Photoprotection scheme

Scheme 1 Photochemical activation of a biomaterial by light-induced cleavage of a photoprotective group.

Investigation of different a-amino photoprotective groups and conditions for their re-movalf resulted in selecting the a-methyl-substituted (6-nitro-l,3-benzodioxol-5-yl)methyl-oxycarbonyl (a-methyl-substituted Npoc) groupt 4 (Scheme 3). It can be quantitatively removed by photolysis under neutral conditions in 3-4min. 

Scheme 3 Summary of photo-triggered cycloaddition reactions useful in biological applications, a Reactions between aUcenes and photochemically generated 1,3-dipoles or 1,3-ienes. b SPAAC between photoprotected cyclooctynes and azides...

Scheme 4.1 The photoprotective reaction cascade interrelating members of the tridachiahydro-pyrone family. Inset known antioxidant molecules used for comparison in these studies...

The observed trend of antioxidant power decreasing in the order 44 > 43 > 9 is consistent with the theory that the origin of the photoprotective properties of the compounds lies in their ability to absorb UV radiation, thus preventing it from causing cell damage. This is most easily explained with reference to Scheme 4.1. As the parent compound of the tridachiahydropyrone family, 44 has the potential to undergo three discrete sequential photochemical transformations, namely a selective double bond isomerisation to form 43, followed by a 6k conrotatory... 

The above cyclic mechanism has been carefully scrutinized by many workers in the past few years, and, in fact, it has been concluded that it alone cannot fully account for the high photoprotective efficiency of the parent amine molecule. The nitroxyl radical itself is a radical scavenger but is not as effective as hindered phenols in competing with oxygen for alkyl radicals. To account for this deficiency in the cyclic mechanism it has been suggested, and indeed confirmed, by many workers, that hindered piperidine stabilizers and their derived nitroxyl radicals form weakly bonded localized complexes with hydroperoxides in the polymer (Scheme 8). This mechanism raises the local concentration of nitroxyl radicals in regions where alkyl radicals are generated after the... 

Photolysis of a 0.1-mM solution of the photoprotected thymidine 65 in a 1 1 methanol/water mixture at 365 nm resulted in the release of thymidine (66), carbon dioxide, and the photolabile o-nitrostyrene derivative 67 (Eq. (69.33).The photorelease is believed to occur through a P-ehmination mechanism from the aci-nitro intermediate 67 shown in Scheme 10. 

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