[OH:1][CH2:2][C@H:3]1[NH:7][C:6](=[O:8])[CH2:5][CH2:4]1.[CH:9](=O)[C:10]1[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=1.C1(C)C=CC(S(O)(=O)=O)=CC=1.C(=O)([O-])O.[Na+]>C1(C)C=CC=CC=1.O>[C:10]1([C@@H:9]2[N:7]3[C:6](=[O:8])[CH2:5][CH2:4][C@H:3]3[CH2:2][O:1]2)[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=1A suspension of 148.5 g of (S)-5-hydroxymethyl-pyrrolidin-2-one in 891 mL of toluene is treated with 172.1 mL of benzaldehyde at room temperature. p-Toluenesulphonic acid (2.94 g) is added and the reaction mixture stirred at reflux for 20 hours with azeotropic removal of water. The reaction mixture is treated with 500 mL of a 5% solution of sodium hydrogen carbonate in water. The organic layer is separated and washed once with 500 mL of a 40% solution of sodium bisulphite solution followed by 2×250 mL of water. The organic layer is dried with sodium sulphate, filtered and the solvent removed to give an oil. Fractional distillation in vacuum produces pure (3R,7aS)-3-phenyl-tetrahydro-pyrrolo[1,2-c]oxazol-5-one.
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