The First Formation of (1Z)‐1‐Alkylidene‐1H‐isobenzofuranium Amides and 1H‐Inden‐1‐ones: Acid‐Promoted 5‐exo Cyclization and Hydration/Aldol Condensation Reactions of o‐Ethynylbenzophenones

Abstract (1Z)‐1‐(2,2‐Dimethylpropylidene)‐1H‐isobenzofuranium bis(trifluoromethylsulfonyl)amides were synthesized through 5‐exo cyclization reactions between sterically encumbered o‐alkynylbenzophenones and bis(trifluoromethylsulfonyl)imide (Tf2NH). It was confirmed that the five‐membered‐ring isobenzofuranium amide isomerized to give the corresponding benzopyrylium amide, the six‐membered‐ring framework compound, in quantitative yield. Treatment of less encumbered o‐alkynylbenzophenones with Tf2NH at ambient temperature resulted in acid‐promoted hydration and sequential intramolecular aldol condensation reactions to afford 3‐aryl‐1H‐inden‐1‐one derivatives in good yields. The proposed reaction mechanism was strongly supported by the reaction behaviour of 4‐chloro‐ and 5‐methoxy‐2‐ethynylbenzophenone derivatives as substrates with Tf2NH, leading to the formation of the corresponding 3‐aryl‐1H‐inden‐1‐ones.