Photoenolization review

For example, 7-phenylbenzocyclobuten-7-ol (568) (obtained from 563) when heated with maleic anhydride in toluene gave acid lactone (571) in 75% yield (Scheme 131). This result is consistent with an endo-[4 + 2] cycloaddition of the dienophile to the ( )-dienol (569) followed by lactonization of cycloadduct (570). The cycloadduct (570) was, furthermore, obtained in 80% yield by the photoenoliza-tion of o-methylbenzophenone (572) in the presence of maleic anhydride at room temperature. It follows that the photoinduced 1,5-hydrogen shift of (572) must have generated the same ( )-dienol (569) as that obtained by the thermal ring opening of (568). This and other aspects of photoenolization have been reviewed. ... 

Other Processes. - The quantum yield for the release of benzoic acid from 2,5-dimethylphenacyl benzoate is temperature dependent in benzene solution. At room temperature = 0.22, while at 50°C the value rises to 0.28. A much greater effect is observed in methanol or ethanol, when there is a threefold increase in the quantum yield. The authors suggest that the reaction in heated methanol enhances the -photoenol population. Givens and Lee have reviewed the use of the j9-hydroxyphenacyl moiety as a photoprotecting group for biological substrates. 

Norrish type II cleavage Photoenolization (s. a. 3-Elimination, photo-enolization-induced) review 31,128 Photoflnorination 32, 484 Photo-Friedel-Crafts synthesis, intramolecnlar 32, 975 mem-Photo-Fries rearrangement, skeletal 31,685 Photoisomerization... 

The singlet excited state (V) was excluded as the intermediate imder-going isomerization due to the long lifetime of the first transient. The photoenol (VI) which absorbs in the visible, reverts back to the ketone in the dark. Such behavior is a common phenomenon in photochemistry. In general, systems which assume color upon irradiation, but revert to colorless in the dark, are said to be phototropic (for a review on phototropic systems see Dessauer and Paris, 1963). 

---