Diradicals precursors, ketones

Photocycloadditions are common and usually involve diradical intermediates photo-excited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and the triplet (n, it ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

The classic precursors used for generating diradicals are cyclic, bicychc and polycyclic diazenes. However, diradicals have also been made by Norrish type I photochemical extrusion of CO from cychc ketones, by thermal cleavage of vinyl and divinylcyclopropanes, by pinacol reactions of diketones, by Bergman-type cyclisations of endiynes, by several types of photoelectron transfer reactions and in other ways. Most synthetic applications have started with a derivative of 2,3-diazabicyclo[2.2.1]hept-2-ene which on heat-... 

Enediynyl ethyl ethers like 12 have been applied by Wang s group as precursors of enyne-ketenes 13, which underwent the Moore cyclization reaction to form diradicals. The intermediate 14 and (after 1,5-hydrogen shift) new diradical 15, form, upon the cascade transformations, the final products mainly chromanol 16 and spiro ketone 17. The latter is a result of subsequent intramolecular reaction of intermediates o-quinones methide and spiro ketone (not illustrated). ... 

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