A. 1-Bromo-3-chloro-2,2-dimethoxypropane. In a good hood, a 1-l., three-necked, round-bottomed flask equipped with magnetic stirrer and reflux condenser is charged with 300 ml. of anhydrous methanol, 111 g. (1.00 mole) of 2,3-dichloro-1-propene (Note 1), and a few drops of concentrated sulfuric acid. With stirring, 178 g. (1.00 mole) of N-bromosuccinimide is added in small portions through the condenser. After the final addition, the reaction mixture is stirred for another hour at room temperature before 5 g. of anhydrous sodium carbonate is added to neutralize the catalyst. The solution is stirred for an additional 15 minutes and poured into a large separatory funnel containing 300 ml. of water. The lower, organic layer is removed, and the aqueous layer is extracted with two 500-ml. portions of pentane. The combined organic extracts are washed twice with an equal volume of water, dried over anhydrous magnesium sulfate, filtered, and evaporated, giving a white semicrystalline mass, which is dissolved in refluxing pentane (250 ml.). The solution is cooled in an acetone–dry-ice bath for 30 minutes, yielding 89–99 g. (41–45%) of the white crystalline ketal, m.p. 69.5–70.5° (Note 2).

B. 3,3-Dimethoxycyclopropene. A 500-ml., three-necked, round-bottomed flask is equipped with a magnetic stirrer, a gas-inlet tube, a thermometer, and an acetone–dry-ice condenser topped with a drying tube containing sodium hydroxide pellets. An acetone–dry-ice bath is placed under the flask, and ammonia is condensed into the flask from a commercial cylinder. When 350–400 ml. of ammonia has condensed, the inlet tube is replaced by a stopper, and a small piece (0.5 g.) of potassium metal is added to the ammonia. The cooling bath is removed, and ca. 0.05 g. of anhydrous iron(III) chloride is added. When the ammonia reaches reflux temperature, the blue color of the dissolved potassium turns to gray, and the remainder of the potassium (11.7 g., 0.300-g.-atom total) is added in 0.5-g. pieces at such a rate that a gentle relux is maintained. The stopper is then replaced by an addition funnel containing a solution of 1-bromo-3-chloro-2,2-dimethoxypropane (21.7 g., 0.100 mole) in 50 ml. of anhydrous diethyl ether, which is added to the gray potassium amide–ammonia suspension over a period of 15 minutes, during which time the mixture is maintained at −50° to −60° with the cooling bath (Note 3). After 3 hours at this temperature, solid ammonium chloride (10.8 g., 0.20 mole) is added with stirring. Ammonia is allowed to evaporate by removing the cooling bath, and during the course of the evaporation it is replaced with 350 ml. of anhydrous ether. When the reaction temperature reaches ca. 0°, the resulting brown solution is filtered from inorganic salts and placed in a 500-ml., round-bottomed flask (Note 4). The ethereal solution is then subjected to a vacuum (50–80 mm.) applied through a carbon tetrachloride–dry-ice condenser (ca. −25°), while the flask is immersed in an ice bath. After 4–5 hours, when the quantity of residue seems to remain constant, the dry-ice condenser is replaced with a distilling head. The pressure is decreased to 1–2 mm., the receiver is maintained at −78° with a cooling bath, and distillation yields 4.0–6.5 g. (40–65%) of 3,3-dimethoxycyclopropene as a clear liquid (Note 5). This material has been purified further,² but it can be used directly in the next step. If it is stored, it should be kept below 0°.

C. Cyclopropenone. A stirred solution of 3.0 g. (0.030 mole) of 3,3-dimethoxycyclopropene in 30 ml. of dichloromethane, cooled to 0°, is treated dropwise with 5 ml. of cold water containing 3 drops of concentrated sulfuric acid. The reaction mixture is stirred at 0° for an additional 3 hours before 30 g. of anhydrous sodium sulfate is added in portions, with stirring, to the 0° solution. The drying agent is removed by filtration, and the solvent is evaporated at 50–80 mm. with a water bath maintained at 0–10°. The brown, viscous residue is then distilled at 1–2 mm, at a water bath temperature of 10°. The distillate, a mixture of methanol and dichloromethane, is collected in a receiver cooled to −78°. A new receiver is attached, and the bath temperature is gradually raised to 35° (Note 6), yielding 1.42–1.53 g. (88–94%) of cyclopropenone as a white solid, b.p. 26° (0.46 mm.), m.p. −29 to −28° (Note 7).

Cyclopropenone prepared in this way is quite pure and suitable for most chemical purposes. It can be repurified by crystallization from 3 volumes of ethyl ether at −60° using a cooled filtering apparatus. The residual ethyl ether is then removed by evaporation at 1–2 mm. and 0°; very pure cyclopropenone is obtained in 60–70% recovery from the distilled material.