Selective synthesis of dihydro- and dehydrogenated spirocyclopenta[b]indoles via acid-modulated [3+2] cycloaddition reaction

p-TsOH catalyzed [3+2] cycloaddition of 3-benzoylvinylindoles with 3-hydroxy-3-(indol-3-yl)indolin-2-ones in acetonitrile gave functionalized 3,4-dihydrospiro[cyclopenta[b]indole-1,3′-indolin]-2′-ones in good yields and with high diastereoselectivity. In the presence of FeCl_3, the similar reaction of 3-benzoylvinylindoles with 3-hydroxy-3-(indol-3-yl)indolin-2-ones in toluene afforded dehydrogenated spiro[cyclopenta[b]indole-1,3′-indolin]-2′-ones in good yields. On the other hand, p-TsOH catalyzed [3+2] cycloaddition of 3-benzoylvinylindoles and 2-hydroxy-2-(indol-3-yl)-indene-1,3-diones gave 3,4-dihydrospiro[cyclopenta[b]indole-1,2′-indene]-1′,3′-diones, which could be oxidized to dehydrogenated spiro[cyclopenta[b]indole-1,2′-indene]-1′,3′-diones by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).