Scheme 2.

The synthesis of analogs with a cyclic metal bonding group or a rigid linker.^a

^aReagents and conditions: (a) CDI, DIPEA, THF, 25 °C, 20 h; 14, 25 °C, 20 h, 43%; (b) CDI, DIPEA, THF, 25 °C, 20 h; 16, 25 °C, 20 h, 22%; (c) TFA, CH₂Cl₂, 25 °C, 30 min, 100%; (d) ClCO₂Et, Et₃N, THF, 25 °C, 1 h; NH₂OTBS, THF, 25 °C, 1 h; (e) TFA, CH₂Cl₂, 25 °C, 30 min, 74% for 2 steps; (f) OCNCH₂CH₂Cl, CH₂Cl₂/MeCN, 40 °C, 20 h, 89%; (g) NaN₃, TBAI, DMF, 45 °C, 48 h, 59%; (h) N-hydroxypropiolamide, CuSO₄, sodium ascorbate, EtOH/t-BuOH/H₂O (2/1/1), 25 °C, 20 h, 28%.