DE1924844A1 - Process for the preparation of trimethyl-2-cyclohexen-1-ones - Google Patents

Description

Badische Anilin- & Soda-Fabrik AG 1924844

Our reference: G.Z. 26 185 Ste / Be 6700 Ludwigshafen, May 14, 1969

Process for the preparation of! Drimethyl-2-cyelöhexen-l-one

The present invention relates to a process for the preparation of trimethyl-2-cyclohexen-1-ones by the action of strong aqueous acids on corresponding 6-alkoxy-5 _f 6-dihydro-4H-pyrans.

It is known from US Pat. No. 3,287,372 to use 2-isopropyl-5-methyl-2-cyclohexen-1-one in such a way that you first 2-ithoxy-3,4-dihydro-6-isobutyl-4-iaethyl - 2H-pyran Action of aqueous acids on 3,7-dimethyl-octan-5-one-l-al hydrolyzed and then in a second stage the isolated product by the action of aqueous alkali to give 2-isopropyl-5-methyl-2-cyclohexen-1-one cyclized.

It has now been found that irimethyl-2-Gyelohexen-l-one the general formula

R ²

in which R ¹ and R are different and represent a methyl group or a hydrogen atom, can be prepared in a simple manner if one uses 6-alkoxy-5,6-dihydro-4H-pyrans of the general formula

R ²

OH ₃ -CH ₂

12 3

in which R and R have the abovementioned meaning and R stands for an aliphatic hydrocarbon radical, allows strong acids to act ^{at temperatures between 0 and 250 ° C.}

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According to the new process, the trimethyl-2-cyclohexen-l-ones obtained from the associated 6-alkoxy-5,6-dihydro-4H-pyrans in one step in excellent yields and in high purity. It was surprising that 2-cyclohexen-1-ones were obtained under the reaction conditions in the process according to the invention because it is known from the US patent mentioned that when 2-ethoxy-3,4-dihydro-6-isobutyl-4-methyl-2H-pyran is acted upon by aqueous acids the formation of the corresponding 2-isopropyl-5-methyl-2-cyclohexen-1-one is not observed.

In the 6-alkoxy-5,6-dihydro to be used as starting materials

■ 5. 4H-pyrans, the aliphatic ^ hydrocarbon radical R in the 'generally 1 to 20, preferably "1 to 6 carbon atoms. It can carry noch.inerte substituents such as halogen atoms, alkyl groups, however, are preferred.

Strong acids are used for the process according to the invention. As a rule, those strong acids are used whose dissociation constant at 20 ^° C. is more than 10 ", preferably more than 10. Mineral acids such as sulfuric acid, phosphoric acid, perchloric acid, hydrogen chloride, hydrogen bromide, hydrogen iodide are also used acids, in general, those which have a higher than the above-mentioned dissociation constant used), for example, sulfonic acids such as p-toluenesulfonic acid or oxalic acid. in general, using the strong acids as aqueous solutions. the concentration of the strong aqueous acid is advantageous at least 0.05 n, in particular 0.1 n. " The upper concentration limit can vary within a wide range. It is advantageous to use 0.1 to about 15 N, in particular 1 to about 5 normal strong aqueous acids. The weight ratio of starting material to the strong aqueous acid is generally 5: 1 to 1: 10, preferably 2: 1 to 1: 5 to be used, the anhydrous acids generally being used in amounts of 0.1 to 50 percent by weight, based on the starting material.

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Jür the implementation applies to temperatures between 0 and 250 ⁰ G, preferably between 20 ° and 10O ⁰ C, to. The reaction is generally carried out "at atmospheric pressure. However, it is also possible to use elevated pressure," for example up to 10 atmospheres or reduced pressure, for example 600 torr. Depending on the reaction temperature and the concentration of the strong aqueous acid, the reaction generally takes from 0.01 "to 24 hours.

When aqueous acids are used, the reaction is generally carried out without the use of inert organic solvents. However, it is also possible to use inert organic solvents in addition to the aqueous acid, water-soluble ones being inert organic solvents are preferred. Suitable organic solvents are, for example, lower aliphatic carboxylic acids, lower aliphatic alcohols, cyclic ethers, lower aliphatic sulfoxides. Examples include acetic acid, Propionic acid, chloroacetic acid, methanol, ethanol, isopropanol, Isobutanol, tetrahydrofuran, dioxane, dimethyl sulfoxide .. The weight ratio of strong aqueous acid to inert organic solvent is generally 5! 1 to 1: 5.

The reaction using essentially anhydrous acids is advantageously carried out in an inert organic solvent, preference being given in addition to the solvents mentioned aromatic, optionally halogenated hydrocarbons, such as benzene, toluene, xylene, chlorobenzene, or lower aliphatic ones Ethers, e.g. diethyl ether, can be used.

The 6-alkoxy-5,6- * dihydro-4H-pyrans to be used as starting materials are obtained, for example, by adding suitable vinyl ethers to corresponding oC, ß-unsaturated in a manner known per se Oxo compounds added (see Houben-Weyl, methods of Organic Chemistry, Volume 6, (1966), Part 4, pages 355 ff.) In general, vinyl ethers are used which carry the above-mentioned radical R- on the oxygen atom. Suitable starting materials will be obtained, for example, by addition of alkyl vinyl ethers and 3-methyl-2-hexen-4-one or by addition of 2-methyl-l-penten-3-one and alkyl isopropenyl ethers. For the diene synthesis,

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the dieoG, β-unsaturated oxo compound and the vinyl ether are generally reacted in a molar ratio of 1: 1 "to 1:20, preferably 1: 2" to 1: 5. The components can be reacted as such or in the presence of an inert solvent. Suitable solvents are "for example aliphatic ^ or aromatic, optionally nitrated or halogenated hydrocarbons, ethers with 4 to 10 carbon atoms, lower aliphatic ^ sulfoxides. Examples include: ligroin, cyclohexane, benzene, toluene, chlorobenzene, nitrobenzene, diethyl ether, tetrahydrofuran, dioxane, dimethyl sulfoxide. the diene addition is generally carried out at temperatures between 100 and 300 ⁰ C, preferably between 150 and 25O ⁰ C. as a rule, is carried out at pressures of from 1 atm to 200 atm, for example when the pressure at the sets the respective reaction temperature in the closed reaction vessel as the sum of the partial pressure of the reactants.

It is advantageous to add a polymerization inhibitor such as hydroquinone or hydroquinone monomethyl ether to the starting materials for the diene addition to stabilize.

The diene addition takes 1 to 50 hours, depending on the reaction temperature in claim. The reaction mixture obtained is worked up, for example, by fractional distillation.

The 6-alkoxy-5,6-dihydro ^ 4H-pyrans to be used as starting materials are characterized by interesting odor notes and are therefore suitable as fragrances, e.g. in perfumes, soaps, detergents and other cosmetic products.

Leave the trimethyl-2-cyclohexen-l-ones obtained according to the invention dehydriereii in high yield to 2,3,6-trimethylphenol. The dehydrogenation is described in German patent applications P 16 68 874.7 and P 17 93 037.9. The thereby obtained 2,3,6-Trimethylphenol is practically free from isomers and by-products and is therefore excellent as an intermediate for manufacture of vitamin E. Vitamin E can be produced from 2,3,6-trimethylphenol, for example, in such a way that one first the 2,3,6-trimethylphenol, e.g. according to the procedure

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the German patent specification 1 022 209 or the Czechoslovak one Patent specification 101 750, converted into trimethylquinone, then hydrogenated the trimethylquinone to trimethylhydroquinone, for example by the method of German Patent 683 908 and finally the trimethylhydroquinone obtained for Converts vitamin E, e.g. according to the method of Belgian patent specification 583 648.

example 1

A mixture of 100 g of 3,6-dimethyl-2-ethyl-6-methoxy-5,6-dihydro-4H-pyran, which was obtained in the manner described in paragraph 2, and 100 ml of 5 η hydrochloric acid is added for 1 hour stirred at 50 ⁰ C. The initially inhomogeneous mixture is homogenized, and then 2,3,6-trimethyl-2-cyclohexen-1-one separates out as an organic layer. After adding 250 ml of diethyl ether, the ethereal layer is separated from the aqueous layer and dried with sodium sulfate. After the ether has been stripped off, the residue is ^{distilled over at 89 to 90 °} C. and 12 torr. 73 g (90% of theory) of 2,3,6-trimethyl-2-cyclohexen-1-one are obtained.

A mixture of 300 g of 2-methyl-1-penten-3-one, 1,000 g of methyl isopropenyl ether and 2 g of hydroquinone is kept in a shaking autoclave at a temperature of 200 ^° C. for 12 hours under a nitrogen atmosphere. The reaction mixture obtained is fractionated after cooling to room temperature, distilled, ^{880 g of methyl isopropenyl ether being recovered at a temperature between 36 and 39 °} C. at atmospheric pressure and 129 g of 2-methyl-1-penten-3-one at a pressure of 50 torr. Finally, 270 g of 3,6-dimethyl-2-ethyl-6-methoxy-5,6-dihydro-4H-pyran distilling over at 12 Torr and a temperature of 72-74 ⁰ C. Based on 2-methyl-1-penten-3-one, the conversion is 57 ° and the yield is 91 % of theory.

Example 2

A mixture of 200 g of 3,4-dimethyl-2-ethyl-6-ätho.xy-5,6-dihydro-4H-pyran, which was obtained as described in paragraph 2, and

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200 ml of 5 η hydrochloric acid is stirred for 30 minutes at 50 ⁰ G. * The initially inhomogeneous mixture is homogenized and then 2,5,6-trimethyl-2-cyclohexen-1-one separates out as an organic layer. After adding 500 ml of diethyl ether, the organic layer is separated off and dried with sodium sulfate. After the ether has been stripped off, the residue is distilled. 141 g (94 % of theory) of 2,5 _} 6-trimethyl-2-cyclohexen-1-one with a boiling point of 76 ⁰ G at 12 torr are obtained.

A mixture of 600 g of 3-methyl-2-hexen-4-one, 1,200 g of ethyl vinyl ether and 2 g of hydroquinone is kept under a nitrogen atmosphere in a shaking bomb for 20 hours at a temperature of 200 ° C. After cooling, the reaction mixture is kept fractionally distilled. at normal pressure at 36 ⁰ C 880 ethyl vinyl ether and at a pressure of 50 Torr at 73 to 78 ⁰ C 115 g of 3-methyl-2-hexen-one 4-recovered. Finally, distilled at 12 Torr at 85 to 87 ⁰ C 770 g of 3 «4-dimethyl-2-ethyl-6-ethoxy-5,6-dihydro-4H-pyran over. Based on 3-methyl-2-hexen-4-one, the conversion in this reaction is 81 $> and the yield $ 96.5.

Example 3

A mixture of 50 g of 3,4-dimethyl-2-ethyl-6-ethoxy-5,6-dihydro 4H-pyran and 100 g of 4n-phosphoric acid is under for 6 hours Heated to reflux. After cooling, 100 ml of benzene are added to the reaction mixture. The organic layer is separated and dried with sodium sulfate. After stripping off the benzene and distilling the residue, 35 g of 2,5,6-trimethyl-2-cyclohexen-1-one are obtained.

Example 4

50 g of 3,4-dimethyl-2-ethyl-6-ethoxy-5,6-dihydro-4H-pyran are dissolved in 100 ml of ethanol and, after the addition of 200 g of aqueous 15% hydrochloric acid, the mixture is stirred for 16 hours held at 25 ^° C. After extraction with 250 ml of benzene, the organic layer is dried with sodium sulfate. According to Abdestilla-

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tion of the solvent and distillation of the residue give 34 g of 2,5,6-trimethyl-2-cyclolaexen-1-one.

Example 5

50 g of 3,6-Mmethyl-2-ethyl-6-ethoxy-5,6-dihydro-4H-pyran are dissolved in 200 ml of benzene. Within an hour "is initiated a weak stream of hydrochloric acid in the solution under stirring at 25 ⁰ C. After one hour of standing at 25 ⁰ C, the reaction mixture is fractionally distilled to give 32 g of 2,3,6-trimethyl-2-cyclohexen-1. on.

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Claims (12)

- 8 - 'O.Z. 26 185 claims 1. Process for the preparation of trimethyl-2-cyclohexen-l-ones the general formula 12th
in which R and R are different and represent a methyl group . or are a hydrogen atom, characterized in that one is based on 6-alkoxy-5 »6-dihydro-4H-pyrans of the general formula CH, -CH / xlW ~~ \ J XXq 12 % in which R and R have the abovementioned meaning and R- stands for an aliphatic hydrocarbon radical, allow strong acids to act ^{at temperatures between 0 and 25O 0 C,} 2,6-alkoxy-5,6-dihydro-4-pyrans of the general formula 12th in which R and R are different and for a methyl group or a hydrogen atom and R ^ "is an aliphatic Eohlenwasserstofrrest with 1 to 20 carbon atoms stands. Badische Anilin- & Soda-FabrilE AGr 4L 0098 477 1890