Ethyl (2-formamido-5-chlorothiazol-4-yl)glyoxylate (10.0 g) was added to the 1N 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).
