EDC·HCl and Potassium Salts of Oxyma and Oxyma‐B as Superior Coupling Cocktails for Peptide Synthesis

Nowadays, DIC is the most widely used carbodiimide for solid‐phase peptide synthesis, while EDC · HCl is mostly used only for solution‐phase synthesis. In this paper, we report new coupling cocktails containing EDC · HCl in combination with potassium salts of OxymaPure and Oxyma‐B (i.e., K‐Oxyma and K‐Oxyma‐B, respectively). These reagent cocktails gave spectacular purity compared to DIC/classical N ‐hydroxylamine derivatives in the solid‐phase peptide synthesis of the Aib‐enkephaline (Aib = 2‐aminoisobutyric acid) pentapeptide (H‐Tyr‐Aib‐Aib‐Phe‐Leu‐NH 2 ), a hindered peptide. Furthermore, we found that the EDC · HCl/K‐Oxyma combination can be used with DMF, THF, or MeCN as the solvent. The optimized cocktail gave less racemization than benzotriazole derivatives, but slightly more than OxymaPure and Oxyma‐B during stepwise solution‐phase peptide synthesis.