Industrial process for the production of L-carnitine

Details Industrial process for the production of L-carnitine Industrial process for the production of L-carnitine

2001-05-07

2003-05-20

Richter, Johann (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C560S170000

Reexamination Certificate

active

06566552

ABSTRACT:

FIELD OF THE INVENTION
The invention described herein relates to the synthesis of L-carnitine.
The subject of the invention is a process for obtaining this product, which can be easily implemented on an industrial scale.
STATE OF THE ART
Carnitine contains an asymmetry centre and can therefore exist in the form of two enantiomers, designated R-(−)-carnitine and S-(+)-carnitine, respectively. Of these, only R-(−)-carnitine is present in living organisms where it acts as a carrier for the transport of fatty acids across the mitochondrial membranes.
It is therefore essential that only R-(−)-carnitine be administered to patients undergoing regular haemodialysis treatment or treated for cardiac or lipid metabolism disorders.
In view of the substantial biological and pharmaceutical interest in this molecule, many studies have been conducted with a view to its synthesis.
The known techniques of large-scale synthesis of L-carnitine include:
i) the optical resolution of a racemic mixture: this technique involves the use of a resolving agent in an equimolar amount and the separation of the unwanted enantiomer. This procedure leads to the loss of 50% of the starting product.
ii) Stereospecific hydration of crotonobetaine or &ggr;-butyrobetaine by a microbiological method (U.S. Pat. No. 4,708,936). This microbiological synthesis procedure entails the risk of imperfect reproducibility, of possible alterations of the strain used, and of possible biological contamination of the product.
iii) Enantioselective reduction of a butyric 4-chloro-3-oxoester by means of mono- or bimetallic ruthenium catalysts. This yields the corresponding 3-hydroxy derivative which, by reaction with trimethylamine and hydrolysis of the ester group, is converted to L-carnitine.
The reduction reaction mentioned in iii) has been the subject of several studies.
For example, patent EP-B-295109 describes the reduction of a 4-chloro-3-oxobutyrate with a catalyst containing ruthenium bound to a chiral diphosphine which in turn is bound to a bisnaphthalenic system; the valence of the metal is completed by a combination of halogens and triethylamine. The reaction is carried out at 30° C. with a hydrogen pressure ranging from 40 to 100 kg/cm
2
, with substrate:catalyst molar ratio of 1000:1, in a reaction time of 16-20 hours. The optical yield below 67% and the lengthy reaction times and high pressures involved make the process industrially unacceptable.
In patent application EP-A-339764, L-carnitine is obtained by means of a process comprising the reaction of a 4-halo-3-oxobutyrate with the above-mentioned ruthenium-based catalyst: the reaction is carried out at approximately 100° C., at a mean pressure of 70-100 kg/cm
2
, with a substrate:catalyst molar ratio ranging from 1000 to 10,000:1. The process described once again presents the disadvantage of having to operate at high pressure values. In addition, the overall carnitine yield with this method was modest (46%). Similar results are reported in Tetrahedron Letters, 29, 1555, (1988).
The synthesis methods described above not only present modest yields and, in the first case, also lengthy reaction times, but also involve operating at high hydrogen pressures, which increases the cost of the process and the safety precautions to be adopted. This problem becomes crucial when moving over from laboratory- to industrial-scale production.
A number of studies describe the reduction of &bgr;-ketoesters by means of ruthenium complexes catalysts, operating at moderate pressure values; the results. however, are unsatisfactory in terms of yields and/or reaction times, and these processes therefore cannot be applied on an industrial scale. In Tetrahedron Letters, 32, 4163, (1991), the reduction of 4-chloro-3-oxobutyrate with 4-atm. hydrogen pressure is described. The reduced product has an enantiomeric purity inferior to that obtained when operating at high pressure, and the reaction times are rather lengthy (6 h). The lower enantiomeric purity leads to a greater loss of L-carnitine yield. In EP-A-573184, the reduction of a terbutylic ester of the same substrate is carried out at a pressure of 10-15 kg/cm
2
: the reaction is completed in two hours with an unsatisfactory yield and enantiomeric purity.
Analysis of the above-mentioned technique reveals the lack of a process for the synthesis of L-carnitine which is easily and efficiently reproducible on an industrial scale. In particular, what is lacking is an L-carnitine synthesis process comprising the enantioselective catalytic reduction of 4-halo-3-oxobutyric derivatives of such a nature as to be carried out on an industrial scale with high yields and high enantiomeric purity and operating in moderate pressure conditions.
SUMMARY OF TILE INVENTION
The present invention discloses a process for the industrial production of L-carnitine, comprising the enantioselective reduction of an alkyl 4-chloro-3-oxbutyrate or 4-chloro-3-oxobutyramide. The optically active 3-hydroxy derivative thus obtained is reacted with trimethylamine, obtaining crude L-carnitine, which is then finally purified. The catalyst used for the reduction is a complex of ruthenium bound to a penta-atomic bis-heteroaromatic system. The reduction reaction, performed in controled conditions of hydrogen pressure, substrate concentration, temperature, and substrate:catalyst molar ratio, enables 4-chloro-3-hydroxybutyrate or 4-chloro-hydroxybutyramide to be obtained in a high yeild. The process described, which leads to L-carnitine being obtained, is easily appilcable on an industrial scale.
DETAILED DESCRIPTION OF THE INVENTION
The subject of the invention described herein is a process for the synthesis of L-carnitine. The first step in achieving the object of the invention consists in the enantioselective catalytic reduction of an alkyl 4-chloro-3-oxobutyrate or 4-chloro-3-oxobutyramide, according to the following diagram:
where:
Y═OR
1
, NH—R
1
, N(R
1
R
2
) in which R
1
is H or
R
1
,R
2
, equal or different=alkyl C
1
-C
10
alkylaryl and reaction of formula (II) derivatives with trimethylamine, with formation of L-carnitine.
The preferred starting substrate is ethyl 4-chloro-3-oxobutyrate (ethyl &ggr;-chloro-acetoacetate).
The reduction reaction catalyst consists of a ruthenium complex bound to a penta-atomic bis-heteroaromatic system. This structure corresponds to one of the two formulas (III) or (IV).
where:
A=S, O, NR
3
, N-aryl, N—CO—R
3
R
3
=alkyl C
1
-C
10
, alkylaryl, aryl
Q=alkyl C
1
-C
4
, phenyl
R, R′, equal or different=optionally alkyl-substituted phenyl, C
1
-C
6
alkyl, C
3
-C
8
cycloalkyl, or R and R′ together form a 4-6 atom phosphorocyclic system
X and L, equal or different, have the following meanings:
X=halogen, alkylsulphonate, arylsulphonate
L=halogen, aryl, &pgr; aryl, olefin system, &eegr;
3
allyl system, such as, for example, the 2-methylallyl system, carboxylate group, such as, for example, acetate or trifluoroacetate.
What is meant by the &pgr; aryl group is a type of direct co-ordination with the aromatic electron system, without any direct bonding of a carbon atom of the ring with the metal.
The formula (III) and (IV) compounds are described in patent application WO 96/01831, incorporated herein for reference.
In particular, the preference is for the use of catalysts where A represents S (3,3′-bisthiophenic structure), X represents halogen, particularly iodine, and L is an aryl system. The preferred catalyst is {[Ru (p-cymene) I (+) TMBTP] I}, represented by formula (V).
The reduction of alkyl 4-chloro-3-oxobutyrate or 4-chloro-3-oxobutyramide is done at a hydrogen pressure ranging from 2 to 7 bar, at a temperature ranging from 90 to 150° C., and with a substrate:catalyst molar ratio ranging from 5,000:1 to 30,000:1.
According to a preferred realisation of the invention, the reduction is performed at a hydrogen pressure of 5 bar, at a temperature of 120° C., and with a substrate:catalyst molar ratio between 10,000:1 and

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