To a cooled solution of oxalyl chloride (315 mg) in methylene chloride (2.5 mL) was added dropwise a solution of DMSO (0.30 mL) in methylene chloride (3.0 mL) at −78° C. The mixture was stirred at −78° C. for 30 min, 1,2-dibenzyl 3-tert-butyl (2S,3S)-5-hydroxypiperidine-1,2,3-tricarboxylate of step 1m (900 mg) in methylene chloride (4 mL) was added dropwise. The mixture was stirred at −78° C. to −60° C. for 1 h. Triethylamine (620 mg) in methylene chloride (2.5 mL) was added. The mixture was allowed to warm to RT during a period of 2 h. The mixture was diluted with ethyl acetate (75 mL) and washed with brine (2×25 mL). The organic layer was dried over Na2SO4 and concentrated under reduced pressure. The residue was flash chromatographed on silica gel column to afford 1,2-dibenzyl 3-tert-butyl (2S,3S)-5-oxopiperidine-1,2,3-tricarboxylate (570 mg). MS (ESI): 490.3 (M+Na+); 368.2 (M+H+—COO(t-Bu)).
