(rac)-8-Benzyl-4-phenyl-2,8-diaza-spiro[4.5]decane-1,3-dion (3.0 g, 9.0 mmol, synthesis described in example 156) was dissolved in dry tetrahydrofuran (50 mL) and triphenylphosphin (3.1 g, 11.7 mmol), methanol (0.374 g, 11.7 mmol) and diethylazodicarboxylate (5.35 mL, 40% in toluene, 11.7 mmol) were added sequentially and stirred at room temperature overnight. The solvent was evaporated and the residue was purified by chromatography on silica gel (dichloromethane/methanol/triethyl amine 98:1:1) to yield the intermediate (rac)-8-benzyl-2-methyl-4-phenyl-2,8-diaza-spiro[4.5]decane-1,3-dion (2.57 g, 82%).
