Scheme 2.

Synthesis Routes of the TEMPO-Substituted Campthotecin Derivatives^a

^a (a) H₂SO₄, MeOH, FeSO₄, H₂O₂ 30%, then Mn₂O, RT, 2 h (yield 74%); (b) CH₃COOH, H₂O, reflux, 1 h (yield 75%); (c) TEMPOamine (6), ytterbium triflate, molecular sieves 4 Å, CH₂Cl₂, RT, 16 h (yield 22%); (d) HCOOH, H₂O₂ 30%, 0 °C, 16 h (yield 75%); (e) 6, HBTU, HOBt, DIPEA, THF, RT, 16 h (yield 67%).