Optically active diastereomer salts of tetrahydro-2-furoic acid

Abstract: Optically active tetrahydro-2-furoic acid is obtained by (i) reacting (±)-tetrahydro-2-furoic acid with an optically active amine resolver of the following formula (I): wherein R₁ is C₁-C₈ alkyl group and R₂ represents a C₁-C₁₂ alkyl group having at least 6 carbon atoms or an aryl group, provided that R₁ and R₂ are different from each other, thus producing an optically active diastereomer salt, and (ii) decomposing the diastereomer salt.

Description: The present invention relates to a process for preparing an optically active tetrahydro-2-furoic acid, and, more particularly, to a process for preparing an optically active tetrahydro-2-furoic acid which comprises reacting (±)-tetrahydro-2-furoic acid with a specific type of optically active amine resolver to produce an optically active diastereomer salt, followed by decomposition of the diastereomer salt. In recent years, requirements for the use of optically active compounds in the fields of pharmaceuticals, agro-chemicals, and foods is increasing. Conventionally, preparing optically active compounds involves difficulties in terms of production cost and manufacturing processes. Because of this, DL compounds have been used in many cases instead of optically active compounds. This, however, may bring about problems where one of the optical isomers is inactive or exhibits adverse side effects as in the case of thalidomide. For this reason, production of optically active pharmaceuticals is an important subject. An optically active tetrahydro-2-furoic acid which is the subject of the present invention is expected to be used as an important pharmaceutical raw material as reported by P. C. Belanger et al. [ Can . J . Chem . 61 , 1383 (1983)] and by JP-A-207387/1986. C ∨ervinka et al (Collection Czechoslovak Chem. Commun. 51 (1986) 404-407) discloses the addition of (-)-quinine to (±)-tetrahydro-2-furoic acid to form a salt which was decomposed with sulphuric acid to provide (S)-(-)-tetrahydro-2-furoic acid. FR-A-2262652 discloses the resolution of (±) cyclic dicarboxylic acids by addition of enantiomers of optically active bases thereto to form diastereomer salts which are subsequently hydrolyzed with aqueous acid. Specified bases include quinine, brucine and 1-phenylethylamine. EP-A-0010874 discloses the resolution of (±) cyclopropane carboxylic acids by addition of enantiomers of optically active amines to form diastereomer salts, which are subsequently hydrolyzed with acid. The preferred amines are α-alkylated benzylamines, especially 1-α-methylbenzylamine (ie 1-phenylethylamine). One of the conventionally known processes for resolving optically active tetrahydro-2-furoic acids is that proposed by P. C. Belanger et al. This process involves the use of brucine dihydrate, which is an expensive and highly toxic substance. In addition, its recovery rate is very low. Because of these reasons, a great deal of improvement has to be done for the process to be industrially applied. In this situation, for the purpose of developing a more practical process, the present inventors have undertaken extensive studies, including investigations on the resolving agents, solvents, reaction conditions, and methods of resolver recovery. As a result, the present inventors were successful in preparing the target optically active tetrahydro-2-furoic acid by reacting (±)-tetrahydro-2-furoic acid with an optically active amine resolver having the formula (I) hereinafter given to produce an optically active diastereomer salt and by decomposing the diastereomer salt. In this process, the resolver is recovered at a high yield and a high purity and a higher reaction concentration could be used. A mass-production of the optically active tetrahydro-2-furoic acid was also realized by the use of the process. Accordingly, the present invention provides a process for preparing an optically active tetrahydro-2-furoic acid comprising: (a) reacting in an organic solvent (±)-tetrahydro-2-furoic acid with an optically active amine resolver of the following formula (I): wherein R 1 is a C 1 - C 5 alkyl group and R 2 represents a C 6 - C 12 alkyl group or an aryl group, provided that R 1 and R 2 are different from each other, thus producing a mixture of optically active diastereomer salts; (b) cooling the reaction mixture to preferentially precipitate the less soluble of said salts; (c) separating said precipitated salt from the mother liquor; (d) recrystallizing said separated salt of step (c) from an organic solvent; (e) treating said recrystallized salt of step (d) with a water-immiscible organic solvent and an aqueous alkaline solution to pH 11-14; (f) separating the organic and aqueous layers formed in step (e); (g) treating said separated aqueous layer of step (f) by addition of mineral acid to pH 1-2; (h) extracting the desired optically active tetrahydro-2-furoic acid from step (g) with an organic solvent; (i) treating the mother liquor of steps (c) and (d) with an organic solvent and an aqueous alkaline solution to pH 11-14; and (j) recovering the amine resolver from the organic layers of steps (f) and (i). Other objects, features, and advantages of this invention will appear more fully from the following description of preferred embodiments. In the process of the present invention (±)-tetrahydro-2-furoic acid is first reacted with an optically active amine resolver of the formula (I) in a solvent to produce a diastereomer salt. The ratio of (±)-tetrahydro-2-furoic acid to the optically active amine resolver used in the reaction usually is in the range of 2:1 to 1:1.5, with the particularly preferable ratio being 1.5:1 to 1:1. In the formula (I) which represents the optically active amine resolver used in the present invention, the group R 1 is an alkyl group having 1-5, preferably 1-3, carbon atoms. The group R 2 may be an alkyl group having 6 to 12 carbon atoms, preferably 6-10, and includes a cycloalkyl group such as cyclohexyl or a branched alkyl group, or may be an aryl group such as phenyl group or naphthyl group. The aryl group may have an substituent such as an alkyl group on its ring. The optically active amines used in the present invention include, but not limited to, 1-cyclohexylethylamine, 1-phenylethylamine, 1-(1-naphthyl)ethylamine, and 1-p-tolylethylamine. Suitable reaction solvents are esters such as methyl acetate and ethyl acetate; alcohols, such as methanol and ethanol; ketones such as acetone: nitriles such as acetonitrile; halogenated hydrocarbons such as methylene chloride, chloroform, and monochlorobenzene; ethers such as tetrahydrofuran and dioxane; and aromatic hydrocarbons such as toluene and benzene. These solvents may be used independently or as a mixture of two or more. The reaction temperature can be from room temperature to the boiling point of the solvent used. A preferable method is to heat the reaction mixture to 40-70°C to make it into a homogeneous solution, and then to gradually cool it to a temperature at which the diastereomer salt precipitates. The precipitated diastereomer salt is separated by, eg, filtration, and subjected to recrystallization using, eg, the same solvent which is used when producing the diastereomer salt, thus obtaining the target high purity diastereomer salt. For decomposing the diastereomer salt, an alkaline solution such as an aqueous solution of sodium hydroxide or potassium hydroxide is added to it to make the pH to 11-14. After separating the organic layer, the water layer is washed with an organic solvent to eliminate impurities, adjusted to pH 1-2 by a mineral acid such as a dilute hydrochloric acid or sulfuric acid, and extracted with an organic solvent to obtain the target optically active tetrahydro-2-furoic acid. The product thus produced has a sufficient purity and can be used for the next reaction in most cases. If required, the product can be further purified by a suitable means such as distillation. There are no specific restrictions as to the solvent which is used in the extraction so long as the same is not miscible with water. The use of the same solvent as used for the production of diastereomer salt is desirable for a more integrated design of the process and for effective recovery of the solvent. The resolver used in the reaction is recovered from the mother liquor obtained from the diastereomer salt reaction mixture. After the addition of water and extraction of the diastereomer salt, an organic solvent and alkaline solution is added to make the pH to 11-14. Water is separated off and the organic layer obtained from the diastereomer salt decomposition reaction is added to the organic layer, followed by washing with water, drying, and evaporating the solvent. The residue thus obtained is purified by distillation to recover the resolver at a high purity and a high yield. EXAMPLES In the Examples, the optical purity (enantiometric excess: e.e.) of diastereomer salts are determined by the following method. Ether was added to the diastereomer salt crystals prepared in Examples. Several drops of concentrated hydrochloric acid were then added to the mixture while vigorously stirring, followed by an addition of an excess amount of diazomethane and stirring for about 10 minutes. The mixture was dried over anhydrous magnesium sulfate. Insoluble components were separated by filtration and the filtrate was subjected to HPLC analysis using a SUMIPAX OA 2000 column (4.6 mm I.D. x 250 mm, 5 µm). A deaerated mixed solvent of hexane-ethylene dichloride (750:150) was used as a mobile phase at a flow rate of 1.2 ml/min. Detection was made by UV light (220 nm). When the standard sample was analyzed under these conditions R-form eluted at 7.64 minute and S-form at 8.41 minute. Example 1 A mixture of (S)-(-)-1-phenylethylamine (12.12 g), (±)-Tetrahydro-2-furoic acid (11.61 g), and tetrahydrofuran (100 ml) was heated under refluxing. After complete dissolution, the mixture was gradually cooled and allowed to stand still at room temperature for two hours. Precipitated crystals were collected by filtration to obtain primary crystals (10.74 g). The primary crystals were twice recrystallized from tetrahydrofuran (134 ml for each recrystallization) to produce 5.97 g diastereomer salt as colorless needle-like crystals. The optical purity of this compound was found to be 98.0% e.e. by HPLC analysis. (R)-(+)-Tetrahydro-2-furoic acid was obtained by decomposing the salt. Examples 2-11 Reactions were carried out in the same manner as in Example 1, except for using the conditions, the solvents, and the solvent amount listed in Table 1. The products had optical purities shown in Table 1. Example Solvent Yield (%) e.e. (%) Kind Amount (ml) 2 Methylene chloride 100 44 47 3 Tetrahydrofuran 80 46 58 4 Acetone 80 41 52 5 Toluene 300 52 61 6 Monochlorobenzene 100 53 54 7 Ethyl acetate 120 50 65 Chloroform 80 8 Ethyl acetate 120 43 62 Methylene chloride 60 9 Toluene 120 46 64 Methylene chloride 30 10 Ethanol 20 34 67 Chloroform 70 11 iso-Propanol 20 43 68 Chloroform 70 (R)-(+)-Tetrahydro-2-furoic acid was obtained by decomposing the salts prepared in the above examples. Examples 12-14 (S)-(-)-1-(1-Naphthyl)ethylamine (59 mg) and (±)-tetrahydro-2-furoic acid (40 mg) were treated with solvents shown in Table 2 in the same manner as in Example 1. Yields and optical purities of the primary crystals are shown in Table 2. Example Solvent Yield (%) e.e. (%) 12 Tetrahydrofuran 44 55 13 Dioxane 55 58 14 iso-Propanol 51 36 (R)-(+)-Tetrahydro-2-furoic acid was obtained by decomposing the salts prepared in the above examples. Examples 15-16 (R)-(+)-1-(1-Naphthyl)ethylamine (236 mg) and (±)-tetrahydro-2-furoic acid (160 mg) were treated with solvents shown in Table 3 in the same manner as in Example 1. Yields and optical purities of the secondary crystals are shown in Table 3. Example Solvent Yield (%) e.e. (%) 15 Tetrahydrofuran 52 83 16 Acetone 46 85 (S)-(-)-Tetrahydro-2-furoic acid was obtained by decomposing the salts prepared in the above examples. Example 17 (S)-(-)-1-(p-Tolyl)ethylamine (47 mg) and (±)-tetrahydro-2-furoic acid (40 mg) were treated with the solvent shown in Table 4 in the same manner as in Example 1. The yield and optical purity of the primary crystals are shown in Table 4. Example Solvent Yield (%) e.e. (%) 17 Acetone 33 68 (R)-(+)-Tetrahydro-2-furoic acid was obtained by decomposing the salt. Example 18 Methylene chloride (550 ml) and ethyl acetate (1,100 ml) were added to (±)-tetrahydro-2-furoic acid (116.1 g). (S)-(-)-1-phenylethylamine (121.2 g) was added dropwise, and the mixture was heated under refluxing for 15 minutes. After gradually cooling the resulting reaction mixture to 20°C over 2 hours, precipitated crystals were separated by filtration. The filtrate was used for recovering the amine. The crystals were dried to obtain 111.5 g of the primary crystals. The optical purity of the crystals was measured by HPLC analysis (yield: 47%, optical purity: 59% e.e.). Methylene chloride (550 ml) and ethyl acetate (1,100 ml) were added to the 111.5 g of primary crystals and the mixture was heated under refluxing for 15 minutes. After gradually cooling the resulting reaction mixture to 20°C over 2 hours, precipitated crystals were separated by filtration. The filtrate was used for recovering the amine. The crystals were dried to obtain 80.7 g of the secondary crystals. The optical purity of the crystals was measured by HPLC analysis (yield: 34%, optical purity: 91% e.e.). The same procedure as above was repeated on 80.7 g of the secondary crystals to produce 75.9 g of ternary crystals (yield: 32%, optical purity: 99% e.e.). Water (20 g) and methylene chloride (160 ml) were added to the 75.9 g of ternary crystals. To the mixture were added 45. 9 g of 28% aqueous solution of sodium hydroxide to make the pH to 13, thus decomposing the diastereomer salt. The water layer was separated, and washed with methylene chloride. Methylene chloride layers were collected for the recovery of the amine. After an addition of methylene chloride (160 ml), the washed water layer was adjusted to pH 1 with concentrated hydrochloric acid (40.9 g) and separated. The water layer was further extracted four times with methylene chloride (160 ml for each extraction). Methylene chloride layers were combined, concentrated, and distilled under reduced pressure (108°C, 10 mmHg; 1.35 kPa) to produce 34.8 g of optically active (R)-(+)-tetrahydro-2-furoic acid (yield: 30%, optical purity: 99% e.e.). The primary, secondary, and ternary filtrates were mixed and extracted twice with 100 ml of water. The water layer was combined and to this was added 200 ml of methylene chloride. The mixture was adjusted to pH 13 with 28% aqueous solution of sodium hydroxide (99.8 g) and the water layer and methylene chloride layer was separated. The water layer was again extracted with 200 ml of methylene chloride. The methylene chloride layers were combined, and to this was added the methylene chloride layer obtained when decomposing the diastereomer salt. The mixture was concentrated, and distilled under reduced pressure (65°C, 10 mmHg; 1.35 kPa) to produce 112.7 g of (S)-(-)-phenylethylamine (recovery rate: 93%). The specific rotation of the recovered product was measured and found to be [α] D = -38.4°C (neat), which was almost the same as the value [α] D = -38.5°C (neat) for the fresh (S)-(-)-phenylethylamine used in the experiment. The recovered (S)-(-)-phenylethylamine was repeatedly used for the experiment and found to give the same results as the fresh one. Example 19 Monochlorobenzene (1,200 l) was added to (±)-tetrahydro-2-furoic acid (116.1 kg). To this mixture (S)-(-)-1-phenylethylamine (121.2 kg) was added dropwise over 2 hours. The mixture was heated to dissolution and reacted at 65°C for 30 minutes. After gradually cooling to the resulting reaction mixture 20°C over 6 hours, precipitated crystals were separated by centrifugation to obtain 144.4 kg of wet primary crystals. A portion of the crystals was dried to determine the yield and to measure optical purity by HPLC analysis (yield: 41%, optical purity: 74% e.e.). Monochlorobenzene (1,200 l) was added to the 144.4 kg of the wet primary crystals and heated to dissolution. After reaction at 74°C for 30 minutes, the solution was gradually cooled to 20°C over 6 hours. The precipitated crystals were separated by centrifugation to obtain 116.4 kg of wet secondary crystals. A portion of the crystals was dried to determine the yield, and to measure optical purity by HPLC analysis (yield: 33%, optical purity: 92% e.e.). The same procedure as above was repeated on the 116.4 kg of wet secondary crystals to produce 83.4 kg of wet ternary crystals. 73.5 kg of dry ternary crystals were obtained by drying the wet crystals (yield: 31%, optical purity: 98 % e.e.). Decomposition of diastereomer salt was carried out using the 73.5 g of dry ternary crystals in the same manner as Example 18 to produce 33.7 kg of optically active (R)-(+)-tetrahydro-2-furoic acid (yield: 29%, optical purity: 98% e.e.). The amine was recovered in the same manner as in Example 18 to obtain 116.4 kg of (S)-(-)-phenylethylamine (recovery rate: 96%) having a specific rotation of [α] D = -38.4°C (neat). As described above, according to the process of the present invention a high purity, optically active tetrahydro-2-furoic acid can be prepared at a high yield using an amine resolver which is less toxic and more readily available than brucine used in conventional processes. In addition the amine resolver can, be recovered at a high yield and high purity.

Claims: A process for preparing an optically active tetrahydro-2-furoic acid which comprises: (a) reacting in an organic solvent (±)-tetrahydro-2-furoic acid with an optically active amine resolver of the following formula (I): wherein R 1 is a C 1 -C 5 alkyl group and R 2 represents a C 6 -C 12 alkyl group or an aryl group, provided that R 1 and R 2 are different from each other, thus producing a mixture of optically active diastereomer salts; (b) cooling the reaction mixture to preferentially precipitate the less soluble of said salts; (c) separating said precipitated salt from the mother liquor; (d) recrystallizing said separated salt of step (c) from an organic solvent; (e) treating said recrystallized salt of step (d) with a water-immiscible organic solvent and an aqueous alkaline solution to pH 11-14; (f) separating the organic and aqueous layers formed in step (e); (g) treating said separated aqueous layer of step (f) by addition of mineral acid to pH 1-2; (h) extracting the desired optically active tetrahydro-2-furoic acid from step (g) with an organic solvent; (i) treating the mother liquor of steps (c) and (d) with an organic solvent and an aqueous alkaline solution to pH 11-14; and (j) recovering the amine resolver from the organic layers of steps (f) and (i). A process as claimed in Claim 1, wherein the same organic solvent is used in steps (a), (d) & (h). A process as claimed in Claim 1 or Claim 2, wherein R 1 is methyl and R 2 is selected from cyclohexyl, phenyl, 1-naphthyl and tolyl. A process as claimed in Claim 3, wherein the amine resolver is (S)-(-)-1-phenylethylamine. A process as claimed in any one of the preceding claims, wherein the organic solvent of step (a) is selected from methyl acetate, ethyl acetate, methanol, ethanol, acetone, acetonitrile, methylene chloride, chloroform, monochlorobenzene, tetrahydrofuran, dioxane, toluene, benzene and mixtures of two or more thereof. A process as claimed in any one of the preceding claims, wherein in step (a) the reaction mixture is heated at 40-70°C to form an homogenous solution. A process as claimed in any one of the preceding claims, wherein the ratio of (±)-tetrahydro-2-furoic acid to amine resolver in step (a) is 2:1 to 1:1.5. A process as claimed in Claim 7, wherein said ratio is 1.5:1 to 1:1.

Inventors: ISHIGURO MASAJI, IWATA HIROMITSU, MIYAHARA OSAMU, NAKATSUKA TAKASHI, NAKAYAMA KIYOHARU, NOGUCHI KENICHI, ISHIGURO, MASAJI, IWATA, HIROMITSU, MIYAHARA, OSAMU, NAKATSUKA, TAKASHI, NAKAYAMA, KIYOHARU, NOGUCHI, KENICHI
Applicants: NIPPON SODA CO, SUNTORY LTD, NIPPON SODA CO., LTD., SUNTORY LIMITED
Application year: 1990
Main classification: C07B57
IPC codes: C07D307, C07B57