To a solution of (R)-3-(tert-butoxycarbonyl)-2,2,4-trimethyloxazolidine-4-carboxylic acid 4 (129.6 mg, 0.5 mmol) in DMF (1 mL) was added HATU (209 mg, 0.55 mol) and DIPEA (0.436 mL, 2.5 mmol). After being stirred at rt for 10 min, the reaction mixture was treated with 4-(octylthio)-3-(trifluoromethyl)benzohydrazide 3a (174 mg, 0.5 mmol) in THF (2 mL) and was left stirred overnight. Aqueous NaHCO3 solution was added and the mixture was extracted with EtOAc. The organic layer was washed with brine, dried over anhydrous Na2SO4, and evaporated under reduced pressure to give a residue, which was purified by SiO2 column chromatograph (n-hexane/EtOAc=3:7) to give (R)-tert-butyl 2,2,4-trimethyl-4-(2-(4-(octylthio)-3-(trifluoromethyl)benzoyl)hydrazinecarbonyl)oxazolidine-3-carboxylate 5a (247 mg, 84%). HPLC retention time on a C18 column (30×4.6 mm, 3.5μ) was 3.08 min with gradient 10-95% acetonitrile-H2O (0.1% TFA) in 3.5 min as mobile phase. MS (ESI, M+H+)=590.5
