C([O:3][C:4](=O)/[CH:5]=[CH:6]/[C:7](=[C:13](/[NH2:15])\[CH3:14])/[C:8]([O:10][CH2:11][CH3:12])=[O:9])C>CN(C)C=O>[CH2:11]([O:10][C:8]([C:7]1[CH:6]=[CH:5][C:4](=[O:3])[NH:15][C:13]=1[CH3:14])=[O:9])[CH3:12]A solution of intermediate (5a), (E)-4-[1-amino-eth-(Z)-ylidene]-pent-2-enedioic acid diethyl ester (51 g, 225 mmol) in N,N-dimethylformamide (250 mL) was heated at 175° C. for 24 hours to give a dark brown solution. The resulting solution was allowed to cool and a pale brown precipitate slowly formed. The precipitate was removed by filtration and the solids washed with dichloromethane (75 mL) and hexane (100 mL) to give a pale yellow powder. The solids were dried in vacuo (60° C.) to give the title compound as a pale yellow powder, 14.45 g (36%)
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