A mixture of (6S,7S)-7-(tert-butoxycarbonyl)-5-azaspiro[2.5]octane-6-carboxylic acid of step 1 g (25 mg), BOP (45 mg), 1-(3-methylphenyl)piperazine (176 mg) and di-isopropylethylamine (70 μL) in DMF (500 μL) was stirred at RT overnight. Formaldehyde solution (0.5 M in THF/MeCN (1:1), 600 μL) was added to the mixture followed by NaBH(OAc)3 (0.25 M in THF/MeCN (1:1), 1000 μL). The resulting mixture was stirred overnight. The solvents were removed under reduced pressure. The residue was dissolved in ethyl acetate (5 mL). The solution was washed with NaHCO3 (7.5%, 3×1 mL). The organic phase was dried over MgSO4 and filtered and concentrated. The residue was used directly in the next step reaction without further purification.
