Step 1: In a dry reaction flask, N-tert-butoxycarbonyl-4-piperidone (19.9 g, 0.1 mol), triphenyl phosphite (34.1 g, 0.11 mol), and triethylamine (15.2 g, 0.15 mol) were dissolved in 160 mL of dichloromethane, and the temperature of the reaction system was controlled to be between -25°C and -20°C. Subsequently, a dichloromethane solution (30 mL) of bromide (17.6 g, 0.11 mol) was slowly added dropwise under stirring. After the dropwise addition, the reaction was gradually warmed up to room temperature with continued stirring. The progress of the reaction was monitored by thin layer chromatography (TLC). Upon completion of the reaction, the solvent was removed by reduced pressure distillation and the residue was treated with 300 mL of pre-cooled 95% ethanol, stirred and filtered to collect the solid. The filtrate was dried and extracted by adding 150 mL of toluene and 40 mL of water, the toluene layer was washed with saturated brine, and the organic phase was dried and concentrated to give the crude product. Purification by column chromatography afforded N-tert-butoxycarbonyl-1,2,5,6-tetrahydropyridine-4-bromide in 80% yield (calibrated by internal standard method).
