[CH3:1][O:2][C:3](=[O:19])[CH:4]([NH:11][C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])P(OC)(OC)=O.CC1(C)[O:26][C:25](=O)[CH:24]=[C:23]([CH3:28])O1.N1C(C)=CC=CC=1C>C1(C)C=CC=CC=1>[CH3:1][O:2][C:3]([CH:4]1[C:23]([CH3:28])=[CH:24][C:25](=[O:26])[N:11]1[C:12]([O:14][C:15]([CH3:16])([CH3:17])[CH3:18])=[O:13])=[O:19]A solution of tert-butoxycarbonylamino-(dimethoxy-phosphoryl)-acetic acid methyl ester (20.0 g, 67 mmol, 1 equiv), 2,2,6-trimethyl-[1,3]dioxin-4-one (43.96 ml, 47.8 g, 336 mmol, 5.0 equiv), and 2,6-lutidine (11.71 ml, 10.77 g, 101 mmol, 1.5 equiv) in toluene (200 ml) was heated at reflux for 16 h. The reaction mixture was allowed to cool to 23° C. and was concentrated. The residue was purified by flash column chromatography (5% ethyl acetate-dichloromethane initially, grading to 8% ethyl acetate-dichloromethane) to give 3-methyl-5-oxo-2,5-dihydro-pyrrole-1,2-dicarboxylic acid 1-tert-butyl ester 2-methyl ester (14.74 g, 86%) as a yellow oil.
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