The present invention provides a process for the preparation and purification of thiol-containing maytansinoids comprising the steps of: (1) reductive hydrolysis of a maytansinoid C-3 ester with a reducing agent selected from the group consisting lithium trimethoxyaluminum hydride (LiAl (OMe)3H), lithium triethoxyaluminum hydride (LiAl(OEt)3H), lithium tripropoxyaluminum hydride (LiAl (OPr)3H), sodium trimethoxyaluminum hydride (NaAl (OMe)3H), sodium triethoxyaluminum hydride (NaAl(OEt)3H) and sodium tripropoxyaluminum hydride (NaAl(OPr)3H) to yield a maytansinol (2) purifying the maytansinol to remove side products when present (3) esterifying the purified maytansinol with a carboxylic acid to yield a mixture of an L- and a D-aminoacyl ester of maytansinol (4) separating the L-aminoacyl ester of maytansinol from the reaction mixture in (3) (5) reducing the L-aminoacyl ester of maytansinol to yield a thiol-containing maytansinoid and (5) purifying the thiol-containing maytansinoid.
