Undec-10-en-5-ynyllithium, generated from the corresponding iodide by low-temperature lithium - iodine exchange, undergoes two sequential 5-exo cyclizations when warmed to room temperature to afford 2-methyl-1-cyclopentylidenecyclopentane in 85% yield. Attempted tandem cyclization of an analogous diacetylenic alkyllithium was less successful.