Scheme 1. Systematic Scheme for Preparation of Target Conjugated 20a–k.

Reagents and condition: (i) CH₃COOCH₂CH₂Br, K₂CO₃, acetone, reflux at K₂CO₃, acetone reflux at 60–70 °C for 48 h. Yield: 99%. (ii) NH₂NH₂·H₂O, RT for 8 h. Yield: 98%. (iii) RNCS (18a–k), EtOH, reflux for 6 h at 60–70 °C. Yield: 95%. (iv) EDC·HCl, cat. HOBt, dry DMF, RT for 3–5 h. Yield: 78–98%.