PROCESS FOR THE PREPARATION OF L-CARNITINE

Abstract: Process for the preparation of L-carnitine wherein: a) The ester of 4-azido-3-ketobutyric acid, with formula: N₃-CH₂--CH₂-CO₂R (II) wherein R is an alkyl or arylalkyl radical, is placed in contact, in the liquid phase, in the presence of a source of carbon and of a phosphate buffer, with a microorganism, to yield the ester of 4-azido-3-hydroxybutytic acid of formula: wherein R has the above-reported meaning, in the form of the (3R) isomer only; b) the compound so obtained is submitted to hydrolysis of the ester group, in the presence of a base, to yield (3R)-4-azido-3-hydroxybutyric acid, with formula: c) (3R)-4-azido-3-hydroxybutyric aid is submitted to hydrogenation of azido group with hydrogen gas, and in the presence of a hydrogenation catalyst, to yield -(3R)-4-ami­ no-3-hydroxybutyric acid [(-)-GABOB], with formula: d) The so-obtained compound is separated and recovered by crystallization; e) (3R)-4-amino-3-hydroxybutyric acid is submitted to methylation to yield L-carnitine compound (I).

Description: The present invention relates to a process for the synthesis of L-carnitine EMI1.1 Compound (I), also known as vitamin BT, is a hydroxyaminoacid stimulating the mitochondrial oxidation of fat ty acids, productin energy at muscular level. Its most known use is as a tonic, although it has been recently used in other therapeutical fields, such as the chronic hemodialysis, the correction of lipoproteinic pattern, the suppression of cardiac arrhytmia during the course of infarct, and the infusional therapy. Compound (I) can be isolated by known processes from natural substrates, such as meat extract, with an end yield of 2 - 1%, that renders the same process economica ly disadvantageous, and however not much suitable on a com mercial scale. According to Carter and Bhattacharyye [J. Am. Chem. Soc. 75, 2503 (1953)] D,L-carnitine is obtained by chemical synthesis from benzaldehyde and epichlorohydrin through a plurality of steps, with an overall yield of 20 25%. According to U.S. Pat. 3,135,788 D,L-carnitine chloride salt is prepared from epihalogenohydrin and trimethylamine -through a plurality of steps, with the production of 3-halogeno2-hydroxypropyl trimethylammonium chloride, which is then reacted with potassium or sodium cyanide to yield a 3 cyano-2-hydroxypropyl trimethylammonium chloride. The compound so obtained is then hydrolized to produce D,L-carnitine. According to said known processes, the chemical synthesis of carnitine is carried out by starting from non optically active compounds or racemic mixtures with the formation both of L and D isomers of carnitine. In the last steps of the process, the separation is then carried out of the L-isomer from the reaction mixture by fractional crystallization using optically active acids. Such an operating way involves a maximum yield of 50 and thus renders the same process not so much advantageous from the economical point of view. In the technical and patent literature,processes for the microbiological synthesis of L-carnitine have recently been described. According to U.K. Patent 2,078,742 to Sigma Tau, the synthesis of L-carnitine is carried out by means of the hydroxylation of r-butyrine with spores of Neurospora crassa. According to French Patent 2,398,046 L-carnitine is obtained by means of the oxidation of D,L-carnitine to the corresponding 3-ketoderivative, and of the subsequent stereospecific reduction to L-carnitine, by means of the use of enzymic systems induced in Pseudomonas. B. Zhan et al describe in J. Am. Chem. Soc. 1983, 105, 5925-5926, a method for synthesizing L-carnitine by means of the microbiological stereospecific reduction of the esters of 4-chloro-3-ketobutyric acid and esters of 4-cloro3-hydroxybutyric acid of (3R)-configuration. The product thus obtained is then treated with trimeth ylamine and ethanol at 800C, and by subsequent acidic hydro lysis. Said processes show, relatively to the chemical syn thesis methods, the advantage of directing the reaction, in a specific way, towards the formation of L-isomer on ly. However, the number of required treatments, the use of valuable raw materials and above all the reaction yields, in the overall poor, render onerous these processes known. It is therefore the purpose of the present invention a process for the preparation of L-carnitine which is free or substantially free from the above mentioned drawbacks. Accordingly, L-carnitine EMI3.1 is prepared according to the present invention by means of a process characterized in that: a) The ester of 4-azido-3-ketobutyric acid, with formula: EMI3.2 wherein R is an alkyl or an alkylaryl, is placed in contact in the liquid phase, in the presence of a car bon source and of phosphate buffer, with a microorganism to yield the ester of 4-azido-3-hydroxybutyric acid with formula: EMI3.3 wherein R has the above-reported meaning, in the form of its (3R) isomer; b) the compound so obtained is submitted to the hydro lysis of the ester group in the presence of a base, to yield (3R)-ii-azido-3-hydroxybutyric acid, of formula: EMI3.4 c) (3R)-4-azido-3-hydroxybutyric acid is submitted to hydrogenation of the azido group with hydrogen gas and in the presence of a hydrogenation catalyst, to yield-(3R)-4-amino-3-hydroxybutyric acid ((-)-GABOB], with formula: EMI4.1 d) the compound so obtained is separated and recovered by crystallization; e) (3R)-4-amino-3-hydroxybutyric acid is submitted to meth ylation in order to yield L-carnitine compound (I). In the step (a) of the process of the present invention, the ester of 4-azido-3-Retobutyric acid, to be represented by the formula EMI4.2 wherein R is an alkyl or an alkylaryl radical with a number of carbon atoms of from 1 to 10, generally with a num ber of carbon atoms of from 7 to 8 is submitted to microbiological reduction. According to the present invention, compound (II) is placed in contact, in the liquid phase, in the presence of a source of carbon and of a phosphate buffer, with a microorganism to yield the ester of 4-azido-3-hydroxybutyr ic acid with formula: EMI4.3 wherein R has the above-indicated meaning, in the form of the only (3R) isomer. Microorganisms suitable to this purpose are selected among Saccharomyces cerevisiae, Aspergillus niger, Geo trichium candidum, Streptomyces, Epicoccus. The amount of microorganisms used in the reduction of compound (II) is such as to have a weight ratio of compound (II)/microorganism comprised within the range of from 1:50 to 1:5. The enzymatic reaction is carried out in an aqueous medium, in the presence of a carbon source, selected among those known in the art, and of a phosphate buffer such as e.g. sodium, ammonium or potassium phosphate at a pH of from 5 to 10. The temperatures at which the reaction is carried out range generally from 100C to 400C and the corresponding reaction times range from 1 hour to 6 hours. The reaction is preferably carried out with a microorganism belonging to the genus Saccharomyces cerevisiae, in the presence of glucose, ad a pH of from 6.5 to 7.5, at a temperature of 370C and for a three hours time. At the end of the reaction the settled matter is filtered off from the reaction mixture, or it is removed by centrifuging, and is washed with water. The supernatants are combined and eXtracted with a solvent selected among ethyl acetate, CH2Cl2, CH Cl3, hexane. The solvent is evaporated off and the residue is thus recovered. At the NMR analysis with chiral shift reactants, said residue results to be constituted by (3R) enantiomer of the ester of -azido-3-hydroxybutyric acid compound (III)]. In the step (b) of the present invention, the compound (III) is hydrolized at its ester group, by means of one of known techniques. The reaction is generally carried out in the liquid phase with a base selected among sodium, potassium or calcium hydroxide at room temperatures (20-250C); and for a time of from 1 to 5 hours. At the end of such a time period, the alcohol formed during the hydrolysis reaction is separated and recovered. The aqueous solution is then made acid and the residue is recovered by extraction with an organic solvent selected among ethyl acetate, CH2Cl2, CHCl3, hexane. In the step (c) of the present invention, the h-azi do-3-hydroxybutyric acid (IV) so obtained -from the preceding step is hydrogenated at its azido group with hydrogen gas and in the presence of a hydrogenation catalyst, to yield the compound (V),4-amino-3-hydroxybutyric acid, also named (-)-GABOB, useful as an antiepileptic and hypotensive agent. The hydrogenation catalysts used to that purpose may be constituted by any hydrogenation catalysts known from the art, e.g., catalysts of noble metals. In the preferred embodiment however, such catalysts as Pd/C or PtO2 are used. The amount of catalysts used in the hydrogenation of the compound (IV) is generally such as to have a weight ratio of compound (IV)/catalyst of from 100/1 to 20000/1. The hydrogenation of compound (IV) is then carried out in the liquid phase, with the catalyst either suspended or dissolved depending of the particular catalyst used, in an inert organic solvent. Solvents suitable to that purpose are hydrocarbons, alcohols, ethers and esters or organic acids, liquid under the conditions under which the hydrogenation reaction is carried out. Specific examples of solvents belonging to the above indicated classes are acetic acid and ethanol. The temperatures at which the hydrogenation reaction is carried out may generally vary within the range of from 100C to 600 C, and the corresponding reaction times range generally from 20 hours to 20 minutes. The reaction is typically carried out at room temperatures (20-250C) and in this case the time required to complete or to essentially complete the reaction is of about 4 hours. The hydrogenation reaction may be carried out under a hydrogen pressure equal to or higher than the atmospheric pressure and in general it is not convenient to exceed a value of about 50 atmospheres. By operating under the previously indicated conditions compound (V) is obtained in a yield of 95% relatively to the starting substrate. At the end of the reaction, the catalyst is removed by extraction, filtering or by other suitable techniques, and the solvent is removed, generally by evaporation. The compound (V) is then recovered from the reaction mixture by crystallization from water, from alcohol or from water-alcohol mixtures. In the step (e) compound (V) is transformed into Lcarnitine by methylation. In this step, any known technique may be used. In practice, the reaction is carried out by using a solution of methyl iodide in methanol and potassium hydroxide. After a reaction time of 36 hours at a temperature of 600 C, the solvent is evaporated to dryness and the residue is extracted four times with a phenol/water solution. The combined extracts are placed in a separation funnel containing diethyl ether, and the phenol/diethyl ether layer is separated from water, and washed with water. The combined aqueous extracts are eluted through a column containing an ion-exchange resin, and the eluate is evaporated to dryness under reduced pressure. By this process, crystals of L-carnitine are obtained with a yield of 80% and with an optical purity of 95-100%. The compound (II) of the present invention may be prepared by means of known processes, by starting from the corresponding ester of 4-chloro-3-ketobutyric acid or of 4-bromo-3-ketobutyric acid, and sodium azide. The reaction is typically carried out by placing in contact the said compounds in a water-alcohol mixture, at a temperature of about 700 C, and for a time of the order of three hours. The organic solvent is then evaporated off and the compound (II) is recovered from the reaction mixture. The following experimental Examples are illustrative and not limitative of the invention. Example 1 Preparation of the ester of 4-azido-3-ketobutyric acid Into a glass flask of 3 1 in volume, equipped with stirring means, 248 g (1 mol) of octyl ester of 4-chloro3-ketobutyric acid, 715 g (1.1 mol) of sodium azide and 1500 ml of a water-methanol (1:1) mixture are loaded. The solution is maintained at a temperature of 700C for a time of about 3 hours. At the end of said time period, 1000 ml of water are added and the whole mixture is subsequently extracted 3 times with 1000 ml of organic solvent, such as ethyl acetate or CH2Cl2. The solvent is evaporated to dryness and 220 g are obtained of the compound (II) (R = C8H17), with a purity, as analyzed by NMR, of 95%. Example 2 Preparation of the ester of 4-azido-3-hydroxybutyric acid Into a glass flask, of 3 1 in capacity, provided with stirring means, 1 1 of an aqueous solution containing 100 g of D-glucose and 10 g of Na2HPO4, and 250 g of breadmaking yeast (Saccharomyces cerevisiae) are loaded. To the said suspension, kept stirred, 8.0 g of the compound (II) are added over a time period of about 10 minutes, with the temperature being controlled at values lower than 370C. At the end of such time, the suspension is maintained at a temperature of 370C for about 3 hours. Said suspension is then exotracted twice with 500 ml of solvent, either ethyl acetate or CH2Cl2. At the end, the solvent is evaporated to dryness. An amount of 7.5 g is obtained of the octyl ester of 4-azido-3-hydroxybutyric acid compound (III)]. On NMR analysis with chiral shift reactants, said compound results to be constituted by the (3R) enantiomer only. Example 3 Preparation of 4-azido-3-hydroxybutyric acid Into a glass flask, of 1 1 in capacity, 25.5 of the octyl ester of 4-azido-3-hydroxybutyric acid as obtained as disclosed in Example 2; 10 g of potassium hydroxide, 500 ml of water and 200 ml of methanol are loaded. The reaction mixture is boiled over two hours, is concentrated in vacuo to remove methanol, is cooled and octyl alcohol is extracted with 200 ml of CH2Cl2. The aqueous solution is made acid to pH 1 withX6N hydrochloric acid and is extracted twice with 200 ml of CH2C12. The residue obtained by evaporating the solvent is constituted by acid (IV), in an amount of 13.4 g (90%). Example 4 Preparation of 4-amino-3-hydroxybutyric acid (C-)-GABoB] Into a hydrogenation reactor of 0.5 1 in capacity, provided with stirring means, 114.6 g (0.1 mol) of the compound (IV), as prepared according to the preceding Example 3; 150 ml of glacial acetic acid and 500 mg of PtO2 are charged, and the reaction is carried out under a hydrogen pressure of 4 atmospheres over three hours. At the end of this time period, the mixture is evaporated to dryness, the residue is collected with water and is filtered to remove the catalyst. The filtrate is crystallized from water-alcohol to separate 4-amino-3-hydroxybutyric acid (V). An amount of 11.9 g is obtained (yield 100%). The com pound has [a]200 = - 21 (Cl, H20), thus resulting enan d tiomorphically pure. Example 5 Preparation of L-carnitine A solution of 5.8 g of methyl iodide in 60 ml of meth anol is added to a solution of 1.2 g of compound (V) and 2.2 g of potassium hydroxide in 10 ml of water. The solution is maintained at the temperature of 600 C over night. At the end, the solution is evaporated to dryness, and the residue is collected with 50 ml of water. The aqueous solution is extracted 4 times with 50 ml of 80 -phenol. The combined phenolic extracts are washed with 20 ml of water, and are then added to 600 ml of diethyl ether. In a separation funnel, the aqueous layer is separated from the ether-phenol layer, and is subsequently extracted four times with 50 ml of water. The combined aque ous extracts are washed with 150 ml of diethyl ether and are eluted through a column containing an anionic resin. The alkaline fraction of the eluate is then concentrated in vacuo up to dryness. The residue is filtered by using an ethanol-acetone (2:3) solution, and 0.97 g (80%) of L-carnitine are obtained. 200 The compound has [a] : - 30 (Cl, H20), thus re- sulting enantiomorphically pure. Example 6 Preparation of (3R) 3-hydroxy-4-azidobutyric acid, key intermediate in the synthesis of R-carnitin from ethyl-,benzyl-, and phenethyl 4-azido3-oxobutyrates and microbial reduction Ethyl-, benzyl-, and phenethyl 4-azido-3-oxobutyrates were prepared from the corresponding A-chloro derivatives by treatment with sodium azide, using exactly the same dilutions and the same molar ratio and the same experimental conditions described for the preparation of the octyl ester obtaining nearly the same yields. The yeast reduction of ethyl-, benzyl-, and phenethyl 4-azido-3-oxobuty rate to 4-azido-3-hydroxy esters was carried on as reported for the octyl ester, using exactly the same molar concentratiors. The reduced materials were isolated as reported, in the same yields. By means of H NMR studies with chiral shift reagents, the ethyl 4-azido3- hydroxybutyrate was shown to contain ca 90% of the (3R) enantiomer. Similarly, the benzyl-and the phenethyl derivatives were shown to contain over 95% of the (3R) enantiomers. The conversion of the ethyl-,benzyl-, and phenethyl 4-azido-3-hydroxybuty rates into 4-azido-3-hydroxybutyric acid was performed by basic treatment under conditions identical in terms of dilutions and molar ratio and experimental conditions as reported for the octyl ester, obtaining the same yields.

Claims: Claims 1. Process for the preparation of L-carnitine EMI13.1 characterized in that: a) the ester of 4-azido-3-ketobutyric acid, with formu la: EMI13.2 wherein R is an alkyl or an alkylaryl radical, is placed in contact in the liquid phase, in the presence of a source of carbonium and of a phosphate buffer, with a microorganism, to yield the ester of 4-azido-3- hydroxybutyric acid, with formula: EMI13.3 wherein R has the above-reported meaning, in the form of its (3R) isomer; b) The compound so obtained is submitted to the hydrolysis of its ester group in the presence of a base,to yield -(3R)-4-azido-3-hydroxybutyric acid, with formula: EMI13.4 c) (3R)-4-azido-3-hydroxybutyric acid is submitted to the hydrogenation of its azido group with hydrogen gas and in the presence of a hydrogenation catalyst, to yield (3R)-4-amino-3-hydroxybutyric acid [(-)-GABOB], with formula: EMI13.5 d) the compound so obtained is separated and recovered by crystallization; e) (3R)-4-amino-3-hydroxybutyric acid so obtained is sub mitted to methylation to yield L-carnitine compound (I). 2. Process according to claim 1, wherein R is an alk yl or an alkylaryl group, with a number of carbon atoms ranging from 2 to 10. 3. Process according to claim 2, wherein R has preferably a number of carbon atoms ranging from 7 to 8. 4. Process according to claim 1, characterized in that in the step (a) the microorganism belongs to genera Saccharomyces, Aspergillus, Geotrichium, Streptomyces or Epicoccus. 5. Process according to claim 4, characterized in that the microorganism is Saccharomyces cerevisiae. 6. Process according to claim 1, characterized in that in the step (a) the reaction is carried out in the liquid phase, in the presence of a carbon source, at a pH of from 5 to 10, at a temperature of from 100C to 400C and for a time of from 3 hours to 10 hours. 7. Process according to claim 6, wherein the source of carbon is D-glucose. 8. Process according to claim 6, wherein said reaction is carried out at a pH of from 6.5 to 7.5, and at a temperature of 370C and for a time of three hours. 9. Process according to claim 1, characterized in that in the step (b) the base is sodium hydrate, potassium hydrate or calcium hydrate. 10. Process according to claim 1, wherein the hydrolysis reaction in the step (b) is carried out in the liquid phase, at a temperature of from 250C to 700C and for a time of from 3 to 10 hours. 11. Process according to claim 10, wherein said reac tion is carried out at boiling temperatures for a time of 3 hours. 12. Process according to claim 1, wherein the hydrogenation catalyst in the step (e) is a catalyst of noble metal. 13. Process according to claim 12, wherein said catalyst is platinum dioxide. 14. Process according to claim 1, wherein said hydrogenation reaction in the step (e) is carried out in the liquid phase, in an inert solvent, at a temperature of from 100C to 600C, for a time of from 20 hours to 20 minutes, and under a hydrogen pressure ranging from the atmospheric pressure up to about 50 atmospheres. 15. Process according to claim 14, wherein the solvent is an hydrocarbon, an alcohol, an ether or an ester or an organic acid. 16. Process according to claim 15, wherein the solvent is either ethanol or acetic acid. 17. Process according to claim 14, wherein said hydro genation reaction is carried out at room temperature (200 C - 250C), for a time of the order of 4 hours. 18. Process according to claim 1, characterized in that the separation in the step (d) is carried out by crystallization from water, alcohol or from water-alcohol mixtures. 19. Process according to claim 1, characterized in that in the step (c) the methylation is carried out with methyl iodide and alkali in methanol.

Inventors: BENEDETTI, ALBERTO, CARMENO, MASSIMO, CASATI, PAOLO, FUGANTI, CLAUDIO, GRASSELLI, PIERO
Applicants: CONSIGLIO NAZIONALE DELLE RICERCHE, SCLAVO S.P.A.
Application year: 1985
Main classification: C07C101
IPC codes: C07C229, C07B31, C12P7, C07C227, C07C67, C12P13