In 10 ml of DMF, HCl, H-D-Trp-NH2 (8.9 mmol; 1 eq.), Boc-D-Trp-OH (2.98 g; 9.8 mmol; 1.1 eq.), NMM (2.26 ml; 2.1 eq.) and BOP (4.33 g; 1.1 eq.) were added successively. After 1 hr, the mixture was diluted with ethyl acetate (100 ml) and washed with saturated aqueous sodium hydrogen carbonate (200 ml), aqueous potassium hydrogen sulfate (200 ml, 1M), and saturated aqueous sodium chloride (100 ml). The organic layer was dried over sodium sulfate, filtered and the solvent removed in vacuo to afford 4.35 g of Boc-D-Trp-D-Trp-NH2 as a white solid.
