(1E,5E)-Cyclopentadeca-1,5-dien-3-yne (1c), which represents the first macrocyclic 1,5-dien-3-yne, can be obtained by thermal- or butyllithium-induced fragmentation of the corresponding 1,2,3-selenadiazole 8. The (E,E)-dienyne functionality causes a geometrical strain E(g), which enhances the reactivity in addition (1c -> 2,13) and cycloaddition (1c -> 10) reactions and lowers the isomerization barrier to the unstrained (E,Z)-configuration 1d (E(g) = 0). A slow process 1c -> 1d occurs even at ambient temperatures within several weeks. (C) 2010 Elsevier Ltd. All rights reserved.