Expanding the scope of sustainable peptide synthesis through post-linear synthesis reactions

The replacement of N,N-dimethylformamide (DMF) in solid-phase peptide synthesis (SPPS) is critical for improving the sustainability and safety of peptide research. While N-butyl-2-pyrrolidinone (NBP) has emerged as a viable alternative for linear peptide synthesis, its applicability to post-linear synthesis modifications remains underexplored. Here, we demonstrate the use of NBP as an alternative solvent for key on-resin transformations required in peptidomimetic synthesis. Orthogonal protecting group strategies were successfully implemented, enabling selective removal of Cys(SIT), Cys(Acm), Cys(Trt), and Lys(Dde) under mild conditions. Notably, SIT deprotection achieved 92% conversion in a single 4-hour treatment, while Acm and Trt were selectively removed using iodine or N-halosuccinimides, respectively. Lys(Dde) was efficiently cleaved in the presence of Cys(Acm) using 4% hydrazine in NBP. Furthermore, on-resin cyclisation via disulfide bond and 1,4-disubstituted triazole formation proceeded effectively in NBP, as did N-terminal acetylation under mild conditions. These findings establish NBP as a practical and greener alternative to DMF for diverse post-linear synthesis modifications, supporting the development of sustainable SPPS protocols.