[C:1]([O:5][C:6]([N:8]1[CH2:15][CH2:14][C:11]2([CH2:13][CH2:12]2)[CH2:10][CH:9]1[C:16]([OH:18])=[O:17])=[O:7])([CH3:4])([CH3:3])[CH3:2].[CH3:19]COCC>CCOCC.CO.[Si](C=[N+]=[N-])(C)(C)C>[CH2:12]1[C:11]2([CH2:14][CH2:15][N:8]([C:6]([O:5][C:1]([CH3:4])([CH3:2])[CH3:3])=[O:7])[CH:9]([C:16]([O:18][CH3:19])=[O:17])[CH2:10]2)[CH2:13]1To an ice cooled solution of 6-(tert-butoxycarbonyl)-6-azaspiro[2.5]octane-5-carboxylic acid (racemic mixture) (D1) (500 mg, 0.78 mmol) in Et2O/MeOH (10/5 ml), TMSCHN2 2M sol in Et2O (1.5 ml, 2.94 mmol) was added and the resulting mixture was stirred 2 hrs at 0° C. then 18 hrs at RT. After solvent evaporation the crude residue was loaded onto SPE-Si cartridge (10 g) eluting with DCM. Collected fractions after solvent evaporation afforded the title compound (D4) (420 mg) as a clear oil.
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