5-Oxaproline: α-Ketoacid-Hydroxylamine (KAHA) Amide-Forming Ligation Reagent

(S)-2-Boc-isoxazolidine-3-carboxylic acid is used in chemoselective ligations with &alpha-ketoacids (KAHA ligation). The KAHA ligation occurs between a peptide segment bearing an N-terminal 5-oxaproline and an another peptide segment bearing an C-terminal α-keto acid in the presence of 0.1M oxalic acid at 60 °C, giving a synthetic protein bearing a homoserine moiety. The method using 1 is expected to have a powerful impact on the synthesis of bioactive peptides and proteins.