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150 mg 1,8-Bis-dimethylamino-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]decane-2,4-dione (PREPARATION EXAMPLE 1) is dissolved in 3 ml tetrahydrofuran, 75 mg triethylamine is added, and the mixture is cooled to 0° C. 44 mg ethyl chloroformate is added and the resulting suspension stirred for 3 hours, thereby warming to room temperature. Then the mixture is partitioned between water and ethyl acetate, the organic layer separated and washed with brine, dried over anhydrous sodium sulfate, and evaporated. The residue is purified by chromatography on silica gel with ethyl acetate/heptane (1:1) as a solvent. Thus, 145 mg Carbonic acid 1,8-bis-dimethylamino-2-oxo-3-(2,4,6-trimethyl-phenyl)-1,8-diaza-spiro[4.5]dec-3-en-4-yl ester ethyl ester is obtained as a solid, mp 105° C. 1H-NMR (CDCl3): 6.83 ppm (s, 2H), 4.02 ppm (q, 2H), 3.05 ppm (m, 2H), 2.94 ppm (s, 6H), 2.80 ppm (m, 2H), 2.43 ppm (s, 6H), 2.40 ppm (m, 2H), 2.23 ppm (s, 3H), 2.17 ppm (s, 6H), 1.86 ppm (m, 2H), 1.08 ppm (t, 3H).

1. temperaturewarming to room temperature
2. customThen the mixture is partitioned between water and ethyl acetate
3. customthe organic layer separated
4. washwashed with brine
5. dry with materialdried over anhydrous sodium sulfate
6. customevaporated
7. customThe residue is purified by chromatography on silica gel with ethyl acetate/heptane (1:1) as a solvent