Chemical modification method of a diastereomer and separation method thereof

Abstract: The present invention discloses a separating and purifying (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone comprising the steps of: reacting a diastereomer mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone represented by the formula (I) and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone represented by the formula (II) with a reagent which reacts with the hydroxyl groups at least equimolar to the compound represented by the formula (II) to thereby convert the compound represented by the formula (II) into a (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone derivative represented by the formula (III) (where R represents a group modifying the hydroxyl group) by predominantly modifying the hydroxyl group in the formula (II), and separating the compound of the formula (I) from the compound of the formula (III) through the difference of the solubility in water or an organic solvent. According to the invention, (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone is easily separated with a high yield.

Description: The present invention relates to a separation and purification of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone, which is useful as a material for synthesis of optically active compounds such as pigments, from a mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and its diastereomer (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone. As a method of separating (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone from its diastereomer mixture with (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone, there has been known, among others, a method of separation by silica gel column chromatography, a method of separation by counter current distribution and a method of separation by crystallization (Japanese Patent Publication No.7277/'83). These separation methods, however, had a number of shortcomings such as requiring a large amount of silica gel and eluent and, in case of the counter current distribution method, a large amount of a highly dangerous solvent such as ether. In case of the crystallization method, a solvent such as ether and an extremely low temperature of approximately -70°C are required. The product obtainable by the crystallization method is poor and several times of recrystallization are essential, which results in a poor recovery percentage, and thus the method is hardly economical. Accordingly, there has been a strong need for a separation method industrially advantageous. It is an object of the present invention to provide a chemical modification method for modifying (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone preferentially to (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone from a diastereomer mixture of both. It is another object of the present invention to provide a method for separating and purifying (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone easily with a high yield. These objects have been achieved by an economical separation and purifying method wherein (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone can be recovered easily with a high yield by reacting a diastereomer mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone with a reagent capable of reacting with hydroxyl groups in an amount not less than equimolar to (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone to thereby modify predominantly the hydroxyl groups of (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone to convert it into a derivative having a different solubility compared to (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone. The present invention is, in one aspect, to provide a chemical modification method which comprises reacting a diastereomer mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone represented by the formula ( I ) and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone represented by the formula ( II ) with a reagent in a quantity at least equimolar to the compound represented by the formula ( II ) to thereby convert the compound represented by the formula ( II ) into a (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone derivative represented by the formula ( III ) (where R represents a group modifying the hydroxyl group), by predominantly modifying the hydroxyl group in the formula ( II ) rather than the hydroxyl group in the formula ( I ). The present invention is, in another aspect, to provide a method for separating and purifying (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone comprising the steps of: preparing a reaction mixture according to the method as described above and separating the compound of the formula ( I ) from the compound of the formula ( III ) through the difference of the solubility in water or an organic solvent. As a reagent for the reaction with hydroxyl groups under basic conditions may be used, for example, alkoxyalkyl chlorides such as methoxymethyl chloride, t-butoxymethyl chloride, 2-methoxyethoxymethyl chloride, 1-ethoxyethyl chloride; alkyl or allylsilyl chlorides such as trimethylsilyl chloride, triethylsilyl chloride, t-butyldimethylsilyl chloride, isopropyldimethylsilyl chloride, t-butyldiphenysilyl chloride; alkyl or allylsulfonyl chlorides such as methanesulfonyl chloride, ethanesulfonyl chloride, p-methylphenylsulfonyl chloride, phenylsulfonyl chloride, benzylsulfonyl chloride; alkanoyl chlorides such as acetyl chloride, propanoyl chloride, butanoyl chloride, pentanoyl chloride, hexanoyl chloride, 2-methylpropanoyl chloride, pivaloyl chloride, 3-methylbutanoyl chloride, 3,3-dimethylbutanoyl chloride; unsubstituted or substituted benzoyl chlorides such as benzoyl chloride, 2-methylbenzoyl chloride, 2-chlorobenzoyl chloride, 2,4-dichlorobenzoyl chloride, 2,6-dichlorobenzoyl chloride, 2-acetoxybenzoyl chloride, 3,4,5-trimethoxybenzoyl chloride; substituted acetyl chlorides such as phenylacetyl chloride, diphenylacetyl chloride; substituted carbonyl chlorides such as cyclohexanecarbonyl chloride, 9-fluorenecarbonyl chloride; acid anhydrides such as acetic anhydride, propionic anhydride, lactic anhydride, valeric anhydride, hexanoic anhydride, benzoic anhydride, succinic anhydride, maleic anhydride, phthalic anhydride; alkyl carbonates such as dimethyl carbonate, diethyl carbonate; alkyl chlorocarbonates such as methyl chlorocarbonate, ethyl chlorocarbonate, and these may be used either alone or in combination of two or more. For the reaction under acidic conditions it can be used, among others, 2,3-dihydrofuran, 3,4-dihydro-2H-pyrane, 2-methoxypropene and ethylvinyl ether, and these may be used either alone or in combination of two or more. The quantity of any such reagents reacting with hydroxyl group should be at least one equivalent per the quantity of (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone contained in the mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone. When its quantity used is more than one equivalent, it is advisable to check the progress of the reaction by means of gas-chromatography, since, if the reaction time is prolonged, the compound ( I ) reacts to decrease the recovery percentage of the unconverted compound ( I ) with increasing length of the reaction time. The reaction may be conducted in various solvents such as methylene chloride, hexane, acetonitrile, ethyl acetate, benzene, toluene, xylene, acetone, tetrahydrofuran, isopropylether, ethylether and dimethylformamide, and these may be used either alone or in combination of two or more. When the reaction is conducted under basic conditions, there may be used as base, for example, nitrogen-containing heterocyclic compounds such as pyridine, 4-dimethylaminopyridine, imidazole, picoline, lutidine, collidine; aromatic amines such as N, N-dimethylaniline; tertiary amines such as triethylamine, trimethylamine; tetramethyl urea and sodium hydrogencarbonate, and these may be used either alone or in combination of two or more. The reaction may be conducted by dissolving the mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone in a solvent and adding the reagent which reacts with hydroxyl groups dropwise or otherwise or dripping its solution in a solvent after addition of a base. When the reaction is conducted under acidic conditions, use of a catalyst is essential. Examples of the catalyst are pyridinium p-toluenesulfonate, p-toluenesulfonic acid, hydrochloric acid, sulfuric acid, phosphoric acid and their salts, and these may be used either singly or in combination of two more. The quantity of the catalyst may be in a range of 0,001∼0.5 equivalent. The reaction may be conducted by dissolving the mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S , 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone in a solvent and adding a proper catalyst after addition of the reagent which reacts with hydroxyl groups. The reaction temperature is variable according to the kind of the solvent or the catalyst used, but may be in a range of from -70 °C to a reflux temperature of the solvent. Generally, the lower the reaction temperature, the larger the difference in reaction velocity between (4R, 6R) -4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone. The reaction time, too, is variable depending on such factors as the kind of the reagent reactive with hydroxyl group, solvent, base, catalyst used and the quantities and the temperatures thereof, but may generally be in a range of 1∼100 hours. If the reaction rate is too low, the reaction may be accelerated by addition of an excessive quantity of the reagent which reacts with hydroxyl groups. It is sometimes the case that derivatives ( IV ) of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone with its hydroxyl group modified occur, but this has no particular influence on the present invention, except for a slight drop of recovery percentage of the compound ( I ) since such derivatives can be removed by the method described later. The reaction can be stopped by, for example, adding a saturated aqueous solution of sodium bicarbonate when its progress is confirmed by means of, for example, gas chromatography. If, in conducting the reaction described above, a large amount of a strong base or acid should be used, the methyl group in 6-position of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone is racemized to form (4R, 6S) or (4S, 6S) isomers. This results in a decrease of the recovery percentage of (4R, 6R) isomer and, worse, an enantiomer is produced as the reaction product. Then, the separation and purifying method will be described. After stopping the reaction by addition of an aqueous solution of sodium bicarbonate to the liquid reactant, the organic phase is first separated and then extraction is made from the aqueous phase by the use of an organic solvent. Then, the organic phase is washed with an acidic aqueous solution or an aqueous solution of copper sulfate to remove the base contained in the organic phase. Thereafter, (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone once dissolved in the aqueous phase is re-extracted with the organic solvent. By mixing all organic phases and concentrating them under reduced pressure, a crude reaction product containing the compounds ( I ), ( III ) and ( IV ) can be obtained. When the removal of the base is made by the concentration under reduced pressure, a crude reaction product containing the compounds ( I ), ( III ) and ( IV ) can be obtained. If the product obtained contains the compound ( I ) only, not containing ( III ) and ( IV ), the subsequent procedure is not necessary. The crude product is first dissolved in organic solvent-water and after separation of the water phase, reextraction is done from the organic phase using water. The organic solvent is required to get an efficient extraction of the compounds ( III ) and ( IV ) but less suited for extraction of the compound ( I ). As such solvents, pentane, hexane, heptane, cyclohexane, benzene, toluene, xylene, petroleum ether and mixtures thereof may be used. The aqueous phases obtained are then mixed and the compound ( I ) is then extracted with ethyl acetate for subsequent dissolving in the organic phase. The organic phase may then be dried by the use of anhydrous sodium sulfate and (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone is obtainable after subsequent filtration and concentration. When maleic anhydride, or phthalic anhydride is used as a reagent for reaction with hydroxyl group, water is first added to the liquid reactants to stop the reaction, then the organic phase is separated and thereafter the extraction is made using an organic solvent. All organic phases are mixed and then concentrated under reduced pressure to obtain a product containing the compound ( I ). Example 1 10 g (64 mmol) of a mixture (80:20) of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone were dissolved in 200 ml of methylene chloride, then 4.0 g (51 mmol) of pyridine were added under stirring at -30 °C and then 5.9 g (42 mmol) of benzoyl chloride were added dropwise. Next, 200 ml of a saturated aqueous solution of sodium bicarbonate were added after stirring for 20 hours at -30°C and after separation of the organic phase, the extraction was conducted from the aqueous phase 3 times using 200 ml of ethyl acetate. The organic phases thus separated were then mixed, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure for removal of pyridine. The resulting oily substance was dissolved in 1 liter of water and 1 liter of hexane and after separation of the aqueous phase, the remainder was extracted from the hexane phase using another 1 liter of water. From the resulting 2 liters of the aqueous phase, extraction was done 3 times using 2 liters of ethyl acetate and 4.8 g (recovery: 48%) of 4-hydroxy-2,2,6-trimethyl cyclohexanone was obtained after drying over anhydrous sodium sulfate, filtration and concentration under reduced pressure. The result of analysis made by gas-chromatography showed that the diastereomer purity was 100%. Conditions of gas-chromatography analysis: OV-210, 10%/Uniport HP, mesh 80/100, 2m x 3mm, 100 °C , N 2 , 0.6 kg/cm 2 Recovery (%): [Recovered (4R, 6R) + (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone]/[(4R, 6R) + (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone used as a material] x 100 Diastereomer purity (%): [(4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone/[(4R, 6R) + (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone] x 100 Example 2 1.0 g (6.4 mmol) of a mixture (80:20) of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone was dissolved in 20 ml of methylene chloride, then 0.27 g (3.2 mmol) of pyridine was added under stirring at -30°C and thereafter 0.38 g (2.7 mmol) of benzoyl chloride was added dropwise. After stirring for 20 hours at -30 °C , 20 ml of a saturated aqueous solution of sodium bicarbonate were added. After separation of the organic phase, extraction was done 3 times from the aqueous phase using 20 ml of ethyl acetate. The organic phases were mixed, dried over anhydrous sodium sulfate, filtered and then concentrated under reduced pressure for the removal of pyridine. The oily substance obtained was dissolved in 100 ml of water and 100 ml of hexane and after separation of the aqueous phase, extraction was done once from the hexane phase using 100 ml of water. From the resulting 200 ml of the aqueous phase, extraction was done 3 times and after drying over anhydrous sodium sulfate, filtration and concentration under reduced pressure, 0.63 g (recovery: 63%) of 4-hydroxy-2,2,6-trimethyl cyclohexanone was obtained. The result of gas-chromatography analysis showed that the diastereomer purity was 98%. Example 3 An experiment was made in the same way as in Example 2, except that the quantities of pyridine and benzoyl chloride were changed to 0.21 g (2.7 mmol) and 0.31 g (2.2 mmol), respectively. The quantity of 4-hydroxy-2,2,6-trimethyl cyclohexanone obtained was 0.71 g (recovery: 0.71 %). The result of gas-chlomatography analysis showed that the diastereomer purity was 96%. Example 4 Experiments were made in the same way as in Example 2, except that 2-methyl propanoyl chloride, pivaoyl chloride, 3-methylbutanoyl chloride, 2-methylbenzoyl chloride, 2-chlorobenzoyl chloride and 2-acetoxybenzoyl chloride were used, respectively, as acid chlorides instead of benzoyl chloride. The results were as shown in Table 1. Example 5 Experiments were made in the same way as in Example 2, except that hexane, acetonitrile, ethyl acetate and toluene were used, respectively, as solvents instead of methylene chloride. The results were as shown in Table 2. Example 6 Experiments were made in the same way as in Example 2, except that triethylamine was used as a base instead of pyridine and the reaction temperature was changed from -30°C to 15°C and 0°C, respectively. The results were as shown in Table 3. Example 7 Experiments were made in the same way as in Example 2, except that the reaction temperature was changed to 15°C, 0 °C, -15 °C and -60°C, respectively. The results were as shown in Table 4. In Tables 1∼4, HTH represents 4-hydroxy-2,2,6-trimethyl cyclohexanone. Methylene chloride Hexane Acetonitrile Ethyl acetate Toluene Temperature ( °C ) -30 15 -30 -30 -30 Acid chloride (g) 0.38 0.61 0.67 1.15 0.83 (mmol) 2.7 4.3 4.8 8.2 5.9 Pyridine (g) 0.26 0.41 0.45 0.78 0.56 (mmol) 3.2 5.2 5.8 9.8 7.1 Reaction time (hr) 20 44 20 20 21 Recovered HTH (g) 0.63 0.52 0.58 0.50 0.57 Recovery (%) 63 52 58 50 57 Diastereomer purity (%) 98 98 98 98 98 Pyridine Triethylamine Triethylamine Temperature ( °C ) -30 15 0 Benzoyl chloride (g) 0.38 0.59 0.85 (mmol) 2.7 4.2 6.0 Base (g) 0.26 0.51 0.73 (mmol) 3.2 5.0 7.2 Reaction time (hr) 20 20 26 Recovered HTH (g) 0.63 0.63 0.66 Recovery (%) 63 63 66 Diastereomer purity (%) 98 98 98 15°C 0°C -15°C -30°C -60°C Benzoyl chloride (g) 0.72 0.58 0.54 0.38 0.43 (mmol) 5.1 4.1 3.8 2.7 3.1 Pyridine (g) 0.48 0.39 0.36 0.26 0.29 (mmol) 6.1 4.9 4.6 3.2 3.7 Reaction time (hr) 23 24 20 20 26 Recovered HTH (g) 0.41 0.50 0.58 0.63 0.67 Recovery (%) 41 50 58 63 67 Diastereomer purity (%) 98 98 98 98 98 Example 8 10 g (64 mmol) of a mixture (80:20) of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone were dissolved in 100 ml of toluene and this solution, after addition of 2.5 g (26 mmol) of maleic anhydride, was kept at the room temperature under stirring for the dissolution thereof. Then, under stirring at -20 °C , 2.9 g (28 mmol) of triethylamine were added dropwise. After further stirring for 24 hours at -20 °C , 100 ml of water were added to the liquid reactant for the separation of the toluene phase. Further, extraction was made from the aqueous phase by the use of toluene (100 ml x 5). The resulting organic phases were mixed, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure, and 6.5 g (recovery: 65%) of 4-hydroxy-2,2,6-trimethyl cyclohexanone were thus obtained. The result of the gas-chromatography analysis showed that the diastereomer purity was 98%. Example 9 An experiment was made in the same way as in Example 8, with the reaction temperature changed to 0 °C . The quantity o f 4-hydroxy-2,2,6-trimethyl cyclohexanone obtained was 7.0 g (recovery: 70%). The diastereomer purity was 96%. Example 10 10 g (64 mmol) of a mixture (80:20) of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone were dissolved in 100 ml of toluene, then 5.69 g (38.4 mmol) of phthalic anhydride were added and stirring was continued at room temperature to dissolve it. Then, 4.66 g (46 mmol) of triethylamine were added dropwise at room temperature. After stirring for 24 hours at room temperature, 100 ml of water were added to the liquid reactant and the toluene phase was separated. Further, extraction was done from the aqueous phase by the use of toluene (100 ml x 5). The resulting organic phases were mixed, dried over anhydrous sodium sulfate, filtered and concentrated under pressure and 5.5 g (recovery: 55%) of 4-hydroxy-2,2,6-trimethyl cyclohexanone were thus obtained. The result of the gas-chromatography analysis showed that diastereomer purity was 97%. As seen from the above, the present invention enables separation of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone with a high yield and with ease.

Claims: A chemical modification method which comprises reacting a diastereomer mixture of (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone represented by the formula ( I ) and (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone represented by the formula ( II ) with a reagent in a quantity at least equimolar to the compound represented by the formula ( II ) to thereby convert the compound represented by the formula ( II ) into a (4S, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone derivative represented by the formula ( III ) (where R represents a group modifying the hydroxyl group), by predominantly modifying the hydroxyl group in the formula ( II ) rather than the hydroxyl group in the formula ( I ). A method for separating and purifying (4R, 6R)-4-hydroxy-2,2,6-trimethyl cyclohexanone comprising the steps of: preparing a reaction mixture according to the method of claim 1 and separating the compound of the formula ( I ) from the compound of the formula ( III ) through the difference of the solubility in water or an organic solvent.

Inventors: HASEGAWA JUNZO, IWASAKI AKIRA, MAEMOTO SHUNICHI, HASEGAWA, JUNZO, IWASAKI, AKIRA, MAEMOTO, SHUNICHI
Applicants: KANEGAFUCHI CHEMICAL IND, KANEGAFUCHI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: C07C49
IPC codes: C07C67, C07C49, C07C45, C07C69, C07B57