Alternately, combine (S)-4-benzyl-3-phenylacetyl-2-oxazolidinone (8.0 g, 27 mmol) and tetrahydrofuran (50 mL). cool in a dry-ice/acetone bath. Add dropwise a solution of sodium hexamethyldisilazide (30 mL, 1.0 M in tetrahydrofuran, 30 mmol). After 30 minutes, add allyl iodide (9.1 g, 54 mmol) and then replace the bath with a dry-ice/carbon tetrachloride bath. After 1.5 hours, quench the reaction by the addition of a saturated aqueous sodium chloride solution, extract with diethyl ether, and separate the layers. Extract the aqueous layer three times with diethyl ether. Combine the organic layers, extract with an aqueous 1 M hydrochloric acid solution, a saturated aqueous sodium bicarbonate solution, dry the organic layer over Na2SO4, filter, and evaporate in vacuo to give a residue. Dissolve in ethyl acetate, extract with a saturated aqueous sodium thiosulfate solution, dry the over Na2SO4, filter, and evaporate in vacuo to give a residue. Chromatograph the residue on silica gel eluting with 10% ethyl acetate/hexane. Combine the product containing fractions, evaporate, and recrytallize from chloroform/hexane to give (S)-4-benzyl-3-(2-phenylpent-4-enoyl)-2-oxazolidinone.
