Photocycloadditions of Carbonyl Compounds to Alkenes

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol, 

Polycyclic oxetanes are obtained in good yields in intramolecular carbonyl-olefin cycloadditions, in an analogous way as the corresponding alicyclic systems are formed in intramolecular enone-olefin additions. Two applications are given in (4.78)492) and in (4.79)493). 

Benzoquinones can also serve as carbonyl compounds in this reaction. The deriving spirooxetanes can be converted to phenols (4.80)494). 

The mechanism of oxetane formation is similar to the one discussed for cyclobutane formation in chapter 4.3.3. The 1,4-diradicals can be efficiently trapped with molecular oxygen. The resulting 1,2,4-trioxanes are interesting synthetic intermediates (4.81) 495 . 

Imides 496) (4.82) and esters 497) (4.83) can also form oxetanes in inter- and intramolecular reactions. 

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