METHOD FOR RACEMIZATION OF CHRYSANTHEMIC ACID OR ITS ESTER

Abstract: Racemization of optically active, particular­ ly (-)-form of chrysanthemic acid or its ester of the formula: wherein R is, for example, hydrogen or an alkyl group, is effected by contacting it with boron tribromide or aluminium tribromide in the presence of an azo compound. This racemization is useful from a commercial point of view.

Description: The present invention relates to a method for racemization of chrysanthemic acid or its ester. More particularly, it relates to a method for racemi­ zation of optically active chrysanthemic acid or its ester of the formula: wherein R is hydrogen; an alkyl group which may be substituted with a cycloalkyl group or with an aryl group, said alkyl group having the total carbon atoms of from l to 20 including the substituent; or a cycloalkyl group which may be substituted with an alkyl or with an alkoxy group, said cycloalkyl group having the total carbon atoms of from 5 to 20 including the substituent, which comprises contacting the acid or its ester with a boron bromide compound or an aluminium bromide compound in the presence of an azo compound. Chrysanthemic acid, i.e. 2,2-dimethyl-­ 3-(2-methyl-l-propenyl)cyclopropane-l-carboxylic acid, constitutes an acid component of esters well-known as so-called pyrethroidal insecticides, such as pyrethrin, allethrin, phthalthrin, etc., which are utilized as low mammalian toxic, quickly effective insecticides, and is useful as intermediate of these esters. Chrysanthemic acid has four isomers, that is, two geometrical isomers, i.e. cis and trans forms, which respectively have two optical isomers, i.e. (+) and (-) forms. It has been known that, in general, among the isomers the esters composed of trans-form acid exhibit stronger insecticidal activity than those composed of the corresponding cis-form acid, furthermore, the esters composed of (+)-form acid exhibit exceedingly higher activity than those composed of the corresponding (-)-isomer. In general, chrysanthemic acid is industrial­ ly produced as a mixture of cis and trans forms, each of which is in the form of racemic mixture, namely, as (±)-form. Optical resolution of the thus-synthesized acid by means of an optically active organic base is conducted to obtain the (+)-form acid which is utilized for the preparation of insecticidal compounds with a high activity. Wherein, the remaining (-)-isomer after the optical resolution is little useful, since the esters composed thereof is almost inactive. Ac­ cordingly, it is a problem to be solved in the pro­ duction of the (+)-form acid, particularly in a commercial scale, that the (-)-form acid should be racemized with a high efficiency, so as to be utilized again as the material for the optical resolution mentioned above. Racemization of optically active chrysanthemic acid is difficult, since it possesses two asymmetric carbon atoms exhibited by * marks, as shown above, at the l- and 3-positions. Some methods for racemization have so far been studied. The methods known are, a method in which (-)-trans-chrysanthemic acid is oxydized at its C₃-substituted isobutenyl group to convert to a keto­ alcohol group, and the acid group at the C₁-position is converted to a lower alkyl ester, which is then subjected to a reaction with an alkali metal alcoholate in a solvent (U.S. Patent No. 3,282,984); a method in which (-)-trans-chrysanthemic acid is irradiated with ultraviolet rays in the presence of a photosensitizer (U.S. Patent No. 3,657,086); a method in which optically active chrysanthemic acid is converted to the corre­ sponding acid halide and then is brought into contact with a Lewis acid (U.S. Patent Nos. 3,989,750 and 4,l82,906); and a method in which optically active chrysanthemic acid is converted to acid anhydride and then is brought into contact with iodine (U.S. Patent No. 4,485,257). After an extensive study, the inventors have now found that optically active chrysanthemic acid or its ester of the formula (I) is able to racemize conveniently and in high yield by a treatment with a boron bromide compound or an aluminium bromide com­ pound in the presence of an azo compound. This in­ vention is established on the basis of such finding. According to the present invention, the optically active chrysanthemic acid or its ester is able to racemize readily and in high yield and the method of the present invention is very convenient for racemization , particularly, in a commercial scale. More­ over, the present invention enables direct utilization, with high efficiency, of (-)-chrysanthemic acid or its ester, which is separated off in the procedures of optical resolutions. The presence of the azo compound facilitates to lessen an amount of the bromide to be employed and shorten a period of time for racemization. Furthermore, various rationalizations are able to make in commercial production, since the azo compound is not so self induction-decomposed that it is easily handled. Racemization method always gives a trans­ rich reaction product regardless of isomeric composition of the starting material. Since insecticidal activity of pyrethroidal esters in the trans form is generally higher than that of the corresponding esters in the cis form, the above characteristic feature of the racemization method is of great advantage. Thus, the racemization method may be also applied to the con­ version of the racemic cis isomer or of a mixture of the racemic cis and trans isomers of the acid or its ester to the corresponding racemic trans-rich isomer. The method of the present invention will more fully be described hereinafter. In the present invention, any of the four optical isomers of chrysanthemic acid or its ester is able to use solely or in mixtures of isomers of the starting material. Namely, the starting material of any degree of the optical purity is employed. Needless to say, however, it is preferred to use, as the start­ ing material, (-)-form or rich in (-)-form. As chrysanthemic acid ester, mention may be made of, for example, methyl chrysanthemate, ethyl chrysanthemate, propyl chrysanthemate, butyl chry­ santhemate, cyclohexyl chrysanthemate, cyclohexylmethyl chrysanthemate and benzyl chrysanthemate. Boron bromide and aluminium bromide to be used in the present process are, for example, boron tribromide and aluminium tribromide. They are used in such amount as l/l000 - l/4 mol, preferably, l/200 - l/8 mol per mol of chrysanthemic acid or ester thereof. Azo compound to be employed includes azo-­ nitriles such as azobisisobutylonitrile, 2,2′-azobis­ (2,4-dimethylvaleronitrile), l,l′-azobis(cyclohexane-­ l-carbonitrile), 4,4′-azobis-4-cyanopentanoic acid, 2-phenylazo-2,4-dimethyl-4-methoxyvaleronitrile and 2-cyano-2-propylazoform; azo-esters such as azobisiso­ butanol diacetate, methyl azobisisobutyrate and ethyl azobisisobutyrate and alkylazo compounds such as azo-­ t-butane. Azo-nitriles and azo-esters are preferable. The azo compound is used, in general, in such an amount as from l/l0 to 5 mol, preferably l/4 to 2 mol, based upon a mol of the boron bromide compound or the aluminium bromide compound employed. The present racemization reaction is prefer­ ably carried out in the presence of an inert solvent. The solvents include saturated aliphatic hydrocarbons, aromatic hydrocarbons and their halide compounds, ethers, etc. Reaction temperature varies depending on the azo compounds employed. The temperature ranges usually from -20°C to boiling point of the chrysanthemic acid or its ester or boiling point of the solvent when it is employed. Temperature is usually 0°C - l00°C. Time for reaction varies depending on amounts of bromide and of an azo compound and reaction temper­ ature, too, but usually ranges from a few minutes to 7 hours. In carrying out the present racemization reaction in the presence of azo compound, the following procedure is generally employed. One is the procedure in which the bromide compound is added into a mixture of the starting material to be treated and an azo compound in a solvent, and the other is the procedure in which an azo compound and a bromide compound are in parallel added into the starting material to be treated in a solvent. The proceeding of the reaction can be checked by any of measurement of the optical rotation, gas-chromatography, etc. As described above, by the method of the present invention, racemization of the (-)-isomer of optically active chrysanthemic acid or its ester is readily and economically accomplished in a commercial scale. The thus racemized product may be subjected to optical resolution procedures to obtain the useful (+)-isomer of chrysanthemic acid or its ester. Moreover, the racemization method of the present invention can be also applied to conversion of the racemic cis isomer or of a mixture of the racemic cis and trans isomers of the chrysanthemic acid or its ester to the corresponding racemic trans-rich isomer being more useful. The method of the present invention will further be described in the following examples. EXAMPLE l In a 50 ml flask, there were charged (-)-­ cis-chrysanthemic acid (l.79 g), n-heptane (20 ml), and azobisisobutylonitrile (50 mg) under a nitrogen atmosphere. Solution of boron tribromide (87 mg) in n-heptane was added with stirring for 30 minutes at 80°C. After the reaction, dil. hydrochloric acid was added to the reaction mixture with stirring to decompose the catalyst. The separated organic layer was extracted with a l0 % aqueous sodium hydroxide solution (4.8 g). The aqueous layer was acidified with diluted hydrochloric acid and extracted twice with toluene. The toluene layer was washed with water and dried over sodium sulfate. The solvent was evaporated and the residue was distilled (b.p. ll0 - ll9°C/2.5 mmHg) to obtain l.6l g of distillate. The IR spectrum of the product was identical with that of chrysanthemic acid. Gas chromatography assay after conversion to (+)-2-octyl ester gave the following result; (+)-cis, 3.7 %; (-)-cis, 6.6 %; (+)-­ trans, 44.6 %; (-)-trans, 45.l %. EXAMPLE 2 In a 50 ml flask, there were charged chrysanthemic acid (composition: (+)-cis, l.8 %; (-)-­ cis, l8.3 %; (+)-trans, ll.2 %; (-)-trans; 68.7 %) (l.80 g), benzene (8 ml), and azobisisobutylonitrile (58 mg) under a nitrogen atmosphere. Solution of boron tri­ bromide (l87 mg) in benzene was added with stirring for 30 minutes at 80°C. After the reaction, the reaction mixture was treated by the same method as described in EXAMPLE l to obtain chrysanthemic acid (l.53 g). Gas chromato­ graphy assay after conversion to (+)-2-octyl ester gave the following result: (+)-cis, 3.l %; (-)-cis, 3.4 %; (+)-trans, 44.5 %; (-)-trans, 49.0 %. EXAMPLE 3 In a 50 ml flask, there were charged chrysanthemic acid (composition: (+)-cis, l.8 %; (-)-­ cis, l8.3 %; (+)-trans, ll.2 %; (-)-trans, 68.7 %) (3.l8 g), dioxane (l5 ml), and azobisisobutylonitrile (76 mg) under a nitrogen atmosphere. Solution of aluminium tribromide (260 mg) in toluene was added with stirring for 30 minutes at 80°C. After the reaction, the reaction mixture was treated by the same method as described in EXAMPLE l to obtain chrysanthemic acid (2.73 g). Gas chromato­ graphy assay after conversion to (+)-2-octyl ester gave the following result: (+)-cis, 3.2 %; (-)-cis, 3.5 %; (+)-trans, 4l.9 %; (-)-trans, 5l.4 %. EXAMPLE 4 In a 50 ml flask, there were charged ethyl chrysanthemate (composition: (+)-cis, 2.5 %; (-)-cis, l4.8 %; (+)-trans, ll.9 %; (-)-trans; 70.9 %) (4.43 g), benzene (20 ml), and azobisisobutylonitrile (l50 mg) under a nitrogen atmosphere. Solution of aluminium tribromide (370 mg) in benzene was added with stirring for 30 minutes at 80°C. After the reaction, icewater was added to the reaction mixture with stirring to decompose the catalyst. Separated organic layer was concentrated under reduced pressure. To the residue was added a l0 % aqueous sodium hydroxide solution (20 g) and the mixture was refluxed for 3 hours. The aqueous layer was washed with toluene. The aqueous layer was acidified with diluted hydrochloric acid and extracted with toluene. The toluene layer was washed with water and dried over sodium sulfate. The solvent was evaporated and the residue was distilled (b.p. ll0 - ­ ll9°C/2.5 mmHg) to obtain 3.34 g of distillate. The IR spectrum of the product was identical with that of chrysanthemic acid. Gas chromatography assay after conversion to (+)-2-octyl ester gave the following result: (+)-cis, 3.5 %; (-)-cis, 3.5 %; (+)-trans, 43.0 %; (-)-trans, 50.5 %. EXAMPLE 5 In a 50 ml flask, there were charged (-)-­ cis-chrysanthemic acid (l.l5 g), dioxane (l5 ml), and methyl azobisisobutylate (45 mg) under a nitrogen atmosphere. Solution of boron tribromide (60 mg) in toluene was added with stirring for 30 minutes at 80°C. After the reaction, the reaction mixture was treated by the same method as described in EXAMPLE l to obtain chrysanthemic acid (l.0l g). Gas chromato­ graphy assay after conversion to (+)-2-octyl ester gave the following result: (+)-cis, 3.8 %; (-)-cis, 8.l %; (+)-trans, 44.0 %; (-)-trans, 44.l %. EXAMPLE 6 In a 35 ml flask, there were charged cis-­ chrysanthemic acid (l.68 g), benzene (l6.8 ml), and azobisisobutylonitrile (ll.5 mg) under a nitrogen atmosphere. Solution of boron tribromide (l2.5 mg) in benzene was added with stirring for l0 minutes at 60°C and the reaction mixture was stirred at the same temperature for l hour. After the reaction, diluted hydrochloric acid was added to the reaction mixture with stirring to decompose the catalyst. Gas chromatographic analysis showed that this product contained l.63 g of chrysan­ themic acid (cis, 6.0 %; trans, 94.0 %). EXAMPLE 7 In a 35 ml flask, there were charged cis-­ chrysanthemic acid (l.6 g), dioxane (20 ml), and methyl azobisisobutylate (63 mg) under a nitrogen atmosphere. Solution of boron tribromide (85 mg) in toluene was added with stirring for 30 minutes at 80°C. After the reaction, the reaction mixture was treated by the same method as described in EXAMPLE l to obtain chrysanthemic acid (l.4 g). Gas chromato­ graphy assay gave the result as follows: cis, l0.9 %; trans, 89.l %. EXAMPLE 8 In a 50 ml flask, there were charged ethyl chrysanthemate (composition: cis, 35 %; trans, 65 %) (4.43 g), benzene (20 ml), and azobisisobutylonitrile (l50 mg) under a nitrogen atmosphere. Solution of aluminium tribromide (370 mg) in benzene was added with stirring for 30 minutes at 80°C. After the reaction, icewater was added to the reaction mixture with stirring to decompose the catalyst. The separated organic layer was concentrated under reduced pressure. To the residue was added a l0 % aqueous sodium hydroxide solution (20 g) and the mixture was heated under refluxing for 3 hours. The aqueous layer was washed with toluene. The aqueous layer was acidified with diluted hydrochloric acid and extracted with toluene. The toluene layer was washed with water and dried over sodium sulfate. The solvent was evaporated and the residue was distilled (b.p. ll0 - ll9 °C/2.5 mmHg) to obtain 3.35 g of distillate. The IR spectrum of the product was identical with that of chrysanthemic acid. Gas chromatography assay gave the result as follows: cis, 7.0 %;trans, 93.0 %.

Claims: 1. A method for racemization of optically active chrysanthemic acid or an ester thereof of the formula: wherein R is hydrogen; an alkyl group which may be substituted with a cycloalkyl or aryl group, said alkyl group and any substituent having a total of 1 to 20 carbon atoms; or a cycloalkyl group which may be substituted with an alkyl or alkoxy group, said cycloalkyl group and any substituent having a total of 5 to 20 carbon atoms, which method comprises contacting the acid or ester thereof with a boron bromide compound or an aluminium bromide compound in the presence of an azo compound. 2. A method for the conversion of racemic cis isomer or of a mixture of the racemic cis and trans isomers of chysanthemic acid or an ester thereof as defined in claim 1 to the corresponding racemic trans-rich isomer, which method comprises contacting said racemic cis isomer or mixture of racemic cis and trans isomers with a boron bromide compound or an aluminium bromide compound in the presence of an azo compound. 3. A method according to claim 1 or 2 wherein the boron bromide compound is boron tribromide. 4. A method according to claim 1 or 2, wherein the aluminium bromide compound is aluminium tribromide. 5. A method according to any of claims 1 to 4, wherein the azo compound is at least one selected from the group consisting of azonitriles, azoesters and alkylazo compounds. 6. A method according to any one of the preceding claims wherein (+)-chrysanthemic acid or ester is separated from the resulting racemic mixture and converted into a pyrethroidal insecticide in manner known per se. 7. A method according to claim 6 wherein the trans (+)-isomer is separated and converted.

Inventors: FUKAO, MASAMI, SUZUKAMO, GOHFU
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1987
Main classification: C07C61
IPC codes: C07C67, C07B61, C07C51, C07B31, B01J31, C07C61, C07B55, C07C69