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The general procedure for the synthesis of ethyl 2-(1,4-dioxaspiro[4.5]dec-8-ylidene)acetate from 1,4-cyclohexanedione monoethyleneglycol ketal was carried out as follows: triethyl phosphonate (44.8 g, 200 mmol) was dissolved in THF (30 ml) at 0 °C, followed by the addition of a 1 M solution of sodium bis(trimethylmethylsilylamino)THF (200 ml). The reaction mixture was stirred at room temperature for 0.5 h and then cooled to 0°C again. A solution of 1,4-cyclohexanedione monoethylene glycol ketal (15.6 g, 200 mmol) in THF (50 ml) was added slowly dropwise. After the dropwise addition, the reaction solution was continued to be stirred at room temperature for 18 hours. Upon completion of the reaction, the mixture was cooled to 0 °C and the reaction was quenched with cold aqueous citric acid, followed by extraction with EtOAc. The organic phase was washed sequentially with saturated aqueous NaHCO3 solution, brine and dried with Na2SO4. The dried organic phase was filtered and concentrated, and the crude product obtained was purified by silica gel column chromatography with CH2Cl2/EtOAc gradient elution to finally obtain the target product, ethyl 2-(1,4-dioxaspiro[4.5]deca-8-ylidene)acetate (21 g, 91% yield).

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