Scheme 1.

Convergent synthetic strategy from three peptide segments of approximately equal size used for the total chemical synthesis of DKP ester insulin by ‘one pot’ native chemical ligations (R = –CH₂CH₂CO–Ala–COOH). The amino acid sequences are given in the single letter code. Color-coding corresponds to the peptide segments used in the synthesis. The key ester moiety is boxed in the top panel. The illustrations for ester insulin follow the convention used in Sohma et al. Angew Chem 2010.²⁴