To a solution of oxalyl chloride (0.18 ml) in dichloromethane (10 ml) was added dropwise a solution of dimethyl sulfoxide (0.3 ml) in dichloromethane (2 ml) at -60° C. After stirring at -70° C. for 30 minutes, to the mixture was added dropwise a solution of (2S,4R)-1-allyloxycarbonyl-2-t butyldimethylsilyloxymethyl-4-hydroxypyrrolidine (0.6 g) in dichloromethane (5 ml) at -60° C. After stirring at -70° C. for 30 minutes, to the mixture was added dropwise triethylamine (1.3 ml) and the resulting mixture was allowed to warm to ° C. and diluted with ethyl acetate (150 ml) and brine. After adjusting pH to around 3 with 10% hydrochloric acid, the organic layer was separated, washed with brine, dried over magnesium sulfate and evaporated. The residue was chromatographed on silica gel (50 ml) eluting with 15% ethyl acetate in hexane to give (2S)-1-allyloxycarbonyl-2 -t-butyldimethylsilyloxymethyl-4-oxo-pyrrolidine (0.54 g).
