Synthesis of cyclic imide containing peptide products

A method of synthesizing a peptide product comprising at least one cyclic imide group of formula (I) or a salt or solvate thereof:

• wherein
• R1 is a bridge of one or two atoms lengths,
• R2 is an amino acid side chain,
• * denotes an asymmetric C atom, and
• (*) denotes an optionally asymmetric C atom, comprising the steps:
• (a) coupling a synthesis building block of formula (II):
• wherein
• X is a base-labile amino protecting group,
• Y is an unprotected carboxy or carboxamido group,
• * denotes an asymmetric C atom, and
• Z is a carboxy group,
• to a peptide product of formula (III)
• wherein
• R2 is an optionally protected amino acid side chain,
• R3 is a peptidic residue bound to a solid phase carrier, and
• (*) denotes an optionally asymmetric C atom under conditions wherein the cyclic imide group of formula (I) is formed,
• (b) cleaving off the amino protecting group X,
• (c) optionally continuing the peptide synthesis, and
• (d) optionally purifying the peptide product (I).