Capping of unprotected amino groups during peptide synthesis

A method for the solid-phase synthesis of lixisenatide SEQ ID NO:1), the method comprising coupling cycles of amino acid building blocks of lixisenatide to an amino acid chain,

• wherein said amino acid building blocks comprise an unprotected C-terminal carboxyl group and a protected N-terminal amino group comprising an Fmoc protecting group,
• wherein said amino acid chain comprises an unprotected N-terminal amino group, and
• wherein at least one coupling cycle comprises the steps:
• (a) coupling the amino acid building block C-terminally at the unprotected N-terminal amino group of the amino acid chain, so that an amide bond is formed between the amino acid chain and the amino acid building block,
• (b) contacting the product obtained in step (a) with a capping reagent comprising a capping compound, wherein the capping compound binds to an unprotected N-terminal amino group of the amino acid chain to which no building block has been coupled in step (a), and
• (c) de-protecting the N-terminal amino group of the amino acid building block,
• wherein the method comprises sufficient coupling cycles to produce lixisenatide (SEQ ID NO:1), and
• wherein step (b) is performed after coupling of the amino acid building block at positions Arg(20), Glu(17), Gln(13, Leu(10) or Gly(4) of the lixisenatide sequence.