The invention relates to the synthesis of peptides, proteins and related bioconjugates, and in particular, to such synthesis using a peptide ligation method whereby a C-terminal salicylaldehyde ester peptide is reacted with an aminoacyl-N-hydroxl peptide. The invention also relates to the synthesis of cyclic peptides, including serinyl- or threonyl-containing cyclic peptides. The invention further relates to a solid phase synthesis of C-terminal salicylaldehyde ester peptides.