Conformational control in activation of an enediyne

In the bicyclo[7.3.1]tridec-4-ene-2,6-diyne framework characteristic of calicheamicin, DFT calculations predict that the chair conformer should be much more reactive toward cycloaromatization compared to the boat form. A functionalized derivative of this framework with an added two-atom bridge to enforce the boat conformation was synthesized and shown to be stable at 23 °C. Cleavage of the bridge releases the conformational lock and cycloaromatization proceeds with t_1/2 42.5 min/23 °C, presumably through the chair conformation. This confirms the prediction based on computation and points to a new principle for triggering the enediyne toxins.