Studies on a Dithiane-Protected Benzoin Photolabile Safety Catch Linker for Solid-Phase Synthesis

Substituted benzoinyl systems 8a-g, differing either in the substitution pattern, type of resin matrix used, or resin loading capacity, were prepared and the kinetics of their photolytic cleavage of Fmoc-beta-alanine examined on resin. The linker systems 6a-g were assembled in near-quantitative yield using Corey-Seebach dithiane addition. The dithiane group that serves as a safety catch against premature photoreaction was removed by either oxidation or alkylation. Analytical methods that include FTIR and (13)C gel-phase NMR spectroscopy were used for rapid reaction monitoring and sample characterization on resin. A survey of different substituted systems 8c-f for releasing Fmoc-beta-alanine confirmed that the 3-alkoxybenzoin linker photocleaves most rapidly to give the highest yield (tau(1/2) = 6.7 min; 98% yield). Lowering the resin loading from 0.59 mmol/g in 8a to 0.26 mmol/g in 8b improved the cleavage kinetics to tau(1/2) = 2.6 min, 92% yield. Tentagel resin 8g exhibits similar photocleavage kinetics in both organic and aqueous media and when compared to the polystyrene counterpart, 8a. The 3-alkoxybenzoin linker 6a was also loaded with aryl carboxylic acids (12h,i) and hindered Fmoc-protected amino acids (12j-l) with varying degrees of success (57-100%) and dithiane deprotected (70-80%, 13h-l) followed by photocleavage with comparable efficiencies (89-93% after 60 min).