Process for the preparation of cyclic ketones

Abstract: The invention provides a process for the preparation of cyclic ketones of the general formula in which R represents an optionally substituted alkyl or cycloalkyl group and R¹ and R² independently represent an optionally substituted alkyl, cycloalkyl or aryl group, which comprises heating a compound of the general formula in which X represents a chlorine or bromine atom and R is as defined above, with a compound of the general formula in which R¹ and R² are as defined above, in the presence of an organic acid. Certain cyclic ketones of formula (I) are useful as intermediates in the preparation of certain fungicidally active cyclopentane derivatives.

Description: This invention relates to a process for the preparation of certain cyclic ketones, certain of which are useful as intermediates in the preparation of certain fungicidally active cyclopentane derivatives. Org. Synth., 53 , (1973), p. 48 et seq describes two methods for the preparation of 4,4-dimethylcyclohex-2-en-1-one. The first method consists of a base-promoted annelation reaction utilising methyl vinyl ketone and 2-methylpropanal (isobutyraldehyde). However, the yields obtained under these conditions are not particularly good and numerous byproducts are generated. A thorough study of this base-catalyzed condensation and the byproducts produced has been reported in J. Org. Chem., 44 , No 23, (1979), pp. 4050-4055. The second method involves converting 2-methylpropanal to an enamine, such as 1-(2-methylpropenyl)pyrrolidine, condensing the enamine with methyl vinyl ketone, hydrolysing the resulting Diels-Alder adduct and finally cyclising under acidic conditions. Better yields are obtained by this method than the first method but the second method is somewhat lengthy and complicated. J. Org. Chem., 45 , (1980), pp. 5399-5400 describes a third method for the preparation of 4,4-dimethylcyclohex- 2-en-1-one and certain other unsaturated cyclic ketones by condensing methyl vinyl ketone with 2-methylpropanal (or certain other aldehydes) under acidic conditions. However, in addition to the problems already stated, all these methods require the use of methyl vinyl ketone which is toxic and unstable and therefore difficult to handle. Moreover, vinyl ketones as a class generally exhibit properties of toxicity and instability. It has now been found that certain cyclic ketones can be prepared in good yield via a process which does not require the use of methyl vinyl ketone. According to the present invention there is therefore provided a process for the preparation of 4,4-dimethylcyclohex-2-en-1-one or a compound of the general formula in which R represents an optionally substituted alkyl or cycloalkyl group and R¹ and R² independently represent an optionally substituted alkyl, cycloalkyl or aryl group, which comprises heating 4-chlorobutan-2-one or a compound of the general formula in which X represents a chlorine or bromine atom and R is as defined above, with a compound of the general formula in which R¹ and R² are as defined above, in the presence of an organic acid. When the compounds of formula I contain an alkyl substituent group, this may be linear or branched and may contain up to 12, preferably up to 6, and especially up to 4, carbon atoms. A cycloalkyl group may contain 3 to 8, especially 3 to 6, carbon atoms. When any of the foregoing substituents are designated as being optionally substituted, the substituent groups which are optionally present may be any one or more of those customarily employed in the development of pesticidal compounds and/or the modification of such compounds to influence their structure/activity, persistence, penetration or other property. Specific examples of such substituents include, for example, halogen atoms, nitro, cyano, hydroxyl, cycloalkyl, alkyl, haloalkyl, alkoxy, haloalkoxy, amino, alkylamino, dialkylamino, formyl, alkoxycarbonyl, carboxyl, alkanoyl, alkylthio, alkylsulphinyl, alkylsulphonyl, carbamoyl and alkylamido groups. When any of the foregoing substituents represents or contains an alkyl substituent group, this may be linear or branched and may contain up to 12 preferably up to 6, and especially up to 4, carbon atoms. It is preferred that R represents a C₁₋₆ alkyl or C₃₋₆ cycloalkyl group. More preferably, R represents a C₁₋₄ alkyl group. It is also preferred that R¹ and R² independently represent a C₁₋₄ alkyl or C₃₋₆ cycloalkyl group. More preferably, R¹ and R² independently represent a C₁₋₄ alkyl group. It is especially preferred that R represents a methyl, ethyl or propyl group, R¹ and R² both represent a methyl group and X represents a chlorine atom. Preferably, the organic acid is an organic sulphonic acid such as methyl sulphonic acid or, especially, para-toluenesulphonic acid. It is further preferred that the reaction is carried out in a solvent which has the ability to remove water azeotropically from the reaction mixture. Suitable solvents include aromatic solvents and chlorinated solvents, such as 1,2-dichloroethane. The reaction is conveniently carried out at the reflux temperature of the solvent used. It is also preferred that the compound of formula II is prepared by reacting ethene with ethanoyl chloride or a compound of the general formula in which R and X are as previously defined, in a solvent in the presence of AlX₃ where X is as defined above. The solvent is preferably a chlorinated solvent, such as 1,2-dichloroethane, and it is particularly preferred that the solvent is the same as that to be used in the subsequent reaction of the compound of formula II thus obtained with a compound of formula III. Conveniently, the reaction is carried out at a temperature in the range from 0° to 30°C, preferably 5° to 25°C. Most preferably, the compound of formula II is prepared in situ. Preferably, it is then used in the reaction with a compound of formula III without further purification. Of the cyclic ketones that may be prepared by the process of the invention, 4,4-dimethylcyclohex-2-en-1-one is useful as an intermediate in the preparation of certain fungicidally active cyclopentane derivatives which form the subject of co-pending patent application EP-A2-0267778. Some of the intermediate compounds and process steps in the process used to synthesise compounds of EP-A2-0267778 from 4,4-dimethylcyclohex-2-en-1-one are the subject of copending European patent application nos. EP-A1-0359305 and EP-A-0357404. The invention is further illustrated by the following Examples. Example 1 Preparation of 4,4-dimethylcyclohex-2-en-1-one (R=H, R¹=R²=CH₃) Ethanoyl chloride (78.5g, 1 mole) was added over a period of 30 minutes to a suspension of aluminium chloride (140g, 1.05 moles) in 1,2-dichloroethane (390ml) whilst maintaining the temperature below 20°C by cooling. Ethene (35g, 1.25 moles) was then passed into the solution at 5°-10°C over a period of 2 hours and the subsequent mixture worked up by slowly pouring into 2M hydrochloric acid (900ml). The organic phase was separated and 2-methylpropanal (50.4g, 0.7 moles) and para-toluenesulphonic acid (0.4g, 21mmoles) were added. The mixture was then refluxed with azeotropic water removal until no more water separated. The organic phase was then washed with 10% (w/v) sodium hydroxide (250ml) by stirring at 60°C for 15 minutes to destroy any traces of 4-chlorobutan-2-one and the solvent flashed. Fractionation gave 70g 4,4-dimethylcyclohex-2-en- 1-one, b.pt. 70-72°C at 2666.44 Pa (20mm Hg). (Yield: 80%) NMR: (in CDCl₃ solvent, tetramethylsilane as reference). Characteristic peaks at:- δ (ppm): 1.16 (6H, singlet), 1.92 (2H, triplet, J 7Hz), 2.40 (2H, triplet, J7Hz), 5.76 (1H, doublet, J10Hz), 6.58 (1H, doublet, J10Hz). Example 2 Preparation of 2,4,4-trimethylcyclohex-2-en-1-one (R=R¹=R²=CH₃) Aluminium chloride (60.6g, 0.454 moles) was added to 1,2-dichloroethane (200ml) and the mixture cooled to about 0°C. Propanoyl chloride (40g, 0.432 moles) was then added over a period of 20 minutes and the temperature of the reaction mixture allowed to rise to 20°C to give a clear pale orange solution. This solution was stirred for 30 minutes and ethene (12g, 0.432 moles) was then bubbled into the solution over a period of about 1½ hours. The mixture was then quenched in a mixture of concentrated hydrochloric acid (65 ml) and ice (330 ml) and the organic phase separated and backwashed with water (2 x 50 ml). 2-Methylpropanal (22g, 0.3 moles) and para-toluenesulphonic acid (1.65 g, 8.64 mmoles) were then added and the mixture refluxed with azeotropic water removal until no more water separated. The organic phase was then washed with 10% (w/v) sodium hydroxide (200 ml) and the solvent removed. Distillation gave 30 g 2,4,4-trimethylcyclohex-2-en-1-one, b.pt. 40°C at 40.00 Pa (0.3mm Hg). (Yield: 72%) Example 3 Preparation of 2-ethyl-4,4-dimethylcyclohex-2-en-1-one (R=C₂H₅, R¹=R²=CH₃) Using 1-chlorohexan-3-one (20 g, 0.148 moles), 2-methylpropanal (10.8 g, 0.148 moles), para-toluenesulphonic acid (0.6 g, 3.0 mmoles) and 1,2-dichloroethane (100 ml), 6 g 2-ethyl-4,4-dimethylcyclohex-2-en-1-one, b.pt. 55°C at 53.33 Pa (0.4 mm Hg), (Yield: 27%) were prepared by a method entirely analogous to those described in Examples 1 and 2.

Claims: A process for the preparation of 4,4-dimethylcyclohex-2-en-1-one or a compound of the general formula in which R represents an optionally substituted alkyl or cycloalkyl group and R¹ and R² independently represent an optionally substituted alkyl, cycloalkyl or aryl group, which comprises heating 4-chlorobutan-2-one or a compound of the general formula in which X represents a chlorine or bromine atom and R is as defined above, with a compound of the general formula in which R¹ and R² are as defined above, in the presence of an organic acid. A process according to claim 1 in which R represents a C₁₋₆ alkyl or C₃₋₆ cycloalkyl group. A process according to claim 1 or claim 2 in which R¹ and R² independently represent a C₁₋₄ alkyl or C₃₋₆ cycloalkyl group. A process according to any one of the preceding claims in which R represents a methyl, ethyl or propyl group, R¹ and R² both represent a methyl group and X represents a chlorine atom. A process according to any one of the preceding claims in which the organic acid is an organic sulphonic acid. A process according to any one of the preceding claims in which the organic acid is para-toluenesulphonic acid. A process according to any one of the preceding claims in which the compound of formula II is prepared by reacting ethene with ethanoyl chloride or a compound of the general formula in which R and X are as defined in any one of the preceding claims, in a solvent in the presence of A1X₃ where X is as defined above. A process according to any one of the preceding claims in which the compound of formula II is generated in situ .

Inventors: BRINER PAUL HOWARD, BRINER, PAUL HOWARD
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1991
Main classification: C07C49
IPC codes: C07C45, B01J31, C07C49, B01J27, C07B61