Direct observation of ESR spectra of bicyclic nine-membered enediynes at ambient temperature

The ESR spectra for synthetic bicyclo[7.3.0]epoxydodecadienediynes in solution at room temperature are steady. These spectra originate from the Masamune-Bergman cyclization of bicyclo[7.3.0]epoxydodecadienediynes to p-benzyne biradicals and the equilibrium between the two forms. Comparison of the ESR spectra of the unlabeled and 13C-labeled nine-membered enediynes indicated that the spectra are not directly due to the p-benzyne biradicals but rather to more stable secondary radical intermediates.