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.
Carnitine contains an asymmetry centre and can therefore exist in the form of two enantiomers, designated R-(xe2x88x92)-carnitine and S-(+)-carnitine, respectively. Of these, only R-(xe2x88x92)-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-(xe2x88x92)-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 xcex3-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 30xc2x0 C. with a hydrogen pressure ranging from 40 to 100 kg/cm2, 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 100xc2x0 C., at a mean pressure of 70-100 kg/cm2, 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 xcex2-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/cm2: 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.
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.
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:
Yxe2x95x90OR1, NHxe2x80x94R1, N(R1R2) in which R1 is H or
R1,R2, equal or different=alkyl C1-C10 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 xcex3-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, NR3, N-aryl, Nxe2x80x94COxe2x80x94R3 
R3=alkyl C1-C10, alkylaryl, aryl
Q=alkyl C1-C4, phenyl
R, Rxe2x80x2, equal or different=optionally alkyl-substituted phenyl, C1-C6 alkyl, C3-C8 cycloalkyl, or R and Rxe2x80x2 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, xcfx80 aryl, olefin system, xcex73 allyl system, such as, for example, the 2-methylallyl system, carboxylate group, such as, for example, acetate or trifluoroacetate.
What is meant by the xcfx80 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 (3,3xe2x80x2-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 150xc2x0 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 120xc2x0 C., and with a substrate:catalyst molar ratio between 10,000:1 and 15,000:1
The concentration of the substrate in the reaction mixture also contributes towards obtaining the reduced product in a high yield and with high-grade optical purity. This concentration ranges from 5 to 15 g of substrate per 100 ml of solvent, and the preferred concentration is 10 g/100 ml.
The reaction mixture may advantageously contain catalytic amounts of a base.
The moderate hydrogen pressure conditions (on average 5 bar) make it possible to operate with simpler reactors and with less stringent safety conditions compared to similar reactions described in the known technique requiring a pressure of 100 atmospheres.
The process described herein proves easily reproducible on an industrial scale and does not require the use of any additional expedients, such as, for example, the use of acid co-catalysts.
The process according to the invention yields optically active alkyl 4-chloro-3-hydroxybutyrate or 4-chloro-3-hydroxybutyramide with a yieldxe2x89xa795% and with e.e. ranging from 95 to 97%. As a result of the transformations described here below, these results make it possible to obtain the L-carnitine end product with an overall yield of 65-70%.
Furthermore, high substrate:catalyst ratios make it possible to operate with low amounts of catalyst, thus contributing to cost savings in the reaction process.
The alkyl 4-chloro-3-hydroxybutyrate or 4-chloro-3-hydroxybutyramide obtained by catalytic reaction are subsequently converted to L-carnitine reaction of formula (II) derivatives by reaction with trimethylamine.
The reduced derivatives are reacted with trimethylamine with formation of L-carnitine alkyl ester or alkyl amide. This reaction is performed preferably at temperatures from 55 to 90xc2x0 C. for time periods ranging from 1 to 70 hours. Particularly satisfactory results are obtainable when operating at 65xc2x0 C. for 60 hours, or at 80xc2x0 C. for 24 hours. According to the type of reactor used, stirring and loading conditions, reaction times of even 1-2 hours have been observed at 80xc2x0 C.
The reaction entails the substitution of the 4-chloro with the 4-trimethylamine group for and hydroylsis of the ester or amide group, with the formation of crude L-carnitine, which is then purified to make it suitable for pharmaceutical use.
The purification can be done with known methods such as, for example, chromatography, extraction with solvents, ultrafiltration, and other equivalent methods.
Thanks to the advantages identified above, such as low pressure, high yields, and high-grade optical purity, and the use of limited amounts of catalyst, the invention described herein makes it possible to produce L-carnitine efficiently and economically in large-scale industrial plant.
The invention is further described by means of the following examples.