C1(COC(=O)[NH:10][CH2:11][CH2:12][CH2:13][N:14]([CH2:24][C:25]([O:27][C:28]([CH3:31])([CH3:30])[CH3:29])=[O:26])[C:15]2[C:18](=[O:19])[C:17](=[O:20])[C:16]=2OCC)C=CC=CC=1.C1CC=CCC=1>[Pd].C(O)C>[CH3:29][C:28]([O:27][C:25](=[O:26])[CH2:24][N:14]1[CH2:13][CH2:12][CH2:11][NH:10][C:16]2[C:17](=[O:20])[C:18](=[O:19])[C:15]1=2)([CH3:31])[CH3:30]To a cooled (20° C.) flask containing 10% palladium on carbon (5.25 g) under nitrogen was added [3-[[(1,1-dimethylethyloxycarbonyl)methyl]-(2-ethoxy-3,4-dioxo-1-cyclobuten-1-yl)amino]propyl]carbamic acid phenylmethyl ester (5.25 g, 11.8 mmol) in ethanol (500 mL), followed by 1,4-cyclohexadiene (11 mL, 0.12 mol) over 5 min. After 5 hours, the suspension was filtered through Celite® with generous ethanol washing (1 L). The ethanol was evaporated and the residue was dissolved in dichloromethane and purified by flash chromatography (7.5 cm diameter, gradient elution with 2.5-3% methanol in dichloromethane) to yield (8,9-dioxo-2,6-diazabicyclo[5.2.0]non-1(7)-en-2-yl)acetic acid 1,1-dimethylethyl ester as a white solid (2.59 g, 82%, mp 167°-168° C.); IR (KBr, cm-1): 3200, 1800, 1720; MS (EI): 266 (M+, 42), 210 (33), 166 (37), 165 (100), 154 (58), 138 (37), 70 (58); 1H NMR (DMSO, 400 MHz): δ 8.62 (br s, NH), 4.38 (s, 2H), 3.36-3.27 (m, 4H), 1.91 (m, 2H), 1.40 (s, 9H).
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