Potassium borohydride (0.41 g) dissolved in distilled water (12 cc) is added at a temperature in the region of 20° C. to a suspension of 2-(7-methoxy-1,8-naphthyridin-2-yl)-3-(5-methyl-2-oxohexyl)-1-isoindolinone (3.0 g) in ethanol (50 cc), and the suspension is stirred for 16 hours at a temperature in the region of 20° C. The reaction mixture is then poured into a mixture of distilled water (300 cc) and dichloromethane (150 cc) which has been cooled to a temperature in the region of 0° C. The aqueous phase is separated after settling has occurred, and re-extracted with dichloromethane (2×150 cc). The organic phases are combined, washed with distilled water (3×25 cc), dried over magnesium sulphate, filtered and then concentrated to dryness under reduced pressure (2.7 kPa) at 40° C. The product obtained is recrystallized twice in diisopropyl ether. 3-(2-Hydroxy-5-methylhexyl)-2-(7-methoxy-1,8-naphthyridin-2-yl)-1-isoindolinone (2.0 g), m.p. 110° C., is thereby obtained.
