N-(t-butoxycarbonyl)-O-ethyl-N-methyl-3-oxoserine (Intermediate 7-1, 2.3 g), 1-(5-methyl-1,2-oxazol-3-yl)methanamine (1.0 g), WSC.HCl (2.4 g), HOBt.H2O (1.9 g), and chloroform (24 mL) were stirred overnight at room temperature. A saturated aqueous solution of sodium hydrogen carbonate was added to the reaction mixture, and the mixture was extracted with chloroform. The extract was dried over anhydrous magnesium sulfate, and the desiccant was filtered out. Then, the solvent was distilled off under reduced pressure, and the resulting residue was purified by OH type silica gel column chromatography (gradient elution with chloroform/methanol=98/2→92/8) to obtain 3-({[N-(t-butoxycarbonyl)-O-ethyl-N-methyl-3-oxoseryl]amino}methyl)-5-methyl-1,2-oxazole (pale yellow oil) (2.1 g, 67%).
