(i) Pd/C, H2, EtOAc; (ii) a) (BrCH2CO)2O DMF, rt, 1 h; b) NaN3, DMF, rt, 24 h (93% yield from 7a); (iii) 1-acetylimidazole, DMF, 24 h (quantitative); (iv) CuI, L-ascorbate, DIPEA, CH3CN/tBuOH/H2O, rt (73% yield); (v) TFA-HS(CH2)2SH-H2O, rt, 1 h (76% yield).