Scheme 3.
Reagents and conditions: (a) Et3N, DMF/H2O, 80 °C, 72 h; (b) 4 M HCl in 1,4-dioxane, 1,4-dioxane, rt, 3 h; (c) 1a or 1b, EDC, HOBt, NMM, DMF, rt, 15 h.
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