C(O)(=O)C.[CH:5]([NH2:7])=[NH:6].[O-]CC.[Na+].O=[C:13]1[CH2:18][CH2:17][N:16]([C:19]([O:21][C:22]([CH3:25])([CH3:24])[CH3:23])=[O:20])[CH2:15][C:14]1=[CH:26]N(C)C>C(O)C>[C:22]([O:21][C:19]([N:16]1[CH2:17][CH2:18][C:13]2[N:6]=[CH:5][N:7]=[CH:26][C:14]=2[CH2:15]1)=[O:20])([CH3:25])([CH3:23])[CH3:24]Formamidine acetate (472 mg, 4.52 mmol) in 15.0 mL of absolute ethanol was treated with sodium ethoxide (21 wt % solution in ethanol; 1.7 mL). After 30 min, a solution of the product from Step A above (1.15 g) in 8 mL of absolute ethanol was added, and the mixture was warmed at reflux for 18 h. The dark solution was cooled to room temperature and concentrated under reduced pressure, and the residue was partitioned between ethyl acetate and water. The organic layer was separated, washed with brine, dried over magnesium sulfate and concentrated to an orange oil. Purification by flash chromatography (silica gel; 2% methanol/dichloromethane as eluant) afforded the title compound as a light yellow gum.
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