Photoremovable protecting groups

Fig. 1 Caging fragrance molecules to a photoremovable protecting group (a) and releasing the fragrance molecules by exposure to sunlight (b).

The most extensively used photoremovable protecting groups are based on o-nitrotoluene derivatives, which, on exposure to light, undergo intramolecular H-atom abstraction to form photoenols (Scheme 9), followed by the elimination of their protected or caged substrate. ... 

PHOTOREMOVABLE PROTECTING GROUPS BASED ON PHOTOENOLIZATION 49 Table 1 Quantum yields for photoelimination from 18... 

In 2003, Banerjee et al. designed an efficient photoremovable protecting group for the release of carboxylic acids based on similar p-elimination from photoenols (Scheme 14). They showed that o-alkyl acetophenone derivatives with various ester groups in the p-position release their ester moiety in high chemical yields. The authors proposed that the photorelease took place as shown in Scheme 14 but did not support the mechanism with transient spectroscopy. Formation of 21, which is expected to be the major product in the reaction, was not confirmed, and thus, the authors speculated that 21 undergoes polymerization to yield oligomers. 

Scaiano and coworkers designed the two new photoremovable protecting groups 27 and 28, based on ketoprofen 25. Photolysis of 27 in aqueous solutions releases ibuprofen in quantum yields of 0.75 (Scheme 19). Furthermore, the rate constant for the release of ibuprofen from 27 was measured to be 5 x 10 s Thus, 27 can release ibuprofen both rapidly and in high quantum yields. 

Recently, Falvey and coworkers have designed photoremovable protecting group... 

Jones et al. have designed photoremovable protecting group 32 for aldehydes that is based on a somewhat similar structure to The authors proposed that the... 

L = Sulfonate esters or phosphonate esters 44 L = Carbonates and carbamates Scheme 26 Photoremovable protecting groups 33, 43, and 44. 

PHOTOREMOVABLE PROTECTING GROUPS BASED ON PHOTOENOLIZATION 57 Table 3 Quantum yields for photorelease from 44... 

We have designed photoremovable protection groups 45, 46, and 47, which release... 

In comparison, photoremovable protecting groups such as 69 also release their protected molecule via intramolecular lactonization, but the photorelease is initiated by cis-trans isomerization. Cis-trans isomerization has been extensively used to turn molecular switches off and on, but there are a few examples where cis-trans isomerization has been used to activate photorelease. Initially, Stoermer showed that photoinduced cis-trans isomerization of trans-69 yields 71, presumably via intramolecular lactonization of cis-lQ (Scheme 37). In a similar way. 

Even though most commonly used photoremovable protecting groups in applications are based on o-nitrobenzyl derivatives, the mechanistic details of photorelease from nitrobenzyl derivatives were not known. Recently, Wirz et al. and Corrie et al. investigated the mechanism. The mechanism of photorelease from the photo-enols formed by irradiating 2-nitrobenzoyl compounds is more complex than for o-alkyl arylketones. 

Steiner et al. designed photoremovable protecting group 103 for photolithographic in situ DNA-chip synthesis (Scheme 49). " These protecting groups have a... 

Robles and Bochet showed that nitrotoluene derivatives 106 (Scheme 50) can be used as a photoremovable protecting group for aldehydes. Irradiation of 106 released aldehydes in good yields, and the authors demonstrated that aldehydes such as phenylacetalaldehyde and citronellal, which are important for flavor and fragrances, are released efficiently from 106. 

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