A solution of t-butyl nitrite (1.7 g) in tetrahydrofuran (10 ml) was dropwise added to a solution of benzhydryl 2-t-butoxycarbonylmethoxyimino-2-(2-aminothiazol-4-yl)acetate (syn isomer) (5 g) in tetrahydrofuran (50 ml) at 50° to 53° C. under stirring and the mixture was stirred at the same temperature at 25 minutes. The reaction mixture was poured into a mixture of ethyl acetate and water and the organic layer was separated. The organic layer was washed with brine and dried over magnesium sulfate. The solution was evaporated and the residue was subjected to column chlomatography on silica gel. The elution was carried out with a mixture of benzene and ethyl acetate (19:1) and the eluates containing the object compound were evaporated to give benzhydryl 2-t-butoxycarbonylmethoxyimino-2-(4-thiazolyl)acetate (syn isomer) (2.2 g).
