Trigger Mechanisms for the Cycloaromatization of Enediynes

Triggering Through Transformation of Enediyne to Eneyne-Cumulene 

There are several approaches to the activation of oxygen-containing enediynes based on the acid-catalyzed conversions of nonreactive precursors [341-344]. 

The first approach is triggering through the ene-allene rearrangement caused by decarboxylation of methoxymalonate terminating one acetylene branch of enediyne. An enediyne model 3.744 in basic medium produces enyne-allene 3.745. The latter undergoes the Myers-Saito cyclization and generates toluic diradicals 3.746 and through a cascade 

It is noteworthy that these reactions include Myers-Saito cyclization through the diradical of s,p-type tolyl diradicals 3.746, 3.752A, and 3.758. Shibuya and coworkers [341-344] concluded that the diradical 

The diradical structure and the zwitterionic structure have been described as contributing structures to a resonance hybrid. However, the zwitteronic structure has also been described as an excited state of the diradical [345]. In the diradical structures, the odd electron on the benzene ring occupies a sigma orbital while the benzylic odd electron occupies a p orbital which makes a classical conjugative interaction impossible. However, this benzylic p electron can delocalize onto the aryl 

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