To a solution of 3-(4-chloro-2,6-dimethyl-phenyl)-1,4-dihydroxy-8-methoxy-1,8-diaza-spiro[4.5]dec-3-en-2-one (140 mg, 0.40 mmol) and triethylamine (0.1 ml, 72 mg, 0.71 mmol) in THF (3 ml) at 0° C. was added a solution of ethyl chloroformate (0.05 ml, 52 mg, 0.48 mmol) dissolved in THF (2 ml) dropwise. The suspension was stirred at 0° C. for 30 minutes. Then the reaction mixture was poured into cold (0° C.) water and thoroughly extracted three times with ethyl acetate. The combined organic layers were washed with water and brine, dried over sodium sulfate and concentrated. The raw material was purified by column chromatography on silica gel (ethyl acetate/cyclohexane 1:4). Yield: 70 mg of carbonic acid 3-(4-chloro-2,6-dimethyl-phenyl)-1-ethoxycarbonyloxy-8-methoxy-2-oxo-1,8-diaza-spiro[4.5]dec-3-en-4-yl ester ethyl ester (title compound P1ii.115) as a colorless gum.
