NaH (1.26 g, 52.5 mmol) was added at RT to a solution of tert-butyl 3-(S)-benzyl-2-oxo-1,4,8-triazaspiro[4,5]decane-8-carboxylate (12.1 g, 35 mmol) in THF (250 ml). After stirring for 1 h at 0° C., benzyl chloride (4.9 g, 38.5 mmol) was added dropwise, and the mixture was allowed to warm to RT and heated for 60 h under reflux. Subsequently, benzyl chloride (2.4 g, 19 mmol) was again added and the mixture was heated under reflux for a further 16 h. The reaction mixture was treated with aqueous saturated NH4Cl solution (20 ml) and extracted with EA (3×50 ml). The combined organic phase was dried over Na2SO4, filtered and the solvent was distilled off. Workup was carried out by means of column chromatography and afforded tert-butyl-1,3-(S)-dibenzyl-2-oxo-1,4,8-triazaspiro[4,5]decane-8-carboxylate in a yield of 13.5 g (88%).
