Total Syntheses of Tubulysins

The total syntheses of tetrapeptides tubulysins D (1b), U (1 c), and V (1d), which are potent tubulin polymerization inhibitors, are described. The synthesis of Tuv (2), an unusual amino acid constituent of tubulysins, includes an 1,3-dipolar cycloaddition reaction of chiral nitrone D-6 derived from D-gulose with N-acryloyl camphor sultam (-)-9 employing the double asymmetric induction, whereas the synthesis of Tup (20), another unusual amino acid, involves a stereoselective Evans aldol reaction of (Z)-boron enolate generated from (S)-4-isopropyl-3-propionyl-2-oxazolidinone with N-protected phenylalaninal and a subsequent Barton deoxygenation protocol. We accomplished the total syntheses of tubulysins U (1c) and V (1d) by using these methodologies, in which the isoxazolidine ring was used as the effective protective group for gamma-amido alcohol functionality. Furthermore, to understand the structure-activity relationship of tubulysins, we synthesized tubulysin D (lb) and cyclo-tubulysin D (le) from 2-Me and 20, and ent-tubulysin D (em-id) from eitt-2-Me and ent-20, respectively. The preliminary results regarding their biological activities are also reported.