Radical-Based Cycloaromatization Processes

20) Once again, a favorite reaction (or the bain...) for many an organic chemistry undergraduate students. One of the authors (A)B) enjoyed immeasurably in the mid-80 s learning about this reaction as an undergraduate and the conceptually and intuitively beautiful symmetrical Woodward-Hoffmann rules that accompany it and act as the theoretical fabric on which this reaction is based. 

The influence of ring strain on the reactivity of the Bergman reaction is very important. Nicolaou and Smith [107] suggested that the crucial factor for determining the reactivity is the interatomic distance between diyne termini (popularly known as the distance theory). The influence of stereoelec-tronic effects is also important, with the observation that chlorine substitution in the vinyl position retards the rate of the Bergman cyclization [108]. 

21) The first total synthesis of calicheamycin was reported by Nicolaou and coworkers [105] in 1992. 

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

CpRu(PMe3)2CI, NH4PF6, MeOH, reflux, then room temperature 

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