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Step 4) 2-(tert-butoxycarbonyl)hexahydrocyclopenta[c]pyrrol-5(1H)-one (0.9 g, 4 mmol) was dissolved in EtOH (20 mL), and NaBH4 (0.82 g, 21.7 mmol) was added in batches. The reaction mixture was stirred at room temperature for 4.5 h and then concentrated under vacuum. The residue was diluted with EtOAc (50 mL) and washed sequentially with H2O (50 mL) and brine (50 mL). The resulting organic phase was dried over anhydrous Na2SO4 and concentrated in vacuum to afford tert-butyl 5-hydroxy-hexahydrocyclopenta[c]pyrrole-2(1H)-carboxylate as a yellow oil (0.99 g, 100% yield), which was used directly in the subsequent reaction.

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