Combine (S)-N-benzyl-N-methyl-2-[N'-(t-butoxycarbonyl)-allylamino]-3-phenyl-propionamide (10.04 g, 24.5 mmol), and pyridine (0.13 ml) in dichloromethane/methanol (300 mL/30 mL). Cool to -78° C. Pass ozonized oxygen through the solution until a persistent light blue color is obtained. Pass nitrogen through the solution until the blue color dissipates. Add dimethyl sulfide (55 mL). Allow the reaction mixture to warm to ambient temperature and stir for 16 hours. Concentrate in vacuo to obtain a residue. Dilute the residue with ethyl acetate and extract with water. Separate the layers, dry the organic layer over MgSO4, filter, and evaporate in vacuo. Chromatograph on silica gel eluting with 10% ethyl acetate/hexane to give the title compound: TLC Rf =0.53 (silica gel, 10% ethyl acetate/hexane).
