Improved processes for the preparation and purification of maytansinoid esters, especially thiol and disulfide-containing maytansinoids are described. In one aspect the process comprises a process of making a maytansinoid ester comprising forming an anion of maytansinol or a maytansinoid bearing a free C-3 hydroxyl moiety and reacting the anion with an activated carboxyl compound to thereby produce the maytansinoid ester.
