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Step 2: Sodium cyanoborohydride (NaBH3CN, 1.26 g, 20 mmol) was slowly added to a solution of tert-butyl N'-(tetrahydropyran-4-ylidene)hydrazine carboxylate (1c) in 50% acetic acid (70 mL) of the product obtained in step 1. The reaction mixture was stirred at room temperature for 1.5 h, followed by neutralization with 1N sodium hydroxide (NaOH) solution and extraction with dichloromethane (DCM). The organic phase was washed with saturated sodium bicarbonate (NaHCO3) solution, dried over anhydrous sodium bicarbonate, and concentrated under reduced pressure to give tert-butyl 2-(tetrahydro-2H-pyran-4-yl)hydrazine carboxylate as a white solid (1d, 4.3 g, about 100% yield). Trifluoroacetic acid (TFA, 23 g, 0.2 mol) was added to a solution of 1d in dichloromethane (30 mL) and the reaction was stirred at room temperature for 2 hours. After completion of the reaction, the solvent was evaporated under reduced pressure to give the product 1e (6.8 g), which was used directly in the subsequent reaction without further purification.

[1] Journal of Medicinal Chemistry, 2004, vol. 47, # 9, p. 2180 - 2193
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