Scheme 2.

Convergent synthesis of ligand 1 and intermediate 8. Conditions: (a) tetrahydro-4H-pyran-4-one, NaBH₃CN, Bu₄NBr, Et₃N, DMF, 36 h, 57%; (b) HCl/EtOAc, 1 h, then NaOH, 93%; (c) 4a or 4b, DIEA, DMF, 70 °C, 16 h, 58% and 99% (crude) for 11a and 11b, respectively; (d) 11a, Raney nickel, N₂H₄·H₂O, EtOH, 45 °C, 2 h, 63% or 11b, NH₄Cl, Zn, EtOH/EtOAc, 16 h, 90%; (e) triphosgene, Et₃N, THF, 45 °C, 2 h, 60% and 52% for 1 and 8, respectively.