Bipyridyl enediyne

While incorporating a second ring in the enediyne complex reduces the c-d distance between the acetylenic branches, conformational changes may alter the situation with respect to the activation barrier of the Bergman cyclization. The ligand, macrocyclic bipyridyl enediyne 3.882, was synthesized in two steps [398, 399] by the condensation of dibromide 3.870 with 3,3 -dihydroxy-6,6 -dimethyl-2,2 -bipyridyl 3.881 (Scheme 3.120) [24a, 400]. 

The bipyridyl group in bipyridyl enediyne 3.882 exists in the tran-soid conformation, where the atoms of the terminal acetylenic branches are separated from each other. After reaction with the metal ion to produce the chelate 3.883 the ligand adopts a cisoid conformation (Scheme 3.121[24a]). 

Scheme 3.121 Activation of bipyridyl enediynes by chelation to (Konig).

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