Thermal and Electron-Transfer Induced Reactions of Enediynes and Enyne-Allenes, 9. Electron-Transfer versus Acid Catalysis in Enediyne Cyclizations†‡

Four novel aryl-substituted enediynes 3 have been synthesized. While their cyclization to the Bergman products 4 could be accomplished at temperatures of 240–320°C, α-functionalization rather than cyclization was observed during anodic oxidation and in the presence of one-electron oxidants. In contrast, enediyne 1a was cyclized in the presence of two one-electron oxidants to the indenone 2a as indicated in the literature; mechanistic investigations clearly indicate the occurrence of an acid-catalyzed mechanism.