C(=[N:8][CH:9]([P:22]([O:26][CH3:27])([O:24][CH3:25])=[O:23])[C:10]([O:12][CH2:13][C:14]1[CH:19]=[CH:18][C:17]([O:20][CH3:21])=[CH:16][CH:15]=1)=[O:11])C1C=CC=CC=1.O.C1(C)C=CC(S(O)(=O)=O)=CC=1.C1CCCCC1>CCOCC>[NH2:8][CH:9]([P:22]([O:26][CH3:27])([O:24][CH3:25])=[O:23])[C:10]([O:12][CH2:13][C:14]1[CH:15]=[CH:16][C:17]([O:20][CH3:21])=[CH:18][CH:19]=1)=[O:11]p-Methoxybenzyl N-benzylidene-α-amino-dimethylphosphonoacetate (9.78 g) in ether (50 ml) is added over 20 minutes to a stirring solution of p-toluenesulfonic acid monohydrate (4.76 g) in ether (150 ml). Cyclohexane (100 ml) is added and the solvents are decanted off. The residue is washed with more 2:1 ether-cyclohexane, which is again decanted off. The residue is partitioned between methylene chloride and 1 M dipotassium hydrogen phosphate (50 ml). The organic portion is separated, washed with water, dried, and evaporated in vacuo to give p-methoxybenzyl α-aminodimethylphosphonoacetate.
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