Ethyl (2-formamido-5-chlorothiazol-4-yl)glyoxylate (10.0 g) was added to the 1 N aqueous solution of potassium hydroxide (76.1 ml) at ambient temperature and stirred for 10 minutes. The reaction mixture was adjusted to pH 2.0 with 10% aqueous hydrochloric acid under ice cooling. A solution of 1-tert-butoxycarbonylethoxyamine (8.4 g) in tetrahydrofuran (35 ml) was added to the stirred suspension of the above mixture in pyridine (13.9 ml) at ambient temperature and stirred for 4 hours at same temperature. The resulting mixture was adjusted to pH 7.5 with a saturated aqueous solution of sodium bicarbonate, and washed with ethyl acetate. The separated aqueous layer was adjusted to pH 2.0 with 10% aqueous hydrochloric acid and extracted with ethyl acetate. The extracts were washed with a saturated aqueous solution of sodium chloride, dried over magnesium sulfate and evaporated in vacuo. The residue was washed with diisopropyl ether and collected by filtration to give 2-(2-formamido-5-chlorothiazol-4-yl)-2-(1-tert-butoxycarbonylethoxyimino)acetic acid (syn isomer) (2.31 g).
