Scheme 4.

Reagents and conditions: a) (1) Ac₂O, pyridine, 50 °C; (2) piperidine, DCM; b) HATU, DIPEA, DMF; c) Pd/C, H₂, NH₄OAc, DCM/MeOH; d) multiple rounds of automated solid phase peptide synthesis; e) TFA/H₂O/Phenol/TIPS; f) (1) BnBr, DIPEA, DMF; (2) piperidine, DMF; g) HATU, DMF; h) (1) Zn, CuSO₄ (sat.), Ac₂O/THF/HOAc; (2) NH₂NH₂-H₂O, HOAc, DCM/MeOH; (3) SO₃-Et₃N, DMF, 55 °C; (4) H₂, Pd/C, DCM/MeOH; i) base catalyzed hydrolysis