[C:1]1([CH2:7][O:8][CH2:9][C@@H:10]([C:12]([OH:14])=[O:13])[NH2:11])[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.C(O[C:19]([CH3:22])([CH3:21])[CH3:20])(=O)C.Cl(O)(=O)(=O)=O.C([O-])([O-])=O.[Na+].[Na+]>CCOCC>[CH3:20][C:19]([O:13][C:12](=[O:14])[C@H:10]([CH2:9][O:8][CH2:7][C:1]1[CH:2]=[CH:3][CH:4]=[CH:5][CH:6]=1)[NH2:11])([CH3:22])[CH3:21]To a suspension of O-phenylmethyl-L-serine (50 g; 0.26 mol) in t-butyl acetate (1000 mL; 7.49 mol) 70% perchloric acid was added (50 mL; 0.58 mol). The solution was maintained under stirring for 72 h at 250° C. under inert atmosphere. The reaction mixture was diluted with Et2O (300 mL), then a 10% Na2CO3 aqueous solution was slowly added until pH 9 was reached. The organic layer was separated, dried over Na2SO4 and concentrated. The residual t-butyl acetate was distilled under reduced pressure (40° C.0.1 mm Hg). The desired product was obtained (54.2 g; 0.21 mol). Yield 83%.
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