Model Studies for Newo-Nitrobenzyl Photolabile Linkers: Substituent Effects on the Rates of Photochemical Cleavage

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Abstract
Both a model phenacyl and o-nitrobenzyl photolabile linker from the literature along with four new o-nitrobenzyl linkers were prepared and the kinetics of their photolytic cleavage examined in solution. The linkers were prepared by amidation of the carboxylic acid anchoring tether with benzylamine, and the cleavable benzylic substituent was chosen to be either acetic acid or acetamide. Irradiation of the linkers in four solvents (methanol, p-dioxane, and aqueous buffer ± dithiothreitol) at 365 nm and analysis via HPLC afforded kinetic rates of cleavage suitable for comparative purposes. The phenacyl linker was found to cleave slowly under aqueous conditions with no detectable cleavage being observed in the organic solvents. Known o-nitrobenzyl linker 4 showed modest rates of cleavage in aqueous and organic solvents. Incorporation of two alkoxy groups in the benzene ring to generate the veratryl-based linker 13a increased the rate of cleavage dramatically, and introduction of an additional benzylic methyl group (13b) increased the rate of cleavage by an additional 5 fold. Increasing the length of the anchoring carboxylic acid tether from acetic to butyric acid (19) improved the cleavage kinetics modestly in organic media and slightly diminished the rates in water. The amide model linker 21 cleaved from 3 to 7 times faster than the corresponding ester linkage 19. An amide-generating linker 26 was prepared, and its performance to generate photolabile solid supports was briefly examined. The stability of the linker and subsequent cleavage upon photolysis from the support of an isotopically enriched 4-thiazolidinone was demonstrated by gel phase 13C NMR.

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