A mixture of 2-(7-chloro-1,8-naphthyridin-2-yl)-3-hydroxy-isoindolin-1-one (1.56 g.) and n-butyl isocyanate (0.99 g.) in anhydrous acetonitrile (30 cc.) is heated at the reflux temperature for 2 hours. The solution obtained is treated with decolourising charcoal at the reflux temperature, filtered hot, after which it is cooled to a temperature of about 20° C. The product which crystallises is filtered off and washed with acetonitrile (4 cc.). After drying, 2-(7-chloro-1,8 naphtyridin-2-yl)-3-n-butylaminocarbonyloxy-isoindolin-1-one (0.96 g.), melting at 152° C. and then at 174° C., is obtained.
