Protecting Groups Cleaved by Light

The major limitations to photodeavage are (a) the relative inefficiency of some phototransitions (low quantum yield) leading to long irradiation times (b) the high reactivity and indiscriminate reactions of some photoexcited states and (c) the reaction of some photocleavage products with the substrate. Obviously the substrate cannot contain a photoexcitable functional group. 

The use of phenacyl esters as photolabile protecting groups in organic synthesis is rare save for their use in the protection of phosphates (see section 7.3.6). 

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Figure 4.7 exhibits examples of diblock copolymers with irreversible photo-response. Polymer 30 contains photolabile protecting groups attached to carboxylic groups in the side chains. Upon UV irradiation, photosolvolysis of the pyrenylmethyl ester occurs, 1-pyrenemethanol is cleaved from the polymer chain and carboxylic add groups are released. As a consequence, the hydrophobic block turns into a hydrophilic PMAA block. Core-shell micelles formed by 30 disappeared after irradiation with UV light at 365 nm. This design was further validated with other chromophores (polymers 31-34). °... 

Carboxyl protection. Esters of the title reagent 1 are cleaved photochemically under specific conditions. The nitro group, while existing mainly in the nitronate form due to resonance interaction of the p-amino group, is therefore photoinactive. By addition of TfOH or Cu(OTf)2 to remove the resonance the susceptibility to decomposition by UV light revives. 

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