Photooxygenation, [2+2] and [4+2]

The great synthetic potential of peroxides accounts for the wide interest of scientific community to the photooxygenation reaction that represents one of the most versatile and simple procedures to prepare a variety of peroxides. Generally the reaction is carried out in the presence of a sensitizer whose role is important to determine the type of mechanism. The use of a sensitizer such as a dye whose energy of excited triplet state can be transferred to the ground state oxygen produces singlet oxygen (Type II photooxygenation) [1]. This species adds to unsaturated substrates by [4+2] or [2+2] or ene-mode leading to endoperoxides or dioxetanes or allylic hydroperoxides, respectively (Scheme 30.1) [2-4].