DD151443A5 - PROCESS FOR PREPARING 3,3-DIMETHYL-CYCLOPROPAN-1,2-DICARBOXYL ACID ESTERS - Google Patents

Abstract

The present invention relates to a novel process for the preparation of 3,3-dimethyl-cyclopropane-1,2-dicarboxylic acid esters of the formula I, in which R and R 'have the meaning given in the description, which is characterized in that 1 , 3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid ester of the formula II with zinc, if appropriate in the presence of a Verduennungsmittels, at temperatures between 20 and 200 degrees C and converts new 1,3-dichloro-2,2 -dimethyl-propane-1,3-dicarboxylic acid ester d. Formula II and methods and intermediates for their preparation.

Description

22 1758 -4-

Berlin, 14.5.1930 57 418/12

Process for the preparation of 3,3-dimethyl-cyclopropane-1,2-dicarboxylic acid esters

the invention

The invention relates to a novel process for preparing known 3α-dimethylcyclopropane-1,2-dicarboxylic acid esters.

Known e technical solu conditions

It has already been disclosed that cis and / or _trans-3} obtained 3-Mmethyl-cyclopropane-1,2-dicarboxylic acid dimethyl ester, v / hen men maleic and / or fumaric acid dimethyl ester with diphenylsulfonium isopropylid at - 70 C (compare J _e Arru Chem. Soc. 89 (1967), 3912-3914; ibid. 9 (1973), 3970-3976).

Diphenylsulfonium isopropylid is prepared by reaction of diphenyl-ethyl-sulfonium tetrafluoroborate with Lithiumdiisopropyl8jnid, followed by alkylation with methyl iodide and re-reaction with lithium diisopropylamide at -70 ⁰ C.

In place of diphenylsulfonium isopropylidene, it is also possible to use oil diphenylphosphonium isopropylidene. for the synthesis of 3,3-dimethyl-cyclopropane-1,2-dicarboxylic acid esters starting from maleic acid and / or fumaric acid esters are used (see Tetrahedron Lett » I978 1847-1850) . ·

22 17 58 -2-. 05/14/1980

57 418/12

The reactions with said ylides are carried out because of their sensitivity to aerated acid and moisture under an inert gas atmosphere and using carefully dried solvents; the same applies to the production of ylides; this are strong bases such _e B 'butyllithium and / or lithium diisopropylamide, used''.

The known synthesis methods for 3,3-dimethylcycloprop8Ji-1-dicarboxylic acid esters, which are carried out using the abovementioned ylides, are likely to be due to the high. Expenses - even in the production of precursors for the production of 31-3-dimethylcyclopropane-i, 2-dicarboxylic acid esters in the industrial sector be little suitable.

Because of the potential importance of 3,3-dimethyl-cyclopropane-1,2-dicarboxylic acid derivatives as intermediates for the preparation of pyrethroids, there has now been interest in a simpler and less expensive process, according to which these compounds in good x \ usbeuten and in high purity should be produced «

For the preparation of 3,3-dimethyl-cyclopropane-1, 2-di ~ carboxylic acid, a method has become known in which this compound by reacting 1-bromo-2,2-dimethyl-3-ethoxycarbonyl-propane-carboxylic acid with Potassium carbonate in alcohol and subsequent treatment of the reaction mixture with aqueous hydrochloric acid is obtained (see J "Chem. Soc _e (London) £ 7 (1899), 48-61 _β

22 17 58 -3- 14.5.1980

57 418/12

However, this process gives 3,3-dimethyl-cyclopropane-1,2-dicarboxylic acid only in unsatisfactory yields; Moreover, the starting product according to the prior art can be obtained only in low yield and is also heavily contaminated by by-products »

Object of the invention. ,

It is therefore the object of the invention to overcome the described disadvantages of the known methods. "

Essence of the invention

The invention has for its object to provide a novel process for the preparation of 3> 3-dimethylcyclopropane-1, 2-dicarboxylic acid esters ·

The subject of the present invention is therefore

(1) a novel process for the preparation of 3,3-dimethylcyclopropane-1,2-dicarboxylic acid esters of the formula I.

(D

H ₃ C CH ₃

in less.

R and S ^{f are the} same or different and are alkyl ^, characterized in that 1,3 ~ dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid ester of the formula II

22 1758 -4- 14.5.1980

.57 418/12

RO-OC-CH- C -CH-CO-OR '(II) Cl CH ₃ Cl -

in which . , ·. ·. ...

R and R ^{1 are the} same or different and are alkyl,

with zinc, optionally in the presence of a diluent, at temperatures between 20 and 200 C.

(2) Hay is also the 1,3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid ester of formula II

RO-OC-CH-- C -CH-CO-OR '(II)

Cl CH ₃ Cl

in which

R and R 'are the same or different and are alkyl;

The intermediates of the formula II can also be prepared by a novel process (3), which is characterized in that 2,2-dimethyl-propane-1,3-dicarboxylic acid derivatives of the formula III

CHO

1 ?

R'-CO-CH ₂ -C-CH ₂ -CO-R * (III)

CH

in which

R is hydroxy or alkoxy and

221758 «5- 'η. 5.1980

57 413/12

2 - 1

R is hydroxy or together with R is an anhydride oxygen atom,

with an acid chloride, optionally in the presence of a catalyst, and with a chlorinating agent at temperatures zVdLschen 0 and 150 C and converts the thus formed 1,3-BiChIOr ^, 2 ~ dimethyl-propane-1,3-dicarboxylic acid derivatives of formula IV

t 3

R ³ ~CO-CH-C-QH-CO-R ⁴ (IV)

Cl CH ₃ Cl

in which

R is chlorine or alkoxy and

R 1 is chlorine or together with R is an anhydride oxygen atom,

esterified by reaction with alcohols by conventional methods,

(4) hay are also the 1,3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid derivatives of the formula IV '

CH ₃

³⁴

Cl CH ₃ Cl "(IV)

2 2 17 5 8 - 6 -

in which

R is chlorine or alkoxy and

4 3

R is chlorine or together with R for an anhydride oxygen atom.

Surprisingly, by the process (1) according to the invention, combined with the process described under (3) for the preparation of new intermediates, 3,3-Diniethyl-cyclopropane-i, 2-dicarboxylic acid ester of the formula (I) substantially simpler and cheaper than by known methods produced in very good yields and in high purity. The new process is therefore more suitable than the hitherto known processes for the industrial preparation of 3,3-dimethyl-cyclopropane-i, 2-dicarboxylic acid esters.

If in process (3) the starting compound used is, for example, 3,3-dimethylpropane-1,3-dicarboxylic anhydride and the chlorinating agent is sulfuryl chloride, and the esterification agent is methanol, the course of the reaction in process (3) and the reaction of the resulting 1, 3-dichloro-2,2-dimethyl-propane-i, 3-dicarboxylic acid dimethyl ester with zinc in process (1) are sketched by the following Formelschem ^:

Le A 19 661

22 1758

H ₃ C-CH ₃ . , , ">

SO ₀ Cl ₀ Tl CH _x OH

^{2 2} c / S ⁵

₃ GH ^ O-OC-C-C-CH-CO-OCH, ^Ζχι _v CH, 0-0C _v , CO-OCH

P it'-> 7->

Cl CH ₃ Cl

· ". ^: " ·. · .. .. ·: .h, c

The novel (1,3-dichloro) -2,2,3-dimethylpropane-1,3-dicarboxylic acid esters to be used (3) are preferably defined by formula (II)

R is C ₁ -C ₄ -alkyl], in particular methyl or ethyl,

Examples include:

1 f 3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid dimethyl ester, diethyl ester, di-n-propyl ester, diisopropylate, di-i-butyl ester, di-1-one iso-butyl ester, di-sec-butyl ester, di-tert-butyl ester, methyl ethyl ester, methyl propyl ester, methyl isopropyl ester and diethyl-n-butyl ester,

receives the new 1 _r 3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid ester by the above under (3) surrounded surrounded by the reaction of 2,2-dimethyl-propane

Le A 19 661

22 1 758 - 8 -

1 f 3-dicarboxylic acid derivatives of the formula III. praise) with an acid chloride, if appropriate, in the presence of a catalyst, and with a chlorinating agent which is optionally identical to the acid chloride, at temperatures between 0 and 150 ⁰ C,

Examples of acid chlorides which are to be used in process (3) are phosphorus pentachloride, phosphorus oxychloride, phosphorus trichloride, thionyl chloride, sulfuryl chloride and phosgene. Preferably, phosphorus pentachloride, thionyl chloride and sulfuryl chloride are used.

Process (3) is optionally carried out using catalysts. As such, compounds come into consideration, which are usually used in the preparation of carboxylic acid chlorides as catalysts. These include in particular tertiary and heterocyclic amines such as, for example, triethylamine, N, N-dimethyl-aniline and pyridine and ammonium salts such as, for example, tetraethylammonium chloride, carboxylic acid amides such as, B. Dimethylformamide, dimethylacetamide and N-methyl-pyrrolidone, and further phosphorus compounds, e.g. Triphenylphosphine, triphenylphosphine oxide or hexamethylphosphoric triamide,

As a chlorinating agent, which in. Process (3) can be used, for example, sulfuryl chloride and elemental chlorine may be mentioned.

In a preferred variant (a) of process (3) is 2,2-dimethyl-propane-1,3-dicarboxylic acid anhydride

Le A 19 661

2 9 1 7 ^ ft

(β, ß-DimethyIglutarsäureanhydrid) with about the equimolar amount (o, 9 to 1.2 mol) of phosphorus pentachloride heated to a temperature between 8o and 14o ° C and at this temperature about 2 molar equivalents (1.9 to 2.2 mol ) Chlorine introduced into the mixture or 2 to 3 molar equivalents of sulfuryl chloride, added dropwise.

In a second preferred variant (b) of process (3) 2,2-dimethyl-propane-1 / S-dicarboxylic acid anhydride at temperatures between "Mo and 1 5o C with excess sulfuryl chloride (3 to 5 moles per mole Anhydride) added dropwise.

In a third preferred variant (c) of process 13) is a 2,2-dimethyl-propane-1, 3-dicarboxylic acid ester (mono-ester) (ß, ß-dimethylglutaric acid monoester) with about 1.2 to 3 molar equivalents of thionyl chloride , optionally in the presence of one of the above-mentioned catalysts, heated to a temperature between 4o and 9o C and treated at about 8o C with about 2 molar equivalents of 0.9 to 2.2 mol per mol of ester) Sulfurylchlorid dropwise.

In a fourth preferred variant (d) of process (3), 2,2-dimethyl-propane-1,3-dicarboxylic acid (β, β-dimethylglutaric acid) having about 2 molar equivalents: (1.9 to 2.2 Mole per mole of acid) phosphorus pentachloride at temperatures between 1o and 5o C-reacted. In the reaction mixture about 2 molar equivalents M, 9 to 2.2 mol) of chlorine are introduced at a temperature between 8o and 14o ° C and the mixture is a few hours at. Stirred 7o to 9o ° C.

Le A 19 661

22 1758 -ί-

The according to the process variants (3 a) to (3 d) obtained 1,3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid derivatives of the formula (IV) can by. Distillation or by careful removal of volatile components at moderately elevated temperature and greatly reduced pressure 'so-called' Andestillieren 'be cleaned. However, the intermediates of the formula (IV) can also be esterified without intermediate isolation, if appropriate after distilling off excess halogenating agent, by customary methods by reaction with alcohols at temperatures between 10 and 8 ° C.

For purification and isolation of the esters of formula (II) is either concentrated and the residue in a water-immiscible solvent such as, B. Ether, or the>: esterification mixture is diluted with water and with a water-immiscible solvent, such as, B. Ether or toluene extracted. The organic phase is washed neutral,

2q dried and concentrated. The remaining products in the residue can be purified by vacuum distillation.

The in process (3) resulting new 1,3-dichloro-2, ^^ dimethyl-propane-i, .3-dicarboxylic acid derivatives are by. Formula (.1.V) defined. Preferably it is in it

P,. for chlorine or C ₁ -C ₄ -alkoxy, in particular methoxy

Or ethoxy, and

4 3

R.. for chlorine or together with R for an anhydride oxygen atom.

Le A 19 661

22 1758

As examples may be mentioned;

1,3-dichloro-2 _/ 2-dimethyl-propane-1,3-dicarboxylic acid dichloride, 1,3-di-chloro-2,2-dimethyl-propane-1,3-dicarboxylic acid anhydride, 1.3 Dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid chloride T-methyl ester, ethyl ester, n-propyl ester, β-propyl ester, n-butyl ester, isobutyl ester, sec-butyl ester and tert-butyl ester.

The 2,2-dimethyl-propane-1,3-dicarboxylic acid derivatives to be used as starting materials in process (3) are defined by formula (III.). Preferably, B _{1 is} hydroxy, or C ₁ -C ₄ -alkoxy, especially methoxy or ethoxy, and

2 1

R for hydroxy or together with R for an anhydride oxygen atom,

Examples include:

2,2-dimethyl-1,3-dicarboxylic acid, 2,2-DimethyT-1,3-dicarboxylic acid anhydride, 2, 2-dimethyl-propane-1, 3-dicarboxylic acid ester -monomethy! - monoethyl ester, mono-n-propyl ester, mono-iso-propyl ester, mono-n-butyl ester, mono-iso-butyl ester, mono-sec-butyl ester and mono-tert-butyl ester,

2/2-Dimethyl-propane-i, S-dicarboxylic acid deriyate of formula (ΠΙ) are known (see J.O.rg.Ch.em. J_5 (195o), 85o and 855).

The process (1) according to the invention is preferably carried out using diluents. Particularly suitable are aprotic-polar sols. These include in particular

Le A 19 661

22 17 58

bonsäureamide such. Dimethylformamide, dimethylacetamide and N-methyl-pyrrolidone, sulphoxides and sulphones, e.g. Dimethylsulfoxide and tetramethylene sulfone, phosphoric acid amides such as z, B, hexamethylphosphoric triamide and ethers such as, glycol dimethyl ether, digiycol dimethyl ether, tetrahydrofuran and dioxane, and nitriles such as e.g. Acetonitrile and propionitrile.

The reaction temperature in the process (1) according to the invention is generally between 2o and 2ooo C, preferably between 8o and 15o C. The reaction is generally carried out at atmospheric pressure or a pressure adapted to the vapor pressure of the diluent.

For carrying out the process (1) according to the invention, to 1 mol of 1, 3-dichloro-2,2-dimethyl-propanl, 3-dicarboxylic acid ester (II) is between 1 and 2.5 mol, preferably between 1.5 and 2.2 Moles of zinc.

In a preferred embodiment of the invention ^; According to the process (1), the zinc is used in one of the abovementioned

2q given diluent to the required reaction temperature, preferably heated to 9o to 1lo ° C and treated dropwise with an ester of formula (II). The reaction mixture is stirred for a few hours at 1oo to 12o C after the exothermic reaction has subsided, cooled and filtered with suction. For purification and isolation of the reaction product, the filtrate is mixed with a water-immiscible solvent such as toluene, washed with dilute hydrochloric acid and then with water, dried and concentrated. The reaction

Le A 19 661

22 17 58 - 13

can through. Vacuum distillation to be cleaned. The boiling point is used for characterization,

The produced by the inventive process S / S-dimethyl-cyclopropari-i, 2-dicarboxylic acid esters can be used as intermediates for the preparation of insecticidally and acaricidally active pyrethroids (see Pestic Be 1_ (1976), 492-498;.. Tetrahedron Lett, 1978, 1847-1850),

Le A 19 661

22 17 58

Preparation Examples

Example 1 ; CH-,

CH O-CO-CH-C CH-CO-OCHo

3 ι,. - 3

Cl CH ₃ Cl

A mixture of 28 _f 4 g (o, 2 mol) ß, ß-dimethylglutaric anhydride and 41.6 g (o, 2 mol) of phosphorus pentachloride is heated to 115-12o C for half an hour. Then 8 g (o, 4 mol) of chlorine are introduced at l15-12o ° C and it is allowed to cool to 2o ° C. 3oo ml of methanol are then added dropwise to this mixture at 2 ° C., and the mixture is stirred at 2 ° C. for 12 hours. Then the methanol is removed. The residue is taken up in 3oo ml of ether. The ether solution is washed once with 2oo ml of 2% sodium carbonate solution and twice with 2oo ml of water, then dried over sodium sulfate and concentrated. The residue is distilled. This gives 48 g (93.5% of theory) of dimethyl 1,3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylate in the form of a colorless oil of boiling point 99-1o2 ° C / 1 mm Hg,

Example 2: "

* V "o

^2o CHoO-CO-CH C-CH-CO-OCH.,

- * III ~>

Cl CH ₃ Cl

A mixture of 28.4 g (ο, 2 mol) of β, ß-dimethylglutaric anhydride and 41.6 g (o _f 2 mol) .Phosphorpentachlorid is heated to l25-13o ° C for half an hour. Then 68 g (0.5 mol) of suiduryl are added to this reaction mixture.

Le A 19 661

2 21 7 58

Chloride added dropwise at a temperature of 125 C. After the sulfuryl chloride addition is completed, it is stirred at 13 ° C. for an hour and then allowed to cool to 2 ° C. Now 3oo ml of methanol are added dropwise to the reaction mixture and it is stirred for 12 hours at 2o ° C. Then the methanol is removed and the residue is mixed with 3oo ml of ether. The ether solution is washed once with 3oo ml of 2% sodium bicarbonate and twice with 2oo ml of water each time and then concentrated. The residue is distilled. This gives 42 g (82% of theory) of dimethyl 1,3-dichloro-2,2-dimethyl-propane-1/3-dicarboxylate in the form of a colorless oil, boiling point 1oo-lo2 ° G / 2. mm Hg.

Example 3; a)

To a melt of 28.4 g (o, 2 mol) ß, ß-dimethylglutaric anhydride are added dropwise at 125-l3o ° C Ho g (o, 8 mol) of sulfuryl chloride in the way that the reaction temperature is always between 125 and 13o C. (The large excess of sulfuryl chloride is necessary because at the high reaction temperature losses of sulfuryl chloride are caused by evaporation). After the addition of sulfuryl chloride is complete, the mixture is stirred at 13o ° C for Io minutes. Then it is distilled. This gives 4o g of 1, 3-dichloro-2 _/ 2-dime-f-hyl-propane-1, 3-dicarbonsäurea.nh.ydrid, which is directly further processing.

Le. A 19 661

2 2 1 7 58 - 16 -

CH ₃

b) CH ₃ O-CO-CH C-CH-CO-OCH ₃

Cl CH ₃ Cl

To 4o g (ο, 19 mol) of the 1,3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic anhydride prepared by process 3 a) are added dropwise at 25 C 3oo ml of methanol and 2 ml of concentrated sulfuric acid, then is heated under reflux for 2 hours. Subsequently, the procedure is as described in Example 1. This gives 42 g (82% of theory) based on ß, ß-dichloro-2,2-dimethylpropane <-1, 3-dicarboxylic acid dimethyl ester in the form of a colorless oil of boiling point 99-1o4 ° C / 2 mm Hg ,

Example 4:

CHOO-CO-CH C CH-CO-OCH.,

O Il I -J.

Cl CH ₃ Cl

87 g (0.5 mol) of β, β-dimethyl-glutaric acid monomethyl ester (see, for example, S. Burch et al., J. Chem. Soc. (London) 1952 , 1364) are added dropwise to 15 ml of thionyl chloride and then refluxed for 1 hour , 148.6 g (1.1 mol) of sulfuryl chloride are then added dropwise, with further boiling, over a period of about 3 hours, and the mixture is boiled for a further 2 hours after the end of the addition. The reaction mixture is freed from excess thionyl chloride in vacuo at about 5o C and then added dropwise to 25o ml of methanol, the temperature rises to the boiling point of methanol. The mixture is stirred for 1 hour and then distilled off the solvent in vacuo. The residue is dissolved in 4oo ml of toluene and washed once with sodium

Le A 19 661

2217 58

bicarbonate solution and washed twice with water. The organic phase is dried over sodium sulfate and concentrated in vacuo. The residue is distilled under high vacuum. This gives 94 g (73% of theory) of 1,3-dichloro-2 _/ 2-dimethyl-propane-1,3-dicarboxylic acid dimethyl ester as a colorless oil with the boiling point 8o-85 ^O C / o, o5 mm Hg.

Example 5:

1 Un o

  '!

.1, step: CH.O-CO-CH: C CH-CO-Cl

Cl CH ₃ Cl

87 g (o, 5 mol) of β, β-dimethyl-glutaric acid (s representation. SF, Birch et al., J, Ch.em, Soc (. London) 3952, 1364) to be 15o ml of thionyl chloride was added dropwise and then Boiled under reflux for 1 hour. 148.6 g (1.1 mol) of sulfuryl chloride are then added dropwise to further boiling within about 3 hours and the mixture is boiled for a further 2 hours after the end of the addition. The reaction mixture is freed from excess thionyl chloride in vacuo at about 5o ° C and then distilled in vacuo. Ma, n receives in this way Io4 g (% of theory) of 1,3-dichloro-2,2-diroethyl-propane-l, 3-dicarboxylic acid methyl ester chloride in the form of a colorless oil having the boiling point 12 8-132 ° C / 1o mm Hg,

CH ₃ 2 "stage: CH 2 O-CO-CH-C - CH-CO-OCH ^.

• 5 ι I -5.

Cl CH ₃ Cl

A mixture of 65.4 g (o, 25 mol) of the ester chloride described above and 15o ml of methanol is 2 hours

Le A 19 661

22 1758

Roughened at room temperature, poured into 5oo ml of water / and extracted 3 times with 1oo ml of toluene. The organic phase is washed first with 5o ml of saturated sodium bicarbonate solution and then with 50 ml of water, dried over sodium sulfate and evaporated in vacuo. The residue is distilled in vacuo. This gives 41.6 g (81% of theory) of dimethyl 1,3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylate having a boiling point of 134-138 ° C / 1o mm Hg.

Example 6 ; CH ₃

CH 2 O-CO-CH C CH-CO-OCH-.

Cl CH ₃ Cl

Ztx 131 g (o / 63 mol) of phosphorus pentachloride are added in portions 48.1 g (o, 3 mol) ß, ß-dimethyl-glutaric acid so that the temperature does not rise above 5o C. In the resulting mixture 44.7 g (o, 63 MoJ) chlorine is introduced at II0 C within 3 hours and stirred at 80 ⁰ C for 2 hours. After distilling off the volatiles, the product is added dropwise to 15o ml of methanol and stirred for 1 hour. The reaction mixture is mixed -2o with 300 ml of water and shaken twice with 25o ml of toluene. The organic phases are washed first with 100 ml of saturated sodium bicarbonate solution and then twice with 1 ml of water, dried over sodium sulfate and evaporated in vacuo. The residue is distilled in vacuo.

This gives 6.7 g (79 % of theory) of dimethyldichloro-1,3-dichloro-2,2-dimethy1-propane-1,3-dicarboxylate having a boiling point of 128-131 ° C./7 mm Hg.

Le A 19 661

22 17 58 - 19 -

Example 7:

CH ₃ O-COv y CO-OCH ₃

To a suspension of 13 g (O / 2 g-atom) zinc dust in 6o ml of dimethylformamide is added dropwise at about 1oo C 25.6 g (o, 1 mol) of 1,3-dichloro-2,2-dimethyl-propane-1 , 3-dicarboxylic acid. Upon addition, the temperature of the reaction mixture rises to about 12o ° C. After the end of the exothermic reaction, the temperature is kept for 2 hours at about 1oo ° C and then cooled to room temperature. The mixture is treated with 2OO ml of toluene and filtered with suction from the inorganic material. The filtrate is shaken once with 5o ml of 1o% hydrochloric acid and then twice with 1oo ml of water, it is dried over sodium sulfate and the solvent is distilled off in vacuo. By vacuum distillation of the residue to obtain 13.6 g (73% of theory) of dimethyl S ^ -dimethyl-cyclopropane-i, 2-dicarboxylate in the form of a colorless liquid, with the boiling point. 1o2-1o6 ° C / 1o mm Hg.

Le A 19 661

Claims (1)

claims 1) A process for the preparation of B ^^ dim propane-1,2-dicarboxylic acid esters of the formula I. RO-OC in which R and R ^{1 are the} same or different and for Alkyl, characterized in that 1, 3-dichloro-2,2-dimethyl-propane-1,3-dicarboxylic acid ester of the formula II CH ₃ RO-OC-CH C-CH-CO-OR ¹ (II) Cl "CH ₃ Cl in which R and R ^{1 are the} same or different and for Alkyl, with zinc, optionally in the presence of a diluent, at temperatures between 2o and 2oo C. Le A 19 661