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 SnCl2.