At 0° C., a solution of 10.5 g of 3-methyl-2-oxohept-5-ynephosphonic acid dimethyl ester in 70 ml of dimethoxyethane is added dropwise to a suspension of 1.81 g of sodium hydride in 180 ml of dimethoxyethane; the mixture is stirred for one hour at 0° C. and then 7.4 g of finely pulverized N-bromosuccinimide is added thereto. The mixture is agitated for 30 minutes at 0° C., combined with a solution of 11.4 g of (1R,5S,6R,7R)-3,3-ethylenedioxy-7-benzoyloxy-6-formylbicyclo[3.3.0]octane in 90 ml of dimethoxyethane, and agitated for 2 hours at 0° C. The reaction mixture is poured on saturated ammonium chloride solution and extracted with ether. The organic extract is washed neutral with water, dried over magnesium sulfate, and evaporated under vacuum. Chromatography of the residue on silica gel yields, with hexane/ether (3+2), 8.2 g of the unsaturated ketone as a colorless oil.
