4-Methylpentanoic acid (2.4 g) and 1,8-diazabicyclo[5.4.0]undec-7-ene (3.05 g) are added at a temperature in the region of 20° C. to a solution, maintained under an argon atmosphere, of 3-chloro-2-(7-chloro-1,8-naphthyridin-2-yl)-1-isoindolinone (6.6 g) in anhydrous dimethylformamide (60 cc), and the suspension obtained is stirred for 24 hours at a temperature in the region of 20° C. Distilled water (500 cc) and dichloromethane (150 cc) are then added. The aqueous phase is separated after settling has occurred and then re-extracted with dichloromethane (2×150 cc). The organic phases are combined, washed with distilled water (3×50 cc), dried over magnesium sulphate, filtered and then concentrated to dryness under reduced pressure (2.7 kPa) at 60° C. The oily residue is purified by chromatography on silica gel (150 g) contained in a column 3.5 cm in diameter [eluant: dichloromethane/methanol (98.2 by volume)]. Elution is first performed with 200 cc of solvent: the corresponding eluate is discarded; elution is then performed with 900 cc of solvent: the corresponding eluate is concentrated to dryness under reduced pressure (2.7 kPa) at 40° C. After recrystallization in ethanol, 2-(7-chloro-1,8-naphthyridin-2-yl)-3-oxo-1-isoindolinyl 4-methylpentanoate (4 g), m.p. 147° C., is obtained.
