Scheme 1. The synthetic pathway of compounds 6a–6i. Reagents and conditions: i: CH₃COOH, EtOH, reflux, 8 h; ii: NaBH₄, MeOH, room temperature (r.t.), 10 h; iii: ClCH₂COCl, TEA/THF, ice-bath, 5 h; iv: NH₂NH₂·H₂O, EtOH, r.t., 4 h; v: (1) CS₂/KOH, EtOH, reflux, 10 h; (2) HCl, pH 4–5; vi: K₂CO₃, acetone, r.t., 8 h.