In the using thiopyridyl ester activation strategies, peptide fragments of typically four residues or less were coupled typically to C-terminal glycine and proline residues to minimize potential for epimerization. Fragment couplings were typically performed after preparation of the thiopyridyl ester (e.g., 30) using PySSPy, triphenylphosphine, and 2-mercaptopyridine, all in 30-fold excess in dichloromethane. The C-terminal carboxylates were protected as tert-butyl esters (e.g., 28 and 31). The side chains were protected respectively as benzyl ethers and carbamates as well as nitro and N-toluenesulfonyl guanidine groups to enable selective tertbutyl ester removal and liberation of the C-terminal carboxylate using trifluoroacetic acid.