**3.** *O***-isoacylpeptide method**

Various methods are reported for the efficient synthesis of long-chain and difficult peptide sequences. These peptides have a tendency of aggregation and low solubility, making its synthesis and purification laborious. Native chemical ligation (NCL) renders a highly effective and powerful method for the preparation of long-chain peptides [3]. This book introduces a novel "*O*-isoacylpeptide" method for the effective synthesis of long and difficult peptide sequences as in [4]. *O*-isoacylpeptides having ester bonds can be converted into the parent peptides under physiological condition *via* the *O* to *N* intramolecular migration. In addition a new approach called segment condensation with no racemization using the *O*-isoacylpeptide method is also introduced excellently [5]. This segment condensation can be treated as a substitute for NCL method.
