Methyl N-(triethylammoniosulfonyl)carbamate → Methyl 5-methyl-2-(4-nitrophenyl)-4,5-dihydro-1,3-oxazole-4-carboxylate

The Burgess reagent (0.60 g, 2.5 mmol) was added to a solution of N-(4-nitrobenzoyl)threonine methyl ester obtained in Example (97a) (0.57 g, 3 mmol) in THF (20 mL), and the mixture was stirred at 60° C. for two hours. The reaction solution was concentrated under reduced pressure, and the residue was purified by silica gel column chromatography (elution solvent: ethyl acetate/hexane=1/4, 1/2, 1/1, 1/2) to obtain 1.06 g of the title compound as a colorless solid (100%).