To a solution of 3-ethyl 2-(phenylmethyl) 8-{[(1,1-dimethylethyl)(dimethyl)silyl]oxy}-2-azaspiro[4.5]decane-2,3-dicarboxylate (111) (5.73 g, 12.05 mmol) in THF (60 mL) was added glacial acetic acid (1.38 mL, 24.0 mmol) followed by TBAF (24 mL) as a 1 M solution in THF. The reaction was stirred at room temperature for 72 h. The reaction was partitioned between EtOAc and water (250 mL) each, the organic layer was separated and washed with saturated NaHCO3 (100 mL) and dried (MgSO4) and concentrated in vacuo. The reaction was found to be incomplete by TLC. The residue was dissolved in dry THF (60 mL), cooled to 0° C. and treated with HF-pyridine (1.6 mL, 18.0 mmol), warmed to room temperature and stirred for 2 h under nitrogen. The reaction was poured into saturated NaHCO3 (100 mL) and solid potassium carbonate was added until gas evolution ceased. Extracted with EtOAc (2×150 mL), the organic layers were combined and washed with 0.1 N HCl (100 mL), dried (MgSO4) and concentrated in vacuo to afford the title compound in quantitative yield as a yellow oil which was used in subsequent reactions without additional purification.
