Substrates 1,3-dienes, photooxygenation

C. Conjugated Diene Systems as Substrates of Photooxygenation Reactions... 

Substrates for Diene Photooxygenation Carbocyclic Substrates Heterocyclic Substrates . . 25-3... 

The main types of substrates investigated so far are polycyclic aromatic compounds, aryl substituted carbo- and heterocyclic pentadienes, cyclic 1,3-dienes, furans, and olefins. It has turned out that type II photooxygenation of these compounds in solution occurs via the oxygen-activation mechanism. 

Until recently, the hypothesis that the termination reaction of type II photooxygenation reactions occurs between the substrate and an excited light absorber-oxygen complex seemed to be well established. The typical products obtained from cyclic 1,3-dienes and olefins (see Fig. 1 and Sect. IV) could only be made by photochemical reactions. 

Singlet molecular oxygen (lC>2) is a very small electrophilic reagent that can produced by photosensitization and react easily with dienes and alkenes in [4 + or [2 + 2] cycloadditions or ene-reactions. In order to develop synthetic applic tions of photooxygenations, numerous studies have recently been developed wi chiral substrates and especially in the presence of chiral auxiliaries. 

On this ground, DCA was found a suitable sensitizer to induce the photooxygenation of a great variety of organic compounds such as alkenes [84,94-98], alkynes [99,100], sulfides [84,98,101], dienes [84], sulfoxides [102], cycloalkanes [103,104], cycloalkenes [105,106], epoxides [107,108], aziridines [109], allenes [110], dioxenes [111], p-dioxins [111,112], aromatic substrates [113], tertiary amines [114], and of great interest from the mechanistic point of view, sterically hindered olefines [97,115-117]. 

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