Scheme 4.
Reagents and conditions: (a) suitable R-Br, K2CO3, anhydrous DMF, reflux, 1–4 h; (b) (i) dry THF, CH3COONa, 0 °C then triphosgene, reflux, 2 h; (ii) NH3 33%, 0 °C, 1 h; (c) Lawesson’s reagent, anhydrous toluene, reflux, 5 h.
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