To a cooled (−78° C.) solution of 1-benzyl 4-tert-butyl (2S)-2-{benzyl[2-(iodomethyl)prop-2-en-1-yl]amino}succinate from step 1c (9.2 g) in THF (50 mL) was added dropwise LiHMDS (1.0 M in THF, 20.2 mL) at −78° C. during a period of 30 min. The mixture was stirred at −78° C. for 1 h, and then was allowed to warm to −30° C. during 3 h. The reaction mixture was quenched with 10% citric acid (10 mL) and diluted with brine (100 mL). The mixture was extracted with ethyl acetate (4×75 mL). The combined organic layers were dried over MgSO4 and concentrated under reduced pressure. The residue was purified through Combiflash (hexane and ethyl acetate: gradient 0 to 5% during 12 min) to give the desired 2-benzyl 3-tert-butyl (2S,3S)-1-benzyl-5-methylenepiperidine-2,3-dicarboxylate (3.45 g). MS (ESI): 422.3 (M+H+).
