[CH3:1][CH:2]1[CH:7]([CH3:8])[NH:6][CH2:5][CH2:4][N:3]1[C:9]([O:11][CH2:12][C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)=[O:10].C=O.[C:21](O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+]>ClCCl>[CH3:1][CH:2]1[CH:7]([CH3:8])[N:6]([CH3:21])[CH2:5][CH2:4][N:3]1[C:9]([O:11][CH2:12][C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)=[O:10]To a solution of phenylmethyl 2,3-dimethyl-1-piperazinecarboxylate (mixture of four stereoisomers) (3.105 g, 12.5 mmol) in dichloromethane (90 mL) at 0° C. was added formaldehyde (1.409 mL, 37% water solution, 18.75 mmol) followed by sodium triacetoxyborohydride (3.445 g, 16.26 mmol). The reaction mixture was allowed to warm to room temperature and stirred for 2 h before being diluted with dichloromethane and washed with 1N NaOH solution. The organics were washed with brine, dried (MgSO4) and evaporated to yield phenylmethyl 2,3,4-trimethyl-1-piperazinecarboxylate (mixture of all four stereoisomers) (3.04 g, 93%). Phenylmethyl (2R,3R)-2,3,4-trimethyl-1-piperazinecarboxylate (365 mg) and phenylmethyl (2S,3S)-2,3,4-trimethyl-1-piperazinecarboxylate (420 mg) were separated out by a combination of preparative and chiral chromatography. Absolute configuration was assigned using Ab Initio vibrational circular dichroism. LCMS: (M+H)+: 263.3.
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