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Synthesis of compounds (29–32). Reagents and conditions: (i) SnCl2.2H2O, HCl/CH3COOH, 65 °C, 5 h, yield: 65%; (ii) HBTU, TEA, DMF, room temperature, 30 min, yield: 41%; (iii) TFA/CH2Cl2, 24 h, room temperature, yield: 51%; (iv) HBTU, TEA, DMF, 3 h, room temperature, yield: 15–18%. For details of compounds characterization, see the experimental section.
