A solution of 11 g (42 mmol) of diethyl 2-(cyclohexylamino)-vinyl-phosphonate in 60 ml of tetrahydrofuran are added dropwise under argon in the course of 10 min to a suspension of 2.52 g (84 mmol) of 80% strength sodium hydride in 60 ml of anhydrous tetrahydrofuran. 9.9 g (35 mmol) of the compound from Example VIII dissolved in 100 ml of tetrahydrofuran are added dropwise at reflux. The mixture is allowed to boil under reflux for 90 min and is cooled, treated with water and extracted several times with ethyl acetate. The combined organic phases are washed with saturated sodium chloride solution and concentrated in vacuo. The residue is dissolved in 300 ml of toluene and heated under reflux for 1 h with 22.9 g (182 mmol) of oxalic acid dihydrate in 350 ml of water. The mixture is cooled, the phases are separated and the aqueous phase is extracted twice with ethyl acetate. The combined organic phases are washed with saturated sodium chloride solution, dried over sodium sulphate and concentrated to dryness in vacuo. The residue is dissolved in dichloromethane and filtered through 400 g of silica gel 230-400 mesh in a glass suction filter. The mixture is eluted without sucking dry using petroleum ether/ethyl acetate 1:1, the eluate is concentrated in vacuo and the residue is stirred thoroughly in ether.
