The synthesis was carried out according to the scheme shown in FIG. 4 (R═CH2Ph). To a suspension of Boc-D-Phe-OH intermediate I-1 (7.96 g, 30.0 mmol), D-Leu-OBn p-TsOH intermediate I-2 (11.80 g, 30.0 mmol), HOBt monohydrate (4.46 g, 33.0 mmol) and DIEA (8.53 g, 66.0 mmol) in anhydrous THF (250 mL) cooled in an ice-water bath was added EDCI (6.33 g, 33.0 mmol) in four portions over 20 minutes with 5 minutes between each addition. The suspension was stirred overnight from a starting temperature of 0° C. to room temperature. After evaporation of THF, the residue was dissolved in ethyl acetate and washed sequentially with 10% citric acid, saturated NaHCO3 and water. The organic phase was dried over sodium sulfate and evaporated under reduced pressure. The residue was dissolved in DCM, passed through a silica gel plug and eluted with 20% ethyl acetate in hexanes. The eluant was evaporated to give the pure product, Boc-D-Phe-D-Leu-OBn, intermediate I-3 (12.40 g, 88%) as a clear oil. LC-MS: m/z=469 (MH).
