To a solution of 4-hydroxy-8-methoxy-1-methyl-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]dec-3-en-2-one (238 mg, 0.72 mmol), triethylamine (0.15 ml, 109 mg, 1.08 mmol) and 4-dimethylaminopyridine (2 mg) in tetrahydrofuran (10 ml) at 0° C. was added ethyl chloroformate (0.075 ml, 85 mg, 0.79 mmol) dropwise. The suspension was stirred at 0° C. for one hour. The reaction mixture was diluted with ethyl acetate and water, the layers separated, the aqueous phase extracted with ethyl acetate, the combined organic phases washed with brine, dried over sodium sulfate and concentrated. The residue was purified by chromatography on silica gel (ethyl acetate/heptane 5:1). Yield: 145 mg of carbonic acid ethyl ester 8-methoxy-1-methyl-2-oxo-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]dec-3-en-4-yl ester (title compound P1.2) as a white solid, mp 134-136° C.
