GB2054570A - Process for the preparation of optically active aryloxyalkanoic acid compounds - Google Patents

Abstract

Optically active aryloxyalkanoic acid derivatives of the formula <IMAGE> wherein Ar represents an optionally substituted monocyclic or bicyclic aromatic radical containing from 6 to 10 carbon atoms, R represents an alkyl radical containing from 1 to 4 carbon atoms, and A represents a -COOR1, <IMAGE> or -CO-Z radical, in which Z is halogen, are prepared by reacting an aqueous or aqueous-organic solution of an optically active alkali metal salt of a 2-halogenoalkanoic acid with an aqueous solution of an alkali metal salt of a phenol of the formula Ar-OH at an elevated temperature under a pressure lower than the vapour pressure of water at the reaction temperature. The aryloxyalkanoic acid derivatives having the D configuration are useful as herbicides.

Description

SPECIFICATION Process for the preparation of optically active aryloxyalkanoic acid compounds This invention relates to a process for the preparation of optically active aryloxyalkanoic acids and of derivatives of such acids. More particularly, it relates to the preparation of aryloxyalkanoic acids, and derivatives of such acids, which have a very high content of the isomer possessing the D configuration, and which can be used as herbicides.

The optically active compounds which can be prepared in accordance with the process of the invention are of the general formula:

wherein Ar represents a monocyclic or bicyclic aromatic hydrocarbon radical containing from 6 to 10 carbon atoms (e.g. phenyl, naphth-1-yl or naphth-2-yl), optionally substituted by one to three substituents, which may be identical or different, selected from halogen (preferably chlorine or bromine) atoms, alkyl radicals containing from 1 to 4 carbon atoms (preferably methyl), a phenoxy radical which is optionally substituted by one to three substituents, which may be identical or different, selected from halogen (preferably chlorine) atoms, alkyl radicals containing from 1 to 4 carbon atoms (preferably methyl), the trifluoromethyl radical and the nitro radical, and a pyridyloxy radical which is optionally substituted by one to three substituents, which may be identical or different, selected from halogen (preferably chlorine) atoms, alkyl radicals containing from 1 to 4 carbon atoms (preferably methyl) and the trifluoromethyl radical, A represents a -COOR1,

or -COZ radical, wherein R1 represents a hydrogen atom, one equivalent of a cation of an organic or inorganic base (e.g. one equivalent of a cation of an alkali metal or alkaline earth metal or of an optionally substituted ammonium cation) or an alkyl radical containing from 1 to 1 2 carbon atoms, which is optionally substituted by one or more halogen atoms or hydroxy radicals, R2 and R3, which may be identical or different, each represent a hydrogen atom, an alkyl radical containing from 1 to 6 carbon atoms which is optionally substituted by one or more hydroxy, unsubstituted amino or alkoxy radicals containing from 1 to 4 carbon atoms, a phenyl radical which is optionally substituted by one or more substituents selected from halogen atoms, alkyl radicals containing from 1 to 4 carbon atoms and alkoxy radicals containing from 1 to 4 carbon atoms or an alkenyl radical containing from 2 to 4 carbon atoms, Z represents a halogen (preferably chlorine) atom, and R represents an alkyl radical containing from 1 to 4 carbon atoms (preferably methyl).

It is to be understood that, in this specification and the accompanying claims alkyl and alkenyl radicals, and alkyl moieties of alkoxy radicals may be straight- or branched-chain.

The invention more particularly relates to the preparation of optically active aryloxyalkanoic acids, and salts of such acids, of the general formula:

wherein Ar and R are as hereinbefore defined and M represents an alkali metal cation (e.g.

sodium or potassium or a hydrogen atom.

The compounds of formula I and those of formula II have an asymmetrically substituted carbon atom. Accordingly, each of these compounds can exist in two enantiomeric forms, one of which has the D absolute configuration and the other of which has the L absolute configuration.

(According to another, equivalent, system of nomenclature, the letters R and S are used instead of D and L respectively, in order to identify the absolute configurations. In this specification and the accompanying claims the first of these systems of nomenclature, i.e. using the letters D and L is used).

A compound of formula I (or of formula II) which contains equal proportions by weight of the isomer having the D absolute configuration and of the isomer having the L absolute configuration is the optically inactive racemic compound.

In this specification and the accompanying claims the term "optically active compound" is to be understood as meaning a compound which predominantly (by weight) or completely comprises one of the optical isomers of the compound. The term "optical purity" of an optically active compound, i.e. a compound consisting predominantly of one of the two isomers, is to be understood as meaning the percentage by weight of the preponderant isomer present in the compound. The term "optically active compound of high optical purity" is to be understood as meaning a compound in which the percentage by weight of the preponderant isomer is at least 90%.

It is known that numerous compounds of formulae I and II exhibit excellent herbicidal properties and that several of them are currently marketed for this use; these compounds especially include: 2-(2-methyl-4-chlorophenoxy)propionic acid (i.e. Mecoprop), 2-(2,4,5-trichlorophenoxy)propionic acid (i.e. Fenoprop), 2-(2,4-dichlorophenoxy)propionic acid (i.e. Dichlorprop) and 2-(2-methylphenoxy)propionic acid, these acids being generally marketed in the form of their sodium or potassium salts or of their amine salts or of their esters with lower alkanols, or 2-( < x-naphthoxy)-N,N-diethylpropionamide (or Napropamide) and methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propionate (i.e. Diclofop-methyl).

These compounds have hitherto generally been marketed in their racemic form. However, it has been observed that for several of the compounds mentioned above, one of the isomers exhibits a very much higher herbicidal activity, at the same dose, than that of the corresponding racemic compound. It thus appears very desirable to' have available a process which makes it possible to prepare these herbicidal compounds in their optically active form, consisting predominently, or completely, of the isomer exhibitirig the better herbicidal activity. In the case of the herbicidal compounds named above, it has been found that the isomer having the D configuration is responsible for almost the whole of the herbicidal activity, whilst the isomer having the L configuration exhibits virtually no herbicidal activity.

The invention thus relates more particularly to the preparation of the optically active derivatives of compounds of formulae I and II, the said derivatives consisting completely or predominantly of the isomer having the D configuration.

It has been proposed, in British Patent Specification No. 1114040, to prepare the isomer, having the D configuration, of certain 2-phenoxypropionic acids by reaction an alkali metal salt of 2-chloropropionic acid, the salt being dextrorotatory in water, with an alkali metal salt of a phenol in the presence of an inert organic solvent of high boiling point, such as an aromatic hydrocarbon, at the boiling point of the solvent, and then acidifying the alkali metal salt thus obtained by treating it with a strong acid, to give D-2-phenoxypropionic acid. According to this process, the conversion of the alkali metal 2-chloropropionate to the alkali metal D-2phenoxypropionate takes place with a configuration inversion of the Walden inversion type.

The process described and claimed in British Patent Specification No. 1114040 makes it possible to obtain good yields of 2-phenoxypropionic, acids having a very high D isomer content.

For practical purposes, using the technique described in the said patent, it is preferred to use an hydros reactants and/or to remove the water present in the reaction mixture by azeotropic entrainment before starting the reaction.

Industrially, the use of an hydros reactants present s serious disadvantages. It is expensive because it requires prior dehydration of the reactants. Furthermore certain reactants, and in particular sodium 2-chloropropionate, are difficult to !prepare in the anhydrous state.

In addition, several of these anhydrous reactants are solids, which are difficult to use and which give heterogeneous reaction mixtures which are difficult to stir and are therefore of low reactivity.

Finally, the elimination of water by azeotropic entrainment requires the use of an auxiliary solvent which must be recovered and recycled.

The present invention proposes to overcome the disadvantages associated with the use of anhydrous reactants and azeotropic removal of water from the reaction mixture.

The present invention provides an improved process for the preparation of optically active aryloxyalkanoic acids, and their derivatives, of general formulae I and II, which makes it possible to prepare these compounds from aqueous or aqueous-organic solutions of optically active alkali metal 2-halogenoalkanoates, which are optionally prepared in situ. The process of the invention permits the preparation of optically active aryloxyalkanoic acids and derivatives thereof in excellent yield from an optically active alkyl 2-halogenoalkanoate or from an optically active alkali metal 2-halogenoalkanoate. The compounds prepared have a high degree of optical purity.

The present invention provides a process for the preparation of an optically active aryloxyalkanoic acid compound of general formula I, wherein the various symbols are as hereinbefore defined, which comprises reacting, at an elevated temperature and a pressure lower than the vapour pressure of water at the reaction temperature, an aqueous or aqueous-organic solution of an optically active alkali metal salt of a 2-halogenoalkanoic acid, containing from 3 to 6 carbon atoms, with an aqueous solution of an alkali metal salt of a phenol of the general formula:: Ar-OH Ill wherein Ar is as hereinbefore defined, to form an optically active alkali metal salt of formula II wherein M represents an alkali metal cation, optional acidification of the salt to form the corresponding acid of general formula II wherein M represents a hydrogen atom and optionally converting by methods known per se the acid obtained into an optically active compound of formula I wherein A is other than a group -COOR, in which R, is a hydrogen atom.

It will be appreciated that the optically active 2-halogenoalkanoic acid from which the alkali metal salt used as starting material is derived necessarily has a hydrogen atom attached to the 2-position carbon atom.

The aqueous-organic solution is preferably aqueous-alcoholic. The alcohol used in the aqueous-alcoholic solution of the alkali metal salt of the 2-halogenoalkanoic acid generally contains from 1 to 4 carbon atoms and is preferably methanol.

By the expression "methods known per se" as used in this specification and the accompanying claims is meant methods heretofore used or described in the chemical literature.

It is to be understood that pressures referred to in this specification and the accompanying claims are absolute pressures, i.e. the value zero corresponds to a complete vacuum.

The temperature to be used for the process of the invention can be selected in accordance with the reactivity of the alkali metal salt of the phenol of general formula Ill and of the alkali metal 2-halogenoalkanoate used. It will, for example, be appreciated that the reaction temperature will not necessarily be the same in the case of an alkali metal 2-chloropropionate as in the case of an alkali metal 2-bromopropionate, because these compounds have rather different reactivities.

The reaction temperature will be selected to be sufficiently high to allow the reaction to take place under satisfactory conditions, but it must remain below the temperature at which thermal degradation of the product or of the reactants employed in the reaction, and possible autoracemisation of the opticaily active compounds, becomes significant.

When the process of the invention is carried out using sodium 2-chloropropionate as the alkali metal 2-halogenoalkanoate, good results are obtained by working at temperatures from 70 to 1 60 C.

Preferably, the conditions used for carrying out the process according to the invention are those indicated below in paragraphs a, b, c, d, e and f. Combinations of the preferred conditions may of course be used.

(a) The 2-halogenoalkanoic acid is preferably 2-chloropropionic acid.

(b) The preferred alkali metal salts of the starting reactants used are, respectively, the sodium salt of 2-chloropropionic acid and the sodium salt of the phenol of general formula Ill; a compound of formula II is then obtained in which R represent a methyl radical and M represents a sodium atom.

(c) The reaction is carried out under a pressure less than or at most equal to atmospheric pressure and preferably of from 0.1 to 1 bar. The pressure chosen must be lower, the lower is the reaction temperature which has been chosen. By way of example, if the reaction is carried out at 80-90"C, the pressure is advantageously of the order of 0.1 to 0.7 bar.

(d) The aqueous or aqueous-alcoholic solution of the alkali metal salt of the 2-halogenoalkanoic acid is preferably a highly concentrated solution, near the saturation limit.

If sodium 2-chloropropionate is used, the solution advantageously contains at least 20% by weight of sodium 2-chloropropionate, the remainder consisting of water and, optionally, an alcohol, e.g. a lower alkanol.

The upper limit of the amount of sodium 2-chloropropionate present in the solution corresponds to the saturation limit of sodium 2-chloropropionate in the solution under the temperature and pressure conditions used. Advantageously, the solution contains from 20 to 50% by weight of sodium 2-chloropropionate.

(e) The aqueous solution of the alkali metal salt of the phenol of general formula III is preferably a solution of high concentration, near the saturation limit. It advantageously contains at least 50% by weight of the alkali metal salt of the phenol of general formula III. The upper limit of the amount of this salt used corresponds to the saturation limit of the solution under the temperature and pressure conditions used. Preferably, a solution containing from 50 to 90% by weight of the sodium salt of the phenol of general formula Ill is used. The solution can be prepared by saponifying the phenol of general formula Ill with an industrial aqueous sodium hydroxide solution.The solution obtained is preferably concentrated to near its saturation limit by heating at a temperature of from 70 to 1 60 C under a pressure lower than the vapour pressure of water at the temperature used. Advantageously, the heating is carried out using the same temperature and pressure conditions as for the process according to the invention.

(f) The alkali metal salt of the 2-halogenoalkanoic acid preferably consists predominantly of the isomer having the L configuration (advantageously, it contains at least 90% of the L isomer).

Because the reaction according to the invention takes place with a Walden inversion, with a very slight reduction in the optical purity, compounds of general formulae I and II consisting predominantly of the isomer having the D configuration are thus obtained. (It is of course possible, where desired, to prepare compounds of general formulae I and II consisting predominantly of the enantiomer having the L configuration by starting from an alkali metal salt of the 2-halogenoalkanoic acid which is predominantly the D isomer).

The optically active alkali metal salt of the 2-halogenoalkanoic acid, used as a starting material in the process of the invention, can be prepared by a method known per se in a separate operation, by saponifying an optically active alkyl 2-halogenoalkanoate with an inorganic base, e.g. sodium hydroxide solution. The alkali metal salt can also be prepared in situ, in accordance with a method known per se e.g. British Patent Specification No. 1,1 14,040, in which an alkyl 2-chloropropionate is run into the reaction mixture containing the phenol and an inorganic base, e.g. sodium hydroxide solution, whilst maintaining the temperature between 5 and 35'C.When these conditions are used, it has been observed that an increase in temperature to above the higher value just indicated results both in a very great lowering of the yield of alkali metal 2chloropropionate, due to the formation of the alkali metal lactate, and in a rapid lowering of the optical purity of the alkali metal 2-chloropropionate formed. Since the saponification reaction is exothermic, it is thus necessary to cool the reaction mixture in order to maintain its temperature at between 5 and 35"C throughout the duration of this reaction, and this is a serious disadvantage in the case of an industrial manufacturing process.

It has been found that, according to a variant of the process according to the invention, it is possible to carry out the preparation in situ of the optically active alkali metal salt of the halogenoalkanoic acid at an elevated temperature (above 40"C and preferably from 70' to 1 60'C) whilst avoiding the abovementioned disadvantages.

According to this variant of the process of the invention (which variant also forms part of the invention), the optically active alkali metal salt of the 2-halogenoalkanoic acid (preferably 2chloropropionic acid) is prepared in situ in the aqueous solution of the alkali metal (preferably sodium) salt of the phenol of general formula III, at an elevated temperature (preferably from 70 to 1 60 C), by introducing into the solution, simultaneously and in substantially stoichiometric amounts, an aqueous alkali metal hydroxide solution (preferably sodium hydroxide solution) and an optically active alkyl 2-halogenoalkanoate of the general formula

wherein Hal represents a halogen, preferably chlorine, atom, R is as hereinbefore defined, and R4 represents an alkyl radical containing from 1 to 6 carbon atoms, under a pressure which is greater than the vapour pressure (or boiling pressure) of the alkyl 2-halogenoalkanoate of general formula IV, but less than the vapour pressure of water, at the temperature used. By "substantially stoichiometric amounts" is meant amounts such that the molar ratio of the two reactants introduced (sodium hydroxide solution and alkyl 2-halogenoalkanoate) is from 0.8 to 1.2 and is preferably from 1 to 1.2. Under these conditions it is found that although the temperature is high, there is virtually neither any formation of alkali metal lactate nor a reduction in the optical purity of the optically active compounds (such as the alkyl halogenopropionate and the alkali metal halogenopropionate) present in the reaction mixture and that the- resulting product is obtained with excellent optical purity and in excellent yield.

For this variant, it is advantageous to use, as the starting reactants, an aqueous sodium hydroxide solution near the saturation limit, and an alkyl 2-halogenoalkanoate of general formula IV, which consists predominantly of the isomer having the L configuration, is optically laevorotatory (the measurement being carried out in the absence of a solvent) and preferably contains at least 90% by weight of the L isomer. Thus, an aqueous-alcoholic solution of the optically active metal salt of the halogenoalkanoic acid is obtained in situ, and this salt reacts, at the rate at which it is formed, with the alkali metal salt of the phenol, under the conditions indicated above.

To carry out the preparation of the alkali metal salts of the 2-halogenoalkanoic acids in situ, the same temperature conditions can be used as in the process described above, for example temperature of from 70 to 1 60'C.

The pressure to be used for this variant of the process according to the invention varies within limits which can be determined from the vapour pressure diagrams of water and of the optically active alkyl 2-halogenoalkanoate used.

By way of example if methyl 2-chloropropionate is used and the process is carried out at 90"C, this pressure must be between 0.24 bar (the vapour pressure of methyl 2-chloropropionate at 90"C) and 0.69 bar (the vapour pressure of water at 90"C).

If it is desired to carry out the process at a lower pressure, it can be advantageous to replace the methyl 2-chloropropionate by a less volatile ester such as, for example, ethyl, propyl or butyl 2-chloropropionate.

The process according to the invention can be carried out discontinuously as indicated in the Examples which follow. It can also be carried out continuously, by gradually adding an aqueous or aqueous-organic solution of the alkali metal 2-halogenoalkanoate (or, in accordance with the variant described above, gradually adding the alkyl 2-halogenoalkanoate and the sodium hydroxide solution) to the aqueous solution of the alkali metal salt of the phenol of general formula Ill, in accordance with the conditions indicated above, and separating the reaction product.

The process according to the invention provides, initially, the alkali metal salts of general formula II, wherein M represents an alkali metal atom, and thereafter, by acidification of these salts, the corresponding free acids (the compounds of general formula II wherein M represents a hydrogen atom).

The various other compounds of general formula I can be obtained from these free acids and their salts by methods known per se: acid halides of general formula I, wherein A represents a -COZ radical, wherein Z is as hereinbefore defined, can be obtained by reaction of the corresponding free acid of formula I with a halogenating agent such as SOCI2, PCl5 and PAL3; the acid halides may be converted to the corresponding amides (i.e. compounds of general formula I, wherein A represents the

radical wherein R2 and R2 are as hereinbefore defined) by reacting the acid halides with an amine of the general formula HNR2R3, wherein R2 and R3 are as herein before defined;; the free acids or acid halides of general formula I may be converted to esters (i.e. compounds of general formula 1, wherein A represents a -COOR, radical, wherein R, represents an optionally substituted alkyl radical as hereinbefore defined) by esterification by by means of an appropriate alkanol. Other esters can also be obtained by direct transesterification by means of an appropriate alkanol.

Certain salts of the free acids of general formula II are obtained in the course of the process of the invention (alkali metal salts, principally sodium and potassium salts). Other salts can be obtained from the free acids of general formula II by saponification with an appropriate inorganic or organic base.

The process according to the invention can be used for the preparation of the various optically active compounds of general formula I. It is very particularly suitable for the preparation of the following compounds as optically active compounds of high optical purity in which the preponderant isomer is the D-isomer: 2-(2-methyl-4-chlorophenoxy)propionic acid, 1-naphthoxypropionic acid and 2-[4-(2,4-dichlorophenoxy)phenoxy]propionic acid.

It is to be understood that the organic solvents in the aqueous-organic solutions used in the process of the present invention are those which are miscible with water, have a boiling point lower than the boiling point of water at the pressure used for the reaction and which are solvents for the alkali metal salt of the 2-halogenoalkanoic acid in aqueous-organic solution.

The following Examples illustrate the invention.

It will be noted from these Examples that the process according to the invention makes it possible to obtain, from alkali metal 2-chloropropionates or alkyl 2-chloropropionates containing 94 to 98% of L isomer, 2-phenoxypropionic acids in which the content of D isomer is generally from 91.5 to 95.9%.

It will also be noted from the Examples that the yield of phenoxypropionic acid is in general from 96 to 98% if the starting material is sodium 2-chloropropionate and from 91 to 95% if the starting material is methyl 2-chloropropionate.

Example 1 A 500 ml reactor equipped with a stirrer and an outlet tube fitted with a condensor (thus making it possible to condense the vapours evolved) is used, the whole being connected to a system which makes it possible to work under reduced pressure.

2-Methyl-4-chlorophenol (171 g, 1.2 mols) and an aqueous sodium hydroxide solution (88 g, comprising 44 g of sodium hydroxide and 44 g of water) are introduced into the reactor and the mixture is heated, with stirring, for 15 minutes at 90'C under a pressure of 0.25 bar (188 mm Hg).

An aqueous solution (200 ml) containing sodium 2-chloropropionate [130.59, 1 mol; having an optical rotation [a120= + 3.90 (C = 0.1, water)] and water (125 g) is then run into the above mixture in the course of 2 hours 1 5 minutes, at a constant rate, whilst maintaining the temperature at 90"C and the pressure at 0.25 bar. When the addition is complete, the reaction mixture is kept at 90"C and under a pressure of 0.25 bar for 1 5 minutes, with stirring.

The mixture obtained is then run into water (550 ml), after which it is acidified with 1 ON hydrochloric acid so as to adjust the aqueous mixture to pH 6. It is then washed with perchloroethylene (4 x 200 ml) and the aqueous and organic phases are then separated. The aqueous phase is adjusted to pH 0.5 and 2-(2-methyl-4-chlorophenoxy)propionic acid (hereinafter referred to as MCPP acid) is precipitated and then extracted with perchloroethylene (2 X 300 ml). The organic extracts are combined and the solvent is distilled off under reduced pressure, giving MCPP acid (199.5 g, 0.93 mol) of 100% purity according to titration with alkali, i.e. a yield of 93% relative to the sodium 2-chloropropionate starting material. 9.6 g of acid are recovered from the first organic phase separated off, thereby raising the total yield to 97%.

The MCPP acid obtained has an optical rotation [a]2g= + 25.86' (C = 0.1, acetone). On the basis of the theoretical optical rotation of optically pure (D) MCPP acid, measured under the same conditions, i.e. (0i12D0= + 28.15 , the MCPP acid obtained consists of 95.9% of the D isomer, the remainder being L isomer, and its optical purity is thus 95.9%.

On the basis of the theoretical optical rotation of optically pure L sodium 2-chloropropionate, namely La]20= + 4 (C = 0.1, water), the sodium 2-chloropropionate used as the starting material consisted of 98% of the L isomer (the remainder being D isomer), and its optical purity was thus 98%.

Comparative experiment The procedure described in Example 1 is followed, using the same amounts of the same reactants and working under the same conditions, except that instead of working under a pressure of 0.25 bar, the process is carried out under atmospheric pressure (1.013 bar), which is greater than the vapour pressure of water at 90"C (0.69 bar). Under these conditions, the total yield of MCPP acid is 92% and its optical rotation is (a]20= + 20.8 (C = 0.1, acetone), corresponding to an optical purity of 87%.

Example 2 The process is carried out under the conditions described in Example 1, with only the following differences: the temperature is 1 50'C (in place of 90"C) and the pressure is 1.013 bar (760 mm Hg) in place of 0.25 bar. MCPP acid (205 g), of 100% purity according to the titration with alkali is thus obtained, and a further amount of MCPP acid (2 g) is recovered from the first organic phase separated off, making a total yield of 96.5%: optical rotation Coil2,0, + 24.8 (C = 0.1, acetone), corresponding to an optical purity of 94%.

Example 3 The process is carried out under the conditions described in Example 2, with only the following difference: the temperature is 130"C in place of 150"C. MCPP acid (200 g) is thus obtained and a further amount of this acid (9.5 g) is recovered from the first organic phase separated off, making a total yield of 97.6%: optical rotation (a]%0= + 24.52 (C = 0.1, acetone), corresponding to an optical purity of 93.5%.

Example 4 The process is carried out under the conditions described in Example 1, with only the following differences: the pressure is 0.26 bar (in place of 0.25 bar), and the aqueous solution of sodium 2-chloropropionate is replaced by an aqueous methanolic solution consisting of sodium 2-chloropropionate (130.5 g, 1 mol), methanol (42.0 g) and water (93.0 g).

MCPP acid (206 g) is thus obtained and a further amount of this acid (2.4 g) is recovered from the first organic phase separated off, making a total yield of 97.2%: [a]2g= + 24.63 (C 0.1, acetone), corresponding to an optical purity of 93.7%.

The aqueous methanolic solution used as starting material was prepared in a separate operation, by treating methyl 2-chloropropionate (122.5 g), of (aJ20= - 24.85" (in the absence of solvent and of 99.9% purity), with an aqueous 30% w/v sodium hydroxide solution (133 g, 1 mol) and methanol (10 g) at a temperature below 35"C, under normal pressure.

On the basis of the theoretical optical rotation of optically pure L methyl 2-chloropropionate, i.e. [a120= - 27.8' (in the absence of solvent), the methyl 2-chloropropionate used as the starting material consisted of 94.7% of the L isomer (optical purity 94.7%).

Example 5 The process is carried out under the conditions described in Example 4, with only the following differences: the temperature is 1 35'C (in place of 90"C) and the pressure is 1.013 bar (in place of 0.26 bar).

MCPP acid (a total of 208.4 g, total yield 91.7%) of optical rotation [a]2g= + 24.0 (C = 0.1, acetone), corresponding to an optical purity of 92.7%, is obtained.

Example 6 The same apparatus as in Example 1 is used. 2-Methyl-4-chlorophenol (171.5 g, i.e. 1.2 mols, taking into account the purity of 99.8%) and a 50% w/v aqueous solution of sodium hydroxide (88 g, 1.1 mols) are introduced into the reactor and the mixture is heated to 90"C under a pressure of 0.4 bar and is kept at this temperature and pressure for 20 minutes, with stirring.

The following are then run into the mixture simultaneously, in stoichiometric amounts: methyl 2-chloropropionate of 98.5% purity (124 g, 1 mol), having an optical rotation [a]2g= - 25.3' (in the absence of solvent), and a 50% w/v aqueous solution of sodium hydroxide (80 g, 1 mol). The two reactants are run in over a period of 1 hour. During the whole of this period, the temperature is kept at 90"C and the pressure at 0.4 bar, and the reaction mixture is then kept at this temperature and this pressure for a further 30 minutes, with stirring.

This mixture is then run into water (550 ml); thereafter it is acidified with 1 ON hydrochloric acid, so as to adjust it to pH 5.5, and is washed with perchloroethylene (4 X 200 ml). The organic phase, which contains the excess of 2-methyl-4-chlorophenol and a small amount of MCPP acid, is separated off. The aqueous phase is acidified with 1 ON hydrochloric acid to pH 0.5 and the 2-(2-methyl-4-chlorophenoxy)propionic acid is precipitated and then extracted with perchloroethylene (2 X 300 ml). The organic extracts are combined and the solvent is distilled off under reduced pressure, and MCPP acid (200 g) having a purity of 99;6%, determined by titration with alkali, and an optical rotation [a]2g= + 24.2 (C = 0.1, acetone) is obtained.A further amount of MCPP acid (1.7 g) is recovered from the first organic phase separated off.

Optical purity (i.e. % content of D isomer) of the MCPP acid obtained: 92.8%, optical purity (i.e. % content of L isomer) of the methyl 2-chloropropionate starting material: 98.5%, and total yield of MCPP acid relative to methyl 2-chloropropionate: 94%.

Examples 7 to 9 The procedure followed is as described in Example 6, starting from the same amounts of the same reactants, but varying the temperature and the pressure which are maintained whilst running in the methyl 2-chloropropionate and the aqueous sodium hydroxide solution, as well as varying the period over which these reactants are added.The results obtained are shown in the Table below: ADDITION OF THE REACTANTS MCPP ACID Example Temperature ' Pressure Total % of D No. ( C) (bars) Period yield, % isomer 7 80 0.4 2 hrs 92.5 92.7 8 90 0.4 1 hr 45 95.6 93.8 9 130 1.013 1 hr 20 93.0 91.5 Example 10 The same apparatus as in Example 1 is used. a-Naphthol (172.8 9, 1.2 mols) and a 50% w/v aqueous solution of sodium hydroxide (88 g, 1.1 mols) are introduced into the reactor and the mixture is heated to 100"C under a pressure of 0.3 bar and is kept at this temperature and this pressure for 1 5 minutes, with stirring.

An aqueous methanolic solution (265.5 g) of sodium 2-chloropropionate, having the same composition as in Example 4 and prepared as described in Example 4, from the same methyl 2chloropropionate, is then run into the reaction mixture in the course of two hours. After the addition is complete, the mixture is kept at 100"C for 30 minutes.

Water (600 ml) is added to the mixture and the pH is adjusted to 5.5 by adding 10N hydrochloric acid. The free naphthol is extracted with perchloroethylene (6 X 100 ml) at 85"C.

Perchloroethylene (500 ml) is added to the aqueous phase, which is kept at 85'C, before adjusting the pH to 0.5 by adding 1 ON hydrochloric acid. The organic phase is separated off and washed with hot water (2 x 100 ml), and the solvent is then removed by evaporation to constant weight in vacuo at 135"C.

1-Naphthoxypropionic acid (202 g) is obtained which melts at 1 25'C, is 99.8% pure according to titration with alkali, and has an optical rotation [a]20= - 39.2' (C = 0.01, acetone): yield 93.5%.

The optical purity of the compound obtained is increased by two successive recrystallisations from toluene, and 1-naphthoxypropionic acid, having an optical rotation (0L12D0= - 45.6' (C = 1, acetone) and melting at 1 28'C, is thus obtained. A third recrystallisation does not increase the optical rotation and it is probable that the compound thus obtained consists entirely of the isomer having the D absolute configuration.

Example Ii The same apparatus as in Example 1 is used. 4-(2,4-Dichlorophenoxy)phenol (306 g, 1.2 mols) and a 50% w/v aqueous sodium hydroxide solution (88 g, 1.1 mols) are introduced into the reactor and this mixture is heated at 90"C under a pressure of 0.3 bar for 1 hour.

An aqueous methanolic solution (265.5 g) of sodium 2-chloropropionate prepared as described in Example 4 is run into the above mixture in the course of 1 hour 30 minutes. When it has all been run in, the mixture is kept at 90"C for 30 minutes.

The mixture is then run into water (2,500 ml), and the pH is adjusted to 6 by addition of iON hydrochloric acid. The mixture is washed with xylene (6 X 300 ml) and the aqueous and organic phases are then allowed to separate. The aqueous phase is adjusted to pH 0.5 and 2-(4-(2,4- dichlorophenoxy)phenoxy]propionic acid precipitates and is extracted with xylene (2 x 300 ml).

The organic extracts are combined and the solvent is distilled under reduced pressure, and 2-(4- (2,4-dichloro-phenoxy)-phenoxyjpropionic acid (293 g, 0.896 mol), of 99.7% purity as determined by titration with alkali, is obtained; the yield is thus 89.6% relative to the methyl 2chloropropionate starting material. A further amount of acid (14 g) is recovered from the first organic phase separated off (total yield: 93.9%).

The acid obtained has an optical rotation [0il2D0= + 15.6 (C = 0.1, chloroform).

A portion of the acid (30 g) is dissolved in methanol (250 ml) and after addition of sulphuric acid (0.5 ml) the mixture is heated at the boil for 2 hours whilst removing the water formed, and a part of the methanol, by distillation. Xylene (200 ml) is then added and the methanol is removed by distillation. The resulting solution of the methyl ester in xylene is washed with an aqueous sodium bicarbonate solution to neutralise the sulphuric acid and the unesterified acid.

After removing the xylene by distillation under reduced pressure, methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propionate (29 g) of optical rotation Eai2D0= + 26 (C = 0.1, chloroform) is obtained, representing a yield of 92.7%.

Claims (23)

1. A process for the preparation of an optically active aryloxyalkanoic acid compound of the general formula:

wherein Ar represents a monocyclic or bicyclic aromatic hydrocarbon radical containing from 6 to 10 carbon atoms, optionally substituted by one to three substituents, which may be identical or different, selected from halogen atoms, alkyl radicals containing from 1 to 4 carbon atoms, a phenoxy radical which is optionally substituted by one to three substituents, which may be identical or different, selected from halogen atoms, alkyl radicals containing from 1 to 4 carbon atoms, the trifluoromethyl radical and the nitro radical, and a pyridyloxy radical which is optionally substituted by one to three substituents, wich may be identical or different, selected from halogen atoms, alkyl radicals containing from 1 to 4 carbon atoms and the trifluoromethyl radical, A represents a -COOR1,

or -COZ radical, wherein R, represents a hydrogen atom, one equivalent of a cation of an organic or inorganic base or an alkyl radical containing from 1 to 1 2 carbon atoms, which is optionally substituted by one or more halogen atoms or hydroxy radicals, R2 and R3, which may be identical or different, each represent a hydrogen atom, an alkyl radical containing from 1 to 6 carbon atoms which is optionally substituted by one or more hydroxy, unsubstituted amino or alkoxy radicals containing from 1 to 4 carbon atoms, a phenyl radical which is optionally substituted by one or more substituents selected from halogen atoms, alkyl radicals containing from 1 to 4 carbon atoms and alkoxy radicals containing from 1 to 4 carbon atoms, or an alkenyl radical containing from 2 to 4 carbon atoms, Z represents a halogen atom, and R represents an alkyl radical containing from 1 to 4 carbon atoms, which process comprises reacting, at an elevated temperature and a pressure lower than the vapour pressure of water at the reaction temperature, an aqueous or aqueous-organic solution of an optically active alkali metal salt of a 2-halogenoalkanoic acid, containing from 3 to 6 carbon atoms with an aqueous solution of an alkali metal salt of a phenol of the general formula; Ar-OH Ill wherein Ar is as hereinbefore defined, to form an optically active alkali metal salt, wherein M represents an alkali metal cation, of a compound of the general formula::

(wherein Ar and R are as hereinbefore defined and M represents a hydrogen atom or an alkali metal cation,) optional acidification of the salt of formula II wherein M represents an alkali metal cation to form the corresponding acid of general formula II wherein M represents a hydrogen atom and optionally converting by methods known per se the acid obtained into an optically active compound of formula I wherein A is other than a group -COOR, in which R, is a hydrogen atom.

2. A process according to claim 1 in which Ar represents a phenyl radical substituted by one to three substituents, which may be identical or different, selected from halogen atoms and the methyl radical, R represents the methyl radical and A represents a carboxy group, and the optically active alkali metal salt of the 2-halogenoalkanoic acid is dextrorotatory in water.

3. A process according to claim 1 or 2 in which the aqueous-organic solution of the alkali metal salt of the 2-halogenoalkanoic acid is an aqueous-alcoholic solution.

4. A process according to claim 1 or 3 for the preparation of a compound of general formula II depicted in claim 1 wherein Ar, R and M are as defined in claim 1.

5. A process according to claim 1, 2, 3 or 4 in which the optically active alkali metal salt of the 2-halogenalkanoic acid is prepared in situ in the aqueous solution of the alkali metal salt of the phenol of general formula Ill, by introducing into the solution, simultaneously and in substantially stoichiometric amounts, an aqueous alkali metal hydroxide solution and an optically active alkyl 2-halogenoalkanoate of the general formula:

wherein Hal represents a halogen atom, R is as defined in claim 1 and R4 represents an alkyl radical containing from 1 to 6 carbon atoms, at an elevated temperature, under a pressure which is greater than the vapour pressure of the alkyl 2-halogenoalkanoate of general formula IV but less than the vapour pressure of water, at the temperature used.

6. A process according to claim 5 in which the elevated temperature is from 70 to 160"C.

7. A process according to claim 5 or 6 in which the alkali metal hydroxide solution is sodium hydroxide solution.

8. A process according to any one of claims 1 and 3 to 7 in which the alkali metal salt of the 2-halogenoalkanoic acid consists predominantly of the isomer having the L configuration.

9. A process according to claim 8 in which the alkali metal salt of the 2-halogenoalkanoic acid contains at least 90% of the L isomer.

10. A process according to any one of claims 1 and 3 to 9 in which the 2-halogenoalkanoic acid is 2-chloropropionic acid.

11. A process according to any one of the preceding claims in which the optically active alkali metal salt of 2-chloropropionic acid and the alkali metal salt of the phenol are sodium salts.

1 2. A process according to any one of the preceding claims in which the reaction between the sodium salt of the phenol and the optically active sodium salt of the chloropropionic acid is carried out at a temperature of from 70 to 160"C under a pressure less than or at most equal to atmospheric pressure.

13. A process according to claim 12, in which the pressure is from 0.1 to 1 bar.

14. A process according to claim 11 or 1 2 in which the aqueous or aqueous alcoholic solution of optically active sodium 2-chloropropionate contains at least 20% by weight of sodium 2-chloropropionate.

1 5. A process according to claim 14 in which the aqueous or aqueous-alcoholic solution contains from 20 to 50% by weight of sodium 2-chloropropionate.

1 6. A process according to any one of the preceding claims in which the aqueous solution of the alkali metal salt of the phenol of general formula Ill contains at least 50% by weight of the alkali metal salt.

1 7. A process according to claim 1 6 in which the aqueous solution contains from 50 to 90% by weight of the sodium salt of the phenol.

18. A process according to claim 1 6 or 1 7 in which the optically active sodium 2chloropropionate consists of at least 90% by weight of the isomer having the L configuration.

1 9. A process according to any one of the preceding claims in which the aryloxyalkanoic acid compound of general formula I is 2-(2-methyl-4-chlorophenoxy)propionic acid, 1-naphthoxypropionic acid or 2-[4-(2,4-dichlorophenoxy)phenoxyjpropionic acid of high optical purity in which the D-isomer is the preponderant isomer.

20. A process according to claim 1 substantially as hereinbefore described.

21. A process according to claim 1 substantially as hereinbefore described with especial reference to any one of Examples 1 to 11.

22. An aryloxyalkanoic acid compound of general formula I when prepared by a process claimed in any one of claims 1 to 21.

23. 2-(2-Methyl-4-chlorophenoxy)propionic acid, 1-naphthoxypropionic acid and 2-(4-(2,4 dichlorophenoxy)phenoxy]propionic acid of high optical purity in which the preponderant isomer is the D-isomer when prepared by a process claimed in any one of claims 1 to 21.