Scheme 1. General Synthetic Procedure To Prepare 1,2,4-Oxadiazole Derivatives 3a–f.

Reagents and conditions: (a) NH₂OH HCl, K₂CO₃ in CH₃OH, reflux, 100–90% yield; (b) N-Boc-Inp-OH, DIPEA, HBTU in DMF dry, 80 °C, 60–80% yield; (c) TFA/CH₂Cl₂ 1:1, 2 h, quantitative yield.