ABSTRACT:The reaction profile of the Bergman cyclization of enediynes is written as a cubic polynomial expansion about the critical distance. Using the "simple sewing" approximation, a relationship is derived that expresses the energy of activation as a function of the critical distance, with the effect of other geometric and electronic factors accounted for in the expansion coefficients. A training set of 10 representative enediynes were selected and input parameters were calculated by density functional theory at the B3LYP/6-311G* level of theory. All calculations were corrected for zero-point energy. All singlet biradical calculations were also checked for wave function stability. Calibration curves were then constructed that correlate the activation energy of cyclization of the molecules in the test set with the critical distance parameter.
It has been shown that the enediyne anticancer antibiotics e.g., calicheamicin, neocarzinostatin and others cleave DNA via the putative intermediate 1,4-diradical formed in the Bergmann cyclization and are thus useful for cancer chemotherapy. The pharmacological activity of these drugs is based, in general, on the activation of the pharmacophore and the subsequent cyclization leading to the formation of a radical, the rate of which is, in part, based on the terminal distance of the 1,5-diyne functionality, also known as the critical distance. But the critical distance alone cannot govern the rate of cyclization of the enediynes. A theoretical model has been developed to predict the rate of cyclization, and the thus the pharmacological activity, of these antibiotics based on the critical distance and the energy of activation.
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