Insight into Acid Driven Formation of Spiro-[oxindole]xanthenes from Isatin and Phenols

One-pot synthesis of symmetrical and non-symmetrical spiro[oxindole]xanthenes was achieved by reacting excess of acid labile and stable p-substituted phenols with isatin in the presence of p-TsOH in refluxing p-xylene. The intermediary 3,3-bis(2-hydroxy-5- substitutedphenyl)oxindoles undergo ring forming dehydration, generating the targeted spiro compounds. Gaining mechanistic insight allowed the selection of suitable reaction times and conditions to optimize the yields of the more challenging non-symmetrical spiro[oxindole]xanthenes.