Scheme 1

Synthesis of key amine intermediate 13. a 3, K₂CO₃, abs. DMF, r.t., 16 h, 81%; b, abs. DMF/MeOH (1:1), r.t., 16 h, 41%; c DMEAD, PPh₃, abs. DMF/THF (1:1), 0 °C to r.t., 16 h, 65%; d l-cysteic acid, NaBH₃CN, abs. DMF/MeOH (1:1), r.t., 16 h, 41%; e taurine, HATU, HOBt, DIPEA, abs. DMF, r.t., 2 h, 70%; f 11, sodium ascorbate, CuSO₄, THPTA, ^tBuOH/H₂O (1:1), r.t., 16 h, 78%; g NaN₃, abs. DMF, 50 °C, 3 h, 60%