Scheme 1.
Synthesis of compound 4. Reagents and conditions: (a) Ac2O, NEt3, DMF (cat.), DCM, 25 °C, 1 day; (b) oxalyl chloride, NEt3, DMF (cat.), DCM, 25 °C, 5 h, then piperazine, DCM, NEt3, DMAP, 0/25 °C, 30 min.
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