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Reagents and conditions: (a)4 picolinaldehyde, dry DMF, TFA, molecular sieves 3 Å, 110 °C, N2 to O2, 21 h; (b) picolinaldehyde, dry DMF, TFA, molecular sieves 3 Å, 110 °C, N2 to O2, 18 h; (c) (1) H2SO4, AcOH, reflux, 17 h, (2) RNH2, imidazole, pyridine, 120 °C, 6 h.
