Synthesis of substituted 3-[(4-oxocyclohexa-2,5-dien-1-ylidene)-hydrazinylidene]-1,3-dihydro-2<Emphasis Type="Italic">H</Emphasis>-indol-2-ones and their reactions with hydrogen chloride

4-Hydroxyphenylhydrazones of isatin derivatives were synthesized and oxidized with lead compounds to the corresponding azines. The introduction of isatin fragment in a quinoid system increases the redox potential of the system compared with 1,4-benzoquinone thus presumably resulting in the prevalence of 1,4-addition process. By an example of the reaction of N-methylisatins with hydrogen chloride it was shown that the reaction proceeded in keeping with the theoretically predicted direction. Azines, derivatives of isatin unsubstituted at the nitrogen atom, do not react with hydrogen chloride because of the presence of a strong intramolecular hydrogen bond.