Diketopiperazine formation may occur after deblocking the second amino acid, the penultimate residue of the peptide to be synthesized, especially in case of a C-terminal proline (generally: in case of Nα-alkylated amino acids). Losses due to this reaction resulting in premature cleavage from the resin can be considerably reduced by simultaneous coupling and liberation of the amino group from the trifluoroacetate.