Intramolecular [4 + 2] Cycloaddition Reactions of Diarylacetylenes:  Synthesis of Benzo[b]fluorene Derivatives via Cyclic Allenes

2-Propynyldiarylacetylenes undergo thermal intramolecular [4 + 2] cycloaddition to give benzo[b]fluorene derivatives in good yields. The hybridization of the tether connecting the reacting alkynes has a pronounced effect on the course of the reaction. Theoretical calculations and isotopic labeling studies support a mechanism which involves the generation of a cyclic allene intermediate that evolves to the final benzo[b]fluorene.