3,5-Dimethyl-4-carbethoxy-2-cyclohexen-1-one (CAS NO.: ) and (CAS NO.: ) could be produced through the following synthetic routes.

A. 3,5-Dimethyl-4-carbethoxy-2-cyclohexen-1-one. In each of three 500-ml. Erlenmeyer flasks is placed 210 ml. (210 g., 1.61 mole) of ethyl acetoacetate. The flasks are placed in an ice-salt bath and chilled to 0°; to each flask there is then added 45 ml. (35.2 g., 0.78 mole) of acetaldehyde. When the contents of the flasks have cooled to -5° to 0° there is added to each flask, with shaking, a solution of 2 ml. of piperidine in 5 ml. of absolute ethanol. The flasks, the contents of which become cloudy in a short time because of the separation of water, are kept in the ice-salt bath for 6 hours. The reaction mixtures are then combined in a 1-l. flask and placed in an icebox. About 24 hours later, 3 ml. of piperidine in 5 ml. of absolute ethanol is added with shaking, and the flask is replaced in an icebox until the next day. The addition of 3 ml. of piperidine in 5 ml. of absolute ethanol is repeated once more, and the mixture is again returned to the icebox for 24 hours. The mixture is then allowed to stand for at least 1 day at room temperature. At some point during this reaction period the mixture should crystallize as a mass of yellow-white needles. This product is crude ethyl ethylidenebisacetoacetate.

The crude bis ester is melted on a steam cone and poured into a 3-l. round-bottomed flask containing 600 ml. of glacial acetic acid, 40 ml. of concentrated sulfuric acid, and approximately 10 g. of small chips of porous plate. The mixture is heated under reflux for 1 hour. There is a copious evolution of carbon dioxide which should be directed to a gas-absorption trap since it is accompanied by acetic acid vapors. The mixture is poured, with mechanical stirring, into 2 l. of ice water in a 4-l. beaker. Enough ether is added to allow separation of the layers, and the organic layer is returned to the beaker with 1.2–1.3 l. of water. With the aid of good stirring the mixture is neutralized by slow addition of solid sodium carbonate until the effervescence ceases. The layers are separated, and the material so obtained may be used immediately for the preparation of 3,5-dimethyl-2-cyclohexen-1-one. To obtain 3,5-dimethyl-4-carbethoxy-2-cyclohexen-1-one, the ether solution is washed with 100 ml. of 5% sodium hydroxide solution and then with 100 ml. of water containing 2 ml. of acetic acid, is dried over anhydrous magnesium sulfate, and distilled through a short column under reduced pressure. After a very slight fore-run the main fraction is collected at 135–155°/10 mm. This is redistilled through a moderately good column, preferably of the Widmer or Vigreux type. The product is 3,5-dimethyl-4-carbethoxy-2-cyclohexen-1-one, b.p. 136–138°/9 mm. The yield is 220–234 g. (47–50%).

B. 3,5-Dimethyl-2-cyclohexen-1-one. The ether solution of crude 3,5-dimethyl-4-carbethoxy-2-cyclohexen-1-one obtained as described above is transferred to a 3-l. round-bottomed flask. The ether is removed on a steam cone, perferably with the aid of an aspirator, and there are added 1140 ml. of water, 60 ml. of ethanol (95%), and 130 g. of sodium hydroxide. The mixture is shaken continuously and heated on a steam cone until the alkali dissolves, and heating is continued on a steam cone with frequent shaking until the ester dissolves. The solution is then refluxed for 15 minutes.

The flask is cooled with a stream of water while a solution of 100 ml. of concentrated sulfuric acid in 200 ml. of water is added slowly and cautiously (Note 7). The acidified mixture is heated under reflux for 15 minutes, allowed to cool, and the layers are separated. The crude product is diluted with 100 ml. of ether and washed successively with two 100-ml. portions of 5% sodium hydroxide solution and with 100 ml. of water containing 5 ml. of acetic acid. The ethereal solution, after drying over anhydrous magnesium sulfate, is distilled through a short column under reduced pressure. The product boiling at 84–86°/9 mm. is 3,5-dimethyl-2-cyclohexen-1-one; the yield is 155–165 g. (52–55%)