To an aqueous solution (10 ml) of sodium chlorite (7.13 g, 78.8 mmol), and sulfamic acid (7.65 g, 78.8 mmol) was added a solution of the compound 12 (15.0 g, 39.4 mmol) in tetrahydrofuran (630 ml) under ice-cooling, and the mixture was stirred at room temperature for 1 hour. After water (250 ml) was added to the reaction solution, precipitated crystals were filtered. Washing with diethyl ether afforded 3-benzyloxy-5-(4-fluoro-benzylcarbamoyl)-4-hydroxy-pyridine-2-carboxylic acid 13 (14.0 g, 90%) as a colorless crystal.
