A.
3,5-Dimethyl-4-carbethoxy-2-cyclohexen-1-one. In each of three
500-ml. Erlenmeyer flasks (Note
1) is placed
210 ml. (210 g., 1.61 mole) of ethyl acetoacetate (Note
2). 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 (Note
3). 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 (Note
4). 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 (Note
5), 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%).