Photochemical reactions applied to the synthesis of helicenes and helicene-like compounds

• Photocyclization is a selective and versatile method for the preparation of helicenes.
• Helicene synthesis with the photolytically supported Vollhardt is reported.
• Both reactions are key steps in the synthesis of helicenes containing heterocycles.
• The preparation of enantiopure helicenes is reported.

Helicenes are composed of ortho annellated benzene moieties. Similar compounds contain heterocyclic or dihydrobenzene rings or smaller rings such as cyclopentadiene or cyclobutadiene (as part of benzocyclobutene units). The present article resumes photochemical reactions used for the preparation of these compounds. A very important and generally applicable reaction used for the synthesis of helicenes and helicene-like compounds is the photocyclization of stilbene subunits followed by oxidation. This reaction is often highly regioselective. The reaction can be conducted in the way that the formation of the helicene structure is favored. This selectivity is caused by the sum of the free valence numbers in the different positions of the stilbene precursor. Very fascinating structures are obtained with the cobalt catalyzed Vollhardt reaction which is photolytically supported. Helicenes are chiral and different methods of asymmetric synthesis were applied to the preparation of these compounds. A very convenient method is optical resolution using HPLC which is now currently used.

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