Dimethylketene β-lactone dimer could be produced through the following synthetic routes.

In a 500-ml. three-necked flask equipped with a thermometer, a mechanical stirrer, and a reflux condenser are placed 200 g. (1.43 moles) of tetramethyl-1,3-cyclobutanedione and 50 g. of chlorobenzene. The mixture is heated to 135° with stirring while a total of 1.8 g. of reagent grade anhydrous aluminum chloride is added in 0.3-g. portions over a 3-hour period. After the addition is complete, heating is continued for an additional 5 hours.

The reaction mixture is then cooled to 35–40° and poured into a stirred solution of 230 g. of sodium chloride and 6.0 g. of sodium acetate in 600 ml. of water at 40°. The mixture is stirred for 15 minutes and then is transferred to a separatory funnel; the layers are separated, and the lower, aqueous layer is discarded. The crude product is distilled at reduced pressure through a stainless steel spinning-band column. The yield of the β-lactone dimer of dimethylketene is 122–132 g. (61–67%); b.p. 69–71.5° (14 mm.). The product may be redistilled and the fraction boiling at 119.5–120° (150 mm.), n²⁰D 1.4380, collected.

Notice: Dimethylketene β-lactone dimer is a mild but deceptively persistent lachrymator.