General peptide coupling procedure B. The protected amino acid (1 eq) was suspended in DCM (0.1 M), and Et3N (5 eq) was added, followed by Piv-Cl (1.1 eq). The reaction stirred at 0℃ for 1h. The amine salt (1 eq) was then added. The reaction stirred at 0℃ for 1h, then at room temperature overnight. Water was added, and the product was extracted with DCM. The combined organic layers were washed with saturated NH4Cl, saturated NaHCO3, and brine, dried over Na2SO4, filtered, and concentrated in vacuo to give the crude product. The crude material was purified by flash chromatography on a silica gel column to give the corresponding dipeptide.