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A methanol (40 mL) solution of tert-butyl 4-oxopiperidine-1-carboxylate (5.0 g, 25.1 mmol) was added to the reaction flask, followed by cyclopropylamine (1.4 g, 25.1 mmol), triethylamine (10.0 mL, 75.3 mmol) and zinc chloride (0.3 g, 2.5 mmol). The reaction mixture was stirred at 60 °C for 7 h. Then sodium cyanoborohydride (4.8 g, 75.3 mmol) was added in batches. The reaction mixture was continued to be stirred at 25 °C for 17 hours. Upon completion of the reaction, the solvent was removed under vacuum and the residue was partitioned between water (250 mL) and ethyl acetate (200 mL). The aqueous layer was further extracted with ethyl acetate (2 x 200 mL) and the organic layers were combined, dried over sodium sulfate and concentrated under vacuum. The residue was purified by column chromatography (basic activated alumina, eluent: 10% to 30% hexane solution of ethyl acetate) to afford the target product tert-butyl 4-(cyclopropylamino)piperidine-1-carboxylate (5.3 g, 88% yield) as a gel. The product data are shown in Table 2.

[1] Patent: WO2017/21730, 2017, A1. Location in patent: Page/Page column 46
[2] Journal of Medicinal Chemistry, 1996, vol. 39, # 19, p. 3769 - 3789
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[4] Patent: WO2007/16610, 2007, A2. Location in patent: Page/Page column 57; 58
[5] Patent: EP3147283, 2017, A1. Location in patent: Paragraph 0106