A new photoremovable protecting group absorbing above 500 nm : (6-Hydroxy-3-oxo-3H-xanthen-9-yl)methyl and its derivates

A new water-soluble photoremovable protecting group for carboxylic acids and phosphates
with high molar extinction coefficients (ελmax ~ 25 000 dm3mol-1cm-1) in the visible region
(above 520 nm) was designed and tested. The suggested concept based on the photochemistry
of coumarinyl PPG proved to work also for the compounds derived from 6-hydroxy-3-oxo-
3H-xanthen-9-yl)methyl. The introduction of an additional aromatic core to the coumarinyl
unit resulted in a substantial shift of the absorption towards longer wavelengths and also
caused a drop in pKa of the phenolic protons to about 6, which then caused the anionic form of
the molecule to be the prevalent species at neutral pH. This is of advantage since it improves
the solubility of these compounds in aqueous media and also because the anionic forms of the
cages are even further red-shifted with respect to the neutral ones.
The model cages released the protected bromide, acetate and/or diethylphosphate upon
irradiation by visible light in neutral aqueous solutions (the cleavage of the free anions was
indicated by a drop in pH and confirmed by NMR).
The quantum yields of photodeprotection of the model compounds were relatively low but
still comparable to those of coumarinyl cages. The quantum yields could probably be
enhanced by a suitable substitution. The efficiency of the photoreaction is also likely to be
higher in the case of cages of other phosphates and/or acyls of biochemical interest, which are
better leaving groups than diethyl phosphate or acetate.
In the dark, the model compounds were stable for several hours (up to 1 day) in aqueous
solutions at room temperature. The stability should thus be sufficient for potential
biochemical applications.
The fluorescence lifetime of one of the model compouds, 9-(bromomethyl)-6-hydroxy-3Hxanthen-
3-one was found to be (331 ± 30) ps.
The last problem that remains to be solved is an unequivocal identification of the final
photoproduct and of the reaction intermediates. The first step of the photoreaction is most
probably analogous to that of coumarinyl cages, i.e. the elimination of the protected species
(free acids) and the nucleophilic attack of the resulting cation by water, which leads to the
formation of the primary photoproduct, 6-hydroxy-9-(hydroxymethyl)-3H-xanthen-3-one.
This compound is in equilibrium with its enol tautomer and also with another tautomer:
3,6-dihydroxy-9H-xanthene-9-carbaldehyde. One or more of these tautomers react further to
give a final product, in which the original chromophore is restored again (testified by UV-Vis
spectroscopy and NMR). The UV-Vis spectrum of the final photoproduct overlaps with the
spectrum of 6-hydroxy-3H-xanthen-3-one but the NMR analysis revealed that the two compounds
were not identical. Further analyses and attempts to identify the photoproduct and the reaction
intermediates are still in progress.