Title of Invention

"A PROCESS FOR PREPARING (R)-4- CYANO-3- HYDROXYBUTYRIC ACID ESTER"

Abstract

The present invention relates to a process for preparing (R)-4-cyano-3-hydroxybutyric acid ester derivatives and more particularly, to a process for preparing optically pure (R)-4-cyano-3-hydroxybutyric acid ester derivatives expressed by formula (1) in high yield by performing cyanation and sequential esterification of (S)-3,4-epoxybutyric acid salt as a starting material. In said formula, R represents linear or branched alkyl group with 1~5 carbon atoms or benzyl group.

Full Text

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for preparing (R)-4-cyano-3-hydroxybutyric acid esters and more particularly, to a novel process for preparing optically pure (R)-4-cyano-3-hydroxybutyric acid esters expressed by the following formula (1) in high yield by performing cyanation and sequential estenficarton of (S)-3,4-epoxybutyric acid salt as a starting material ( Formula Removed) wherein R represents a linear or branched alkyl group having 1-4 carbon atoms or a benzyl group. Description of the Prior ArtOptically active (R)-4-cyano-3-hydroxybutyric acid ester expressed by the following formula (1) is useful as a key intermediate for hypolipidemic agents. Various methods for preparing hypolipidemic agents from optically active (R)-4-cyano-3-hydroxybutyric acid ester have been reported as introduced hereinafter. U.S. Patent No. 4,681,893 describes a process for the preparation of hypolipidemic agent, atorvastatin, from (R)-4-cyano-3-hydraxybutyric acid ester. The use of (R)-4-cyano-3-hydroxybutyric acid ester as starting material is described in preparation of Cl-981 which is an inhibitor of 3-hydroxy-3- methylglutarylcoenzyme A(HMG-CoA) reductase in Tetrnlicdroii Letters, 33, 2279-22S2 (1992). As described above, there is a need for a simple and inexpensive method for the preparation of (R)-4-cyano-3-hydroxybutyric acid ester due to its usefulness for hypolipidemic agents. A large number of syntheses have been developed. For example, U.S. Patent No. 4,611,067 describes a method for preparation of (R)-4- cyano-3-hydroxybutyric acid ester by the synthetic pathway from ascorbic acid. Ascorbic acid is degraded to afford 4-bromo-3-hydroxybutyric acid ester through D-threonate potassium salt which is then reacted with an appropriate reagent to protect the hydroxy function to silyl group prior to reaction with sodium cyanide in dimethyl sulfoxide for 16 hours to afford (R)-4-cyano-3-hydroxybutyric acid ester. However, this conventional method requires multi-step synthesis and long reaction time for cyanation as well as difficulty in removing dimethyl sulfoxide which is used as a solvent. It generates large amount of side-products such as acrylate by dehydration due to the high acidic hydrogen on α-position for the carboxy group and the corresponding acid by hydrolysis of ester group. In addition, it is not applicable to produce various esters. SUMMARY OF THE INVENTION The inventors made extensive efforts to provide more economical and effective preparing methods of (R)-4-cyano-3-hydroxybutyric acid ester which is an essential intermediate for drugs. As a result, it was realized that the use of (S)-3,4-epoxybutyric acid salt as a starting material, followed by cyanation and sequential esterification could provide a novel and inexpensive preparing method of (R)-4-cyano-3-hydroxybutyric acid ester with excellent optical purity and yield. The introduction of cyano functional group to (S)-3,4-epoxvburvnc acid sail in the present invention has not only been tried to afford (R)-4-cyano-3-hydroxybutyric acid ester and but also provided a novelty, applicability and easiness in the method of preparing the same. Therefore, the purpose of this invention is to provide the synthetic method of optically pure (R)-4-cyano-3-hydroxybutyric acid ester, as the target product, in maximum yield as well as high purity simultaneously inhibiting the side-reaction, hydrolysis or formation of acrylate which are usual problems in the conventional methods. Detailed Description of the Invention The present invention relates to a process for preparing optically pure (R)-4-cyano-3-hydroxybutyric acid esters expressed by the following formula (1) in high yield by performing cyanarion and sequential esterification of (S)-3,4-epoxybutyric acid salt as a starting material, ( Formula Removed) wherein R represents a linear or branched alkyl group having 1~4 carbon atoms 0r a benzyl group. The present invention is explained in more detail as set forth hereunder. The process enables preparing optically pure (R)-4-cyano-3-hydioxybutyrk acid esters economically due to use of aqueous condition in cyanation and introducing various ester groups by esterification without any side reaction. The procedure described in this invention for the preparation of (R)-4- cyano-3-hydroxybutyric acid ester is briefly shown in the following reaction scheme 1, Scheme 1 ( Scheme Removed) wherein M represents an alkali metal atom, an alkaline earth metal atom or an ammonium ion; R represents a linear or branched alkyl group having I to 4 carbon atoms or a benzyl group. (S)-3,4-epoxybutyric acid salt used as a starting material in the present invention can be prepared from (S)4-halo-3hydroxybutyric acid or ester thereof. For example, (S)-3,4-epoxybutyric acid salt is prepared by reacting (S)-4-halo-3hydroxybutyric acid with 1 ~ 50% of sodium hydroxide at -50 ~ 50 "C for 0.5 ~ 10 hours. Over 95% conversion of (S)-4-halo-3-hydroxybutyric acid to (S)-3,4-epoxybutyric acid salt can be detected by NMR. The formation of a carboxylate anion in the present invention is very important. Generally, a method of (R)-4-cyano-3-hydroxybutyric acid ester is prepared by nucleophilic reaction of cyanide toward (S)-4-halo-3-hydroxybutyric acid ester. A hydrogen on α-position of ester group is so acidic that il is very reactive with a base to produce acrylate by dehydration. However, the formation of a carboxylate anion decreases the acidity of hydrogen on a-position of carboxylate and thus, there is no side reaction such as dehydration which is shown in conventional method. Cyanation in the present invention is conducted with 1.0 ~ 5.0 equivalents of a cyanation reagent at the temperature of 0 - 100°C for 0.5 ~ 5 hours. The cyanation reagent is selected from the group consisting of potassium cyanide, sodium cyanide and alky I ammonium cyanide having 1 ~ 4 carbon atoms. Solvent used in cyanation can be water or a mixture of water and organic solvent. The organic solvent is selected from the group consisting of acetonitrile, dichloro methane, chloroform, linear or branched alcohol having I - 4 carbon atoms, telrahydrofuran, benzene and toluene. Esterification in the present invention is conducted by reacting cyanation reaction mixture with an acid such as sulfuric acid, hydrochloric acid, phosphoric acid or nitric acid, followed by concentration of solvent prior to reaction with ROH at 0 ~ IOO°C wherein R represents linear or branched alkyl group having I ~ 4 carbon atoms or benzyl group. (R)-4-cyano-3-hydroxybutyric acid ester with over 90% of yield and 99.8% of optical purity is obtained by performing the process described in the present invention. The following examples are intended to be illustrative of the present invention and should not be construed as limiting the scope of this invention denned by the appended claims. Example 1 : Preparation of (S)-3-acetoxy-4-bromoburyric acid To a 2 / of three-necked flask equipped with reflux condenser, thermometer and mechanical stirrer were charged (S)-3-hydroxy--butyrolactone (102 g) and 30 % of hydrobrornic acid in acetic acid (675 g, 2.5 eq.). The mixture was stirred al 40T for 3 hours. After cooling the reaction mixture, methylene chloride (1000 ml.) was added to it. The reaction mixture was washed with aqueous solution of sodium acetate. The organic layer was separated, dried over magnesium sulrate, and concentrated to afford (S)-3-acetoxy-4-bromobutyric acid (213 g, 95%). 'H-NMR(D2O ppm) : δ 2.1(S, 3H, CHCOO), 2.8 - 2.9(dd, 2H, CH2COOH), 3.5 ~ 3.6(dd, 2H, BrCH2CH), 5.3 ~ 5.4(m, 1H, BrCH2CH) Example 2 : Preparation of (S)-3,4-epoxyhuryric acid sodium salt To a 5 / flask was charged (S)-3-acetoxy-4-bromobutyric acid obtained from example I. IN aqueous solution of sodium hydroxide (3 /, 3 mol) was slowly added by keeping the temperature below 0 C. The reaction mixture was stirred at 0°C for 1 hour. Fairly pure (S)-3,4-epoxybutyric acid sodium salt was detected by NMR. 'H-NMR(D2O ppm): δ 2.3 ~ 2.5(m, 2H, CH2-CO2Na), 2.6 ~ 2.9(m, 2H), 3.2 ~ 3.3 (m, 1H) 13C-NMR(D2O, ppm): δ 40.87(-CH2-CO2Na), 48.24 (4-CH2), 51.08 (3-CH), 179.41 (-CO2Na) Example 3 : Preparation of (S)-3,4-epoxybutyric acid potassium salt To a 5 / flask was charged (S)-3-acetoxy-4-bromobutyric acid obtained from example 1. IN aqueous solution of potassium hydroxide (3 /, 3 mol) was slowly added while keeping the temperature below 0°C. The reaction mixture was stirred for 1 hour at 0°C. Fairly pure (S)-3,4-epoxybutyric acid potassium salt was detected by NMR. 'H-NMR(D2O ppm) : δ 2.3 - 2.5(m, 2H, CH2-CO2K), 2.6 ~ 2.9(m, 2H), 3.2 ~ 3.3 (m, 1H) 13C-NMR(D2O): δ 40.87(-CH2-CO2K), 48.24 (4-CH2), 51.08 (3-CH), 179.41 (-CO2K) Example 4 : Preparation of (S)-3,4-epoxybutyric acid calcium sail To a 2 / of three-necked flask equipped with thermometer, pH meter and mechanical stirrcr were charged distilled water (I /), (S)-3-acetoxy-4-bromobu lyric acid (90 g, 0.4 mol) and calcium hydroxide (45 g, 0.6 mol). The reaction mixture was stirred at 0 - 5°C for 2 hours to afford (S)-3,4-epoxybutyric acid calcium salt . Over 99% of conversion was detected by NMR. 'H-NMR(D2O, ppm) : δ 2.3 - 2.5(m, 2H, CH2-CO2Ca), 2.5 ~ 2.S(m, 2H, 4-H), 3.2 -3.3 (m, IH,3-H) Example 5 : Preparation of (S)-3,4-epoxybutyric acid telrabutylammonium salt To a 2 / of three-necked flask equipped with thermometer, pH meter and mechanical stirrer were charged distilled water (1 /), (S)-3-acetoxy-4-bromobutyric acid (90 g, 0.4 mol) and tetrabutylammonium hydroxide, 1.0 M in methanol (1.2 /, 1.2 mol). The reaction mixture was stirred at 0 ~ 5°C for 2 hours to afford (S)-3,4-epoxybutyric acid tetrabutyl ammonium salt. Over 99% of conversion was delected by NMR. 1M-NMR(D2O, ppm): δ 2.2 ~ 2.3(m, 2H, CH2-CO2NBu4), 2.5 - 2.8(m, 2H, 4-H), 3.2 -3.3(m,lH,3-H) Example 6 : Preparation of (S)-3,4-epoxy butyric acid triclhylamine salt To a 2 / of three-necked flask equipped with thermometer, pH meter and mechanical stirrer were charged distilled water (I /), (S)-3-acetoxy-4-bromobulyric acid (90 g, 0.4 mol) and triethylamine (121 g, 1.2 mol). The reaction mixture was stirred at 0 ~ 5°C for 2 hours to afford (S)-3,4-epoxyburyric acid triethylamine salt. Over 99% of conversion was detected by NMR. 'H-NMR(D2O, ppm):δ 2.2 - 2.4 (m, 2H, CH2-CO2NEb), 2.5 - 2.8(m, 2H, 4-H), 3.1 -3.2 (m, 1H,3-H) Example 7 : Preparation of (S)-3,4-epoxyburyric acid diisopropylamine salt To a 2 / of three-necked flask equipped with thermometer, pH meter and mechanical stirrer were charged distilled water (I /), (S)-3-acetoxy-4-bromobutyric acid (90 g, 0.4 mol) and diisopropylamine (121 g, 1.2 mol). The reaction mixture was stirred at 0 ~ 5°0 for 2 hours to afford (S)-3,4-epoxybutyric acid diisopropylamine salt. Over 99% of conversion was detected by NMR. 'H-NMR(D2O, ppm) : δ 2.2 - 2.3(m, 2H, CH2-CO2NH2iPr2), 2.5 ~ 2.8(m, 2H, 4-H), 3.1 ~ 3.2 (m, 1H, 3-H) Example 8 : Preparation of (S)-3,4-epoxybutyric acid t-butylamine sail To a 2 / of three-necked flask equipped with thermometer, pH meter and mechanical stirrer were charged distilled water (1 /), (S)-3-acetoxy-4-bromobutyric acid (90 g, 0.4 mol) and t-butylamine (88 g, 1.2 mol). The reaction mixture was stirred at 0 ~ 5°C for 2 hours to afford (S)-3,4-epoxybutyric acid t-burylamine salt. Over 99% of conversion was detected by NMR. 1H-NMR(D2O, ppm) : δ 2.1 ~ 2.4 (m, 2H, CH2-CO2NH,Bu-/), 2.5 ~ 2.8(m, 2H, 4-H), 3.1 -3.2(01,1 H, 3-H) Example 9 : Preparation of (R)-4-cyano-3-hydroxybutyric acid 30% of aqueous sodium cyanide solution (163 g, 1.0 mol) was added to (S)-3,4-epoxybutyric acid sodium salt obtained from example 2 dissolved in water. The reaction mixture was stirred at 60°C for 3 hours. Over 99% of conversion was detected by NMR. 1H-NMR(D2O, ppm) : δ 2.3 (d, 2H, CH2), 2.5 - 2.7 (m, 2H, CH2), 4.1 (m, I H, CH-OH) The reaction mixture containing (R)-4-cyano-3-hydroxy butyric acid sodium salt was acidified to pH 1 with concentrated sulfuric acid. The reaction mixture was condensed, dissolved in ethanol and filtered. The filtrate was concentrated to give (R)-4-cyano-3-hydroxyburyric acid which was used for esterification without further purification. 'H-NMR(D2O, ppm): δ 2.5 ~ 2.8 (m, 4H, 2CH2), 4.3 (m, 1H, CH-OH) Example 10: Preparation of (R)-4-cyano-3-hydroxybutyric acid Tetraethylammonium cyanide (156 g, 1.0 mol) was added into (S)-3,4-epoxybutyric acid sodium salt (1.0 mol) obtained from example 2 dissolved in water. The reaction mixture was stirred at 60°C for 3 hours to prepare (R)-4-cyano-3-hydroxy butyric acid sodium salt. Over 99% of conversion was delected by NMR. 1H-NMR(D2O, ppm) : δ 2.3 (d, 2H, CH2), 2.5 ~ 2.7 (m, 2H, CH2), 4.1 (m, IH, CH-OH) The reaction mixture containing (R)-4-cyano-3-hydroxybutyric acid sodium salt was acidified to pH I with concentrated sulfuric acid. The reaction mixture was condensed, dissolved in ethanol and filtered. The filtrate was concentrated to give (R)-4-cyano-3-hydroxybutyric acid which was used for esterification without further purification. IH-NMR(D2O, ppm): δ 2.5 ~ 2.8 (m, 4H, 2CH2), 4.3 (m, IH, CH-OH) I Example 11: Preparation of (R)-4-cyano-3-hydroxybutyric acid 30% aqueous sodium cyanide (163 g, 1.0 mol) was added into (S)-3,4-epoxybutyric acid calcium salt (1.0 mol) obtained from example 4 dissolved in water. The reaction mixture was stirred at 60°C for 3 hours to prepare (R)-4-cyano-3-hydroxybutyric acid calcium salt. Over 99% of conversion was detected by NMR. Concentrated sulfuric acid was added into the reaction mixture to adjust pH I. The reaction was concentrated in vacuo. The residue was dissolved in ethanol and filtered. The filtrate was concentrated to afford (R)-4-cyano-3-hydroxybutyric acid which was used for esterification without further purification. H-NMR(D2O, ppm) : δ 2.5 ~ 2.8 (m, 4H, 2CH2), 4.3 (m, 1H, CH-OH) Example 12 : Preparation of (R)-4-cyano-3-hydroxybutyric acid 30% aqueous sodium cyanide (163 g, 1.0 mol) was added into (S)-3,4-epoxybutyric acid tetrabutylammonium salt (1.0 mol) obtained from example 5 dissolved in water. The reaction mixture was stirred at 60°C for 3 hours to prepare (R)-4-cyano-3-hydroxybutyric acid tetrabutylammonium salt. Over 99% of conversion was detected by NMR. Concentrated sulfuric acid was added into the reaction mixture to adjust pH 1. The reaction was concentrated in vacuo. The residue was dissolved in ethanol and filtered. The filtrate was concentrated to afford (R)-4-c:yano-3-hydroxy butyric acid which was used for esterification without further purification. HINMR(D2O, ppm) : δ 2.5 ~ 2.8 (m, 4H, 2CH2), 4.3 (m, 1H, CH-OH) Example 13 : Preparation of (R)-cyano-3-hydroxybutyric acid methyl ester (R)-4-cyano-3-hydroxybutyric acid (1.0 mol) in methanol (500 ml) and cone. sulfuric acid (5 g) were refluxed for 5 hours. The reaction mixture was neutralized with sodium carbonate and filtered. The filtrate was concentrated ui vacuo to afford (R)-4-cyano-3-hydroxy butyric acid methyl ester (130 g, 91 %). 1H-NMR(D2O, ppm) : δ 2.6 - 2.7 (m, 4H, 2CH2), 3.7 (s, 3H, OCH3), 4.4 (m, 1H, CH-OH) Example 14: Prepararion of (R)-cyano-3-hydroxybutyric acid ethyl ester (R)-4-cyano-3-hydroxybutyric acid (1.0 mol) in ethanol (500 ml) and conc, sulfuric acid (5 g) were refluxed for 5 hours. The reaction mixture was neutralized with sodium carbonate and filtered. The filtrate was concentrated in vacuo to afford (R)-4-cyano-3-hydroxybutyric acid ethyl ester (141 g, 90%). 'H-NMR(D2O, ppm) : δ 1.2 (t, 3H, CH3), 2.5 ~ 2.6 (m, 4H, 2CH2), 4.1 (q, 2H, OCH2), 4.3 (m, 1H, CH-OH) Example 15 : Preparation of (R)-cyano-3-hydroxybutyric acid isopropyl ester (R)-4-cyano-3-hydroxybutyric: acid (1.0 mol) in isopropanol (500 ml) and cone, sulfuric acid (5 g) were refluxed for 5 hours. The reaction mixture was neutralized with sodium carbonate and filtered. The filtrate was concentrated in vacuo to afford (R)-4-cyano-3-hydroxybutyric acid isopropyl ester (163 g, 90%). 'H-NMR(D2O, ppm): δ 1.4 (d, 6H, 2CH3), 2.6 (m, 4H, 2CH2), 4.4 (m, 1H, CH-OH), 5.1 (m,lH,CH-(CH3)2) Example 16 : Preparation of (R)-cyano-3-hydroxyburyric acid isobutyl ester (R)-4-cyano-3-hydroxybutyric acid (1.0 mol) in isobutanol (500 ml.) and cone, sulfuric acid (5 g) were refluxed for 5 hours. The reaction mixture was neutralized with sodium carbonate and filtered. The filtrate was concentrated in I vacuo to afford (R)-4-cyano-3-hydroxybu lyric acid isobutyl ester (157 g, 92%). 'H-NMR(D2O, ppm) : δ 0.8 (d, 6H, 2CH3), 1.9 (m, 1H, CH-(CH3)2), 2.6 ~ 2.7 (m, 4H, 2CH2), 3.9 (d, 2H, O-CH2), 4.3 (m, 1H, CH-OH) Example 17: Preparation of (R)-cyano-3-hydroxybutyric acid benzyl ester Benzylalcohol (216 g, 2 eq.) and cone, sulfuric acid (5 g) were added to (R)-4-cyano-3-hydroxybutyric acid (1.0 mol) dissolved in acetonitnle (500 ml') and refluxed for 5 hours. The reaction mixture was neutralized with sodium carbonate and filtered. The filtrate was concentrated in vacuo to afford (R)-4-cyano-3-hydroxybutyric acid benzyl ester (157 g, 92%) which was purified by column chromatography on a silica gel. 'H-NMR(D2O, ppm) : δ 2.6 ~ 2.7 (m, 4H, 2CH:), 4.3 (m, IH, CH-OH), 5.2 (s, 2H, CH2-Ph), 7.2 ~ 7.3 (m, 5H, Ph) The process for preparing (R)-4cyano-3-hydroxybutyric acid from (S)-3,4-epoxybutyric acid salt by performing cyanation and sequential esterification described in the present invention provides less by-products, much faster reaction time and higher yield than conventional methods. And also it provides various ester compounds by esterification. We Claim: 1. A process for the preparation of (R)-4cyano-3-hydroxybutyric acid ester expressed by the formula (1) which comprises: (A) cyanation of (S)-3,4-expoxybutyric acid salt expressed by the formula (2) to afford the compound expressed by the formula (3); and (B) esterification of the compound expressed by the formula (3), (Formula Removed) wherein M represents an alkali metal atom, an alkaline earth metal atom or an alkylammonium ion having 1-4 carbon atoms; and R represents a linear or branched alkyl group having 1-4 carbon atoms or a benzyl group. 2, A process as claimed in claim 1, wherein the reagent in cyanation is selected from the group consisting of sodium cyanide, potassium cyanide and alkylammonium cyanide having 1-4 carbon atoms. 3. A process as claimed in claim 1, wherein the solvent in cyanation is water or a mixture of water and organic solvent. 4. A process as claimed in claim 3, wherein the said organic solvent is selected from the group consisting of acetonitrile, dichloro methane, chloroform, linear or branched alcohol with 1-4 carbon atoms, tetrahydrofuran, benzene and toluene. 5. A process as claimed in claim 1, wherein the cyanation is conducted at the temperature of 0 ~ 100°C. 6. A process for the preparation of (R)-4-cyano-hydroxybutyric acid ester expressed by the formula (1) substantially as herein described with reference to the foregoing examples.

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in-pct-2001-00591-abstract.pdf

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