To a solution of ethyl 7-fluoro-4-hydroxy-2-neopentyl-1-oxo-1,2-dihydro-3-isoquinolinecarboxylate (5.12 g, 31.9%) (from Example 1(1), 3.21 g, 10 mmol), 1-butanol (1.4 ml, 15 mmol) and tributylphosphine (5.0 ml, 20 mmol) in tetrahydrofuran (30 ml) was added 1,1′-(azodicarbonyl)dipiperidine (5.05. g, 20 mmol) and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated, under reduced pressure and the residue was purified by silica gel column chromatography to give ethyl 4-butoxy-7-fluoro-2-neopentyl-1-oxo-1,2-dihydro-3-isoquinolinecarboxylate (3.55 g, 94.2%) as an oil.
