Unraveling the Photodeprotection Mechanism of Anthraquinon-2-ylmethoxycarbonyl-Caged Alcohols Using Time-Resolved Spectroscopy

Anthraquinone (AQ) compounds have been used as photolabile protecting groups (PPGs) to protect alcohols, ketones, and carboxylic acids. However, because of the lack of direct spectroscopic information for the transient species and intermediates related with the deprotection reaction(s), the photorelease mechanism(s) of these systems are still largely unknown. In this contribution, we detail a time-resolved spectroscopic investigation using anthraquinone-protected galactose (1) and adenosine (2) to investigate the photodeprotection reaction mechanism(s) of these kinds of AQ-PPGs. It was found that, in THF-H2O solvents, the ketyl radical species generated for 1 and 2 was found to be a reactive intermediate that then formed a dihydroxyanthracene species. The photodeprotection then occurred with the accompaniment of a decarboxylation process. These results provide an improved understanding for how AQ-PPGs work and will help assist in the design and applications of selected anthraquinone derivatives as a PPG platform, especially in aqueous environments more relevant for use in biological systems.