Preparation of 2-(7-hydroxy-1,8-naphthyridin-2-yl)-5-chlorophthalimide (7 g.), m.p. 320° C., by reacting 2-amino-7-hydroxy-1,8-naphthyridine (9.5 g.) with 4-chlorophthalic anhydride (21.5 g.) in acetic acid (450 cc.) for 1 hour at 116° C. Preparation of 2-(7-chloro-1,8-naphthyridin-2-yl)-5-chlorophthalimide (6.4 g.), m.p. 280° C., by reacting phosphorus oxychloride (70 cc.) with 2-(7-hydroxy-1,8-naphthyridin-2-yl)-5-chlorophthalimide (7 g.) in the presence of dimethylformamide (0.7 cc.). By reacting potassium borohydride (0.75 g.) with 2-(7-chloro-1,8-naphthyridin-2-yl)-5-chlorophthalimide (6.4 g.) in a mixture (300 cc.) of dioxan and methanol(50-50 by volume), a mixture (5.2 g.) of 2-(7-chloro-1,8-naphthyridin-2-yl)-5-chloro-3-hydroxy-isoindolin-1-one and 2-(7-chloro-1,8-naphthyridin-2-yl)-6-chloro-3-hydroxy-isoindolin-1-one is obtained. This mixture is recrystallised firstly from dichloroethane (700 cc.) and then a second time from the same solvent (315 cc.). A product (1.51 g.) is thus obtained and is recrystallised successively from bromoform (38 cc.) and then from a mixture (104.5 cc.) of dichloroethane and ethanol (91-9 by volume). 2-(7-Chloro-1,8-naphthyridin-2-yl)-5-chloro-3-hydroxy-isoindolin-1-one (0.65 g.) is thus obtained. Preparation of 2-(7-chloro-1,8-naphthyridin-2-yl)-5-chloro-3-phenoxycarbonyloxy-isoindolin-1-one (1.6 g.), m.p. 220°-230° C., from 2-(7-chloro-1,8-naphthyridin-2-yl)-5-chloro-3-hydroxy-isoindolin-1-one (1 g.) and phenyl chloroformate (1.36 g.) in anhydrous pyridine (15 cc.).
