Evaluation of unexpected protecting group removal in solid-phase peptide synthesis: Quantified using continuous flow...

As peptides gained interest as new drugs in the past years, their synthetic routes had been the subject of review and improvement. Fmoc/tBu-based solid-phase peptide synthesis (SPPS) is the most convenient technique, and the implementation in continuous flow has allowed for single-pass coupling and deprotection reactions. The focus of this research is to evaluate the relationship between undesired solvent-promoted reactions and final crude purity, by studying volume changes of a variable bed flow reactor through the synthesis. Based on the temperature, typical solvents for SPPS such as dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) can cause unwanted Fmoc removal during wash steps. It was found that for every millilitre of DMF used at 80°C, up to 1 μmol of Fmoc-protected peptide is deprotected, leading to additional impurities. This effect can, however, be minimised by adding additives such as HOBt, which reduces such unwanted deprotection to just 0.1 μmol/ml.