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 degrees C, alpha-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.