A jacketed reactor is loaded with a solution of N-[2-(1,1-dimethylethoxy)-2-oxoethyl]-L-tryptophan 1,1-dimethylethyl ester (5 g; 13.35 mol) prepared at point B, in CH3CN (25 mL). To the solution cooled to −80° C. is added ethylene oxide (13 mL; 0.26 mol) and ytterbium triflate (0.83 g; 1.34 mol). The mixture is then slowly warmed to room temperature then, after 15 hours, diluted with water (50 mL) and extracted with Et2O (3×50 mL). The organic phases are evaporated to dryness to obtain a crude which is purified by flash chromatography (eluent n-hexane/EtOAc 7:3) to give the desired product as a pale yellow oil (4.32 g; 10.32 mol).
