To a suspension of 4-hydroxy-8-methoxy-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]dec-3-en-2-one [prepared according to WO09/049,851] (67.0 g, 211.7 mmol) in acetone (900 ml) was added potassium carbonate (35.1 g, 254.1 mmol), followed by benzyl bromide (35.3 ml, 50.7 g, 296.4 mmol) dropewise. The suspension was stirred at reflux for one hour, then poured on ice water and ethyl acetate. The resulting precipitate was filtered off, dissolved in methylene chloride, dried over sodium sulfate, concentrated and dried over phosphorus pentoxide under vacuum at 50° C. overnight to afford a first crop of product as a white solid (55.8 g). The layers of the mother liquor were separated, the aqueous phase extracted with ethyl acetate, the combined organic phases washed with brine, dried over sodium sulfate and concentrated. The residue was suspended in diethyl ether, filtered and dried to further deliver 22.6 g of product. Yield: 78.4 g of 4-benzyloxy-8-methoxy-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]dec-3-en-2-one (compound P3.4) as a solid, mp 184-186° C.
