Enediynes have received significant attention since their discovery as potent antitumor antibiotics. Most of this attention has been directed towards studying the Bergman cyclization, which is the proposed key step in the mechanism of their biological activity.
This work investigates another potentially useful and facile transformation of enediynes, which have significant potential in material science, molecular electronics and metallocene catalyst ligand synthesis. We found that aryl substituted benzannulated enediynes undergo a 5-exo-dig radical cyclization mediated by tributyltin hydride. The reaction is regio and stereoselective and leads predominately to (E) –benzannulated fulvenes with tributyltin residue on the endocyclic methylene group.
The Sn-substituted fulvenes can be either hydrolyzed, or transformed into more complex substituted fulvenes through reactions of vinyl stannanes of the vinylstannane functionality, e.g. Stille coupling or iodination. The fulvene moiety can further be elaborated into a substituted indene by addition of a nucleophile to the exocyclic double bond followed by the trapping of the cyclopentadienyl anion with an electrophile.
Utility of the above processes has been highlighted by the synthesis of selectively deuterated fulvenes and indenes where deuterium was selectively incorporated as either radical, electrophile or nucleophile in different stages of the reaction
