4-Chloromethyl-2-methoxy-1-pent-2-ynyloxy-benzene (28 g, 0.12 mol) and N-hydroxyphthalimide (19.5 g, 0.12 mol) are dissolved in 180 ml of N,N-dimethylformamide. The reaction mixture is heated to +70° C. and potassium hydroxide (24 ml of a 5 M solution in methanol, 0.12 mol) is added at this temperature. The reaction is stirred for 1 hour at +70° C., subsequently cooled to room temperature and poured on water. This mixture is stirred for one further hour and filtered. The resulting crystalls are washed with water and recrystallized from methanol/acetone (8:1) to yield 2-(3-methoxy-4-pent-2-ynyloxy-benzyloxy)-isoindole-1,3-dione as colourless crystalls.
