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Synthesis of the target compounds (5a1-3, 5b1-3, 5c1-3). Reagents and conditions: (i) N-ethyl-N-methylcarbamoyl chloride, TEA, 95 °C, 12 h; (ii) H2, MeOH, 10% Pd-C, 4 h, 4 bar (for 3a, 3b); (iii) H2, MeOH/HCl, 10% Pd-C, 6 h, 4 bar (for 3c); (iv) TBTU, NMM, dry DMF, 0 °C → r.t., N2 atm, 20 h.
