Pentamethylene chain conformational effects for the Bergman cyclization of the 11-membered ring enediyne, (3Z)-3cycloundecene-1,5-diyne, 2, are examined theoretically with unrestricted Becke, three-parameter, Lee-Yang-Parr/6-31 G(d,p) calculations. A C 1 symmetric enediyne conformation was found to be the global minimum, where its nonsymmetric pentamethylene chain prevented p-orbital alignment of the acetylene groups for C-C s bond product formation. The Bergman cyclization of 2 was found to be conformationally dependent. In a Curtin-Hammett type process, the C 1 symmetric 2 inverts to one of the C S or C 2 symmetric conformers required for the Bergman cyclization, which produced a C S or C 2 symmetric 1,4-diradical intermediate. The activation energy for the cyclization is slightly higher to reach the C 2 symmetry diradical compared with the C S symmetry diradical. a Enthalpies at 298 K: ΔH 298 = Δ(E 0 + H corr ), E 0 = the total electronic energy, H corr = E tot + k b T, E tot = internal thermal energy, k b = Boltzmann constant. b Energies at 298 K: Δ(E 0 + ZPE).