Development of Thiol-Labile α-Amino Protecting Groups for Efficient Solid-Phase Peptide Synthesis

Recently, a thiol-labile α-amino-protecting group, 2,4-dinitro-6-phenyl-benzenesulfenyl (DNPBS), is introduced for solid-phase peptide synthesis (SPPS). DNPBS-SPPS effectively mitigates major side reactions-such as aspartimide formation and α-C racemization-that commonly occur in conventional Fmoc-SPPS. However, N-DNPBS-protected histidine is unstable during the base-catalyzed peptide bond coupling, leading to histidine redundant insertion byproducts (Nat. Commun. 2023, 14, 5324). In this study, modifications to the nitrobenzenesulfenyl protecting group are explored and their impact on chemical stability and thiol-mediated deprotection kinetics are systematically evaluated. It is found that introducing a C6 substituent and replacing the phenyl group with a pyridine ring significantly modulate both stability and thiol susceptibility. Notably, 3-nitro-2-pyridinesulfenyl (Npys) outperforms DNPBS in SPPS. N-Npys-protected histidine exhibits markedly greater stability than its N-DNPBS-protected counterpart, effectively preventing histidine redundant insertion byproducts commonly observed in DNPBS-SPPS. This study provides valuable insights into optimizing nitrobenzenesulfenyl-based protecting groups for more efficient SPPS.