In an effort to improve the stability of labile lactone ring and water solubility of camptothecin, five novel spin-labeled camptothecin derivatives were synthesized in quantitative yield by a simple modification of the carbodiimide method using the combination of scandium triflate (Sc (OTf) 3) and 4-dimethylaminopyridine (DMAP), and the in vitro pharmacokinetic determination of the lactones of representative compound 13a showed that the biological ...