GB2044755A - Derivatives of D-2-hydroxy-4- methylmercaptobutyric acid, a Process for their Production and their Use - Google Patents

Abstract

Derivatives of D-2-hydroxy-4- methylmercaptobutyric acid of the formula <IMAGE> wherein A represents sodium, potassium or one equivalent of magnesium or calcium or a methyl group and R represents hydrogen or an acetyl group; are useful in fodder, as a food or as a medicine where they may act as methionine supplements. They may be prepared by treating D- methionine with nitrous acid and reacting the D-2-hydroxy-4- methylbutyric acid formed with an oxide, hydroxide, carbonate or bicarbonate of the appropriate metal to form the salt or with methanol to form the ester and/or an acetylating compound to form the acetylated product. The calcium salt of D-2- hydroxy-4-methylmercaptobutyric acid is particularly useful in pharmaceutical preparations for patients having renal insufficiencies.

Description

SPECIFICATION Derivatives of D-2-hydroxy-4-methylmercaptobutyric Acid, a Process for Their Production and Their Use The present invention relates to derivatives of D-2-hydroxy-4-methylmercaptobutyric acid, a process for their production, and their use. More particularly, the present invention is directed to derivatives of D-2-hydroxy-4-methylmercaptobutyric acid corresponding to the following general formula:

wherein A represents sodium, potassium, one equivalent of magnesium or calcium or a methyl group and R represents hydrogen or an acetyl group. The present invention also relates to a process of producing these compounds and to their use as components of fodder, food or medicine.

The calcium salt of DL-2-hydroxy-4-methylmercaptobutyric acid may be used in large amounts as a constituent of industrially produced mixed fodders for animal feeding, particularly for the feeding of poultry. Furthermore, it may also be used in the treatment of renal insufficiency in so-called "renal diets" in admixture with other a-hydroxy- and/or a-ketocarboxylic acids in the form of pharmaceutical preparations. The calcium salt of DL-2-hydroxy-4-methylmercaptobutyric acid acts as a precursor of methionine, an essential amino acid.

Since methionine in natural proteins occurs exclusively in the L-configuration, the DL-2-hydroxy4-methylmercaptobutyric acid must be converted in the organism into L-methionine.

In general, with very few exceptions, amino acids are only normally nutritionally active when in their naturally occurring L-configuration. Even in the rare cases where the corresponding D-form is recognized as having any biological activity at all, this activity is less than that of the corresponding Lform. In any case, there is no known case where the nutritional activity of a D-amino acid has exceeded that of the corresponding L-amino acid.

Since a-hydroxyacids (e.g. a-hydroxycarboxylic acids) are similar to the corresponding a-amino acids (only the hydroxy groups has to be substituted by an amino group), and since it is known in biochemistry that a-hydroxyacids (e.g. a-hydroxycarboxylic acids) can be converted into their corresponding a-amino acids (and vice versa), it was generally thought until now that the L-form of the a-hydroxyacids would also be the more nutritionally active form.

Unexpectedly, it has now been found that the derivatives of D-2-hydroxy-4methylmercaptobutyric acid of the present invention are more effective nutritionally than the corresponding racemate (that is, the mixture of 50% of the D-form and 50% of the L-form).

Accordingly, therefore, these derivatives of the D-form 2-hydroxy-4-methylmercaptobutyric acid (i.e.

free from the L-form) must, surprisingly, be very much more effective than the corresponding L-form.

In one embodiment of the process of the present invention, a compound corresponding to formula I is prepared wherein A represents sodium, potassium, or an equivalent of calcium or magnesium. This process comprises treating D-methionine which is substantially free from the L-form with nitrous acid and then reacting the D-2-hydroxy-$methylmercaptobutyric acid thus formed with an oxide, hydroxide, bicarbonate or carbonate of sodium, potassium, calcium or magnesium. In another embodiment, a compound of formula I wherein A represents a methyl group may be prepared by treating D-methionine which is substantially free from the L-form with nitrous acid and then reacting the D-2-hydroxy-4-methylmercaptobutyric acid thus formed, or a salt thereof, with methanol in the presence of an esterification catalyst.Thionyl chloride is a preferred esterification catalyst.

In another embodiment, compounds of Formula I wherein R represents acetyl may be prepared by acetylating a compound of formula I wherein R represents hydrogen or D-2-hydroxy-4methylmercaptobutyric acid.

Compounds of formula I may also be prepared by treating D-methionine which is substantially free from the L-form with nitrous acid. The D-2-hydroxy-4-methylmercaptobutyric acid thus formed may then be converted to an ammonium salt with an amine, the ammonium salt subsequently being crystallised from a solvent and converted to the free acid. The thus purified free acid may then be reacted with a compound of sodium, potassium, calcium or magnesium which will form a salt with the acid or with methanol and/or acetylated. The amine is preferably dicyclohexylamine.

In the production of the derivatives of the present invention, there must first be prepared the pure enantiomer D-2-hydroxy-4-methylmercaptobutyric acid. This can be done, for example, by dissolving or suspending from 0.25 to 5 moles, preferably from 0.6 to 1 moles of D-methionine in from 5 to 50, preferably from 10 to 20 percent by weight sulphonic acid. It is then treated at a temperature of from -Sto +600C, preferably at from 0 to +50C, for from 1 to 10 hours, preferablyforfrom 1 to 3 hours, with an aqueous solution of from 0.5 to 5 moles, preferably of from 1 to 1.4 moles of sodium nitrite per mole of D-methionine used.Subsequently, the reaction mixture is allowed to stand for from 5 to 24 hours, preferably for about 1 2 hours, at a temperature of from 20 to 500 C, preferably from 20 to 250C for after-reaction. The solution is then saturated with sodium chloride and extracted with a water immiscible, inert organic solvent, preferably with diethyl ether. After evaporation of the solvent from the extract, crude D-2-hydroxy-4-methylmercaptobutryic acid is obtained.

This can be directly converted into the desired derivatives.

It is generally more advantageous, however, to purify the crude D-2-hydroxy-4methylmercaptobutyric acid before producing the desired derivative. This can be done, for example, by reacting it with an amine to form an ammonium salt which is easier to purify. Suitable amines are, for example, alkyl amines, e.g. methyl amine, ethyl amine, butyl amine, hexyl amine, dimethyl amine, diethyl amine, dipropyl amine, dibutyl amine, dihexyl amine or cycloalkyl amines, e.g. cyclohexylamine and dicyclohexylamine.

The reaction product formed with dicyclohexylamine is particularly favourable since the dicyclohexyl ammonium salt can be purified very easily by recrystallization, e.g. from a mixture of ethyl acetate and petroleum ether.

The pure D-2-hydroxy-4-methylmercaptobutyric acid can be literated from the purified ammonium salt by the addition of a strong acid, e.g. 40 percent by weight sulphuric acid, the aqueous solution subsequently being extracted with a water immiscible, inert organic solvent, e.g. diethyl ether.

After the evaporation of the solvent from the extract, pure D-2-hydroxy-4-methylmercaptobutyric acid is obtained.

To produce the sodium, potassium, magnesium or calcium salt, the acid is dissolved in the least possible amount of water and treated with approximately an equivalent amount of the corresponding hydroxide, oxide, bicarbonate or carbonate, e.g. sodium hydroxide, sodium oxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, potassium bicarbonate, potassium carbonate, calcium oxide, calcium hydroxide, calcium bicarbonate, calcium carbonate, magnesium oxide, magnesium hydroxide, magnesium bicarbonate or magnesium carbonate. The salt solution obtained, in a given case after treatment with activated carbon, is evaporated to dryness at reduced pressure, whereby the pure D-form of the desired salt is obtained.

The D-2-hydroxy-4-methylmercaptobutyric acid or the salts thereof can be converted into the D2-hydroxy-4-methylmercaptobutryic acid methyl ester by reaction with methanol in the presence of an esterification catalyst, such as hydrochloric acid, sulphuric acid orp-toluene sulphonic acid. If salts of D-2-hydroxy-4-methylmercaptobutyric acid are used for this reaction, it is necessary to usa greater than stoichiometric amounts of the (mineral) acid. The esterification proceeds particularly well if the D2-hydroxy-4-methylmercaptobutyric acid or the salt is added at a low temperature in a mixture of thionyl chloride and methanol. The ester may be isolated from this mixture in a conventional way by taking it up in water and extraction with an organic solvent.

The D-2-hydroxy-4-methylmercaptobutyric acid methyl ester can be acetylated to the 2-acetoxy derivative with acetyl chloride or acetic anhydride, preferably in the presence of a tertiary amine, such as pyridine or 4-dimethylamino pyridine. The reaction can be carried out in the presence or absence of a diluent. The isolation of the D-2-acetoxy-4-methylmercaptobutyric acid methyl ester may take place in a conventional way by treatment with water and extraction with an organic solvent.

In addition to the compounds mentioned in the Examples, other compounds within the scope of the present invention include D-2-acetoxy-4-methylmercaptobutyric acid sodium salt; D-2-acetoxy-4methylmercapto-butyric acid potassium salt; D-2-acetoxy-4-methylmercaptobutyric acid calcium salt and D-2-acetoxy-4-methylmercaptobutyric acid magnesium salt.

The derivatives of D-2-hydroxy-4-methylmercaptobutyric acid of the present invention, particularly the calcium salt, are suitable as a methionine supplement in feeds and mixed feeds, foods, nutrients and balanced synthetic diets and for the production of pharmaceutical compositions having a nutritionally anabolic acting principle.

Thus, the compounds of the present invention can be used with conventional fodder, e.g. grass, oat shell meal, alfalfa, silage, corn, grain sorghum, oats, rice, rice meal, soybean meal, wheat millfeed, gluten meal, cottonseed meal, hay, barley feed, barley mixed feed, distillers dried grain, peanut meal, or dried molasses beet pulp.

The compounds of the present invention can be used in the feed of various animals, e.g. cattle, sheep, goats, pigs, horses, cats, dogs and poultry, e.g. chickens and turkeys. The compounds of the present invention can be used, for example, in an amount of from 0.01 to 5% of the feed on a dry weight basis.

Using the derivatives of the present invention, for example, it is possible to supplement mixed feeds having a substantially lower proportion of the calcium salt of 2-hydroxy-4methylmercaptobutyric acid, since the D-form is nutritionally more effective than the previously used racemate. A further advantage of the use of the pure D-form is that the circuitous metabolic route followed by the less effective L-form is avoided hence relieving the animals metabolism.

Nutrients, especially for example those which are composed predominantly of soya protein may also be supplemented with the D-2-hydroxy-4-methyimercaptobutyric acid derivatives of the present invention. Since soya protein, which in many countries of the world represents a basic nutrient, contains only a little natural methionine, a diet based predominantly on this protein can result in methionine insufficiency and consequently malnutrition.

The derivatives of the present invention are also very useful for the production of balanced synthetic diets (e.g. for the provisions of astronauts). Here as well, of course, nutritional superiority is of th'e primary importance. Odour, taste and solubility are also important since these diets are frequently taken in as aqueous solutions or pastes. The derivatives of the present invention are tasty, have a pleasant odour and are sufficiently water soluble.

The derivatives of the present invention, especially the calcium salt of D-2-hydroxy-4methylmercaptobutyric acid, are particularly significant for the production of medicines, particularly special preparations for patients having renal insufficiencies. The idea behind such "renal diets" is to load the organism with as little nitrogen or "ammonium-containing materials" as possible, since the excretion of ammonia (in the form of urea) is not possible or is only possible with great difficulty (for example using an artifical kidney). On the other hand, various L-a-amino acids are essential for humans and thus must be supplied in the food. Recent intensive investigations have shown that it is possible to replace several essential amino acids by the corresponding a-hydroxy and/or a-keto acids.These acids use the ammonia which is produced in other metabolic processes in the animal and which is toxic if there Is a renal insufficiency. The ammonia is thus detoxified to a certain degree with the simultaneous formation of the amino acids essential for life.

Until now in these types of "renal diets", the racemic calcium salt of DL-2-hydroxy-4-methylmercaptobutyric acid has nearly always been used instead of the essential L-methionine. In the case of renal insufficiency, anything which leads to an addition load on the kidneys should, of course, be avoided, and thus it is most important to replace the racemate by the nutritionally more effective enantiomer free D-form.

The present invention is further illustrated by the following examples. Unless otherwise indicated, all the parts and percentages quoted are used on a weight basis.

The compositions can comprise, consist essentially of, or consist of the stated materials, and the processes can comprise, consist essentially of, or consist of the stated steps.

Example 1 596.9 grams (4 moles) of D-methionine were dissolved in 3,430 grams of 10% sulphuric acid and treated at a temperature of from 0 to +50C within 2 hours with an ice cooled solution of 345 grams (5 moies) of sodium nitrite in 500 ml of water. This mixture was allowed to warm to a temperature of from 20 to 250C overnight, saturated with sodium chloride and extracted subsequently four times with 600 ml of ether each time. After drying over sodium sulphate, the solvent was distilled off from the combined ether phases. There remained behind an oily residue of 96.3 grams (14.4% of the theoretical yield) of crude D-2-hydroxy-4-methylmercaptobutyric acid.

The residue was taken up in ether and treated with dicyclohexylamine until no further salt crystallized out. The ammonium salt which crystallized out was recrystallized several times from a mixture of ethyl acetate and petroleum ether (the ammonium salt was dissolved in 600 ml of ethyl acetate and precipitated by adding 1400 ml of petroleum ether). There were finally obtained 100.2 grams (52.4% of the theoretical yield) of analytically pure dicyclohexyl ammonium salt.

The salt was hydrolyzed with 111 grams of 40% sulphuric acid. After the addition of sodium sulphate, the salt pulp was filtered off and washed four times with ether. The combined filtrate was separated into an organic phase and an aqueous phase. The latter was extracted another three times with ether. Then the combined ethereal solutions were dried with magnesium sulphate, filtered and evaporated, whereupon 42 grams of 95% D-2-hydroxy-4-methylmercaptobutyric acid were formed.

This had a melting point of from -1 to +50C and a specific rotation [a]5 of + 12.90 (c='1 ; water).

The acid was dissolved in 100 ml of water and treated with a suspension of 9.9 grams of calcium hydroxide in 100 ml of water. After a treatment with activated carbon, the aqueous solution was evaporated to dryness in a water jet vacuum. There remained behind 45.4 grams (88% of the theoretical yield) of the calcium salt of D-2-hydroxy-4-methylmercaptobutyric acid which had a specific rotation [(X]D5 of +24.30 (c=1; water).

Example 2 42 grams of 95% D-2-hydroxy-4-methylmercaptobutyric acid produced as described in Example 1 Were dissolved in 100 ml of water and treated with a suspension of 5.4 grams of magnesium oxide in 100 ml of water. After a treatment with activated carbon, the aqueous solution was evaporated to dryness in a water jet vacuum. There remained behind 43.2 grams (88% of the theoretical yield) of the magnesium salt of D-2-hydroxy-4-methylmercaptobutyric acid.

C1oHr8o8s2Mg (322.7) Calculated: C 37.21% H 5.62% S 19.87% Mg 7.53% Found: C 37.42% H 5.90% S 19.67% Mg 7.42% Example 3 1 5.8 grams of 95% D-2-hydroxy-4-methylmercaptobutyric acid produced as described in Example 1 were dissolved in 40 ml of water and treated with 20 ml of 5 N soda lye. After treatment with activated carbon, the aqueous solution was evaporated to dryness in a water jet vacuum. There remained behind 1 5.0 grams (87% of the theoretical yield) of the sodium salt of D-2-hydroxy-4methylmercaptobutyric acid.

C5Hg03SNa (172.2) Calculated: C 34.87% H 5.27% S 18.62% Na 13.36% Found: C 34.71% H 5.53% S 18.49% Na 13.10% Example 4 1 5.8 grams of 95% D-2-hydroxy-4-methylmercaptobutyric acid produced as described in Example 1 were dissolved in 40 ml of water and treated with 20 mi of 5 N potash lye. After a treatment with activated carbon, the aqueous solution was evaporated to dryness. There remained behind 1 6.9 grams (90% of the theoretical yield) of the potassium salt of D-2-hydroxy-4-methylmercaptobutyric acid.

C5H9QSK(188.3) Calculated: C 31.89% H 4.82% S 17.03% K 20.76% Found: C 32.01% H 5.02% S 16.95% K 20.51% Example 5 8.3 grams (0.07 mole) of thionyl chloride were added at a temperature of -50C to 20 ml of absolute methanol and stirred for 1 5 minutes.

10.15 grams (0.03 mole) of the calcium salt D-2-hydroxy-4-methylmercaptobutyric acid, produced in accordance with Example 1, were dissolved at an elevated temperature in 25 ml of absolute methanol. The solution was dropped into the above-described reaction mixture at a temperature of -5 OC. Subsequently, it was treated with ether and water and separated into an organic phase and an aqueous phase. The latter was extracted three times with ether. The combined organic phases were dried over sodium bicarbonate and sodium sulphate, filtered and concentrated. There remained behind 8.7 grams (88.3% of the theoretical yield) of D-2-hydroxy-4-methylmercaptobutyric acid methyl ester.

Example 6 8.7 grams (0.053 mole) of D-2-hydroxy-4-methylmercaptobutyric acid methyl ester, produced according to Example 5 were dissolved in 30 ml of absolute ether and 4.19 grams (0.053 mole) of pyridine. At OOC, 4.16 grams (0.053 mole) of acetyl chloride were added dropwise with stirring. After stirring overnight, the precipitated pyridine hydrochloride was filtered off and washed with ether. The ether solution was washed once with 10 ml of 2 N-sulphuric acid and three times, each time with 10 ml of water, dried over magnesium sulphate and concentrated.

There remained behind 8.98 grams (82.2% of the theoretical yield) of D-2-acetoxy-4methylmercaptobutyric acid methyl ester.

Example 7 In a feeding test with male chickens, the activity of the D-form of the calcium salt of 2-hydroxy-4 methylmercaptobutyric acid was tested and compared with that of the racemate, i.e. the DL-form. For each test grQup, 21 animals were used. The test lasted 6 days (from the 8th to the 14th day after hatching). The average starting weight of the animals was 75 grams.

The basic diet for the feeding test contained a semi-purified soya protein to which there was added a mixture of amino acids, none of which were sulphur-containing amino acids. This basic diet had the following composition (always in percent by weight): Corn starch (granulated) 31.15 Dextrose 25.00 Soya meal (48.3% raw protein) 31.00 Amino acid mixture (described 5.00 below) Corn oil 2.50 Cellulose 1.00 Dicalcium phosphate 2.20 CaCO3 1.00 NaCI 0.40 NaHCO3 0.50 MnSO4 0.05 ZnCO3 (100 ppm) + Choline chloride 0.10 Vitamin mix 0.10 100.00 Amino Acid Mixture L-Arginine . HCI 0.29 L-Histidine . HCI . H20 0.11 L-Lysine .HCI 0.29 L-Tyrosine 0.11 L-Tryptophane 0.04 L-Phenylalanine 0.13 L-Methionine L-Cystine L-Threonine 0.16 L-Leucine 0.25 L-lsoleucine 0.15 L-Valine 0.17 Glycine 0.15 L-Proline 0.10 L-Glutamate 3.05 5.00 Test group I was fed with only this basic diet which had no sulphur-containing amino acids. Test group II was additionally fed with 0.16 percent by weight (based on the basic diet), of the calcium salt of DL-2-hydroxy-4-methylmercaptobutyric acid. Test group Ill was fed with, in addition to the basic diet, 0.1 6 percent by weight (based on the basic diet) of the calcium salt of D-2-hydroxy-4methylmercaptobutyric acid.

The average daily growth per animal and the feed utilization were used as test criteria. The test results are shown in the following table.

Average Daily Food Utilization Difference Growth Per (g increase Compared Test Group Animal (g) per kg food) to 1(g) 5.9 448.6 11 12.3 602.5 153.7 Ill 12.9 615.3 166.9

Claims (14)

Claims

1. A derivative of D-2-hydroxy-4-methylmercaptobutyric acid which is substantially free from the L-form and which corresponds to the following general formula:

wherein A represents sodium, potassium, an equivalent of magnesium or calcium or a methyl group and R represents hydrogen or an acetyl group.

2. A derivative of D-2-hydroxy-4-methylmercaptobutyric acid substantially as described with particular reference to any of the Examples.

.

3. A process for preparing a compound as claimed in Claim 1 wherein A represents sodium, potassium, or an equivalent of calcium or magnesium which comprises treating D-methionine which is substantially free from the L-form with nitrous acid to form D-2-hydroxy-4-methylmercaptobutyric acid and then reacting the D-2-hydroxy-4-methylmercaptobutyric acid with an oxide, hydroxide, bicarbonate or carbonate of sodium, potassium, calcium or magnesium.

4. A process for preparing a compound as claimed in Claim 1 wherein A represents methyl which comprises treating D-methionine which is substantially free from the L-form with nitrous acid to form D-2-hydroxy-4-methylmercaptobutyric acid and then reacting the D-2-hydroxy-4methylmercaptobutyric acid or a salt thereof with methanol in the presence of an esterification catalyst.

5. A process as claimed in Claim 4 wherein the catalyst is thionyl chloride.

6. A process for preparing a compound as claimed in Claim 1 wherein R represents acetyl which comprises acetylating a compound of formula I as defined in Claim 1 wherein R represents hydrogen or D-2-hydroxy-4-methylmercaptobutyric acid.

7. A process as claimed in Claim 6 wherein the acetylation is carried out with acetyl chloride or acetic anhydride in the presence of a tertiary amine.

8. A process of preparing a compound as claimed in Claim 1 which comprises treating Dmethionine which is substantially free from the L-form with nitrous acid to form D-2-hydroxy-4 methylmercaptobutyric acid, converting the free acid thus formed to an ammonium salt with an amine, crystallizing the ammonium salt from a solvent, converting the ammonium salt to the free acid and then reacting the thus purified free acid with a compound of sodium, potassium, calcium or magnesium which will form a salt with the purified free acid or reacting with methanol and/or acetylating.

9. A process as claimed in Claim 4, wherein the amine is dicyclohexylamine.

10. A process for preparing a compound of formula I as defined in Claim 1 substantially as described with particular reference to any one of the Examples.

11. A compound corresponding to formula I as defined in Claim 1 when prepared by a process as claimed in any of Claims 3 to 10.

12. A food or pharmacological composition comprising a compound as claimed in any of Claims 1,2 or 11.

13. A process of supplying methionine to an animal which comprises feeding the animal a compound as claimed in any of Claims 1, 2 or 11.

14. A process as claimed in Claim 13 wherein the animal is a mammal.

1 5. A process as claimed in Claim 14 wherein the mammal has a renal insufficiency.

1 6. A process as claimed in Claim 13 wherein the animal is poultry.