Esters as Enolates 1,4-Cyclohexanedione and Meerweins Ester

The utility of base catalyzed condensations of esters to give jS-ketoesters is well known. A straightforward example of this reaction is the intermolecular cyclization of diethyl succinate giving 2,5-dicarbethoxy-l,4-cyclohexanedione, which can in turn be easily decarboxylated to give 1,4-cyclohexanedione. 

A second example of the use of esters as enolates is the formation of Meerwein s ester, an intermediate in the synthesis of substituted adamantanes (6). Dimethyl 

5-Dicarbethoxy-1,4-cyclohexanedione (7) A 500-ml, three-necked, round-bottom flask is fitted with a condenser (drying tube) and arranged for magnetic stirring. A solution of sodium ethoxide is prepared in the flask by the addition of sodium (9.2 g, 0.4 g-atom) in small pieces to 90 ml of absolute ethanol. The mixture is heated at reflux for 3 hours (oil bath) to complete the reaction. Diethyl succinate (34.8 g, 0.2 mole) is added to the hot solution in one portion exothermic ) and the mixture is refluxed for 24 hours. (A pink precipitate forms and persists during the reflux.) 

At the conclusion of the reflux period, ethanol is removed on a rotary evaporator. Sulfuric acid (200 ml of 2 A solution) is added to the residue, and the mixture is stirred for 3-4 hours. The solid product is collected by filtration, washed with several portions of water, and allowed to dry in the air. It is then dissolved in 150 ml of boiling ethyl acetate, then filtered hot. The filtrate is cooled in ice, affording 16-17 g of 2,5-dicarbethoxy-l,4-cyclohexanedione, mp 126-129° as off-white crystals. 

4-Cyclohexanedione (5) 2,5-Dicarbethoxy-l,4-cyclohexanedione (10 g) is suspended in a solution of 34 g of 85 % phosphoric acid, 250 ml of water, and 5 ml of ethanol in a 500-ml round-bottom flask. The mixture is refluxed for 5 days (or until all the solid material has dissolved), cooled, and extracted six times with 100-ml portions of chloroform (or better, continuously extracted with chloroform). The combined chloroform extracts are dried (sodium sulfate) and the solvent is removed (rotary evaporator). The residue on distillation affords 1,4-cyclohexanedione, bp 130-133720 mm. The product solidifies and may be recrystallized from carbon tetrachloride, mp 11-19°. 

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