An Unexpectedly Mild Thermal Alder−Ene-Type Cyclization of Enallenes

A mild, thermal Alder−ene reaction of enallenes has been developed. The allenic double bond acts as the “ene” and generates a carbon−carbon bond to an unactivated olefinic “enophile” in DMF at 120 °C to give [n.3.0] bicyclic systems (n = 3−5) in good yields. Except for a minor [2 + 2] cycloaddition byproduct, the reaction proceeded with complete atom economy, as there is no requirement of a catalyst or additional reactants, and no waste products are formed in the process.