To a stirred solution of (4S,5S)-4-[2-(4-amino-phenyl)-ethyl]-2,2,5-trimethyl-oxazolidine-3-carboxylic acid tert-butyl ester (120 mg) and 5-ethoxypyrazine-2-carboxylic acid (72.4 mg) in THF (3 ml) were added N-methylmorpholine (158 μl) and TBTU (230 mg). The reaction mixture was capped and shaken at 50° C. overnight. The crude reaction mixture was concentrated in vacuo and the residue was purified by flash chromatography (silica gel, gradient: 0% to 70% EtOAc in hexane) to afford (4S,5S)-4-(2-{4-[(5-ethoxy-pyrazine-2-carbonyl)-amino]-phenyl}-ethyl)-2,2,5-trimethyl-oxazolidine-3-carboxylic acid tert-butyl ester as a white solid (167 mg, 96%). MS (ISP): 507.3 ([M+Na]+), 502.3 ([M+NH4]+), 485.3 ([M+H]+), 429.3 ([M-C4H8]+).
