C([O:3][C:4](=[O:20])[CH:5]([NH:17]C=O)/[CH:6]=[CH:7]/[CH2:8][P:9]([O:14]CC)([O:11]CC)=[O:10])C>Cl>[NH2:17][CH:5](/[CH:6]=[CH:7]/[CH2:8][P:9]([OH:14])([OH:11])=[O:10])[C:4]([OH:20])=[O:3]8.22 g of E-2-formylamino-5-diethylphosphono-3-pentenoic acid ethyl ester are dissolved in 170 ml of 6N hydrochloric acid and the whole is heated under reflux for 22 hours. After concentration in vacuo, the oily residue is taken up in a small amount of ethanol and the mixture is again concentrated by evaporation in vacuo. This procedure is repeated a furthertwice. The resulting residue is dissolved in 15 ml of ethanol and 20 ml of ethanol/propylene oxide (1:1) are added dropwise. The resulting brownish-coloured precipitate is filtered off and purified by ion exchangechromatography (Dowex 50W×8/H2O). After concentration and lyophilisation, E-2-amino-5-phosphono-3-pentenoic acid is obtained in the form of a white amorphous powder, 1H-NMR (D2O): 2.39 (dd, 2H, C(5)--H); 4.27 (d, 1H, C(2)--H); 5.53 (m, 1H, C(3)--H); 5.87 (m, 1H, C(4)--H), m.p. after recrystallisation from ethanol/water 191°-192°.
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