a. N-Boc-4-piperidone (50 g, 0.25 mol) and zinc powder (49.2 g, 0.75 mol) were suspended in a mixture of tetrahydrofuran (500 mL) and aqueous ammonium chloride (2/3, 500 mL) under nitrogen protection. The reaction temperature was controlled between -10 and 10 °C and 3-bromo-1-propene (109.3 g, 0.90 mol) was added slowly dropwise. After the dropwise addition, the temperature was slowly increased to 15-40 °C and the reaction was continued with stirring for 2-5 hours. The progress of the reaction was monitored by thin layer chromatography (TLC, unfolding agent ratio of petroleum ether/ethyl acetate = 2/1, v/v). Upon completion of the reaction, the reaction mixture was extracted with ethyl acetate (100 mL x 3). The organic phases were combined, dried with anhydrous sodium sulfate, filtered and concentrated under reduced pressure to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent ratio of petroleum ether/ethyl acetate = 30/1 to 10/1, v/v) to afford the colorless oily target compound 4-allyl 1-Boc-4-hydroxypiperidine (60 g) in 99.9% yield.
