General peptide coupling procedure D. The protected amino acid (1 eq) was dissolved in DMF (0.3 M), and the solution was cooled to 0C. HATU (1.2 eq) was added, followed by the amine (1 eq), then Et3N (10 eq). The reaction stirred 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.