Pentamethylene chain conformational effects for the Bergman cyclization of the 11-membered ring enediyne, (3Z)-3-cycloundecene-1,5-diyne,     are examined theoretically with unrestricted B3LYP/6-31G(d,p) calculations. A C1 symmetric enediyne conformation was found to be the global minimum, where its asymmetric pentamethylene chain prevented p-orbital alignment of the acetylene groups and C-C s bond formation. The Bergman cyclization of the 11-membered enediyne was found to be conformationally dependent and stepwise. In a Curtin-Hammett type process, the C1 symmetric   conformer of the 11-membered enediyne inverts to one of the CS or C2 symmetric conformers required for the Bergman cyclization, which led to a CS or C2 symmetric 1,4-diradical intermediate. The activation energy for the cyclization is slightly higher to reach the C2 symmetry diradical compared to the CS symmetry diradical.

Towards the development of new anticancer drugs: The Bergman cyclization of an 11-membered ring-constrained enediynes

244th National Meeting (2012)

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