To a solution of [(S*)-2-(4-Benzyloxy-phenyl)-1-((S*)-5-oxo-2,5-dihydro-furan-2-yl)-ethyl]-carbamic acid tert-butyl ester (11.4 g, 27.7 mmol) in THF (35 ml) and DMPU (5 ml, 42 mmol, 1.5 eq) at −78° C. is added dropwise lithium-bis-(trimethylsilyl)-amide (55 ml 1M solution in THF, 55 mmol, 2 eq). After stirring at −78° C. for another 45 min methyliodide is added dropwise and the mixture is stirred another 3 h at −78° C. Propionic acid (10.3 ml, 138 mmol, 5 eq) is added followed by water (10 ml). After warming up to 0° C. a 10% solution of citric acid (72 ml) is added. The reaction mixture is extracted with EtOAc. The organic layer is washed with bicarbonate, 0.1N sodium sulfite and brine, dried over magnesium sulfate and evaporated. Purification by chromatography on silica (hexane/EtOAc 9/1 to 4/1) followed by recrystallization from ether/hexane gives white crystals (8.14 g).
