PACKING FOR USE IN RESOLUTION

Abstract: The packing for use in resolution proposed for the liquid chromatography for particularly the opti­ cal resolution of racemic compounds is prepared by reacting a silica gel carrier with a so-called silane treating agent to introduce a spacer part therein and chemically bonding it with a copper salt of a carboxylic or thiocarboxylic acid selected from the group consisting of optically active D- and L-2-­ acetizinecaroxylic acid, proline, hydroxyproline and allohydroxyproline. The steric bulkiness, the distance between the silica gel and the optically active group and the degree of hydrophobicity are specified so as to obtain an excellent resolving power.

Description: Technical Field: The present invention relates to a new packing for use in resolution, particularly in liquid chromatography for optical resolution of racemic compounds. The packing proposed by the present invention is prepared by reacting a silica gel carrier with a so-called silane treating agent to introduce a spacer part therein and chemically bonding it with an optically active D- or L-group. Background Art: Packings for use in optical resolution, comprising silica gel chemically bonded with optically active proline or hydroxyproline, have a carboxyl group coordination-bonded or ionically bonded with a copper ion as described in G. Gübitz et al. "J. High Resolut. Chromatogr. and Chromatogr. Comm." 2 , 145 (1979), K. Sugden et al. "J. Chromatogr." 192 , 228 (1980) and V. A. Davankov et al. "Angew. Chem. Int. Ed. Engl." 21 , 930 (1982). These packings are resolve the racemic compounds taking advantage of a difference in the free energy of the mutual action of the respective antipodes of amino acids with the coordinatable racemic compounds. In using the conventional packings for resolution, the optically active amino group is bonded with the silane treating agent which acts as the spacer and, therefore, the spacer exerts a significant influence on the resolving capacity and synthetic techniques have been greatly restricted in designing such packings. As described by V. A. Davankov et al. in "Chromatographia" 13 , 677 (1980), a process wherein L-hydroxyproline containing a straight-chain alkyl group having 7 to 16 carbon atoms is adsorbed on a reversed phase silica gel column to effect the optical resolution has problems that the groups to be introduced into the amino group are limited to hydrophobic groups and the reactivity and the selection of solvent are difficult. Similar optical resolution packings are disclosed in the specifications of Japanese Patent Laid-Open No. 96062/1983 and corresponding West German Patent Laid-Open No. 3,143,726. After intensive investigations made for the purpose of further improving the capacities of the known packings, the inventors have found that the resolving power is greatly influenced by the steric bulkiness of the silane treating agent, the distance between the silica gel and the optically active group and the degree of hydrophobicity in said resolution mechanism. The present invention has been completed on the basis of this findings. Disclosure of Invention: The present invention provides a packing for use in resolution comprising a substance of the following general formula (I) : wherein at least one of Y¹, Y² and Y³ represents a silica gel and siloxane bond with the silica gel and the balance represents a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an aralkyl group having 7 to 20 carbon atoms, a halogen atom, a hydroxy group, an alkoxy group having 1 to 20 carbon atoms or any desired combination of them, and W represents one of X¹ and X² represents a hydroxy group and the other represents an optically active group of the formula: where A is a substituted or unsubstituted alkylene group having 2 or 3 carbon atoms and B¹ is a metal carboxylate group, refers to an asymmetric carbon atom. The packing of the present invention serves to reduce the time required for effecting the chromatography remarkably as compared with that of the conventional processes without damaging the resolving power. Further, the flow rate of the solvent necessitated in the chromatography can be reduced remarkably and, therefore, the pressure applied to the packing can be reduced and deterioration of the column due to crushing of silica gel can be reduced significantly. The resolution packing of the present invention is packed in a metal or glass cylinder in an ordinary liquid chromatograph for use in an optical resolution of racemic compounds. A resolution packing (2) proposed by the present invention is obtained by chemically modifying silica gel by reacting it with a silane treating agent, i.e. [2-(7-oxabicyclo-[4.1.0]hept-3-yl)ethylene]silane and chemically bonding this product with any of copper salts of optically active D- or L-2-azetidinecarboxylic acid, proline, hydroxyproline and allohydroxyproline. The inventors have found that when [2-(7-oxabicyclo-[4.1.0]hept-3-yl)ethyl]silane is used as the silane treating agent, the steric control of the fixed optically active ligand and an increase in the hydrophobicity are simultaneously realized to improve the resolving power far more remarkably than that obtained by using the conventional resolving agent. The resolution packing (2) of the present invention has been developed on the basis of this finding. The resolution packing of the present invention comprising the substance of the above general formula (I) comprises the silica gel (A) bonded with the optically active group (C) through the silane treating agent (B). The respective components of the packing will now be illustrated. (A) Silica gel: The starting silica gel has a particle diameter of 0.1 to 1000 µm and a pore diameter of 10 to 10,000 Å, preferably a particle diamter of 1 to 100 µm and a pore diameter of 50 to 5,000 Å. Now, the description will be made on the silane treating agent used for forming the packing of the general formula I-(1) and the optically active group. B) The silane treating agent forming the spacer part may result from a compound having the following formula 2-(3,4-epoxycyclohexylethyl)trimethoxysilane, C) Optically active group: The optically active group R constituting the characteristic part of the packing I-(1) of the present invention is represented by the following general formula (III)-(1): wherein A represents an unsubstituted or substituted alkylene group having 2 or 3 carbon atoms, B represents a metal thiocarboxylate group, and refers to asymmetric carbon atom. More specifically, a D- or L-compound selected from the group consisting of optically active 2-azetidinecarboxylic acid, proline, hydroxyproline and allohydroxyproline is used. Generally, easily available L-compounds are used. The thiocarboxylic acid is converted into its metal salt by reacting its alkyl, aryl or aralkyl ester with a metal salt of hydrogen sulfide such as sodium hydrosulfide or potassium hydrosulfide in an anhydrous solvent. The conversion may be effected also by a process disclosed in I. Shahak et al. "J. Am. Chem. Soc." 95 , 3440 (1973). Now, the description will be made on the silane treating agent and the optically active group of the packing I-(2) of the present invention. (B) Silane treating agent: The silane treating agent constituting the characteristic part of the packing of the present invention is represented by the following general formula (II)-(2): wherein Y, Y′ and Y″ have the same meaning as in the above formula (II)-(1) with the proviso that at least one of them represents a halogen atom, a hydroxy group or an alkoxy group having 1 to 20 carbon atoms. (C) Optically active group: The optically active group R forming the characteristic part of the packing of the present invention is represented by the following general formula (III)-(2) wherein A has the same meaning as in the formula (III)-(1), B¹ represents a metal carboxylate group, and * refers to an asymmetric carbon atom. More specifically, a D- or L-compound selected from the group consisting of optically active 2-azetidinecarboxylic acid, proline, hydroxyproline and allohydroxyproline is used. Generally, easily available L-compounds are used. The resolving agent of the present invention may be prepared from the above-mentioned starting materials (A), (B) and (C) by treating silica gel with silane and then bonding the silica gel with the optically active group or by previously bonding the silane treating agent with the optically active group and then reacting the resulting product with the silica gel. The reaction for the conversion of the carboxyl group or its ester into the metal thiocarboxylate may be conducted either before or after the bonding with the silica gel. The metal salt may be converted into a corresponding copper salt by exchange reaction with a copper salt of a strong acid. As described above, the resolution packing of the present invention comprises the silica gel chemically bonded with the optically active group through the silane treating agent. The resolution packing is suitable for use as a liquid chromatographic packing particularly for the optical resolution of amino acids. The following synthesis examples of the resolution packings of the present invention, examples thereof and comparative examples will further illustrate the present invention, which by no means limit the invention. Synthesis Example 1 Silica gel was dried by heating to 120 to 150°C in dry nitrogen stream for 2 to 10 h. 20 g of the dry silica gel was suspended in 100 mℓ of anhydrous benzene. 6 g of glycidoxypropyltrimethoxysilane was added to the suspension and the mixture was heated under reflux in dry nitrogen stream. The reaction was carried out for 5 to 10 h while methanol thus formed was removed from the reaction system. After completion of the reaction, the mixture was cooled to room temperature and filtered through a glass filter. The resulting modified silica gel was washed with anhydrous benzene and dried at 40°C in vacuum. 4.1 g (0.02 mol) of L-proline benzyl ester was dissolved in 250 mℓ of anhydrous dimethylformamide. 20 g of the silica gel having glycidoxypropylsilyl group was added to the above obtained solution to obtain a suspension, which was then agitated at 90°C for 4 h. The suspension was cooled to room temperature. A solution of 1 .2 g (0.021 mol) of sodium hydrosulfide in methanol was added to the suspension and the mixture was agitated at room temperature for 19 h to replace the benzyl ester group with sodium thiocarboxylate. The obtained, modified silica gel was filtered, washed with methanol and poured into a solution of 6 g of copper sulfate in 50 mℓ of pure water to obtain a copper salt. The copper salt was filtered and washed with pure water to obtain a silica gel chemically bonded with the copper salt of L-2-pyrrolidinecarbothioic acid. The structural formula of this product was estimated to be as follows: wherein one or both of R and R′ represent a methyl group or both or either of them is the same silica gel as above. Example 1 Various racemic amino acids were optically resolved with the packing obtained in Synthesis Example 1. The packing obtained from wholly porous silica gel having an average particle diameter of 10 µm and an average pore diameter of 100 Å in the above Synthesis Example 1 was placed in a stainless steel high-performance liquid chromatographic column (25 cm x 0.46 cm). Racemic amino acids were optically resolved at a flow rate of 2 mℓ/min at 50°C using a 5 x 10⁻⁴M aqueous copper nitrate solution as the solvent to obtain excellent resolution effects as shown in Table 1. The pressure applied to the packing was 30 kg/cm²G. The terms k′, α and Rs are defined as follows: capacity factor (k′) = [(retention time of antipode) - (dead time)] (dead time) separtion factor (α) = volume ratio of antipode adsobed more strongly volume ratio of antipode adsorbed less strongly resolution factor (Rs) = 2 x (distance between a peak of more strongly adsorbed antipode and that of less strongly adsorbed antipode) (total band width of both peaks) Comparative Example 1 Amino acid were optically resolved under the same conditions as in Example 1 using a column filled with a silica gel having a copper corboxylate of the following formula as the modifying group. The capacity factor of phenylalanine K′ L and k′ D were 28.8 and 13.9, respectively. The capacity factor of tryptophane k′ L and k′ D were as high as 74.8 and 23.8, respectively, which were practically undesirable. When the flow rate of the solvent was increased to 5 mℓ/min so as to reduce the time, the pressure applied to the packing was elevated to 90 kg/cm²G and the silica gel was crushed in the continuous operation. wherein both or either of R and R′ represents a methyl group or both or either of them is the same silica gel as above. Synthesis Example 2 A silica gel was dried by heating to 120 to 150°C in dry nitrogen stream for 2 to 10 h. 20 g of the dry silica gel was suspended in 100 mℓ of anhydrous benzene. 8 g of 2-(3,4-epoxycyclohexylethyl)trimethoxysilane was added to the suspension and the mixture was heated under reflux in dry nitrogen stream. The reaction was carried out for 5 to 10 h while methanol thus formed was removed from the reaction system. After completion of the reaction, the mixture was cooled to room temperature and filtered through a glass filter. The resulting modified silica gel was washed with anhydrous benzene and dried at 40°C in vacuum. 6.2 g (0.03 mol) of L-proline benzyl ester dried in vacuum was added to 250 mℓ of anhydrous dimethylformamide and the mixture was stirred at 90°C to obtain a solution. 20 g of the modified silica gel obtained as above was added to this solution and the mixture was agitated at 90°C for 4 h. The mixture was then cooled to room temperature. 1.9 g (0.033 mol) of sodium hydrosulfide dissolved in anhydrous methanol was added thereto and the mixture was agitated at room temperature for 19 h to convert the benzyl ester group into sodium thiocarboxylate. The obtained, modified silica gel was filtered through a glass filter. The filter cake was washed with methanol to remove excessive sodium L-prolinethiocarboxylate and then it was added to a solution of 12 g of copper sulfate in 100 mℓ of pure water to obtain the corresponding copper salt. The structural formula of the product was estimated to be as follows: and wherein both or either of R and R′ represents a methyl group or both or either of them is the same silica gel as above. Example 2 The packing obtained from wholly porous silica gel having an average particle diameter of 10 µm and an average pore diameter of 100 Å in the above Synthesis Example 2 was placed in a stainless steel high-performance liquid chromatographic column (25 cm x 0.46 cm). Racemic amino acids were optically resolved at a flow rate of 2 mℓ/min at 50°C, using a 2.5 x 10⁻⁴M aqueous copper nitrate solution as the solvent to obtain excellent resolution effects as shown in Table 2. The pressure applied to the packing was 32 kg/cm²G. Synthesis Example 3 A silica gel was dried by heating to 120 to 150°C in dry nitrogen stream for 2 to 10 h. 20 g of the dry silica gel was suspended in 100 mℓ of anhydrous benzene. 8 g of trimethoxy[2-(7-oxabicyclo[4.1.0]-hept-3-yl)ethyl]silane was added to the suspension and the mixture was heated under reflux in dry nitrogen stream. The reaction was carried out for 5 to 10 h while methanol thus formed was removed from the reaction system. After completion of the reaction, the mixture was cooled to room temperature and filtered through a glass filter. The resulting modified silica gel was washed with anhydrous benzene and dried at 40°C in vacuum. 5.6 g of sodium salt of L-proline was dissovled in 250 mℓ of anhydrous dimethylformamide at 90°C. About 20 g of the above-mentioned silica gel containing 2-(3,4-epoxycyclohexylethyl)silyl group was added to this solution to obtain a suspension, which was agitated at 90°C for 4 h. The suspension was cooled to room temperature and the resulting modified silica gel was filtered, washed with methanol and added to a solution of 12 g of copper sulfate in 100 mℓ of pure water to obtain a copper salt. This salt was filtered again and washed with pure water to obtain silica gel chemically bonded with the copper salt of L-proline. The structural formula of the product was estimated to be as follows: and wherein both or either of R and R′ represents a methyl group or both or either of them is the same silica gel as above. Example 3 The packing obtained from wholly porous silica gel having an average particle diameter of 10 µm and an average pore diameter of 100 Å in the above Synthesis Example 3 was placed in a stainless steel high-performance liquid chromatographic column (10 cm x 0.46 cm). Racemic amino acids were optically resolved at a flow rate of 2 mℓ/min at 50°C using a 5 x 10⁻⁴M aqueous copper nitrate solution as the solvent to obtain excellent resolution effects as shown in Table 3.

Claims: 1. A packing for use in resolution, comprising a substance of the following general formula (I): wherein at least one of Y¹, Y² and Y³ represents a silica gel and siloxane bond with the silica gel and the balance represents a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an aralkyl group having 7 to 20 carbon atoms, a halogen atom, a hydroxy group, an alkoxy group having 1 to 20 carbon atoms or any desired combination of them, and W represents one of X¹ and X² represents a hydroxy group and the other represents an optically active group of the formula: Where A is a substituted or unsubstituted alkylene group having 2 or 3 carbon atoms and B¹ is a metal carboxylate group, and * refers to an asymmetric carbon atom. 2. A method for optical resolution of racemic compounds by chromatography using a chromatographic packing, characterized by using a packing as defined in claim 1.

Inventors: ICHIDA, AKITO, YUKI, YOICHI
Applicants: DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1984
Main classification: B01J20
IPC codes: C07B57, B01J20