6.3 g (22 mmol) of diethyl [methoxy-(4-methoxyphenyl)-methyl]-phosphonate are dissolved in 40 ml of DMF under a nitrogen atmosphere. 5.1 g (45 mmol) of potassium tert.butoxide are added batchwise at −40° C. and the mixture is then stirred for another 30 minutes at −10° C. Then 3.84 g (20 mmol) of 5-nitroisatine are added. The mixture is stirred for 1 hour at ambient temperature and then poured onto ice water containing 20 ml of saturated potassium hydrogen sulphate solution. The precipitate is suction filtered and chromatographed over silica gel (dichloromethane/methanol=10:1).
