A. Dimethyl 3-chloro-2-pentenedioate. A dry, 500-ml., three-necked, round-bottomed flask fitted with a ground-glass stopper, a condenser provided with a gas bubbler, a gas-inlet adapter attached to a nitrogen (or argon) source, and a magnetic stirring bar, is charged with 60.0 g. (0.297 mole) of diethyl acetone-1,3-dicarboxylate (Note 1). A steady, gentle flow of nitrogen is started through the reaction vessel (Note 2), and 65.0 g. (0.313 mole) of phosphorus pentachloride (Note 3) is added in thirteen, approximately equal portions through the stoppered joint to the neat diester at 3-minute intervals with vigorous stirring (Note 4). After the addition is complete, the reaction mixture is warmed to 40° in a water bath for 30 minutes. The red solution is cooled in an ice bath and poured onto ca. 100 ml. of ice in a 500-ml. Erlenmeyer flask immersed in an ice bath. A 1 : 1 mixture of water and dichloromethane is used to rinse traces of the product from the reaction vessel into the Erlenmeyer flask, and the resulting mixture is stirred for 15 minutes (Note 5). After separating the two layers, the aqueous phase is extracted with three 100-ml. portions of dichloromethane, and the combined organic extracts are dried over anhydrous sodium sulfate. Filtration through glass wool and removal of solvents with a rotary evaporator affords ca. 60 g. of a red oil, which is placed in a 500-ml., round-bottomed flask containing 20 ml. of concentrated sulfuric acid in 300 ml. of anhydrous methanol (Note 6), and the solution is refluxed using a heating mantle for 18 hours. Excess methanol (200 ml.) is distilled, and the residual yellow solution is cooled to room temperature and poured into 100 ml. of water. Sodium chloride is added to saturation, and the solution is extracted with eight 100-ml. portions of diethyl ether. The combined extracts are washed successively with 150 ml. of aqueous saturated sodium hydrogen carbonate and 150 ml. of aqueous saturated sodium chloride, dried over anhydrous sodium sulfate, and filtered. Concentration of the extract with a rotary evaporator affords a yellow oil which is distilled, yielding 33.5–34.4 g. (59–60%) of dimethyl 3-chloro-2-pentenedioate² as a colorless liquid, b.p. 50–60° (0.02 mm.) (Note 7).

B. Dimethyl 2,3-pentadienedioate. A 500-ml., three-necked, round-bottomed flask, equipped with a gas-inlet adapter, a 50-ml. addition funnel, a ground-glass stopper, and a magnetic stirring bar, is charged with 27.0 g. (0.145 mole) of the diester from Part A and 100 ml. of anhydrous tetrahydrofuran (freshly distilled from sodium). The flask is flushed with nitrogen (or argon), and a positive pressure is maintained while the contents are cooled to 0° in an ice–salt bath and stirred with an efficient motor. Triethylamine (22 ml., 0.16 mole, freshly distilled from calcium hydride) is added through the addition funnel over a 10-minute period, the gas-inlet adapter is replaced with a calcium chloride tube, and the mixture is stirred at 0–5° for 18 hours (Note 8). The precipitate is removed by vacuum filtration and washed with three 100-ml. portions of anhydrous diethyl ether. The combined filtrate and washings are washed successively with three 75-ml. portions of 0.1 N hydrochloric acid and 100 ml. of aqueous saturated sodium chloride, dried over anhydrous sodium sulfate, filtered, and concentrated with a rotary evaporator. The residual oil is distilled (Note 9), yielding 13.3–13.9 g. (61–64%) of dimethyl 2,3-pentadienedioate³ (Note 10), b.p. 58° (0.02 mm.).