Polycyclic aromatic systems, photocyclization reactions

There are a large number of photocyclizations reactions of polycyclic aromatic systems that all proceed through antiaromatic transitions states. The cyclization " of cis-stilbene (105) and the related diarylethylene are classical examples of this reaction,... 

Two types of reaction are observed for thioketones that do not have parallels in ketone photochemistry. The first is photocyclization in thioketones with polycyclic aromatic groups (4.104), where the sulfur atom forms a new bond to the aromatic system. The second is photo-oxidation by singlet oxygen to give the corresponding carbonyl compound 4.10S), possibly via a 1,2,3-dioxathietane formed as a result of initial cycloaddition. 

A new synthetic approach to polycyclic aromatic compounds has been developed based on double Suzuki coupling of polycyclic aromatic hydrocarbon bis(boronic acid) derivatives with o-bromoaryl aldehydes to furnish aryl dialdehydes. These are then converted to larger polycyclic aromatic ring systems by either (a) conversion to diolefins by Wittig reaction followed by photocyclization, or (b) reductive cyclization with trifluoromethanesulfonic acid and 1,3-propanediol (Eq. (12)) [30]. 

Pholocyclization reactions of 3-chlorothiophenes have been utilized to prepare a variety of thiophene-containing polycyclic aromatic hydrocarbons. Recently, the preparation of the novel ring system, naphtho(2 ,r 43Ithieno[23-c]naphtho[2,l-/]quinoline, has been reported using a photocyclization reaction <01JHC137>. 

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