Scheme 2.

Synthesis of Protease Inhibitors 15–28^a

^a Reagents and conditions: (a) iBuNH₂, EtOH, 80 °C, 3 h; (b) aq. Na₂CO₃, CH₂Cl₂, 0 °C to rt, 6 h; (c) Et₃N, CH₂Cl₂, 0 °C to rt, 6 h; (d) NaBH₄, MeOH, 0 °C, 30 min; (e) TFA, CH₂Cl₂, rt, 1 h; (f) (COCl)₂, rt, overnight; (g) Et₃N, THF, 0 °C to rt, 6 h; (h) SnCl₂·2H₂O, EtOAc, 70 °C, 3 h; (i) Pd-C, HCO₂NH₄, EtOAc, rt, overnight.