A process for the production of carnitine from b-lactones
The invention relates to methods for the production of L-carnitine as well as L-carnitine
with a unique impurity profile.
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, D-carnitine, 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.
Further, attempts have been made to synthesize L-carnitine directly from chiral
precursors. A group of potential precursors are chiral cyclic lactones. Since methods for
obtaining chiral lactones are known in principle, L-carnitine is available upon hydrolysis
of the lactone ring.
US 5,473,104 discloses a process for the preparation of L-carnitine from (S)-3-
hydroxybutyrolactone. The process is a two-step process, wherein in a first step (S)-3-
hydroxybutyrolactone is converted into the corresponding hydroxy-activated lactone,
whilst maintaining the ring structure. In a second step, the ring of the activated lactone
is opened and the trimethylammonium group is introduced with trimethylamine.
Altogether, the reaction is relatively complicated because it requires the activation of an
intermediate with harsh chemicals.
CH 680 588 A5 discloses a process for producing L-carnitine from a b-lactone
precursor, wherein a chiral 2-oxetanone is converted into L-carnitine in a two-step
process. In a first step, 4-(chloromethyl)-2-oxetanone is subjected to a hydrolysis step,
in which the ring is opened and 4-chloro-3-hydroxybutyric acid is obtained. In a
subsequent step, the acid is converted into L-carnitine with trimethylamine. However,
the reaction is a two-step reaction, and thus relatively labor- and time-consuming.
Further, reactions in multiple steps are generally more susceptibly to variations and
associated with a relatively low product yield.
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 for 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 L-carnitine,
which overcomes the above-mentioned drawbacks. Specifically, the problem is to
provide an efficient and simple process for the production of L-carnitine.
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
expensive 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 have a high be atom economy and
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 carnitine, wherein a b-lactone,
which is a 4-(halomethyl)oxetane-2-one, is converted into carnitine with trimethylamine
(TMA), wherein the b-lactone is not subjected to a hydrolysis step before being
contacted with the trimethylamine. The hydrolysis step, can be any hydrolysis step, for
example under acidic or basic conditions, which opens the b-lactone ring. However,
esters are commonly hydrolysed under basic conditions in a basic hydrolysis.
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. The use of 4-
(chloromethyl)oxetane-2-one is preferred. Preferably, the b-lactone is a chiral b-lactone
and the carnitine is L-carnitine. L-carnitine is available when the (R)-B-lactone is used.
According to the invention, the b-lactone ring is opened in a basic hydrolysis reaction
and the halogen atom is substituted by a trimethylamine group in a nucleophilic
substitution reaction. This is achieved in a novel one-step pathway. The halogenated b-
lactone can be converted into L-carnitine without a 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 : Synthesis of L-carnitine by cleavage of a cyclic U-lactone.
The prior art requires a two step pathway, which is disclosed in CH 680 588 A5. In a
first step, the halogenated b-lactone is hydrolysed, usually under basic conditions, to
obtain 4-halo-3-hydroxybutyric acid. In a second step, the acid is converted into Lcarnitine
with TMA. This two-step approach was used in the art, 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 and
efficiency.
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 chloro hydroxy butyric acid 5, which can cyclize giving the
hydroxybutyrolactone 6 or after elimination of water forming the furanone 7 . Formation
of hydroxy butyric 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 1 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-B-lactone 4 to intermediate 13, which is hydrolyzed with sodium hydroxide to Lcarnitine
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 Lcarnitine
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 (TMA) are 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, for example a solution or suspension, of metal hydroxide and
trimethylamine, or by adding the metal hydroxide and 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 simultaneously, 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 of the invention, 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 of the invention, 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.
In a preferred embodiment of the invention, 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.
In a preferred embodiment of the invention, 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 5 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 energy-efficient.
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 of the invention, 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 fe/f-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 to 4 equivalents, preferably 1. 1 to 4 equivalents, more 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.
In a preferred embodiment of the invention, 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 (eg 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 of the invention, 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 of the invention, 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 10 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 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 initial
total amount of b-lactone. The yield refers to the chiral yield or to the total yield.
In principle, chiral monohalogenated b-lactones for carrying out the inventive ring
opening reaction are known in the art. For example, the b-lactones can be obtained by
hydrochlorination of non chiral precursors with tributyltinhydride as disclosed in CH 680
588 A5.
In preferred embodiment of the invention, the chiral 4-(halomethyl)oxetane-2-ones are
obtained according to a [2+2] cycloaddition reaction in the presence of a chiral catalyst.
Specifically, the chiral b-lactone is obtained by a novel [2+2] cycloaddition of ketene
with an aldehyde X-CH2-CHO, wherein X is selected from CI, Br and , in the presence
of a chiral catalyst.
Ketene (ethenone, formula C2H20 ) is a colorless gas, which is highly reactive due to
two adjacent double bonds in the molecule.
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.
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.
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 triamine or salen. The chiral phosphine is
preferably SEGPHOS, TUNEPHOS or BINAP. The chiral amide is preferably a
bissulfonamide. The chiral catalyst may also be a derivative of any of the above.
In a preferred embodiment, 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 preferred embodiment, 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 te/f-butyl, 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.
A preferred group of catalysts comprises a central Al(lll) atom, to which two sulfonamide
groups and one additional residue, which may be an organic or inorganic residue, are
attached. Thereby, the Al(lll) is coordinated by the respective N-atoms of the
sulfonamide groups. The sulfonamide groups may be substituted, preferably with with
aryl or alkyl groups. Preferably, they are linked to each other through a bridging group.
Chirality is conferred to the catalyst either by chiral nitrogen atoms of the sulfonamide
groups or by C-atoms of the bridging group. For example, such catalysts are described
by Nelson et al., 1999.
In another preferred embodiment, 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.
The process according to the invention may comprise an additional purification step,
whereby the L-carnitine is subjected to an electrodialysis and a subsequent
recrystallization treatment. Such techniques are generally known to the skilled person.
Electrodialysis (ED) is a membrane technology used to purify organic products in liquid
mixtures. The ED can be used to reduce the salt concentration in a mixture in a
discretionary way. The driving force for this separation is an electric field over the
membranes. Pressure driven membrane processes such as Reverse Osmosis,
nanofiltration, ultrafiltration or microfiltration can be applied to concentrate/retain organic
compounds. The salts will only be partially concentrated/retained, depending on the
type of membrane used.
The betaine (L-carnitine) can be isolated and purified using methods known in the art.
An excess of tertiary amine as well as parts of the water used as solvents can be
removed by distillation, preferably under reduced pressure. The excess amine may be
recovered and recycled.
A salt byproduct is preferably removed by membrane technology (e.g. Electrodialysis,
reverse osmosis, nanofiltration, ultrafiltration or microfiltration), advantageously after
removing the volatile compounds as described above. The betaine (L-carnitine) can be
isolated by conventional methods, e.g. by distilling off the water from diluate obtained
after electrodialysis followed by recrystallization.
The inventive process 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 Lcarnitine
can be obtained without using tin organic compounds or other toxic reactants,
which would be problematic in a food and feed product. The use of precious metal
catalysts is not necessary. Alternative pathways are available which provide more
flexibility for carrying out the process.
Specifically, compared to the process of CH 680588 A5, the inventive process is carried
out in a one-step reaction, whereas the prior art process is carried out in a two-step
reaction. Further, the inventive process requires relatively low amounts of a base
compared to a classical ester hydrolysis. The TMA can be recycled and reintroduced
into the process. The inventive reaction can be carried out without increased
temperatures and at normal pressure. In summary, the inventive process is highly
efficient regarding energy, time and use of chemicals.
Another aspect of the present invention is an L-carnitine which is obtainable by a
process as described supra. Said L-camitine is characterized by possessing a unique
impurity profile. Specifically, the L-carnitine according to the invention exhibits
hydroxycrotonic acid as the main impurity. Preferably, the amount of hydroxycrotonic
acid is equal or less than 0.1 wt-%, more preferably in the range of 0.5 - 0.1 wt-% and
most preferably in the range of 0.5-0.005 wt-%, while other impurities are negligible.
Due to the presence of hydroxycrotonic acid as main impurity, the L-carnitine according
to the invention is superior to the state of the art. Since hydroxycrotonic acid is non¬
toxic, non-carcinogenic and non-mutagenic (Ames-Test negative, LD50 (rat) > 2000
mg/kg bw), it does not have to be removed before adding the L-carnitine into food and
feed compositions.
The unique impurity profile of the L-carnitine is a direct result of the process according
to the invention. It is not achievable with the two-step, state of the art processes
according to e.g. US 5,473,1 04 (which furthermore starts with different educts
compared to the invention) or CH 680 588, since said state of the art processes are
prone to resulting in a variety of different, often hazardous side products.
Examples
L-carnitine was produced from chloroethanal and ketene. The reaction pathway
shown in scheme 4 below
Molecular Weight =78.50
Molecular Formula =C2H3CI 0
15N0 3
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 UVand
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 flourometric detection.
Example : 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 LiCI0 4 (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 inline IR
(characteristical 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 MgS0 4 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 L-carnitine
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 Lcarnitine
(over 2 steps, dissolved in the aq. phase) in approx. 30% conversion with an Lcarnitine
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 <-1 0°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 : Reaction in an organic solvent
4-(chloromethyl)oxetane-2-one (10 wt% in organic solvent Ethanol) 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 reaction
for 1 h at 0°C followed by warming up to room temperature yields L-carnitine (over 2
steps) in approx. 22% conversion with an L-carnitine assay of 84.8%. Main side product
is hydroxycrotonic acid.
Example 7 : Reaction in an organic solvent
4-(chloromethyl)oxetane-2-one (10 wt% in organic solvent Ethanol) is added to a
mixture of 2.5 eq. of TMA (10-40 wt% in H2O) and 1.2 - 1.4 eq. of NaOH. The reaction
for 1 h at 0°C followed by warming up to room temperature yields L-carnitine (over 2
steps) in approx. 22% conversion with an L-camitine assay of 84.8%. Main side product
is hydroxycrotonic acid.
Example 8: Reaction in a biphasic system
4-(chloromethyl)oxetane-2-one ( 0 wt% in organic solvent DCM or toluene) is added to
a mixture of 2.5 eq. of TMA (10-40 wt% in H20 ) and 1.2 - 1.4 eq. of NaOH. The twophase
reaction at 0°C followed by reaction for 1 h at room temperature yields Lcarnitine
(over 2 steps, dissolved in the aq. phase) in approx. 30% conversion with an Lcarnitine
assay of 85%. Main side product is hydroxycrotonic acid.
Example 9 : General procedure for salt removal via ED
The setup used to carry out the ED treatments consisted of an ED miniplant equipped
with a stack with 10 pairs of PES-Membranes of 64 cm2. The experiments were carried
out in batch-mode; however, a continuous operation mode can be also implemented. 3
pumps were responsible to circulate the concentrate (waste water stream), dilute
(product stream) and electrolyte (service stream) solutions to the membrane stack. The
flux of these 3 streams was adjusted and measured with 3 rotameters. In order to
guarantee a maximisation of the process yield, a control of pH and temperature in the
concentrate and dilute streams was implemented. During the ED-experiments pH,
electrical conductivity, temperature and flux of these 3 streams are controlled und
recorded.
The above described setup was also used to desalt and purify L-carnitine from a liquid
reaction mixture. The yield of L-carnitine obtained under optimized conditions was 88 -
94%. The diluate stream containing the product is evaporated to dryness in a rotavapor
under vacuum.
Example 10: General procedure for recrvstallization
A laboratory reactor is charged with 100g of carnitine and 300g of ethanol. The reactor
is heated up to 65°C and stirred until all carnitine has been dissolved. Afterwards the
reactor temperature is set to 37°C. At 37°C seed crystals of pure L-camitine are added.
The reactor temperature is cooled down to 20°. And 900g of acetone are added within 2
hours. Afterwards the suspension is cooled down to 10°C. At 10°C the solids are
isolated and washed with acetone and dried at 55°C and <100mbar.
As a result, 86.1 g of a crystalline-white dry solid were obtained. The solid comprised 99
% (w/w) of total carnitine and 0.03-0.01 % (w/w) of hydroxycrotonic acid. The
enantiomeric purity was 99.60% (e.e.). The residual solvent content was 349 mg/kg
ethanol and 386 mg/kg acetone. The total yield of L-carnitine was 88.6%.
Literature:
Calter, Catalytic, Asymmetric Dimerization of Methylketen, J. Org. Chem. 1996,
6 1, 8006-8007.
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.
Shen et al., Catalytic Asymmetric Assembly of Stereo-Defined Propionate Units:
An Enantioselective Synthesis of (-)-Pironetin, J. Am. Chem. Soc. 2006, 128, 7436-
7439.
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 b-lactone, which is a 4-
(halomethyl)oxetane-2-one, is converted into carnitine with trimethylamine (TMA),
wherein the b-lactone is not subjected to a hydrolysis step before being contacted
with the trimethylamine.
2. The process of claim 1, wherein a basic hydrolysis and addition of trimethylamine
(TMA) are carried out in one process step.
3. The process of at least one of the preceding claims, wherein the basic hydrolysis is
carried out with a metal hydroxide, preferably sodium hydroxide.
4. The process of claim 3, wherein the b-lactone is brought into contact with the metal
hydroxide and the trimethylamine essentially at the same time.
5 . The process of claims 3 or 4 , wherein the amount of the metal hydroxide is 1. to 1.6
equivalents, preferably 1.2 to 1.4 equivalents, based on the initial amount of b-
lactone.
6. The process of at least one of claims 3 to 5, wherein the b-lactone is brought into
contact with an aqueous solution comprising the metal hydroxide and the
trimethylamine.
7 . The process of at least one of claims 3 to 6 , 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.
8. The process of at least one of the preceding claims, wherein the reaction is carried
out at a temperature between -20°C and 40°C, preferably between 0°C and 25°C.
9. The process of claims 1, 2 or 8, wherein basic hydrolysis is mediated by the TMA
and no additional base is added for basic hydrolysis.
0.The process of at least one of the preceding claims, wherein the reaction is carried
out at enhanced pressure, preferably in an autoclave.
1 .The process of at least one of the preceding claims, wherein the TMA is recycled
during the process.
12. The process of at least one of the preceding claims, wherein the b-lactone is a chiral
b-lactone and the carnitine is L-carnitine.
13. The process of at least one of the preceding claims, comprising an additional step, in
which the L-carnitine is purified via a combination of electrodialysis and subsequent
recrystallization.
1 .The process according to claim 13, whereby the recrystallization is effected in an
organic solvent.
15. The process of at least one of the preceding claims, comprising a preceding step, in
which the b-lactone is obtained in a [2+2] cycloaddition of ketene with an aldehyde
X-CH2-CHO, wherein X is selected from CI, Br and I , in the presence of a chiral
catalyst.
16. The process of claim 15, wherein the chiral catalyst is a Lewis acid-Lewis base
bifunctional metal catalyst or an organic phosphine catalyst.
17. L-carnitine, obtainable by a process according to at least one of claims 13 and 14.
18. L-carnitine, characterized by having an amount of hydroxycrotonic acid of equal or
less than 0.1 wt-%, more preferably in the range of 0.5 - 0.1 wt-% and most
preferably in the range of 0.5-0.005 wt-%.