Scheme 1.
Reaction steps involved in the synthesis of compounds 5a–h. Reagents and conditions: (i) PdCl2, CH3CN, reflux, 3 h; (ii) NH2OH.HCl, pyridine, EtOH, reflux, 5 h; (iii) TFA, CH3CN, reflux, 2 h; (iv) (CF3CO)2O, THF, reflux 5 h.
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