Cycloaromatization 1,4-benzenoid diradical

The cycloaromatization of enediynes, having a structure like 1, proceeds via formation of a benzenoid 1,4-diradical 2, and is commonly called the Bergman cyclization. It is a relatively recent reaction that has gained importance especially during the last decade. The unusual structural element of enediynes as 1 has been found in natural products (such as calicheamicine and esperamicine) which show a remarkable biological activity... 

The key transformation of 3-ene-l,5-diynes is a thermal rearrangement that was disclosed in the early 1970s by Darby et al. and Lockhart and Bergman, and which is commonly called the Bergman cycloaromatization (Scheme 19.3). In the classical Bergman experiment, the deuterated enediyne 6 was transformed into the isomer 7 or, in the presence of a hydrogen donor, into deuterated benzene 9. This reaction is believed to proceed through a diradical benzenoid species (a 77-benzyne). ... 

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