4-Substituted-2-hydroxybutanoates and a process for producing them

Abstract

The present invention provides an optically active 4-substituted-2-hydroxybutanoate represented by the general formula: ##STR1## wherein X is Cl, Br, I or Cn, R is alkyl, and * shows an asymmetric carbon.The compounds which are useful as starting materials for medical agents, are obtained by a process comprising ring-opening optically active .alpha.-hydroxy-.gamma.-butyrolactone and esterifying the compound obtained.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optically active 4-substituted-2-hydroxybutanoates which are useful as starting materials for medical agents and to a process for producing them.

2. Descriptions of the Prior Art

Optically active 4-substituted-2-hydroxybutanoates of the present invention are useful as starting materials of many medical agents. However, there has been no effective process so that racemic methyl 4-chloro-2-hydroxybutanoate, racemic methyl 4-bromo-2-hydroxybutanoate and the like were used (European Patent Laid-open Application No. 233728, etc.).

Recently, when racemic compounds are used as medical agents, it is necessary to examine the physiological activity of two enantiomorphs, because the physiologically activities may be different. Further, when a compound having a stereostructure has especially strong physiological activity, it is earnestly required to construct only a compound having the desired stereostructure, considering the efficiency and safety.

SUMMARY OF THE INVENTION

The inventors of the present invention have developed optically active 4-substituted-2-hydroxybutanoates which become useful compounds as starting materials of pharmaceuticals, and they found an effective process for producing the esters.

Namely, the present invention provides an optically active 4-substituted-2-hydroxybutanoate represented by the general formula: ##STR2## wherein X is Cl, Br, I or CN, R is alkyl of 1-10 carbon atoms, and * shows an asymmetric carbon.

Further, the present invention provides a process for producing said optically active 4-substituted-2-hydroxybutanoates represented by the general formula (I), which comprises ring-opening an optically active .alpha.-hydroxy-.gamma.-butyrolactone as an intermediate compound of the formula: ##STR3## wherein * shows an asymmetric carbon, and esterifying the compound obtained.

The following description illustrates this invention more specifically.

Optically active 4-substituted-2-hydroxybutanoates of the present invention are represented by the above general formula (I). As these compounds, the following compounds are exemplified:

R-methyl 4-chloro-2-hydroxybutanoate, R-ethyl 4-chloro-2-hydroxybutanoate, R-propyl 4-chloro-2-hydroxybutanoate, R-butyl 4-chloro-2-hydroxybutanoate, R-pentyl 4-chloro-2-hydroxybutanoate, S-methyl 4-chloro-2-hydroxybutanoate, S-ethyl 4-chloro-2-hydroxybutanoate, S-propyl 4-chloro-2-hydroxybutanoate, S-butyl 4-chloro-2-hydroxybutanoate, S-pentyl 4-chloro-2-hydroxybutanoate, R-methyl 4-bromo-2-hydroxybutanoate, R-ethyl 4-bromo-2-hydroxybutanoate, R-propyl 4-bromo-2-hydroxybutanoate, R-butyl 4-bromo-2-hydroxybutanoate, R-pentyl 4-bromo-2-hydroxybutanoate, S-methyl 4-bromo-2-hydroxybutanoate, S-ethyl 4-bromo-2-hydroxybutanoate, S-propyl 4-bromo-2-hydroxybutanoate, S-butyl 4-bromo-2-hydroxybutanoate, S-pentyl 4-bromo-2-hydroxybutanoate, R-methyl 4-iodo-2-hydroxybutanoate, R-ethyl 4-iodo-2-hydroxybutanoate, R-propyl 4-iodo-2-hydroxybutanoate, R-butyl 4-iodo-2-hydroxybutanoate, R-pentyl 4-iodo-2-hydroxybutanoate, S-methyl 4-iodo-2-hydroxybutanoate, S-ethyl 4-iodo-2-hydroxybutanoate, S-propyl 4-iodo-2-hydroxybutanoate, S-butyl 4-iodo-2-hydroxybutanoate, S-pentyl 4-iodo-2-hydroxybutanoate, R-methyl 4-cyano-2-hydroxybutanoate, R-ethyl 4-cyano-2-hydroxybutanoate, R-propyl 4-cyano-2-hydroxybutanoate, R-butyl 4-cyano-2-hydroxybutanoate, R-pentyl 4-cyano-2-hydroxybutanoate, S-methyl 4-cyano-2-hydroxybutanoate, S-ethyl 4-cyano-2-hydroxybutanoate, S-propyl 4-cyano-2-hydroxybutanoate, S-butyl 4-cyano-2-hydroxybutanoate, S-pentyl 4-cyano-2-hydroxybutanoate and the like.

The optically active 4-substituted-2-hydroxybutanoates of the present invention are obtained by ring-opening an optically active .alpha.-hydroxy-.gamma.-butyrolactone represented by the above formula (II) which is obtained by optical resolution wherein racemic .alpha.-hydroxy-.gamma.butyrolactone is transesterified with ethyl acetate, a fatty acid vinyl ester, a triglyceride or the like in the presence of an esterase produced by a microorganism or an esterase from an animal as a catalyst, and esterifying the compound obtained.

The racemic .alpha.-hydroxy-.gamma.-butyrolactone as a raw material can be easily obtained from .alpha.-bromo-.gamma.-butyrolactone by a method of Goal et al. (Organic Preparations and Procedures Int., 17, 91 (1985)).

As fatty acid vinyl esters used int he transesterification reaction, vinyl acetagte, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl laurate, etc. can be used. As triglycerides, triacetin, tripropionin, tributyrin, tricaproin, trilaurin etc. can be exemplified. These compounds are commercially available without any difficulty.

The following table shows commercially available esterases.

TABLE______________________________________Trade name Origin Seller or Maker______________________________________Lipase PS Pseudomonas fluorescens Amono Pharmaceu- tical Co., LtdLi-pase CES Pseudomonas sp Amono Pharmaceu- tical Co., LtdLipase AP Aspergillus niger Amono Pharmaceu- tical Co., LtdLipase M Mucor javanicus Amono Pharmaceu- tical Co., LtdLipase CE Humicola lanuginosa Amono Pharmaceu- tical Co., LtdLipase F-AP Rhizopus javanicus Amono Pharmaceu- tical Co., LtdLipase II Porcine Pancreas Sigma Chemical Co., LtdLipase VIII Geotrichum Candidum Sigma Chemical Co., LtdLipase X Rhizopus delamar Sigma Chemical Co., LtdLipase Chromobacterium Viscosum Toyo Jozo Co., LtdLipase A Aspergillus niger Novo Industi A/SLipase Rhizopus niveus Nagase Biochemi- cals, Ltd.Lipase B Pseudomonas fragi Sapporo Beer Co.______________________________________

In addition to these esterases, the enzymes produced from microorganisms can be used. These microorganisms which produce the enzymes having the above reaction ability can be used regardless of their species and genus. As such microorganisms, the genera Pseudomonas, Arthrobacter, Acromobacter, Alcaligenes, Aspergillus, Chroaobacterium, Candida, Mucor, Rhizopus, etc. can be exemplified.

In these microorganisms, particularly the genus Pseudomonas is preferred.

The transesterification reaction is conducted by mixing a racemic .alpha.-hydroxy-.gamma.-butyrolactone with ethyl acetate, a fatty acid vinyl ester or a triglyceride and efficiently contacting the mixture with an esterase. The reaction temperature is suitably from room temperature (about 10.degree. C.) to 150.degree. C., especially and preferably from 20.degree. to 45.degree. C. The reaction time is from 1 to 1000 hours. .alpha.-hydroxy-.gamma.-butyrolactone and ethyl acetate, the fatty acid vinyl ester or the triglyceride are suitably mixed in the ratio from 1:0.5 to 1:10 by mole, preferably 10.5 by mole.

When necessary, n-hexane, n-heptane, benzene, toluene, ethyl ether, etc. can be used as a solvent by which esterase activity is not inhibited. The solvent may be omitted.

After the transesterification reaction is conducted, the esterase can be removed by conventional filter operation and used again, as it is. After concentrating the filtrate, optically active .alpha.-hydroxy-.gamma.-butyrolactone and an optically active .alpha.-acyloxy-.gamma.-butyrolactone can be separated by vacuum distillation or column chromatography, respectively. When these lactones have insufficient optical purities, each lactone is re-transesterified (in the case of the acyl compound, after the acyl group is hydrolyzed) to obtain a high optically pure compound.

The optically active .alpha.-hydroxy-.gamma.-butyrolactone obtained can lead to the production of optically active 4-chloro-2-hydroxybutanoate or optically active 4-bromo-2-hydroxybutanoate by reaction with ethyl alcohol saturated with hydrogen chloride or hydrogen bromide.

Further, optically active .alpha.-hydroxy-.gamma.-butyrolactone can lead to the production of optically active 4-iodo-2-hydroxybutanoate by reaction with trimethylsilyl chloride and sodium iodide or trimethylsilyl iodide.

Moreover, optically active .alpha.-hydroxy-.gamma.-butyrolactone can lead to the production of optically active 4-cyano-2-hydroxybutanoate by reaction with trimethylsilylcyanide. Otherwise, it can lead to the production of optically active 4-cyano-2-hydroxybutanoate by cyanizing optically active 4-halo-2-hydroxybutanoate.

By using the above operation, the R- and S-compounds of optically active 4-substituted-2-hydroxybutanoate can be obtained, respectively.

Optically active 4-substituted-2-hydroxybutanoates of the present invention are useful and applicable compounds as starting materials of medical agents. For example, optically active guanine derivatives (U.S. Pat. No. 4,495,190 etc.) which are useful as physiologically active materials can be obtained. Otherwise, as to calcium channel blockers (Japanese Patent Laid-open Application No. 61-87503, etc.), optically active materials by which more effective activity is expected can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate the present invention more specifically, but these will not always be precise in practical applications.

Example 1

(i) Production of R-(+)-.alpha.-hydroxy-q-butyrolactone

A mixture of 15.3 g of racemic-.alpha.-hydroxy-.gamma.-butyrolactone, 6.5 g of vinyl acetate and 5 g of lipase PS (manufactured by Amano Pharmaceutical Co., Ltd.) was put into a 100 ml flask. The mixture was stirred for 6 hours at room temperature. After lipase PS was removed by filtration, the filtrate was concentrated and about 40 g of liquid was obtained. The liquid was column-chromatographed (toluene/ethyl acetate (10/1)), and 5.2 g of S-(-)-.alpha.-hydroxy-.gamma.-butyrolactone ([.alpha.].sub.D -57.6.degree. (c 1.22, CHCl.sub.3), 99% ee) and 7.2 g of R-(+)-.alpha.-acetyloxy-.gamma.-butyrolactone ([.alpha.].sub.D +32.6.degree. (c 0.87, CHCl.sub.3), 70% ee) were separated, respectively.

R-(+)-.alpha.-acetyloxy-.gamma.-butyrolactone was dissolved into a mixed solvent of 50 ml of 1,4-dioxane and 30 ml of ethanol, and 3.8 g of potassium carbonate was added. The mixture was stirred for 5 hours at room temperature. After the mixture was neutralized with hydrochloric acid, the salt separated was removed by filtration. Moreover, after removing ethanol, the filtrate was purified by distillation, and 6.1 g of R-(+)-.alpha.-hydroxy-.gamma.-butyrolactone was obtained.

(ii) Production of R-ethyl (+)-4-chloro-2-hydroxybutanoate

1.6 g of R-(+)-.alpha.-hydroxy-65 -butyrolactone (90% ee) was dissolved into 200 ml of ethanol, and the solution was put into a 300 ml three-necked flask. Hydrogen chloride gas was blown into the three-necked flask, and the solution was stirred in a saturated condition at room temperature overnight. After hydrogen chloride gas was removed thoroughly by blowing nitrogen through the solution, ethanol was distilled away, and the residue was extracted with ether. The ether layer was washed with sodium bicarbonate and then with a saturated saline solution. After the ether layer was dried on anhydrous magnesium sulfate, ether was distilled away. The residue was distilled under reduced pressure to obtain 2.1 g of the desired R-ethyl (+)-4-chloro-2-hydroxybutanoate. The physical properties of the compound are as follows.

Boiling point: 112.degree. C./17 mmHg

Specific rotation: [.alpha.].sub.D +6.1.degree. (C1.9, CHCl.sub.3)

Example 2

Production of S-ethyl (-)-4-chloro-hydroxybutanoate

1.6 g of S-(-)-.alpha.-hydroxy-.gamma.-butyrolactone (90% ee) was dissolved into 200 ml of ethanol, and the solution was put into a 300 ml three-necked flask. Hydrogen chloride gas was blown into the three-necked flask, and the solution was stirred in a saturated condition at room temperature overnight. After hydrogen chloride gas was removed thoroughly by blowing nitrogen through the solution, ethanol was distilled away, and the residue was extracted with ether. The ether layer was washed with sodium bicarbonate and then with a saturated saline solution. After the ether layer was dried on anhydrous magnesium sulfate, ether was distilled away. The residue was distilled under reduced pressure to obtain 2.0 g of the desired S-ethyl (-)-4-chloro-2-hydroxy butanoate. The physical properties of the compound are as follows.

Boiling point: 101.degree. C./13 mmHg

Specific rotation: [.alpha.].sub.D -6.7.degree. (C1.2, CHCl.sub.3)

Example 3

Production of R-ethyl (+)-4-iodo-2-hydroxybutanoate

20 ml of an acetonitrile solution of trimethylsilyl chloride was slowly added into 50 ml of an acetonitrile solution containing 1.3 g of R-(+)-.alpha.-hydroxy-.gamma.-butyrolactone (70% ee) and 5.7 g of sodium iodide. After the solution was heated under reflux for 14 hours, 10 ml of ethanol was added dropwise and the solution was stirred at room temperature for 10 minutes.

Acetonitrile was distilled away, and ether was added to the residue. The ether layer was washed with sodium bicarbonate and then with a saturated saline solution. After the ether layer was dried on anhydrous magnesium sulfate, ether was distilled away. 0.73 g of the desired R-ethyl (+)-4-iodo-2-hydroxybutanoate was obtained. Specific rotation: [.alpha.].sub.D +4.0.degree. (C1.15, CHCl.sub.3)

Claims

1. A process for producing an optically active 4-substituted-2-hydroxybutanoate represented by the formula (I) ##STR4## wherein X is Cl or Br, R is alkyl or 1-10 carbon atoms, and * shows an asymmetric carbon which comprises contacting a compound of the formula: ##STR5## wherein * shows an asymmetric carbon with alkyl alcohol of 1-10 carbon atoms saturated with hydrogen chloride or hydrogen bromide to perform the following reactions (1) ring-opening of the compound of the formula (II), (2) introducing a Cl ion or a Br ion derived from said hydrogen chloride or hydrogen bromide into the fourth position of thus obtained ring-opened intermediate of the compound of the formula (II) and (3) introducing an alkyl group derived from said alkyl alcohol into the carbonyloxy portion of thus obtained ring-opened intermediate of the compound of the formula (II) in one step.

2. A process according to claim 1, wherein the alkyl alcohol is ethanol.

3. A process for producing an optically active 4-substituted-2-hydroxybutanoate represented by the formula (I) ##STR6## wherein X is I or CN, R is alkyl of 1-10 carbon atoms, and * shows an asymmetric carbon which comprises contacting a compound of the formula: ##STR7## wherein * shows an asymmetric carbon with alkyl alcohol of 1-10 carbon atoms containing trimethylsilyl iodide or trimethylsilyl cyanide to perform the following reactions (1) ring-opening of the compound of the formula (II), (2) introducing a I ion or CN ion derived form said trimethylsilyl compound into the fourth position of thus obtained ring-opened intermediate of the compound of the formula (II) and (3) introducing an alkyl grouped derived from said alkyl alcohol into the caronyloxy portion of thus obtained ring-opened intermediate of the compound of the formula (II) in one step.

4. A process for producing an optically active 4-substituted-2-hydroxybutanoate represented by the formula (I) ##STR8## wherein X is I, R is alkyl of 1-10 carbon atoms, and * shows an asymmetric carbon which comprises contacting a compound of the formula ##STR9## wherein * shows as asymmetric carbon with trimethylsilyl chloride and sodium iodide in a solvent to perform the following reactions (1) ring-opening of the compound of the formula (II) and (2) introducing a I ion derived from said sodium iodide into the fourth position of thus obtained ring-opened intermediate of the compound of the formula (II), and contacting the thus obtained I ion-introduced ring-opened intermediate with alkyl alcohol of 1-10 carbon atoms to introduce an alkyl group derived from said alkyl radical into the carbonyloxy portion of said I ion-introduced ring-opened intermediate.

5. A process according to claim 4, wherein the alkyl alcohol is ethanol.

6. A process according to claim 4, wherein the contacting of the compound (II) with trimethylsilyl chloride and sodium iodide is carried out in a solvent of acetonitrile.