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Reagents and condition: (a) 2, 4-pentanedione, B2O3, (n-BuO)3B, EtOAc, 85 °C, 30 min, n-BuNH2, 105 °C, 30 min, 1 N HCl, 50%; (b) B2O3, (n-BuO)3B, ethyl acetate, 85 °C, 30 min, piperidine, 85 °C, 30 min, 0.5 N HCl, 50 °C, 30 min, 55% (c) MeOH, 1 N NaOH, 50 °C, 1 h, 50% formic acid, 75 %.
