2-(1′-Hydroxyethyl)-anthraquinone as a photolabile protecting group for carboxylic acids

Using anthraquinone 2-yl ethyl (Aqe) as a photolabile protecting group for carboxylic acids, five caged compounds, Aqe esters of p-methoxybenzoic acid (1a), o-methylbenzoic acid (1b), benzoic acid (1c), p-nitrobenzoic acid (1d) and N-acetyl-l-tryptophan (1e), have been prepared, and their photochemistry was investigated. Upon 350 nm light irradiation, three caged compounds 1a–c in methanol solutions can efficiently release the corresponding carboxylic acids, their quantum yields ranging from 0.12 to 0.08. The intramolecular triplet–triplet energy transfer and the intramolecular electron transfer between triplet anthraquinone and the caged acids may occur, leading to a low efficiency of caged compounds 1d and 1e, respectively. Furthermore, based on quenching experiments, HPLC and spectral analysis, the mechanism of the uncaging reaction was suggested.