| Abstract
| - The synthesis of two new aromatic cyclic enediynes 1a and 1b using a double Wittig condensation,followed by bromination and di-dehydrobromination, is reported. The structures were determinedusing X-ray crystallography. The observed C1−C6 distances of the enediyne moiety in the oxygen-and sulfur-containing eleven-membered rings are quite short with 3.44 and 3.50 Å, respectively,indicating the highly strained nature of these compounds. Isodesmic calculations at the B3LYP/6-31G* level of theory indicate a strain energy of 8.2 and 5.8 kcal/mol, respectively. Despite thehighly strained ring system, the compounds were found to be stable under thermal, photochemical,and electron-transfer conditions. Using BLYP/6-311+G**/BLYP/6-31G* calculations, the origin ofthis unusual stability was traced to the high energy of the 1,4 benzyne diradicals formed viaBergman cyclization. For the radical cation, both the C1−C6 and the C1−C5 cyclization modepreviously reported for acyclic analogues of 1 were found to be energetically prohibitive.
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