Scheme 4.
Reagents and conditions: (i) (a) ethylenediamine, EtOH, 70 °C, 24 h; (b) Ac2O, Py, rt, 24 h, 83% for 13 for, 81% for 14, 73% for 15, 71% for 16; (ii) NaOMe, MeOH, 86% for 17, 86% for 18, 88% for 19, 85% for 20.
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