A method of chemical ligation of peptides that requires no side chain protecting groups and no activation of the C-.alpha. carboxyl group is presented. The method consists of three steps. In the first step, initiation, a masked glycoaldehyde ester is enzymatically or chemically coupled to the C-terminal carboxylic acid of an sidechain unprotected first peptide. In the second step, ring formation, the masked aldehyde ester of the first peptide is unmasked, and then reacted with the N-.alpha. amino acid of a second sidechain unprotected peptide to form a ring structure. In the third step, rearrangement, the O-acyl ester linkage transfers at higher pH to an N-acyl linkage on the ring to form a peptide bond.