A mixture of 2-[5-benzyloxycarbonylamino-2-(2-nitro-benzenesulfonylamino)-pentanoylamino]-3-naphthalen-2-yl-propionic acid methyl ester, 40, (2.4 g, 3.63 mmol), dibromoethane (3.8 mL, 4.36 mmol), potassium carbonate (5.0 g, 36.3 mmol) in DMF (50 mL) is stirred at 55° C. for 18 hours. The reaction mixture is cooled to room temperature, treated with 1M KHSO4 and the resulting solution is extracted with ethyl acetate. The crude product is purified over silica (sequential elution with EtOAc/hexanes mixtures 1:2, 1:1, 100% EtOAc, then EtOAc with 5% MeOH) to afford 2.76 g of the desired product.
