To tert-butyl 4-aminooxycrotonate (10.0 g) were added ethanol (150 ml) and water (150 ml), followed by gradually adding (2-formamidothiazol-4-yl)glyoxylic acid (11.0 g) with stirring. During the addition, the mixture was adjusted to pH 5 to 5.5 with 10% aqueous sodium hydroxide, and the stirring was continued at ambient temperature for 2 hours. After removal of the ethanol, to the remaining aqueous solution was added ethyl acetate, followed by adjusting to pH 7.5 with 10% aqueous sodium hydroxide. The aqueous layer was separated and washed with ethyl acetate. Thereto was further added ethyl acetate, followed by adjusting to pH 2.0 with 10% hydrochloric acid. The separated organic layer was washed with a saturated aqueous sodium chloride and then dried over magnesium sulfate. Removal of the solvent gave a residue, which was pulverized with n-hexane and tetrahydrofuran, and collected by filtration. To this substance were added ethanol (50 ml) and water (30 ml), followed by adjusting to pH 7.5 with 10% aqueous sodium hydroxide. The precipitated substance was collected by filtration, washed with a mixed solvent of water and ethanol (1:1 by volume), followed by addition of water and ethyl acetate, and adjusting to pH 2.0 with 10% hydrochloric acid. The organic layer was separated, washed with a saturated aqueous sodium chloride and then dried over magnesium sulfate. Removal of the solvent gave a residue, which was pulverized with n-hexane and tetrahydrofuran to obtain 2-(trans-3-tert-butoxycarbonylallyloxyimino)-2-(2-formamidothiazol-4-yl)acetic acid (syn isomer) (12.01 g).
