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(A,B) Synthesis of compounds 1a–1b (A) and 1c–1d (B). Reagents and conditions: (i) according to Ref. [19]; (ii) AcOH/H2O 5:1, 40 °C, 48 h; (iii) TEMPO, Fe(NO3)3·9H2O, KCl, air flux, DCE, r.t., 24 h; (iv) (1) 15% TFA, DCM, r.t., 1 h; (2) column purification with Dowex Marathon C (H+ form) resin, 10%Py/H2O (eluent).
