Synthesis of the Deacetoxytubuvaline Fragment of Pretubulysin and its Lipophilic Analogues for Enhanced Permeability in Cancer Cell Lines

Abstract

In the last two decades, tubulysins have emerged as alternatives to microtubule depolymerizing agents such as colchicine and vinblastine, which are well-established anticancer agents. However, the complex structure of tubulysins has always posed a challenge for synthetic chemists to scale up the production of these compounds. We report a new strategy for the practical gram-scale synthesis of a (4R)-4-[(tert-butoxycarbonyl)amino]-5-methylhexanoic acid through regioselective cleavage of a chiral aziridine ring with a vinyl Grignard reagent to afford tert-butyl [(1R)-1-isopropylbut-3-en-1-yl]carbamate, which was subjected to regioselective hydroboration–oxidation with 9-BBN. The resulting (4R)-4-[(tert-butoxycarbonyl)amino]-5-methylhexanoic acid was successfully transformed into the deacetoxytubuvaline fragment of pretubulysin or its highly lipophilic methyl-substituted thiazole and oxazole analogues for incorporation into pretubulysins. Increasing the lipophilicity of tubulysin or pretubulysin molecules should enhance their cell permeability and cytotoxicity in cancer cell lines.