This invention relates to a novel process for the resolution of an acid mixture (hereinafter: threo acid mixture) containing (+)-threo-3-/r2-amiπophenyl)thio7- -2-hydroxy-3-(4-methoxyphenyl)propionic acid /herein¬ after: (+)-threo acid7 and (-)-threo-3-/ 2-aminophenyl)- thio7-2-hydroxy-3-(4-methoxypheπyl)propionic acid /herein- after: (-)-threo acid7.

The (+)-threo acid is an important raw material of the therapeutically active (2S,3S)-3-acetoxy-5-dimethyl- aminoethyl-2-(4-methoxypheπyl)-2,3-dihydro-l,5-benzothiaz- epin-4(5H)-one hydrochloride (generic name: diltiazem), a known antianginal drug acting as a calcium antagonist. There are several processes known from the litera¬ ture for the preparation of the (+)-threo acid.

A resolution process using L-lysine as resolving agent is described in the published German patent appli- cation (DE-OS) No. 3,337,176. However, the particularly costly L-lysine is used in a 4-fold excess and in addi¬ tion, it is difficult and expensive to recover L-lysine in its acid form from the mother liquors.

According to an other method /Helv. Chim. Acta 67 , 916 (1984)7, the racemic threo acid is resolved by using cinchonidine in ethanol. This method is characterized by the use of an extraordinarily high amount of alcohol as solvent and by a 48-hours time demand of crystallization.

According to the method described in the United

States patent specification No. 4,416,819, an equivalent amount of ( +) - oC -pheπylethylamine each is employed for both forms of the racemic acid in order to resolve the racemic threo acid in water. In our own examinations, a

(+)-threo acid with an optical purity of only 87 %, n

Λ 7η = +302 (c = 0.3, ethanol), was obtained in a yield of 79 % when the resolution and subsequent crystalliza¬ tion of the diastereomeric salt were carried out by using the method cited above.

In the process described in the Hungarian patent speci ication No. 193,230, the acid addition salt of 0.5 to 0.6 molar equivalent of (÷ -^ -pheπylethylamine is used as resolving agent; a method, useful to enrich the valuable (+)-threo acid in the remaining threo acid mix¬ ture is also published in the above specification.

In the course of working out the present invention, it has been aimed to develop a process providing the (+)- -threo acid economically, in a high optical purity and good yield by using simple technological steps as well as a cheap and easily available resolving agent.

The invention is based on the recognition that D- -(-)-phenylglycine amide /hereinafter: (-)-PGA7 and L-(+)-phenylglycine amide /Hereinafter: (+)-PGA7, res- pectively form a slightly (less) soluble diastereomeric salt with the (-)-threo or (+)-threo acid, respectively. Thus, the present invention essentially relates to a

novel process for the resolution of an acid mixture ("threo acid mixture") containing (+)-threo-3-/(2-amino- phenyl)thio7-2-hydroxy-3-(4-methoxyphenyl)propionic acid / ^π (+)-threo acid"7 and (-)-threo-3-/2-aminophenyl)thio7- -2-hydroxy-3-(4-methoxyphenyl)propionic acid / ^π (-)-threo acid"7, which comprises using D-(-)-phenylglycine amide or L-(+)-phenylglycine amide, respectively as resolving agent.

Although (+)-PGA and (-)-PGA used for separating the threo acid mixture can be prepared in any optional way (see e.g. the German patent specification (DE-PS) No. 2,547,548), a particularly economical process for the preparation of the above resolving agents is also described in the present invention namely, racemic or nearly racemic c -phenylglycine amide (hereinafter: PGA mixture) can be separated to its enantiomers in a good yield and satisfying purity by using (-)-threo acid con¬ sidered as a side product of no use up to the present; whereas the racemic or nearly racemic threo acid mixture can be resolved to its enantiomers in a suitable quality by using the aqueous solutions containing (+)-PGA or (-)-PGA, respectively obtained according to the process of the present invention.

Thus, by using this most preferred variation (embodi- ent) of the present invention, the preparation of (+)- -threo acid can be solved (realized) in a very economical way without any demand on an expensive, optically active

resolving agent namely, the resolving agent required to prepare (+)-threo acid can be obtained by utilizing (-)- -threo acid, which is accumulated in large amount or con¬ tinuously formed, respectively in the preparation of the (+)-threo acid.

It should be noted that (+)-PGA also forms a slight¬ ly water-soluble diastereomeric salt with (+)-thrεo acid, however, this resolution process bears no economical im¬ portance. It should also be remarked that the process of the present invention is not aimed to obtain solid (+)-PGA and (-)-PGA from the aqueous solutions containing (+)-PGA or (-)-PGA, respectively, since the (+)-threo acid can be resolved in an aqueous medium, too and on the other hand, PGA is partially decomposed under heat effects accompany¬ ing its recovery.

Racemic or nearly racemic ©^-phenylglycine amide used as starting substance can be prepared according to e.g. 3. Am. Chem. Soc. i , 78 (1949). The resolution of the threo acid mixture can be achieved by using any method commonly used for resolu¬ tion. It has been found to be preferred first to dis¬ solve the threo acid mixture in water by the means of sodium hydroxide solution at 50 to 80 °C, then to pour to the mixture ethanol in an amount being nearly equal to that of the water used, thereafter to add portionwise the corresponding (-)- or (+)-PGA hydrohalide, respec-

tively, preferably hydrochloride in a solid or dissolved form in an excess of 5 to 15% calculated for the enaπ- tio er to be separated. The precipitated diastereomeric salt is filtered, dissolved in an 8 to 12-fold amount of water and decomposed by adding aqueous hydrochloric acid (pH = 2 to 4). The corresponding (+)- or (-)-threo acid, respectively is obtained as a precipitate.

The mother liquor remaining after separation of the diastereomeric salt is acidified (pH = 2 to 4) by aqueous hydrochloric acid and the solution is stirred under cool¬ ing by ice-water. The precipitate (i.e. the oppositely rotating enantiomer -of the threo acid obtained by decom¬ posing the salt) is filtered.

The remaining mother liquor contains the starting resolving agent thus, it can be used in further resolu¬ tion steps (cycles).

The PGA mixture can be resolved by using any method commonly employed in resolution processes. According to a process found to be most advantageous in our investiga- tions, an acid addition salt, preferably hydrochloride of the starting d -phenylglycine amide is dissolved in water at 50 to 80 C and an aqueous solution of (-)-threo acid prepared with an alkaline metal hydroxide, preferably sodium hydroxide and maintained at 50 to 80 °C is portion- wise added (if necessary, the pH value of both solutions is previously adjusted to 7). The solution obtained is allowed to cool to 20 - 30 °C and then further cooled by ice-water. The precipitate is filtered.

The filtrate is an aqueous solution containing (A- -PGA, which can directly be used as a resolving agent.

The precipitate filtered out is (-)-threo acid, which may be repeatedly be used for separation. The process of the invention is illustrated in detail by the following non-limiting Examples. Example 1

Preparation of an aqueous solution containing (+)- -c -phenylglycine amide A solution prepared from 3.6 g (0.113 mole) of (-)- -threo acid with 22 ml of 0.5 N sodium hydroxide and 22 ml of water, clarified by activated carbon and heated to 70 °C is portionwise added to a solution of 3.7 g (0.0198 mole) of < .-pnenylglycine amide hydrochloride prepared in 40 ml of water at 70 °C. (If necessary, the pH value of both above solutions is adjusted to 6.5-7.5 by adding hydrochloric acid or sodium hydroxide solution, respec¬ tively before combining them.) The solution cooled down to room temperature is cooled by ice-water for additio- nal 30 minutes. During crystallization the pH value is several times controlled and maintained between 6.5 and 7.5; when necessary, the pH value is appropriately ad¬ justed as described above.

The precipitate is filtered to obtain 5.0 g of dia- stereomeric salt containing (-)-PGA-(-)-threo acid as the main bulk, / 7g° = -215° (c = 0.3, water).

The filtrate contains 1.37 g (0.0091 mole) of (+)-

-PGA, c 7p ⁿ = +92°. Example 2

Preparation of an aqueous solution containing (-)- - f^-phenylglycine amide 5.0 g of the diastereomeric salt filtered out as described in the preceediπg Example is decomposed in 100 ml of water at 70 °C in such a way that the solution is heated to the boiling point while adjusting the pH value of the solution to 3 by adding 1 N hydrochloric acid. The mixture is cooled and let stand at room tempera ture, then cooled by ice-water for 1 hour and the preci¬ pitate is filtered to give 3.4 g of (-)-threo acid, /£7Q° = -335° (c = 0.3, ethanol).

The filtrate contains 1.48 g (0.0099 mole) of (-)- -PGA hydrochloride.

The /<Z7Q° value of the filtrate is -82°. Example 3

Preparation of (-)-threo-3-/r2-aminophenyl)thio7-2- -hydroxy-3-(4-methoxyphenyl)propionic acid /£-)- threo acid7

40 ml of ethanol and a solution of 1.0 g (0.0054 mole) of (-)-PGA hydrochloride (/^7g° = -92° (1 N HC1)7 in 10 ml of water are portionwise added to the solution containing 3.2 g (0.01 mole) of racemic threo acid in a mixture of 30 ml of water and 5 ml of 1 N sodium hydroxid solution at 70 °C. If necessary, the pH value of the solution obtained is adjusted to 7 after combining. The

solution is allowed to cool while stirring. From the be¬ ginning of crystallization, a cooling by ice-water is employed for 30 minutes. The precipitate is filtered and the filtrate is set aside for further working up (process- ing). In this way 2.4 g of (-)-PGA-(-)-threo acid dia¬ stereomeric salt are obtained, I Oη = -218.7° (c = 0.3, water). After recrystallization of this diastereomeric salt from 85 ml of water, 2.1 g of product are obtained,

/eZ7p ⁰ = -16° (c = 0.3, ethanol)

The diastereomeric salt obtained as described above is dissolved in 50 ml water at 70 °C while adjusting the pH value of the solution to 3 by 1 N hydrochloric acid. After letting the mixture stand for 1 hour, the mixture is cooled by ice-water for 30 minutes, then the precipi- tate is filtered to give 1.1 g (68.8 %) of the title com¬ pound, m.p.: 136 - 138 °C, /JL7^° = -330.5° (c = 0.3, ethanol) .

From the mother liquor set aside after separation of the diastereomeric salt, (+)-threo-3-/r2-aminophenyl)- thio7-2-hydroxy-3-(4-methoxyphenyl)propionic acid /!+)- -threo acid7 is obtained as follows.

After adjusting the pH value of the remaining mother liquor to 3 by 1 N hydrochloric acid, the mixture is crystallized first at room temperature for 1 hour, then under cooling by ice-water for 30 minutes. After filter¬ ing the precipitate, 1.5 g (93.8 h of (+)-threo acid are obtained, / 7g° = 260.6° (c = 0.3, ethanol).

Example 4

Preparation of (+)-threo-3-/r2-aminophenyl)thio7-2- -hydroxy-3-(4-methoxyphenyl)propionic acid +)- -threo acid7

6.4 g (0.02 mole) of racemic threo acid are dissolved, in a mixture of 17 ml of water and 43 ml of 0.5 N sodium hydroxide solution at 70 C and then 80 ml of methanol and 2.0 g (0.011 mole) of (+)-PGA = +92° (1 N HC1)7 dissolved in 20 ml of water are portionwise added. If ne¬ cessary, the pH value of the solution is adjusted to 7 after combination. The solution is allowed to cool down while stirring. The -mixture is left to stand for 1 hour from the beginning of crystallization, then it is cooled by ice-water for 30 minutes, thereafter the precipitate is filtered to obtain 3.5 g of product /T+ )-PGA-(+)-threo acid diastereomeric salt7.

The diastereomeric salt obtained as described above is dissolved in 70 ml of water by heating the solution to its boiling point and adjusting its pH value to 3 by 1 N hydrochloric acid. Subsequently, the mixture is cooled by ice-water for 1 hour, then the precipitate is filtered and the filtrate is set aside for further working up (processing). In this way 2.1 g (65.6 %) of the title compound are obtained, .p.: 138 - 139 °C, Λ 7η = +346° c = 0.3, ethanol).

The mother liquor set aside after decomposing the diastereomeric salt can be used in a further resolution

step as follows.

3.2 g (0.01 mole) of racemic threo acid are dis¬ solved at 80 C in a mixture containing 30 ml of water and 21.5 ml of 0.5 N sodium hydroxide solution. To this solution, the mother liquor set aside after decomposing the diastereomeric salt is added, the pH value of which has previously been adjusted to 7 by 1 N sodium hydroxide solution. If necessary, the pH value of the solution ob¬ tained is adjusted to 7 after combination. This solution is allowed to cool down under stirring. After beginning of crystallization, cooling by ice-water is used for 30 minutes, then the precipitate is filtered to give 2.0 g of product / +)-PGA-(+)-threo acid diastereomeric salt7. The diastereomeric salt obtained as described above is dissolved in 30 ml of water by heating the solution to its boiling point while adjusting its pH value to 3 by 1 N hydrochloric acid. Thereafter, the mixture is cooled at room temperature for 1 hour, then by ice-water for 30 minutes and the precipitate is filtered to yield 1.3 g (81 %) of the title compound, m.p.: 134 - 136 °C, /«"7p° = +327° (c = 0.3, ethanol).