A study on the impact of catharanthine C10 and C12 indole substituents on the biomimetic Fe(III)-mediated coupling with vindoline led to the discovery and characterization of two new and substantially more potent derivatives, 10'-fluorovinblastine and 10'-fluorovincristine. In addition to defining a pronounced and la unanticipated substituent effect on the biomimetic coupling, fluorine substitution at C10', which minimally alters the natural products, was found to uniquely enhance the activity 8-fold against both sensitive (IC50 = 800 pM, HCT116) and vinblastine-resistant tumor cell lines (IC50 = 80 nM, HCT166/VM46). As depicted in the X-ray structure of vinblastine bound to tubulin, this site resides at one end of the upper portion of the T-shaped conformation of the tubulin-bound molecule, suggesting that the 10'-fluorine substituent makes critical contacts with the protein at a hydrophobic site uniquely sensitive to steric interactions.