To a solution 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) (1.0 g, 2.46 mmol) in dioxane (40 ml) was added bromomethyl-cyclopropane (1.257 ml, 1.78 g, 13.16 mmol) and potassium tert-butoxide (1.50 g, 13.37 mmol). The reaction mixture was stirred at 100° C. for 5 days, then poured on water and extracted with ethyl acetate. The combined organic phases were washed with brine, dried over sodium sulfate and concentrated. The residue was suspended in ethyl acetate/heptane 1:5, stirred overnight, filtered and dried to afford a first crop of product as a white solid (350 mg). The mother liquor was concentrated, and the residue purified by chromatography on silica gel (dichloromethane/acetone 10:1) to further deliver 205 mg of product. Yield: 555 mg of 4-benzyloxy-1-cyclopropylmethyl-8-methoxy-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]dec-3-en-2-one (compound P3.8) as a solid, mp 119-121° C.
