Oxindole was found to react readily with thionyl chloride to give (in an excellent yield) the isolable sulfine (13a), which on heating (refluxing acetonitrile) gave isoindigo (15a). The dark violet 3‐sulfinato‐oxindole (13a) readily reacted with 2,3‐dimethylbutadiene to give a colorless cyclo‐adduct (14a). The sulfine also reacted readily with various nucleophilic reagents, thus, thioloacetic acid