To a stirred solution of (2R,5R)-4-benzyl-5-hydroxymethyl-2-methyl-piperazine-1-carboxylic acid tert-butyl ester (0.96 g, 3 mmol) in DCM (10 mL) externally cooled in an ice-methanol bath was added triethylamine (0.91 g, 1.25 mL, 9 mmol) followed, dropwise over 0.05 h, methylsulfonyl chloride (0.38 g, 0.26 mL, 3.3 mmol). The mixture was stirred at 0-20° C. for 2 h, then pyrazole (0.27 g, 4 mmol) was added in one portion and stirring continued at 20° C. for 7 days. Mixture was poured into saturated aqueous NaHCO3 and extracted with DCM (3×20 mL). Combined organic extracts were dried (Na2SO4) then evaporated in vacuo to give an oil. Chromatography (SiO2, 0-100% Et2O in petrol) gave the title compound (0.24 g). MS: [M+H]+=371.
