To 250 mg of 3-(2,5-dimethyl-phenyl)-4-hydroxy-8-methoxy-1,8-diaza-spiro[4.5]dec-3-en-2-one (compound C1, preparation example P1, step P1.5) in 2.5 ml of tetrahydrofuran were added 160 μl of Hünig's base and 85 μl of chloromethyl ethyl ether. After stirring for 20 hours at room temperature, water and ethyl acetate were added and the layers were separated. The aqueous phase was extracted with ethyl acetate, the combined organic phases dried with sodium sulfate, filtered and concentrated. Chromatography (heptane/acetone 4:1) yielded 72 mg of 3-(2,5-dimethyl-phenyl)-4-ethoxymethoxy-8-methoxy-1,8-diaza-spiro[4.5]dec-3-en-2-one (title compound A61) as a solid, mp: 150-152° C.
