Ethanol → Ethyl 6-(4-fluorobenzyl)-4-hydroxy-5-oxo-5,6,7,8-tetrahydro-2,6-naphthyridine-1-carboxylate 2-oxide

To ethyl 6-(4-fluorobenzyl)-4-hydroxy-5-oxo-5,6,7,8-tetrahydro-2,6-naphthyridine-1-carboxylate (0.5 gm, 1.09 mmol; see Example 8, Step 4) in glacial acetic acid (25 mL) at room temperature under nitrogen was added, with stirring, aqueous peroxide (30% by wt) (1.24 mL, 10.9 mmol). The reaction was warmed to 100° C. and stirred for 1.5 hours. The reaction was allowed to cool, ethanol (1 mL) was added and volatile components were removed under reduced pressure. The resulting oil was placed under high vacuum for 16 hours, then used as is. Alternatively, after cooling, water can be added and the volatile components removed under reduced pressure. The residue can be partitioned between CHCl3 and saturated Na2SO3. The organic extract can be dried with Na2SO4, filtered, and concentrated under vacuum to provide the title compound.