A. 6-Chloromethyl-2,2-dimethyl-1,3-dioxen-4-one. A three-necked, 500-mL, round-bottomed flask is fitted with a nitrogen inlet, a rubber septum, and a 125-mL dropping funnel. The flask is flame-dried, flushed with nitrogen and charged with diisopropylamine (22.0 mL, 0.16 mol) and 100 mL of tetrahydrofuran (THF) (Note 1). This solution is cooled in an ice bath, and the dropping funnel is charged with a solution of butyllithium (80.0 mL of a 1.88 M solution in hexane, 0.15 mol), which is added dropwise over 15 min (Note 2). The resulting solution is cooled to approximately −75°C in a dry ice–acetone bath and treated with a solution of 2,2,6-trimethyl-1,3-dioxen-4-one² (16.0 g, 0.11 mol) in tetrahydrofuran (20 mL) dropwise over 20 min (Note 3). During the addition, a fine yellow suspension forms. The enolate solution is stirred at −75°C for an additional 15 min and then transferred via cannula to a 1-L flask containing hexachloroethane^{3 4}; ⁵ (39.0 g, 0.16 mol) (Note 4) in tetrahydrofuran (150 mL) at −50 to −55°C (dry ice–acetone bath) over 30 min. When the addition is complete, any residual enolate is transferred with an additional portion of tetrahydrofuran (20 mL). The resulting reaction mixture is allowed to warm slowly to −25°C over 30 min and poured into ice-cold aqueous 10% hydrochloric acid (200 mL), and the mixture is briefly shaken to discharge the red color. The organic layer is separated, and the aqueous layer is extracted with ether (2 × 100 mL). The combined organic extracts are washed with saturated aqueous sodium bicarbonate solution (100 mL), saturated aqueous sodium chloride solution (100 mL), dried over sodium sulfate, and concentrated under reduced pressure to afford 31.50–35.50 g of an oily solid. Column chromatography on Florisil (100–200 mesh, 400 g) (Note 5) and elution with hexane (1 L) and 20% ethyl acetate–hexane (2 L), gives 12.17–12.67 g (63–65%) of the desired product as a yellow oil (Note 6),(Note 7),(Note 8).

B. 6-Diethylphosphonomethyl-2,2-dimethyl-1,3-dioxen-4-one. A 500-mL, three-necked flask is outfitted as above, flushed with nitrogen, and charged with potassium tert-butoxide (21.0 g, 0.187 mol) and dimethylformamide (200 mL) (Note 9). The stirring mixture is cooled in an ice bath and treated with diethyl phosphite (26.7 g, 0.193 mol). The resulting solution is stirred in the ice bath for 20–40 min and then treated dropwise with a solution of 6-chloromethyl-2,2-dimethyl-1,3-dioxen-4-one (11.00 g, 0.062 mol) in tetrahydrofuran (50 mL) over 20 min. The resulting purple solution is stirred for an additional 15 min at 0°C and treated with concentrated hydrochloric acid dropwise until the purple color is discharged (ca. 6 mL). The resulting mixture is filtered by suction through Celite (Note 10), and the collected solids are washed with tetrahydrofuran (50 mL). The combined organic portions are treated with several grams of anhydrous potassium carbonate and filtered, and the tetrahydrofuran is removed with a rotary evaporator. Dimethylformamide and excess diethyl phosphite are removed by distillation at 0.4 mm with the bath temperature maintained below 50°C (Note 11). The residue is diluted with ethyl acetate (200 mL) and placed in the refrigerator at 0°C overnight. The solid that precipitates is removed by filtration, and the filtrate is concentrated under reduced pressure to ca. 75 mL and purified by flash chromatography (Note 12) and (Note 13) on 700 g of Florisil (9 × 22-cm column). Elution with 3 L of 1 : 1 ethyl acetate–hexane, 3 L of 3 : 1 ethyl acetate–hexane, and then 3 L of 100% ethyl acetate affords 8.30–8.56 g (48–50%) of 6-diethylphosphonomethyl-2,2-dimethyl-1,3-dioxen-4-one (Note 14) and (Note 15). Mixed fractions may be rechromatographed to afford an additional 2–4% of product.