Intramolecular ene reaction of 1,6-fullerenynes: a new synthesis of allenes.

[Structure: see text] Thermal treatment of 1,6-fullerenynes bearing an alkyl group on the terminal carbon of the alkyne moiety leads quantitatively to new allenes through a reaction mechanism involving an intramolecular ene process. This reaction outcome is in contrast to that recently found for free terminal alkynes which form cyclobutene derivatives through a [2+2] cyclization mechanism.