Methyl 7-benzyl-4-(3-bromo-4-fluorophenyl)-2-methyl-5-oxo-1,4,5,6,7,8-hexahydro[1,7]naphthyridine-3-carboxylate → 6-benzyl-9-(3-bromo-4-fluorophenyl)-5,6,7,9-tetrahydrofuro[3,4-b][1,7]naphthyridine-1,8(3H,4H)-dione

A solution of the product from Example 39A (3.1 g, 6.3 mmol) in chloroform (50 mL) was cooled to 0° C., treated with 90% pyridinium tribromide (2.45 g, 6.9 mmol), warmed to abmient temperature, stirred for 16 hours and washed with water. The chloroform layer was isolated, dried (MgSO4), filtered, refluxed for 16 hours and cooled in an ice bath. The resulting precipitate was collected by filtration and dried to provide the title compound (2.1 g) as tan crystals.