He also worked on the synthesis of isatin. Dimerization of isatin with KBH in methanol yield Indirubin. The Sandmeyer methodology is the oldest and straightforward way for the synthesis of isatin. To date, only few articles concerning the ring expansion of isatin derivatives has been reported. Notably, it is used to synthesize isatin. Indigo can be converted to isatin and then to oxindole. The ketone group of isatin (1) is nonenolizable and has interesting properties. Isatin suffers nucleophilic addition on carbonyls at "C-2" and "C-3" positions.