Title of Invention

"PROCESS FOR THE PREPARATION OF 4,4-DIFLUORO-3-OXOBUTANOIC ACID ESTERS"

Abstract

The present invention relates to a process for the preparation of a compound of the formula (I), wherein R isCM2 alkyl, by the contact of a compound of the general formula (II). wherein RI and R2 are each, independently, CM2 alkyl; or RI and R2 join together with the nitrogen atom to which they are attached to form an alicyclic aniine ring containing 4 to 7 carbon atoms or a morpholine ring, with an acetic acid ester of the general formula (III) CH3COOR, wherein R is as defined under formula (1), in the presence of a base.

Full Text

PROCESS FOR THE PREPARATION OF 4, 4-DIFLUQRO-3-OXOBUTANOIC ACID ESTERS The present invention relates to a novel process for preparing 4,4-difluoromethyl-3-oxo-butanoic acid esters. These esters are useful for preparing 3-difluoromethyl-4-pyrazole carboxylic acid esters which are important intermediates for the manufacture of pyrazole carboxanilide fungicides. Various pyrazole carboxanilide fungicides and their preparation are described for example in US 5,498,624 and in WO 01/42223. The preparation of many of these fungicides requires the use of an ester of 3-difluoromethyl-l-methyl-4-pyrazole carboxylic acid. In US 5,498,624 the ethyl ester of this carboxylic acid is prepared by the reaction of methyl hydrazine and ethyl 2-(ethoxymethylene)-4,4-difluoromethyl acetoacetate in ethanol. This latter compound is prepared by the method described in JACS, 73, 3684 (1951), which involves the condensation of ethyl orthoformate and acetic anhydride with ethyl difluoroacetoacetate. The synthesis of methyl and ethyl difluoroacetoacetates, also known as the methyl and ethyl esters of 4,4-difluoro-3-oxo-butanoic acids, by reacting the corresponding fluorinated esters with acetic acid esters under basic conditions has been known since a long time and described in, for example, JACS, 69, 1819 (1947) and JACS, 75, 3152 (1953). When a weaker base, such as sodium ethylate, is used, the yields of this reaction are not satisfactory for commercial large-scale production processes. For example, the yield when using sodium ethylate is only 35%, as described in JACS, 69, 1819 (1947). It is known in the literature, that yields of those type of reactions can be increased by using a much stronger base, such as sodium hydride, see for example in JACS, 75, 3152 (1953), there it is reported that the yield can be increased up to 75-85% when using sodium hydride as a base. However, it is undesirable to use sodium hydride for commercial productions, because it is dangerous to work with on a large scale and presents the hazard of large amounts of explosive hydrogen gas. An alternative synthesis route is decribed in EP-A-694526. Here, methyl and ethyl polyfluoroacetoacetates are prepared by the reaction of a polyfluoro carboxylic acid chloride or anhydride with a carboxylic acid chloride in the presence of a tertiary amine base, such as pyridine. The reaction is completed by addition of an alcohol, such as methanol or ethanol. This synthesis route can be used conveniently for the production of trifluoroacetoacetates witt average yields of 52%, but it is unsatisfactory for the production of difluoroacetoacetates. The difluoroacetic acid chlorides or anhydrides are not sufficiently stable under these conditions. For example, EP-A-694526 describes the synthesis of methyl 2-difluoroacetylbutanoate by the reaction of difluoroacetic anhydride with butyryl chloride. The yield for this reaction is only 25% of theory. Such low yields are not acceptable for commercial production of chemical compounds. The aim of the present invention is therefore to provide a novel general process for the preparation of esters of 4,4-difluoro-3-oxo-butanoic acid, by means of which it is possible to prepare such compounds in high yields and good quality, by a simple reaction procedure and with low expenditure, without the above-mentioned disadvantages of the known processes. Thus, according to the present invention there is provided a process for the preparation of a compound of the formula (I) wherein R is €1-12 alkyl, which comprises contacting a compound of the general formula (II) wherein R| and R2 are each, independently, C\.n alkyl; or RI and RZ join together with the nitrogen atom to which they are attached to form an alicyclic amine ring containing 4 to 7 carbon atoms or a morpholine ring; with an acetic acid ester of the general formula (III) CH3COOR (HI), wherein R has the meaning given above, in the presence of a base. R is a branched or unbranched alkyl group containing from 1 to 12 carbon atoms and is, for example, methyl, ethyl, ??-propyl, /i-butyl, wo-propyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, ;z-hexyl, //-heptyl, n-octyl, H-nonyl,, n-decyl, /z-undecyl or n-dodecyl. Conveniently it is methyl or ethyl. RI and R2 are branched or unbranched alkyl groups containing from 1 to 12 carbon atoms and are, for example, methyl, ethyl, w-propyl, /i-butyl, wopropyl, sec-butyl, wo-butyl, ter/-butyl, H-pentyl, /i-hexyl, w-heptyl, «-octyl, n-nonyl,, w-decyl, /j-undecyl or n-dodecyl. They may be the same or different. Typically they are both methyl or both ethyl. Alternatively, RI and R2 join together with the nitrogen atom to which they are attached to form an alicyclic amine ring containing 4 to 7 carbon atoms or a morpholine ring. Examples of such alicyclic amine rings are pyrrolidine and piperidine. When RI and R2 join with the nitrogen atom to which they are attached to form a ring, the ring is conveniently a pyrrolidine or morpholine ring. R is preferably C\.& alkyl, more preferably methyl or ethyl. In a preferred embodiment RI and R2 are each, independently, Q-s alkyl; or RI and R2 join together with the nitrogen atom to which they are attached to form an alicyclic amine ring containing 4 to 7 carbon atoms or a morpholine ring. In one further preferred embodiment RI and R2 are each, independently, Ci-g alkyl, preferably both methyl or both ethyl. In another further preferred embodiment RI and R2 join together with the nitrogen atom to which they are attached to form an alicyclic amine ring containing 4 to 7 carbon atoms or a morpholine ring. In a particular preferred embodiment RI and R2 join with the nitrogen atom to which they are attached to form a pyrrolidine or morpholine ring. The process is conveniently carried out in a solvent, which may be an excess of the acetic acid ester (III) or a different solvent or a mixture of both. If it is a mixture of both, the acetic acid ester acts as a cosolvent. Suitable 'different' solvents include C|-Cg alcohols; aromatic or halogenated aromatic solvents such as toluene, xylene and chlorobenzene; and ethers such as tetrahydrofuran, dioxane and terf-butylmethylether. When the acetic acid ester (III) is used as the solvent or as a cosolvent, it is employed in a large excess, typically in excess of 10 molar equivalents (preferably 10-30 molar equivalents) of the compound of formula (II). Any suitable base may be used in the process of the invention, but it will usually be an alkoxide base, typically an alkali metal alkoxide base, such as an alkali metal CM alkoxide base. Examples are sodium methoxide, sodium ethoxide and sodium terf-butoxide. Preferably the base is sodium methoxide or sodium ethoxide. In order to optimise the yield of product (I), the amount of base used is from 1 to 4 molar equivalents of the compound of formula (II). The process is conveniently carried out at a temperature in the range of 15°C to 80°C, for example, from 45°C to 80°C, and typically from 50°C to 70°C. Thus, when an ethanolic solution of an alkoxide base is used with ethyl acetate as a cosolvent, the process maybe carried out from anywhere between ambient temperature and the reflux temperature of the combined solvents. The time the process takes will depend upon, inter alia, the scale of the preparation and the temperature at which it is carried out. For example, it may take from half an hour to 24hours. Typically a laboratory preparation on a less than a molar scale may take from 1 to 6 hours. Conveniently, the process is carried out by dissolving a compound of formula (II) in an acetic acid ester of formula (III), optionally in the presence of another solvent. An alcoholic or other solvent solution of the base is then added with stirring at ambient or elevated temperatures. The mixture is then heated to 50 to 70°C until the reaction is complete. After cooling, the mixture is poured into an acidified ice-water mix, and extracted with a suitable solvent such as diethyl ether or ethyl acetate. The product may then be recovered from the organic extract by washing with brine, evaporating the solvent and, if necessary, purifying the residual product by distillation under reduced pressure. The invention also embraces embodiments wherein mixtures of 4,4-difluoromethyl-3-oxo-butanoic acid esters are produced. For example, the use of ethylacetate as ester and sodium methoxide as base, leads to a mixture of 4,4-difluoromethyl-3-oxo-butanoic acid ethyl ester and 4,4-difluoromethyl-3-oxo-butanoic acid methyl ester. Compounds of the general formula (II) H ,NR1R2 (I]), wherein RI and R2 are each, independently, Q.^ alkyl; or RI and R2 join together with the nitrogen atom to which they are attached to form an alicyclic amine ring containing 4 to 7 carbon atoms or a morpholirie ring; may be prepared by the method described in JP-A-06228043. This involves the fluorination of an N,N-disubstituted dichloroacetic acid amide, the A^W-disubstituted dichloroacetic acid amide being prepared by the reaction of dichloroacetyl chloride with a secondary amine. The methodology is summarised in the following schematic diagram. ((Figure Remove) Acetic acid esters of the general formula (III) CH3COOR (III), wherein R is CM2 alkyl, are known and commercially available. The following non-limiting examples illustrate the invention in more detail. EXAMPLE 1 Preparation of 2,2-dichloro-NJsI-dimethyl acetamide. In a sulfonation flask, a solution consisting of dichloroacetyl chloride (1 lOg; 0.75mol) and toluene (100ml) was slowly added, over a period of Ihour, to a solution of dimethylamine (68g; l.Smol) and toluene (1.21) initially at 0°C, maintaining the temperature of the reaction mixture at below 10°C throughout. The reaction mixture was stirred for a further SOminutes at 0-5°C and was then gradually diluted by toluene (11). The organic phase was washed consecutively with water (IxSOOml), hydrochloric acid (5% solution; 2x500ml), water (IxSOOml), a saturated sodium bicarbonate solution (2x500ml) and finally brine (IxSOOml) and was then dried over sodium sulfate. Evaporation furnished a residue, which was distilled at high vacuum to yield 2,2-dichloro-7V,7/-dimethylacetamide as a colourless oil. Yield 78.6g (67.2%); b.pt. 65-67°C at 0.3mbar. EXAMPLE 2 Preparation of 2,2-difluQro-NJSf-dimethylacetamide. In a sulfonation flask, a mixture of 2,2-dichloro-7V,AL(Jimethylacetamide (23.4g; O.lSmol), spray dried potassium fluoride (26.Ig; 0.45mol) and diethylene glycol (150ml) was healed to 183°C at 160mbar in a distillation apparatus filled with a VIGREUX column (10cm). Under these conditions, the desired producl was distilled as a colourless oil over Ihour. Yield 12.3g (66.7%); b.pt. 105-108 °C at 160mbar. EXAMPLE 3 Preparation of 4,4-difluoro-3-oxo-butanoic acid ethyl ester. In a sulfonation flask, 7V,A^-diethyl-2,2-difluoroacetamide (1.51g; lOmmol) was dissolved in ethyl acetate (20ml) before ethanolic sodium ethoxide (15ml of a 21% solution; 40.2mmol) was added dropwise. The resulting mixture was stirred at 60°C for 6hours. After cooling, the mixture was poured into ice-water (20ml), acidified with hydrochloric acid (10%) and extracted with ethyl acetate. The organic phase was washed with brine, dried over sodium sulfate and evaporated in a water jet vacuum. The residue was purified by distillation under reduced pressure to give the desired 4,4-difluoro-3-oxo-butanoic acid ethyl ester in the form of a colourless oil. Yield 1.09g (66%); b.pt. 50-53°C at ISmbar. EXAMPLE 4 Alternative preparation of 4.4-difluoro-3-oxo-butanoic acid ethyl ester. In a sulfonation flask, sodium ethoxide in ethanol (79ml of a 21% solution; 0.243mol) was added dropwise to a solution of 2,2-difluoro-7V,jV-dimethyl-acetamide (27.2g; 0.22mol) in ethylacetate (460ml). The reaction mixture was heated at reflux temperature for Ihour and the disappearance of the starting material was monitored by GC. The reaction mixture was then poured in to ice-water (800ml), acidified with hydrochloric acid (10%) and then extracted twice with ethylacetate (2x200ml). After separation, the organic layer was washed with brine (200ml), dried over sodium sulfate and concentrated under reduced pressure (40°C at lOOmbar). Ethyl 4,4-difluoro-3-oxo-butanoic acid ethyl ester was obtained as a dark oil (34.8g; 72%) containing some ethanol as an impurity; the purity of the product was established as ca.75% by the use of GC. EXAMPLE 5 Preparation of a mixture of 4,4-difluoro-3-oxo-butanoic acid ethyl ester and 4,4-difluoro-3-oxo-butanoic acid methyl ester, In a sulfonation flask, sodium methoxide in methanol (165.7 g of a 30 % solution; 0.92 mol) was added dropwise to a solution of 2,2-difluoro-A^Af-dirnethyl-acetarnide (98.5 g; 0.8 mol) in ethylacetate (1570 ml) at 60 °C. The reaction mixture was heated at reflux temperature for 3 hours and the disappearance of the starting material was monitored by GC. The reaction mixture was then poured into cold hydrochloric acid ice-water (3 %, 1100 ml), and then extracted twice with ethylacetate (640ml). The combined organic layers were concentrated under reduced pressure (40°C at 1 SOmbar). A mixture of 4,4-difluoro-3-oxo-butanoic acid ethyl ester and 4,4-difluoro-3-oxo-butanoic acid methyl ester was obtained as a dark oil containing 81 % ethyl ester and 19 % methyl ester (121.8 g; 90% combined yield for both esters) containing some ethylacetate as an impurity. According to the present invention it is possible to prepare compounds of formula I in good yields and with little effort. A special advantage of the process according to the invention is that the starting compounds of formula II are readily obtainable and easy to handle. A further special advantage of the process according to the invention is that the starting compounds of formula III are commercially available, inexpensible and easy to handle. CLAIMS 1. A process for the preparation of a compound of the formula (I) wherein R is C\-\2 alkyl, which comprises contacting a compound of the general formula (II) wherein RI and R2 are each, independently, CM 2 alkyl; or Rj and Ra join together with the nitrogen atom to which they are attached to form an alicyclic amine ring containing 4 to 7 carbon atoms or a morpholine ring, with an acetic acid ester of the general formula (III) CH3COOR (III), wherein R is as defined under formula I, in the presence of a base. 2. A process according to claim 1 wherein R is methyl or ethyl. 3. A process according to claim 1 wherein RI and RT arc both methyl or both ethyl. 4. A process according to claim 1 wherein Rt and R2 join together with the nitrogen atom to which they are attached to form a pyrrolidine or morpholine ring. 5. A process according to claim 1 which is carried out in a solvent, the solvent being an excess of the acetic acid ester (III) or a different solvent or a mixture of both. 6. A process according to claim 1 wherein the different solvent is a Q-Cs alcohol; an aromatic or halogenated aromatic solvent; or an ether. 7. A process according to claim 1 wherein the amount of acetic acid ester (III) used is in excess of 10 molar equivalents of the compound of formula (II). 8. A process according to claim 1 wherein the base is an alkali metal alkoxide. 9. A process according to claim 1 wherein the alkali metal alkoxide is sodium methoxide or sodium ethoxide. 10. A process according to claim 1 which is carried out at a temperature in the range of 15°C to 80°C.

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