Development of Novel Amino Acids Containing N‐mercaptophenetyl (MPE)‐type Auxiliary and Orthogonal Protecting Groups for Sequential Ligation of Multiple Peptides

Chemoselective peptide ligation using ligation auxiliaries is a powerful methodology to extend the scope of native chemical ligation beyond ligation at Cys. Herein, we report the development of novel amino acids containing an N ‐mercaptophenethyl (MPE)‐type auxiliary and protecting groups (i. e., tert ‐butyloxycarbonyl (Boc), 1,3‐thiazolidine (Thz), and allyloxycarbonyl (Alloc)). These developed amino acids can be introduced into a peptide via 9‐fluorenylmethoxycarbonyl‐based solid‐phase peptide synthesis, enabling deprotection and native chemical ligation in one pot. The combination of Thz‐type and Alloc‐type acids enables sequential peptide ligation, which is applicable to the synthesis of peptides branched from a Lys side chain. We also found that the multiple N ‐MPE groups are efficiently cleaved upon treatment with tris(2‐carboxyethyl)phosphine and morpholine in the presence of SnCl 2 .