chiral phosphines, such as BINAP, SEGPHOS (5,5 -
Bis(diphenylphosphino)-4,4'-bi-1 ,3-benzodioxole) or TUNEPHOS ((R)-1 , 13-
Bis(diphenylphosphino)-7,8-dihydro-6H-dibenzo[f,h][1 ,5]dioxonin; CAS 301847-89-
2) or derivatives thereof.
In another preferred embodiment, a central aluminum (III) is coordinated with a
chiral amide, preferably an amide comprising two amide groups in a chelate
complex. Preferably, the ligand is a bissulfonamide and the catalyst is a
bissulfonamide aluminum complex. Such complexes and their use for [2+2]
cycloadditions are disclosed in Kull and Peters, 2007. The chiral catalysts
disclosed therein, especially in scheme 1 and scheme 3, are incorporated herein
by reference. The Lewis acid may be derived from an organoaluminum compound,
specifically an alkylaluminum compound, such as DIBAL
(diisobutylaluminumhydride) or triethylaluminum. Preferred amide catalysts are
shown in scheme 2 e). Other useful amide catalysts, such as the aluminium
complex of ( 1R, 2R)-1 ,2-N,N-bis(2,4,6-triisopropylbenzenesulfonylamino)-1 ,2-
diphenylethane, are disclosed by US6040465 and JP1 1255759 and also
incorporated herein by reference.
BINAP is an abbreviation for the organophosphorus compound 2,2'-
bis(diphenylphosphino)-1 ,1'-binaphthyl (CAS number of (S)-form: 76189-56-5).
BINAP is a chiral ligand, which is used in asymmetric synthesis. It consists of a
pair of 2-diphenylphosphinonaphthyl groups linked at the 1 and 1' positions. In
formula (I) below, BINAP is the compound in which all R are H.
In another embodiment of the invention, the Lewis acid/Lewis base bifunctional
catalyst has the formula
Co(ll)L L2 or Co(lll)LiL 2Y , wherein
L is salen or a derivative thereof,
L2 is quinine or quinidine or a derivative thereof and
Y is a monovalent anion,
wherein L and L2 are covalently linked, preferably via an ester bond.
In a preferred embodiment, the monovalent anion Y comprises antimony and is
more preferably [SbFe] . Such Lewis acid/Lewis base bifunctional catalysts
comprising central cobalt atoms are disclosed by Lin et al. (2007a, 2007b) and
Chidara and Lin (2009, see scheme 2b) below). Specifically, herein incorporated
by reference is the Lewis acid/Lewis base bifunctional catalyst disclosed by Lin et
al (2007a), Fig. 1 (page 568). Further incorporated by reference is the specific
asymmetric bifunctional catalyst disclosed in scheme 2 of Lin et al. (2007a) on
page 569, which carries a central cobalt atom. These catalysts are also studied in
Lin et al (2007b). In these catalysts, a central cobalt atom, preferably cobalt (II) or
(III), is bound to a salen and quinidine ligand, both of which are covalently linked to
each other via an ester bond. The catalyst thus comprises a central metal atom as
a Lewis acid, which provides electrophilic activation of the aldehyde. The chiral
Lewis base, which is quinidine, provides asymmetric induction in the reaction. The
salen moiety binds the cobalt atom as a planar chelating agent. Further
incorporated by reference is the specific catalyst disclosed by Chidara and Lin
(2009). As shown in scheme 3 of this publication, the catalyst comprises a central
cobalt (III) ion, which is bound to a salen ligand which is covalently attached to
quinine or quinidine by an ester bond. Since the cobalt is in oxidative state (III), it
carries a positive charge, and thus an anion is comprised, which is [SbFe] . The
general methods and conditions of the use of the catalysts according to Lin and
co-workers are preferably applied according to the invention.
In another highly preferred embodiment of the invention, the catalyst is a chiral
organic phosphine. Usually, such catalysts comprise in one molecule one, two or
more phosphor atoms and one or more aromatic ring systems. Amongst such
phosphines, BINAPHANE ((R,R)-1 ,2-Bis[(R)-4,5-dihydro-3H-binaphtho(1 ,2-c:2',1'-
e)phosphepino]benzene; CAS 25331 1-88-5; see scheme 2d)) is preferred, either
in the R- or S-form. The development and use of BINAPHANE is disclosed by
Mondal et al., 2010. Preferably, the general catalytic process conditions disclosed
therein are used according to the invention and incorporated herein by reference.
When using a chiral phosphines catalyst, the addition of a Lewis acid is in principle
possible, but not necessary.
a)
b)
Scheme 2: preferred catalysts: a) (trimethylsilyl)quinidine (TMSQ, Calter et al.,
1996), b) Lewis acid/Lewis base bifunctional catalysts with central cobalt atom
(Chidara and Lin, 2009), c) catalyst with central aluminum (III) attached to
triamine compound and additional ligand according to Nelson et al., 1999, d) (R)-
BINAPHANE (Mondal et al., 2010), e) Al-bissulfonamide catalysts according to
Kull and Peters, 2007.
From the catalysts mentioned above, the catalysts based on chiral alkaloids in
combination with a lithium salt and the chiral phosphine catalyst are especially
preferred, because they are relatively easily available and thus eligible for
synthesis in an industrial scale.
In preferred embodiment, the catalyst is recycled. The catalyst can then be reused
in multiple reactions. The reaction mixture or the solvent can be separated from
the catalyst by known methods, such as pervaporation, nanofiltration, membrane
separation, membrane filtration, electrodialysis, diafiltration, reverse osmosis,
liquid chromatography (LC), HPLC, extraction, crystallization and the like. It is also
preferred to immobilize the catalyst on a solid phase, such as a silica support. For
example, a catalyst having a free vinyl group, such as a quinine or quinidine vinyl
group, could be attached to a silica support by hydrosilylation.
Another subject of the invention is a process for the production of a chiral b-
lactone, wherein the chiral b-lactone is obtained by a [2+2] cycloaddition of ketene
with an aldehyde X-CH2-CHO, wherein X is selected from CI, Br, I and
trimethylamine, in the presence of a chiral catalyst. The b-lactone is an L-carnitine
precursor and thus a valuable intermediate in the synthesis of L-carnitine. The
reactants, catalysts, conditions etc. regarding this embodiment are selected as
outlined above for the synthesis of L-carnitine.
In a preferred embodiment of the invention, the aldehyde X-CH2-CHO is obtained
in a preceding step from 2,4,6-(X-CH2)3- 1,3,5-trioxane, wherein X is selected from
CI, Br, I and trimethylammonium. It was found that halogenated ethanal can be
obtained from the corresponding halogenated ,3,5-trioxane. In a preferred
embodiment, the halogenated trioxane, which carries one halogen atom at each
methyl group, is obtained in a preceding step from 2,4,6-methyl-1 ,3,5-trioxane.
Preferably, this reaction is carried out with molecular chlorine. A preferred reaction
pathway is shown in the left side of scheme 3 below.
In specific embodiments of the invention, the b-lactone is 4-(chloromethyl)oxetane-
2-one, 4-(bromomethyl)oxetane-2-one or 4-(iodomethyl)oxetane-2-one, and the b-
lactone is converted to 4-[(trimethylammonium)methyl]oxetanone-2-one.
Preferably, the reaction is carried out with trimethylamine (TMA).
The b-lactones obtained according to the inventive process are converted into Lcarnitine.
The reaction requires opening of the b-lactone ring.
In a specific embodiment of the invention, the b-lactone is 4-
[(trimethylammonium)methyl]oxetanone-2-one, and is hydrolyzed in a ring opening
reaction in order to obtain L-carnitine directly. The ring opening reaction can be
carried out according to known methods, for example in the presence of a base,
preferably in sodium hydroxide solution.
In preferred embodiments of the invention, the b-lactone is 4-
(chloromethyl)oxetane-2-one, 4-(bromomethyl)oxetane-2-one or 4-
(iodomethyl)oxetane-2-one. The use of 4-(chloromethyl)oxetane-2-one is
preferred. The b-lactone is a chiral b-lactone. L-carnitine is available when the (R)-
b-lactone is used. The lactone ring is opened by hydrolysis and the halogen atom
is substituted by a trimethylammonium group in the presence of trimethylamine
(TMA).
The b-lactone obtained according to the inventive reaction may be isolated or
purified before the ring opening reaction. Alternatively, the reaction product of the
cycloaddition may be subjected to the ring opening reaction without purification of
the b-lactone, if required after a pre-treatment, such as a quenching or
neutralization step. In this embodiment, it is preferred to add the reaction product
comprising the b-lactone to an aqueous mixture comprising a metal hydroxide and
TMA. Preferably, this reaction is carried out in a two-step pathway according to the
state of the art or in a novel one step pathway.
In the two step pathway, which is disclosed in CH 680 588 A5, the halogenated b-
lactone is hydrolysed, usually under basic conditions, in a first step to obtain 4-
halo-3-hydroxybutyric acid. In a second step, the acid is converted into L-carnitine
with TMA.
Surprisingly, it was found according to the invention that the hydrolysis of the b-
lactone and reaction with TMA can be carried out in one step. Preferably, the b-
lactone is not subjected to a basic hydrolysis step before being contacted with the
trimethylamine. In the reaction, the b-lactone ring is opened and the halogen atom
is substituted with a trimethylamine group in a nucleophilic substitution reaction.
The halogenated b-lactone can be converted into L-carnitine without a basic
hydrolysis before the TMA addition. The TMA can be brought into contact with the
b-lactone together with an additional base for basic hydrolysis, or the reaction can
be carried out without an addition of an additional base at all, or an additional base
for basic hydrolysis might be added after bringing the b-lactone in contact with the
TMA. Scheme 1 below shows an exemplified inventive reaction for the production
of carnitine, in which a chlorinated b-lactone is brought into contact with a
combination of TMA and aqueous NaOH as a hydrolytic base.
Scheme 1: Synthesis of L-carnitine by cleavage of a cyclic b-lactone.
The prior art requires a two step pathway for ring opening of b-lactones, which is
disclosed in CH 680 588 A5. In a first step, the halogenated b-lactone is
hydrolysed under basic conditions to obtain 4-halo-3-hydroxybutyric acid. In a
second step, the acid is converted into L-carnitine with TMA. This two-step
approach was used, because in a one-step reaction numerous side reactions were
observed or expected, which concur with the desired reaction and inhibit carnitine
formation or at least strongly reduce the yield. The side reaction and side
products, which are observed and would be expected when carrying out the basic
hydrolysis and the halogen substitution with TMA in one single step, are
summarized in scheme 2 below. Scheme 2 illustrates all the side reactions which
occur, or could occur in theory, when 4-(chloromethyl)oxetane-2-one is reacted
with NaOH and TMA. Scheme 2 thus shows the reaction pathways, which are
observed in one single reaction batch. Some of the products, such as the lactone
13, may be transient intermediates. Other compounds, especially hydroxycrotonic
acid 8, crotonobetaine 10 and the cyclic lactone 6 and furanone 7 are competitive
end products. When analyzing the product mixture of a reaction, it was found that
the main impurities within this synthesis are hydroxycrotonic acid 8 and
crotonobetaine 10. In principle, the 4-(chloromethyl)oxetane-2-one 4 can enter two
reaction pathways in the presence of NaOH and TMA. The first pathway starts
with basic hydrolysis of the beta lactone 4 to chlorohydroxybutyric acid 5, which
can cyclize giving the hydroxybutyrolactone 6 or after elimination of water forming
the furanone 7 . Formation of hydroxybutyric acid 8 proceeds via intermediate 9,
which results from elimination of water from compound 5 . Additionally, furanone 7
can also be formed by cyclization reaction of intermediate 9 . Crotonobetaine 10
can be obtained by either L-carnitine 1 eliminating water or by compound 9
reacting undergoing nucleophilic substitution of the chloride by trimethylamine.
Also epoxy acid 11 can be formed from L-carnitine 1 or 5 by intramolecular
nucleophilic substitution of chloride or ammonium by the alcohol group. As both
the primary alkylhalogenide in 5 and the ammonium group in L-carnitine 1
represent good leaving groups, a side reaction is their nucleophilic substitution by
hydroxide giving the diol 12. The second pathway starts with the amination of the
chloro-R-lactone 4 to intermediate 13, which is hydrolyzed with sodium hydroxide
to L-carnitine 1. Especially by having not the right reaction conditions, L-carnitine 1
can also undergo further reactions such as cyclization and elimination giving side
products 6 and 7 or the above mentioned elimination yielding compound 10.
Scheme 2 : Potential reactions of 4-(chloromethyl)oxetane-2-one upon contact with
a combination of NaOH and TMA. According to the invention, side reactions can
be suppressed and carnitine 1 is obtained as the main product.
In summary, scheme 2 shows that a multitude of reactions occurs, or would at
least be expected, when carrying out a basic hydrolysis of the b-lactone and the
nucleophilic substitution reaction with TMA at the same time in one batch. The
skilled person would not have expected that both reactions could be carried out
efficiently at the same time in the same batch, i . e. that the addition of the TMA
and an additional base together would yield carnitine in high amounts. In contrast,
he would have expected that especially hydroxycrotonic acid 8 and crotonobetaine
10 and cyclic lactones 6 and 7 would be obtained at significant high yields. Indeed,
in initial experiments it was found that the addition of a combination of NaOH with
TMA to the b-lactone precursor did not yield L-carnitine in relevant amounts, but
various side products as shown in scheme 2 instead. Surprisingly, it was found in
further experiments that upon variation of the process conditions (as outlined
further below and as shown in the examples), a selective shift of the overall
reactions towards L-carnitine production in high amounts occurred. It is unusual
that two different process steps can be combined in one single step in a reaction,
which is as complicated as outlined above and as illustrated by scheme 2.
According to the invention, the basic hydrolysis (ring opening reaction) and
reaction with trimethylamine are preferably carried out in one process step. An
additional base different from TMA may be added for the basic hydrolysis.
Alternatively, the conditions can be adjusted such that the base TMA itself triggers
the basic hydrolysis. In this embodiment, it is not necessary to add an additional
base.
In a preferred embodiment, an additional base is added, which is preferably a
metal hydroxide. In this embodiment, the b-lactone should be brought into contact
with the additional base and with the trimethylamine essentially at the same time.
Preferably, the additional base and the trimethylamine are added at the same
time, preferably in the form of a mixture, such as a solution or suspension, of metal
hydroxide and trimethylamine, or by adding a metal hydroxide solution whilst
passing gaseous TMA through the reaction mixture.
When added at the same time, the metal hydroxide triggers the basic hydrolysis
and the trimethylamine reacts with the b-lactone by replacing the halogen atom in
a nucleophilic substitution. The term "essentially" expresses, that it is not
necessary that both components are added precisely at the same time. In
principle, both components can be added to the reaction mixture one after the
other within a short time span. However, the metal hydroxide should be added
before the trimethylamine has considerably reacted in a nucleophilic substitution,
or vice versa the trimethylamine should be added before the metal hydroxide has
considerably reacted in the ring opening reaction. Thus, both components also can
be added one after the other, as long as it is ensured that both reactions are
carried out simultanously, or at least that 90% or 95% of the reactions are carried
out simultaneously. Especially when it is ensured that the reactions do not proceed
or proceeds slowly, for example due to a low temperature, it is possible to add one
component first and the second component subsequently. When adding the metal
hydroxide before the TMA, it should be ensured that no basic hydrolysis occurs
before the TMA is added, or that only a neglectable basic hydrolysis occurs, for
example of less than 5% of the total b-lactone.
In a preferred embodiment, the basic hydrolysis is carried out by adding a metal
hydroxide, preferably sodium hydroxide. In principle, the basic hydrolysis is an
ester hydrolysis reaction and reactants know in the art can be used for this step.
Thus the basic hydrolysis can also be carried out with other bases, for example
potassium hydroxide, lithium hydroxide, calcium hydroxide or magnesium
hydroxide.
Preferably, the solvent used according to the invention is water. Alternatively, the
reaction can be carried out in a two-phase system comprising water and an
organic solvent. In another embodiment, the reaction may be carried out without
water in an organic solvent, for example an alcohol, such as ethanol. In this
embodiment, a base is added which is free of water or essentially free of water.
In a preferred embodiment, the amount of the additional base, especially the metal
hydroxide, is 1. 1 to 1.6 equivalents, preferably 1.2 to 1.4 equivalents, based on the
initial amount of b-lactone. As outlined above, the basic hydrolysis is an ester
hydrolysis reaction, which is in principle well known in the art. However, the basic
hydrolysis of an ester according to the state of the art is commonly carried out with
a high surplus of a base, for example with a metal hydroxide, such as sodium
hydroxide, in a surplus of about 3 to 4 equivalents. Surprisingly, it was found
according to the invention that the yield of carnitine is low when such a high
stoichiometric excess of a base is added. According to the invention, it was found
that a low surplus of a base is advantageous for selectively obtaining carnitine and
for suppressing the formation of side products.
Preferably, the reaction is carried out at a temperature between -20°C and 40°C,
preferably between 0°C and 25°C, preferably at about 0°C and/or about 25°C. In a
preferred embodiment, the temperature is increased during the process, for
example from about 0°C to about 25°C. In a preferred embodiment, the reaction is
carried out at normal pressure. Thus, energy can be saved, which is important for
industrial scale production.
Preferably, the b-lactone is brought into contact with an aqueous solution
comprising a metal hydroxide and TMA. The concentration of the metal hydroxide
in the aqueous solution may be between 1 and 20 wt.%, preferably between 2 and
10 wt.%. The concentration of the TMA in the aqueous solution may be between 2
and 15 wt.%, preferably between 3 and 10 wt.%. The b-lactone may be provided in
pure form or in an aqueous solution, for example at a concentration between 1 and
80%, preferably between 5 and 50%. It is preferred that the reaction of the b-
lactone with TMA and metal hydroxide in aqueous solution is carried out at room
temperature or between 0 and 40°C. The reaction time may be between 20
minutes and 5 hours, preferably between 30 minutes and 3 hours. In this
embodiment, enhanced pressure is not necessary. Thus, the reaction can be
carried out at low temperatures and without enhanced pressure and is energyefficient.
Preferably, the b-lactone is added to the aqueous solution comprising TMA and a
metal hydroxide. The b-lactone or b-lactone comprising aqueous solution may be
added slowly, for example over a time span of 10 minutes to 4 hours, preferably
dropwise.
In another preferred embodiment, a solution of the b-lactone in an organic solvent
is provided and mixed with an aqueous solution comprising TMA and a metal
hydroxide. In this embodiment, the reaction is proceeds in a biphasic system.
Preferred organic solvents are ferf-butylmethylether (MTBE), dichloromethane
(DCM), dichloroethylene (DCE), chloroform, chlorobenzene or toluene. However,
other solvents are also appropriate which form a separate organic phase and
which do not interfere with the reaction. In theory, chlorinated solvents might react
with TMA. Although this was not observed, it would be acceptable, if the
production of carnitine is not severely inhibited. The concentration of the b-lactone
in the organic solvent may be between 2 and 50 wt.%, preferably between 5 and
20 wt.%. In this embodiment, a surplus of about 1.5 to 4 equivalents, preferably
between 2 and 3 equivalents of TMA, may be used. The two-phase reaction can
be carried out at low temperatures, for example between -20 and 40°C, or
between 0 and 25°C, preferably at 0°C.
Preferably, the TMA is recycled during the process. Since TMA is available in
gaseous form, it can be led through the reaction fluid, collected and recycled. In
the reaction medium, dissolved TMA can be separated from the mixture after
reaction is finished (e. g . by distillation) and reintroduced in the process.
Preferably, the TMA is reintroduced into the reaction pathway in a cyclic process.
TMA is commercially available in the form of a pure gas (Fluka Chemicals) or in
the form of an aqueous solution of 10 to 40 wt.%. The amount of TMA in the
reaction mixture may be between 1 and 3 equivalents, preferably between 1 and
2.5 equivalents. However, the amount and excess of TMA is less critical than the
amount of metal hydroxide, because it can be recycled during the reaction and
reintroduced into the reaction chamber.
In a preferred embodiment, the reaction mixture consists of the b-lactone, water,
metal hydroxide and TMA. Additional components may be present at a level below
1% or below 2%. When only using this composition, the reaction mixture is simple
and side reactions are minimized.
In a specific embodiment, the basic hydrolysis is mediated by the TMA and no
additional base is added for basic hydrolysis. Preferably, this reaction is carried
out at enhanced pressure and/or at least in part at enhanced temperature. In a
specific embodiment, the solvent is ethanol and the reaction intermediate product
is an ethylester of carnitine, which is subsequently hydrolyzed to carnitine. In a
specific embodiment of the invention, the solvent is an alcohol and the reaction
product is an ester, which is subsequently subjected to a basic hydrolysis.
In this embodiment without an additional base, it is preferred to carry out the
reaction at enhanced pressure, preferably in an autoclave. For example, the
pressure may be between 2 and 200 bar, especially between 5 and 150 bar or
between 0 and 100 bar. The application of enhanced pressure is preferred when
the reaction is carried out without an additional base for basic hydrolysis. The
hydrolysis reaction with the weak base TMA, which is gaseous, is promoted upon
increased pressure.
In this embodiment without an additional base and at enhanced pressure, it is
preferred to carry out the reaction at least in part at enhanced temperature, for
example between 50°C and 120°C, more preferably between 80°C and 100°C.
The initial temperature may be below 0°C and may be raised during the reaction.
In a preferred embodiment of the invention, the yield of L-carnitine is at least 75%,
more preferably at least 80%, most preferably at least 85 or at least 90%, based
on the total amount of b-lactone. The yield refers to the chiral yield or to the total
yield. Preferably, the yield of L-carnitine in the overall reaction starting from the
ketene and aldehyde is at least 30%, more preferably at least 40% or 50%, based
on the total amount of aldehyde. The yield of L-carnitine based on total carnitine
obtained is preferably at least 75%, more preferably at least 85%.
Scheme 3: Exemplified routes for L-carnitine synthesis.
In scheme 3 , a preferred reaction route according to the invention is disclosed.
Chloroethanal can be obtained in a preceding step from the corresponding
trioxane (see upper pathway). The trioxane can be obtained by chlorination of a
trimethyltrioxane. The chloroethanal can react with ketene in a [2+2] cycloaddition
to obtain the corresponding chiral b-lactone. L-carnitine is obtained upon basic
hydrolysis of the b-lactone in the presence of TMA. Alternatively, the chiral b-
lactone can be obtained from a trimethylammonium aldehyde precursor (see
bottom pathway). For example, the trichloromethyltrioxane can be converted to the
corresponding trimethylammonium trioxane, which subsequently reacts with
ketene to the b-lactone. Upon basic hydrolysis and addition of TMA, L-carnitine is
obtained.
The inventive process thus solves the problems underlying the invention. The
process is relatively simple and economic and requires only a low number of
process steps. Thus side reactions are avoided and the total yield and
enantiomeric yield are high. The L-carnitine can be obtained without using tin
organic compounds or other toxic reactants, which would be problematic in a food
or feed product. The use of precious metal catalysts is not necessary. Alternative
pathways are available which provide more flexibility for carrying out the process.
When comparing the overall reaction to the reaction of Song et al. (scheme 1), it is
obvious that the overall process requires less process steps and is more
economic. The method of the invention does not require toxic tin organic
chemicals, which renders the method more appropriate for producing the food
additive carnitine. It is a further advantage of the reaction of the invention that
alternative pathways are available, which each yield the desired product.
According to the invention, a precursor molecule can either be aminated before or
after the cyclization step.
Examples
L-carnitine was produced from chloroethanal and ketene. The reaction pathway is
shown in scheme 4 below.
Molecular Weight =78.50 Molecular Weight =120.54 ,
Molecular Formula =C2H3CIO Molecular Formula =C4H5CI02 Molecular Weight =161.20
Molecular Formula =C7H15N03
Scheme 4: Reaction pathway of carnitine synthesis according to examples 1
and 2.
Analytical methods:
The reaction and the ED are monitored by HPLC on a cation exchange column
with UV- and conductometric detection.
Assay carnitine: HPLC, cation exchange column, UV and conductivity detection
eluent: acidified water/acetonitrile, using both D- and L-carnitine as a standard.
Enantiomeric purity: the product is derivatized using a chiral, fluorescent reagent.
The reaction mixture is analyzed by HPLC using an ODS-column and flourimetric
detection.
Example 1: Synthesis of b-lactone
A TMSQ catalyst (see scheme 2b) above) was prepared according to the method
of Michael A. Calter, J. Org. Chem. 1996, 6 1, 8006-8007. The catalyst was used in
the following [2+2] cycloaddition reaction. In a 500 ml double jacketed reactor
(equipped with over head stirrer, cryostate for cooling, nitrogen inlet; ketene dip
tube) under nitrogen atmosphere, methylene chloride and a solution of a
chloroacetaldehyd in methylene chloride (10.0 g dissolved in 135 g DCM) are
charged. The solution is cooled to -50°C followed by addition of 5.16 g (TMS
Quinine, dissolved in 55.17 g methylene chloride) and 4.09 g LiCIO (dissolved in
54.1 g DCM and 18.0 g THF). Ketene is bubbled through the solution (7 g/h) for 2
h. The reaction is followed by online IR (characteristic wave number of product
approx 1832). The reaction is quenched with saturated aqueous bicarbonate
solution (579.1 g). After separation of layers, the organic layer is dried with MgSO4
and evaporated to dryness in vacuo. The crude b-lactone is used for the next step
without further purification.
Example 2 : Conversion of reaction product to L-carnitine
The crude product is added to an aqueous solution of NaOH and TMA (water
95.0 g, NaOH 7.3 g , TMA 45% in water 20.8g.) at 0°C. The reaction is stirred at
that temperature for 1 h and warmed up to room temperature. Stirring is continued
for 1 h. HPLC and IC quoted 40% conversion to carnitine (over 2 steps) with an Lcarnitine
assay of 85.5.
Example 3: Reaction in a biphasic system
4-(chloromethyl)oxetane-2-one (10 wt% in organic solvent DCM or toluene) is
treated with a mixture of 2.5 eq. of TMA (10-40 wt% in H20) and 1.2 - 1.4 eq. of
NaOH. The two-phase reaction at 0°C followed by reaction for 1 h at room
temperature yields L-carnitine (over 2 steps, dissolved in the aq. phase) in approx.
30% conversion with an L-carnitine assay of 85%. Main side product is
hydroxycrotonic acid.
Example 4 : Reaction without NaOH
A solution of lactone in water (50 wt%) is treated with 1.2 eq. of TMA at <-10°C
and autoclaved. The reaction mixture is heated to 90°C. HPLC and IC quoted
carnitine (over 2 steps) with an L-carnitine assay of 82%. Main side product is
hydroxycrotonic acid.
Example 5: Reaction at low temperature
An aqueous solution of sodium hydroxide ( 1.4 eq) and TMA ( 1.2 eq) is prepared
and cooled to 0°C. At that temperature the b-lactone is added within 1 h. The
reaction mixture is stirred further for 1 to 2 h, warmed up to room temperature and
analyzed. HPLC and IC quoted 23% conversion to carnitine (over 2 steps) with an
L-carnitine assay of 84.6%. Main side product is hydroxycrotonic acid.
Example 6 : Conversion of reaction product to L-carnitine
To the crude product an aqueous solution of NaOH and TMA (water 95.0 g, NaOH
7.3 g , TMA 45% in water 20.8g,) is added at 0°C. The reaction is stirred at that
temperature for 1 h and warmed up to room temperature. Stirring is continued for
1 h. HPLC and IC quoted 40% conversion to carnitine (over 2 steps) with an Lcarnitine
assay of 85.5%.
Example 7 : Synthesis of L-carnitine
In a pre-cooled double jacketed 2 L reactor (with overhead stirring, addition funnel,
internal temperature control and inline IR, purged with nitrogen) 3.38 g of LiCI0 4 in
14.5 g THF and 43.4 g methylene chloride were charged followed by the addition
of TMSQ in methylene chloride (2 g in 100 ml DCM). When the inner temperature
reached -30°C aldehyde addition was started (Amount: 198 g of 20 wt-%
chloroacetaldehyd in DCM, dosage range: 198 g within 40 minutes) followed by
ketene gas addition (ketene flow: 32 g/h; time of dosage: 47 min). The reaction is
followed by online IR (characteristic wave number of product approx. 1832). The
reaction is quenched with aqueous bicarbonate solution (10%, 254 g). After
separation of layers, the organic layer is dried with MgS0 (NMR sample showed
32% conversion to beta-lactone, cyclohexane was added as internal standard) and
evaporated to dryness in vacuo. The crude b-lactone is used for the next step as
for example 6 above without further purification. HPLC and IC quoted 22%
conversion to carnitine (over 2 steps) with an ee of 62%.
Example 8: Synthesis of L-carnitine
In a pre-cooled double jacketed 2 L reactor (with overhead stirring, addition funnel,
internal temperature control and inline IR, purged with nitrogen) 3.38 g of LiCI0 4 in
50 ml acetonitrile were charged followed by the addition of TMSQ in methylene
chloride (2 g in 100 ml DCM) and 3.5 ml of N,N'-diisopropylethylamine. When the
inner temperature reached -30°C aldehyde addition was started (Amount: 198 g of
20 wt-% chloroacetaldehyd in DCM, dosage range: 198 g within 50 minutes)
followed by ketene gas addition (ketene flow: approx. 26.5 g/h; time of dosage:
approx. 50 min). The reaction is followed by inline IR (characteristic wave number
of product approx. 1832). The reaction is quenched with aqueous bicarbonate
solution (75g bicarbonate in 400ml water). After separation of layers, the organic
layer is dried with MgS0 (NMR sample showed 49% conversion to beta-lactone,
cyclohexane was added as internal standard) and evaporated to dryness in vacuo.
The crude b-lactone is used for the step next as for example 6 without further
purification. HPLC and IC quoted 33% conversion to carnitine (over 2 steps) with
an ee of 76%.
Example 9: Synthesis of L-carnitine
In a pre-cooled double jacketed 2 L reactor (with overhead stirring, addition funnel,
internal temperature control and inline IR, purged with nitrogen) 3.38 g of LiCI0 in
50 ml acetonitrile were charged followed by the addition of TMSQ in acetonitrile
(0.5 g in 100 ml DCM) and 7.0 ml of N,N'-diisopropylethylamine. When the inner
temperature reached -30°C aldehyde addition was started (Amount: 198 g of 20
wt-% chloroacetaldehyd in DCM, dosage range: 198 g within 50 minutes) followed
by ketene gas addition (ketene flow: approx. 26g/h; time of dosage: approx. 60
min). The reaction is followed by inline IR (characteristic wave number of product
approx. 1832). The reaction is quenched with aqueous bicarbonate solution (75g
bicarbonate in 300ml water). After separation of layers, the organic layer is dried
with MgS0 4 (NMR sample showed 24% conversion to beta-lactone, cyclohexane
was added as internal standard) and evaporated to dryness in vacuo. The crude
b-lactone is used for the next according to example 6 step without further
purification. HPLC and IC quoted 20% conversion to carnitine (over 2 steps) with
an ee of 58%.
Example 10: Synthesis of L-carnitine
In a pre-cooled double jacketed 2 L reactor (with overhead stirring, addition funnel,
internal temperature control and inline IR, purged with nitrogen) 15 ml of LiBF4-
solution ( 1M in acetonitril) and 4 1 ml acetonitrile were charged followed by the
addition of TMSQ in acetonitrile (2 g in 100 ml DCM). When the inner temperature
reached -30°C aldehyde was added to the solution (39.5 g of 20 wt-%
chloroacetaldehyd in DCM). Ketene gas was bubbled through the solution (23.6
g/h) for 30 minutes. The reaction is followed by inline IR (characteristic wave
number of product approx. 1832). The reaction is quenched with aqueous
bicarbonate solution. After separation of layers, the organic layer is dried with
MgS0 (NMR sample showed 81% conversion to beta-lactone, cyclohexane was
added as internal standard) and evaporated to dryness in vacuo. The crude b-
lactone is used for the next as for example 6 step without further purification.
HPLC and IC quoted 49% conversion to carnitine (over 2 steps) with an ee of
81%.
Examples 11 to 13: Synthesis of L-carnitine
A catalyst was prepared according to Nelson et al., 1999. To a solution of the bissulfonamide
ligand (3.2 g , 5 mmol, 0.05 equiv.) in dry DCM (42.7 g) in a 3-neck
flask under argon at RT, trialkylaluminum solution (7.5 mmol, 0.075 equiv.) was
added dropwise (AIEί : 7.5 ml_ of a 1 M solution in hexane, AIMbb: 3.7 mL of a 2 M
solution in toluene), leading to gas evolution, but no significant heat evolution. The
resulting, faintly yellowish solution was stirred at RT for 2 hours and then diluted
with DCM (21 3 g).
A pre-cooled 2 L reactor with cooling mantle, overhead stirring, addition funnel,
internal temperature control and inline IR, purged with nitrogen, was cooled to the
indicated temperature and then charged with the activated catalyst solution and a
solution of chloroacetaldehyde (20% w/w in DCM, 1 equiv). Both solutions were
added through teflon tubing, using argon pressure. When the indicated internal
temperature was reached, ketene gas was bubbled into the solution through a
teflon tube. After the end of the ketene addition, the mixture was stirred for another
1 h at the indicated temperature before being drained from the reactor into a
vigorously stirred sat. ammonium chloride solution. This mixture was stirred for 30
min or until gas evolution ceded. The phases were separated and the organic
phase was dried over MgS0 4 (2 min stirring). The solvents were removed under
reduced pressure (bath temperature 45-48 °C) yielding an orange oily or semisolid
residue. To determine the yield, cyclohexane (840 mg, 10 mmol) was added
as internal standard for NMR (significant signal: multiplet at 4.75 ppm, CHO).
Yields for the lactone were determined by NMR using 10 mmol cyclohexane as an
internal standard, yields for carnitine were determined by quantitative HPLC with
IC-detector. All reactions were carried out with 5 mol% ligand and 7.5 mol% AIR3.
The reaction conditions and results for three different experiments are summarized
in table 1 below.
Table : Summary of conditions and results
Literature:
Calter, Catalytic, Asymmetric Dimerization of Methylketen, J. Org. Chem.
1996, 6 1, 8006-8007.
Chidara and Lin, Reaction Rate Acceleration Enabled by Tethered Lewis
Acid-Lewis Base Bifunctional Catalysis: A Catalytic, Enantioselective [2+2] Ketene
Aldehyde Cycloaddition Reaction, Synlett 2009, 10, 1675.
Kull, Peters, Practical Enantioselective Synthesis of b-Lactones Catalyzed by
Aluminum Bissulfonamide Complexes, Adv. Synth. Catal. 2007, 349, 1647-1652.
Lin et al., A Lewis acid-Lewis Base Bifunctional Catalyst from a New Mixed
Ligand, Org. Lett. 2007, 9, 4, 567.
Lin et al., Predicting the R/S Absolute Configuration in Asymmetric
Bifunctional Catalysis (ABC), Tetrahedron Letters 2007, 48, 5275.
Mondal et al., Phosphine-Catalyzed Asymmetric Synthesis of b-Lactones
from Arylketones and Aromatic Aldehydes, 2010, Org. Lett., Received Jan. 12,
2010.
Nelson et al., Catalytic Asymmetric Acyl Halide-Aldehyde
Cyclocondensations. A Strategy for Enantioselective Catalyst Cross Aldol
Reactions, J. Am. Chem. Soc. 1999, 121 , 9742.
Nelson and Spencer, Enantioselective b-Amino Acid Synthesis Based on
Catalyst Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reactions, 2000,
Angew. Chem. Int. At. Ed. 39, 1323-1324.
Nelson et al., 2002, Divergent Reaction Pathways in Amine Additions to b-
Lactone Electrophoresis. An Application to b-Peptide Synthesis. Tetrahedron
2002, 58, 7081-7091 .
Santaniello et al., Chiral Synthesis of a Component of Amanita muscaria, (-)-
4-Hydroxypyrrolidin-2-one, and Assessment of its Absolute Configuration, J.
Chem. Res. (S) 1984, 132-133.
Shen et al., Catalytic Asymmetric Assembly of Stereo-Defined Propionate
Units: An Enantioselective Synthesis of (-)-Pironetin, J. Am. Chem. Soc. 2006,
128, 7436-7439.
Song et al., Mew Method for the Preparation of R-Carnitine, Tetrahedron
Asym. 1995, 6 , 1063.
Wynberg and Staring, Asymmetric Synthesis of S- and R-Malic Acid from
Ketene and Chloral, J. Am. Chem. Soc. 1982, 104, 166.
Zhu et al, Cinchona Alkaloid-Lewis Acid Catalyst Systems for
Enantioselective Ketene-Aldehyde Cycloadditions, J. Am. Chem. Soc. 2004, 126,
5352-5353.
CLAIMS
1. A process for the production of L-carnitine, wherein a chiral b-lactone carnitine
precursor is obtained by a [2+2] cycloaddition of ketene with an aldehyde XCH2-
CHO, wherein X is selected from CI, Br, I and trimethylammonium, in the
presence of a chiral catalyst.
2 . The process of claim 1, wherein the chiral catalyst is a Lewis acid-Lewis base
bifunctional metal catalyst or an organic phosphine catalyst.
3. The process of claim 2, wherein the metal is selected from lithium, sodium,
potassium, magnesium, calcium, silver, cobalt, aluminum, copper, nickel,
chromium, iron, tin, zinc and boron.
4 . The process of at least one of the preceding claims, wherein the chiral catalyst
comprises a Lewis base selected from chiral amines, chiral phosphines, chiral
amides and chiral alcohols.
5. The process of claim 4 , wherein the chiral amine is an alkaloid, preferably
quinine or quinidine, a triamine or salen, or wherein the chiral phosphine is
SEGPHOS, BINAP or TUNEPHOS, or wherein the chiral amide is a
bissulfonamide, or wherein the chiral catalyst is a derivative of any of the
above.
6 . The process of at least one of the preceding claims, wherein the chiral Lewis
acid/Lewis base bifunctional catalyst is a chiral alkaloid in combination with a
lithium salt.
7. The process of at least one of the preceding claims, wherein the chiral catalyst
is (trimethylsilyl)quinine in combination with lithium perchlorate, or wherein the
chiral catalyst is BINAPHANE.
8. The process of at least one of the preceding claims, wherein the chiral Lewis
acid/Lewis base bifunctional catalyst has the formula
Co(ll)LiL2 or Co(lll )LiL 2Y . wherein
L is salen or a derivative thereof,
l 2 is quinine or quinidine or a derivative thereof and
Y is a monovalent anion,
wherein l and 2 are covalently linked, preferably via an ester bond,
or wherein the chiral Lewis acid/Lewis base bifunctional catalyst is an Al(lll)
complex with at least one chiral triamine ligand.
9. The process of at least one of the preceding claims, wherein the aldehyde XCH
2-CHO is obtained in a preceding step from 2,4,6-(X-CH 2)3- 1,3,5-trioxane,
wherein X is selected from CI, Br, I and trimethylammonium.
0.The process of at least one of the preceding claims, wherein the b-lactone is 4-
[(trimethylammonium)methyl]oxetanone-2-one, and wherein the b-lactone is
hydrolyzed in a ring opening reaction in order to obtain L-carnitine.
11.The process of at least one of the preceding claims, wherein the b-lactone is a
chiral 4-(halomethyl)oxetane-2-one, which is subsequently converted into
L-carnitine in a method comprising hydrolysis of the b-lactone and reaction with
trimethylamine (TMA).
12. The process of claim 11, wherein the b-lactone is not subjected to a hydrolysis
step before being contacted with the trimethylamine.
13. The process of at least one of claims 11 and 12, wherein a basic hydrolysis
and reaction with trimethylamine are carried out in one process step.
14. The process of at least one of claims 11 to 13, wherein the b-lactone is brought
into contact with an aqueous solution comprising a metal hydroxide and
trimethylamine, or wherein a solution of the b-lactone in an organic solvent is
provided and mixed with an aqueous solution comprising TMA and a metal
hydroxide.
15.A process for the production of a chiral b-lactone, wherein the chiral b-lactone
is obtained by a [2+2] cycloaddition of ketene with an aldehyde X-CH2-CHO,
wherein X is selected from CI, Br, I and trimethylamine, in the presence of a
chiral catalyst.