1,4-benzenoid diradical

The biological activity of calicheamicin 4 (simplified structure) is based on the ability to damage DNA. At the reaction site, initially the distance between the triple bonds is diminished by an addition reaction of a sulfur nucleophile to the enone carbon-carbon double bond, whereupon the Bergman cyclization takes place leading to the benzenoid diradical 5, which is capable of cleaving double-stranded DNA." ... 

The names p-xylylene and p-quinodimethane might indicate two different electronic states of the same species. Etymologically, the former is a benzenoid diradical (paramagnetic), while the latter is its isomeric quinonoid structure (diamagnetic). Actually, those names are frequently used interchangeably, regardless of the electronic state. 

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... 

Phenylenyl monoradical (C13H9) is a strictly peri-condensed PAH6 with the methyl radical as an excised internal structure and both have e = 0. The diradical C22H12 isomer, triangulene, is a strictly peri-condensed benzenoid hydrocarbon and has trimethylenemethane diradical as an excised internal structure both triangulene and trimethylenemethane diradicals have two eigenvalues of s = 0. 

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). ... 

In a series of papers Dias has derived expressions for resonanee-strueture counts of several classes of mono- and diradical benzenoid hydrocarbons. The considered classes belong to polymer graphs which were extensively studied some... 

It attacks the DNA, provoking lethal double-strand cuts to the genetic material with transformation of the diradical to a benzenoid system. 

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