Photolabile protecting groups for nucleosides: Mechanistic studies of the 2-(2-nitrophenyl)ethyl group

The photochemistry of several 2-(2-nitrophenyl)ethyl-caged compounds includ ing caged thymidine nucleosides was studied by nanosecond laser flash photo lysis and stationary illumination experiments with quantitative HPLC analys is for quantum yields and product distribution, Effects of solvent basicity and acidity were investigated by varying the H2O content and HCl concentra tion. respectively, in MeCN/H2O mixtures. For all compounds 1 - 7 investiga ted, intramolecular H abstraction by the nitro group from the exocyclic a-p osition with respect to the aryl moiety was found to be the primary process . The protolytic dissociation equilibrium of the resulting aci-nitro compou nd was kinetically characterized in the 0.1 - 10 mus time region. In genera l, two reaction channels compete for the ati-nitro compound anti its anion: beta -elimination of the caged compound occurs from the anion, while from the undissociated aci-nitro compound, a nitrosobenzene derivative is formed with no release of the caged compound. The yield ratio of these two reacti on channels can be controlled through shifts in the protolytic dissociation equilibrium of the aci-nitro compound. In solutions with either low basici ty (H2O-free MeCN) or high acidity (higher concentration of HCl in H2O/MeCN ), two as yet unidentified products are formed, each one specifically for o ne of the mentioned conditions.