Scheme 1. Reagents and conditions: (a) NH₂NH₂·H₂O (4 equiv.), CH₃CO₂H (1 equiv.), EtOH, reflux 4 h; (b) Et₃N (1.2 equiv.), Ac₂O or TFA (1 equiv.), CHCl₃; (c) Et₃N (1.2 equiv.), carboxylic acid chloride (1 equiv.), CHCl₃; (d) carboxylic acid (1 equiv.), EtO₂CCl/Et₃N, CHCl₃.