5.11 g of trimethylsulfonium iodide (23.23 mmol, 4 eq.) are dissolved with stirring in 20 mL of DMSO in a 250 mL three-necked flask under a nitrogen atmosphere. 2.61 g of potassium tert-butoxide (23.23 mmol, 4 eq.) are added and the reaction medium is stirred for 3 hours. 2 g of ethyl 4-(4-tert-butoxycarbonylmethylenecyclohexyl)benzoate dissolved in 5 mL of DMSO are added and stirring is continued for 2 days. 2.56 g of trimethylsulfonium iodide (11.61 mmol, 2 eq.) are dissolved in 20 mL of DMSO in a 25 mL three-necked flask under a nitrogen atmosphere, and 1.0 g of potassium tert-butoxide (11.61 mmol, 2 eq.) is added with stirring. After 3 hours, this reaction medium is added to the 250 mL three-necked flask and stirring is continued for 18 hours. Ethyl acetate is added. The organic phase is washed with saturated aqueous ammonium chloride solution, dried over magnesium sulfate and filtered. The residue is chromatographed on silica gel with a gradient of ethyl acetate in heptane ranging from 10% to 50%. 0.159 g of tert-butyl 6-(4-ethoxycarbonylphenyl)spiro[2.5]octane-1-carboxylate is obtained.
