To a solution of oxalyl chloride in DCM (2 M, 3 mL, 6 mmol) and DCM (5 mL) at −78° C. under nitrogen was added, dropwise with stirring, a solution of DMSO (1.0 mL, 1.09 g, 14 mmol) in DCM (5 mL) and mixture was stirred at −78° C. for 0.4 h. A solution of (2R,5R)-4-benzyl-5-hydroxymethyl-2-methyl-piperazine-1-carboxylic acid tert-butyl ester (1.12 g, 3.5 mmol) in DCM (5 mL) was added dropwise over 0.2 h and mixture stirred at −78° C. for 0.6 h. Triethylamine (3.9 mL, 2.83 g, 28 mmol) was added dropwise over 0.2 h and stirring was continued for a further 0.3 h. Water (10 mL) was added and mixture was warmed slowly to 20° C. over 16 h. Organic phase was separated and aqueous phase was extracted with DCM (2×10 mL). Combined organic extracts were dried (Na2SO4) and evaporated in vacuo to give a paste. This was partitioned between 1:1 Et2O+40-60 petroleum ether (50 mL) and water (3×50 mL) and organic phase was dried (Na2SO4) and evaporated in vacuo to give an oil. Chromatography (SiO2; gradient elution with 0-20% EtOAc in 40-60 petroleum ether) gave the title compound (0.275 g) as an oil. MS: [M+H]+=319.
