p-Nitromandelic Acid as a Highly Acid-Stable Safety-Catch Linker for Solid-Phase Synthesis of Peptide and Depsipeptide Acids

p-Nitromandelic acid as a safety-catch linker for Boc/Bzl-SPPS of base-labile compounds like peptides and depsipeptides is described. This linker permits acidic removal of side-chain protection groups from the resin. For cleavage from the solid support, the p-nitro group was reduced with tin(II) chloride. After washing off the reducing agents, the (depsi)peptide acids with or without the side-chain protection schemes were obtained by microwave irradiation at 50 °C with 5% TFA in dioxane.